Download Saturn lecture

Saturn and it’s Rings
And while we’re at it, a little about
those other giants too
Outline
• Why bother
• Interiors of the gas giants
• Heat sources
• Atmospheres
• Storms
• Rings
Why bother?
• Even though they’re way out there, the giants
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have a far greater presence in our solar system
than any off the terrestrial planets combined
Jupiter’s gravity in particular has affected solar
system evolution
Need to understand how they formed to
understand how the solar system and Earth
formed
Giant Interiors
• the main constituents are hydrogen and helium, which
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made up 98% of the primordial ingredients in the solar
nebula
Pressure and temperature conditions were such that
methane, ammonia and water were stable in solid form
As you move out from the sun, each planet has less
hydrogen and helium and more ice
• The giants also have
giant cores, ~10-15 times
the size of Earth
Heat source
• Jupiter and Saturn both emit more energy
than they receive. Uranus emits the same
amount and Neptune just a little more,
but their great distance means they also
have an internal heat source
• Theory 1: As the planets cool from their
original accretion, their outer layers
contract
-gravitational energy
• Theory 1 works out okay for Jupiter, but not for
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Saturn. Saturn actually releases 3 times more
energy than it receives from the sun
Therefore, theory 2: helium rain
Saturn is cold enough that helium liquefies in its
outer layers and falls inward due to gravitational
attraction. As the helium falls through the liquid
hydrogen, it generates heat from friction, which
is in turn radiated away.
• Sounds crazy, but this theory would predict that
Saturn has less helium in it’s outer layers and
that’s exactly what is
observed
Gaseous
Exteriors
• The gaseous exteriors are much like the interiors, more
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hydrogen and helium, but more so on Jupiter and
Saturn, and Saturn is more depleted in helium due to
helium rain
Ammonia, methane and water ice are also present
The latter three are also some of the constituents in the
cloud layers
Winds
• zonal
jets: layers we see on the giants’ exteriors are
counterflowing eastward and westward jets
• These jets are fast, with Saturn’s equatorial jet being the
fastest at 500m/s
• banded clouds restricted to specific latitudes, perhaps due to
fast rotation speeds and lack of topography, such as
continents
Storms
• The Great Red Spot is 10 degrees wide (about
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as wide as Earth, outer edges have been clocked
at 400m/s, and is at least as old as first
observations of Jupiter
High pressure centers, occur between banded
cloud layers
Saturn also has storms, apparently every ~30
years, close to it’s orbital period
Neptune’s Great Dark Spot was drifting ~15
degrees toward the equator at last look
Theories on Storms
• Hurricane model (convective cells), with
energy recharged from interiors
• Shear instability: energy from the regions
between two zonal jets moving in opposite
directions
• Absorb small buoyancy driven eddies to
coalesce into a large storm
Planetary Rings
Rings in General
Saturn’s Rings
• Initially discovered by Galileo in 1610, though
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unknowingly
Christian Huygens realized they were rings in 1659
Oribital period: 5.6 hrs for inner rings, 14.2hrs for outer
rings
D, C, B, A, F, G and E rings (with A, B, and C visible from
Earth)
In equatorial plane
Thickness: ~20m
Total mass of ring system ~10^21kg
Formation of Rings
• Two theories:
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-remains of a broken satellite, unable to recoalesce
-particles never able to combine to form a
satellite in the first place (sub nebula)
Essential component of both: Roche’s limit (a.k.a
tidal stability limit), where tidal forces exceed
self-gravitational forces (depends on density and
internal strength of satellite)
Size and composition
• Range: mm (sand grains) to boulders and small
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satellites, but typically cm scale
Mostly water ice, reflecting 50-60% of incident
light, but some darker particles that may
represent silicate material
Ring Dynamics
• Interactions between small satellites and the
rings have various consequences:
– A 2:1 resonance between the B ring and Mimas forms
the Cassini division
– A 7:6 resonance between the A ring and Janus forms
the outer edge of the A ring
– The F ring is “herded” by two small satellites,
Prometheus and Pandora (“Shepard moons”)
– The gravitational influence from Pandora and
Prometheus also makes “braids” and other
instabilities in the F ring
Spokes!
• Occur in B ring
• Appear to be
Short lived
• Has something
To do with synchronous
Orbit?
The E Ring
• Particle size 1 micron
• Too small to be long lasting, therefore
recharged
• Enceladus! The peak density of this ring is
near Enceladus, evidence for the theory
that the ring is recharged by Enceladus’
geologic activity
A Final (maybe) Word about
Impact Cratering…
• “G” of Shoemaker-Levy 9
• We’ve seen an impact
structure on every other
planetary surface, so
here’s the last kind: