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JOVIAN Ganymede Callisto Io Europa Amalthea Himalia Thebe Elara JOVIAN SATURNIAN URANIAN NEPTUNE Ganymede Titan Oberon Triton Callisto Rhea Titania Proteus Io Iapetus Ariel Nereid Europa Dione Umbriel Larissa Amalthea Tethys Miranda Galatea Himalia Enceladus Puck Despina Thebe Mimas Portia Thalassa Elara Hyperion Juliet Naiad Metis Phoebe Belinda Pasiphae Epimetheus Cressida Carme Janus Rosalind Sinope Prometheus Desdemona Lysithea Pandora Bianca Ananke Helene Ophelia Adrastea Atlas Cordelia Leda Telesto Calypso Pan SEE figure of different sizes, Encyclopedia of Solar System, p. 436. Outer planet icy satellites are any of the celestial bodies in orbit around Jupiter, Saturn, Uranus, Neptune, or Pluto. All composed mainly of some frozen volatile, usually water ice May also be methane, carbon dioxide, or sulfur dioxide at least 60 satellites (more undoubtedly exist, we just haven’t found them yet!) I. History none was known until after the invention of the telescope 1610: Galileo found 4 Jovian satellites (“Galilean satellites”: Io, Europa, Ganymede, and Callisto) best early evidence for Sun-centered (heliocentric) solar system 1655: Huygens found Titan 1680's: Cassini found Rhea, Iapetus, Dione, Tethys 1980's and 1990's: small satellites found in Pioneer and Voyager flybys most recently, U1 and U2 discovered in 1997 around Uranus International Astronomical Union names them... Numbers given to them in order of discovery Generally named after figures in mythologies associated with planet’s name SURFACE FEATURES also assigned... e.g., Iapetus = people and places from Chanson de Roland (medieval French) Mimas = people and places from Malory’s Le Morte d’Arthur Ganymede = Gods, heros and places from ancient Egypt Rhea = people, places from African, Asiatic, S. American creation myths Miranda = humans from Shakespeare’s The Tempest, his place names 1 II. Physical Properties motion of a satellite around a primary defines an ellipse with three elements: 1. semimajor axis 2. eccentricity = departure of orbit from circle 3. angle of inclination angle of intersection btwn plane of orbit and plane of spin equator regular orbit = prograde = same direction as primary also low eccentricity and inclination majority of satellites irregular orbit = retrograde = opposite sense of motion or highly eccentric or highly inclined most believed to be captured objects SEE Encyclopedia of the Solar System, p. 439, for summary table most satellites present same hemisphere toward their primaries, This is a result of tidal evolution… gravitational force exerted on near side > far side therefore, you get a bulge/distortion which lags behind rotation as satellite rotates, the bulge moves with respect to the satellite this induces internal friction, which creates heat spin rate is sacrificed to create heat, so rotation slows longest axis of bulge locks onto line btwn primary and satellite because that’s the lowest energy state you end up with a “despun” satellite in synchronous rotation happens very quickly, so most satellites are in synch. rotation 10,000-10 my. (see Rothery, p. 16) once believed to be dead worlds w/o heat sources recent work has re-written the book tidal interactions provide heat sources non-H2O ice components change melting points and viscosities III. Satellite Formation A. Theory think back to formation of solar system... denser materials toward center (e.g., Si, Al, Fe, Ti, Ca, etc. on Moon) because they have the highest melting temperatures so inner planet satellites are generally denser outer planet satellites mostly lighter things that condensed at colder T C-based material makes up Phobos and some Saturnian and Jovian Water ice+silicates form most Jovian satellite (except Io, which has lost H2O) Saturnian and Uranus are ice+silicates+methane (CH4) + ammonia(NH3) Neptune and Pluto: solid forms of N, carbon monoxide (CO), and dioxide (CO2) all generally less dense than inner solar system because of high fraction of volatiles contaminants lower the melting temperature of the ice e.g., salts, ammonia, methane, sulfates, carbonates How close can you have a satellite? satellites can’t accrete close to the surface of a planet, because they’ll be attracted by gravity to the planet’s surface 2 Roche limit = distance (about 2.44 x radius of primary) at which tidal forces exerted on the satellite equal internal gravitation forces of the satellite satellite must be far enough away that tidal forces don’t destroy it satellites can’t accrete within this limit satellite system form like mini-solar systems (e.g., on Jupiter, Neptune) have density gradients as a function of distance from primary therefore, there should be more ice as you get further out from primary BUT this doesn’t work on Saturn or Uranus retrograde satellites are captured asteroids or large planetesimals