Astr 1050     Fri., Apr. 23, 2004
   Today:  Extra Credit Articles
Continue with the Solar System
Start Ch 17., Terrestrial Planets
  Recall: Nice webpage your classmate provided http://www.nationalgeographic.com/solarsystem/splash.html

Extra-Solar Planets
Hard to see faint planet right next to very bright star
Two indirect techniques available
(Like a binary star system but where 2nd “star” has extremely low mass)
Watch for Doppler “wobble” in position/spectrum of star
Watch for “transit” of planet which slightly dims light from star
About 110 planets discovered since 1996   See http://exoplanets.org/
Tend to be big (³Jupiter) and very close to star    (easier to see)

Characteristics of “Planets”
Two types of planets
Terrestrial Planets: small, rocky material, inner solar system
Jovian Planets: large, H, He gas, outer solar system
Small left-over material, “debris”, that provides “fossil” record of early conditions
Asteroids     mostly between orbits of Mars and Jupiter
Comets     mostly in outermost part of solar system
Meteorites –  material which falls to earth

Slide 4

Patterns in Composition
Terrestrial Planets
Relatively small
Made primarily of rocky material:
Si, O, Fe, Mg  perhaps with Fe cores
(Note – for earth H2O is only a very small fraction of the total)
Jovian Planets
Relatively large
Atmospheres made of H2, He, with traces of CH4, NH3, H2O, ...
Surrounded by satellites covered with frozen H2O
Within terrestrial planets inner ones tend to have higher densities
(when corrected for compression due to gravity)
       Planet Density Uncompressed Density
(gm/cm3)        (gm/cm3)
Mercury     5.44          5.30
Venus     5.24          3.96
Earth     5.50          4.07
Mars     3.94          3.73
(Moon)     3.36          3.40

Equilibrium Condensation Model
Start with material of solar composition material
  (H, He, C, N, O, Ne, Mg, Si, S, Fe ...)
Material starts out hot enough that everything is a gas
May not be exactly true but is simplest starting point
As gas cools, different chemicals condense
First high temperature chemicals, then intermediate ones, then ices
Solids begin to stick together or accrete
snowflakes Þ snowballs (“Velcro Effect”)
Once large enough gravity pulls solids together into planetesimals
planetesimals grow with size
At some point wind from sun expels all the gas from the system
Only the solid planetesimals remain to build planets
Composition depends on temperature at that point (in time and space)
Gas can only remain if trapped in the gravity of a large enough planet

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

Evidence of Assembly Process?    Craters

Craters evident on almost all small “planets”

Clearing of the Nebula
Radiation pressure  (pressure of light)
See present day effects in comets
Most important effect
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).  There will be a few exam questions!!!

Four Stages of Planetary Development

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
Þ 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 = ½ 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

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

Venus