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Greenhouse effect. Provides right conditions for liquid water on surface. Most of O2 on Earth atms today is from photosynthesiscreased by life from CO2 and sunlight. O2 began to build up in the air around 2 bil yrs ago (reached modern levels less than 500 mil yrs ago). Earth’s atms provides protection from harmful radiation (gamma rays and x rays are absorbed by O2 very high in atms, UV light absorbed by ozone in stratosphere). Greenhouse gases(CO2, H2O, methane) absorb and trap infrared radiation (heat). Effects warm Earth’s surface by 31 C. Earth’s liquid water: 70% of earth’s surface, vital for all known forms of life, only planets w abundant liquid water on its surface. Water exists on other worlds generally as ice/water vapor. Earth has liquid surface water because the surface is the right temp and pressure for liquid water (optimal distance from sun, atms provides moderate greenhouse effect). Venus today:very thick CO2 atms w a surface pressure 90x Earth’s. Reflective clouds contain droplets of sulfuric acid, slow rotation produces v weak surface winds. HOT AND INHOSPITABLE. Doesn’t cool off at night, sulfuric acid rain evaporates before touching ground. No weather, only slow steady breeze. Venus got hot from massive amounts of CO2 in Venus’ atms creating a very strong greenhouse effect. Venus’ atms is dominated by CO2 w negligible H2O, while Earth’s atms is only 0.03 CO2 and surface is covered by H2O. Most of Earth’s CO2 is locked away in carbonate rocks (like limestone).On Venus,water evaporated into the atms, too hot for liquid water. Solar UV broke H2O into H and O atoms (no ozone!). The solar wind strips away very light hydrogen atoms. Mars: has most earth-like surface.Iron oxide in soil makes it reddish. Rotation period of 24 hrs 37 min. Atms is just 1% of Earths (greenhouse effect only raises temp by 6 deg; its COLD! (-87 to -5C)). Martian dust storms can envelop the entire planet! Martian seasons: mars’ axis tilt is 25 deg, nearly the same as Earth’s. Mars has seasons. Polar ice caps grow in winter and retreat in summer. Mars atms is colder and lower pressure than Earth.There are currently water-ice clouds and polar ice caps on Mars. Frost forms on surface rocks in the cold winters. Ice just below surface which sublimated within days of being exposed. Fresh craters also expose subsurface ice. Warm-season flows might be evidence of salty liquid water active on Mar’s today. Mars must have had a thicker atms when it was young (higher pressure, stronger greenhouse effect, surface liquid water). But Mars was too small(geological activity stopped, atms escaped to space, planet froze). Earth is habitable due to distance (right distance from sun for liquid water to exist) and size (large enough to remain geologically active, plate tectonics and volcanic outgassing are important to the carbon cycle). Jovians: rapid rotation.Jupiter/Saturn (brownish, ~10 hr rotation, ~10 earth dia, ~100-300 Earth mass) vs Uranus/Neptune (bluish, ~17 hr rotation, ~4 earth dia, ~15 earth masses). Jupiter/Saturn: gas giants. Mostly hydrogen and helium, atms ~1000km thick, liquid h interiors (high pressures, liquid metal at v high pressures), rocky