Module 10: Terrestrial Worlds

atmosphere

(air) relatively thin layer of gaseous matter that surrounds the body's surface

magnetosphere

(magnetic fields) generated by the rotation and convection of molten metals in the body's outer core

lithosphere

(rocks) the solid soils, sediments, and rocks of the body's crust and upper mantle

Earth Facts

Radius: Equatorial = 6,378 km (3,963 miles) Polar = 6,356 km (3,949 miles) The difference of 22 km (14 miles) is caused by the rotation of the Earth. The resulting shape of the Earth is an oblate spheroid. Shape deduced by ancient Greeks and Romans. Aristotle observed shape of Earth's shadow during lunar eclipse and change in stars at horizon with changing latitude. Fernando de Magellan's expedition (1519-1522): 1st to circumnavigate the globe, proof that the Earth is round. The overall oblate shape of the Earth was established by French Academy expeditions between 1735 and 1743. Circumference Eratosthenes (Greek, ~200 B.C.): measured the circumference of the Earth to be 250,000 stadia or ~40,000 km (see the complete story of this in your text) measured by satellite to be 40,070 km Mass: 5.97 x 10^24 kg (6.6 sextillion tons) Applying Newton's modification to Kepler's third law for the Earth-Moon system: (MꚚ+ M Moon) P^2 = a^3 where MꚚ is Earth's mass and M Moon is the Moon's mass, in solar masses. Assuming that the mass of the Moon is far less than the mass of the Earth, M Earth = a^3/ P^2 For Earth-Moon distance of 380,000 km (2.53 x 10^-3 AU) and period of 27.3 days (7.48 x 10^-2 year), MꚚ = (2.53 x 10^-3 AU)^3 / (7.48 x 10^-2 year)^2 = 2.9 x 10^-6 solar masses = 5.8 x 10^24 kg (accepted value 5.98 x 10^24 kg) Volume: 1 x 10^12 km^3 (2.60 x 10^11 miles^3) Density: 5.5 g/cm^3 average density = total mass/ volume Earth's average density is the highest in the solar system, resulting from its metallic core and rocky mantle. Jupiter is 318 x more massive than Earth, but has a density of less than ¼ that of Earth because it is made primarily of low mass materials that are usually gases on Earth. The Earth's mean density and total mass were determined by the English physicist and chemist Henry Cavendish in about 1797. It was later ascertained that the density of rocks on the Earth's surface is significantly less than the mean density, leading to the assumption that the density of the deeper parts of the planet must be much greater. Knowing the Earth's mass MꚚ = 5.98 x 10^24 kg and the diameter of the Earth DꚚ = 12,756 km (RꚚ = DꚚ/2 = 6,378 km). the average density of the Earth can be calculated: average density = mass/volume = MꚚ /(4/3 · π RꚚ^3) = 5.98 x 10^24 kg /(4/3 π (6,378 km)^3) = 5.5 x 10^12 kg/km^3 = 5.5 g/cm^3 Acceleration due to gravity at the Earth's surface Determined from Newton's Second Law: F=ma Universal Law of Gravitation F(g)=GMm/r^2 These two equations should be put together and reduced to give the formula used to get the acceleration due to gravity. If a=g, and the forces are equal, then F= ma = GMm/r^2 now cancel the "m" on both sides and you get a = GM/r^2 Knowing the Earth's mass, the gravitational constant "G", which is 6.67 X 10^-11 N·m^2/kg^2 MꚚ = 5.98 x 10^24 kg and the radius of the Earth RꚚ = DꚚ /2 = 6,378 km the surface gravity of the Earth can be calculated: g = 9.8 m/s^2 Escape speed: 11.2 km/s

Mercury's Formation and History

Scientists once thought that Mercury's high density from its richness in iron compared with the other terrestrial planets' could be explained by its accretion from objects made up of materials derived from the extremely hot inner region of the solar nebula, where only substances with high freezing temperatures could solidify. The more volatile elements and compounds would not have condensed so close to the Sun. However recent exploration has revealed significant amounts of volatile material in the surface of Mercury. To accommodate these observations, modern theories of the formation of the solar system propose that the components of the bodies that accreted into Mercury likely were derived from a wide part of the inner solar system. Indeed, Mercury itself may have formed anywhere from the asteroid belt inward; subsequent gravitational interactions among the many growing proto-planets could have moved Mercury to its current location. There is currently no definitive theory to explanation for Mercury's high average density. Some planetary scientists have suggested that during Mercury's early epochs, after it had already differentiated into a less-dense crust and mantle of silicate rocks and a denser iron-rich core, a giant collision stripped away much of the planet's outer layers, leaving a body dominated by its core. This event would have been similar to the collision of a Mars-sized object with Earth that is thought to have formed the Moon. However, other processes may have been primarily responsible for Mercury's high density. Perhaps the materials that eventually formed Mercury experienced a preferential sorting of heavier metallic particles from lighter silicate ones because of aerodynamic drag by the gaseous solar nebula. Perhaps, because of the planet's nearness to the hot early Sun, its silicates were preferentially vaporized and lost. Each of these scenarios predicts different bulk chemistries for Mercury. In addition, in-falling asteroids, meteoroids, and comets and implantation of solar wind particles have been augmenting or modifying the surface and near-surface materials on Mercury for billions of years. Because these materials are the ones most readily analyzed by telescopes and spacecraft, the task of extrapolating backward in time to an understanding of ancient Mercury, and the processes that subsequently shaped it, is formidable. Mercury would have a similar history to the Moon, but one in which the planet cooled off more slowly to become geologically inactive. On the presumption that Mercury's craters were produced by the same populations of planetesimals, asteroids, and comets that struck the Moon, most of the craters would have formed before and during an especially intense period of bombardment in the inner solar system, which is well documented to have ended on the Moon about 3.8 billion years ago. Apart from occasional cratering and the formation of surface scarps from the continued cooling and shrinking of Mercury's interior, Mercury's surface is essentially geologically dead. On the other hand, there are many indications that Mercury's interior is very much geologically alive even today. Its dipolar magnetic field seems to require a core that is still at least partially molten in order to sustain the required field generating dynamo. Measurements of Mercury's gravitational field by Messenger have been interpreted as proving that at least a region of the outer core is still molten. It is likely that the outer shell of Mercury's iron core remains molten because of contamination, for instance, with a small proportion of sulfur, which would lower the melting point of the metal, and of radioactive potassium, which would augment production of heat. Also, the planet's interior may have cooled more slowly than previously calculated as a result of restricted heat transfer. Perhaps the contraction of the planet's crust, so evident about the time of formation of Caloris, pinched off the volcanic vents that had yielded such prolific volcanism earlier in Mercury's history.

Venus Surface Features

Similar in size and density to Earth, the surface of Venus has impact, tectonic, and volcanic features. Composed of igneous rocks which would appear a dull gray if viewed on Earth, but when viewed in situ, have a yellow tint due to filtering of the shorter (bluer) wavelengths of by the thick Venusian atmosphere. The upper color photo below from the Venera 13 lander shows plates of rock thought to be basalt, with dark soil between some of them. The lower photo has been corrected to show the color of the surface as it would appear under direct sunlight. This site is probably typical of the plains on Venus. Tectonic Features: Heat flow from the interior creates mountains, rifts, and patterns of fractures, while the sluggish winds sculpt the surface in subtler ways. Tectonic Features on Venus Alpha Regio (bright region center top), is a topographic upland ~ 1300 km across, characterized by multiple sets of intersecting trends of structural features such as ridges, troughs, and flat-floored fault valleys. Maxwell Montes The bright radar feature is the highest mountain on Venus, rising almost 11 km(6.8 miles) above mean planetary radius; higher than Mt. Everest on Earth. Its broad ridges and valleys suggest that the topography resulted from compression. The prominent circular feature in eastern Maxwell is Cleopatra, a double-ring impact basin about 100 km (62 miles) in diameter and 2.5 km (1.5 miles) deep. Rifts on Venus A 'half crater' located in the rift in Beta Regio on Venus. It has been cut by many extensional faults since it was formed by the impact of a large asteroid. The fault valley is up to 20 km (12 miles) wide and apparently quite deep. Dunes on Venus The tight pattern of bright and dark ripples in the center of this image is an area where loose material was sculpted by the gentle surface winds into dunes. The bright streaks of material curve away from small hills, revealing which way the winds were blowing. Some studies suggest the dunes on Venus are about 40- to 80-m tall, whereas similar wind-blown, or aeolian, dunes on Earth typically grow to a height of 200-m or more. The extreme atmospheric pressure and low wind speeds at the surface of Venus may create an environment more similar to dunes found on the floor of Earth's oceans. Underwater, where particles move differently, dunes tend to be much smaller and more like those on Venus. Impact Craters Due to the very high surface temperature and atmospheric pressure, meteorite impact craters on Venus differ in some ways from those found on other planets and moons. Fewer smaller craters than found on Mercury or the Moon: small meteors burn up in thick atmosphere. Many chain/multiple impact craters: incoming meteors break apart in thick atmosphere Extensive lava flows and small ejecta patterns around craters: high surface temperatures may increase surface melt while thick atmosphere and low-speed surface winds keep ejecta close to impact. Lava Flow from Impact Crater Perhaps because of the high surface temperature, meteorite impacts cause large amounts of rock to melt and flow like lava. If this molten material spills from the crater, it can form flows like those shown here around the crater Markham. Volcanic Features: most common features on Venus Venus has more volcanoes than any other planet in the Solar System (>1600 volcanos). Volcanic eruptions are a major force reshaping the landscape with 65% of surface covered with volcanic plains. Venus may have had a major global resurfacing event about 500 million years ago. Inferred from the density of impact craters on the surface. Differences in the types of erupted magma and the eruption rate lead to a wide variety of surface features, including giant shield volcanoes, domes, large flow structures, and rilles. Data from ESA's Venus Express suggests volcanic activity on Venus took place in the recent geological past and may still be happening on the planet today. Volcano with a "hotspot" Signature The image above is an elevation model of the volcano Idunn Mons, with a diameter of 200 km (124 miles). ESA's Venus Express probe revealed that Idunn Mons is a "hotspot," meaning it radiates high levels of infrared light compared to the surrounding area and suggesting that lava flowed at the spot recently, and that the area is still warm. Combining infrared data with much higher-resolution radar images from the NASA Magellan mission, scientists created a high-resolution geologic mapping of the recently active (in the geological sense) volcanic structure. Volcanic "Pancake" Domes on Venus Typically a few tens of km in diameter and about 1 km (0.6 mile) high and remarkably circular in shape. Flat-topped and steep-sided, they appear to have formed when a mass of high-viscosity lava was extruded from a central vent and spread outward for a short distance in all directions before solidifying. Volcanic Corona on Venus This region, roughly 100 km (60 mi) on a side, shows a gigantic structure known as a corona. Such features are believed to form when plumes of rising hot material in the mantle push the crust upwards into a dome shape, which then collapses in the center as the molten magma cools and leaks out at the sides, leaving a crown-like structure: the corona. Magellan acquired this view of Venus during its first mapping cycle around the planet in 1990 and 1991. Canali on Venus Canali are long, sinuous, lava-carved channels similar to the sinuous rilles found on the Moon. The arrows in this image point out one such example. These features can extend up to 6,800 km (4,200 miles) on Venus (longer than on any other planet) and have remarkably constant widths, which can be as much as 3 km (2 miles). They probably formed when very hot low-viscosity lava erupted onto the surface.

