Geology Test #1

Shocked Quartz

Shocked quartz looks very different from normal quartz. The planar deformation features form when quartz grains are suddenly subjected to pressures of 5-8 GPa (~50,000-80,000 atmospheres)

Simple Crater

Simple craters are, well, simple They are bowl shaped, with clean rims Small craters (on Earth, <2-4 km) are bowl shaped

Complex Craters

Complex craters often have terraced rims caused by slumping and gravitational collapse of the fractured rim. The floor is often flat and may be filled with impact melt. A central peak or central ring is caused by rebound of the excavated floor following compression Larger impacts form complex craters which have slumped or terraced rims and often have central peaks

Cratering Process

1) Compact and Compression 2) Shock Wave, Rarefaction Wave, and Ejecta Curtain (Excavation) 3) Modification

Types of Boundaries

1) Divergent Boundary 2) Transform Boundary 3) Convergent Boundary

Types of Volcanism

1) Mafic Volcanism 2) Felsic Volcanism

Types of Meteorites

1) Stony Meteorites (chondrites/achondrites) 2) Iron Meteorites 3) Stoney Irons

Models of Formation

1)Core accretion 2)Gravitational instability

How do we know the Earth has an iron core?

1)Earth is too dense to be made of just silicate rocks. 2) Earth's moment of inertia suggests much of it's mass is near the center-NOT a uniform sphere. 3) Primitive meteorites (building blocks for the planets) are much richer in iron and nickel than crustal or mantle rocks-where's the missing iron?

Causes of Volcanism

1. Magmas form in the mantle due to decrease in pressure or addition of volatiles (e.g., water). 2. Magmas ascend towards surface because they are less dense than the surrounding rock. 3. As pressure decreases, volatiles in magma expand to produce bubbles. This lowers the density of the magma, causing it to ascend faster. 4. Magmas erupt onto the surface of the volcano or are carried into the atmosphere by hot gases The type of eruption is determined by the viscosity of the magma and its volatile content

Plate Tectonic Theory

12 rigid outer plates moving over a softer (but still solid) upper mantle; Continents carried along for the ride Mantle convection drives this process, hot buoyant mantle rises and cold dense mantel sinks and is heated, process repeats

Earth Internal Structure

A relatively thin crust is overlain by the rocky upper and lower mantle, which overly the liquid iron outer core and solid iron inner core "Xenoliths" contained in some volcanic rocks provide us with direct samples of the mantle. However, these xenoliths only sample the upper few hundred kilometers of the mantle Mantle melt has a different composition than the actual mantle composition (peridite)

Vesicle

A small cavity in a glassy igneous rock, formed by the expansion of a bubble of gas or steam during solidification

Transform Boundary

At transform boundaries, one plate moves horizontally past another. The friction of the two plates sliding past each other can produce large earthquakes The San Andreas fault in California is an example of a transform plate boundary

Basaltic Lavas

Basaltic lavas are usually very hot (~1000-1250 oC) and so are very fluid. They usually have fairly low contents of dissolved gases like water vapor or CO2 Very fluid - erupts at 1000 to 1200°C Low silica and high temperature means low viscosity Flows very quickly and covers large areas Low SiO2 composition

P and S Waves

Because S-waves cannot travel through liquids, when these waves intersect the liquid outer core they are blocked. This creates a "shadow zone" where no S-waves from distant earthquakes are observed. The shadow zone reveals that the outer core is liquid, and constrains the size of the core Velocity of P- and S-waves increases with increasing depth, because the rocks become denser as they are squeezed by the weight above.At the core/mantle boundary, shear wave velocity goes to zero, because shear waves cannot travel through liquids. This is how we know the outer core is liquid

Galilean Satellites

Calisto, Ganymede, Europa, Io Orbit Jupiter

Stony Meteorites

Chondrites are mixtures of silicate and metal phases, and most contain chondrules. These represent different objects formed directly from the solar nebula with little subsequent processing. Achondrites have experienced melting and differentiation processes occurring on or within a planetesimal

Cinder Cones

Cinder or scoria cones are basically piles of volcanic rubble. Volcanic ash and bombs (large blocks) are thrown into the air by expanding gases, and fall to Earth along ballistic paths, forming a pile of material around the volcanic vent The vessicular basalt above is a lunar sample collected by the Apolo astronauts. The dark-haloed craters to the right may be volcanic vents similar to terrestrial cinder cones The lunar "soil" collected by the Apollo astronauts contains small, round "droplets" of orange volcanic glass. These droplets may have formed from the lunar equivalent of fire fountains. However, the lunar interior is very dry, so whereas terrestrial fire fountains are driven by release of H2O vapor, on the moon such fountains may have been driven by release of CO2 or SO2 gas The Mars Pathfinder missions revealed a landscape littered with volcanic boulders, many of which appear to contain vesicles very similar to those found in many terrestrial lavas. Chemical analysis reveals Martian lavas are primarily basaltic in composition

Absolute Ages Caution

Crater densities can provide reliable absolute ages for planet surface that can be independently calibrated (the moon), and good relative ages for different surfaces from the same planet. Comparing different planets or moons, however, is very difficult. Every object is unique in terms of population of impactors (e.g., comet/asteroid ratio), velocity of impacts, crustal strength, etc. We have models to do this, but we don't really know how accurate our models are

Divergent Boundary

Divergent boundaries occur at zones of upwelling mantle Produces new oceanic crust Volcanism and shallow earthquakes Ex. Mid-Atlantic ridge, Iceland Ocean crust becomes older the further away it is from the ocean ridges. This is one of the most important observations that lead to plate tectonic theory

Divergent Margin Volcanism

Divergent margins and areas of mantle upwelling (mantle plumes like Hawaii) As hot mantle rises in response to spreading plates or in mantle plumes, the decrease in pressure causes the mantle to melt, forming basaltic magmas Decompression Melting

Kinetic Energy

E = (MV^2)/2

How did Earth develop the layered structure it has today?

Earth's evolution from an aggregate of rocks dust to the layered system today started with extensive melting As the Earth melted, iron and nickel "sank" to form the core. The lighter elements (Si, Al, O) "floated" to the top, cooled and formed the outer crust Between the core and crust is the mantle, >90% Fe, Mg, Si, and O

Felsic Magma

Felsic magmas typically have higher concentrations of dissolved gases, which have difficulty escaping due to the high viscosity of the magma

Felsic Volcanism

Felsic volcanism produces material that has much higher silica content (up to ~70% SiO2) but lower magnesium. Silicic magmas are colder(~700 oC) and have a much higher viscosity Felsic magma Explosive

Decompression Melting

HOT ROCK MOVING UPWARD MELTS As depth decreases(moves upward), the hot rock changes from solid to molten rock

Plate tectonics on other planets?

Hallmarks of plate tectonics include long, linear chains of mountains, deformation, volcanism On Earth, two types of crust, thick, old continental crust and thin, younger oceanic crust

Mafic Volcanism

Mafic volcanism occurs where mafic (or basaltic) lavas are produced. These lavas have modest silica contents (~45-50% SiO2) and relatively high magnesium content (~10% MgO) Basaltic Lavas Effusive

Lava Flows

Many flow fields, like the one shown to the left, can extend for 10s of kilometers from their vent. In some instances, however, they can travel much farther. Once lava erupts onto the surface, it begins to cool and solidify The large lunar lava flow shown to the left extends for over 600 kilometers. Many Martian lava flows (below) also extend over 100 kilometers When lavas erupt, the surface of the lava flow quickly cools and solidifies to form a solid crust. This crust can help insulate the still-molten lava below the crust (lava insulated underneath the crust) Lava protected by a crust can remain hot and molten for a long time. Periodically, the lava may break through the crust and flow over the surface, extending the flow field. One such breakout is shown above. This process may repeat over and over, slowly extending the lava flow over great distances

Subduction Zone Volcanism

Melting at subduction zones occurs when wet sediments and altered crust release water into the mantle, lowering the melting point Lavas from subduction zones are richer in water and silica than lavas from mid-ocean ridges or mantle plumes One plate subducts because it is denser and colder than the other plate Flux melting occurs

Mars v. Mercury

Mercury is about ½ the mass of Mars, but is that the only reason for the difference in geologic activity recorded on these two worlds? Compared to Mars, Mercury's core is much larger, and the mantle much thinner Because heat-producing elements like U, Th, and K are concentrated in the rocky portion of planets, a thinner mantle means less heat production and potentially less mantle convection. And, as we'll learn next, convection is what drives volcanism

Planetary Magnetic Fields

Mercury: Weak field, about ~1% the strength of Earth's field. Venus: No magnetic field. It is thought that Earth's rotation helps "stir" our liquid outer core, so maybe Venus' very slow rotation doesn't allow this stirring. Earth: Our magnetic field varies in strength and direction, periodically flipping its polarity. Mars: Mars doesn't have a magnetic field today, but it did in the past. As Mars' interior cooled down, convection in the core stopped, shutting down the dynamo

Flux Melting

Occurs at subduction zones water released which reduces the melting point and lowers the temperature, changing the rock from solid to molten release of water and volatiles which makes an explosive volcanism higher SiO2 composition

Lava Tubes

Often, lava flows in lava tubes. When the eruption wanes, the lava may drain from the tube, leaving long, narrow caves like the one shown below Lava tubes also insulate the flowing lava, allowing it to travel great distances Collapsed lava tubes, where the roof has caved in to expose the tube, are visible on many terrestrial bodies, including the moon (left) and Mars (below)

Magnetic Field

On Earth, oceanic crust records the Earth's magnet field as it forms at mid-ocean ridges. Variations in the field over time result in oceanic crust having a "striped" pattern. Mars doesn't have a magnetic field today, but it did early in its history. Are the magnetic "stripes" preserved in the martian crust a result of early plate tectonics on Mars? The Earth's magnetic field protects us from harmful ionized particles from space Earth's magnetic field resembles a bar magnet The outer core is rapidly convecting due to heat escaping from the core to the mantle. A "stray" magnetic field (probably from the Sun) interacts with the moving iron in the core to produce an electric current that is moving about the Earth's spin axis, yielding a magnetic field—a self-exciting dynamo Convection in the outer core generates the Earth's magnetic field

How do scientists peer inside a planet?

On Earth, seismic waves generated by earthquakes can help us "image" the interior much like X-rays allow to peer inside the human body

Plate Movement

On Earth, the Hawaiian Islands are the result of a deep mantle plume. As the Pacific plate moves over the plume, volcanoes are carried away from the plume and go extinct, while a new volcano forms. This results in an age progression-Hawaii is the youngest island, and the islands get progressively older to the northwest 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 volcanos 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

Pangea

Over 200 million years ago, all the continents were assembled into one "supercontinent"-Pangea ("All land") By ~150 Ma, the continents had started to break apart. The Atlantic ocean was born where rifting occurred The continents are still moving today, carried by currents within the mantle

P-Waves

P waves will pass through both a solid and a liquid. Velocity increases as material becomes more dense, atoms more tightly packed Travel in dialations and go side to side Velocity increases as material becomes more dense, atoms more tightly packed Travel faster

Seismographs

P-Wave arrives first S-Wave arrives second Difference between the waves is the S/P Interval Greater Interval means that the Earthquake is further away Find the Earthquake by triangulation Variations in travel time with distance constrain how seismic velocity changes with depth

Conduction

Planet interiors cool as heat is conducted from the interior to the surface, and then radiated into space. Conduction is controlled by surface area, which for a sphere is given by:4piR^2 In contrast, the amount of heat stored or produced in a planet's interior is a function of composition and volume, which is given by:4piR^3/3 The surface area-to-volume ratio therefore decreases as radius increases. Smaller planets have a higher (surface area)/volume ratio and so their interiors cool faster than larger planets

Planetary Surfaces

Planetary surfaces are shaped by both external processes (e.g., asteroid and comet impacts) and internal processes, such as melt generation and mantle and core convection. These internal processes are driven by heat, both primordial and radiogenic. The structure and dynamics of a planet's interior plays a critical role in the evolution of the surface

Rampart Craters

Rampart craters like those shown here are common on Mars, but are absent on the Moon and Mercury. The ejecta blankets look suspiciously like mud splatter The prevailing theory is that rampart craters form when an impactor strikes a target with frozen volatiles, such as water ice, beneath the surface. The force of impact melts the ice, turning the rock briefly to mud

S-Waves

S (shear) waves only travel through solids. Liquids have zero shear strength and so shear waves cannot pass through liquids Travel up and down like a wave Travel slower

Aitken Basin

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.

Moment of Inertia

The Earth's rotation axis changes like a spinning top. This precession takes ~26,000 years, and is caused by gravitational torques from other planets and the sun. From the rate of precession, we can calculate Earth's moment of inertia. A planet's moment of inertia is controlled not only by a planet's mass, but also by how the mass is distributed within a planet The moment of inertia of different planets combined with their mass and density provides insights into their internal structure For example, we know that Mercury has a very large dense (metal) core, much larger in proportion to the size of Mercury than the core of Earth or Venus. In contrast, the moon has a very small core comprising less than 2% of the moons' volume

Mars Plate Tectonics

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.

Sudbury Basin

The Sudbury basin in Canada is the remains of a nearly 2 billion year old impact crater Today, it is the site of one of the world's largest platinum mines Shatter cones (right) are a give-away that Sudbury formed by a large impact event

Convergent Boundary

The heavier (usually older) plate sinks beneath the lighter plate = SUBDUCTION Subduction is associated with volcanic islands, deep ocean trenches, coastal mountain ranges and earthquakes. Much more dangerous than divergent margins Much more dangerous than transform boundaries Andes Mountains, Japan Volcanic Arc

Why have some planetary bodies remained geologically active longer than others?

The main controls are size and the presence or absence of internal sources of heat. Internal heat comes primarily from stored heat (from the time of planetary accretion), and heat produced by radioactive decay. The abundance of radioactive element like uranium is therefore important

Mars, Mercury, Earth, Moon

The surface of Mercury is uniformly heavily cratered, suggesting a very old age. In contrast, large variations in crater density for different regions on Mars suggest a more varied and extended history of geologic activity, including large volume, long-lived volcanism Earth is an active planet, with ongoing mountain building, earthquakes, and volcanism The moon and Mercury appear geologically "dead". Earth is still very much alive

Venus Plate Tectonics

The surface of Venus is also divided into highlands (thick crust) and lowlands (thinner crust). But, the entire surface appears to be roughly the same age-about 200 million years Although Venus has plenty of volcanic features, we do not see the linear patterns of volcanism or deformation characteristic of plate tectonics. The distribution of elevations can provide clues to the presence or absence of plate tectonics. On Earth, we have two different types of crust (continental and oceanic) with very different elevations. On Venus, there only appear to be one type of crust One possibility is that Venus experiences periodic catastrophic overturns. During periods of inactivity, heat builds up in the interior. Once the heat build-up is too large, mantle upwelling produces a giant pulse of volcanism that resurfaces the entire planet. Venus is active, but it doesn't have plate tectonics

Viscosity

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion ex: syrup has a high viscosity Viscosity is controlled by the composition and temperature of a melt

Eruptive Style is controlled by

Viscosity, or how fluid a melt is Volatile content or how much gas (water vapor and CO2 a melt initially contains) Viscosity is controlled by the composition and temperature of a melt

Volcanism

Volcanism is one of the most important processes that shape the surfaces of rocky or icy worlds Volcanism on all of the terrestrial worlds is similar in many respects (e.g., composition of most lavas), but important differences also exist that are influenced by planet size, atmospheric thickness and other variables

Shield Volcanism

When erupted lava is very fluid (low viscosity) it can spread out and flow great distances, creating a broad volcanic edifice with gently slopes. This type of volcano is called a shield volcano and is very common on Earth (Mauna Loa and Mauna Kea volcanoes, Hawaii) and Mars (Olympus Mons)

Gene Shoemaker

carefully mapped Meteor Crater, Arizona (before the name gave the answer away) Evidence he documented included: -Iron meteorite fragments -Doubled-over layers of rock (ejecta blanket) -Highly fractured, or breciated rock -Impact glass, shocked quartz, and other evidence for very high pressures

Structure of the Earth

crust: ~10-70 km thick, solid, intermediate composition mantle: ~2800 km thick, (mostly) solid, ultramafic composition outer core: ~2200 km thick, liquid, mostly iron inner core: ~1500 km thick, solid, mostly iron

Risk of a Comet

look at aphelion distance, perihelion distance, see if intersects with Earth's orbit, also look at the inclination of the orbit

Venus v. Mars

one major difference between Venus and Mars is that Venus has a very thick CO2 atmosphere that creates a runaway greenhouse effect with hot surface temperatures while Mars a very thin atmosphere with a cold surface. Another difference between Mars and Venus is that mars surface has many craters while venus' surface has less craters than mars does. These differences between Mars and Venus may tell us that the greenhouse effect has had different effects on each planet. On Venus, the very thick CO2 atmosphere causes very hot temperatures and pressures. On Mars, the very thin atmosphere results in very little heat retention from the sun which is what makes the surface so dry and cold with a low pressure. Mars is inactive while Venus is active because of "zamboni" processes

Difference between inner and outer planets

the inner planets are primarily made up of rocks while the outer planets are made up of gases like hydrogen and helium. This means that the inner planets are more dense than the outer planets. Additionally, the inner planets are smaller, have little or no moons, and are warmer in temperature. The outer planets are bigger, have many moons, and are colder in temperature. These differences occurred because of the process of core accretion. When the planets formed of rock, metal, and ice reach a mass of 4-5 times the Earth, gravity becomes strong enough to retain gases. The density of the protoplanetary disk decreases the further away from the sun which means that the more materials condensed to form solids, so outer planets were able to grow larger than the inner planets could.

Evidence for Big One Theory

-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

How can we learn about the formation of our solar system?

1) Planetary Science: The sizes, orbits, compositions, and physical properties of the planets in our own solar system provide clues to their origin 2) Meteoritics: Meteorites provide samples of the oldest objects in the solar system, dating to the time the planets 3) Astronomy: We can learn about how stars form by looking at ongoing star formation elsewhere in our galaxy.

Converting Relative Ages to Absolute Ages

1) The size distribution of potential impactors (e.g., comets or asteroids) 2) The frequency of impacts 3) How impactor size scales with crater size 4) How all this has evolved over time

Satellites

1)Capture of passing objects 2)Accretion from impact ejecta 3)Co-accretion from disks surrounding growing proto-planets (similar to formation of the solar system in miniature)

The basic stages of solar system formation

1)Collapse of giant gas/dust cloud 2)Formation of rotating disk 3)Condensation of solids and formation of planetesimals 4)Accretion of planetesimals to form "embryos" 5)Runaway growth

Outer Planets

1)Rocky cores several Earth masses in size 2)Dense atmospheres of H2 and He similar to sun, but enriched in heavier gases relative to solar composition.

Kepler's Laws

1)The orbit of a planet is an ellipse with the Sun at one of the two foci. 2)A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. 3)The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit

Major dilemmas with forming the outer solar system

1)Uranus and Neptune are too big. There probably wasn't enough stuff out at 19 AU (Uranus) and 30 AU (Neptune) to form giant planets. 2)There are too many comets, Kuiper belt objects, and other icy worlds at the far edges of our solar system. 3)Jupiter must have formed quickly to disrupt the asteroid belt and slow the growth of Mars, but planets should form more slowly in the outer solar system. 4)Lots of comets and Kuiper belt objects have highly elliptical, highly inclined orbits. It is thought that the structure of the asteroid belt is strongly controlled by gravitational interactions with Jupiter. But, if the inner planets formed more quickly than the outer planets, how could Jupiter influence what was going on in the inner solar system if it hadn't formed yet?

Data to explain

1.Planets isolated 2.Orbits ~circular / in ~same plane 3.Planets (and most moons) travel along orbits in same direction.... same direction as Sun rotates (counter-clockwise viewed from above) 4.Most (not all) planets rotate in this same direction 5.Solar System highly differentiated: Terrestrial Planets (rocky, dense with density ~4-5 g/cm3), Jovian Planets (light, gassy, H, He, density 0.7-2)

Uranus

13x Earth mass Uniform through out; no rocky core 11 rings, 27 satellites -212 C at surface 18 hour rotation, 84 year orbit Spins axis inclined almost 90 degrees

How far away is the sun?

150 million km, 1 AU

The Moon

1738 km radius ~1% the mass of the Earth Lower density (3.3 g/cm3 vs. 5.4 g/cm3) The lunar crust can be divided into the dark-colored lowlands (Mare) and light-colored, heavily cratered highlands The light colored highlands are older because are they are more heavily cratered Frequent impactsfor ~500 million years after formation of the solar system Bombardment still continuing today

Mercury

2440 km radius 6% the mass of Earth Average distance from sun:57.9 million km(0.39 AU) Dense (5.43 g/cm3)

Jupiter

318x the mass of Earth Larger than all other planets combined Low density (1.33 g/cm3)- gas giant Over 39 known satellites It is NOT a failed star. Jupiter is big, but the minimum mass for nuclear fusion is ~75x that of Jupiter Jupiter has a dense atmosphere composed mostly of hydrogen and helium, though methane and other gases are also present Jupiter rotates very rapidly (a "day" is only 9.9 hours). This rapid rotation combined with convection driven by heat from the interior helps produce the stark atmospheric banding, severe and long-lived storms (e.g., the Great Red Spot) and unimaginable winds At the equator, winds can reach velocities of up to 150 m/s (540 km/hr)

Cretaceous-Tertiary mass extinction

65 million years ago the dinosaurs and up to 50% of all species on Earth suddenly went extinct The Cretaceous-Tertiary mass extinction wiped out a significant fraction of all species living at the time Sometimes it pays to be lucky. Walter and Louis Alvarez wanted to find out how long it had taken for a thin layer of clay at the K-T boundary to form by measuring the abundance of a rare element (iridium) that can be found in cosmic dust. What they found were iridium levels way too high to have come just from cosmic dust. The iridium-rich layer can be found all around the globe. A large asteroid rich in iridium had to have hit Earth right at the time of the K-T extinction The asteroid impact that killed the dinosaurs at the end of the Cretaceous would have thrown massive amounts of dust into the atmosphere, blocking the sun and preventing photosynthesis Consider the frictional heating that occurs when objects enter the Earth's atmosphere. Now consider what happens when debris from the Chicxulub impact enters the atmosphere all around the world, all at once. The impact would have triggered global fires, killing any animals that couldn't hide underground or underwater. Those that survived would have faced famine and a vastly altered climate The Chicxulub crater is almost 200 km in diameter. It formed 65 million years ago, right at the K-T (Cretaceous-Tertiary) boundary

Earth

7900 mile (12756 km) diameter 23 degree axis tilt (seasons!) Surface temps -73 to 48 C(-100 to 120F) Thick atmosphere, mild greenhouse effect Liquid water - lots! - at surface plate tectonics liquid metal core the perfect place for life to evolve Geologically active core, mantle, crust Solid inner core, liquid outer core Magnetic field that protects from solar winds

Saturn

95x the mass of the Earth Very low density (0.69 g/cm3)-Saturn would float in water if you could find a big enough bathtub Massive atmosphere of H, He, and methane (similar to Jupiter) Saturn's rings are composed of countless small icy particles, ranging in size from ~1 meter to < 1 micrometer The rings may be the result of the catastrophic breakup of a satellite due to impact Gaps or grooves in the rings are the result of gravitational interactions with "shepard" satellites Like Jupiter, Saturn has a large array of major and minor satellites. Most of these are icy worlds composed of water, CO2, and methane ice. The largest and most intriguing is Titan, which shows that appearances can be deceiving.It's bland, featureless appearance is the result of a thick, hazy nitrogen atmosphere that also contains methane and hydrocarbons. Encladeus

Asteroids

A major clue to the composition of asteroids comes from their reflectance spectra (basically, their color).Different types of materials (e.g., silicate minerals, metal, organic compounds) absorb different wavelengths of light Asteroids show great variety in their reflectance spectra, indicating that not all asteroids are the same composition Over 7000 asteroids with Earth-crossing orbits have been identified. Fortunately, we've probably found most of the large ones (>1 km in size), and none of these seem to pose an immediate threat. However, there are many smaller objects still to be found, and there is no way of knowing when one will hit

Newton's Theory of Gravity

Although Kepler's laws provide an elegant description of how the planets orbit around the sun, they are empirically derived and do not provide an explanation as to why the planets orbit as they do. For this, we needed to wait over a half century for Sir Isaac Newton(1643-1727) to publish his work on gravity and the laws of motion F = m1a = Gm1m2/r2 As we orbit the sun, we are constantly accelerating towards it, but centrifugal force essentially causes us to miss Because it is gravity that controls the orbits of all the objects in the Solar System, the orbital period of one object around another of much greater mass is a function not only of mean orbital distance, but also the mass of the object being orbited. In detail, this relationship is given by:P2 = (4pi^2a^3) /GM Where G is the Gravitational constant = 6.67x10-11Nm2kg^-2, a = semimajor axis (in meters) M = mass (in kg) of the planet or star being orbited. Note that N stands for "newton", a unit of force that has units of kgms^-2.

Iron Meteorites

As the name implies, these are made mostly of iron, along with nickel. These derive from planetesimals that were large and hot enough to experience differentiation, or density separation of metal and silicate components.

Asteroid Belt

Between the orbits of Mars and Jupiter lies the asteroid belt, composed of thousands of bodies ranging from less than a km in size up to nearly 1000 km Most meteorites derive from here 4 Vesta is the second largest asteroid in the asteroid belt, with a diameter of 525 km It is thought to be the source of a special class of achondrite meteorites call Eucrites Vesta may be the source of a certain class of meteorites that came from an object that underwent early partial melting Vesta is thought to have a differentiated structure, with a core, mantle, and crust. This requires that Vesta was initially hot and at least partly molten

Kuiper Belt

Beyond the Gas Giants - Icy worlds include Pluto, Charon and the Kuiper Belt objects (mostly from ~30-55 AU, although "Trans-Neptune Objects" extend much farther), Comets (Kuiper Belt and Oort Cloud - extends to over 100,000 AU!), and other icy denizens of deep space. There are probably over a trillion comets > 1 km. There was too little material in the distant fringes of the proto-planetary disk to generate the objects that populate this region today. These objects were formed closer to the sun, but "cast out" by Jupiter and Neptune

Calisto

Calisto (shown here) has a radius larger than our moon (2403 km vs 1738 km), though it is much less massive because of its low density (1.85 g/cm3), reflecting its composition of a mixture of rock and ice Its surface is heavily cratered, revealing that Calisto has been inactive for a long time.

Comets

Comets are typically small (<10 km) collections of ice and rock that probably represent the planetesimals that accreted to produce the cores of the giant planets. Comets can provide important clues to the formation of the outer solar system. Because comets are mostly small and distant, they are usually invisible until they venture into the inner portions of the Solar System Coma: When a comet approaches the sun, it begins to heat up and ices are sublimated (turned to gas). For water ice (the most abundance ice in comet nuclei) this starts at ~3 AU. The gas forms a transient cometary atmosphere called the coma. During this process, dust is also released. The dust reflects sunlight, making the coma visible The gas tail fluoresces when the gas is ionized by sunlight, and points away from the sun. The gas is driven away from the sun by radiation pressure The dust tail generally points back along the path of the comet. The dust particles follow orbits similar to comet nucleus Comets leave behind a trail of dust that spreads out to occupy the entire orbit of the comet. If this orbit intersects that of the Earth, it produces a meteor shower. Several annual meteor showers are related to known short-period comets, including the Perseids in August (Swift-Tuttle) and the Orionids in October (Comet Halley). Comet Nucleus: The central, solid portion of a comet, usually hidden behind the coma. These range in size from ~100 m up to >40 km. They are composed of a collection of rock, dust and water ice as well as ices of carbon monoxide, carbon dioxide, methane, ammonia. Other trace compounds include amino acids and other organic compounds

Compact and Compression (Step 1)

Compact and compression occur as the impactor makes contact. Shock waves pass into the target and disrupt it. Peak pressures can exceed 100 GPa, and both impactor and target may vaporize or melt

Why we (as scientists) are happy to be living in a shooting gallery

Craters can reveal a great deal of information to planetary geologists: 1)The relative number of craters constrains the ages of planetary surfaces. Older surfaces have more craters. 2)The shapes of craters provide clues to the strength and thickness of a planet's crust and lithosphere. 3)Ejecta blankets provide clues to the presence or absence of water or ice beneath the surface. 4)Large craters create deep holes in the crust, exposing material that otherwise would not be visible. This allows geologists to examine crustal layering, or how composition changes with depth.

