Review 2 Astronomy 150, April 27 th 2005

Moon’s Formation Giant-Impact theory –A Mars-sized object hits the Earth and the Moon is created from the debris –Lack of volatiles – impact was a high energy event and volatile elements evaporated –Lack of iron core – the Moon was created from Earth’s mantle debris – Earth was already differentiated  iron was in the core  low amounts of iron in the ejected debris –Similar Oxygen isotope ratios  similar composition

Mars 4.5OriginOrigin by Accretion 4.5 – 4.0DifferentiationFormation of the original crust 3.9 – 3.8Late Heavy Bombardment Formation of Hellas Basin and the heavily cratered southern highlands 3.8 – 0.6Geological Activity Formation of the Tharsis region and the Northern volcanic plains NowThe Big ChillLow-level cratering and wind erosion Mineral Olivine – quickly dissolves in water  it cannot last for long in wet environments Southern Hemisphere –Mineral Hematite – created in hot, wet environments –Evaporative basins – rocks get saltier as you move down the crater (as the Salt Lake) Lower gravity, lack of plate tectonics  broad, tall shield volcanoes

Venus Thick atmosphere: –no continuous meteoritic bombardment (like on the Moon)  jagged rocks –Lack of simple craters  more complex craters, impact basins Rocks – basalts  geologically active world  lava flows, volcanoes, but no plate tectonics  “blob” tectonics Surface remelts every 0.5 billion years How do we know that it went through 5 planetary stages?

Atmospheres Primary atmospheres – Hydrogen and Helium –Why don’t we have H & He atmospheres? The Sun, combustion/freezing removes atmospheres Secondary atmospheres - main source is outgassing  gasses released when rocks melt Venus – thick, hot atmosphere due to the greenhouse effect  CO 2 stops IR light (heat) emitted from the surface from leaving the planet Earth – very little CO 2  plants! –Rocks also absorb CO 2  rusting –Ozone layer absorbs UV light  UV light does not break up water molecules  we have water and Venus does not Mars – thin atmosphere, no ozone –CO 2 in polar ice caps