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Discover the fascinating geology and atmospheric evolution of the inner planets in our solar system. Learn about the scrambled and fried asteroids, the outer planets and their mini-solar systems of moons, and the enigmatic nature of comets. Explore the characteristics of inner versus outer planets, the relationship between temperature and distance in the solar system, and the sizes of all planets compared to the sun. Finally, delve into the question of how planets retain their atmospheres and the two primary ways they can lose them.
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The Solar System Inner planets; geology, atmosphere evolution The asteroids – scrambled and fried Outer planets and their mini-solar systems of moons Comets: icy dirtballs or dirty iceballs?
How Does a Planet Retain an Atmosphere? • Surface gravity must be high enough and surface temperature must be low enough that the atmosphere molecules don’t leak away during the 4.6 billion years since formation.
Two Ways a Planet Loses Atmosphere: First… • Lighter molecules move faster, because on average Kinetic Energy = Thermal Energy • (½)m<v>2 = (3/2)kT • Molecules are continually bouncing off of each other and changing their speed, but if the average speed is higher, a few may be speedy enough to escape the planet’s gravity • Hydrogen and Helium = 97% of the mass of the solar nebula, and these are the lightest and easiest molecules to lose
The Second way to Lose Atmosphere… • …maybe easier to understand - Impact Cratering! Big comets and asteroids hitting the planet will deposit a lot of kinetic energy which becomes heat, blowing off a significant amount of atmosphere all at once. • This is a Big issue especially in dense areas (inner solar system), and dense times (soon after formation).
The Inner Planets • It’s hot close to the sun. No ices. So only the rocky material (~3% of the solar nebula) could collect. Not hydrogen and helium since escape velocities that are too low for these • Atmosphere histories for each planet are unique…as we’ll see
Mercury • Smallest planet, only 3,000 mi across. • 600F on daylight side, too hot to retain any atmospheric molecules at all. Probably doesn’t help that the sun is so close and solar storms can rack the planet and help carry off any atmosphere too. • Cratering shows it hasn’t had atmosphere for most of solar system’s history
Venus • Has thick CO2 atmosphere, 100 times denser than earth’s. CO2 is the heaviest common molecule. • Greenhouse effect – CO2 transparent to visible light coming down from the sun, but opaque to infrared coming back off the surface, hence heat comes in but can’t easily escape. 900K on surface!! • Let me draw you a picture…
Earth – largest inner planet • Crust divided into tectonic plates which move due to friction against the moving molten mantle underneath. Continental drift animation • . Click here for animation website. • This likely explains why the earth has so little atmosphere. That’s one heck of an impact event.
Atmosphere; initially rich in CO2, methane, no oxygen Why so little atmosphere, and why is CO2 such a tiny % (~0.3% today)? 1. Life took out CO2 and produced O2, and organic and inorganic processes produced CaCO3. Nice! This has been lowering greenhouse gases at the same time the sun has been increasing its luminosity – balance! 2. Moon created by Mars-sized planet impacting early Earth. Evidence: (1) moon crustal age is a little younger than Earth’s, (2) moon composition = Earth mantel composition, (3) moon orbits ~in ecliptic, not equator plane as you would expect if it was formed with the earth.