Exploration of Venus

Since Galileo's discovery of Venus's phases, the planet has been studied in detail, using Earth-based telescopes, radar, and other instruments. Transits of Venus are rare events, occurring in pairs eight years apart with more than a century between pairs. Important early telescopic observations of Venus' transits were conducted in the 1700s. They provided the most accurate method known at that time for determining the distance between Earth and the Sun. Observations of the 1761 and 1769 transits suggested that Venus has an atmosphere. By the time of the 1874 and 1882 transits, photography had developed enough to allow scientists to record on glass plates what they saw through their telescopes. The next transits of Venus will occur in 2117 and 2125. Observations outside of the visible spectrum provided additional information. Ultraviolet: Cloud features were discovered with certainty in 1927-28 in ultraviolet photographs. Infrared: Venus' infrared spectrum showed in 1932 that its atmosphere is composed primarily of carbon dioxide; subsequent infrared observations revealed further details about the composition of both the atmosphere and the clouds. Microwave: Observations began in the late 1950s and early '60s and provided the first evidence of the extremely high surface temperatures on the planet and prompted the study of the greenhouse effect as a means of producing these temperatures. Radio: The most effective way to map Venus' surface has been used from both Earth-based and orbiting radio telescopes. The first successful radar observations of Venus took place at Goldstone and Haystack in 1961 and revealed the planet's slow rotation. By the mid-1980s Earth-based radar technology had advanced such that images from Arecibo were revealing surface features as small as a few km in size. Nevertheless, because Venus always presents nearly the same face toward Earth when the planets are at their closest, much of the surface went virtually unobserved from Earth. Venus has been the target of more than 20 spacecraft missions. The first spacecraft to reach the vicinity of another planet and return data was the U.S. Mariner 2 in its flyby of Venus in 1962. Other early United States missions include: Mariner 5 (1967), Mariner 10 (1974) that continued on to flyby Mercury, Pioneer Venus mission (launched in 1978; orbiter returned data for 14 years), Galileo spacecraft flew by Venus in 1990 on its way to Jupiter; imaged deep into the atmosphere at near-infrared wavelengths and showed the highly variable opacity of the main cloud deck, Magellan was launched in 1989 and the next year entered orbit around the planet, where it conducted radar observations from a polar orbit until late 1994; produced high-quality radar images of about 98 percent of the planet and measured surface topography as well as some properties of its surface materials. Cassini-Huygens spacecraft flew by Venus twice, in 1998 and 1999, on the way to its primary target, Saturn. Soviet Union's planetary exploration program the 1960s to the 1980s also targeted Venus. Venera 4, launched in 1967, was comprised of a flyby spacecraft as well as a probe that entered the planet's atmosphere. Venera 7 (1970) was the first successful soft landing on another planet. Venera 9 and 10 landers (1975) returned from the first images from the surface of another planet, and Venera 9 and 10 orbiters were the first spacecraft placed in orbit around Venus. Venera 15 and 16 (1983) carried the first radar systems flown to another planet that were capable of producing high-quality images of the surface. ESA's Venus Express, launched in 2005, was the first European spacecraft to Venus; carried a camera, a visible-light and infrared imaging spectrometer, and other instruments to study Venus's magnetic field, plasma environment, atmosphere, and surface; mission ended in 2015. Akatsuki, launched in May 2010 and entered orbit in December 2015, was Japan's first successful mission to another planet; carried five cameras, three taking images in infrared, one in ultraviolet, and one in visible light, to study different depths in Venus's atmosphere.

hydrosphere

(water) location and movements of water on and under the surface of the body, as well as the water vapor within the atmosphere

Mercury's Atmosphere

Like the Moon, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. Composed mostly of oxygen, sodium, hydrogen, helium and potassium.

Hydrosphere on Mars

Mars today is a cold, dry desert with an atmosphere so thin that any surface water evaporates or freezes. But spacecraft have found signs of a warmer and wetter past: sulfur-rich salts and other deposits that could only have formed in a water-rich environment, ancient rivers that carved networks of valleys, and evidence of lakes and vast floods. Much of the early water supply remains frozen in polar ice caps and in ice deposits just beneath the surface. The cratered highlands of Mars have many river valleys that formed in a water-rich environment about 4.5 to 3.7 billion years ago. Erosion since then has been slow, leaving these very old features preserved. Ancient River Channel on Mars: Libya Montes Topography The prominent river channel that runs from south to north (left to right in this image) is thought to have cut through the region around 3.6 billion years ago. It apparently originates from the impact crater in the south, breaching its crater wall and flowing towards the north, navigating the hummocky mountains of the local topography. The color-coded topographic view shows relative heights and depths of terrain in the Libya Montes region on Mars. As indicated in the key at top right, whites and reds represent the highest terrain, while blue/purple is the lowest. The color-coded topographic view is based on a digital terrain model of the region, from which the topography of the landscape can be derived. Later, massive outflows of groundwater formed flood channels tens to hundreds of km wide and perhaps over 1,000 km (620 miles) long. Many scientists think Mars' outflow channels formed when large lakes burst their shorelines or when groundwater pressure caused an aquifer to erupt violently to the surface. Similar features found commonly on Earth. Several of the largest outflow channels arise at the eastern end of Valles Marineris, near equator. Typically flow from south to north Imply catastrophic floods, when compared to similar Earth features, with enormous flow rates (> 100 x Amazon) Dated to about 3 billion years ago. Outflow Channels on Mars Outflow channels are broad, winding, sculpted valleys cut by the catastrophic release of liquid water. Some ancient impact craters and canyons contain sedimentary deposits that resemble river deltas. In image below of Eberswalde Crater, an ancient river's meanders grew and cut off their loops as meandering rivers do on Earth. Wind has slowly blown away the floodplain sediments, leaving the channel beds behind as ridges. Sedimentary Deposits in Martian Crater Resembling River Deltas on Earth An ancient river's meanders grew and cut off their loops as meandering rivers do on Earth to form this delta in the bottom of Eberswalde Crater on Mars. Another class of channels is called gullies. These occur mostly at mid-latitudes. They are very young, lacking superimposed craters. Some gullies are located on the walls of craters and tectonic troughs (grabens) and were first interpreted as having formed from discharge of modern ground water or the melting of snow that accumulated thousands of years ago when Mars' climate was different. However some gullies occur on sand dunes, which are not believable modern water sources, and they change year-to-year indicating active processes. Some recent studies show that gullies occur preferentially where seasonal carbon dioxide frost forms, with gully formation likely driven by water-free processes such as rapid sublimation of the carbon dioxide frost. Rapid formation of CO2 gas may act to fluidize overlying sand, causing it to flow rather than avalanche, and thus create a gully. Gullies Most Martian gullies are found within craters or other depressions, and appear to be related to the bedrock. however, some middle-latitude gullies are found on sand dunes. Formed during Mars' most recent history. these features are probably related to rapid sublimation of carbon dioxide frost incorporated into the dunes during the winter. The most recently recognized class of sinuous features possibly formed by water is called "recurring slope lineae," or RSLs. RSLs lack topographic relief and so are not channels, strictly speaking. Instead they are dark streaks that form in spring on sunny slopes, grow in a downslope direction over the spring and summer, and then fade in late summer. Grow at times and places where surface temperate is near or above the freezing point of water. Have all the characteristics expected for thin films of very salty water that creeps down warm slopes over the summer. Recurring Slope Lineae This enhanced-color image shows a slope covered by dark, reddish streaks or "recurring slope lineae" visible in the middle left of the image.

Mercury's Surface Features

Surface Similar to Moon, but missing the Moon's large, dark mare and with evidence of a longer volcanic history. Covered with dead volcanoes, impact craters/basins, and extensive lava flows. Dominated by igneous rocks and metamorphic rocks from igneous sources, implying that the entire surface was molten early in its history. Mercury is larger than the Moon and would cool more slowly Þ longer period of volcanic activity. Between the heavily cratered regions of Mercury's surface lie large expanses of smooth plains. Much of the north polar region of the planet is covered by these plains which a believed to be volcanic basalt deposits several kilometers thick. The image below shows an extensive, smooth volcanic plain near Mercury's North Pole. The volcanic lava flows buried craters, leaving only traces of their rims visible. Such craters are called ghost craters, and there are many visible in this image, including a large one near the center. Wrinkle ridges cross this scene and small troughs are visible regionally within ghost craters, formed as a result of the lava cooling. Mercury's Northern Plains from MESSENGER The smoothness of these plains indicates that these extensive volcanic features are younger than Mercury's rougher surfaces. Note the "ghost" crater that is ~ 100 km in diameter near the center of the image (inside dashed circle, rim barely visible as slightly raised circle) and the wrinkle ridges scattered across the plain. Surface resembles that of Earth's Moon, covered with a layer of pulverized rock and scarred by many impact craters resulting from collisions with meteoroids and comets. The surface would appear greyish-brown to the human eye. The bright streaks in the image below are called "crater rays" and are formed when an asteroid or comet strikes the surface. As time passes, the bright rays darken and become less visible. Like the Moon, Mercury has giant impact basins. The largest is the Caloris Basin, located on Mercury's "hot" longitude and one of the largest impact basins in the solar system and is similar in size and structure to Orientale Basin on the Moon. Unusual geologic features are often found antipodal (180o around planet) from large impact basins. One of the first regions of "weird terrain" was imaged on Mercury by Mariner 10 opposite the Caloris Basin. Similar terrain is found on the Moon opposite Imbrium Basin. The unusual terrain is believed to form when intense seismic waves travel around the planet and through its core to focus on the opposite side of the body, crumpling its surface. Caloris Basin The Caloris Basin is 1300 kilometers (810 miles) in diameter. The impact was so great that the resulting seismic waves traveling through the core and across the surface of Mercury interacted on the side opposite the crater to create a large region of unusual, hilly and furrowed terrain, called "weird terrain." Formation of Weird Terrain on Mercury When an exceptionally large meteorite hit Mercury an estimated 3.85 billion years ago, it sent intense waves around the planet and through its core. They came to a focus on the opposite side of Mercury, disrupting the surface and producing hilly and lineated terrain there. The Caloris Basin was excavated at the impact site, and it now exhibits concentric waves that froze in place after the impact. Numerous long, sinuous cliffs, called lobate scarps, indicate its crust has contracted globally. Length: tens to over a thousand km Height: about 100 m (330 feet) to 3 km (2 miles) As the interior of the planet cooled it contracted. Gravity then forced the crust to adjust to a smaller interior. From the numbers and geometries of the lobate scarps, it appears that the planet shrank in diameter by as much as 7 km (4 miles). Scarps are also found on the lunar surface. Mysterious features named hollows were imaged by both Mariner 10 and MESSENGER. Described as "sub-kilometer scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters," these features are unique to Mercury. Their small sizes, sharp edges, and lack of subsequent impacts indicate a relatively young age. Although there is no definitive theory for the formation of these features, scientists general agree that hollows form when something in Mercury's surface sublimates (turns from solid to gas).

