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Home , Amalthea (moon), Ananke (moon), Callisto (moon), Callisto (mythology), Carme (moon), Elara (moon)

JOVIAN SATURNIAN URANIAN Janus Pandora Helene

SEE figure of different sizes, Encyclopedia of , p. 436.

Outer icy satellites are any of the celestial bodies in orbit around , , , Neptune, or . All composed mainly of some frozen volatile, usually May also be methane, dioxide, or 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: found 4 Jovian satellites (“Galilean satellites”: Io, Europa, Ganymede, and Callisto) best early evidence for -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 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 … 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

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 so inner planet satellites are generally denser outer planet satellites mostly lighter things that condensed at colder T C-based material makes up and some Saturnian and Jovian Water ice+ form most Jovian satellite (except Io, which has lost H2O) Saturnian and Uranus are ice+silicates+methane (CH4) + (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 contaminants lower the melting 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 = 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 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 or large left over from planetary formation most satellites are too small to retain an against thermal escape exceptions: Titan and Triton Ganymede does have a B. Evolution after , 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 in silicates on larger bodies with lots of silicates, this heat was enough to cause differentiation into core and 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, , 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 ? 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 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 are made of? ... 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 (O2) and hydrogen; has 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 is size of CO, with rings that could span US 4 covered with loose, dark particulate matter delivered by No evidence of icy volcanism there, just blankets of smooth, dark material Never hot enough to melt throughout, or form a core Ice and are not completely separated 3. Io bathed in intense : 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 lavas 500 km3 lava erupting per year 100x ! 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 of 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 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 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 among brightest in the solar sysetm -260º F at surface Interior: Metallic core about 1250 km diameter Silicate mantle 100-200 km water and ice “crust” icy plains crossed by darker fractures - “linea” dome-shaped areas are future locations for upwellings few large impact craters, age 10-100 my So something is actively resurfacing this satellite! crater floors are at same elevation as exteriors Pwyll has bright rays across 1000 km, has discontinuous rim 26 km across Callanish is about 100 km diameter Contains series of concentric fractures 5 Small craters found all over possible “dirty geysers” along fracture zones very smooth, little topography <6 impact craters visible chaos terrain: rotated blocks of ice several city blocks in size suggest they’re 3-5 km thick models say 20-30 km thick probably no melt-through wedges = gaps between plates of ice dark brown spots and other features may be silicate-laden water at the surface thin atmosphere of molecular O 1/100 billionth of Earth’s atmosphere! Came from charged particle, , and sunlight particles impacting data suggest metallic core, rocky interio, 100 km water, 10-15 km ice 2x ocean of water LIFE there? Two important issues: 1. Can life originate without sunlight (photosynthesis)? NO (recent work by Guidos, Kirschvink, et al., Science) 2. Can like survive without sunlight? MAYBE models of impacts suggest 25% of impacts deliver material there may also be a radiation-driven ecosystem there it’ll be hard to detect! Mission approved in 2006, arrives 2009 5. Others: 11 known smaller satellites, including 3 discovered by Voyager INSIDE IO Amalthea, dark reddish, small, heavily cratered Red color due to contamination from Io? Adrastea, Metis closest known satellites to Jupiter just outside the outer edge of the ring; may be sources of some of its particles probably rock-ice mixtures OUTSIDE IO Lysithea, Elara, Himalia, Leda have highly inclined normal orbits similar to C and D asteroids small, dark objects Sinope, Pasiphae, Carme, Ananke also have inclined orbits Retrograde orbit direction, probably captured asteroids

