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GEOLOGY 12 CHAPTERS 22 NOTES COMPARATIVE PLANETOLOGY Name __________________ An Overview of the Solar System Basics The solar system consists of the Sun; the nine eight planets, at least 2 dwarf planets, sixty four satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. The inner solar system contains the Sun (Sol), Mercury, Venus, Earth and Mars. The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and the lesser or dwarf planets Pluto and Eris. The asteroid belt separates these two groups. The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. Pluto's orbit deviates the most from the plane of the ecliptic with an inclination of 17 degrees. The planets all orbit in the same direction (counterclockwise looking down from above the Sun's north pole); all but Venus and Uranus also rotate counterclockwise. There are numerous smaller bodies in the solar system: the satellites or moons of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun; and the comets (small icy bodies) which come and go from the inner parts of the solar system. With a few exceptions, the planetary satellites orbit in the same direction (counterclockwise) as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids. Classification The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest: • • • • • there are several moons larger than Pluto and Eris (formerly known as "Xena" or 2003 UB313) and two larger than Mercury; there are several small moons that are probably captured asteroids; comets sometimes fizzle out and become indistinguishable from asteroids; the Kuiper Belt objects and others like Pluto’s moon, Charon, don't fit this scheme well (The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU (astronomical unit = distance from sun to earth) from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets.); The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets". Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions. The bottom line is that many of the bodies are unique; our present understanding is insufficient to establish clear categories. Conventional Categorizations 8 The nine bodies conventionally referred to as planets are often further classified in several ways: 1. BY COMPOSITION (a) terrestrial or rocky planets: Mercury, Venus, Earth, and Mars • The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites. (b) Jovian or gas giants: Jupiter, Saturn, Uranus, and Neptune • The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites. (c) under debate • Pluto and Eris 2. BY SIZE (a) small planets: Mercury, Venus, Earth, Mars and Pluto. • The small planets have diameters less than 13000 km. (b) giant planets or gas giants: Jupiter, Saturn, Uranus and Neptune. • The giant planets have diameters greater than 48000 km. (c) lesser or dwarf planets • Mercury, Pluto and Eris are sometimes referred to as lesser planets (not to be confused with minor planets which is the official term for asteroids) 3. BY POSITION RELATIVE TO THE SUN (a) inner planets: Mercury, Venus, Earth and Mars (b) outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto (c) asteroid belt: between Mars and Jupiter; forms the boundary between the inner solar system and the outer solar system 4. BY POSITION RELATIVE TO EARTH inferior planets: Mercury and Venus • closer to the Sun than Earth • The inferior planets show phases like the Moon's when viewed from Earth. superior planets: Mars thru Pluto • farther from the Sun than Earth • The superior planets always appear full or nearly so. 5. BY HISTORY classical planets: Mercury, Venus, Mars, Jupiter, and Saturn • known since prehistorical times • visible to the unaided eye modern planets: Uranus, Neptune, Pluto and Eris • discovered in modern times • visible only with telescopes Chapter 22 Notes Page 2 The Nebular Disk Theory and The Origin of the Solar System Here is a brief outline of the current theory of the events in the early history of the solar system: 1. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova. 2. As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. 3. The center compresses enough to become a protostar and the rest of the gas orbits around it. Most of that gas flows inward and adds to the mass of the forming star, but since the gas is rotating the centrifugal force prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" or “NEBULAR DISK” around the star. The disk radiates away its energy and cools off. 4. The gas cools off enough for the metal, rock and ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms; the rock condenses a bit later. The lighter elements remain in gaseous form. 5. The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids. Once the larger of these particles get big enough to have significant gravity, their growth accelerates and, very quickly, they form protoplanets. (Much like the dust bunnies under your bed!) 6. About 1 million years after the nebula cooled the sun would have generated a very strong solar wind which swept away all of the gas left in the protoplanetary nebula. If a protoplanet was large enough its gravity would pull in the nebular gas and it would become a gas giant. If not, it would remain a rocky or icy body. 7. At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants. The "planetesimals" slowly collided with each other and become more massive. Once the solar system was mostly clear of debris, planet building ended. 