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Life_on_Mercury_Venus - Life on Mercury& Venus Stephen...

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Unformatted text preview: Life on Mercury & Venus Stephen Eikenberry 26 October 2010 AST 2037 1 Life on Mercury • Can the closest planet to the Sun support life? 2 Mercury’s Orbit and Rotation •Astronomers initially thought Mercury’s rotational and orbital Astronomers periods were the same - same side always faces sun periods •Radar observations showed rotational period = 59 days (orbital Radar period = 88 days!) period •Mercury is not tidally locked to the sun in Mercury the same way as the moon-earth system the •Sun’s gravity and Mercury eccentric orbit Sun’s brought it into semi-synchronous orbit brought •Mercury presents the same face to the sun Mercury every OTHER time around! every 3 Mercury’s Atmosphere • Almost non-existent - high surface temperature and Almost low escape velocity low • no atmosphere means drastic temperature changes no 700 K (day) = 800 F 100 K (night) = -280 F • what little atmosphere there is comes from solar wind what particles (Hydrogen and Helium) particles 4 Mercury’s Surface •Similar to the moon - craters! •Old surface •No plate tectonics •Craters flatter and have thinner Craters ejecta rims than lunar craters due to higher gravity on Mercury than the moon Mercury •Craters not as dense as on the Craters moon - filled by volcanic activity - but not dark like “maria” “maria” Mariner 10 image from mid-1970s 5 Mercury’s Surface • Scarps (or cliffs) are seen Scarps on the surface on •NOT seen on the moon •appear to be about 4 appear billion years old billion •not the result of plate not tectonics tectonics •probably the result of the probably surface cooling, shrinking and splitting at this time and 6 Water on Mercury? • It’s WAY HOT there – don’t be stupid!! • Besides, no atmosphere (water would boil off, right?) • We thought so … • But, only ~1/2 of the surface mapped by Mariner • Radar map made in 1991 shows large reflections at North Pole • Similar to radar signature from ice on Mars polar caps • Believe Mercury has ice at the bottom of craters near the pole • Permanent shade ⇒ permafreeze! 7 Why Venus? • Duh … 8 OK … Really • Physical properties of Venus: • Diameter = 0.95 * Earth • Mass = 0.8 *Earth • Distance from Sun = 0.72 * Earth • Solid surface, with atmosphere (incl. clouds) • Venus is commonly known as our “twin” planet! 9 Venus’s Rotation •Rotation direction is retrograde (opposite that of other terrestrial planets)! •243 day rotation period •Axis is almost exactly perpendicular to orbit •Why? Possibly hit by large body during formation altering spin direction 10 Venus’s Atmosphere •Much more massive Much atmosphere than Earth’s (surface pressure is 90x Earth!) Earth!) •Surface temperature is Surface 730 K (!!!) 730 •Carbon dioxide (96.5%), Carbon Nitrogen (3.5%) Nitrogen •No water - the clouds are No made of sulfuric acid droplets droplets Pioneer UV image taken 1979 11 Venus’s Atmosphere •Fast moving clouds 50-70km above Fast surface surface •Haze 30-50 km •Clear air below 30 km •Upper atmosphere is very windy Upper 400 km/hour 400 •Wind speed decreases lower in the Wind atmosphere atmosphere •Temperature and pressure increase Temperature closer to the surface closer 12 Why is Venus So Hot? • F = L/4π d2 ⇒ Venus receives about twice the solar radiation as Earth per unit area • F_emitted = σ T4 ⇒ equilibrium temperature scales up by (2)1/4 power ⇒ about 20% higher temperature • This is 20% of 300K, or about 60K higher • Expect Venus to be at about 170 °F (but really is more like 900 °F) • • • • Atmosphere is largely CO2 (a greenhouse gas) This traps solar radiation more effciciently Temperature much higher this way What about “cool spots”? Not likely (why?) 13 Runaway Greenhouse Effect • On Earth, most CO2 from atmosphere has been locked up in limestone (CaCO3), slowly getting released/replenished via combo of rain and plate tectonics • If temps higher (i.e. move Earth closer to the Sun, by magic), more water vapor in the air • Water vapor increases infrared absorption from the ground (it’s an EXCELLENT greenhouse gas!) • Temps rise some more, more water evaporates, more greenhouse effect, temps rise more, etc. • Once hot enough, water vapor rises to top of atmosphere, and solar UV light dissociates it into H + O • Lighter H drifts off into space (which is why Venus has 150x the deuterium of Earth!) 14 Venus’s Surface Radar (radio waves) echoes reveal the surface topology •Elevated “continents” make up 8% of the surface •Mostly rolling plains with some mountains (up to 14 km) •No tectonics •Buckled and fractured crust with numerous lava flows 15 A few Soviet spacecraft have landed on Venus in 1970s •Survived only an hour before burning up •little evidence of erosion - young surface •rocks are basaltic and granite •some craters (very few) caused by meteoric impact 16 Venus: Magellan Mission • Satellite orbiting Venus in 1990s • Precision radar mapping of entire surface with ~150yard resolution (better map of all Venus than of all Earth!) 17 Venus’s Surface: Volcanoes and Craters Venus’s •Volcanoes resurface the planet every ~300 million years •Shield volcanoes are the most common (like Hawaiian Islands) • A caldera (crater) is formed at the summit when the underlying lava withdraws •Largest volcanic structures are called coronae - upwelling in the mantle which causes the surface to bulge out - not a full-fledged volcano. •Usually surrounded by other volcanoes •Venus is thought to still be volcanically active today (Magellan lava flows) 18 How did Venus get this way? • Runaway greenhouse raises temps, gets rid of surface water early on • Early plate tectonics brings up internal H2O and CO2 from early limestone into atmosphere • No surface water ⇒ no way of trapping CO2 , so it stays in the atmosphere now (huge pressure, greenhouse high temps) • Internal water lost ⇒ important lubricant for plate tectonics; plate tectonics stops • Volcanoes continue 19 Summary • Mercury has no atmosphere, little water (frozen in caps), and extreme hot/cold temps • Venus is Earth’s twin superficially, but has huge pressure, no water, sulfuric acid for rain, and temps that can melt lead (pretty much everywhere) • Reasons for Venus situation indicate Earth would look like that too if it was at Venus distance from Sun • So … in general it is unlikely that planets this close to a star like the Sun will be able to support life (!) 20 ...
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