Moon's Surface Features

Surface Geology The lowland lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They generally are located within giant impact basins which are filled with layers of basaltic lava, not unlike the basaltic flows of the Columbia River Plateau (Eastern Washington and north-central Oregon) or of Iceland. Fine-grained crystallinity and large holes indicate that this rock crystallized rapidly near the top of a molten lava flow. Grey color of this rock is due to the presence of dark-colored minerals. Low viscosity lava. Believed to have their origins in partially melted areas 100-400 km (60-250 miles) beneath the large meteoroid impact basins. The basalt flows covered areas up to 1200 kilometers (750 miles) away from where they had erupted to the surface. Not uniformly distributed over lunar surface. Nearly 26% of the near side of the Moon is basalt and only 2% of the far side is basalt. Most basalt in either hemisphere is found in areas of lowest elevation, particularly in the very large impact basins. Distribution of Lunar Basalts ~26% coverage of the lunar near side. ~2% coverage of the lunar far side. The lunar highlands are anorthosite, a whitish-colored igneous rock predominantly of calcium-rich plagioclase feldspar, and materials thrown out during the creation of the impact basins. Oldest rocks on the Moon. One explanation for the presence of anorthosite in the lunar crust is based on the assumption that the Moon was once molten. Plagioclase, a relatively light mineral, crystallized as the Moon cooled and solidified. This mineral floated toward the surface and formed anorthosite. Heavier minerals sank and produced the denser interior of the Moon. This ancient crust has been smashed and redistributed by countless meteoric impacts, from giant impact basins to microscopic "zap pits". Secondary Craters Rocks thrown out during the formation of large impact craters often produce smaller, secondary craters when they fall back to the lunar surface. Zap Pits Tiny impact craters produced by small, high velocity particles are common on the exposed faces of lunar rocks. The Moon's surface layer contains fragments of the major lunar rock types: basalt (A), anorthosite (B), and breccia (C), and round glass particles (D) are common. The texture of undisturbed lunar surface layer, called regolith, is somewhat like talcum powder, with clumps of small particles 0.1-0.6 mm (0.004-0.024 inch) in diameter. Before any craft had landed on the Moon, scientists were not certain that the lunar surface would support a spacecraft or astronaut. On February 3, 1966, the Soviet's Luna 9 was the first probe to soft land on the Moon and to transmit pictures from the lunar surface, proving a lunar lander would not sink into a thick layer of dust, as had been feared. Lunar breccias are rocks produced of meteoroid impacts. Evidence of this process can be seen in the countless craters of various sizes which cover the Moon. On the Moon, breccias are the lithified aggregates of angular rock fragments cemented together by the melt generated by meteorite bombardment. Impacting meteorites break up the lunar surface and the pieces are glued together by the heat and pressure of the impact.

Mercury's Topography

Topography Relatively smooth. Lowest point: ~13 km (3 mi) deep on floor of Rachmaninoff Basin. As on the Moon, low topography is generally related to the giant impact basins. Highest point: ~10.8 km (2.5 miles) lies just south of the equator.

Earth's Atmosphere

Atmosphere Contains the air that we breathe and protects us from the blasts of heat and radiation emanating from the Sun. About 480 km (300 miles) thick, but most of it is within 16 km (10 miles) of the surface. Air pressure decreases with altitude. At sea level, air pressure is about 1 bar (14.7 lb/in^2). At 3 km (10,000 feet), the air pressure is 0.7 bar (10 lb/in^2); also less oxygen to breathe. Pressure adequate to allow liquid water to be stable on Earth's surface. Composition of air Nitrogen — 78% Oxygen — 21% Argon — 0.93% Carbon dioxide — 0.04% Trace amounts of neon, helium, methane, krypton and hydrogen, as well as water vapor Atmosphere layers (innermost to outermost) defined by temperature gradient Troposphere temperature decreases with altitude highest density and pressure layer where "weather" occurs Stratosphere temperature increases with altitude contains ozone layer Mesosphere temperature decreases with altitude most incoming meteors burn up in this layer Thermosphere temperature increases with altitude lowest density and pressure interactions with incoming solar photons: disassociate and/or ionize atmospheric molecules Imaginary boundary between atmosphere and outer space "Karman line" about 100 km (62 miles) above Earth's surface gas density dissipates with altitude until almost unmeasurable

biosphere

(living things) all ecosystems and zones of life that occur within the three spheres listed above on Earth, life exists nearly everywhere, in all hospitable spaces on Earth and also in the less hospitable habitats including the extremely hot, high-pressure environments of the deep ocean's hydrothermal vents, in clouds in the sky (where some microorganisms reside), and inside frozen sandstone in the Antarctic, where specialized algae and bacteria thrive

Mars Topography

Although Mars is only half the size of Earth, its largest impact basins, volcanoes, and canyons are far bigger than any found on Earth. Visible on the topographic map of the surface of Mars above are giant volcanoes, deep valleys, impact craters, and terrain considered unusual and even mysterious. Smooth lowlands dominate the north, while the south is covered with rough highlands. Particularly notable are the volcanoes of the Tharsis province, visible on the left in (false-color) red and white, which are taller than any mountains on Earth. Just to the left of center is Valles Marineris, a canyon much longer and deeper than Earth's Grand Canyon. On the right in blue is the Hellas Basin, an impact basin 2300 km wide. Highest point: 21.2 km (13 miles) above the average radius of Mars (peak of Olympus Mons) Lowest point: 8.2 meters (5 miles) below the average radius of Mars (bottom of Hellas Basin) Difference in elevation between the southern highlands and the northern lowlands exceeds 6 km (3.7 miles) in some areas Distance comparable to the elevation difference between Earth's continents and ocean floors. Mars' terrain has three distinct regions: Highlands Predominately in the southern hemisphere Cover 60% of surface Ancient, heavily cratered terrain Resemble lunar highlands Includes Hellas Basin, one of the largest impact craters in the solar system Lowlands Predominately in the northern hemisphere Cover 20% of surface Relatively young, lightly cratered terrain Average elevation 4 km below highlands Contain dune fields, rift valleys, dry riverbeds, water flow patterns Resemble lunar maria Volcanic Highlands Largest area called Tharsis volcanic province Centered near equator Covers 20% of surface Immense bulge the size of North America Volcanic plains, 10 km above surroundings Few impact craters implies young age Crowned by four volcanoes that rise another 15 km.

Moon's Atmosphere

Atmosphere Considered a "surface boundary exosphere." May be the most common type of atmosphere in the solar system. Mercury, the larger asteroids, a number of the moons of the giant planets and even some of the distant Kuiper belt objects out beyond the orbit of Neptune, all may have surface boundary exospheres. Consists of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars or Venus. Extremely low density (comparable to the density of the outermost fringes of Earth's atmosphere where the International Space Station orbits), equivalent to a very good vacuum on Earth. Source of atmosphere: high energy photons and solar wind particles knocking atoms from the lunar surface, chemical reactions between the solar wind and lunar surface material, evaporation of surface material, material released from the impacts of comets and meteoroids, and out-gassing from the moon's interior.

Venus Atmosphere

Atmosphere Thickest atmosphere of the terrestrial planets. About 480 km (300 miles) thick, but 90% of it is within 28 km (17 miles) of the surface. Air pressure decreases with altitude. At average radius, air pressure is about 95 bars (95 x pꚚ). Same pressure found at a depth of about 1 km (0.6 mile) in Earth's oceans. Composition of air Carbon dioxide — 96.5% Nitrogen — 3.5% Trace amounts of sulfur dioxide, carbon monoxide, argon, helium, and water vapor Ozone was discovered by ESA's Venus Express and is located at varying altitudes in the Venusian atmosphere, between 90 and 120 km - compared with 15-50 km on Earth - and is always confined to a rather thin layer, measuring 5 to 10 km across. The ozone layer on Venus is also very tenuous - up to 1000 times less dense than that on Earth. Atmosphere layers (innermost to outermost) defined by temperature gradient Troposphere Temperature decreases with altitude Highest density and pressure Contains 99% of atmosphere by mass 65 km thick Surface winds quite slow (typically > 1m/s), but increase with altitude in this layer. Winds near top show super-rotation, in which the atmosphere circles the planet in four Earth days (much faster than the planet's sidereal day of 243 days) and wind speeds are ~360 km/h (220 mph) or more. Mesosphere Temperature decreases with altitude Extends 60 to 100 km above surface Thermosphere Temperature increases with altitude Lowest density and pressure Extends from 120- to 300-km above surface Circulation of the upper atmosphere is driven by the influx of solar radiation. Having absorbed solar radiation, hot upper atmosphere rises still further, circulating to the night side of the planet where it cools and sinks back to the level of the cloud tops. Region where atmosphere ionized by solar wind to form an ionosphere on the daytime side of the planet. Atmospheric Circulation The main circulation of the Venusian atmosphere is dominated by two huge convection currents (Hadley Cells) in the cloud layers, one in the northern hemisphere and one in the southern hemisphere. Comparison of Atmospheric Circulation: Venus and Earth (a) The slow rotation and uniform surface temperature on Venus create two convection cells: one in the N-hemisphere and one in the S-hemisphere. (b) On Earth, the varying heat sources of land and water coupled with a much faster rotation create three convection cells per hemisphere. Near the poles, these cells are replaced with a cold circumpolar circulation (polar collar) that encircles highly variable circulation called the polar vortex. Circulation in the upper atmosphere above the clouds is driven by daytime solar heating and nighttime radiative cooling. General Circulation in Venus' Atmosphere The main feature is a convection-driven 'Hadley cell', which extends from the equatorial region up to about 60° latitude in each hemisphere. A cold 'polar collar' is found around each pole at about 70° latitude and is characterized by remarkably low temperatures and dense, high clouds. Inside the collar, a thinning of the upper cloud layer forms a complex and highly variable feature, called the polar vortex The upper atmosphere's circulation is driven by solar heating during the Venusian day. For the atmosphere above Venus' thick cloud layer, the planet's slow rotation allows for significant heating during daytime hours and cooling at night. Clouds Clouds obscure the surface of Venus from optical imaging, and reflect about 75% of the sunlight that falls on them. Typical surface light levels are similar to a partly cloudy day on Earth. Composed mainly (75-96%) of sulfuric acid droplets and extend from about 40 to 60 km. Divided into three distinct layers. Below the clouds is a layer of haze down to about 30 km and below that it is clear. Above the clouds there is a high-speed "jet stream" which blows from west to east at about 360 km/h. Wind is fastest at the equator and slows toward the poles, often giving a "V" type pattern in the visible cloud cover. Above this layer, the atmosphere contains light hazes of various aerosol particles, composed chiefly of sulfuric acid and water. Venus' Cloud Tops in UV Light Pioneer Venus Orbiter image of Venus cloud tops in UV light (Feb. 5, 1979). Although Venus's cloud cover is nearly featureless in visible light, UV imaging reveals their distinctive structure and pattern, including global-scale V-shaped bands that open toward the west (left). Slow rotation and a uniform surface heat source create the "V" pattern in the thick cloud layer.

Mars Atmosphere

Atmosphere To our eyes, the sky would be hazy and red because of suspended dust instead of the familiar blue tint we see on Earth. Mars is a planet that shows climate change on a large scale. Although its atmosphere used to be thick enough for water to run on the surface, today that water is either scarce or non-existent. Mars' sparse atmosphere doesn't offer much protection from impacts by such objects as meteorites, asteroids and comets, and is currently too thin to easily support life as we know it, although life may have existed in the ancient past. About 100 x thinner than Earth's, but thick enough to support weather, clouds and winds. Because the atmosphere is so thin, heat from the Sun easily escapes this planet. If you were to stand on the surface of Mars on the equator at noon, it would feel like spring at your feet (75° F or 297 K) and winter at your head (32° F or 273K). It is 95% carbon dioxide. About 480 km (300 miles) thick, but 90% of it is within 28 km (17 miles) of the surface. Air pressure decreases with altitude. At the surface of Mars, atmospheric pressure is ~ 1/100 x Earth's (0.01 x pꚚ). May vary by 30% throughout Mars year because of variations in solar heating. Seasonal pressure change occurs as carbon dioxide freezes and then evaporates from polar caps. During southern hemisphere winters, the global air pressure drops by 30%. Seasonal changes are also affected by Mars' distance from the Sun, and are also the cause of planet-wide dust storms that can obscure the planet's surface. Diurnal temperature changes are quite extreme ranging from -225 F at night to 63 F during the day. The greatest extremes occur in the southern hemisphere. Occasionally carbon dioxide and water vapor clouds form because of the low temperature in the atmosphere. Also frost can form on the ground. Composition of air 95.3% carbon dioxide (< 1% on Earth). 2.7% nitrogen (78% on Earth) 1.6% argon 0.13% oxygen (21% on Earth) 0.07% carbon monoxide 0.03% water vapor (~1% on Earth) minor amounts of nitrogen oxide, neon, hydrogen-deuterium-oxygen, krypton, xenon, and methane. Martian Sunset The robotic rover Spirit was deployed in 2005 to park and watch the Sun dip serenely below the distant lip of Gusev crater. Colors in the above image have been slightly exaggerated but would likely be apparent to a human explorer's eye. Fine Martian dust particles suspended in the thin atmosphere lend the sky a reddish color, but the dust also scatters blue light in the forward direction, creating a bluish sky glow near the setting Sun. Because Mars is farther away, the Sun is less bright and only about two thirds the diameter it appears from Earth. Images like this help atmospheric scientists understand not only the atmosphere of Mars, but atmospheres across the Solar System, including our home Earth. Atmosphere layers (innermost to outermost) defined by temperature gradient Troposphere Temperature decreases with altitude Highest density and pressure 60 km thick (deep by comparison to Earth) Mesosphere Temperature nearly constant Extends 60 to 100 km above surface Thermosphere Temperature increases with altitude Lowest density and pressure Extends from 100 to 200 km above surface Atmospheric gases start to separate from each other at these altitudes, rather than forming the even mix found in the lower atmospheric layers. Atmospheric Circulation The similarity of Mars's rotation period, axial tilt, and atmospheric transparency to Earth's suggests a parallel to Earth's meteorology, but certain features unique to Mars lead to novel departures from familiar behavior. Carbon dioxide, the main atmospheric gas on Mars, condenses on the surface, leading to large variations in surface pressure. Mars is missing liquid water at its surface and experiences dramatic changes in surface temperature between day and night. At noon in the summertime, surface temperatures may reach >300 K. At night, temperatures drop up to 100 K, convection ceases, and the troposphere vanishes. On Earth, the water oceans moderate day-night temperature variations and latitude-dependent temperature gradients. Water has a high latent heat (takes a lot of energy to raise temperature or change phase). Both oceanic circulation and evaporation of ocean water stabilize the differences between solar-heated days and night-time cooling. Large quantities of suspended dust can absorb heat directly from sunlight and provide a distributed source of energy throughout the lower atmosphere. The eccentricity of Mars' orbit causes "seasonal" temperature variations that heat and cool the entire planet. Daily Weather The weather on Mars is pretty much the same each day: Cold and dry with small daily and seasonal changes in temperature and pressure, plus a chance of dust storms and dust devils. Light winds blow from one direction in the morning and then from the reverse direction in the evening. Clouds of water ice hover at altitudes of 20 to 30 km (12 to 18 miles), and clouds of carbon dioxide form at approximately 50 km (30 miles). Because Mars is so dry and cold, it never rains. That's why Mars resembles a desert, much like Antarctica on Earth. Clouds Two types clouds: water vapor w/ carbon dioxide; white appear in low-lying areas in morning near poles in late summer/early fall dust yellowish can be moved by high speed surface winds (>100 mph) to create global dust storm Dust Storm on Mars These Hubble Space Telescope images show the Red Planet before (left) and during (right) the great Martian dust storm of 2001. The fine airborne dust blocks a significant amount of sunlight from reaching the Martian surface. Because the airborne dust is absorbing this sunlight, it heats the upper atmosphere. Seasonal global Mars dust storms have been observed from telescopes for over a century, but this was the biggest storm ever seen in the past several decades.

Earth's Moon and Rings

Natural satellites: one, the Moon Rings: none

Greenhouse Effect on Mars

In contrast to Venus, Mars displays a very small greenhouse effect. Like Venus, the primary gas in the Martian atmosphere is carbon dioxide (a greenhouse gas). However, on Mars the atmosphere is very thin and cannot retain energy from the Sun, creating extreme temperature contrasts between day and night and sun or shade. However, most scientists agree that Mars was much warmer and wetter in the past and even had oceans, which means that the atmosphere was also very different. Evidence for a warmer, wetter past with a thick atmosphere includes: discovery of water-formed minerals on Mars by the Opportunity and Spirit rovers leading to the conclusion that climatic conditions in the distant past allowed for free-flowing water on Mars, morphology of some crater impacts on Mars indicate that the ground was wet at the time of impact (rampart or splosh craters), and observations of both landscape erosion rates and Martian valley networks also strongly imply warmer, wetter conditions earlier than about 4 billion years ago. If the carbon dioxide atmosphere was thicker in the past, then the greenhouse effect could have raised the temperature, at least in some places, to above the freezing point of water. With the higher temperature, running water could have carved out the many channels and outflow valleys that are common on the planet and may have gathered together to form lakes and an ocean. But Mars' size and distance from the Sun doomed its environment to becoming a cold, dry desert. Mars was vulnerable to impacts from asteroids and other bodies early in its history. Large impacts could have blasted away significant amounts of the Martian atmosphere. At one-half the size of Earth, by the time Mars was about 500 million years old, planetary cooling would have frozen the interior enough to shut off the Martian dynamo and its magnetosphere. Today, only local magnetic fields remain. As the magnetic field turned off, the solar wind began stripping Mars of more of its atmosphere. Mars' size would also slow down the rate of volcanic activity and out-gassing. Over several hundred million years, Mars has lost most of its atmosphere. With less atmosphere, atmospheric pressure drops and the greenhouse effect lessens. The surface cools below the temperatures necessary for liquid water to be stable at the surface.

Mercury's Hydrosphere

Like the Moon, Mercury does not possess a thick, permanent atmosphere and cannot support liquid water at its surface. Also like the Moon, frozen water ice has been detected within the permanently shaded interiors of some of the craters near the planet's poles. Water ice on the surface of Mercury or the Moon is exposed directly to vacuum, and will rapidly sublimate and escape into space unless it is kept cold at all times, implying that the ice can never be exposed to direct sunlight. The only location on the surface of Mercury where this is possible is near the poles, where the floors of some craters are deep enough to afford permanent shading.

Mercury's Biosphere

Like the Moon, conditions on Mercury are not favorable for sustaining life because of the absence of liquid water, organic topsoil, and atmosphere. The length of the solar day on Mercury creates prolonged periods of darkness and cold that would present significant challenges to life. Although Mercury does possess a measurable global magnetic field, that field does not provide significant protection from solar wind at the planet's surface.

Exploration of Mars

Mars has intrigued mankind since early times. Its size, red color, and proximity to Earth make it one of the easiest planets to observe from Earth, by telescope, or from space. Easily visible about once every two years (780 days OR the period between oppositions), when it is at perihelion (closest to the Earth): visible all night long, highest in the sky at midnight, and surface features as small as 100 km across can be resolved by Earth-based telescopes (about the same size as objects on the Moon resolved by unaided eye). At maximum brightness, it is the second brightest planet (after Venus). Distance of Mars from Earth (in AU) The image above shows the distance of Mars from Earth (in AU) as a function of time (in years) from 2001 to 2035. The closest approaches occur every 780 days or the time between oppositions. Recall that a planet viewed from Earth is in opposition when it is closest to Earth and appears to be on the opposite side of the Earth from the Sun.