Big One Theory

Current theory suggests an early impact of a Mars-sized body with proto-earth at ~4.5Ga Collision tilted the Earth's spin axis Ejected material formed the moon

Density

Density (mass/volume) is a very important property of planets and other solar system objects, because it provides clues to composition. For example, water has a density of 1 g/cm3. Typical rocks have densities of ~2.5-3.5 g/cm3. The high density of Mercury (5.4 g/cm3) means it must not be made of just rocky material. Something heavier (e.g., metallic iron) must also be present in the planet's interior.

Pluto

Diameter - 1,413 miles (2274 km) - 2/3 size of Earth's Moon (but much lower mass) Orbit: 248 years highly elliptical Light from Sun takes 5.5 hours to reach it Surface of water and methane ice, frozen nitrogen Pluto belongs to a large and growing class of "dwarf planets" located beyond the orbit of Neptune. Poor Pluto lost its "planet" status in 2006 Ultima Thule (right) is the most distant object (44.7 AU) to be visited by a spacecraft (New Horizons). This ~31 km long object is a classical Kuiper Belt object.

Major spectral classes of asteroids

E:Appear to contain the mineral enstatite (MgSiO3).Probably differentiated bodies. S:Stony asteroids. Probably differentiated bodies. C: Carbonaceous, dark. Probably very primitive. M: Metallic, probably mostly iron and nickel. Differentiated bodies. P:Large metallic component. Differentiated bodies. D:Very dark and red. May be related to comet nuclei? Probably very primitive (like C-type)

Elongated Craters

Elongated craters like those shown here are produced by "grazing impacts", where the impactor trajectory intersects the target at less than ~10 degrees

Europa

Europa is denser than Ganymede and Calisto (2.99 g/cm3), suggesting it is made primarily of rock rather than ice. However, it has an icy surface that is only lightly cratered and which is covered with a very complex network of cracks, ridges, and grooves Young (sparsely cratered), complex terranes suggest Europa's surface has been reworked by cryovolcansim Europa's proximity to Jupiter and gravitational interaction with the other Galilean satellites results in tidal heating, which warms Europa's interior. Europa's icy crust may be quite thin and hide an ocean of liquid water several hundred km thick Deep hydrothermal vent which warms the liquid water

Orbital Resonances

Every object in the Solar System exerts a gravitational influence on every other object. These small tugs can be compounded when two or more objects have orbital periods that are simple multiples of one another. We call this situation orbital resonance. Orbital resonances can cause orbits to be particularly stable, as for the Galilean satellites shown here, or they can cause orbits to be unstable Ganymede 4:1, Europa 2:1, Io 1:1, all orbiting around Jupiter Repeated gravitational interactions between small bodies (e.g., asteroids) and larger bodies (e.g., Jupiter) can cause the orbits of the smaller bodies to evolveor change over time. In some cases, new orbits may cross those of planets in the inner Solar System, including Earth

The "Nebular Hypothesis"-Step 2

Flat rapidly rotating disk with a proto-sun As the cloud collapses and spins faster, it flattens into a disk with a protostar in the center Many young stars are surrounded by disks of dust and gas, just as our sun was 4.5 billion years ago. This disk forms due to the accelerated spinning of the gas/dust cloud that results from contraction The proto-planetary disk was heated by the accretion of material falling onto the disk (release of gravitational energy), by friction, and by the growing proto-sun. Numerical models constrained from both astronomical observations and findings from meteorites show that peak temperatures in the inner solar system would have been hot enough to evaporate all solids. Condensation is the process by which solid grains formed as the initially hot solar nebula began to cool (like snow flakes forming in a cloud). Different compounds condense at different temperatures. Silicates(rock-forming minerals) and metals condense at high temperatures. Ices (water, CO2, CH4) condensed in the colder outer portions of the disk, but not inward of the "snow line" Metallic elements (Mg, Si, Fe) condense into solids at high temps. Combined with O to make tiny grains Lower temp (H, He, CH4, H2O, N2, ice) - outer edges (ices form) We can actually see this zonation around other proto-stars! Planetary Compositions reflect this temperature-driven compositional zonation

Ganymede

Ganymede is the largest Galilaean satelite (radius = 2634 km). It is similar in composition to Calisto Although parts of Ganymede's surface are heavily cratered, others are less so. In addition, massive fractures and grooves cut across the planet, suggesting a much more active past than observed for Calisto

The "Nebular Hypothesis" - Step 3

Gas and dust collide to form planetesimals Dust particles=> Softball sized objects Softball sized objects => ~1 km planetesimals Planetesimals => proto-planets

Looking for extra-solar planets

Gravitational pull from orbiting planets causes star to wobble. The larger the planet, the greater the pull. We can detect the presence of a planet by observing the wobble it produces, even when the planet is too faint to see. Other methods include looking for changes in star luminosity as a planet passes across its surface.

Triton

Ice volcanos-- geysers Thin atmosphere (nitrogen, methane) Ridges and valleys, melting

Meteorite

If the meteoroid survives passage through the atmosphere, it becomes a meteorite Majority of meteorites are fragments of asteroids from the asteroid belt

Achondrite and Iron Meteorites

In contrast to chondrites, achondrites and iron meteorites have been subjected to melting and differentiation processes occurring on or within the planetesimals from which came. Surprisingly, although achondrites and iron meteroites come from bodies with a more complicated geologic history than chondrites, they are almost the same age as chondrites Radiometric dating of different types of meteorites and their constituent minerals allows us to place tight constraints on the timing of Solar System formation. The time it took from the condensation of the first solids from the solar nebula to the formation and differentiation of planetesimals 10s or even 100s of km in size was only ~10 Ma Many achondrites have compositions broadly similar to the terrestrial volcanic rock basalt and appear to be partial melts separated from initially chondritic planetesimals. They likely represent the crust of plantesimals that became hot enough to melt early in solar system history. A special class of achondrite meteorites called HED meteorites (for howardite, eucrite, and diogenite) are believed to all derive from the large asteroid 4 Vesta, which has a diameter of ~530 km. Whereas achondrites represent the mantle and crust of differentiated planetesimals, iron meteorites derive from planetesimal cores. If a planetesimal gets hot enough to melt, the dense iron can sink to the middle, even in the weak gravity present in objects 10s to 100s of km in size

Io

Io is a planet where tidal heading has gone mad Io is the most volcanically active body in the solar system Io's high density (3.53 g/cm3) indicates a rocky composition No impact craters. Io's surface is constantly being reworked by volcanism Eruption of Pele volcano, Io. Volcanic plumes from Io's volcanoes can extend hundreds of km into space. Because of Io's volcanism, Io has a tenuous atmosphere composed primarily of sulfur dioxide

Kepler

Johanes Kepler (1571-1630) tried and failed to explain the detailed movement of Mars using circular orbits as envisioned by Copernicus. He eventually derived three simple laws that described planetary motions.

Kirkwood Gaps

Kirkwood gaps in the asteroid belt occur where orbital resonances between the asteroids and Jupiter cause the orbits to become unstable. Over time, asteroids are "cast out" of these orbits

Comets and Asteroids

Leftover building blocks from the inner and outer solar system Clues to Solar System formation-oldest, most primitive objects available "Rogue" bodies-risk assessment Both asteroids and comets can have highly elliptical orbits. Comet orbits also may be highly inclined relative to the Ecliptic plane Both features reflect scattering of these objects due to gravitational interactions with the giant outer planets, especially Jupiter and Neptune. The total mass of the asteroid belt is less than ~0.001x the mass of Earth, although originally there would have been 2-4 Earth masses available to build objects in this region. The rest has been swept out by Jupiter's gravity. Likewise, the population of comets is slowly decreasing as individual comets enter into the inner Solar System to be destroyed (either by impact or evaporation) or are flung out into deep space

Stoney Irons

Macroscopic mixtures of silicate and metal phases. In some cases these may sample the core/mantle boundary of differentiated planetesimals

Meteorite Falls

Meteorite falls are observed falling to Earth, and can be collected soon after impact. This means that falls are generally fresher and less contaminated than finds, and are therefore scientifically more valuable About 90% of fallsare chondritic in composition. About 5% are iron meteorites, 1% are stony-ron, and the rest (~5%) are achondrites.

Meteorite Finds

Meteorite finds are found and collected long after they fall to Earth. In some places, meteorites are easier to find than others because they stick out from their surroundings, as here in the Sahara desert The moving Antarctic ice sheets act as a conveyor belt collecting and concentrating meteorites in certain regions, where they stick out like a sore thumb against the icy landscape about 40% of finds are iron meteorites

Meteoroids

Meteoroids enter the Earth's atmosphere traveling >10 km/s. As they descend, friction causes the meteoroid to heat and surface layers melt and ablate, starting at a height of ~120 km. Ionization of the surrounding atmosphere causes it to glow, producing the streak of light we know as a meteor or shooting star. If the meteoroid survives passage through the atmosphere, it becomes a meteorite Most of the meteors we observe in the night sky are caused by particles roughly the size of a grain of sand. Most of these do not survive the journey. Asteroid collisions and outgassing comets constantly produce dust in the solar system. The interplanetary dust cloud reflects sunlight, making it visible as the zodiacal light. The white fuzzy region extending up from the horizon in this photo is the dust cloud that lies in the plane of the ecliptic. Sunlight reflected from the dust particles makes the cloud visible.

Microtektites

Microtektites are often preserved in deep-sea sediments, and provide a record of cratering processes on Earth. Sometimes tektite fields provide clues to location of impact

Modification (Step 3)

Modification depends on the size of the crater and the strength of the target. Debris may partial fill the cavity, or the walls of the crater may slump to form terraces. Rebound of the central cavity may produce a central peak

Multi-Ring Basins

Multi-ring basins such as Orientale Basin (left) or the Callisto Valhalla Basin (below) form when a very large impact strikes a body with a rigid lithosphere overlying a more plastic asthenosphere The largest impacts produce multi-ring basins

Copernicus

Nicolaus Copernicus (1473- 1543 A.D.) proposed a model of "Heliocentrism", in which the sun is near the center of the Universe, and the Earth and other planets revolve around the Sun at varying rates. This theory provides a simpler, more elegant solution to the problem of planetary retrograde motion. However, Copernicus still believed that the planet paths were perfect circles, which was difficult to reconcile with astronomical observations

Where stars are formed

Observations from astronomy reveal that stars are formed primarily from giant "Molecular Clouds", which are cold, dense (by the standards to space) clouds of dust and gas concentrated in the spiral arms of our Galaxy.

Tungunska impact

On June 30, 1908 a large meteoroid or comet fragment broke apart over northern Siberia, producing an air burst with an energy equivalent to 10-15 megatons of TNT. The explosion was ~1000x as strong as the Hiroshima bomb, and devastated an area roughly 2150 km2. Recent studies suggest a chondritic meteoroid ~50m in size could have produced the Tunguska event We don't need to panic, but a Tunguska-sized event is expected to happen every few hundred years. What if the next impact/explosion isn't over the ocean or some remote region, but a large city? Many scientists are working on ways to "nudge" objects on a collision course out of the way, if we spot them in time

Comet Missions

On November 14, 2014, the Philae lander became the first robotic device to ever land on a comet. The images to the left show the rotating comet as the Rosetta spacecraft (which carried the lander) approaches. Notice the very irregular shape. Landing on a comet is not easy. There's almost no gravity to hold you in place. When the Philae lander touched down, it was supposed to fire several harpoons into the icy surface to anchor it in place. Instead, it bounced along the surface like a beach ball on a windy day before finally coming to rest In 1992, comet Shumaker-Levy 9 broke apart during a close encounter with Jupiter. Two years later, the fragments collided with the giant planet. The "scars" the impacts left on Jupiter were as large as the Earth, and were visible for weeks. The Shumaker Levy impacts provided a unique opportunity to observe the impact process. Scientists studying the ejected plumes learned much about the Jovian atmosphere. Deep Impact mission to Comet Temple 1 - why did NASA spend millions of dollars to crash perfectly good spacecraft? Deep Impact launched on January 12, 2005 and reached Temple 1 on July 4, 2005. A projectile struck the comet, releasing a cloud of dust and gas that was analyzed by the main spacecraft. By looking at the spectra of the debris cloud, we can learn more about what comets are made of. Spectral analyses revealed a higher dust/gas ratio than expected. Released particles were also very fine (like talcum powder). Solids included clay minerals, carbonates, and silicates. Do comets form a rocky crust over time in the inner solar system? Asteroid collisions and outgassing comets constantly produce in the solar system

Ptolemy

Ptolemy (AD 90 - c.a. AD 168) believed that the Earth was at the center of the universe. The relative motions of the sun and planets was associate with nine concentric spheres. Because planetary apparent motion cannot be described by a simple circle, Ptolemy "corrected" his model by adding the concept of "Epicycles" planetary retrograde movement

How old is the solar system?