left over from planetary formation most satellites are too small to retain an atmosphere against thermal escape exceptions: Titan and Triton Ganymede does have a magnetic field B. Evolution after accretion, satellites began to heat up 1. from release of gravitational potential energy 2. from mechanical release of energy during bombardment e.g., Phobos, Mimas, and Tethys have huge impact craters 3. release of heat from radioactive decay in silicates on larger bodies with lots of silicates, this heat was enough to cause differentiation into core and crust 4. tidal interactions frictional energy of spin released as heat during “despinning” results from slight eccentricity in orbits satellites tug on each other to preventthem from being circles orbital periods of satellites within a system turn into multiples of each other due to gravitational interactions Io: Europa: Ganymede have 4:2:1 orbits (Fig. 2, p. 242, Beatty et al) this mutual gravity can cause significant heat production TWO types of erosional processes on satellites: 1. Endogenic = internally-produced 3 e.g. volcanism, tectonics, weathering produced by climate 2. Exogenic = brought on by external agents bombardment impact melting darkens and reddens soilm exposes excavated rx leading hemispheres are brighter; higher micrometoerite flux? leading = sides that lead in direction of orbital motion bimodal ice size distribution on leading side 1-2% fine-grained ice, only on leading side trailing side has only one, blocky size of ice interactions with primary’s magnetosphere implantation of energetic particles and sputtering volatiles are most susceptible alteration by UV photons accretion of particles from rings IV. Satellite Observations How do we know what these planets are made of? Spectroscopy... individual minerals absorb at characteristic wavelengths... so, we can detect mineral species AND ice SEE viewgraphs... V. Individual Satellites A. Jupiter 1. Ganymede largest Galilean satellite heavily cratered dark terrain crossed by brighter grooved terrain contains larger fraction of rocky material very dark and “space weathered” palimpsests = outlines of old, degraded craters with very low topographic relief named after re-used parchment from which writing is incompletely erased probably >4 by old bright grooved terrains have grooves 1/3 – ½ km high and <10 km wide, about 3.5-4 b.y. old maybe caused by slight crustal expansion after thaw-refreeze episode in past grooved terrain is brighter because it’s made of ice and “younger” than dark areas icy volcanism anticipated, but we see little evidecen for it thin atmosphere of molecular oxygen (O2) and hydrogen; has ionosphere signs of ozone also (O3) has bright poles of water ice and polar aurorae (due to impact of charged particles on atmosphere) must be differentiated – has a magnetic core and magnetic field (Galileo) 2. Callisto no evidence for resurfacing at any point in its life relatively uniform, dark terrain saturated with craters craters >150 km and very small craters are lacking probably due to viscous relaxation i.e., the material’s not strong enough to maintain large indentations! Crater Valhalla is size of CO, with rings that could span US 4 covered with loose, dark particulate matter delivered by meteorites No evidence of icy volcanism there, just blankets of smooth, dark material Never hot enough to melt throughout, or form a core Ice and rock are not completely separated 3. Io bathed in intense radiation: electrons, protons, and heavier ions color variations due to different froms of sulfur (allotropes) most volcanically active body in the solar system! uniform distribution of volcanoes, indicating global source of magma High T S plumes (allotropes) erupt like geysers; height aided by low gravity, thin atmosphere Plumes appear to be long-lived, stable; consist of superheated SO2 Some lavas erupt at hotter T (700-1800º K), probably are silicate lavas 500 km3 lava erupting per year 100x Earth! atmosphere is thin and patchy (dense over about 10% at a time) sulfurous; has dense ionosphere Old Faithful would be 35 km high if erupting on Io! Neutral Na and K clouds sputtered from surface global frost of sulfur dioxide surfaces bulges up and down about 100 m surface T 110º-120ºK, but up to 300-600ºK near volcanoes Loki Patera: lava lake Interior: crust probably about 100 km thick magma ocean extends down to perhaps 875 km (radius = 1821 km) dense core of Fe and FeS extends half way to surface magnetic field arises from core much of Io’s mantle must be melted... Io has interior of hot crystal/magma mush, about 40% molten NO impact craters – rapid resurfacing STILL erupting ; plumes 100 km high! calderas 300x100 km (vs Kilauea, 8 x 5 km) see PSR Discoveries article on Io, Feb. 2000 4. Europa Same size and density as our Moon Surface
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