Astr 1050     Mon., Apr. 26, 2004
   Today:  Start Ch 17., Terrestrial Planets
  Recall: Nice webpage your classmate provided http://www.nationalgeographic.com/solarsystem/splash.html

Chapter 17: Terrestrial Planets
Comparative Planetology
Earth
History, Interior, Crust, Atmosphere
The Moon
In particular origin
Mercury
Venus
Mars
Including water (and life ?)

“Comparative Planetology”
Basis for comparisons is Earth
Properties of Earth
Similarities and differences with Mars and Venus help us understand Earth better (e.g., life, greenhouse effect, etc.)
Won’t spend much class time on basic properties (size, gravity, orbital period, length of day, number of moons, etc.) but you should have some relative ideas about these (see “Data Files” in text).

Four Stages of Planetary Development

Timeline

Earth’s Interior

The Active Earth
Plate tectonics, volcanoes, etc., a lot of action!

Earth’s Atmosphere: Greenhouse Effect

The Moon and Mercury
No atmosphere
Cratering is evidence of final planet assembly – lots to be learned from craters

Patterns in Geologic Activity
Judge age of surface by amount of craters:
more craters
Ţ more ancient surface
(for some objects, have radioactive age dates)
Moon “dead” after about 1 billion years
Mercury “dead” early in its lifetime
Mars active through ~1/2 of its lifetime
Venus active till “recent” times
Earth still active
Big objects cool off slower
Amount of heat (stored or generated) proportional to Volume ( so R3)
Rate of heat loss proportional (roughly) to Surface Area          (so R2)
Heat/(Unit Area) µ R3/R2 = R     so activity roughly proportional to R
Same reason that big things taken out of oven cool slower than small things     (cake cools slower than cookies)

What is a crater?
Must think of them as caused by very large explosions from release of kinetic energy of impactor
Like a mortar shell – it isn’t the size of the shell which matters,
its how much energy you get out of the explosion
DO NOT think of them as just holes drilled into surface – think EXPLOSION
Kinetic Energy E = ˝ m v2
v is roughly escape speed of earth
m = mass = volume * density       (Consider a 1 km asteroid)
E
This is ~4500 ´ the size of the largest (~50 Mt)  hydrogen bombs ever built
and this is for a relatively small size asteroid

Formation of an impact crater
Crater caused by the explosion
Impactor is melted, perhaps vaporized
 by the kinetic energy released
Temporary “transient” crater is round
Gravity causes walls to slump inward forming “terraces”
Movement of material inward from all sides (trying to fill in the hole) may push up central peak in the middle.
Final crater is typically ~10 times
 the size of the impactor

Examples of craters on the moon
Images on line at
The Lunar and Planetary Institute:
http://www.lpi.usra.edu/expmoon/lunar_missions.html
Detailed record of Apollo work at:
http://www.hq.nasa.gov/office/pao/History/alsj/frame.html

Superposition
(way to get relative ages)
Newer features are superposed
on top of older ones
Large impact forms basin
Basin floods with lava
Additional impacts occur in mare lava
Over time both crater rate and volcanic activity are declining
Craters less because debris swept up
Volcanism less because moon cooling

Problems with the Condensation Model:
Why is the moon so different than the earth?

Effects of late impacts

Moon: Giant Impact Hypothesis
Explains lack of large iron core
Explains lack of “volatile” elements
Explains why moon looks a lot like earth’s mantle, minus the volatiles
Explains large angular momentum in the earth-moon system

Relative size of core in Mercury

Venus

Expect Venus to be similar to Earth
(but it isn’t!)
Venus only slightly closer to sun, so expect about same initial composition
Venus only slightly smaller than Earth, so expect about same heat flow
Venus atmosphere is dramatically different
Very thick CO2 atmosphere
Virtually no water in atmosphere or on surface
Venus shows relatively recent volcanic activity, but no plate tectonics
Both probably related to its slightly closer position to the sun
which caused loss of its critical water
Thick atmosphere and clouds block direct view so information from:
Orbiting radar missions  (Magellan in early 90’s)
Russian landers (as in previous photo)

