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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 • • 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 • • 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 • • 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 • • 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 • • • 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 • • • • • • 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: • -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 • 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: