2000 March 14, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS Chapters 17, 18, and 19 (the Galilean satellites). - Read TNSS Ch. 20 (Titan) *and* PBD Ch. 7, for Thursday, March 16. - 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: Io, Europa, Ganymede, Callisto ----------------------- All worlds in their own right, revealed by high-resolution imaging, other studies during multiple flybys of Voyager 1 and 2 and the Galileo spacecraft, orbiting Jupiter since 1995. Discovered in 1610 by Galileo Galilei. Also discovered by Simon Marius (famous priority dispute) I've seen them in 7x35 birdwatching binoculars (7 x = magnification; 35 mm = aperture) All are about 4.6th - 5.6th magnitude (i.e. 4.6 < V, < 5.6) but can't be seen by the eye because of the glare from the planet (i.e., the eye has insufficient dynamic range). (Not because of insufficient resolution of eye. What angular distance is the eye's resolution limit?) All four are in circular, equatorial orbits => formed around Jupiter All are in sychronous rotation, like the Earth's Moon, with near & far, leading & trailing hemispheres, since all are fairly near the planet, and so are tidally synchronized, like the Earth's Moon: For Io, Porb = 1.8 days; distance from Jupiter, 5.9 R_J (Jupiter radii) For Callisto, Porb = 16.9 days; distance from Jupiter, 26.3 R_J (Earth's Moon is 60 Earth radii from Earth.) (Rings < 3.0 R_J) Stellar occultations => none have significant atmospheres. Io has some residual gas, from volcanoes, but not gravitationally bound. UV spectra with Hubble Space Telescope => Europa has trace of O_2 atmosphere, probably from sputtering from surface Jupiter's 4 outermost moons: small, irregular, bodies far from planet (> 300 R_J) in eccentric, *retrograde* orbits => captured asteroids Galilean satellites are less dense, farther from Jupiter => formed there, heated by Jupiter; same trend of more refractories near parent body, more volatiles farther away, as with the Sun. => A mini Solar System in many ways! Comparative structure of the Galilean satellites: Io (pronounced EYE-oh): ---------------------- Slightly larger than the Earth's Moon Density similar, too (3.5 g/cm^3) => rocky 1:2:4 orbital resonance with Europa and Ganymede - mimicks Bode's law Causes nearly circular orbit (e=0.004) to be tidally perturbed, and *tidally heated*. => Heat flow from interior is 100 times that of the Earth's Moon, slightly greater than in Earth's most active geothermal areas (e.g., Yellowstone) By far the most volcanically active body known! All identifiable features volcanic. => Over 80 known *active* volcanic features, over a dozen of which are active at any time. Many spew > 200 km into space! Temperatures: typically 300 - 600 K; up to 1400 K Ambient temperature: 130 K. *No* known impact craters: is volcanically resurfaced 100x faster than Earth. Orange/yellow color ("the cosmic pizza", with volcanoes as anchovies): from sulfur or sulfur dioxide Silicate (SiO_2) vs. sulfur volcanism debate: now solved, it's *both* Silicate volcanism: wide variety of flows across surface e.g. Lava lake and island at Loki; comparable in size to Hawai'i. Pillan Patera (resurfaced area size of Arizona between April and August, 1997) Sulfur volcanism: causes plumes, like terrestrial geysers (which spew water and steam) - Deduced from high escape velocities (> 100 m/s) e.g. Prometheus (continuously active since 1979?), Pele plumes Subsurface heat on Earth boils water, causing phase transition into steam; as opposed to direct heating of the magma, in a volcano. (But *no* water on Io: baked or boiled away long ago?) Sputtering: Io is deep in Jupiter's trapped radiation belts; charged particles erode surface Volcanoes and sputtering => provide material for Io plasma torus, trapped in Jovian magnetosphere Galileo to obtain only high-resolution images during its last 1 or 2 orbits, late this year. Will also take magnetometry, to determine whether magnetic field is remnant (like Earth's Moon's) or whether it's dynamic (generated now), from an active dynamo. Europa: ------ About size of the Earth's Moon Similar density, too: Europa's density = 2.97 g/cm^3, vs. Moon's 3.34 g/cm^3 => probably mostly rocky Europa is *very* smooth---literally smoother than a billiard ball! Highest features within about 100 m of average ("sea" level) (compare with Everest [8545 m] and Marianas Trench [-10,600 m]) 3 types of terrain: mottled, bright plains, linear features Differences mainly due to texture (degree of roughness) Very few impact craters (< 6 seen by Voyager): very young surface (10^ 6 - 10^7 yr) (How old is Earth's surface?) Surface is covered in ice (known from spectra), with _cracks_. Cracks show wide variety of types and features, in high-res Galileo images: warping, fracturing, upwelling fluid flow, collapse, & chaos. Reminiscent of Earthquake faulting, and terrestrial ice floes. => Dominant surface process is an icy _tectonism_ => Icy crust < 200 km thick, *much* thinner in some places => Together with smoothness, evidence of a subsurface ocean Not so simple, though: double ridges and triple bands, and dark spots around them, hint at more complex processes => escape of heat from interior. Variety of processes: Explosive heating, from geysers? Also certainly slow, viscous outflows: cryovolcanism (cryo = cold, e.g. cryogenics): Upwellings of water through the ice; this water then froze => internal heating, from tides More evidence for subsurface ocean: Variable magnetic field => inplies electric currents, in sloshing in a salty, subsurface ocean Europa Orbiter: proposed mission, to search for definitive proof of a subsurface ocean with radar, designed to "see" through ice