Chapter 9 notes.docx

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Astronomy & Astrophysics
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Michael Reid

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Chapter 9: Planetary Geology 9.1 Connecting planetary interiors and surfaces  Volcanoes earthquakes, asteroids or comets slamming into the Earth can dramatically reshape the surface of the Earth. Water can also affect the Earth -> water has carved the Grand Canyon.  Surfaces of five terrestrial planets probably looked similar when they were young. All five were made of rocky material that condensed in the solar nebula and all five were subjected early on to the impacts of the heavy bombardment  Today, there are profound differences between the worlds: - Mercury and the Moon show the scars of their battering during the heavy bombardment. They are densely covered by craters, except in areas that appear to be volcanic plains - The surface of Venus is covered by bizarre bulges and odd volcanoes - Mars has the largest volcanoes in the solar system and a huge canyon cutting across its surface  Planetary geology = trying to understand how the differences among the terrestrial planets came to be  Seismic waves = vibrations that travel both through the interior and along the surface after an earthquake. We use them to study the interior of the Earth  All the terrestrial worlds have layers. We can divide the layers according to their density: - Core: The highest density material which primarily consists of metals like nickel and iron - Mantle: Rocky material of moderate density (mostly minerals that contain silicon, oxygen and other elements) - Crust: The lowest density rock like granite and basalt form the world’s outer crust  Earth’s metallic core consists of 2 distinct regions: a solid inner core and a molten outer core (Venus may have a similar core structure)  Gravity pulls the denser water to the bottom, driving the less dense oil to the top in a process of differentiation. Dense metals like iron sink toward the center, driving less dense rocky material toward the surface  We think that the relative proportions of metal and rock should have been similar throughout the inner solar system at the time the terrestrial planets formed, which means we should expect smaller worlds to have correspondingly smaller metal cores  If we assume this is true, we can explain why Mercury has a very large core (a giant impact blasted away its outer rocky layers). Also, we can assume the Moon formed from debris blasted out of Earth’s rocky outer layers. These debris contained little high-density metal and accreted into an object with a very small metal core  The terrestrial world’s interiors were probably molten when differentiation occurred, but today they are almost entirely solid  Not all solid rock is equally strong though  Like all matter built of atoms, rock is mostly empty space and the bonds between the atoms that compose it can break and re-form when subjected to heat or sustained stress  Even solid rock can deform and flow over millions and billions of years  The long-term behavior of rock is similar to silly putty (stretches when you pull slowly but breaks cleanly when pulled rapidly)  Solid rock can slowly deform and flow over millions of years  A planet’s outer layer consists of cool and rigid rock called the lithosphere (which encompasses the crust and part of the mantle)  Under the lithosphere, warmer temperatures make the rock softer allowing it to deform and to flow much more easily. The rigid rock of the lithosphere floats upon the softer rock below  The lithospheric thickness is closely related to a world’s size: smaller worlds tend to have thicker lithospheres  A thin lithosphere is brittle and can crack easily while a thick lithosphere is much stronger and inhibits the passage of any molten rock from below, making volcanic eruptions and the formation of mountain ranges less likely  The weak gravity of a small object is unable to overcome the rigidity of its rocky material so the object retains the shape it had when it was born  However, for a larger world, gravity can overcome the strength of solid rock, slowly deforming and molding it into a spherical shape. In about a billion years, gravity can make any rocky object bigger than 500 km in diameter a sphere  Larger worlds become spherical more quickly, especially if they are molten (or gaseous) at some point in their history  Geological activity represents the ongoing change of the surfaces of the terrestrial worlds  Earth is geologically active (geological processes continue to reshape its surface) while the Moons and Mercury have virtually no geological activity  Interior heat is the primary driver of geological activity  A hot interior contains a lot of thermal energy, which comes from 2 sources: 1. Heat of accretion: The gravitational potential energy of colliding planetesimals is converted into kinetic energy, which upon impact is converted to thermal energy 2. Heat from differentiation: Light materials rise to the surface while dense materials fall to the core, converting gravitational potential energy into thermal energy 3. Heat from radioactive decay: Mass-energy contained in nuclei is converted into thermal energy  Accretion and differentiation deposited heat into the planetary interiors when the planets were very young  Radioactive decay is an ongoing source of heat  Over billions of years, the total amount of heat deposited by radioactive decay has been several times greater than the amount deposited initially by accretion and differentiation  However, the rate of radioactive decay declines with time so it was an even more significant heat source when the planets were younger  Long ago, many violent impacts that occurred during the latter stages of accretion deposited so much energy that the outer layers of the planets began to melt. This started the process of differentiation, which then released its own additional heat. This heat, along with the substantial heat from early radioactive decay, made the interiors hot enough to melt and differentiate throughout  Planetary interiors cool by transporting heat outwards in several different ways: 1. Convection: Hot material expands and rises while cooler material contracts and falls -> heat is transferred upwards 2. Conduction: Refers to the transfer of heat from hot material to cooler material through contact (molecules of materials in close contact constantly collide with each other and the molecules in hot material tend to transfer some of their energy to the slower-moving molecules of cooler material) 3. Radiation: Objects emit thermal radiation, which carries energy away and therefore cools the object.  