Why does Venus have much more CO2 in atmosphere than Earth?
Amount of CO2 in atmosphere on Venus roughly equal to
amount of CO2 in limestone on Earth
With no oceans, don’t have a way to get CO2 out of atmosphere and back into rocks
Runaway effect, because high T causes faster loss of water to space.
If H2O gets into upper atmosphere it is broken down into O, H by UV sunlight
H is so light it escapes to space
On Earth cooler T traps H2O in lower atmosphere (it condenses if it gets to high)
Location closer to the sun pushed Venus “over the edge”  compared to Earth

Surface Relief of Venus from Radar
Venus does show evidence of “recent” volcanism
It does not show linear ridges, trenches, or rigid plates
In a few spots there are weak hints of this – but clearly different

Volcanoes
Sapas Mons
Lava flows from central vents
Flank eruptions
Summit caldera
Size:
250 miles diameter
1 mile high

Lava Channels
Large!
100’s of miles long
1.2 miles wide
High Venus temperatures may allow very long flows
Composition could also be different

Pancake Domes
Pancake domes formed from very viscous lava

“Ticks”
Domes which have partially collapsed?

Corona and a possible model
Corona possibly due to upward moving plume of hot mantle which bow up surface, then spreads out and cools
(as in a “lava lamp”)

Slide 28

Lots of Martian Science Fiction
Best, most recent and scientifically accurate is probably Kim Stanley Robinson’s series:
Red Mars, Blue Mars, Green Mars
Terraforming/colonization of Mars

Mars and the Pattern of Geologic Activity
and Atmospheric Loss
Expect intermediate geologic activity based on size
RMars = 0.53 REarth         RMoon = 0.27 REarth
Earth still active but lunar mare volcanism ended ~3 billion years ago
Expect intermediate atmospheric loss
Smaller size will make atmospheric escape easier
Cooler temperature (farther from sun) will make astmospheric escape harder
In some ways Mars is most “Earth-like” planet
Has polar caps
Has weather patterns
Had (in past) running water
May have had conditions necessary for development of life

Why some atmospheres are lost
Compare velocity of gas atoms (Vgas)  to planet’s escape velocity Vesc
If any significant # of atoms have escape speed atmosphere will eventually be lost
In a gas the atoms have a range of velocities,
with a few atoms having up to about 10
´ the average velocity,
so we need 10
´ Vavg gas < Vesc to keep atmosphere for 4.5 billion years.
In above equations R = planet radius, M = planet mass, T = planet temperature,
m = mass of atom or molecule,   k and G are physical constants
Big planets have larger Vesc (i.e. larger M/RµR3/R) so hold atmospheres better
Earth would retain an atmosphere better than Mercury or the Moon
Cold planets have lower Vgas so hold atmospheres better
Saturn’s moon Titan will hold an atmosphere better than our moon
Heavier gasses have lower Vgas so are retained better than light ones
CO2 or O2 retained better than He, H2, or H
Even with “heavy” gasses like we H2O we need to worry about
loss of H if solar UV breaks H2O apart.  That is what happens on Venus.

Which planets can retain which gasses?

Mars atmosphere today
Pressure is only ~1% of Earth’s
Composition:  95% CO2    3% N2    2% Ar
Water:
Pressure too low for liquid water to exist
Water goes directly from solid phase to gas phase
CO2 (dry ice) acts like this even at terrestrial atmospheric pressure
Water seen in atmosphere
Water seen in polar caps
Evidence of running water in past
Carbon dioxide (CO2)
Gets cold enough for even this to freeze at polar caps
Unusual meteorology, as atmosphere moves from one pole to other each “year”

Mars dust storm

Sand Dunes on Mars
Spacecraft in Mars orbit
Mars Global Explorer
Mars Odyssey
Even though atmosphere is thin, high winds can create dust storms

Water ice clouds

Ancient River Channels?
(note channels older than some craters – by superposition)

Recent liquid water?
(water seeping out of underground “aquifer” ?)

Layered Deposits

Where is the water today?
Much may have escaped to space
Some is locked up in N Polar Cap
Much could be stored in subsurface ice (permafrost)
Mars Missions making progress this semester:
http://www.nasa.gov/vision/universe/solarsystem/mer_main.html
Location of water critical to knowing where to search for possible past life

“Comparative Planetology”
Think about how Venus, Earth, and Mars started out so similarly
Think about what properties led to the very different environments today
Think about how these issues may apply to the future of Earth, and even our prospects for terraforming (and there is a debate about whether we should terraform at all!).