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 our own moon) orbit in this direction, too
Planets are regularly spaced
steps increasing as we go outward

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 7

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

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

Expect Venus to be similar to Earth?
(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

Slide 33

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

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!).

Chapter 18: Worlds of the Outer Solar System
Jupiter
Condensation model
Atmospheric winds
Atmospheric chemistry
Magnetic fields
Other Jovian Planets (Saturn, Uranus, Neptune)
will only cover major differences from Jupiter
Satellites (i.e. Moons)

Jovian Planets

Ice+Rock Core    H+He “Atmosphere”

Details of the atmosphere
Mostly made of H, He
Trace amounts of C, N, O, S
CH4 present as gas
NH3, NH4SH, H2O can condense in colder upper regions Þ clouds
Colors from unknown trace chemicals
Density of gas smoothly increases with depth till point where it is indistinguishable from liquid 
 
Þ no real “surface”
At very high temperatures and pressures hydrogen becomes a “metal” and conducts electricity
 
Þ generates magnetic field

Jupiter as seen by Cassini

Magnetic fields and trapped particles

Aurora on Jupiter

Comparison of Jovian Planets
Variation in distance presumably ultimate causes other effects
P:      Kepler’s third law
T:      Falloff mostly just result of falling solar energy
But Neptune hotter because more internal heat
M:     Clue to details of solar nebula mode
Less material in outer solar system – or perhaps less efficient capture
r:      Should drop with mass because less compression
Works for Saturn vs. Jupiter
Increase for Uranus, Neptune indicates less H, He and more heavy material

Saturn as seen by the Hubble Space Telescope

The Roche Limit
When can tides tear a moon apart?
As a planetary body get close to another object, tidal forces distort the body more and more.
Remember, Earth raises tides on the Moon
just like it raises tides on the Earth
If the distortion gets large enough, the moon will be pulled apart
Happens at “Roche Limit” when moon is
~2.44
´ radius of planet away
At that point, tidal force pulling up on surface of moon is greater than moon’s gravity pulling down
Only matters for objects held together by gravity
Astronaut in orbit will not be pulled apart
Is held together by much stronger chemical forces
Astronaut standing on the outside of the shuttle, hoping the shuttle’s gravity would hold her there, will be pulled away from the shuttle

Rings are individual particles all orbiting separately
Each particle – dust to golf ball to boulder size –
is really a separate moon on its own orbit
Orbit with Keplerian velocities:  high in close, slow farther out
Nearby relative velocities are low – so particles just gently bump into each other – slowly grinding themselves up
Structure in rings largely caused by gravity of moons

Comparison of Rings
All within Roche limit
Details controlled by Resonances and Shepard Satellites

Jupiter as a miniature solar system
Four large moons (Io, Europa, Ganymede, Callisto)
Regular (equatorial, circular) orbits
Pattern of changing density and composition with distance
Inner two (Io, Europa) mostly rocky
Outer two (Ganymede, Callisto) more icy

Io, Europa break rules about activity
Io most volcanically active body in solar system
Europa shows new icy surface with few craters

Tidal heating explains activity
Large tides from Jupiter flex satellites
Friction from flexing heats interiors
Important for Io, Europa, some other outer solar system satellites

Possible H2O ocean on Europa
Tidal heating may keep H2O liquid under ice cover
Perhaps a location where life could evolve
“Europa Orbiter” Mission being planned to determine if ocean exists

Callisto not active

Comparison of Satellites

Titan
Largest moon of Saturn
Has thick atmosphere
Pressure ~ 1 earth atmosphere
Mostly N2, some CH4
Gas held because of low T
UV acting on CH4 Þ smog
Ethane produced – Lakes?
Can “see” surface only in IR
Cassini dropped a probe in early 2005.
“Code of the Lifemaker” by James P. Hogan, good sf

Chapter 19:  Meteorites, Asteroids, Comets
Small bodies are not geologically active
They provide “fossil” record of early solar system
Asteroids
Mostly from region between Mars and Jupiter
Left over small debris from accretion, never assembled into a large planet
Meteorites come mostly from asteroids
Comets
“Stored” on large elliptical orbits beyond planets
Thought to be “planetesimals” from Jovian planet region, almost ejected from solar system in its early history
Meteorites provide only samples besides Apollo
With sample in hand, can perform very detailed analysis:  detailed chemistry; radioisotope age; other isotope info

Asteroids
Most located between Mars and Jupiter
Largest is Ceres
1/3 diameter of moon
Most much smaller
>8,000 known
Total mass << Earth
A few make it to earth
source of the meteorites

The larger asteroids

Are Asteroids Primitive?
Ida (56 km diam.) and its moon Dactyl (1.5 km diam.)
Colors have been “stretched” to show subtle differences
Imaged by Galileo on its way out to Jupiter

Phobos & Deimos:  Two “misplaced” asteroids?
Phobos and Diemos are small (~25 km and ~15 km diam.) moons of Mars
Look like captured asteroids rather than moons formed in place
Are “C” class – i.e. dark “Carbonaceous” type “asteroids”

Types of Meteorites
Three main kinds of meteorites
Carbonaceous chondrites: Most primitive material – dark because of C
Stones Similar to igneous rocks
Irons Metallic iron – with peculiarities

Meteors vs. Meteorites
Meteor is seen as streak in sky
Meteorite is a rock on the ground
Meteoroid is a rock in space
Meteor showers (related to comet orbits) rarely produce meteorites
Apparently most comet debris is small and doesn’t survive reentry
Meteorites can be “finds” or “falls”
For a fall – descent actually observed and sometimes orbit computed
Most have orbits with aphelion in asteroid belt

Large Meteor over the Tetons (1972)

The Leonids  2001
APOD site:  Picture by Chen Huang-Ming

Meteor Showers and Comets
Meteor showers caused by large amount of small debris spread out along comet orbits
Almost none makes it to the ground – no meteorites
Occur each year as earth passes through orbit of comet
Appears to come from “radiant point” in sky
Leonids:  Mid November

Comets:         Hale-Bopp in April 1997

Comet characteristics
Most on long elliptical orbits
Short period comets – go to outer solar system
“Jupiter family” still ~ in plane of ecliptic
“Halley family” are highly inclined to ecliptic
Longer period ones go out thousands of AU
Most of these are highly inclined to ecliptic
Become active only in inner solar system
Made of volatile ices and dust
Sun heats and vaporizes ice, releasing dust
“Dirty snowball” model

Comet structure
Gas sublimates from nucleus
Dense coma surrounds nucleus
Ion tail is ionized gas points directly away from sun
shows emission spectrum
ions swept up in solar wind
Dust tail curves slightly outward from orbit
shows reflected sunlight
solar radiation pressure gently pushes dust out of orbit

Hale-Bopp clearly shows components

Where do comets come from?
Long period comets:  The Oort Cloud
Most (original) orbits have aphelions of  >1000 AU
Need ~6 trillion comets out there to produce number seen in here
Total mass of 38 MEarth
Passing stars deflect comets in from the cloud

Importance of comets
Evidence of solar nebula
Source of H2O and CO2 for earth
Impacts continue
Impacts on Earth
Extinction of the dinosaurs
SL-9 impact on Jupiter

Pluto and Charon