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Moons of Jupiter and Saturn Oct 7, 2015 A wealth of worlds of ice and rocks >170 moons orbit jovian planets Most large Jovian Planet satellites are smaller than our moon. Based on the geological principles controlling Terrestrial Planets, we expect cold, dead worlds, covered by craters… Io Europa (Jupiter) Triton Enceladus Titan (Neptune) (Saturn) NOT SO! Tides: so what? • Tides slow rotations • Synchronous lock moons (and planets!) • Tides distort moon’s shape - continually changing shape heats interior -> geological activity • Tides change the system (cause orbits to circularize) • Tides are stronger around more massive planets • Tides are stronger closer to the planet – Inside a few planetary radii, tides can pull a moon apart (or more likely, prevent it from accreting): (‘Roche tidal zone’) Plus: moons have large percentage of ice! JUPITER SYSTEM Groups of Jupiter moons: • Regular: – Inner satellites or Amalthea group: Metis, Adrastea, Amalthea, and Thebe. – Main group or Galilean moons; • Irregular: – The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit Galilean moons of Jupiter Io, Europa, Ganymede, Callisto Laplace resonances of Io, Europa and Ganymede Laplace resonance: • The superior conjunction between Io and Europa always occurs when Io is at periapsis and Europa at apoapsis. • The superior conjunction between Europa and Ganymede occurs when Europa is at periapsis. • The longitudes of the Io–Europa and Europa–Ganymede conjunctions change with the same rate, making the triple conjunctions impossible. the eccentricities get pumped up to much higher values than if the satellites were not in a resonance -> tidal heating -> geological activity! Ganymede – king of the moons R = 2634.1±0.3 km (0.4 REarth) global geologic map Ganymede: • orbital period is 7 days 3h. • Tidally locked • Orbit radius 1,070,400 km Undersurface water ocean on Ganymede how do we know? • • • • • Ganymede generates its own magnetic field -> produces aurorae – strips of radiant electrified gas that circle Ganymede’s poles Ganymede is close to Jupiter, any changes to Jupiter’s magnetic field directly affect that of Ganymede when Jupiter’s magnetic field shifts due to the planet’s rotation, Ganymede’s aurorae “rock” back and forth -> can make a model, and compare to the observational data scientists surmised that an ocean works against Jupiter’s magnetic pull, causing Ganymede to rock less violently than they had anticipated (March 2015) Callisto • Not fully differentiated (metal, rock and ice are still mixed throughout most of its interior) • No tidal heating (out of resonance) • far out of in Jupiter’s magnetic field Galileo image of cratered plains Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter Callisto base Exploration of Jupiter system • Past: Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, New Horizons (in 2007); • Current: – Juno (arrival 4 July 2016) • Planned: – European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022; – NASA/JPL: Europa Multiple-Flyby Mission (formerly Europa Clipper), set for launch in 2020; SATURN SYSTEM Moons of Saturn: So far, 62 moons have been discovered in orbits around Saturn, and 53 of them have been officially named. From left to right the moons are Epimetheus (113 km/70 miles across), Janus (179 km/111miles), Prometheus (86 km/53 miles) and Atlas (30 km/19 miles). Saturn's Medium-Size Moons (300 — 1,500 km in diameter) Saturn has 6 medium-size moons. In order of increasing distance from the planet, they are: • Mimas • Enceladus • Tethys • Dione • Rhea • Iapetus Mimas Iapetus Tethis Rhea Dione “Pac-Men” • • • High-energy electrons bombard low latitudes on the leading side of a moon, turning a fluffy icy surface into hardpacked ice. The altered surface does not heat as rapidly in the sunshine or cool down as quickly at night as the rest of the surface The surface alteration is occurring more quickly than its recoating by plume particles. Rhea Iapetus Hyperion • • • irregular shape chaotic rotation sponge-like appearance Phoebe Cassini-Huygens (2004-2017)