Radioactive dating of meteorites shows they are ~4.55 billion years old.Earth and the moon formed at the same time as the other planets. Whole process took ~50 million years. Lunar samples - up to ~4.5 Ga(Ga = giga anna = billion years) Meteorites - 4.56 Ga Earth - oldest rocks 3.9 (or 4.4 Ga) Evidence for the initial presence of now-extinct short-lived radioactive isotopes in the early solar system suggest the whole process from start to finish took <50 Ma for the terrestrial planets

Planet 9

Researchers from Cal Tech recently presented evidence that the solar system may have a 9th planet ~1-10x as massive as Earth located well beyond the orbit of Pluto. This planet has not been detected, but it's gravitational influence could explain the odd orbits of some objects in the Kuiper Belt

Spectral Classes of Meteorites

S:Stony asteroids. Probably differentiated bodies. C: Carbonaceous, dark. Probably very primitive. M: Metallic, probably mostly iron and nickel.

Crater Size and Frequency

Size-frequency distributions provide a means of evaluating the relative age of different terrains from the same planet (e.g., the lunar mare and highlands). On the moon, we can calibrate these curves against radiometric absolute ages for samples returned by Apollo The crater size-frequency distribution also provides information about the distribution of the sizes of the objects (e.g., asteroids) responsible for most impacts. Crater size will be a function of impact velocity, mass of the impactor, and the composition/strength of the target The frequency of impacts has declined exponentially over the history of the Solar System. The rate of impacts was much higher in the past, and would have made life on early Earth impossible Many scientists believe that the moon (and Earth) were also subjected to a "late heavy bombardment period" between about 3.8 and 4.0 Ga. Many of moon's large basins formed at this time

Neptune

Small rocky core 4 Rings - unknown composition 13 moons 18 hour rotation / 165 year orbit

Chelyabinsk

Small, weak objects, such as asteroid "rubble piles" or comet fragments (c.f., Shoemaker-Levy 9) may break up and explode in the atmosphere before they hit the ground On February 15, 2013 a bright fireball was seen shooting across Russia. The fireball was brighter than the sun, and observers felt intense heat The shockwave from the superbolide damaged buildings, shattered windows, and injured over 1000 people in the southern Ural town of Chelyabinsk Fragments of the bolide crashed through a frozen lake. Fragments of the bolide were recovered and are chondritic in composition Before breaking apart in the atmosphere, the Chelyabinsk meteor was probably ~20 meters in size. The energy from the explosion was 20-30 times more powerful than the Hiroshima atomic bomb The orbit of the Chelyabinsk meteorite reveals it to belong to a class of Near Earth Asteroids called Apollo asteroids

Tektites

Tektites form as molten ejecta follows ballistic trajectories that can carry the material thousands of miles from the impact site. The aerodynamic shapes result from atmospheric friction.

The "Nebular Hypothesis" - Step 4

Terrestrial planets form from planetesimals Gravitational accretion: planetesimals attract stuff Large protoplanets dominate, grow rapidly, clean up area ( takes ~10 to 25 My) - Runaway growth Gravitational interactions between a large number of initial planetesimals leads to collisions and growth of planetesimals. Runaway growth leads to the formation of a small number of large objects from the initial large population of planetesimals The largest objects grow fastest and clear their orbits by "canibalizing" other planetesimals in their region of space

Solar System Moons

The Martian moons Phobos and Demios are small, irregular bodies (their gravity is insufficient to crush them into a spherical shape). Their surface color suggests they are similar in composition to many asteroids, and they most likely are in fact wayward asteroids that were captured by Mar's gravitational field. Asteroids passing too close to Jupiter can receive a gravitational slingshot which puts them on course for a close encounter with Mars The density of the Galilean moons decreases systematically with increasing distance from the planet. The larger moons of Jupiter and Saturn are now believed to have formed from a disk of material that surrounding these planets as they formed (much like in a miniature proto-planetary disk). However, many of the smaller moons, like the moons of Mars, are captured objects (asteroids or comets) that strayed too close. Neptune's moon Triton is the largest example of a captured object. It has a retrograde orbit, which indicates it cannot have formed in place. Triton likely was originally a Kuiper Belt Object, similar to Pluto Dynamic calculations show that a moon with a retrograde orbit cannot have formed in the same region of space as the planet it orbits, but instead is most likely to have been gravitationally captured during a close encounter Recently, astronomers discovered a massive ring system orbiting a gas giant planet (with the exciting name J1407B) that itself orbits the very young star J1407. Young Jupiter-like planets may be surrounded by circumplanetary disks similar to the protoplanetary disks of material that orbit young stars. Moons like the Galilean satellites may form from these disks

Did the outer planets form at the same time as the inner planets? How long did it take to form the outer planets?

The answer to the first question is that the outer planets probably started forming about the same time as the inner planets, as solids were condensing and accumulating in the proto-planetary disk. How long this took is a much harder question to answer. We don't have any direct samples from the outer planets (or moons) that we can date. Models suggest that formation of the outer planets should have taken much longer (~108 vs. <5x107 yr). However, this appears to conflict with evidence (the asteroid belt) that Jupiter had formed before formation of the inner planets was complete

Impact Melt

The extreme pressures generated by impact can cause rocks at the base of a crater to melt, producing impact melt that may also incorporate material from the impactor. In very large events, such as the Sudburry crater, melt and brecciated rock may form a pseudotachylite

Aphelion

The farthest point from the sun

Core Accretion

The first model involves formation of a solid planet composed of rock, metal and ice. Once this solid body reaches a mass ~4-5x the mass of the Earth, its gravity becomes strong enough to attract and retain the nebular gas, which is mostly made of hydrogen and helium. Even though the density of the disk deceased with increasing distance from the sun, the lower temperatures means than more material (in particular, ices of water, CO2, ammonia, and methane) condensed to form solids. More solids means that planetary embryos of rock and ice could grow larger (albeit more slowly) than in the inner solar system

Orbits of Planets

The orbits of the planets and other Solar System objects are defined by their semimajor axis and their eccentricity, but also their orbital inclination, or the angle their orbit makes with the Ecliptic plane. The planets have low inclination, but comets can have very high inclination For the most part, the planets have orbits with low eccentricity and low inclination- i.e., they have nearly circular orbits all in more or less the same plane. In contrast, asteroids, comets, and Kuiper Belt objects have highly eccentric, highly scattered orbits

Perihelion

The point in a planet's orbit where it closest approaches the Sun

Nuclear Fusion

The pressure and temperature in the sun's core is so high, that hydrogen atoms fuse together to form helium:4 1H => 1 4He The helium atom weighs ~0.7% less than the four hydrogen atoms The "missing" mass is converted into energy (E = MC2). There is lots of energy tied up in chemical bonds. This is what happens when you combine hydrogen and oxygen

Proto-Planetary Disk initially hot

The proto-planetary disk was initially HOT (max temps of 2000 K), especially close to the growing sun Refractory phases called CAIs (calcium-aluminum inclusions) formed by condensing from hot gas Round Chondrules formed by flash heating/melting of dust accumulations, possibly due to shock waves in the disk

Scaling Relationship

The scaling relationship between impactor and average crater diameter will vary from planet to planet. For example, because impact velocities are lower on Mars than on Earth, craters will tend to be smaller. On Earth, the ratio of crater to impactor size is typically ~10:1. A 1 km asteroid will produce a crater 10 km in diameter

Shock Wave, Rarefaction Wave, and Ejecta Curtain (Step 2)

The shock wave is followed by a rarefaction (release) wave. Material moves upwards and outwards in an expanding ejecta curtain, producing a transient bowl cavity

Craters

Today, we consider it self-evident that craters such as these are produced by hyper-velocity impacts of asteroids or comets with planetary surfaces Nearly 200 impact structures have been identified on Earth, and more are discovered every year. Clearly, Earth has not been spared the cosmic pounding that produced the heavily scared lunar surface Asteroids hit the Earth with an average velocity of ~17 km/s, and comets collide at up to 70 km/s (for a head-on collision). Hypervelocity impactsproduce shock waves very similar to those produced by explosions The final shape of the excavated crater will vary with the size of the crater. Most craters are round, regardless of the angle of impact. This is because the energy released from the hypervelocity impact acts like a central point explosion. Only very low-angle impacts look different. Over 7000 asteroids with Earth-crossing orbits have been identified. Fortunately, we've probably found most of the large ones (>1 km in size), and none of these seem to pose an immediate threat. However, there are many smaller objects still to be found, and there is no way of knowing when one will hit

Grand Tack Model

Unless you can tack, you're pretty much at the mercy of the wind, traveling wherever it takes you. For planets forming in a gas disk, that usually means a one-way trip towards the sun

Greenhouse Effect

Venus and Mars are two different extremes of the greenhouse effect Mars atmosphere is too thin now to be able to retain heat Venus' atmosphere is too thick that it retains heat and is super hot less pressure = less CO2 and particles are more spread out so Mars can't retain the heat

Effects of Giant Impacts

Venus has a very slow retrograde rotation (the Venusian day is 243 Earth days), meaning the planet spins on its axis in the opposite direction as it travels around the sun. A giant glancing impact during the formation of Venus could have slowed its rotation. Finally, Uranus' axis of rotation is tilted 98 degrees relative to the plane of the Ecliptic. A giant impact may have knocked the planet on its side, in much the same way that the moon-forming impact that Earth was subjected to is thought to be responsible for Earth's relatively large tilt (23.5 degrees).

Venus

Venus is similar to Earth in size (0.815 Earth mass), composition, and distance to the sun (0.72 AU) In many respects it can be considered Earth's twin...our evil twin Clouds of sulfuric acid and a crushing CO2 atmosphere obscure a surface whose temperature is hot enough to melt lead Nearly the same size as Earth (.95) Slowest rotation of any planet (243 days) Spins backwards Surface temp 377 to 487 C, 710 to 908 F ... hotter than Mercury Cloud covered - radar observations Dry Very thick atmosphere mostly CO2• Surface pressure is 100 times higher than Earth's Runaway greenhouse Venus' dense atmosphere produces a run-away "greenhouse effect", and also obscures the planet's surface from view The Magellan spacecraft revealed Venus' startling and complex surface using radar, which can penetrate the atmosphere What is revealed is a diverse landscape with jagged mountains, abundant volcanic features, but very few craters.

The "Nebular Hypothesis"-Step 1

We start with a large cloud of ~99% gas (mostly H2 and He) and dust. Forces acting to collapse the cloud: Gravity Forces acting to prevent collapse: Gas pressure; turbulence; magnetic fields. If gravity wins, we've started the process of star formation. Diffuse spherical rotating nebula

Interplanetary Dust Particles

What happens to all the dust that escapes comets during their approach to the sun? Although large particles (grain of sand) usually burn up in the Earth's atmosphere, small particles are slowed down by the atmosphere high enough that they do not burn up, and settle slowly down Many IDPs represent comet dust minus the icy components. These particles can be analyzed in laboratories.

Orbit Migration

appears to be important in planetary formation, and may have also been important for our own Solar System We have now identified over 2000 extra-solar planets, and we know of one star with at least 8 planets. But, surprisingly, we've found lots of super-Jupiters very close to their companion star. This does not fit our simple model, because these planets cannot have formed where they are today - too hot and too little material A planet imbedded in a gas cloud is like a fish swimming through molasses. The gas slows the planet down, causing it to spiral closer to the Sun. Once the gas clears locally, other gravitational interactions can cause the planets to change course Initially, interaction with gas in the disk caused Jupiter and Saturn to migrate inward. Later, as the gas began to clear, they changed direction and migrated outward, pushing Uranus and Neptune out as well This is basically what the orbit migration of the large outer planets did to other smaller objects in the solar system. Scattering of the orbits of planetesimals in the outer solar system during migration of Neptune and Uranus leads to formation of comets and Kuiper Belt objects. Oort cloud is a consequence of the outer planets migrating outward interacting with icy planetesimals, scattering due to gravitational interactions, comets can come from any direction

Widmanstaetten Patterns

form when iron- and nickel-rick crystals grow from cooling molten metal Crystal size is related to cooling rate-slow cooling results in big crystals. From this, scientists can estimate the cooling rate (0.5-500 K/ma) and therefore size (less than a few 100 km) of the parent bodies

Eccentricity

measure of how far removed the shape of an ellipse is from a perfect circle. A circle is a special type of ellipse with an eccentricity e = 0. e = [(a2 - b2)/a2]1/2

Chondrite Meteorites

represent primitive mixtures of the various components present in the solar nebula. They provide our best estimate of the composition of the nebula. Refractory phases called CAIs (calcium-aluminum inclusions) formed by condensing from hot gas Round Chondrules formed by flash heating/melting of dust accumulations, possibly due to shock waves in the disk The relative abundances of most elements in chondrite meteorites are very similar to the elemental abundances seen in the Sun's atmosphere, confirming that chondrites are primitive samples of the Solar Nebula. Carbonaceous chondrites like Murchison (shown here) and Tagish Lake contain significant quantities of organic carbon, including amino acids not found on Earth. Some scientists think that the early Earth may have been "seeded" with organic compounds from comets and meteorites, and that these compounds were necessary for the origin of life on Earth

Condensation

the process by which solid grains formed as the initially hot solar nebula began to cool (like snow flakes forming in a cloud)

Mars

~11% Earth's mass 1.5 AU (= ~50% as much solar energy) Extensive past volcanism Evidence for past water Mars was warm and wet for the 1st billion years, but today it is cold and dry Ancient river channels Dune Fields, Wind Streaks, Dust Storms Very thin atmosphere

The Sun

•At the Center (and we do go around it .....) •99.85% mass of Solar System-~2x1030 kg! •92% H / 8% He •Source of solar wind and space Diameter: 1,392,000 km You could fit over 100 Earths side-by-side Surface temperature:5770 K Core temperature:15 million K Powered by nuclear fusion in the core Middle-aged (4.5 billion years old, will probably keep going another 5 billion or so.) Stars (like the sun) are powered by nuclear fusion