Life on Mercury & Venus

Stephen Eikenberry

25 March 2013

AST 2037

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

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Mercury’s Orbit and RotationMercury’s Orbit and Rotation

•Astronomers initially thought Mercury’s rotational and orbital Astronomers initially thought Mercury’s rotational and orbital periods were the same - same side always faces sunperiods were the same - same side always faces sun•Radar observations showed rotational period = 59 days (orbital Radar observations showed rotational period = 59 days (orbital period = 88 days!)period = 88 days!)

•Mercury is not tidally locked to the sun in Mercury is not tidally locked to the sun in the same way as the moon-earth systemthe same way as the moon-earth system•Sun’s gravity and Mercury eccentric orbit Sun’s gravity and Mercury eccentric orbit brought it into semi-synchronous orbitbrought it into semi-synchronous orbit•Mercury presents the same face to the sun Mercury presents the same face to the sun every OTHER time around!every OTHER time around!

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Mercury’s AtmosphereMercury’s Atmosphere

• Almost non-existent - high surface temperature and Almost non-existent - high surface temperature and low escape velocitylow escape velocity

• no atmosphere means drastic temperature changes no atmosphere means drastic temperature changes 700 K (day) = 800 F 700 K (day) = 800 F 100 K (night) = -280 F 100 K (night) = -280 F

• what little atmosphere there is comes from solar wind what little atmosphere there is comes from solar wind particles (Hydrogen and Helium)particles (Hydrogen and Helium)

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Mercury’s SurfaceMercury’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 Craters flatter and have thinner ejecta rims than lunar craters ejecta rims than lunar craters due to higher gravity on due to higher gravity on Mercury than the moonMercury than the moon•Craters not as dense as on the Craters not as dense as on the moon - filled by volcanic moon - filled by volcanic activity - but not dark like activity - but not dark like “maria”“maria”

Mariner 10 image from mid-1970s

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Mercury’s SurfaceMercury’s Surface• ScarpsScarps (or cliffs) are seen (or cliffs) are seen on the surfaceon the surface•NOT seen on the moonNOT seen on the moon•appear to be about 4 appear to be about 4 billion years oldbillion years old•not the result of plate not the result of plate tectonicstectonics•probably the result of the probably the result of the surface cooling, shrinking surface cooling, shrinking and splitting at this timeand 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

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• 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 …

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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!

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Venus’s RotationVenus’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

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Venus’s AtmosphereVenus’s Atmosphere

•Much more massive Much more massive atmosphere than Earth’s atmosphere than Earth’s (surface pressure is 90x (surface pressure is 90x Earth!)Earth!)•Surface temperature is Surface temperature is 730 K (!!!)730 K (!!!)•Carbon dioxide (96.5%), Carbon dioxide (96.5%), Nitrogen (3.5%)Nitrogen (3.5%)•No water - the clouds are No water - the clouds are made of sulfuric acid made of sulfuric acid dropletsdroplets

Pioneer UV image taken 1979

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Venus’s AtmosphereVenus’s Atmosphere•Fast moving clouds 50-70km above Fast moving clouds 50-70km above

surfacesurface

•Haze 30-50 kmHaze 30-50 km

•Clear air below 30 kmClear air below 30 km

•Upper atmosphere is very windy - Upper atmosphere is very windy -

400 km/hour400 km/hour

•Wind speed decreases lower in the Wind speed decreases lower in the

atmosphereatmosphere

•Temperature and pressure increase Temperature and pressure increase

closer to the surfacecloser to the surface

Why is Venus So Hot?• F = L/4d2 Venus receives about twice the solar

radiation as Earth per unit area• F_emitted = 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?)

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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!)

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Venus’s SurfaceVenus’s Surface

Radar (radio waves) echoes reveal the surface topologyRadar (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

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A few Soviet spacecraft have landed on Venus in 1970sA 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!)

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Venus’s Surface: Volcanoes and Venus’s Surface: Volcanoes and CratersCraters

•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 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

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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 (!)

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