Transcript Quantum Well Electron Gain Structures and Infrared Detector Arrays

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 Rotation
•Astronomers 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!)
•Mercury 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 orbit
•Mercury presents the same face to the sun
every OTHER time around!
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Mercury’s 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)
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Mercury’s 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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Mercury’s 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?
• 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!
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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 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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Venus’s Atmosphere
•Much more massive
atmosphere than Earth’s
(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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Venus’s 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 (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 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 ~150yard resolution (better map of all Venus than of all
Earth!)
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Venus’s 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 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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