2000 March 16, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS Ch. 20 (Titan) *and* PBD Ch. 7 - Read TNSS Ch. 21 (Triton, Pluto, and Charon) *and* PBD Ch. 9, for Tuesday, March 21. - Read TNSS Ch. 22 (Icy Satellites) & 23 (Small Worlds), for Thursday, March 23. --------------- The Galilean Satellites (continued): ----------------------------------- Ganymede & Callisto, the fraternal twins (but not in mythology) ---------------------------------------- Ganymede: --------- Larger than Mercury; icy surface, but dirty (albedo, density = 1.94 g/cm^3; water is 1 g/cm^3) ~ 60% silicates, ~ 40% ice Two distinctive types of terrain, light and dark. Like Europa, this is because of texture. About half of planet (darker): very ancient cratered terrain, > 4 Gyr old How do we know, without having been there? Why should one wary of the accuracy of this figure? Many multiple concentric rings, here called *furrows*. Are much more common than in the concentric basin rings of terrestrial bodies (e.g. Mare Orientale on Earth's Moon; Caloris basin on Mercury). => From different material properties of ice: rings frozen in more readily, makes more rings, and slow, glacier-like flow of ice causes infilling, making *palimpsests*. (Palimpsests also used in Sagan's novel Contact: remember how?) Slushy ice flow affects craters in other ways: "dome" craters. Also chains of craters, called "catena" (also seen on Callisto): Perhaps from comets sheared apart by Jupiter, like Shoemaker-Levy 9 (the one observed to hit Jupiter in July 1994). Lighter half of Ganymede shows geological activity: Novel "grooved terrain" => like *folding mountains*, in ice! Subsidence, oozing in some craters: also liquid H_2 O, seeping out of surface? => Cryovolcanism quite common; Age: very uncertain, 10^8 to 4 Gyr. Graben: faulting, but from volcanism. (Rocky graben also are present on Earth and Mars.) Lava cooled, shrank, and cracked. Also genuine faulting! Perhaps not true plate tectonics, though, since little new activity (a situation not unlike Earth's Moon). Callisto: -------- Dirty ice (density = 1.85 g /cm^3), slightly smaller than Ganymede, but: *Densely* cratered: saturation cratered, like parts of the Lunar Highlands. If any more meteorites hit, they would wipe out as many craters as they made! Valhalla impact basin: over 10 concentric rings (Also Asgard) => *Very* ancient surface, among oldest of any large Solar System body, > 4 Gyr (How old is the Earth's Moon's surface?) Some geological evolution, though: smaller craters (in high-res Galileo images) are surprisingly rare => Slow erosion, again from ice flow but little or no cryovolcanism or tectonism. From orbital tracking, can infer gravity field => thought to be undifferentiated (not mixed up, or separated into a dense core and less dense outer layers, like Earth's interior). Lacks tidal heating of Ganymede, which probably made all the difference between the two. BUT: recent (January) surprise detection of a variable magnetic field, like Ganymede and Europa! => From a salty, subsurface ocean?!? Needs confirmation, from new Galileo flyby Summarize structure with table: Short intro to Saturnian satellites: ------------------------------------ 18 known satellites, excluding ring particles. Nearly all icy: Small (r < 100 km), 7 Mid-sized (r < 1000) km (Mimas, Enceladus, Thehys, Dione, Rhea, Hyperion, Iapetus), and Titan (r = 2575 km) Titan is bright (V = 8); many mid-sized can be seen in a respectably sized telescope (e.g., an 8-inch). Titan: ------ Has an atmosphere, 1.5 bars at surface: denser than Earth's. (What is surface pressure of Earth's atmosphere?) Only one other planetary satellite is known to have an atmosphere: which is it? Discovered in 1944 by Gerard Kuiper, with McDonald 82" telescope! Vertical Structure: ------------------- At 40 km altitude: Methane clouds, near condensation point (in other words, might be raining!) Mostly from 60 km to surface (traces high as 200 km): *Orange haze layer*, apparently unbroken. Can't see surface, surface can't see Sun. Aerosols: colloidal particles, suspended in a gas. Particle size: 0.2 - 1.0 microns (from Voyager UV spectrometer). Haze largely made of hydrocarbons: Composition, state, and opacity all reminiscent of Los Angeles smog... Compositon varies with altitude: ---------- N_2: 82 - 99% CH_4: 1 - 6% (still poorly determined, too) Also many organic trace gases: Hydrocarbons (H and C) Ethane C_2 H_6 Acetylene C_2 H_2 Ethylene C_2 H_4 Propane C_3 H_8 etc. Chemically highly reducing, though, so no chance of a fire... Also Nitrogen compounds: Hydrogen cyanide HCN Cyanogen C_2 N_2 etc. Also carbon compounds: CO CO_2 => Titan's atmosphere, by lucky coincidence, is a natural organic chemistry lab. If Titan were near Neptune, it'd be like Triton: much less dense atmosphere, mostly frozen out; Pluto's atmosphere *does* freeze out, near perihelion. Titan: methane and H_2 being broken up, by charged particles from Saturn's magnetosphere. => Should be gone in 10^6 years: how come they're still there? => Must be actively replenished; unknown how. Other possible energy inputs: Solar UV radiation, lightning? (As on Early Earth.) Urey-Miller experiment: pre-biotic chemical evolution Stanley Miller, Harold Urey 1952 Took commonest molecular gases in Universe CH_4, NH_3, H_2 O, & energy (zapped with electricity) => amino acids, which make up proteins, the stuff of life! Comet Shoemaker-Levy 9: "tarry gunk" (Heidi Hammel) Cliff Matthews: HCN polymers: Re-did Miller-Urey experiment, got proteins! "Any young planet will soon be knee-deep in protein" - Matthews. Sagan: tholins Similar experiment, matches spectra of Titan's atmosphere Doubtful that there's life on Titan: it's much too cold (T = 90 K). Still, of interest as a probable copy of early Earth, kept in a deep freeze. Surface of Titan: *not* well known, for obvious reasons. ---------------- Earth-based radars (e.g. Arecibo) at limit of sensitivity: Radar intensity falls off with 1/r^4, since it has to get there _and back_. (1/r^2 times 1/r^2) => Suggests solid surface covered at least partly with liquid: methane or ethane seas, or lakes? Hubble Space Telescope observations: near-infrared light *does* get through haze layer. Can see "continent", size of Australia (or CONUS). Cassini Saturn orbiter: launched 1997, to arrive 2004, prime mission to 2008 (40 orbits). Will pass Titan every orbit, map with radar to < 300 m resolution. Will drop Huygens probe (made by ESA, European Space Agency). Has imaging camera (with headlights). Designed to land, or float!