Astr 1050     Fri., Apr. 30, 2004
   Today: Extra Credit Articles
Ch. 18, Jovian Planets
  Recall: Nice webpage your classmate provided http://www.nationalgeographic.com/solarsystem/splash.html

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 Ž clouds
Colors from unknown trace chemicals
Density of gas smoothly increases with depth till point where it is indistinguishable from liquid 
 
Ž no real “surface”
At very high temperatures and pressures hydrogen becomes a “metal” and conducts electricity
 
Ž generates magnetic field

Jupiter as seen by Cassini

Winds on Jupiter

Winds near the Great Red Spot

Hurricanes exist because
Low Pressure trying to turn winds to the left
almost balance
Coriolis Force trying to turn winds to the right.

Jupiter has multiple cloud decks as air rises in low pressure “zones”
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 Ž clouds
Colors from unknown trace chemicals

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

Effects of T (and E) on Atmospheres
Saturn’s bands much less distinct than Jupiter’s
Temp. lower on Saturn Ž cloud condense lower
Deeper clouds Ž markings less visible
Differences at Uranus and Neptune
Even colder Ž clouds even deeper
So cold CH4 can condense
Little solar energy to drive weather
Uranus has strange seasons – tipped on its side
Neptune has strong internal heat source,
so it still can have weather
Large amounts of heavy elements compared to amount of H, He on Jupiter, Saturn
Large amounts of CH4 gas absorb red,
 make planets appear blue

Implications of M, r for nebula
Relative amount of H, He (compared to heavy elements) drop for Saturn
then drop dramatically at Uranus and Neptune
Why were these outer planets so less efficient at capturing H, He?
Their mass is still great enough to do this, especially given low temperatures
 in the outer solar system
May be a problem of timing
Accretion takes longer in the outer solar system because
The velocities of all objects there are much less
The distances between objects are greater
This is the same reason the periods of the orbits are so long
Uranus and Neptune may have only started to grow to critical size by the time the H, He gas was being driven out of the solar system

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 Ž smog
Ethane produced – Lakes?
Can “see” surface only in IR
Cassini will drop probe in Fall 2004
“Code of the Lifemaker” by James P. Hogan, good sf

Triton
Largest moon of Neptune
In unusual retrograde orbit
Probably captured after it formed
Tides during capture may have caused heating
Does have thin atmosphere
Shows recent “activity”
Not volcanic – rather volatile related
Ices migrate with seasons
“Geysers” caused by heated ices