Mars Biosphere

Mars is similar to Earth in many ways, having many of the same "systems" that characterize our home world. Like Earth, Mars has systems of air, water, ice, and geology that all interact to produce the Martian environment. However, we don't yet know whether Mars ever developed or maintained a biosphere. Although the Martian atmosphere was once thick enough for water to run on the surface, today that water is either scarce or non-existent and the atmosphere is too thin to easily support life as we know it. But, it is possible that Mars could have once harbored life and, some conjecture that life might still exist there today. That possibility has spurred humans to search for evidence of life (ancient or current) on Mars.

Mars Surface Features

Mars' surface is rocky, with canyons, volcanoes, dry lake beds and craters and fine red dust covers most of its surface. Today, wind is the only major force altering its surface, but the rocks and soil and landscape still hold clues to the planet's past. Giant volcanoes, canyons, and impact basins dominate the Martian landscape. Formed one to three billion years ago. Still visible because many of the geologic processes that rework the Earth's surface are either absent or work very slowly on Mars. Some landforms, such as the Olympus Mons volcano, 26 kilometers (15.5 miles) high, and the Valles Marineris canyon system, are the largest in the solar system. Tectonic Features: Heat escaping from the planet's interior not only created melting of the crust and volcanism, but also produced tectonic forces that pulled and pushed the crust. The Tharsis Rise is the largest volcanic and tectonic feature on Mars. As shown in the image below, the flanks of the Tharsis Rise contain vast volcanic plains that have been both pulled apart, to create narrow troughs called graben (right), and pushed together, to create long, narrow, twisting features called wrinkle ridges (left). Wrinkle ridges are common on Mars and were in part created by global forces from the cooling and contraction of the planet's interior. Graben and Wrinkle Ridges The narrow troughs on the right side of the image are graben formed as the Martian crust was stretched. The more narrow twisted features on the lower left corner are wrinkle ridges formed as the crust compressed. Scarps, like those on Mercury and the Moon, are cliffs formed by thrust faults as the rigid crust attempts to shrink as the planet's interior cools and contracts. Largest Scarp on Mars To the left is a Viking Orbiter mosaic of Amenthes Rupes, the largest thrust fault scarp on Mars. The scarp, marked by the red arrow, is up to 3-km high and over 400 km long. The largest tectonic feature on Mars is Valles Marineris, named after the Mariner 9 probe that first photographed it. System of canyons that runs along the Martian surface east of the Tharsis Rise. Up to 8 km (5 miles) deep and 4,500 km (2,800 miles) long. Stretches for nearly a quarter of the planet's circumference. Would stretch from Los Angeles to New York City on Earth. The Grand Canyon could easily fit into one of its side canyons. Second longest rift system in solar system (after mid-ocean rifts on Earth) Valles Marineris began to open along geological faults about 3.5 billion years ago. The faulting was caused by the tectonic activity that accompanied the growth of the giant volcanoes in Tharsis, lying just to the west. As magma pushed into Tharsis from below, the entire region rose, and the surrounding crustal rocks stretched and broke into faults and fractures. As cracks opened, the ground sank and opened paths for subsurface water to escape, undermining the ground and enlarging the fracture zone. The valley's steep, newly exposed walls became unstable, causing landslides that widened the canyon further. It appears the main activity came to a halt roughly 2 billion years ago. Valles Marineris in mosaic of THEMIS infrared images from 2001 Mars Odyssey Long enough to reach from New York City to Los Angeles, this great rift in the Martian crust is named Valles Marineris. Formed out of several parallel, connecting troughs, Valles Marineris dwarfs Earth's Grand Canyon in every respect, being wider, longer, deeper, and older. This mosaic image of Valles Marineris - colored to resemble the Martian surface - comes from the Thermal Emission Imaging System (THEMIS), a visible-light and infrared-sensing camera on NASA's Mars Odyssey orbiter. Built from more than 500 daytime infrared photos, the mosaic shows the whole valley in more detail than any previous composite photo. The smallest details visible in the image are about the size of a football field: 100 meters (328 feet). Impact Craters Martian craters show an unusually wide variation in characteristic shapes and features when compared, for example, to craters seen on the Moon. Some do resemble those seen on the Moon: smaller than 5-km in diameter are usually bowl-shaped, with raised rims and slightly flat floors, just like similarly sized craters on the Moon. larger than about 50 to 70 km in diameter -- resemble flat plains ringed by a circle of hills; similar to their lunar counterparts, although they appear shallower and their rims are often worn down. Others, such as the mid-sized rampart craters, are not observed on the Moon, Venus, or Mercury. Rampart craters, sometimes called splosh craters, suggest presence of permafrost below the surface. Common at equatorial and mid-latitudes on Mars; smaller ones near poles. Craters characterized by a low ridge along its edge and ejecta that form smooth-edged, overlapping lobes (just like you might see if you threw a rock into runny mud). Impact Basins Mars has some of the largest impact basins in the solar system. Generally have a raised central peak and multi-ringed rim structures, like other solar system impact basins. Size of Large Martian Impact Basins This image shows the relative sizes of two Martian impact basins with a map of the United States for scale. Argyre Basin has a diameter of about 1100 km. Hellas Basin (diameter 2300 km) is the largest well-preserved basin on Mars. The Hellas Basin, the largest exposed impact feature on Mars, is one of the largest in the solar system. About 9-km (5.6 miles) deep and 2,300 km (1,430 miles) across. Depth of the Grand Canyon in the United States is roughly 1.6 km (1 mile), which means the depth of Hellas is about 2.5 times greater than the Grand Canyon! Formed from 3.9 to 4.6 billion years ago. In the time since its formation, crater has been infilled by aeolian, fluvial, glacial, and volcanic materials. Topography of the Hellas Basin Extending more than 2300 km east-west, and over 8 km deep, with evidence of volcanic, fluvial, and glacial activity revealed on its floor. Volcanic Features Mars has the largest volcanoes in the solar system. Most are shield volcanoes, but other features such as large volcanic cinder cones, unusual patera structures (term for volcanic crater), and mare-like volcanic plains are also found. Volcanism lasted much longer on Mars than on the Moon and seems to have changed over time. Mare-like plains on Mars are the same age as the lunar mare, roughly 3 to 3.5 billion years old. Volcanism in the highland paterae and mare-like plains on Mars stopped 3 billion years ago, but some of the smaller shields and cones erupted only 2 billion years ago. The giant shield volcanoes are even younger, forming between 1 and 2 billion years ago. The youngest lava flows on Olympus Mons are only 20 to 200 million years old. These flows are very small, however, and may represent the last gasp of Martian volcanism. Unlike the Moon, most Martian volcanism lies outside large impact basins: the mare-like plains are mostly near the largest volcanoes and are not limited to the lowest elevations. Lava flows on Mars are much longer and thicker than typical basalt lava flows on Earth. A large lava flow near Arsia Mons Measurements based on shadow lengths suggest that the flow averages 100 meters (330 feet) thick. The Tharsis Rise is the largest volcanic and tectonic feature on Mars. Huge volcanic plateau near equator of Mars Size of North America Rises 10-km above rest of surface Covered with volcanic plains formed from numerous superposed lava flows Little cratering ⇒ relatively new feature (2-3 billion years) Continent like, but not drifting ⇒ no sign of plate tectonics Olympus Mons lies just off the western flank of Tharsis. Largest shield volcano and second tallest mountain the solar system. 2.5 x the elevation of Mt. Everest above sea level on Earth. Base: 600 km (370 miles) wide covers an area approximately the size of Italy, or about 300,000 km2 (120,000 mi^2) Caldera: 80 km across Height: 26 km higher than surrounding plains and surrounded by a 8 km high cliff. Size implies that Mars did not have plate tectonic activity at the time of its formation. Olympus Mons Oblique view of Olympus Mons, from a Viking image mosaic overlain on MOLA altimetry data (Mars Global Surveyor), shows the volcano's asymmetry. The view is from the NNE; vertical exaggeration is 10:1. The wider, gently sloping northern flank is to the right. The narrower and steeply sloping southern flank (left) has low, rounded terraces, features interpreted as thrust faults. The volcano's basal escarpment (cliff) is prominent.

Mercury's Moons and Rings

Natural satellites: none Rings: none

Venus Moons and Rings

Natural satellites: none Rings: none

Venus Topography

Relatively smooth: according to Magellan data, 80% of the topography is within 1-km (0.62 mi) of the median radius. Total distance from the lowest point to the highest point on the entire surface is about 13 km (8.1 mi), about the same as the vertical distance between the Earth's ocean floor and the higher summits of the Himalayas, about 13 km(8.1 mi). Similarity is to be expected as the maximum attainable elevation contrasts on a planet are largely dictated by the strength of the planet's gravity and the mechanical strength of its lithosphere; these are similar for Earth and Venus. Lowest point: 2.9 km (.8 mi) below the mean level in the rift valley, Diana Chasma. Highest point: ~11 km (6.8 miles), Maxwell Montes in Ishtar Terra.