Convection = most important heat transfer process in the interior  Hot rock from deep in mantle gradually rises, cooling as it moves upward. When it reaches top of mantle, it has transferred its excess heat to its surroundings so it is now cool and it begins to fall. The process created convection cells within the mantle. Mantle convection primarily involves solid, not molten, rock.  Mantle convection takes a long time -> rocks move about 1 cm per year  Mantle convection stops at the base of the lithosphere. At this point, heat continues upward through conduction. When it finally reaches Earth’s surface, it is radiated away into space  A large planet can stay hot inside longer than a small one (the extra rock acts as insulation). This is why Moon and Mercury cooled about a billion years after they formed (their lithospheres thickened as they formed and mantle convection was confined to deeper and deeper layers until it stopped all together -> now these two worlds are geologically dead) and why Earth and Venus are still active.  Interior heat creates a magnetic field which protects Earth’s atmosphere from being stripped away into space  There are 3 requirements for a magnetic field: 1. There must be an interior region of electrically conducting fluid 2. A convection in that layer of fluid 3. At least a moderately rapid rotation  Earth is the only world that meets all three requirements (it is the only terrestrial world with a magnetic field) 9.2 Shaping planetary surfaces  All surface features on worlds can be explained by four major geological processes: 1. Impact cratering = when bowl-shaped impacts are created by asteroids or comets striking a planet’s surface 2. Volcanism = when molten rock or lava erupts from a planet’s interior onto its surface 3. Tectonics = when a planet’s surface is disrupted by internal stresses 4. Erosion = when geological figures are worn down or built up by wind, water, ice and other phenomena of planetary weather  Cratering: - Scarred faces of Moon and Mercury attest to the battering that the terrestrial worlds have taken from planetesimals such as comets of asteroids - Small craters outnumber large ones -> more small asteroids and comets orbit the Sun than large ones - An impact crater forms when an asteroid or comet slams into a solid surface - At such a tremendous speed, the impact releases enough energy to vaporize solid rock and blast out a crater. Debris from the blast shoot high above the surface and then rain down over a large area - Craters are circular because the impacts blasts material in all directions - Craters are usually 10 times as wide as the object that create them and 10-20% as deep as they are wide  Volcanism: - “Volcanism” refers to any eruption of molten lava, whether it comes from a tall volcano of simply rises to the surface through a crack in a planet’s lithosphere - It occurs when underground molten rock (=magma) finds a path to the surface - Molten rock tends to rise for three main reasons: 1. It is generally less dense than solid rock 2. Because most of Earth’s interior is not molten, the solid rock surrounding a chamber of molten rock can squeeze the molten rock, driving it upward under pressure 3. Molten rock often contains trapped gases that expand as it rises, which can make it rise much faster and lead to dramatic eruptions - Molten rock is called magma underground and lava when it hits the surface - Lava can shape 3 different types of volcanic structures: 1. The runniest lavas flow far and flatten out before solidifying, creating vast volcanic plains 2. Thicker lavas tend to solidify before they completely spread out, creating shield volcanoes (which can be very tall but not very steep) 3. Thickest lavas flow far before solidifying and build up tall, steep stratovolcanoes - Lava plains and shield volcanoes are made of basalt, which is a mixture of different minerals that erupts form volcanoes as a high-density but fairly runny lava. Basalt is very common throughout the solar system - Stratovolcanoes are made by lower density volcanic rock that erupts as a thicker lava. This type of lava is common on Earth but rare in the rest of the solar system - Volcanism explains the existence of out atmospheres and oceans - The water and gas that arrived on Earth by way of icy planetesimals became trapped in the interiors of the planet and was expelled in a process called outgassing (when molten rock erupts onto the surface as lava, the release of pressure expels the trapped gases) - All the gas that made the atmospheres of Venus, Earth and Mars was originally released from the planetary interiors by outgassing  Tectonics: - Refers to the “building” of surface features by stretching, compression or other forces acting on the lithosphere - Most tectonic activity is a direct or indirect result of mantle convection - The crust can be compressed in places where adjacent convection cells push rock together - Cracks and valleys form in places where adjacent convection cells pull the crust apart - On Earth, the ongoing stress of mantle convection fractured the lithosphere into more than a dozen plates which move over, under, and around each other in a process called plate tectonics  Erosion: - Refers to the breakdown or transport of materials through the action of ice, liquid or gas - Erosion not only breaks down structures but it also builds things - For example, erosion has piled sediments into layers on the floors of oceans and seas, forming sedimen
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