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Search for Life in the Universe: 

Search for Life in the Universe Chapter 8 Jovian Moons

Announcement: 

Announcement Most of Chapter 8 will be covered today The lecture on Thursday will concentrate on what we have so far learned about Titan from the Cassini mission This subject post-dates your textbook (Cassini reached Saturn in July 2004) See the review “Cassini at Titan” by John Teanby

Outline: 

Outline Jovian Moons Discovery Characteristics Synchronous Rotation Tidal Heating Europa Underground Ocean? Life? Future Explorations Ganymede Callisto

Discovery: 

Discovery Galileo (15641642) Adapted telescope, initially developed for naval purposes, to astronomical use Four largest moons of Jupiter: Io, Europa, Ganymede & Callisto Serendipitous sighting of Neptune (did not appreciate that it was a planet): still important for accurate orbit determination Huygens (16291695) Largest moon of Saturn: Titan Saturn’s ring detached from planet Brilliant physicist: multiple major discoveries preceding Newton Cassini (16251712) Saturn’s ring has a gap: Cassini division Four moons of Saturn: Tethys, Dione, Rhea & Iapetus First director of the Paris Observatory

Characteristics: 

Characteristics Size Larger than Mercury: Ganymede & Titan Larger than Pluto: Io, Europa, Callisto & Triton Many smaller planets, down to asteroid size Orbit Similar to planets around Sun Nearly in the equatorial plane Move in same direction as planetary spin Triton: retrograde rotation Composition & Shape Ice grains in protosolar nebula  accrete into planetessimals together with silicates and irons (unlike the terrestrial planets) Outer moons also contain CH4, NH3 & C2H6 Small moons: irregular shapes like asteroids  deduce capture

Synchronous Rotation: 

Synchronous Rotation Synchronous rotation: Equal spin and orbital periods Tidal force: Unequal pull on both sides of the body Tidal deformation: Response to tidal force Tidal friction: Loss of energy due to constant change of shape Synchronous rotation: Deformation always in the same direction Circularization: Degree of deformation unaltered

Tidal Heating: 

Tidal Heating Frictional heating: Viscous fluid sloshing around  frictional breaking  heating  possible differentiation Sloshing around: Tides on Earth Moon wobble Resonance orbits: Moons cannot all have same orbital period (Kepler’s third law) Moon-moon pulls  ongoing frictional heating Resonance orbits (Io : Europa : Ganymede = 1.8d : 3.6d : 7.2 d) minimize sloshing, but there is some  tidal heating Resonant orbits are elliptical  more heating Strongest effect on closest moon (for Jupiter: Io)

Underground Ocean?: 

Underground Ocean? Evidence Gravity measurements: central metallic core surrounded by 80170 km of water/ice Lack of craters  ice tectonics  liquid below (but could be “fluid” ice, like glaciers) Chaotic terrain: like arctic ice pack, with separating pieces Magnetic field: conducting liquid for internal dynamo & metallic core too cold  brine ocean Tidal heating: computations show it can do the job Estimated size Crust depth: 525 km, based on flooded impact crater Ocean 50150 km deep (cf., < 11 km on Earth)

Life?: 

Life? Environmental requirements Elements and molecules: no problem Liquid water: underground ocean? Energy: need substantial temperature difference to extract energy (2nd law of thermodynamics), e.g., at deep sea vents Biomass Earth: deep sea vents may be origin of life, but bulk of energy comes from photosynthesis Cycling photosynthesizing organisms to the surface? Not applicable: impacts (ended), lightning (no atmosphere) & solar UV (absorbed by ice) Radioactive decay of potassium (K)  energy + H2 + O2 Conclusion: biomass, it at all, is small

Future Explorations: 

Future Explorations Orbiter: Laser altimeter: Tidal deformation with underground ocean: 30 m Tidal deformation of solid ice: 1 m Long-wavelength radar: Hunt for thin spots in the crust Like discovery of underground Lake Vostok in Antarctica Lander: Land at the thin spots of the crust Look for microbes Drilling the ice to the underground ocean: No time soon

Ganymede: 

Ganymede Cratering Dark areas: cratering upon cratering  several byr old Bright areas: far fewer craters and grooves Explanation: “lava” (i.e., water) eruptions followed by freezing Ocean? Magnetic field  convecting core Part of magnetic field varies with Jupiter’s rotation  electrically conducting interior (brine?) Salts found on the surface Heat source Less tidal heating than Europa (larger distance from Jupiter) Large mass  more radioactivity Much less heat than in Europa  thick crust (>150 km?) Much harder to prove the existence of life never mind finding it

Callisto: 

Callisto Cratering Heavily cratered everywhere  no water gushing to the surface Gravity Undifferentiated: mix of ice and rock throughout Induced magnetic field Exists  underground ocean? Heat source? Does not participate in the tidal resonance Radioactive decay: only possibility Shock absorber? No bulge on the antipode of a big impact crater, unlike Mercury Suggests shock absorption by a liquid