Earth-based Telescopic Studies of Mars

Telescopic observations played a major role in understanding Mars before the era of space exploration. Unlike Venus, whose thick haze of clouds obscured its surface, Mars offered views of shifting light and dark markings with its changing seasons. The first known recorded observations were made by Galileo Galilei in 1609. Subsequent telescopic observations include: 1659: Christiaan Huygens made the first useful sketch of Martian surface features. With Huygens' crude telescope, he saw one distinct dark feature on the surface (which is now called Syrtis Major). He observed it move that night and again the next night and concluded that Mars rotated on its axis with a rate of 24 hours. He also believed that Mars might be inhabited, perhaps even by intelligent creatures. He shared that belief with many other scientists who would observe Mars over the years to come. 1666: Giovanni Cassini conducted more careful observations. He concludes the rotation rate is 24 hours and 40 minutes. While there is some question on this matter, Cassini is probably the first to notice that Mars has white spots located near the poles. For the next 300 years people assume these spots are made up of snow, ice or both (we now call these spots "polar caps"). 1672: Huygens is the first to notice a white spot at the South Pole, probably the southern polar cap. 1719: Cassini's nephew Giancomo Miraldi observes "white spots" at the poles, and discovers that the southern cap is not centered on the rotational pole. He observes that the shapes of some dark regions (maria) change over time and considers this as evidence of clouds that sometimes obscure the surface. He also saw changes in the polar caps and speculates this showed evidence of seasons: ice from the polar caps supposedly melted during the "summer" and freeze again during "winter." 1784: In Herschel's paper, entitled "On the remarkable appearances at the polar regions on the planet Mars," he reports the inclination of its axis, the position of its poles, and its spheroidal figure, and suggests estimates of its real diameter and atmosphere. Not all of his observations were accurate: he finds the axial tilt to be 30o when the actual current value is 25.19 o. Herschel also mistakenly assumed that the dark areas on Mars were oceans, and the lighter regions land. When two faint stars passed very close to Mars with no effect to their brightness, Herschel correctly assumed that Mars had a tenuous atmosphere. Herschel confirms Cassini's suspicions that Mars has seasons, based partly on Cassini's observations, partly on his own observations, but also on the fact that Mars has an inclination that is close to the same value as Earth's. Herschel seems to be the first to refer to the maria by the term "sea," however he was not the first to assume that maria actually contained liquid water. He suggests that flooding may explain some of surface changes, though he agreed that clouds could explain some changes. 1860: Emmanuel Liais suggests the variations in surface features are due to changes in vegetation (not flooding or clouds). 1863: Father Pierre Angelo Secchi notices that maria change color. At different times he observed maria with green, brown, yellow and blue colors. 1877: Since the Earth, Jupiter, and Saturn were known to have moons, scientists suspected Mars might have moons as well. However finding them was not easy. Asaph Hall searched for Martian moons and, eventually, discovered two small moons, which were given the names Deimos and Phobos. 1877: Giovanni Schiaparelli makes a map of Mars that showed maria, some of which were connected by thin lines. He wasn't the first to observe them (earlier maps show a few) but he saw more lines than his predecessors. However some observers did not see any lines, and there was some controversy over whether they existed at all. Schiaparelli assumed that these lines were natural landscape features. He gave them the name "canali" which is the Italian word for "groove." However when this word was translated into English, "canali" became "canal," a word with a very different meaning. This simple mistake led many people to speculate about intelligent beings who built canals. Schiaparelli himself was unconvinced and somewhat annoyed that his observations led to such speculation. Years of observations, contradictory theories and scientific debate followed. 1938: On the day before Halloween, Orson Wells produces a radio production of the fictional story "War of the Worlds," a story of Martians invading the earth. The production was so convincing, many people believe there has been a real invasion by Martians. A panic resulted. The canal controversy would not be completely resolved until spacecraft arrived at Mars in the 1960's. 1892: Edward Emerson Barnard observed craters on Mars. This observation was almost completely ignored for over 70 years. 1912: Svante Arrhenius has an alternate suggestion for the Martian surface variation: Mars might be covered with salts, during the winter the salts have a light color. When the polar caps melt in summer, the salts absorb water and develop a darker color. 1952: Gerard Kuiper makes the first attempt to determine the composition of the Martian atmosphere using modern equipment. He discovered spectral lines that indicated the presence of carbon dioxide.

Venus Hydrosphere

With Earth and Venus approximately the same size, and having formed at the same time, scientists believe that both planets likely began with similar amounts of liquid water. Being closer to the Sun, Venus heated up and much of its water evaporated into the atmosphere, where it could then be ripped apart by sunlight and a stronger solar wind, and subsequently lost to space. Today the proportions of water on each planet are extremely different: Earth's atmosphere and oceans contain 100,000 x the total amount of water on Venus. With a surface temperature hot enough to melt lead, no liquid water is found on the surface (even at 90 x Earth's atmospheric pressure).

Moon's Topography

Elevations are on average about 1.9 km higher on the far side than the near side. Lowest point: ~13 km (8.1 mi) deep within the South Pole Aitkin Basin (one of largest impact basins in solar system). Low topography on the Moon is generally related to the giant impact basins, known a mare (maria is the plural form). On the lunar near side, most of the basins are partially infilled with dark lava, creating our familiar view of the Moon. Highest point: ~10.8 km (6.7 miles) lies just to the north-east of the lowest point. High topography, or highlands, on the Moon is generally found on the lunar far side. Note that the highest point on the Earth is at the summit of Mt. Everest 8.85 km above sea level and lower than the highest point on the Moon by ~ 2 km. Everest is a relatively new feature formed by the tectonic collision of two continental plates over the course of about 60 million years. The lunar high point is very ancient, and was possibly formed as ejecta from the enormous South Pole Aitken basin piled up during this cataclysmic event, in a matter of minutes, more than 4 billion years ago. The surrounding terrain gently slopes (about 3° slope) up to the summit. Lunar Surface Dominated by igneous rocks and metamorphic rocks from igneous sources, implying that the entire lunar surface was molten early in its history. Covered with dead volcanoes, impact craters/basins, and lava flows. Very few tectonic features are observed on the lunar surface (small size, rapid cooling). Even the so-called mountain ranges of the Moon are not tectonic in nature: generally circular, they are the raised rims of impact craters and basins. Crater Counts and Dating of Surface Since we have rock samples from many terrains on the Moon and have determined their radiometric ages, it is sometimes possible to infer a relative age for a surface on other solar system objects by counting the number of impact craters on that surface and using the radiometric age for a similarly impacted surface on the Moon. Technique effective if the other body has little erosion or internal activity to re-surface body. Assumes rate of impacts has been approximately constant for several billion years. If so, number of craters is proportional to the length of time the surface has been exposed. Older surfaces have more impact craters than younger surfaces. From Earth and Moon data, impact rate has been almost constant for > 3 billion years and much higher prior to 3.8 billion years ago.

Moon's Formation and History

Formation and History The giant impact hypothesis, the prevailing theory supported by the scientific community, suggests that the Moon formed when an object smashed into early Earth. Known as Theia, the Mars-sized body collided with Earth, throwing vaporized chunks of the young planet's crust into space. Gravity bound the ejected particles together, creating a moon that is the largest in the solar system in relation to its host planet. This sort of formation would explain why: Earth's spin and the Moon's orbit have similar orientations, Moon samples indicate that the Moon's surface was once molten, the Moon has a relatively small iron core, the Moon has a lower density than Earth, and the stable-isotope ratios of lunar and terrestrial rock are identical. There is evidence in other star systems of similar collisions, resulting in debris disks. Giant collisions are consistent with the leading theories of the formation of the Solar System. Geologic activity decreased as the Moon's interior cooled. Impact cratering and early volcanic infills are the primary geologic changes that occurred before the Moon became essentially geologically dead.

Mars Interior

Interior layers: differentiated into crust, mantle, and core Based on its mean density and the bulk chemistry of terrestrial planets, Mars is believed to have a dense metallic core and a silicate mantle. However, because no seismic data exist for Mars, the density profile of its interior and the depth of the core-mantle boundary are not known precisely. Features such as faults, folds, and volcanoes are present at the surface of Mars and were probably driven by processes in the mantle. Surface maps do not show evidence of a globally connected plate tectonic system and the size and positioning of the large shield volcanoes imply that Mars is a single-plate planet. Crust: rocky, 50 km thick Mantle: rocky, ~ 1560 km thick. Iron Core: partially fluid iron-sulfide outer core, possible solid inner core, overall 1800 km thick. Crust 10 - 50 km (31 mi) thick Earth's crust, averaging 40 km (25 mi), is only one third as thick as Mars's crust, relative to the sizes of the two planets. Composed primarily of basaltic rock with silicon and oxygen being the most abundant elements and including smaller amounts of iron, magnesium, aluminum, calcium, and potassium. Red color from oxidized iron. Higher percentages of iron in its crust indicate that Mars may not be as differentiated as other terrestrial planets. Mantle About 1,240 to 1,880 km (770 to 1,170 miles) thick (compared to Earth's at 2,890 km). Composed of silicate rocks, probably with the consistency of soft rocky paste. Core 1,500 to 2,100 km (1,115 mi) radius (compared to Earth's inner + outer core radius of 3470 km) Composed of iron and nickel with about 16-17% sulfur. The iron sulfide core is partially fluid, and has twice the concentration of the lighter elements that exist at Earth's core.

Venus Interior

Interior layers: differentiated into crust, mantle, and core Venus formed at the same time as Earth, in the same general region of the solar system, and is very similar in both size and density. Because of the similarity in size and density, scientists think that Venus has an internal structure quite similar to Earth's. The mathematical models for Venus derived from gravity and magnetic field data from the Venera, Pioneer Venus, and Magellan spacecraft generally agree with this assumption. Features such as faults, folds, and volcanoes are present at the surface of Venus and are probably driven largely by processes in the mantle. However, surface maps do not show evidence of a globally connected plate tectonic system, implying that Venus is a single-plate planet. Extreme surface temperatures bring rock temperatures to one-half their melting points, making brittle fracture less likely and making the crust more buoyant and harder to subduct than on Earth. Gravitational studies imply that Venus does not have an asthenosphere. On Earth, this layer in the upper-mantle is lower viscosity and mechanical weakness and allows crustal tectonic plates to move and interact. Venus probably formed a differentiated core much as Earth did, with the heaviest elements (such as iron) sinking to the middle of the planet. However, it is not known if this core has yet solidified. Limits on magnetic field strength from Magellan magnetometer data shows that the Venus magnetic field is a miniscule 0.000015 times Earth's field. Because of its slow rotation (243 days) and its predicted lack of internal thermal convection, any liquid metallic portion of its core could not be rotating or convecting fast enough to generate a measurable planetary magnetic field. Crust: rocky, 20-50 km thick Mantle: rocky, ~ 2800 km thick. Iron Core: liquid-iron, 3200 km thick. Crust Approximately 50 km (31 miles) thick. Approximately 90% basalt (volcanic) and are different shades of grey, like rocks on Earth. Surface imaged in visible wavelengths by landers and from Earth and orbit in radar frequencies. The absolute ages of materials on the surface of Venus are not known, but the overall density of craters on Venus is lower than on many other bodies in the solar system. Estimates vary, but the average age of materials on Venus is almost certainly less than one billion years and may in fact be substantially less. Mantle About 2800 km (1740 miles) thick (compared to Earth's at 2,890 km). Composed of silicate rocks. High surface temperatures (~melting temperature of lead) prevent study of interior with a seismic network. Core About 3200 km thick (compared to Earth's inner + outer core radius of 3470 km) Composed of heavy element from differentiation of original body, similar to Earth. Structure not currently known. No seismic information to determine if the inner core has solidified. No magnetic field to indicate convection in core.

Moon's Magnetosphere

Magnetosphere Very weak in comparison to that of the Earth. Moon does not currently have a dipolar magnetic field. Observed varying magnetization almost entirely crustal in origin. Early analysis of magnetized moon rocks brought to Earth by Apollo astronauts showed that the Moon must have had a strong magnetic field at least 4.25 billion years ago, which then slowly diminished up until 3.1 billion years ago. Recent analysis of a relatively young rock collected by Apollo astronauts reveals the Moon had a weak magnetic field until 1 billion to 2.5 billion years ago, at least a billion years later than previous data showed. No firm conclusions as to source of early stronger field or for continued weak field.

Moon's Interior Structure

Interior layers: four from outermost to innermost Crust Rocky, 60-100 km thick Mantle Rocky with a radius of ~1160 km (700 km thick). Liquid Outer Core Liquid-iron with a radius of roughly 330 km. Topped by a partially molten boundary layer, ~150 km thick (unlike Earth's outer core) Solid Inner Core Solid, iron-rich inner core with a radius of ~ 240 km. Crust Between 60- and 100-km (38 to 63 miles) thick. Thickest parts are on the far side of the Moon. Early evolution to synchronous rotation allows hottest and densest part of interior to be pulled closer to Earth, creating unequal heat flow to surface. Surface covered with a layer of pulverized rock (regolith). Shallow as 3 m (10 ft) in the maria or as deep as 20 m (66 ft) in the highlands. Product of over 4 billion years of meteoroid impacts. Most prominent surface features are impact craters. Sizes range from Mantle About 700 km (435 miles) thick. Most moonquakes occur about 1,000 km below the surface and occur with monthly periodicities related to tidal stresses caused by the eccentric orbit of the Moon about the Earth. A few shallow moonquakes with hypocenters located about 100 km below the surface have also been detected, but these occur more infrequently, appear to be unrelated to the lunar tides, and may be related to readjustment of voids in fragmented regolith. Quakes of truly tectonic origin seem to be uncommon. Outer Core About 240 km thick Outermost region partially molten iron mixed with a small amount of sulfur. Innermost region molten iron Inner Core Radius of about 240 km (150 miles). Made of iron in the solid phase. Makes up only 20% of the Moon. Small compared to core size of ~ 50% of other terrestrial bodies. Probably related to loss of material to Earth during formation.

Mercury's Interior

Interior layers: four from outermost to innermost Like all large terrestrial bodies, Mercury boasts a crust, mantle, outer core, and inner core, but in different proportions from the classic planets or the Moon. Most of the interior of Mercury is made up of its iron-rich core, which has a radius of about 2,074 km or about 85% of the planet's radius. The outermost part of this core is partly molten or liquid and topped by a solid outer iron/sulfur layer. Mercury's mantle and crust is only about 260 km thick and its lithosphere acts as a single plate. Its surface most resembles that of the Moon, but with a lower topography and more inter-crater volcanic plains. Crust: rocky, 50 km thick Mantle: rocky, 210 km thick. Solid Outer Core Layer: iron-sulfide, 50 to 200 km thick Liquid Core: liquid-iron, 830 km thick. Solid Inner Core: iron-rich, 1240 km thick. Crust Approximately 50 km (31 miles) thick. Composed of igneous, silicate rocks. Rocks are not rich in feldspar like the rocks on the lunar surface. Contain less iron and titanium than lunar or Earth's crustal rocks. Contain more sulfur, about 20 times richer, than the surfaces of Earth, the Moon, and Mars. Mercury seems to have formed in conditions much more reducing—i.e., those in which oxygen was scarce—than other terrestrial planets. Mantle About 210 km (130 miles) thick. Composed of silicate rocks. Core Makes up ~ 85% of planet's radius About 2200 km thick Unique structure for terrestrial planets in our solar system. Outermost region is a thin solid annulus of iron-sulfide, 50 to 200 km thick. Middle region is liquid-iron, 830 km thick. Inner Core is believed to be solid and iron-rich, 1240 km thick.

Venus Magnetosphere

Magnetosphere A global magnetic field deflects charged particles of the solar wind (electrons and protons) as they stream away from the Sun. This deflection creates a magnetosphere - a protective "bubble" around the planet. Today, Venus does not exhibit an intrinsic geomagnetic field. Measurements by orbiting spacecraft have shown that any dipole field originating from within Venus must be no more than 1/8,000 that of Earth's. Current theories of the formation and evolution of the terrestrial planets do support a much more intense, ancient magnetic field on Venus which may have been at least as strong as Earth's a few billion years after Venus was formed. Current lack may be related to: slow rotation In dynamo theory, rotation helps to drive the fluid motions within the planet's interior that produce the field. possibility that Venus' core is fluid but does not circulate or that the core is solid and incapable of supporting a dynamo. However, Venus is partially protected by an induced magnetic field created by interaction between solar wind and the upper atmosphere to create an ionosphere and weak magnetosphere. Pressure exerted by the solar wind compresses the ionosphere to a much lower altitude on the dayside of Venus than on the night side. Density of the ionosphere is also far greater on the dayside than on the night side.

Mercury's Magnetosphere

Magnetosphere Approximately a global magnetic dipole field, offset to the North by 20% of the planetary radius. Magnetic field at the surface has just 1% the strength of Earth's. The field interacts with the magnetic field of the solar wind to sometimes create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface of the planet. When the ions strike the surface, they knock off neutrally charged atoms and send them on a loop high into the sky, creating the planet's exosphere. Mercury's planetary magnetic field largely shields the surface from the supersonic solar wind emanating continuously from the Sun.

Earth's Magnetosphere

Magnetosphere Strongest of the terrestrial planets. First studied by William Gilbert of England during the late 1500s. Earth's geomagnetic field orientation changes over time. Evidence from magnetic stripes on either side of spreading centers (e.g., Mid-Atlantic Ridge) Solar wind particles with charge trapped in magnetosphere and interact with atmosphere to create aurora near magnetic poles.

Mars Magnetosphere

Magnetosphere Today, Mars exhibits no indications of an active global magnetic field, but does have weak magnetic fields in various regions of the planet which appear to be the remnant of a past magnetosphere. Fields of inconsistent strengths measuring at most ~16-40 x less than Earth's were measured by Mars Global Surveyor. In the ancient southern crust, which is undisturbed by giant impacts and volcanism, the field strength is highest. In the northern lowlands, deep impact basins, and the Tharsis volcanic province, the field strength is very low. Indicates that the magnetic field died out before they formed more than three billion years ago. Measurements of the solar tidal deformation of Mars (from Mars Global Surveyor radio tracking data) indicate that at least the outer part of the core is liquid and are also consistent with an entirely liquid core. Map of Mars' Magnetic Field Early in its history, Mars did have a liquid core whose motion generated a magnetic field. Evidence of it was preserved in ancient molten rocks as they cooled. Traces are best preserved in the southern highlands. The absence of magnetic features in the large impact basins indicates that the magnetic field died out before they formed more than 3 billion years ago.

Mars Orbit

Rotation period: 24 hr 37 min Revolution period: 1.881 yr (687 Earth days) Eccentricity of orbit: 0.093 Aphelion distance: 249 x 10^6 km (155 x 10^6 miles) Perihelion distance: 205 x 10^6 km (128 x 10^6 miles) Orbital inclination: 1.85° to ecliptic Orbital semi-major axis: 228 x 10^6 km (1.52 AU)

Venus Biosphere

Many factors make water-based life as we know it unlikely on the surface of the Venus: its location closer to the Sun than Earth makes the rate and effect of solar radiation higher, the extreme greenhouse effect raises temperatures on the surface to nearly 735 K (462 °C), and the atmospheric pressure 90 times that of Earth. A few scientists propose that thermo-acidophilic extremophile microorganisms might exist in the lower-temperature, acidic upper layers of the Venusian atmosphere. However, solar radiation constrains the atmospheric habitable zone to between 51 km and 62 km altitude, within the acidic clouds. It is plausible that microbial life originated on Venus if liquid water existed on its surface prior to the heating of the planet by the runaway greenhouse effect. However, the global re-surfacing that occurred ~500 million years ago would have covered any remaining fossil evidence of such life, if it existed.

Naked Eye Studies of Mars

Mars has been observed by many ancient cultures, but we have no idea who was the first to notice it. For those observing Mars without a telescope, the appearance of the planet changes in a cycle that repeats in a little over two years: Mars first appears as a pale pink object, only visible in the early morning just before dawn (and rather difficult to see at that). It moves relative to the stars, gets brighter over the next year, and rises earlier and earlier. It then abruptly stops and reverses direction. At its brightest, it is the third brightest object in the night sky (only Venus and the Moon are brighter), has a more intense red color, and is visible all night long. After moving the "wrong" direction for some 70 days or so, it stops and reverses direction again. It gradually gets dimmer, is only visible in the evening sky, and sets earlier and earlier. After another year, it again is a pale pink object, this time only visible just after sunset. Shortly after that, it cannot be seen at all. It remains hidden for about one hundred days when the cycle repeats again. Records of Mars observations include the following: Ancient Egyptian astronomers recorded observations since the 2nd millennium BCE and were the first to recognize that the stars seem "fixed" and that the Sun moves relative to the stars. They also noticed five bright objects in the sky (Mercury, Mars, Venus, Jupiter, and Saturn) that seemed to move in a similar manner. Chinese records about the motions of Mars appeared before the founding of the Zhou Dynasty (1045 BCE). Babylonian astronomers made detailed observations of the position of Mars and developed arithmetic techniques to predict the future position of the planet. Ancient Greek philosophers and Hellenistic astronomers developed a geocentric model to explain the planet's motions. In 356 or 357 BC, Aristotle observed Mars passing behind the Moon. We now call such an event an occultation. This convinced Aristotle that Mars was more distant than the Moon. Measurements of Mars' angular diameter can be found in ancient Greek and Indian texts. In the 16th century, Nicolaus Copernicus proposed a heliocentric model for the Solar System in which the planets follow circular orbits about the Sun. In the 17th century, Johannes Kepler updated the model fitting an elliptic orbit for Mars that more accurately fitted the observational data.

Mercury Facts

Mass: 3.3 x 10^23 kg (5.5% x MꚚ) Radius: 2440 km (0.38 x RꚚ) Average Density: 5.43 g/cm^3 Second only to Earth. High value for small size implies that a metal-rich core occupies at least 85% of the planet's radius while Earth's occupies only 50% of its radius. Several theories to explain difference; need additional information for definitive explanation. Acceleration due to gravity: g(Mercury) = 0.38 x gꚚ Escape speed: 4.43 km/s (0.45 x v(escape Ꚛ)) Average surface temperature: Day 700 K (+800°F) Night 165 K (-290°F) Lack of atmosphere provides the extreme variation in surface temperatures. Tilt of Rotation Axis: 2° no seasons due to tilt; see Eccentricity of orbit below

Venus Facts

Mass: 4.9 x 10^24 kg (0.815 x MꚚ) Radius: 6051 km (0.95 x RꚚ); very spherical Average Density: 5.25 g/cm^3 Acceleration due to gravity: g(Venus) = 0.91 x gꚚ Escape speed: 10.4 km/s (0.93 x v(escape Ꚛ)) Average surface temperature: Day = Night 740 K (870° F) Slow rotation, proximity to Sun, and a runaway greenhouse effect created thick layer of clouds that effectively insulate the surface from daytime heating/nighttime cooling. Tilt of Rotation Axis: 177.3° No seasons due to tilt; almost upright, but flipped to other terrestrial planets.

