Geology Test #2

Mars Open Basin Lakes

"Open basin lakes" on Mars provide evidence for long-lived lakes and large volumes of water in the ancient past. These lakes formed in craters that are breached in two places by channels. That means water filled the crater from one channel and then spilled over and out the other side. It is possible that water still flows on the surface periodically today. This series of images shows the formation of what appear to be flows from the flank of Newton Crater. These flows were observed to form during the warmest part of the Martian summer, and may reflect melting of subsurface ice. Dissolved salts could act as an antifreeze.

Why Mercury can't have a magnetic field

1) Geologically inactive-interior cooled long ago, so core should be solid, or if it is still liquid, heat loss to mantle will be slow and convection in the core will be sluggish. 2) Spins too slow (57.8 Earth days); Just as with Venus, slow rotation should prevent development of geodynamo.

Why are the lunar Maria concentrated on the near side of the Moon?

1) The lunar crust is thinner on the near side, so maybe melts had an easier time reaching the surface. 2) Heat-producing elements (K, Th, U) are more concentrated on the near side, so meling might not have occurred in the far-side mantle. Two recent missions have provided new insights into lunar structure and evolution. The Lunar Prospector (let) analyzed the composition of the lunar surface. The Grail mission analyzed the moons topography and gravitational field. The far side of the moon, like the near side, is pock-marked with thousands of craters, including a number of large impact basins. The largest of these is the Aitken Basin near the south pole, revealed in this topographic image as a large depression Unlike the near side, however, most impact basins on the far side are not filled with maria, extensive basalt lows. In addition, the crust appears to be thicker on the far side than on the near side of the moon. By measuring both the topography and gravitational field of the moon, scientists have determined that the lunar crust is thicker on the far side of the moon. In addition, basalts from the near side of the moon are richer in thorium, potassium, and uranium. These are heat-producing radioactive elements. How might these two observations relate to why maria are mainly found on the lunar near-side? The Aitken basin is a very large, very old impact basin on the far side of the moon. The topographic low regions in the center of the basin have a very different composition than other portions of the lunar crust. The basin provides a glimpse into portions of the deeper crust (and maybe the mantle) otherwise inaccessible to us.

Timeline for Events on the Moon

1) The oldest crust (anorthosite from the highlands) dates to ~4.4-4.5 Ga, formed from the initial magma ocean. 2) Most major basins formed during the heavy late bombardment between 4.2 and 3.8 Ga. 3) Flood volcanism occurred mostly between 3.9 and 3.2 Ga, with some scattered volcanism possibly occurring as recently as 1 Ga.

Messenger Questions

1)Why does Mercury have such a high density? 2)What is the geologic history of Mercury? How active was/is it? 3)What is the nature and origin of Mercury's magnetic field? 4)What is the structure of the core? How much of it is molten? 5)Is there ice at the poles, hidden in the shadows? 6)Where does Mercury's very thin atmosphere come from?

Mercury Summary

1)Why does Mercury have such a large core? We don't know. 2)How is this core able to generate a magnetic field similar to Earth's? We don't know. 3)How did Mercury stay volcanically active for an extended period of time despite its small size? We don't know. So does our current confusion about Mercury, how it formed, and how it has evolved represent a scientific failure? No! Science is most exciting when we are confronted with a puzzle. The first step in gaining a better understanding about the world around us is first figuring out what we don't know.

Why does Mercury have such a large core and high metal/silicate ratio? Three hypotheses:

1.)Mercury started out with a composition similar to the other terrestrial planets, but a giant impact after it had differentiated blasted much of the rocky outer layer into space. A giant impact would have greatly heated the planet, possibly forming a magma ocean. DOESNT WORK 2)After Mercury differentiated, the very hot, active early sun heated the surface so much (>2000 K) that much of the rocky outer layer was vaporized and boiled off into space. Obviously, this suggests Mercury was very hot initially. DOESNT WORK 3)Processes operating in the solar nebula (gas drag, interaction with magnetic fields) caused metal and silicate to separate before clumping together to form planetesimals, and Mercury formed in a region with an inherently high metal/silicate ratio. MOST LIKELY

The Moon

1738 km radius ~1% the mass of the Earth Lower density (3.3 g/cm3 vs. 5.4 g/cm3) Low albedo (0.136)-not uniform(highlands are brighter than the Maria)

Photosynthesis

6 H2O + 6 CO2 + sunlight => C6H12O6 + 6 O2 The Earth initially had an atmosphere devoid of oxygen, but photosynthetic bacteria (and much later, plants) produce O2 as a byproduct of storing energy from the sun as glucose. The buildup of oxygen in Earth's atmosphere took several billion years.

Mars Aeolian Processes

Aeolian or wind-driven transport of sediment is an important process that shapes the Earth's surface, particularly in arid regions with little vegetation. Sand dunes are one obvious type of landform produced by wind transport of fine-grained material. Wind transport of sand and dust has played an important role on other solar system objects besides Earth. The Martian sand dunes seen to the right are very similar to the terrestrial dunes shown above

Aeolian Processes

Aeolian processes involve transport of fine-grained material by wind. Sand dunes are an example of an aeolian landform. The size, shape, and orientation of sand dunes can provide a great deal of information about the direction and strength of winds.

Moonquakes

Apollo astronauts installed seismometers on the lunar surface. These seismometers record moonquakes, which are possibly triggered by the tidal stresses produced by lunar libration. Analyses of these moonquakes suggest that the moon does in fact have a (small) core.

Water on the Moon

At Mercury, we discovered that some crater loors near the poles are in perpetual shadow and therefore extremely cold. Radar imaging of these crater suggests they may contain water ice. It therefore should not be surprising that the same thing is seen for the moon. In 1994 data from the Clementine mission suggested the presence of water at the poles, but the data were inconclusive. On October 9, 2009, the LCROSS projectile impacted into the floor of Cabelus crater near the moon's south pole, while a companion satellite observed the resultant plume from orbit. The Cabeus crater has been in shadow for billions of years. The impact lited a plume of line material several kilometers above the lunar surface. This plume contained a large amount of water ice, confirming the presence of water on the moon. Why do we care that there's water trapped at the lunar poles? First, this water probably derived from comet impacts on the lunar surface. By measuring how much water is present, we can learn how frequent comet impacts are on the moon (and Earth). Water on the moon would also be a valuable resource for future lunar colonies. Not only do we need water to live, but we can make O2 to breath as well as rocket fuel out of water. This would be more efficient than transporting these from Earth.

Coriolis Effect

Atmospheric circulation is driven by temperature differences that arise from differences in solar irradiation. However, the Coriolis effect arising from a planet's rotation has a significant influence on patterns of air circulation. Because of Earth's rotation, a packet of air rising at the Equator is actually traveling east (along with the ground underneath) at over 500 m/s. As the air moves north or south, it appears to bend to the east, but this is actually due to the air maintaining its velocity, which is faster than that of an air packet further from the Equator (the air and the ground don't have as far to go to travel around the Earth once every 24 hours). Air masses moving away from the Equator will "bend" towards the East. Air masses moving towards the equator will bend towards the west. The counterclockwise spiral of hurricanes in the northern hemisphere is due to air masses rushing towards the hurricane center, which is an area of low pressure. Rotate clockwise in southern hemisphere

Water on Mercury

Because of Mercury's proximity to the sun, the surface can get extremely hot-up to 450 oC during the day. At night, however, the surface cools drastically because Mercury essentially lacks an atmosphere to retain heat. The temperature plunges to -170 oC. The floors of some craters near the poles are perpetually in shadow. Radar mapping reveals "bright" regions in these craters that may be patches of water ice, possibly derived from comets that have impacted the surface of Mercury. Many craters that are permanently in shadow (and therefore very cold) have "bright" (radar-reflective) floors, consistent with the presence of water ice. Why is this important? The amount of ice stored at the poles can help constrain the rate of comet impacts in the inner solar system. Also, water is an important resource for future exploration. Because there is no atmosphere, there are very large temperature differences between areas in sunlight and in shadow. Regions in permanent shadow can be as cold as 80K, cold enough to trap water vapor as ice.