8. The original atmosphere of the Earth, Venus and Mars consisted of Hydrogen and Helium. Those light elements, however, were heated to escape velocity by solar radiation and thus much of this original atmosphere escaped. The current atmospheres of the Earth, Venus and Mars evolved from other processes. Early bombardment brought some of the materials from which atmospheres and oceans formed. Outgassing, gasses blown out of volcanoes, is another likely source for atmosphere's formation. 9. It is likely that there was NOT enough water released via outgassing to account for the present day oceans. A significant portion of the ocean water may have been delivered to the Earth after it formed by the impact of comets. 10. On Earth, oxygen was produced by photosynthetic organisms (cyanobacteria / blue-green algae / stromatolites) breaking down CO2. 11. Rings around giant planets, such as Saturn, are probably the result of stray planetesimals being torn apart by gravity (tidal forces) when they ventured too close to planet. Chapter 22 Notes Page 3 Planetary Characteristics ROCKY PLANETS Diverse atmospheres cover terrestrial planets. Shells of silicates overlay inner metallic cores. 1. Mercury - The surface of Mercury and the Earth's Moon have very similar, old crusts displaying a nearly complete record of impacts over the last 4 billion years. Caloris Basin is a ringed impact crater of an asteroid that hit 4 billion years ago. No satellites. 2. Venus - An atmosphere so thick that its surface can only be mapped by radar. The surface contains abundant evidence for volcanoes such as Fula Mons which is 1.8 miles high. 3. Earth's Moon - Same age as the earth (4.6 billion years). The moon's crust, with craters, fractures, and lava flows, is four times thicker than earth's. Large mare (“lunar seas”) on the near side of the moon are actually composed of dark basalts. Highlands dominate the far side of the moon. Interior heat from radioactivity caused partial melting and basaltic material rose along fractures, filling basins layer by layer to form the mare. 4. Mars - Shows polar ice caps, large volcanoes (Olympus Mons sours 15 miles above its surface, has a base 335 miles across, and a 45 mile wide caldera), rift canyons extending the length of the United States (Valles Marineris is 150 miles wide and 4 miles deep), shifting sand dunes and other indications of wind erosion, and river canyons that must have been cut by torrential rains. GIANT PLANETS Emitting more energy than they receive from the sun, the gas giants are actually composed of much the same material as the sun. Together they account for more than 90% of the mass of the solar system. 1. Jupiter - Cloudy with high-pressure storms lasting years to centuries. Internal heat drives the turbulence. Jupiter's cloud cover consists of three layers: ammonia crystals over ammonium hydrosulfide over ice crystals. Below the atmosphere is a sea of liquid hydrogen and helium. • Io - Youngest surface in the solar system. Erupts continually. Heated by gravitational tugs (tidal forces) from Jupiter and Europa. Displays three types of plumes due of interactions between its molten silicate interior, sulfurous mantle, and hard sulfur crust. • Europa - Has one of the smoothest surfaces in the solar system - most likely a thin ice crust over a global ocean of water. Streaks that paint the surface were probably fissures in the ice that were filled by upwelling water or soft ice. Their patterns suggest that at one time Europa's ice crust was expanding and cracking on a large scale. Only a few impact craters are found the moon's surface, indicating it is relatively young. • Ganymede - Largest moon of solar system. Fresh white ice was ejected by most recent meteorite impacts. Strips of alternating parallel grooves and ridges indicate crustal movement millions of years ago. Crustal faulting and movement has reworked the surface to leave only the most recent impact craters. • Callisto - Concentric ridges were heaved up by the collision of a huge meteorite. The impact basin has been filled in by ice, and, though later battered by smaller meteorites, is nearly level. Callisto shows a nearly complete record of impacts. (∴No tectonics.) Chapter 22 Notes Page 4 2. Saturn - Saturn's atmosphere is like Jupiter's but less dynamic because it is further from the Sun and, hence, colder. The rings of Saturn are 100,000 km across and as few as ten meters thick • • • Mimas - The ammonia and water ice of Mimas is nearly as rigid as granite. Enceladus - Ices oozed through fissures. Titan - Main gases are nitrogen and methane with methane in three phases (s, l, g). ICY PLANETS Have cores equal to the closer-in giants but have much smaller accretions of gas. Huge mantles of water, methane, and ammonia probably underlay their dense atmospheres. 