Lecture 7 / Rocks: Geology and the Terrestrial Planets Dr. Henry Throop / University of Colorado ASTR 1110, Summer 2000 Announcements: Observing night tonight, 9-11 PM What will we see? Moon Constellations Summer Triangle Ring Nebula -- exploded star Andromeda Galaxy (2.5 Mlyr) Different colors of stars: red & Blue Double stars Terrestrial P's: Mercury, Venus, Earth, Mars Moons: Moon, Phobos, Deimos VG: sizes of them all Planets Big enough to be a sphere Also long enough to differentiate Can be made of ice, rock, or both. Really, ice is just water 'rock' Intro: Planetary formation --------------------------- Lots of dust sticks together from solar nebula, big cloud Have a whole `pudding' of material: from rocks, to ice, to metals, to gasses. Slowly over time (billion years), they settle out: heavy to center, light to outside Just like pudding or salad dressing Scum @ top Peppercorns @ bottom Goop halfway inbetween Planets are _hot_ at the start: Solar nebula is hot Debris smashes into each other, basically big explosions, rock gets vaporized Heavy stuff falls to center, and all the friction keeps it hot. Also radioactivity: Not like a bomb, but like little hands on a fluorescent watch Bigger planets hotter: a) more stuff, and b) it cools slower If planet is too small: a) it's not round, and b) it's boring, since no procs, all frozen. Over time, they'll settle, and cool off. Cooling rate depends on size of planet: like pot roast, biggest cool the slowest. Or like a house: big mansion will cool off faster than a small tent -- even with windows wide open. They'll get _some_ heat from the sun, but not much. Most of the energy is from formation. Layers ------ Differentiation: like salad dressing Core (Iron): molten, v. hot Mantle Lithosphere Crust (Atmosphere): coolest Processes --------- Surfaces & Interiors Can't measure interiors, but can probe them by Earthquakes (planetquakes) Temperature Volcanism Density (ie, physics) Cratering --------- Into rock, ice, mud Dominant process on Moon, Mercury Moon has fewer impacts per year than earth, but is much more cratered! how?! Happens everywhere! A _little_ bit more on larger planets with more gravity, and a little bit more near inside of SS, but basically same. Micrometeorites Happens everywhere, but requires _no_ atmosphere! Will erode footprints on Moon Tectonics --------- `Plate tectonics' -- plates squooshing, like pond scum of skin of milk Dominant process on Earth Requires warm mantle (thus a large planet which hasn't cooled off yet) -> Doesn't exist on cool planets Erosion ------- Wind Ice Water All above require atmosphere. Atmosphere requires a large planet to retain it Volcanism --------- Mt. St. Helens Can only happen on `warm' planets -- basically boiling Mars, Venus, Earth -- no Moon, Merc _but_: large dark splotches -- Maria -- on Moon, are volcanic! Basically oceans of lava We will see tonight! Earth Photos micrometeorites shoeprint bb's into talc Venus Earth Mars Moon Mercury Tecton X X X Volcan X X X Impact X X X X X Eros X X X/2 Collab Q's ---------- Why are there so few impact craters on earth? Erosion is faster on Earth, since it has more atmospher Earth's atmosphere blocks large impactors The moon's gravity attracts more asteroids & comets to it The Earth's magnetic field repels iron-rich asteroids They are there, but we've bulldozed them all away If a planet is smaller (lower mass), what will typically happen with wind & rain erosion? Have less erosion Have more plate tectonics Have more impact craters Have more volcanoes If we had a huge planet, similar to but several times the mass of the Earth... It could have fewer craters because the atmosphere would block them or erode them It could have more craters since it woudl attract more gravitationally It might be expected to have an active crust, with volcanoes constantly pumpung out more gases into atmosphere It would be warmer, because of ongoing differentiation or radioactivity. All of the above Look at these two planets: a) how big is it? b) Does it have an atmosphere? did it use to? c) which one was bigger? Is one an `older' surface? d) which looks more active today? e) If you ran a spacecraft by, what would you like to measure? Gravity, atmosphere, size of planet? Why? Do you think it would be a good place for life? e) guesses as to which is which?