World Weather Formation

Because the equator receives more solar radiation than higher latitudes, the surface warms more, heats the air, which rises and spreads towards the poles in a large convection cell. The Coriolis effect divides this simple convection cell into several smaller cells. On Earth, air rising from the Equator descends again at ~30 o north and south, producing an arid belt containing most of Earth's deserts. Because clouds form mostly where air is moving upwards, the Earth's Hadley cells concentrate cloud formation at certain latitudes (e.g., equator) that correspond to convective upwelling regions. Major Desert Areas of the World: Most deserts found at ~30 oN or ~30 oS. Remember, sinking air = no clouds!

Venura Spacecraft

Between 1961 and 1983 the Soviet Union launched 16 Venera spacecraft to study Venus. Several probes sampled Venus' atmosphere, measuring its composition and variations in temperature and pressure with altitude. Ten Venera probes successfully landed on the surface of Venus, though none survived for more than a few hours. These probes sent back pictures of the surface and analyzed the composition of the Venusian "soil".

Calcite Production

CaO + CO2 = CaCO3 Many different organisms produce calcite (CaCO3). Most of Earth's CO2 is stored as calcite, rather than as a gas in the atmosphere. Life has played an important role in the evolution of Earth's atmosphere. Biogenic calcite production removes CO2 from the atmosphere faster than would occur through inorganic calcite precipitation alone. If all the CO2 locked up as calcite were returned to the atmosphere, the Earth would have a runaway greenhouse similar to Venus, and life would be impossible.

Cloud Formation

Clouds on Earth are composed of water droplets or ice crystals. On Venus, however, clouds are composed mostly of sulfuric acid! Martian clouds may contain H2O ice or CO2 ice. Within the troposphere, thermal convection generates clouds, weather, etc. Convection is driven by temperature differences. The sun heats the ground, which heats the overlying air. Hot air expands and rises because it is less dense than surrounding air. As long as the air remains hotter than its surroundings, it will continue to expand and rise. Hot air moving upwards condenses and forms clouds The amount of water vapor that can exist in air is a strong function of temperature-hot air can hold more moisture. As air rises, it expands and cools by almost 10 oC/km. As the air cools, the relative humidity increases. When the humidity reaches 100%, water vapor condenses as water liquid or ice particles, producing clouds. Whether it is a Texas thunderstorm in August or the cloud belt that stretches across the tropics, clouds form where warm, moist air rises and cools, causing condensation. Rising, cooling air = clouds, possibly precipitation Sinking, warming air = evaporation, dry conditions.

Barchan Dunes

Crescent-shaped, form in areas of limited sand supply and unidirectional winds. Move downwind over a flat surface of pebbles or bedrock The points of the crescent are directed downwind. Sand dunes of various sizes and shapes are common in many parts of the Martian surface. Often, the floors of craters become filled with wind-blown sediment over time, and dune fields will form in the crater depressions.

Mercury Internal Structure

Pre-Messenger proposed internal structure of Mercury - Relative to the Earth, Mercury's metallic core is MUCH larger, making up ~42% of the volume of the planet, compared to ~17% for Earth's core. Crust—100-300 km thick, Mantle—600 km thick, Core—1,800 km radius Based on gravity measurements from the Messenger spacecraft, it appears that Mercury's core is even larger than originally thought (~85% volume of planet), and contains a solid inner core, liquid middle core, and a solid layer of iron sulfide on top.

Deltas

Deltas form where a river enters a static body of water, and sediment is deposited as the velocity decreases. Sand is deposited first, with progressively finer material deposited further into the water body. Mars also has features that resemble delta deposits, as well as lake-bed deposits.This suggests that at one time, Mars also had large bodies of standing water, i.e. lakes or seas. Some scientists have even suggested that much of the northern hemisphere was at one covered with water. Mars may have once had an ocean. To the left is a not-so-exciting picture of a gravel stream bed here on Earth. Flowing water carries away finer particles, leaving the larger, heavier particles behind. The size of particles that make up a streambed (silt, sand, gravel) depends on the rate at which water is flowing.

Mars Dendritic Channels

Dendritic channels suggest a network of streams fed by surface runoff. This, in turn, implies rainfall in the past on Mars. Dendritic channels are mostly found in the oldest, most heavily cratered portions of Mars.

Development of cross-bedding

Dunes advance with the wind. A well-sorted cross-bedded sandstone, combined with the characteristic grain surface feature of aeolian sands, is diagnostic of wind blown sands, The cross-beds can be used to infer paleowind direction Cross bedding features in terrestrial sandstones. Wind or water direction can be deduced from the direction of the cross beds. Grain size and appearance can be used to distinguish features created by wind from those produced by flowing water. This is a Martian outcrop displays cross bedding very similar to terrestrial fossil sand dunes. The cross bedding and the grain size of the particles that make up the layers of rock can provide information on Mars' past climate (wind speed, direction, changes in atmospheric density).

Dust Devils

Dust devils are similar to small tornadoes, and are caused by rising, spinning columns of air. Dust devils are a common late-afternoon feature in both Terrestrial and Martian deserts. These swirling black lines observed in the Martian surface are thought to be tracks left by dust devils as they dance across the Martian surface.

Dust Storms

Dust suspended by dust storms in the Sahara can be transported thousands of kilometers across the ocean. Dust collected in Hawaii comes largely from the deserts of central Asia! On Earth, arid regions with little vegetation cover are susceptible to dust storms, which can grow to enormous size and transport tremendous quantities of dust. On Mars, arid and sparse vegetation pretty much describes the entire planet. Mars images taken before (left) and during (right) a global dust storm. Much harder to see the surface during the dust storm.

Venus and Mars both have atmospheres dominated by CO2. Why is Earth different?

Earth is distinct from Venus and Mars in having an atmosphere dominated by N2 and O2, with very little CO2. Yet Earth and Venus are very similar in size and composition. Why is our atmosphere so different? The answer has to do with the presence of liquid water and of life on Earth. This may seem like a chicken/egg problem, but without life, Earth would probably not be habitable.

Water features on Earth/Mars

Earth is the only planet that has abundant liquid water on its surface today, but evidence suggests that this wasn't always the case. Many of the types of landforms formed or shaped by water on Earth today can be found preserved on the surface of Mars. First we will examine how some of these landforms are created on Earth, then we will examine similar features on Mars. Meandering streams (left), dendritic tributary systems (below), and deltas or distributary networks (below left) are all common landforms produced by water on Earth that are easily recognizable in aerial or satellite photos.

Earth's Atmosphere

Earth's atmosphere can be divided into several layers: Troposphere-region where convective mixing occurs Stratosphere-unique to Earth, a region where temperature increases with height Mesosphere-temperature decreases with height, but not enough to drive convection Thermosphere-increasing temperature due to absorption of UV radiation The high concentration of oxygen (O2) in Earth's atmosphere permits the formation of ozone (O3), a highly reactive molecule containing 3 oxygen atoms. Ozone in the troposphere can form from car pollution and is bad-it causes respiratory problems. Ozone in the stratosphere is good-it absorbs harmful UV radiation from the sun. The absorption of UV radiation by O2 and O3 in the stratosphere produces the warming of the stratosphere that is unique to Earth.

Earth's Magnetic Field

Earth's magnetic field resembles a bar magnet. Where does this field come from? Earth's magnetic field is generated by rapid convection in the molten metal outer core. This rapid convection is driven largely due to cooling of the core by the mantle. Mantle convection doesn't generate the magnetic field, but it helps remove heat from the core, which then drives core convection. The Coriolis force resulting from Earth's rotation also helps organize core convection, resulting in a field that resembles a bar magnet (a dipole field).This also keeps the magnetic poles close to the rotational poles.

Mariner 10

Prior to 2008, Mariner 10 was the only space probe to explore Mercury. Mariner 10 made three fly-bys of Mercury in 1974 and 1975. Mariner 10 was able to image about 45% of Mercury's surface. Mariner 10 also detected Mercury's magnetic field. Mariner 10 data reveal that this field is dipole-shaped, like Earth's field.

Evidence for Water on Mars

Evidence for rivers and lakes on Mars is mostly observed in very old terrains dating from >3.7 billion years ago (the Noachian period). At this time, much of the northern hemisphere may even have been covered by a shallow sea. Around the time of the formation of the Hellas Basin, the planet appears to have grown much colder, and the atmosphere much thinner. There is abundant evidence that Mars once had liquid water on the surface. Noachian dendritic channel networks(below) attest to drainage channels collecting and transporting surface runoff from precipitation. Hesperian Outflow channels (left) attest to massive flash floods and a colder, drier Mars. Further evidence that Mars not only once had water, but standing bodies of water such as lakes. The cracked ground in the photo on the left is a dry lake bed here on Earth. The photo on the right shows a very similar terrain from Mars. Such features have been found in several low-lying areas, and probably represent dry Martian lake beds rich in salts. Today, much of Mars' water is stored as ice at the poles or below the surface. The photo to the left compares a terrestrial debris-covered glacier with a very similar feature on Mars. Mars has many debris covered glaciers. The lobate features surrounding the mountain to the right contain large quantities of water ice. Ice collects as frost on the mountain slopes and flows down and outwards. Rampart craters like those shown here also attest to the presence of water ice beneath the Martian surface. The energy from impacts melts the permafrost, converting it to mud. The liquified ejecta flows over the surface after it lands, resulting in the lobate ejecta curtain seen. Mars has ice caps at both its southern and northern poles. These ice caps are composed mainly of water ice, although in winter a layer of CO2 ice is deposited on top of the water ice in the north, and CO2 frost remains year round in the south. There is enough water in the caps to cover the entire planet in ~20 m of water if the ice were melted. More water is stored as permafrost beneath the surface at mid-latitudes.

Fluvial Processes

Fluvial processes involve the action of flowing water. Water is an important agent of erosion and sediment transport on Earth. River channels and river deltas are examples of fluvial landforms.

Evidence for the Giant Impact theory for origin of the moon

High angular momentum of Earth-moon system Moon and Earth's mantle are very similar in composition Low density of moon (missing its metal component) Evidence for early "magma ocean" on moon The Earth and moon have very different bulk composition. The Earth has much more iron, which is why the Earth has a much higher density. However, the moon is fairly similar in composition to the silicate portion of the Earth. This indicates that the moon-forming impact occurred after the Earth and impactor had differentiated to form a rocky mantle and metal core. Another piece of evidence supporting the giant impact hypothesis is the similarity in oxygen isotopes between the Earth and moon. Studies of meteorites (including samples that came from Mars) indicate that different solar system objects each have a unique isotopic signature. However, the moon and Earth are isotopically identical, suggesting they both formed from the same batch of material.

Earth's Stable Temperatures

How has Earth maintained a stable, habitable temperature over geologic timescales? The answer lies in CO2. Sun has gotten hotter and brighter as it has gotten older, Earth receives 30% more energy from the sun then it did a while ago, liquid water on Earth's surface for over 4 billion years The level of CO2 in the atmosphere reflects a balance between volcanic activity (which adds CO2 to the atmosphere) and chemical weathering (which removes it). If the surface temperature increases, the rate of chemical weathering increases, which lowers the CO2 level in the atmosphere, which cools the planet, which slows chemical weathering, ... Even though the early sun was cooler than today, the Earth started with a much thicker, CO2-dominated atmosphere (stronger greenhouse effect). Over time, the sun has gotten stronger, but this has been balanced by CO2 removal. But, there is very little CO2 still left in the atmosphere. What happens if the sun keeps getting hotter? Yep, the Earth is pretty much doomed (in 500 million to a billion years).

Methane on Mars?

In 2003 and 2004, methane plumes were reportedly detected in the Martian atmosphere, using Earth-based measurements. Methane is quickly destroyed in the atmosphere, so its presence would require a process to constantly replenish it. On Earth, the main sources of methane are biological (e.g., methanogenic bacteria), so the possible presence of Methane on Mars was exciting. Say "hi" to SAM (Sample Analysis at Mars), a highly sensitive package of instruments on board the Curiosity rover, which landed on Mars on March 6, 2012. The rover has been making highly precise measurements of the Martian atmosphere and soil samples. Initially, no organic molecules were found, to the great disappointment of NASA scientists. But wait... The Curiosity rover can sample both the atmosphere and drill into rocks and analyze them for a range of compounds, including organics. SAM has detected lots of compounds that indicate water was once present, but initially found no organic compounds. Then, in 2014, SAM found transient methane spikes in the atmosphere. Sporadic "spikes" of methane in the air suggest a local source, but where is the methane coming from? Sources - Microbes, Olivine Rock + Water, Clathrate storage outgassing, surface organics from cosmic dust, Formaldehyde + Methanol In 2014, SAM also discovered evidence for chlorobenzene on Mars, and a fatty acid

Lunar Structure

Just like the Earth, the moon has a layered structure, with a crust, mantle, and core. However, the moon's core is much smaller than the Earth's in proportion to the total size of the planet. Careful measurement of changes in the moon's orbit suggest that the core is at least partly molten. However, the moon lacks a magnetic field, so no geodynamo is active. Much of what we know about the internal structure of the Earth comes from the study of earthquakes. Similarly, seismometers installed on the lunar surface by Apollo astronauts allow the study of moonquakes

Lava on the Move

Lava will flow and spread when the yield strength of the flow (which is controlled in part by viscosity) is exceeded by the force exerted by gravity. The critical thickness a flow must have on a given slope before it begins to flow is given by:h = Sy/[grtan(a)] where Sy = yield strength; g = gravitational acceleration; r = lava densitya = slope angle

Why do we care so much about ice on Mars, if life needs liquid water to survive?

Life is incredibly adaptable, and can survive in amazingly harsh conditions. Recently, scientists discovered bacterial living underneath several kilometers of ice in Antarctica. If bacteria can survive here, could life also survive underneath the Martian surface? Certain types of extremophile bacteria can survive and even thrive at temperatures well below freezing. They can do this in part because their cells contain a natural antifreeze that prevents water from freezing. Other bacteria can survive in highly salty brines, which freeze at much lower temperatures than fresh water. Watermelon snow is an example of a cryophilic (cold-loving) bacteria (Chlamydomonas nivalis). The snow smells faintly like watermelon, but don't eat it (you'll have long night if you do). Cryophilic organisms could potentially survive on Mars Did life evolve on Mars? Could it still exist, hidden below the surface, even today? On Earth, life has proven remarkably resilient. Bacteria can survive scalding temperatures in hot springs, or the cold and dark beneath the Antarctic ice cap. In 1996, a group of NASA scientists claimed to have found evidence for ancient bacterial life in a Martian meteorite (ALH84001). The evidence included "worm-like" structures seen under high magnification and some organic compounds. Today, most scientists believe the organic compounds found in this meteorite derived from terrestrial contamination, and the worm-like structures have a non-biologic origin. However, the debate regarding possible life on Mars is far from over.