1. Uranus - Hides its atmospheric features beneath a deep layer of hydrogen. Very bland surface. Unlike Jupiter, Saturn, and Neptune, it has no severe turbulence beyond local thunderheads. • Miranda - A complex terrain which may have been repeatedly shattered by collisions and reassembled by gravity. Ammonia and water ices, erupting as lava does on Earth, could have partly smoothed the surface. Has ice cliffs higher than the Earth's Grand Canyon. 2. Neptune - Orbiting in a deep freeze within 60°C of absolute zero, Neptune unleashes winds that may be the fastest in the solar system. Storms form as convection shoots hydrocarbon gases up into colder regions where they condense into bright ices. • Triton - Largest moon 3. Pluto – Formerly the farthest planet from the Sun and by far the smallest. Pluto is smaller than seven of the solar system's moons. Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (1979 until 1999). Pluto rotates in the opposite direction from most of the other planets. Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of solid methane and carbon monoxide. • Charon is named for the mythological figure who ferried the dead across the River Styx into Hades (the underworld). Prior to 1978 it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. Charon's composition is unknown, but its surface seems to be covered with water ice. Charon was formed by a giant impact similar to the one that formed Earth's Moon. 4. Eris – (formerly Xena or 2003 UB313) In Greek mythology, Eris (known as the goddess of discord, strife, and chaos) caused a quarrel among the goddesses that sparked the Trojan War, while her daughter Dysnomia was known as the goddess or spirit of lawlessness. Not surprisingly, the discovery of Eris and the finding that it is larger than Pluto have also caused strife in the astronomical community, forcing some astronomers to produce a strict definition of the term "planet" which eventually led to Pluto losing its status as the "ninth planet" that it had held since its discovery in 1930. Initially nicknamed Xena, the object is currently located at around 98 AU, a distance that is up to more than three times farther out than that of Pluto or Neptune. However, Eris will eventually move in as close as Pluto and Neptune in a 557-year orbit around the Sun. It has an elliptical orbit that is more eccentric than Pluto's. The dwarf planet is also tilted almost 44.2 Chapter 22 Notes Page 5 degrees from the ecliptic so many astronomers assume that encounters with a more massive object moved it into its current, highly inclined orbit. • Dysnomia - On September 10, 2005, astronomers and engineers at the Keck Observatory on Mauna Kea in Hawaii discovered that Eris has a satellite or "moon." The Ecliptic - where most of the planets orbit The Eccentric Orbits of Pluto and Eris Chapter 22 Notes Page 6 Geological Processes One reason surface processes are emphasised both in planetary and conventional Earth based geology is that they are the easiest to observe. Also, they give evidence of the evolution of that planet. You should be aware of what drives these processes, both internal and external. There are various forms of energy that do work on different materials. Source of Energy Related Processes Gravitational Energy accretion of planets impact cratering internal differentiation isostatic adjustment atmospheric circulation hydrospheric circulation tidal heating and orbit evolution Radioactive Decay convection in planets volcanism tectonism planetary differentiation Solar Energy atmospheric circulation hydrospheric circulation solar wind biologic activity Chemical Energy atmospheric evolution rock alteration and weathering condensation of nebula biological activity NOTE: No simple scheme fits all circumstances, but the list should be helpful as a way to understand some of the driving forces. For example, some very complex processes are likely going on surrounding Io, the inner moon of Jupiter. Not only is this satellite thought to be heated by tidal interactions (Jupiter's strong magnetic field and intense radiation belts interact to control the way the sulphur fumes from its volcanoes are removed from the surface) but a huge electric field and strong electrical currents are also likely involved. Be sure to check our website or Google images for color photos of Io. It has appeared on previous exams. Chapter 22 Notes Page 7 Geological Processes (cont’d) Volcanism Volcanic activity occurs on a planet or moon when sufficient heat inside the solid body causes partial melting of the interior rock. Eruptions of the molten rock, called magma, and gases onto the surface produce a variety of volcanic landforms. Surfaces of the terrestrial planets show volcanic mountains of different shapes and sizes, fields of lava flows, lava channels, and ash and cinder deposits. The forms of volcanoes and other volcanic products is controlled by such factors as the composition and fluidity of the magma, nature of the vent, eruption rate and duration, planetary atmosphere, and gravity. -------------------------------------------------------------------------------- Impact Cratering (see text p.500) Impact cratering involves the collision of solid bodies, such as a meteorite into a planet, resulting in roughly circular, excavated holes. Rock material excavated from the crater is called ejecta. This ejecta is distributed radially from the crater onto the surface of the planet as fragmental debris. The size and shape of the crater, and the form and extent of the ejecta depend upon a number of factors: impact energy, strength of the target material, rock compositions, presence of volatiles (such as water), and gravity. -------------------------------------------------------------------------------- Tectonics Tectonics is a general term referring to the large-scale deformation (or change) of rock in response to forces causing faulting and folding. The forces acting upon a rock mass are generally termed tensional (pulling apart), compressional (squeezing together), or shear (parallel sliding). The magnitude and direction of the force (stress), the temperature and confining pressure on the rock, the composition of the rock, and the rate at which the rock is deformed determine how the rock changes in length, shape or volume. Common landforms resulting from tectonic processes are mountain ranges, rift zones, faults, fractured rock, and folded rock masses. -------------------------------------------------------------------------------- Weathering and Erosion or Gradation Gradation is the set of processes that work on surface rocks to break, loosen, move, and finally deposit the smaller pieces. The processes are: weathering (the chemical decay or mechanical break-up of rocks), erosion, transportation (the removal of weathered rocks by moving water, wind, ice, or gravity), and deposition. The presence of water and atmosphere drive the chemical decay of rock. Gravity moves landslides. Moving water, wind, and ice create landforms such as river channels, deltas, beaches, sand dunes, glacial deposits and valleys. On airless, waterless planets, gradation appears to be limited to impact cratering and the pull of gravity. Chapter 22 Notes Page 8 Comparison Charts of Geological Processes TERRESTRIAL PLANETS Erosion and Transportation Weathering Planet chemical decay mechanical break-up Mercury "solar wind" chemical changes meteorite impacts Venus heat, acid yes Earth pressure of ice, water, acid, roots; heating & oxidation, cooling; saltation organics in rivers Moon meteorite impacts, heating & cooling Mars water and carbon dioxide ice oxidation Deposition water wind ice gravity yes yes yes yes yes yes yes yes landslides on innercrater rims, crater ejecta yes landslides, wind streaks yes landslides, deltas, flood plains, beaches, sand dunes, glacial deposits, soil, dust yes landslides on innercrater rims, crater ejecta, regolith yes landslides, crater ejecta, wind streaks, sand dunes, channels, regolith, dust Volcanism Planet lava flows Impact Cratering Tectonics yes yes pyroclastics volcanoes Mercury yes probably Venus yes probably not yes yes yes Earth yes yes yes yes yes, global plate tectonics Moon yes yes yes yes Mars yes yes yes yes yes In addition to the textbook, be sure to check our website or Google images for photos of the various geological processes. They have appeared on previous exams. Chapter 22 Notes Page 9 Model of the Earth’s Interior By Composition V Crust U W Mantle X Y Core Z - 7 km to 70 km thick - silicate rocks - 2900 km thick - ultramafic, ferromagnesian silicates - 3400 km thick - nickel and iron V U W Crust Lithosphere Asthenosphere 7 km to 70 km thick 100 km thick 250 km thick X Mantle 2900 km thick Y Outer Core 2000 km thick Z Inner Core 1400 km thick By Physical properties Oceanic Crust Continental Crust 100 km thick Cool, rigid rock Asthenosphere 250 km thick Hot, plastic rock Mesosphere 2500 km thick Hot, rigid and brittle rock Outer Core 2000 km thick Liquid iron, nickel Inner Core 1400 km thick Solid iron, nickel Lithosphere Compare the cross-section of the Earth with that of the moon and Jupiter. How do we know these models are correct? Although we don’t have “solid proof” scientists study gravitational and magnetic measurements to suggest models for the interior structure of the planets. For the Earth and the moon we have also have the advantage of being able to study seismic waves. (i.e. earthquakes and moonquakes). Chapter 22 Notes Page 10 The following diagrams show a cross-section of the Earth, and a graph of earthquake wave velocities against depth in the Earth. • The Earth’s overall density is 5.5 g/cm3, yet the density of the crust averages only 2.8 g/cm3. This fact implies that the densities of the mantle and core must be greater than that of the crust • Both P and S wave velocities increase as they pass down through layer X or the mantle. This increase in velocity is because with increasing depth, layer X becomes denser. • The S waves have no velocity or stop below a depth of about 2900 km. This is because the S waves are entering a liquid layer. Chapter 22 Notes Page 11 Earth’s Composition and Structure Chapter 22 Notes Page 12