10. Saturn: lord of the rings

    • Saturn has the lowest mass density of any planet in the solar system, low enough for the planet to float on water, and this means that Saturn is primarily composed of the lightest element hydrogen.

    • Saturnís rapid rotation has pushed its lightweight material out in the planetís middle, creating the most pronounced equatorial bulge of any planet.

    • Saturn is just a great big liquid drop, covered by a thin atmosphere of gas, slightly smaller than Jupiter and less than a third of its mass.

    • Liquid hydrogen is compressed inside Saturnís depths to form an electrically conducting, liquefied metal.

    • There is no solid surface anywhere inside Saturn, though it might have a core of melted ice and molten rock that is about ten times as massive as the Earth.

    • Saturn radiates almost twice as much energy as it receives from the Sun, and most of the planetís excess heat is generated by helium raining down into its liquid metallic hydrogen core.

    • Saturnís rings are completely detached from the planet and separated from each other.

    • The rings of Saturn are not solid, but instead composed of innumerable small water-ice particles and larger chunks of water ice.

    • The icy constituents of Saturnís main A, B and C rings are as big as hailstones, snowballs and even icebergs; there are more smaller ones, but the big kind supply most of the ring mass.

    • The total mass of the rings of Saturn is comparable to that of its medium-sized satellite Mimas, which is 396 kilometers across.

    • Saturn has a retinue of diffuse, tenuous, and nearly transparent rings, designated the D, E, F and G rings, that are most likely composed of microscopic ice crystals, smaller than snowflakes and about the size of the dust in your room.

    • Two small moons confine the edges of Saturnís narrow F ring and shepherd the ring particles between them.

    • The icy material in the prominent rings of Saturn has been marshaled into thousands of individual ringlets, resembling ripples on a pond, but with circular, oval and even spiral shapes.

    • Gravitational interaction with nearby, external satellites can sculpt Saturnís ring material into numerous ringlets and produce waves in it.

    • Small moons embedded within Saturnís rings can sweep out gaps, keeping them open and also sharpening their edges.

    • The gaps within Saturnís rings are not completely empty; the Cassini Division contains about 100 ringlets.

    • Enigmatic dark spokes stretch radially across Saturnís rings, moving at constant speed regardless of distance from the planet, in apparent violation of the laws of gravity.

    • Saturnís dark ring spokes consist of microscopic dust-sized particles that may become electrically charged and levitated above the larger ring particles. They might then be swept around Saturn by its rotating magnetic fields.

    • Planetary rings lie closer to a planet than its large satellites, within the Roche limit where the planetís tidal forces will rip a large satellite to pieces and prevent small bodies from coalescing to form a larger moon.

    • The rings of Saturn could have formed when a moon was pulled toward the planet by tidal forces and eventually ripped apart.

    • Small moons embedded in the rings of Saturn might sustain them.

    • Saturnís relatively small moon Enceladus emits jets of ice particles, powdery snow, water vapor and organic compounds, which vent from warm fissures, known as tiger stripes, in the moonís south polar crust.

    • The active jets on Enceladus suggest that tidal effects may make the moon hot inside. The tiger-stripe fractures could rub against each other, creating heat, or open to expose explosive ice to the vacuum of space. The interior ice might be melted into underground seas of liquid water containing organic chemical elements.

    • Saturnís largest moon, Titan, is a planet-sized world with a substantial atmosphere whose surface pressure is about 1.5 times the air pressure at sea level on Earth.

    • Titanís atmosphere is composed of 98.4 percent molecular nitrogen, nearly 1.6 percent methane, and trace amounts of other hydrocarbons; so nitrogen molecules are the main constituents of Titanís atmosphere, as they are in the Earthís air.

    • Hazy smog, composed of complex organic molecules, envelops Titanís atmosphere and hides its surface from view.

    • The Huygens Probe touched down on the surface of Titan on 14 January 2005, detecting methane rainfall and dark narrow riverbeds on the way down. The probe landed at equatorial latitudes, on a damp, moist riverbed littered with pebbles that were apparently rounded by flowing liquid.

    • Radar pulses from the Cassini spacecraft in orbit about Saturn have seen through the haze that shrouds Titan, revealing long, deep meandering channels on Titanís surface, which resemble terrestrial rivers but are attributed to flowing methane or ethane rather than water.

    • The Cassini radar instrument has imaged dark, flat, smooth places with shore-like boundaries. They have been attributed to large lakes of liquid methane and ethane, and the spectral signatures of liquid ethane have been detected in at least one of them.

    • Seasonal variations might account for the fact that there are now about 20 more lakes at high northern latitudes on Titan than high southern ones. Clouds may rain methane during winter in the north, when southern lakes are evaporating in the local summer.

    • Vast dunes accumulate near Titanís equator, shaped by strong winds blowing east to west. Unlike Earthís sand dunes, Titanís dunes are thought to be composed of organic material that has rained down from its smoggy skies.

    • The dunes and lakes on Titan may contain hundreds of times more hydrocarbons than all the oil and gas reserves on Earth.

    • Saturn has six mid-sized icy moons that retain impact craters dating back to their early history; some of them exhibit signs of internal activity and ice volcanism. Impacting objects almost broke the moons Mimas and Tethys apart.

    • A number of unique small, irregularly shaped moons revolve around Saturn with remarkable orbits. The co-orbital moons have almost identical orbits, the Lagrangian moons share their orbit with a larger satellite, and the shepherd moons confine the edges of rings.

    • Saturnís mid-sized moon Hyperion is so light that it must be about half filled with empty spaces, and it tumbles chaotically along its orbit with no definite rotation period or orientation in space.

    • The enigmatic moon Phoebe moves around Saturn in the opposite, retrograde direction to the planetís other mid-sized satellites. Phoebe has sharp-edged craters and a varying brightness that suggest thin dark surface deposits overlying bright water ice.

    • An enormous, exceptionally distant, wide and diffuse ring of Saturn is apparently replenished by micrometeorite impacts with Phoebe; the ejected material might also move in toward Saturn, striking the next innermost moon Iapetus and accounting for the dark side of its two-faced surface.

Copyright 2010, Professor Kenneth R. Lang, Tufts University