Mars Water

Liquid water requires a minimum atmospheric pressure of ~0.006 bar. Below this pressure, liquid water cannot exist. The surface atmospheric pressure on Mars averages 0.0063 bar (ranges from 0.004 to 0.0087 bar), which is very close to the triple point of water. Mars' atmospheric pressure is almost exactly at the water triple point. Coincidence? Although some water was probably lost to space, much is still present as ice, locked in the polar ice caps or frozen underground. Mars' hidden ice may buffer its atmosphere. A major goal of current research is figuring out how much water is present and where it is. This will help constrain models for climate change on Mars as well as determine if liquid water could be present below the surface. In 2008, the Mars Phoenix lander discovered ice just below the surface The Mars Odyssey orbiter has mapped the distribution of water ice in the upper meter of the Martian soil using a technique called gamma ray spectroscopy. The equatorial latitudes are too warm for ice to survive-ice evaporates and water vapor is transported to higher latitudes where is freezes to form frost in the sub-surface.

Mercury's Wrinkle Ridges

Long scarpes and "wrinkle ridges" are abundant on the surface of Mercury. These compressional features are believed to have formed as the planet cooled and contracted. Solids contract as they cool, so as Mercury's interior lost its stored heat, the planet literally got smaller.

Lunar Libration

Lunar libration is a result of the moon's elliptical orbit. Although the moons rotation period matches the orbital period, the acceleration of the moon as it approaches perigee and deceleration as it approaches apogee means that the rotation does not match the orbital speed at any given moment, just the average speed. Because the moon's orbit is elliptical, it does not always travel at the same speed. However, the rate of rotation is constant. Although the moon spins on its axis once every orbital period, at different times it is either spinning faster or slower than it orbits the Earth. This causes the moon to librate. As a result, we can see about 59% total of the lunar surface at different times of the month.

Mariner 4

Mariner 4, launched in 1964, was the first spacecraft to photograph portions of the Martian surface. The photos returned showed a heavily cratered surface not unlike the moon. As luck would have it, the areas photographed did not contain any of the volcanic features or evidence for past water that make Mars such an exciting place.

Liquid Water on Mars

Mars has a thin atmosphere, but it is a cold desert world. The atmosphere is too thin and the surface temperature too cold for liquid water to exist on the surface for any length of time. The average surface temperature of Mars is a chilly -55 oC, which is a little colder than the average temperature in Antarctica. However, the temperature range is very large, from -143 oC at the poles in winter up to a balmy 35 oC on an equatorial summer day. Thus, the temperature does at times get high enough for liquid water to exist. Most of the time, however, the temperature stays well below freezing over most of the planet. Liquid water also requires a minimum atmospheric pressure of ~0.006 bar. Below this pressure, liquid water cannot exist. The surface atmospheric pressure on Mars averages 0.0063 bar (ranges from 0.004 to 0.0087 bar), which is very close to the triple point of water. Mars' atmospheric pressure is almost exactly at the water triple point.

Mars Missions

Mars is probably the most extensively studied planet besides the Earth. Over 40 spacecraft have been sent to explore Mars, both from orbit and from the surface. Unfortunately, roughly half of all these missions have ended in failure, leading some NASA scientists and engineers to jokingly suggest that there is a Martian curse. The Mars Polar Explorer crashed onto the Martian surface in 1999, probably because a software glitch caused the descent engines to cut off while the craft was still 40 meters above the surface. The Mars Climate Observer most likely burned up in the atmosphere due to a software glitch. One of the contractors that built the spacecraft, Lockeed-Martin, used Imperial units of measure (e.g., foot-pounds) instead of metric units (e.g., newtons) in the software code controlling the thrusters. The cause of many mission failures is simply unknown. However, these setbacks are part and parcel of space exploration, and speak to the tremendous technological challenges involved designing spacecraft that can survive the extremely harsh environment of space Despite the many setbacks, our knowledge of Mars has been revolutionized over the past decade by several spectacularly successful missions. The Mars rovers Spirit and Opportunity were designed to last 90 days on the Martian surface. After landing in 2004, these two lasted far longer than planned. The Mars Global Surveyor and Mars Reconnaissance Orbiter have provided detailed images and spectral analysis of the Martian surface

Mars Outflow Channels

Mars outflow channels can be >100 km wide, >1 km deep, and can extend for 1000s of km. Teardrop shaped erosional remnants (below) record the direction of flowing water. These channels are far larger than anything observed on Earth, and are thought to have formed from catastrophic floods caused by groundwater bursting onto the Martian surface. Outflow channels often originate on or near the flanks of volcanoes. The association of outflow channels with volcanic features suggests that geothermal heat may trigger catastrophic floods by melting water trapped as ice below the surface. Outflow channels record catastrophic flash floods. These channels are mostly younger than the dendritic channels found mostly in the highly cratered southern highlands.

Dendritic Networks

Mature rivers are usually fed by many smaller streams. The network of tributaries captures surface precipitation and runoff from a drainage basin. The network of river and stream channels is carved into the land over an extended period of time. The dendritic network of channels shown above is similar to channel networks on Earth, and suggests a network of streams fed by surface runoff. This, in turn, implies rainfall in the past on Mars. These types of drainage networks are only seen in the oldest Martian terrains.

Mercury

Mercury is the smallest planet, but it shares many similarities with the other terrestrial planets in terms of its composition. Mercury's highly cratered surface records information about the early days of the solar system shortly after the planets formed. The history of this period has been erased from the surfaces of Mercury's more geologically active cousins. Mercury is difficult to see from Earth, because it is always close to the sun and so usually obscured by the suns glare. It can sometimes be seen just before sunrise or after sunset. 2440 km radius ~6% the mass of Earth Average distance from sun: 57.9 million km(0.39 AU) Dense (5.43 g/cm3) Heavily cratered Dark (albido = 0.068, similar to the moon)

Mercury's Weird Terrain

Mercury's surface is pock-marked with craters and several large impact basins. The largest basin, the Caloris basin is 1550 km in diameter. "Weird" terrain antipodal to Caloris Basin. Giant impacts such as that responsible for the Caloris basin had global effects on the Mercury crust. The "weird terrain" found antipodal (opposite side of the planet) to the Caloris impact basin may have been formed as shock waves traveled through the planet, converging at the point opposite the point of impact.

Mercury's Cratered Surface

Most of the surface of Mercury is highly cratered, like that of the moon. This indicates that Mercury has been (mostly) tectonically and volcanically inactive for billions of years. Why? 1)Mercury's small size means heat is lost from it's interior quickly (high surface area/volume). 2)Mercury's large core means Mercury has relatively low abundance of radioactive elements like K, U, and Th, which are concentrated in the crust and mantle.

Runaway Greenhouse Effect on Venus

Motor oil is to your car's engine what water is to the heat engine of the Earth's interior. Both lubricate the engines moving parts, keeping the engine from freezing up. The subduction of water into the mantle softens Earth's interior, "lubricating" the motions of the plates. Without water, Earth's interior would be much more sluggish. On Venus, the thick CO2-rich atmosphere produces a runaway greenhouse effect, making the surface way too hot for liquid water to exist. No water on the surface means no way of returning water to the interior of the planet. As water was removed from the interior through volcanism, the mantle became stiffer, preventing plate tectonics. Without plate tectonics, heat builds up in the interior until a catastrophic overturn results in a big pulse of volcanism. So, runaway greenhouse effect = no liquid water at the surface = no plate tectonics because the mantle is too dry and dry = stiff... On Earth, the CO2 concentration in the atmosphere is regulated by the geologic carbon cycle and the biologic carbon cycle. In the geologic carbon cycle, CO2 combines with Ca in aqueous solutions (e.g., seawater) to form CaCO3 (calcite). Subduction of calcite into the mantle removes carbon from the surface. Volcanic eruptions, in turn, return CO2 to the atmosphere. As long as liquid water is present, formation of calcite keeps CO2 levels in check. On Venus, no water on the surface means no way of removing CO2 from the atmosphere by calcite production. No CO2 removal means runaway greenhouse. Runaway greenhouse means no water on the surface...

Mars

My favorite planet (not counting Earth) ~11% Earth's mass 1.5 AU (= ~50% as much solar energy) Extensive past volcanism Evidence for past water Could life have evolved early in it's history?

Tides

Ocean tides are the result of the gravitational attraction of the moon and sun on the ocean. The tides formed by the moon are the lunar tides, and those formed by the sun are the solar tides. Tidal friction is gradually slowing the Earth's rotation. The days are getting longer by a fraction of a second every year. Alignment of Sun and Moon every ~2 weeks= spring tides. Sun and Moon at 90 degrees (half moon) causes neap tides every ~2 weeks Bay of Fundy - 16 meters hi-low the land tilts slightly, each high tide due to the load. Like other waves, tides increase in height as they approach the shore.

Martian Dichotomy

The "Martian Dichotomy" between the rugged, heavily cratered southern highlands and smoother, less cratered northern lowlands is one of the most pronounced features on Mars. Some scientists believe that the northern basin formed from a giant impact. Other scientists believe that early tectonic processes linked to mantle convection could have generated this dichotomy. So, was there plate tectonics on Mars? The answer is a definite maybe.

Olympus Mons

Olympus Mons is far larger than any volcano (or other mountain) on Earth. This is in part due to the lack of plate tectonics on Mars. The crust is stationary, so the volcano can be built over a long period of time. On Earth, a mountain the size of Olympus Mons would collapse under it's own weight. The size of Olympus Mons also requires the Martian crust to be thicker and stiffer than on Earth. Is Mars still volcanically active? Probably. We actually have samples from Mars in the form of "Martian meteorites". Some of these meteorites are crystallized magmas, with "crystallization ages" of ~170-180 Ma, very young!

Sedimentary Rocks

On Earth, sediments transported by wind or water collect in topographic lows (e.g., basins), are compacted by their own weight, cemented together, and become sedimentary rocks. One key feature of sedimentary rocks is their layered structure. Younger deposits are at the top, older deposits at the bottom of this sequence. Most sedimentary rocks on Earth formed from sediments deposited by water, because water is more effective at transporting material than wind currents. On Mars, both satellite images and pictures taken by robotic rovers reveal a variety of sedimentary rocks, attesting to the importance of both fluvial and aeolian processes. Sedimentary rock layers in Holden Crater (above) and exposed in the crater wall of Erebus Crater (right). The Martian rover Opportunityexamined a variety of layered sedimentary rocks. Many of these sediments may initially have been deposited as wind-blown dust. These rocks record a record of Mars' past. Future geologists will be able to study these layers to discover when/how/why Mars' climate changed.

Mercury's Magnetic Field

One of the most surprising discoveries of the Mariner 10 mission was the fact that Mercury does have a magnetic field. 1)Field is a dipole, like Earth's; 2)Field is oriented with rotation axis (Earth's is tilted); 3)Field is stable (hasn't changed between 1974 and 2008)

What happened to Mars' atmosphere?

One possibility is that the atmosphere was eroded away by the solar wind after the geodynamo shut down, turning off the protective magnetic field. Another possibility is that a giant impact (possibly forming the Hellas Basin) "blew off" a large portion of the atmosphere, turning Mars from habitable to inhabitable overnight. Some scientists have suggested that the shockwaves produced by giant impacts can actually blow off portions of a planet's atmosphere. Giant impacts associated with the formation of large basins such as the Hellas Basin could have contributed to the thinning of Mars' atmosphere.

Rising Global Temperatures

Over the past 400,000 years CO2 levels and temperature strongly correlate with one another - atmospheric CO2 acts as Earth's thermostat Over the past 150 years (since the industrial revolution, both CO2 levels and temperature have been rising rapidly. Atmospheric CO2 concentrations are strongly linked to past global temperature variations. What effect will increasing atmospheric CO2 levels have on climate in the future? Climate models predict that the Earth will be vastly different in 2100 if CO2 emissions continue on their current trajectory, with average global temperatures ~4 0C warmer than today. That might not sound like much, but it would produce drastic changes in precipitation and weather patterns. Scientists are not exaggerating when they say this would be catastrophic.

Two equations that have changed the world:

Photosynthesis and Calcite Production

Does (or did) Mars have plate tectonics?

Portions of the Martian crust, like the ocean basins on Earth, show magnetic "stripes". On Earth, these stripes are the result of periodic reversals of Earth's magnetic field combined with the movement of the plates and creation of new crust at mid-ocean ridges. Did a similar process operate on Mars? The Valles Marineris may provide evidence for early-stage, rudimentary plate tectonics on Mars. Lateral offset of geologic features cut by Valle Marineris suggest transform (lateral) motion similar to what we see along Earth faults like the San Andreas. Mars, like Earth, displays a bimodal distribution in surface elevations. On Earth, this distribution is a result of plate tectonics, which results in two different crust types (continental crust and oceanic crust) On Mars, this distribution could be the result of: 1)Early plate tectonics; or 2)Giant impact in the northern hemisphere

Why does Earth have a magnetic field and Venus doesn't?

The slow rotation of Venus isn't the whole reason-Mercury rotates very slowly but it does have a magnetic field. The lack of plate tectonics slows the rate of heat loss from the interior. It is this heat loss that helps drive convection in the core. No water = no carbon cycle = runaway greenhouse = no plate tectonics = slow heat loss from interior = no thermal convection in core = no geodynamo = no magnetic field

Mercury still active

Rachmaninoff basin is 290 kilometers (180 miles) in diameter. Within the inner ring is are smooth plains that overlap the ring. Notice anything missing within these plains? Right, very few craters! These plains, probably formed by lava flows, are much younger than the surrounding terrain. The lava flows covering large impact basins like Rachmaninoff and Raditladi are sparsely cratered, suggesting that they are fairly young. Recent estimates suggest that volcanism on Mercury may have continued until at least 1 billion years ago. Large portions of Mercury are covered by volcanic plains similar to the lunar Mare. Apparently Mercury has had a more extensive period of volcanic activity than we initially thought Mercury volcanoes and evidence for pyroclastic eruptions (ash). Pyroclastic eruptions are volatile-driven (think coke and menthos).

Mercury's Orbit

Relative to the other planets, Mercury's orbit is very eccentric (eccentricity = 0.21). Mercury's perihelion advances by ~2o per century, due to the gravitational interactions between Mercury and the other planets. However, consideration of all the gravitational torques using Newton's laws of gravity and motion only account for about 80" of advance. For centuries, this discrepancy puzzled astronomers. Einstein's theory of relativity accurately predicts the advance of Mercury's perihelion. Mercury proves Einstein was right Planetary orbits are chaotic over long time periods. Mercury's orbit is much more eccentic than the other planets. This eccentricity varies over millions of years due to gravitational interactions with the other, larger planets. Orbital simulations suggest that it is possible (but highly unlikely) (<1%) that over the lifetime of the solar system, Mercury could collide with Venus, the Sun, or be ejected from the solar system. Mercury also spins very slowly on its axis-once every 58.7 Earth days. For a long time it was thought that Mercury was "tidally locked" so that it always showed the same face to the sun, just as the moon always shows the same face to the Earth. However, we now know that Mercury is locked in a 3:2 spin-orbital resonance. In other words, three Mercury days are exactly two Mercury years (one year = 88 Earth days)

Martian Magnetic Field

Remnant magnetism is seen in the oldest crust in the southern highlands, but not in the northern lowlands or in the Hellas Basin (~4 billion years old), suggesting that Mars' dynamo was only active for the first 500 Ma of the planet's history. Without a protective magnetic field, Mars' atmosphere faces the full brunt of the solar wind, and can be torn away in chunks, particularly during periods of high solar activity. Too small to maintain a geodynamo for long, and weak a gravitational field to hold an atmosphere without a magnetic field. This suggests that much of the atmosphere escaped to space over time. The lighter isotope escaped more readily than the heavier one. The Curiosity rover has made precise measurements of the ratios of different isotopes of Ar in Mars' atmosphere. The atmosphere is depleted in the lighter isotope 36Ar relative to 38Ar in comparison to Earth.

How do scientists determine the thickness or composition of planetary atmospheres?

Remote observations using spectroscopy can provide much information. This can be supplemented by direct measurement using gas chromatography or mass spectrometry. We know that light is a form of electromagnetic radiation, and that the color of light depends on its wavelength. Radiation with wavelengths shorter or longer than are visible to the human eye also exist (e.g., ultra-violet or infra-red radiation). Hot objects such as the sun emit light over a continuous range of wavelengths. We call such light sources black-body emitters. When light shines through a gas, electrons in the individual atoms may absorb light of specific wavelengths. The wavelengths correspond to photos with energy equivalent to the difference in energy of different electron quantum states. The absorbed energy boosts the electron to a higher energy state If a continuous light source is examined after it has passed through a gas, a number of dark lines, or absorption lines will be apparent, corresponding to the photon energies absorbed by electrons in the gas. If emission from the gas itself is observed, it will show bright emission lines corresponding to the same wavelengths as the absorption lines. Molecules show more complicated absorption spectra, due to the atoms in a molecule vibrating and rotating with respect to one another. This produces a more continuous band of absorption, rather than a discrete line. Mono-nuclear diatomic molecules such as N2 and O2 do not produce such absorption bands, and so are largely "invisible" unless the elemental absorption or emission lines are observed (usually requires high temperature).

Saltation

Saltationis an important process for wind-driven sand transport. Sand-sized particles are briefly lifted into the air by gusts of wind, but then fall back down to to surface, potentially dislodging other grains in the process. Individual grains thus move in the direction of the wind through a series of "hops". Saltation can reinforce itself: As wind-driven sand grains strike other grains, these are in turn lifted into the air. These then strike new grains, which are lifted... The positive feedback between wind and dust caused by saltation can lead to giant dust storms that can suspend enormous quantities of dust (and lesser amounts of sand) in the atmosphere.

Far side of the Moon

Several large impact basins, but oddly lacking the maria observed on the near side. The near side and far side of the moon are not the same. Maria, for example, make up a significant portion of the near side, but are only found in small patches on the far side. The lunar crust is also thicker on the far side than the near side. Both sides, however, display abundant craters, including many large impact basins.

What have we learned about Mars

So what have we learned? Mars is much more complex than suggested by the early Mariner 4 images. It has volcanoes and tectonically-generated canyons that dwarf those on Earth. It has evidence for past liquid water at the surface. It once had a magnetic field. Mars was once much more "Earth-like" than it is today, both in terms of its climate and its internal dynamics

Explosive Volcanic Eruptions

Some volcanic eruptions produce columns of ash that extend tens of kilometers high, and can carry the ash hundreds or even thousands of kilometers. How is volcanic material carried to such great heights? Ex: Mount St. Helens May 18th, 1980 Expanding gas fragments the rising magma and propels it upwards through the volcano's vent. Initially, upward ascent is due to the momentum of the erupted material. However, left alone, the material would quickly fall back to Earth due to gravity. Entrainment and heating of surrounding air reduces the density of the volcanic cloud. If enough air is entrained and heated, the cloud becomes buoyant and rises like a hot-air balloon. Larger fragments of volcanic material such as pumice along with finer volcanic ash are carried upward by the rising air. As the rising column slows down, the larger fragments rain out of the cloud. Finer particles are carried further down-wind. Column height is controlled by many variables, including eruption rate, temperature difference between magma and atmosphere, and atmospheric density. Welded tuffs (right) are composed of thick accumulations of volcanic ash and pumice fragments (below), and are the result of explosive volcanic eruptions. Pumice is a highly vesicular volcanic rock, indicating that the magma from which it derived contained lots of dissolved gases. No welded tuffs or other evidence for volcanic ash on planets that do not have an atmosphere such as Mercury and the Moon

Sulfur

Sulfur is antifreeze for the core High sulfur content in Mercury's core would lower the temperature at which it turns solid, allowing the core to stay liquid longer.

Apollo Missions

The Cold War between the United States and the Soviet Union, not the quest for cheese, spurred the race to the moon in the 1960s. The safe landing of humans on the moon in 1969 represents one of the great technological triumphs of the 20th century. Neil Armstrong and Buzz Aldrin, not Wallace and Gromit, were the first to have a "Grande Day Out" on the surface of the moon on July 20, 1969. The moon is the only solar system object besides the Earth where humans have set foot. Between 1969 and 1972 six Apollo missions successfully landed on the lunar surface. These missions were a tremendous technological triumph, and returned 382 kg of lunar material for scientific study. This material is sill being studies today. Between 1969 and 1972, the Apollo program landed humans on the moon 6 times. Apollo astronauts collected lunar samples, conducted a wide range of scientific experiments, and installed instruments (seismometers, mirrors for ranging lasers) that greatly improved our understanding of the moon and it's origins. The Apollo program had two major setbacks. On January 27, 1967, a ire on the launch pad killed the crew of Apollo 1, Gus Grissom, Ed White, and Roger Chaffee. On April 11, 1970, an oxygen fuel cell aboard Apollo 13 exploded, crippling the spacecraft. The crew had to use the landing module as a "life rat". Remarkably, despite a damaged heat shield, the crew was able to return safely to Earth on April 17, and the mission was termed a "successful failure" Ater the last Apollo landing in 1972, we did not send another probe to the moon until the 1994 Clementine mission, though both the former USSR and Japan did send probes.

Habitable Zone

The Habitable Zone defines the region around a star where liquid water can exist on the surface of a planet with a suitable atmosphere. Our nearest neighbors, Mars and Venus, exist just outside and inside this zone. Note that the thickness of this zone is subject to debate. For example, Mars could be habitable if it had a thicker atmosphere.

Magellan Spacecraft

The Magellan spacecraft was launched on May 4, 1989. The spacecraft spent four and a half years mapping the surface of Venus with radar. The mission also mapped the gravity field of Venus. Much of what we know about the surface structure of Venus (e.g., volcanoes, paucity of craters) is the result of this mission. The surface of Venus is young (few craters) and covered with wrinkle ridges, volcanoes, and other evidence for recent geologic activity. But...There is no evidence for plate-like features on Venus. Why does Earth have plate tectonics when Venus (similar size, composition) doesn't? Radar mapping of Venus by Magellan reveals numerous volcanic features. The high concentration of sulfur dioxide in Venus' atmosphere may reflect recent volcanic activity. Although both Earth and Venus are volcanically active, Venus differs from the Earth in that it shows no evidence for plate tectonics, which is the major cause of most volcanism on Earth.

