Astr 1050    Fri., Dec. 6, 2002
   Today: Astronomy Articles for Extra Credit
             Finish Ch. 16, The Origin of the Solar System
             Start Ch. 17, Terrestrial Planets—will skip some slides

The Material Available
Inner solar system dominated by silicate rocks
SiO2 (quartz)     Mg2SiO4  Fe2SiO4 (olivine)     etc.
Outer solar system dominated by H2, He, ice (H2O)

Reason for Jovian Planets
Because you cannot condense O by itself (but only in compounds also containing Si, Mg, Fe), you don’t have much material available for making terrestrial planets.  You are limited by the low abundance of Si, Mg, Fe:  Terrestrial planets are relatively small
Once solid H2O becomes available you have lots more material
Starting at Jupiter you can make a big enough core from solid H2O that you can gravitationally hold onto the H and He gas

Growth of the Planetisimals
Once a planetisimal reaches critical size gravity takes over

Growth and Differentiation of Planets
Planet forms from homogeneous mix of material
Planet heats up
“Heat of formation”
(i.e. energy from gravity)
Heat from radioactive decay of U, etc.
Dense material (Fe) sinks to center
Certain “siderophile” elements (like Ni)
Other “lithophile” elements remain behind
Homogeneous model too simple
Final collisions can be big:
Little planetesimals first form bigger ones, then bigger ones collide to form yet bigger ones
Moon may be result of impact of Mars size body as Earth formed       (more later)
First material to condense might separate out early

Heterogeneous Growth of Planets
Fe is among the first materials to condense as nebula cools
Might form iron cores before lower temperature materials condenses
Has implications for separation of lower temperature “siderophiles” during later differentiation

Evidence of Assembly Process?    Craters

Craters evident on almost all small “planets”

Even larger planets typically have some

Clearing of the Nebula
Radiation pressure  (pressure of light)
Will see present day effects in comets
Solar Wind
Strong solar winds from young T Tauri stars
Will see present day effects in comets
Sweeping up of debris into planets
Late Heavy Bombardment
Ejection of material by near misses with planets
Like “gravity assist maneuvers” with spacecraft
Origin of the comets

Patterns and Predictions
Why do different planets have different levels of geologic activity?
Why do different planets have different atmospheres?
What are ages of old “unaltered” planetary surfaces?
Should be similar, and agree roughly with age of Sun
Does composition of asteroids match predictions?
Lower temperature than Mars region:  Hydrated silicates, etc.
What types of minerals do we see in meteorites?
What types of ices and minerals do we see in comets?

Chapter 17: Terrestrial Planets
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, 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 techtonics, volcanoes, etc.

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 of 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

Why do lava flows come out in mare basins?
Mare basins are the lowest areas of the planet
The crust beneath them is badly fractured by the impacts
When do the lavas come out?
Superposition only gives relative ages
Can use crater counts to estimate absolute ages – but need to know crater rates
Apollo missions provided samples from which we have radioactive decay ages

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 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 lost of its critical water
Thick atmosphere and clouds block direct view so information from:
Orbiting radar missions  (Magellan in early 90’s)
Russian landers

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 39

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 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
Boiling point drops with pressure
Freezing point doesn’t change much with pressure
Eventually boiling point reaches freezing point
Water goes directly from solid phase to gas phase
CO2 (dry ice) is 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
Two spacecraft now 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 Global Observer and Mars Odyssey
studying these issues now
Location of water critical to knowing where to search for possible past life