Life on Mercury & Venus Stephen Eikenberry 25 March 2013 AST

Life on Mercury Can the closest planet to the Sun support life? 2

3 Mercury’s Orbit and Rotation Astronomers initially thought Mercury’s rotational and orbital periods were the same - same side always faces sunAstronomers initially thought Mercury’s rotational and orbital periods were the same - same side always faces sun Radar observations showed rotational period = 59 days (orbital period = 88 days!)Radar observations showed rotational period = 59 days (orbital period = 88 days!) Mercury is not tidally locked to the sun in the same way as the moon-earth systemMercury is not tidally locked to the sun in the same way as the moon-earth system Sun’s gravity and Mercury eccentric orbit brought it into semi-synchronous orbitSun’s gravity and Mercury eccentric orbit brought it into semi-synchronous orbit Mercury presents the same face to the sun every OTHER time around!Mercury presents the same face to the sun every OTHER time around!

4 Mercury’s Atmosphere Almost non-existent - high surface temperature and low escape velocityAlmost non-existent - high surface temperature and low escape velocity no atmosphere means drastic temperature changes 700 K (day) = 800 F 100 K (night) = -280 Fno atmosphere means drastic temperature changes 700 K (day) = 800 F 100 K (night) = -280 F what little atmosphere there is comes from solar wind particles (Hydrogen and Helium)what little atmosphere there is comes from solar wind particles (Hydrogen and Helium)

5 Mercury’s Surface Similar to the moon - craters!Similar to the moon - craters! Old surfaceOld surface No plate tectonicsNo plate tectonics Craters flatter and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moonCraters flatter and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moon Craters not as dense as on the moon - filled by volcanic activity - but not dark like “maria”Craters not as dense as on the moon - filled by volcanic activity - but not dark like “maria” Mariner 10 image from mid-1970s

6 Mercury’s Surface Scarps (or cliffs) are seen on the surface Scarps (or cliffs) are seen on the surface NOT seen on the moonNOT seen on the moon appear to be about 4 billion years oldappear to be about 4 billion years old not the result of plate tectonicsnot the result of plate tectonics probably the result of the surface cooling, shrinking and splitting at this timeprobably the result of the surface cooling, shrinking and splitting at this time

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 7 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!

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

10 Venus’s Rotation Rotation direction is retrograde (opposite that of other terrestrial planets)!Rotation direction is retrograde (opposite that of other terrestrial planets)! 243 day rotation period243 day rotation period Axis is almost exactly perpendicular to orbitAxis is almost exactly perpendicular to orbit Why? Possibly hit by large body during formation altering spin directionWhy? Possibly hit by large body during formation altering spin direction

11 Venus’s Atmosphere Much more massive atmosphere than Earth’s (surface pressure is 90x Earth!)Much more massive atmosphere than Earth’s (surface pressure is 90x Earth!) Surface temperature is 730 K (!!!)Surface temperature is 730 K (!!!) Carbon dioxide (96.5%), Nitrogen (3.5%)Carbon dioxide (96.5%), Nitrogen (3.5%) No water - the clouds are made of sulfuric acid dropletsNo water - the clouds are made of sulfuric acid droplets Pioneer UV image taken 1979

12 Venus’s Atmosphere Fast moving clouds 50-70km above surfaceFast moving clouds 50-70km above surface Haze kmHaze km Clear air below 30 kmClear air below 30 km Upper atmosphere is very windy km/hourUpper atmosphere is very windy km/hour Wind speed decreases lower in the atmosphereWind speed decreases lower in the atmosphere Temperature and pressure increase closer to the surfaceTemperature and pressure increase closer to the surface

Why is Venus So Hot? F = L/4  d 2  Venus receives about twice the solar radiation as Earth per unit area F_emitted =  T 4  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 CO 2 (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 CO 2 from atmosphere has been locked up in limestone (CaCO 3 ), 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

15 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

16 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

Venus: Magellan Mission Satellite orbiting Venus in 1990s Precision radar mapping of entire surface with ~150- yard resolution (better map of all Venus than of all Earth!) 17

18 Venus’s Surface: Volcanoes and Craters 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) 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

How did Venus get this way? Runaway greenhouse raises temps, gets rid of surface water early on Early plate tectonics brings up internal H 2 O and CO 2 from early limestone into atmosphere No surface water  no way of trapping CO 2, 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