Mars Facts

Mass: 6.42 x 10^23 kg (0.107 x MꚚ) Radius: 3,397 km (0.53 x RꚚ) Average Density: 3.93 g/cm^3 (Moon=3.3 g/cm^3, Earth=5.5 g/cm^3) Acceleration due to gravity: g(Mars) = 0.38 x gꚚ Escape speed: 5.03 km/s (0.45 x v(escape Ꚛ)) Average surface temperature: 218 K Day 310 K (116° F) Night 150 K (34° F) Tilt of Rotation Axis: 25.2° Seasons similar to Earth's but twice as long. Eccentricity of orbit creates more extreme seasonal variations in S-hemisphere. S-hemisphere closer to Sun in summer and further from Sun in winter.

Moon Facts

Mass: 7.35 x 10^22 kg (1/80 x MꚚ) Radius: 1738 km (0.27 x RꚚ) Average Density: 3.34 g/cm^3 Acceleration due to gravity: g(Moon) = 1/6 x gꚚ Escape speed: 2.4 km/s (0.21 x v(escape Ꚛ)) Average surface temperature: Day 375 K (+216°F) Night 125 K (-234°F) Lack of atmosphere provides the extreme variation in surface temperatures.

Mars Moons and Rings

Natural satellites: Phobos and Deimos Mars' Two Moons: Phobos and Deimos Phobos has a mean radius of ~7 miles. Deimos is about half the size of Phobos. Asaph Hall discovered Deimos and Phobos during one week of observations in August 1877. In Greek mythology, Phobos (fear) and Deimos (panic) were named after the horses that pulled the chariot of the Greek war god Ares, the counterpart to the Roman war god Mars. Among the smallest in the solar system. In synchronous orbit: always present the same face to their planet. Appear lumpy, heavily-cratered, and covered in dust and loose rocks. Among the darker objects in the solar system. Made of carbon-rich rock mixed with ice and are probably captured carbonaceous asteroids. Phobos is the largest and innermost Martian moon. Mean radius of 11 km (7 mi.): 150 x smaller than Earth's Moon. Seven times as massive as Deimos. Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions. Gradually spiraling inward, drawing 1.8 m (6 feet) closer to the planet each century. Within 50 million years, it will either crash into Mars or break up and form a ring around the planet. Orbits 6,000 km (3,700 mi) from the Martian surface. No known moon orbits closer to its planet. Orbits more rapidly than Mars rotates. From the point of view of an observer on the surface of Mars, it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Deimos is the smaller and outer of the two natural satellites of Mars. Mean radius of 6.2 km (3.9 miles): 280 x smaller than Earth's Moon Orbit is nearly circular, close to Mars's equatorial plane and takes 30 hours. From the point of view of an observer on the surface of Mars, it rises in the east and sets in the west (unlike Phobos). Rings: none

Earth's Orbit

Rotation period: 1 day Revolution period: 365.25 days — one year. Orbital semi-major axis: ~150 x 106 km (92,955,807 miles) Average distance from Earth to the Sun is called an astronomical unit, or AU. Earth orbits the Sun 100,000 times closer than the Oort Cloud. Earth's orbit is not a perfect circle; it is an ellipse. Over the course of a year, Earth's closest approach to the Sun, called perihelion, comes in early January and is about 146 km (91 x 106 miles), while the farthest from the Sun Earth gets, called aphelion, comes in early July and is about 152 x 106 km (94.5 x 106 miles). Tilt of rotation axis: 23.5° Seasons due to tilt creating variation in angle of incidence of sunlight.

Venus Orbit

Rotation period: 243 days, retrograde No definitive theory for rotation opposite to rest of solar system. Assuming Venus formed with the solar system's standard counter-clockwise rotation, current ideas include: Sun's gravitational pull on the planet's very dense atmosphere could have caused strong atmospheric tides. Such tides, combined with friction between Venus's mantle and core, could have caused the flip in the first place. Venus' rotation slowed to a standstill and then reversed direction. Taking into account the factors mentioned above, as well as tidal effects from other planets, Venus' axis could have shifted to a variety of positions throughout the planet's evolution before settling into its current rotational state. Revolution period: 225 days Eccentricity of orbit : 0.007 Smallest for any classic planet in our solar system; almost circular orbit. Orbital inclination: 3.4° to ecliptic Orbital semi-major axis: 108 x 10^6 km (0.72 AU)

Moon's Orbit

Rotation period: 27.3 days Revolution period about Earth: 27.3 days Synchronous rotation: orbital and rotational periods equal due to tidal interaction with Earth. Humans never saw "far-side" of Moon until the space age. Orbital Distance from Earth Average = 382,500 km Perigee = 360,000 km (closest approach to Earth) Apogee = 405,000 km (farthest from Earth)

Mercury's Orbit

Rotation period: 59 days Revolution period: 88 days 3:2 rotation-to-orbital periods, known as a spin-orbit resonance, due to tidal interactions with Sun and the eccentricity of Mercury's orbit around Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, Mercury appears to rotate only once every two Mercurian years Þ an observer on Mercury would see only one day every two Mercury years. Eccentricity of orbit : 0.206 Largest for any classic planet in our solar system. Eccentricity of orbit correlated with the 3:2 spin-orbit coupling creates longitudinal seasons on Mercury. The same longitude is facing the Sun at perihelion (closest approach), creating a "hotter" longitude. Orbital inclination: 7° to ecliptic largest for any classic planet in our solar system Orbital semi-major axis: 58 x 10^6 km (0.39 AU)

Exploration from Space

Since the beginning of the space age, Mars has been a focus of planetary exploration for three main reasons: it is the most Earth-like of the planets; other than Earth, it is the planet most likely to have developed indigenous life; and it will probably be the first extraterrestrial planet to be visited by humans. Over the past few decades, the robotic spacecraft we've sent to Mars — orbiters, landers, and rovers — have vastly deepened our understanding of the Red Planet. We've come to know Mars not just as an image through a telescope, but also as a unique and intriguing world. The first Earth spacecraft to successfully visit Mars was Mariner 4 in 1965. It passed near Mars and sent back 22 photographs of the surface. Although fuzzy and grainy, the pictures revealed a dry, rugged, cratered terrain, more similar to parts of the Moon than Earth. Every 26 months there is an opportunity to send a vehicle from Earth to the planet Mars along an efficient, low-energy trajectory with the one-way trip taking six months or more. Since the first successful mission in 1965, forty-five (45) missions have been sent to Mars from Earth. However, more than half of all spacecraft destined for Mars failed before completing their missions and some failed before their observations could begin. To date, the United States and USSR (Russia) are the only countries to successfully land on Mars. Missions have focused primarily on understanding the geology, atmosphere, and habitability potential of Mars.

Earth's Surface and Structure

Total surface area: ~510 x 106 km^2 (197 x 106 miles^2). About 71% is covered by water and 29% by land. Earth is lowland dominated since most of its solid surface is below sea level. Oceans have an average depth of 4 km (2.5 miles). Fresh water exists in liquid form in lakes and rivers and as water vapor in the atmosphere, which causes much of Earth's weather. Interior layers: four from outermost to innermost By the mid-1900s observations of earthquake waves had led to a spherically symmetrical crust-mantle-core picture of the Earth. Crust Composed of the ocean basins and the continents Between five and 75 km (three and 46 miles) deep. The thickest parts are under the continents and the thinnest parts are under the oceans. Made up of several elements: oxygen, 46.6 % by weight; silicon, 27.7 %; aluminum, 8.1 %; iron, 5 %; calcium, 3.6 %; sodium, 2.8 %, potassium, 2.6 %, and magnesium, 2.1 %. The crust and upper region of mantle (layer beneath the crust) are divided into huge plates that float on the lower mantle. The plates are constantly in motion and move at about the same rate as fingernails grow. Earthquakes occur when these plates grind against each other. Mountains form when the plates collide and deep trenches form when one plate slides under another plate. Plate tectonics is the theory explaining the motion of these plates. Mantle About 2,890 km deep (1,800 miles). Composed mostly of silicate rocks rich in magnesium and iron. In the uppermost part of the mantle, intense heat causes the rocks to rise. They then cool and sink back down to the core. This convection — with the consistency of caramel — is thought to be what causes the tectonic plates to move. When the mantle pushes through the crust, volcanoes Outer Core Composed of a nickel-iron alloy in a liquid state. About 2,180 km (1,355 miles) thick. Thought to be the source of Earth's magnetic field. Inner Core Made of iron in the solid phase. Radius of about 1,220 km (about 760 miles). Spins at a different speed than the rest of the planet.

Greenhouse Effect on Venus

a planet with gases in its atmosphere that absorb and emit infrared wavelengths will trap heat near the planet's surface and, generally, raise the surface temperature in a process known as the Greenhouse Effect. Venus has the most powerful greenhouse effect found in the Solar System. The greenhouse agents sustaining it are water vapor, carbon dioxide and sulfuric acid aerosols. 96.5% of the gas in Venus' current atmosphere is CO2. On Venus, 80% of the incoming solar radiation is reflected back to space by the cloud layer, about 10% is absorbed by the atmosphere and only 10% manages to get through it and heat the surface. The thermal radiation emitted by the surface gets trapped by the same CO2-rich atmosphere, raising the surface temperature to 740 K. Venus has experienced a runaway greenhouse effect involving carbon dioxide and water vapor. Most scientists agree that early Venus may have had a global ocean and an atmosphere similar to Earth's early atmosphere. As the brightness of the early Sun increased, the amount of water vapor in the atmosphere increased. Being closer to the Sun than Earth, this would happen to a greater extent on Venus than Earth. Increasing the water vapor content increases the temperature and the rate of evaporation of the ocean, leading eventually to the situation in which the oceans boiled, and all of the water vapor entered the atmosphere. Water vapor is an even more powerful greenhouse gas than carbon dioxide and this caused temperatures to rise even more. Then the surface of Venus got so hot that the carbon trapped in rocks sublimated into the atmosphere and mixed with oxygen to form even more carbon dioxide. With no global magnetic field and proximity to the Sun, the water vapor is broken apart by solar UV radiation and lost from the atmosphere. Water is key in controlling the greenhouse effect on Earth: (1) the oceans absorb a lot of CO2 (2) water and CO2 react with silicate rock to lock the CO2 up in carbonaceous rock; and (3) water nourishes plants, which remove CO2 from the atmosphere. When Venus lost its water, it was no longer able to suppress atmospheric CO2 and was doomed to develop an extreme greenhouse effect.