Astr 1050     Mon., Apr. 25, 2005
   Today:  Solar System Overview

Chapter 16: Origin of the Solar System
Solar Nebula Hypothesis
Context for Understanding Solar System
Extrasolar Planets
Dust Disks, Doppler Shifts, Transits and Eclipses
Survey of the Solar System
Terrestrial Planets
Jovian Planets
Other “Stuff” including apparent patterns with application to the nebular hypothesis

Patterns in Motion
All planets orbit in almost the same plane (ecliptic, AKA Zodiac)
Almost all motion is counterclockwise as seen from the north:
All planets orbit in this direction
*Almost* all planets spin in same direction
with axes more-or-less perpendicular to ecliptic
Regular moons (like Galilean satellites and our own moon) orbit in this direction too
Planets are regularly spaced
steps increasing as we go outward

Spacing of Planets
Regular spacing of planets on a logarithmic scale
Each orbit is ~75% larger than the previous one
Need to include the asteroids as a “planet”

Solar Nebula Model
Planets form from disk of gas surrounding the young sun
Disk formation expected given angular momentum in collapsing cloud
Naturally explains the regular (counterclockwise) motion
Makes additional explicit predictions
Should expect planets as a regular part of the star formation process
Should see trends in composition with distance from sun
Should see “fossil” evidence of early steps of planet formation

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 100 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
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 8

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)
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

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, 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
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)

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