B. Saturn largely water-ice satellites +/- ammonia or other volatiles (Voyager flybys 1980-1981) many are resurfaced low density, high 0. Titan 5150 km diameter – bigger than ! Usually resides within Saturn’s magnetosphere, but no magnetic field of its own Discovered by Huygens in 1655 6 Has atmosphere of N2 and methane (CH4) – probably formed originally from a gas-rich ice Haze of aerosols opaque to visible light – studied with IR Still evolving 94ºK vertical structure Also acetylene (C2H2), propane (C3H8) and hydrogen cyanide (HCN) Has “ethane cycle” like hydrologic cycle on Earth Relatively weak gravity Some rocks in core Some outgassing occurred during differentiation 1. Mimas covered with craters Herschel is 1/3 of its diameter!!!! surface grooves caused by impact? high-rimmed craters (with no slumping) suggest little gravity 2. Enceladas very reflective - pure water ice? extensively resurfaced and grooved terrain like Ganymede’s resurfaced regions have high and fewer craters few impact craters -> surface is less than 1 by old! ice volcanism possible, fueled by the other half of the planet is extensively cratered, 4 by may provide particles that form E ring of Saturn E ring is tenuous collection of icy particles btwn Enceladas and Dione 3. Tethys covered with impact craters is largest known in solar system flatter craters suggest viscous relaxation some regions have fewer craters and higher albedos Ithaca chasm = huge trench 4. Dione same size as Tethys mostly heavily cratered, but some areas show a lot less cratering leading side is 25% brighter than trailing side due to micromedia impacts wispy streaks result from internal activity, emplacement by eruption 5. Rhea no evidence for resurfacing has some bright wispy streaks large craters are non-randomly distributed, though 6. Iapetus one hemisphere is much more reflective than the other don’t know if this is endogenic or exogenic highly inclined orbit less dense than other satellites – maybe more ice in its interior? 7. Hyperion covered with ice mixed with dark rocky stuff has highly irregular, boxy shape and slightly battered appearance may be in chaotic rotation – perhaps due to a recent collision? 7 Hasn’t made it back to tidally locked orbit yet 8. Phoebe outermost satellite dark object, smiliar to C-type asteroids highly inclined, retrograde orbit - captured ? Nearly perfectly spherical 9. Others shepherding satellites: Atlas, Pandora, Prometeus define Saturn’s rings Atlas is sevearl 100 km from outer edge of A-ring co-orbital Janus and Epimetheus, exchange orbits every 4 years inner satellite orbits slightly faster than outer one; when it overtakes the other, they exchange positions orbit either side of F-rings, giving rise to its kinky appearance ? maybe were once parts of a larger body Lagrangians: one associated with Dione and two with Tethys Lagrange points: equilibrium points in the orbit of a body around its primary locations within an object’s orbit where a less massive body can move in identical orbit

C. Uranus major ones are Miranda, Ariel, Umbriel, Titania, Oberon because Uranus’ rotational axis is inclined 98º to plane of solar system, are observed pole-on all composed of water ice, +/- ammonia and methane relatively dark albedo – dark component on surface graphite? carbonaceous material? UV radiation very porous Miranda has coronae, origins unknown Slightly inclined orbit May have undergone heating and differentiation Chevron terrain, of uncertain origin Ariel and Titania covered with grabens Umbriel is heavily cratered and darkest - probably very old surface Also densest – larger fraction of rocky material than the others Oberon also was resurfaced, but it’s heavily cratered Umbriel and Oberon may contain some organic material delivered by asteroids discovered 10 new small satellites 2 shepherding satellites all outside Miranda’s orbit not much other info D. Neptune 1. Triton discovered in 1846 2710 km diameter – larger than Pulto! High reflectivity Interior has mix of rock and ice Only a few impact craters (largest is 27 km wide Mazomba), so surface is relatively young Lots of cryovolcanism – emplacement of slushy ice “lavas” 8 Cantalope skin surface – large circular dimples (cavi) some 25-30 km across Some smooth planitia has appreciable atmosphere and seasons frozen methane (CH4) and N2; also H2O, CO2, and CO has >100 elongated streaks of dark (C-rich?) material across lighter ice – cause unknown course is active plumes of dark stuff discrete clouds and a haze layer, probably photochemical smog lots of wind on the surface 2. Others 6 inner bodies move in highly regular, circular orbits close to Neptune (< 5 planetary radii) Proteus, Larissa, Galatea, Despina, Nereid, and Naiad all discovered by Voyager in Aug. 1989 Proteus is despun, others aren’t – recently captured? irregularly shaped size increase with distance from Neptune Nereid has a highly eccentric orbit (57-385 planetary radii) most eccentric of any known orbit probably a captured object may have surface water frost not much known about surface features mostly gray objects similar to C-asteroids

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