Geology of Terrestrial Planets - Chapter 9 (Oct 23rd).docx

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

1 Tuesday, October 23 , 2012 AST101H1 Astronomy Terrestrial planets: basic properties and interiors, Mercury, Venus, Earth, the Moon and Mars Textbook (pg. 243) Chapter 9 Planetary Geology: Earth and Other Terrestrial Worlds Qualities of Earth that make it suitable for human life: moderate temperature, protective atmosphere, abundant water, and a relatively stable environment (extremely rare in terms of other planets) 9.1 Connecting Planetary Interiors and Surfaces Various processes shape the surface of the planets and Moon over time, some are ongoing e.g. Colorado River shaped the Grand Canyon, Rocky Mountains cut down in size Planetary Geology = The extension of the study of Earths surface and interior to apply to other solid bodies in the solar system, such as terrestrial planets and Jovian planets moons. Most geological activity shaped by processes below the surface What are terrestrial planets like on the inside? Most of our information about Earth comes from seismic waves - Seismic Waves = Earthquake-induced vibrations that propagate through a planet. All terrestrial worlds have layered interiors which are divided by density (differentiation) - Core = The dense central region of a planet that has undergone differentiation (e.g. nickel and iron). -Earth has a solid inner core and a molten outer core. Surprisingly, but the inner core is kept solid by the higher pressure at greater depth despite higher temperature. - Mantle = The rocky layer that lies between a planets core and crust (e.g. mineral like silicon, oxygen, etc.) - Crust = The low-density surface layer of a planet that has undergone differentiation (volcanic rock) Differentiation = The process by which gravity separates materials according to density, with high-density materials sinking and low-density materials rising. Due to the proportions of rock and metal in the inner solar system at the time of formation, we expect smaller worlds to have smaller metal cores, which is a common pattern with some exceptions: - Mercurys core is surprisingly large and the Moons is very small (due to giant impacts which blaster away Mercurys outer layers and the accretion of low-density debris from Earth to form the Moons small core) Lithosphere = The relatively rigid outer layer of a planet; generally encompasses the crust and the uppermost portion of the mantle. - The lithosphere essentially floats on the softer rock (made softer by temperatures) below - Smaller worlds tend to have thicker lithospheres - Thicker lithospheres are much stronger and inhibit the passage of molten rock from below, making volcanic eruptions and formation of mountain less likely Why are big worlds round? The weak gravity of a small object cannot overcome the rigidity of its rocky material, so it retains the same shape as when it was born, but a larger world, gravity can overcome the strength of solid rock, molding it into a spherical shape. Larger worlds become spherical more quickly. What causes geological activity? Geological Activity = Processes that change a planets surface long after formation, such as volcanism, tectonics, and erosion. Earth is geologically active, but the Moon and Mercury are geologically dead because their surfaces look the same as they did billions of years ago. - Interior heat is the primary driver of geological activity Three sources of energy explain nearly all interior heat of terrestrial worlds 1. Heat of Accretion Accretion deposits energy brought in from afar by colliding planetesimals. As they approach a forming planet, their gravitational potential energy is converted to kinetic energy, causing them to accelerate, and upon impact this kinetic energy is converted to heat, adding to planets thermal energy.2 2. Heat from Differentiation Light materials rise to the surface while dense materials fall to the core, converting the gravitational potential energy to thermal energy (via friction). 3. Heat from Radioactive Decay Mass energy contained in nuclei is converted into thermal energy as particles from radioactive isotopes collide with and heat neighbouring atoms. *Radioactive decay is the only source that provides ongoing heat, but the rate declines over time so it was more important when the planets were young. *Combination of three sources explains how interiors ended up with core-mantle-crust structure: accretion caused outside layers to melt, differentiation allowed for layering, and heat from decay added to differentiation Three processes that cool a planet: - Radiation = All objects emit thermal radiation, so planets lose heat to space through radiation from the surface. Because of low temperatures, planets radiate primarily in the infrared. - Conduction = The process by which thermal energy is transferred by direct contact from warm material to a cooler material. In a planet, conduction carries heat through rigid lithosphere to surface. - Convection = The energy transport process in which warm material expands and rises while cooler material contracts and falls. Heat is transferred upward whenever there is strong heating from below. For Earth, convection is the most important heat transfer process in the interior. The process creates convection cells. Mantle convection takes a very long time due to the slow flow of solid rock, but stops at the lithosphere. -Convection Cell = An individual small region of convecting material. Size is the most important factor in planetary cooling, and thus determining geological activity - A large planet stays warmer longer than a small one because the extra rock acts as insulation - Small sizes of Mercury and Moon allowed interiors to cool and mantle convection to slow and then stop - Earth and Venus are still
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