Martian Surface

The Martian crust can be divided into the very old, heavily cratered southern highlands and the younger northern lowlands. Many very large volcanic features, including Olympus Mons, can be found near the Tharsis Bulge. Although Mars appears to be (mostly) inactive today, it clearly has had a more protracted history of volcanism and other tectonic processes than, for example, Mercury. Topographic map of Mars showing heavily cratered southern highlands and less cratered northern lowlands. Mars has far fewer volcanoes than Venus or Earth, and most are concentrated in either the Tharsis or Elysium regions. These regions may overly mantle plumes. On Mars, the thick, rigid lithosphere does not move. It is thought that the very large volcanoes of the Tharsis Region, which far exceed the size of volcanoes on Earth, are the result of a long-lived mantle plume beneath the region. Because the volcanoes stayed over the plume, volcanism could build the volcanoes larger and larger over a long period of time

Messenger Mission

The Messenger space probe was launched in August 2004. Messenger made three fly-bys of Mercury in 2008 and 2009, mapping an additional 30% of the planet. On March 18, 2011, Messenger entered orbit around Mercury and began mapping the planet. Messenger provided much higher resolution images of the planet's surface, and helped constrain the internal structure of the planet as well as the composition of the surface rocks. Messenger ended it's mission on April 30, 2015. Because Mercury is deep in the sun's "gravity well", getting into orbit around Mercury is very difficult. To do this without using literally tons of fuel (weight = money), we use "gravity assists" from Earth and Venus.

Valles Marineris

The Valles Marineris is a giant valley system that extend to the east of the Tharsis Bulge, where Olympus Mons and other giant volcanoes formed early in Martian history. The Valles Marineris is thought to be a giant crack in the Martian crust, formed as the crust beneath the bulge was uplifted. The valley is 4000 km long, 200 km wide, and 7 km deep. The Grand Canyon pales in comparison.

If hydrogen and helium were the main components of the solar nebula, why don't all planets have atmospheres made of these gases?

The answer has to do with the speed with which individual atoms or molecules in a gas move about. These molecules are constantly moving about, and the speed with which they do so is a function of temperature. The distribution of kinetic energy in a gas is well understood and is described as a Maxwell distribution. Now imagine what would happen to an atom at the top of the atmosphere moving particularly fast away from the planet. What would happen? The average speed of gas molecules depends on both temperature and molecular mass. At the same temperature, hydrogen molecules (H2) will move 4x as fast as oxygen molecules (O2). If the average velocity of a gas species is greater than ~1/6 the escape velocity, then that species will be lost from the atmosphere over the age of the solar system. Big planets and cold planets can more easily retain gases than small, hot ones.

Jupiter Atmosphere

The bands of Jupiter's atmosphere are a result of the planet's very rapid rotation (a Jupiter "day" is less than 10 hours), which causes convection cells in Jupiter's atmosphere to break apart into bands much narrower than the Hadley and Ferrel cells on Earth.

Destruction of Ozone Layer

The concentration of ozone in the stratosphere has declined steeply in recent decades. During the southern hemisphere winter, chloro-fluorocarbons (CFCs) in the stratosphere break down ozone molecules, producing the ozone "hole". Because of this, most CFCs have been banned in an effort to prevent further destruction of the ozone layer. UV radiation breaks apart the CFC and free Chlorine molecule breaks apart the ozone into a O2 molecule and Chlorine monoxide molecule

Lunar Mare

The dark regions we see when we look at the moon are the lunar maria (Latin: seas). The maria were so named because early astronomers believed that they were large bodies of water. In fact, the maria are basaltic plains, primarily filling large impact basins. The basalts are significantly younger than the basins themselves. Samples of Mare basalt returned to Earth by the Apollo astronauts have been radiometrically dated. These dates have allowed calibration of relative ages determined by crater counting. Based on these methods, it appears that most maria basalts erupted between 3 and 3.5 billion years ago (Ga), although some Maria are as old as 4.2 Ga, and some may be as young as 1.2 Ga. The Maria are much younger than the anorthositic highlands.

Cyrovolcanism

The eruption of liquid or vapor phases of water or other volatiles (with or without entrained solids) that would be frozen solid at the normal temperature of the icy satellite's surface In 2005, the Cassini spacecraft detected plumes of icy particles emanating from grooved regions on the moon's surface. These are thought to derive from "cold geysers" powered by tidal heating. Internal heat melts pockets of ice, and water pushes up towards the surface. In the vacuum of space, the water boils or freezes-liquid water cannot exist in a vacuum. Plumes of ice crystals are carried upward by the expanding vapor. If "normal" volcanism is like a lava lamp, then cryovolcanism may be more like a juice box. Whereas silicate magma is less dense than solid rock, liquid water is denser than water ice, so why would water push up to the surface? One possibility is that pockets of water are "squeezed" through cracks up towards the surface. Alternatively, other volatiles like ammonia, methane, or CO2 may form bubbles, decreasing the density of the mixture.

Where did all the water go?

The evidence points to abundant liquid water on the surface of Mars for the first ~1 Ga of its history. This requires that the Martian atmosphere was originally much thicker than today, providing more of a greenhouse effect and allowing liquid water to exist at the surface. As the atmosphere thinned, the planet grew colder. Water was trapped in the polar ice caps or under the surface as permafrost. Some water may also have escaped into space after the geodynamo shut down.

Lunar Highlands

The highlands are thought to have formed as the initial magma ocean cooled and solidified. Dense phases like olivine and pyroxene sank to the bottom of the magma ocean. Plagioclase crystals that grew from the magma ocean were less dense than the remaining magma and floated to the surface, forming a crust composed of anorthosite (a rock type mostly composed of plagioclase). As the lunar magma ocean cooled and crystallized, mafic phases like olivine and pyroxene crystallized first. These phases sank and today form much of the lunar mantle. Eventually, feldspar began to crystallize, and floated to the top of the magma ocean, forming the original anorthosite lunar crust.

Moon Formation/Origin

The leading theory for the formation of the moon posits that the proto-Earth was struck by a Mars-sized object about 30 million years after the beginning of solar system formation. Material from both the Earth and the impactor were ejected into orbit around the Earth. This ejected material quickly formed a ring of very hot material around the Earth, which then collected together to form the Moon. Initially, both the moon and Earth may have been largely molten.

Lunar Surface

The lunar crust can be divided into the dark-colored lowlands (Maria) and light-colored, heavily cratered highlands (Terrae). Curiously, the Maria are primarily found only on the near side of the moon. The sample to the left is originally from the lunar highlands. It was collected by Apollo astronauts, and was probably transported to the lowlands as impact ejecta.This rock is an anorthosite. It is composed primarily of anorthite, a calcium rich type of feldspar. The sample to the right is a basalt from the lunar lowlands. Note the very different appearance. Now we can see why the highlands appear lighter in color than the lowlands-it is because of the rocks that make up the different portions of the lunar crust. The highlands and maria have very different compositions. The maria are richer in iron and the highlands are richer in aluminum. These differences reflect the different mechanisms by which these crustal rocks formed. Naturally occurring radioactive elements in volcanic rocks can be used to date the time at which these rocks solidified. The radioactive "parent" isotopes decay over time to their "daughter" isotopes. By measuring the ratio of parent and daughter isotopes, we can calculate the age of minerals in a rock. Samples from the original lunar crust from the lunar highlands are extremely old, nearly 4.5 Ga. This crust must have formed very soon after the original formation of the moon.

Meanders

The maximum flow velocity (maximum erosion) occurs at the outside of each meander The flow at the inside edge is slower, and deposition of a point bar results As erosion and point bar deposition continue, the meander migrates, particularly on a floodplain. Meandering stream channels are common in mature, long-lived streams with relatively slow water velocity and low sediment load. Flash floods are likely to produce a very different type of channel system, such as deeply eroded gullies or braided stream valleys. Meandering stream channels on Earth (left) and similar features on Mars. On Earth, such channels are formed by slowly-flowing water over an extended period of time.

Planetary Atmospheres

The moon has essentially no atmosphere (surface pressure is ~10-14 bar), while Earth's thick atmosphere makes liquid water and life possible. Earth, Venus, and Mars all show absorption features from CO2. However, Earth's atmosphere also contains some other odd molecules: O3 and H2O. Note that diatomic molecules like O2 and N2do not have broad absorption bands, and so are mostly "invisible". So what are the atmospheres of the terrestrial planets made of? Mars and Venus have broadly similar atmospheres composed primarily of CO2 and N2. In contrast, Earth has an atmosphere composed mostly of N2 and O2, with only trace amounts of CO2. In all cases, the atmospheres are very different from the giant planets, which have atmospheres composed of H2, He, and hydrogen compounds (e.g., CH4). Different possible origins for planetary atmospheres include: Primary accretion of solar nebula gas; volcanic outgassing; and late accretion from comets and carbonaceous chondrites.

Moon's Orbit

The moon's orbit lies close to the plane of the ecliptic, but is far removed from Earth's equatorial plane. The moon's orbit is also somewhat elliptical. Although the moon orbits at an average distance of 384,405 km, this distance varies from 363,100 to 405,700 km. Because the moon has an elliptical orbit, its speed varies. The moon travels fastest at perigee and slowest at apogee. Because the moon's rotation, however, stays constant, the moon undergoes libration.

Messenger Results

The surface of Mercury is much richer in the elements sodium, potassium, and sulfur than expected. These three elements are volatile at temperatures >1000K, so if the surface of Mercury was extremely hot early in its history, it should be depleted in these elements (these elements would have been gases and would have escaped into space due to Mercury's weak gravitational field. Sulfur is also important because it dissolves easily in molten iron, and acts like an "antifreeze", keeping the iron molten at lower temperatures than pure iron. If the surface of Mercury is rich in sulfur, the core probably is as well and this may explain why it is still molten We can measure the amount of potassium and thorium in the Mercury crust by measuring the gamma rays that are produced by the radioactive decay of 40K and 232Th. Mercury has a lot more potassium than we expected. Because potassium was a volatile element during solar system formation (it vaporizes at temperatures > 1000 K), Mercury should be depleted in potassium (and other volatile elements like sulfur) if it formed hot.

Greenhouse Effect

The surface temperature of any planet (Earth included) is controlled by the balance between energy received from the sun, heat from the interior (usually a small effect), and the energy radiated back into space. Although most of the energy from the sun arrives as visible light, the Earth, which as approximately a black body, emits in the infrared portion of the spectrum. Why does this matter? Because although the Earth's atmosphere is transparent at visible wavelengths, it is not transparent to infrared light. The glass of a greenhouse, lets visible light from the sun in, but traps infrared light from escaping. Certain gases in the Earth's atmosphere, in particular H2O and CO2, have the same effect as the glass of a greenhouse. They "trap" heat in the atmosphere and keep it from radiating into space. Without this effect, Earth's surface would be ~33 oC colder than it is. Earth's atmosphere is largely transparent to visible wavelengths of light. However, species such as CO2, O3, H2O, and CH4 absorb various wavelengths of infrared light. A portion of this light is radiated back to the surface, and a portion warms the upper atmosphere. In both cases, these "greenhouse gases" act like a blanket, keeping the planet's surface warm.

Earth's Magnetic Field

The thin, uppermost layer of planetary atmospheres becomes partially ionized from absorption of high-energy photons from the sun. This ionized layer interacts with charged particles from the sun (the solar wind) and deflects them, creating a bow shock. On Earth, this interaction between the ionosphere and solar wind is more complicated because the wind also interacts with Earth's magnetic field. On Earth, charged particles captured by Earth's magnetic field are channeled towards the poles, where they bombard with the upper atmosphere to produce glowing aurora. no magnetic field on Venus/Mars so no aurora

Lunar Geological features

The vessicular basalt above is a lunar sample collected by the Apollo astronauts. The dark-haloed craters to the right may be volcanic vents similar to terrestrial cinder cones. Here you can see how basaltic lava flows associated with formation of the mare have partially filled in several large craters. You can also see a number of sinuous rilles. Sinuous rilles can be caused by hot lavas melting and eroding the underlying crust. In some cases, the rilles are likely collapsed lava tubes. Just as we saw on Mercury, wrinkle ridges are common on the moon. Two origins of wrinkle ridges are considered likely. They may be caused by compression of the crust of cooling lavas in the maria (as the lava cools, it contracts) or they may be extrusions of still mobile lava from below the surface crust that has developed elongate cracks through which the materials upwell.

Pancake Domes on Venus

These "pancake domes" on Venus may be similar to volcanic domes on Earth, although they are much larger. On Earth, such domes are formed from highly viscous, high-SiO2 lavas On Earth, high-silica lavas are often formed near subduction zones, but on Venus, direct melting of the Venutian crust (anatexis) may be responsible for these silicic lavas.

Why does a magnetic field matter?

Ultraviolet light causes photo-dissociation of water: H2O => H + OH In the absence of a protective magnetic field, the solar wind will blow the hydrogen into space, slowly drying out a planet's atmosphere. It is interesting to consider that Venus might have started out much more Earth-like than it is today. The sun was initially much cooler than it is today, and Venus likely had oceans just as Earth. Over a period of ~600 million years Venus lost its water, and once the water was gone there was no way to keep greenhouse gases in check. If you want to know what Earth will look like in ~1-2 Ga, look to Venus.

Difference between Earth and Venus Atmospheres

Venus = runaway greenhouse (dense CO2 atmosphere). Earth = habitable CO2 levels kept in check by biological precipitation of calcite.

Venus volcanism and plate tectonics

Venus has LOTS of volcanoes. Over 1600 volcanic features have been identified on Venus. Many of these appear to be very young Venus' surface has very few craters, so it's surface is much younger than that of Mercury or the moon. The craters are evenly distributed over the planet's surface, so the entire crust appears to be roughly the same age, ~500 Ma. This has led to the suggestion that Venus' surface experiences periodic "catastrophic overturns" in between periods of quiescence. On Earth, plate tectonics results in a bi-modal distribution in topography. Ocean crust is much lower in elevation than continental crust. This bi-modal distribution is a signature of plate tectonics, and reflects the different composition of continental and oceanic crust. On Venus, topographic variations show a bell-shaped distribution, suggesting no plate tectonics.

Why is Venus so different from Earth?

Venus is similar to Earth in size (0.815 Earth mass), composition, and distance to the sun (0.72 AU). Yet, Venus' surface and internal dynamics are very different from Earth's. The surface temperature (460 oC) is hot enough to melt lead. The surface is obscured by clouds made of sulfuric acid. 1)Why is the surface of Venus so hot? Runaway greenhouse effect 2)Why does Venus lack plate tectonics? Runaway greenhouse effect? 3)Why does Venus lack a magnetic field? Runaway greenhouse effect?

Lunar Mare Formation

Where did the basaltic lava that lowed over the lunar surface to form the Maria come from?On Earth, meling in the interior occurs when: 1) Temperature increases 2) Convecion results in decrease in pressure; or 3) Water is added to the mantle. The moon is too dry for #3 to be possible, so either convection of the lunar mantle or heating (due to decay of radioactive elements is the likely cause.) Heat-producing elements (K, U, Th) don't partition into either olivine and pyroxene (which sank to the bottom if the magma ocean) or feldspar (which floated to the top). Therefore, these elements would have concentrated in a layer within the mid-mantle that formed from the last liquid to solidify from the magma ocean. Radioactive heating may have caused this layer to remelt a few 100 million years after its initial solidification.

How far away is the moon?

~384,399 km, give or take an inch or two. One of the things the Apollo astronauts left on the lunar surface was a mirror. By shining a laser beam at the mirror and timing how long it takes for the light to return to Earth, scientists can measure the distance to the moon to a precision of a few centimeters. Lunar ranging experiments reveal that the moon is getting farther away from the Earth by about 38 mm per year. At the same time, Earth's rotation is slowing and the days are getting longer by ~15 microseconds (10-6 s) per year. Both effects are the result of tidal friction and the transfer of angular momentum from the Earth to the moon.