Life on Mercury

1
Life on Mercury Venus

• Stephen Eikenberry
• 26 October 2010
• AST 2037

2
Life on Mercury

• Can the closest planet to the Sun support life?

3
Mercurys Orbit and Rotation

• Astronomers initially thought Mercurys
  rotational and orbital periods were the same -
  same side always faces sun
• 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 system
• Suns gravity and Mercury eccentric orbit brought
  it into semi-synchronous orbit
• Mercury presents the same face to the sun every
  OTHER time around!

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Mercurys Atmosphere

• Almost non-existent - high surface temperature
  and low escape velocity
• no 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)

5
Mercurys Surface

• Similar to the moon - craters!
• Old surface
• No plate tectonics
• Craters 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

Mariner 10 image from mid-1970s
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Mercurys Surface

• Scarps (or cliffs) are seen on the surface
• NOT seen on the moon
• appear to be about 4 billion years old
• not the result of plate tectonics
• probably the result of the surface cooling,
  shrinking and splitting at this time

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Water on Mercury?

• Its WAY HOT there dont 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!

8
Why Venus?

• Duh

9
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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Venuss 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

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Venuss Atmosphere

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

Pioneer UV image taken 1979
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Venuss Atmosphere

• Fast moving clouds 50-70km above surface
• Haze 30-50 km
• Clear air below 30 km
• Upper atmosphere is very windy - 400 km/hour
• Wind speed decreases lower in the atmosphere
• Temperature and pressure increase closer to the
  surface

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Why is Venus So Hot?

• F L/4?d2 ? 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 (its 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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Venuss 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

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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

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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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Venuss Surface Volcanoes and Craters

• 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)

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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 Earths 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 (!)