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Life on Jovian Moons Chapter 9 Details of the Jovian (Jovian planet) Moons Habitability – life requirements ! Life requires a source of energy ◦ Closest system, Jupiter, is at 5.2 AU so 1/5.22=1/27 less solar energy ◦ Surprisingly, in the Jovian moon systems the prospects for life are pretty good! The frost line When the Solar System was formed, there was a boundary beyond which it was sufficiently cold to form and retain water ice ! Jovian planets rich in water ice ! Terrestrial planets very dry, some water retained by early comets The Jovian Moon System • Jupiter is outside the ‘frost line’, where water ice can form • The ice grains contained H, so a large amount of mass came together • When large enough, gravity took over from collisions in building a bigger body (‘direct gravitational accumulation’) • 4 large moons, the ‘Galilean Moons’ Formation: ! Many of the larger Jovian moons orbit nearly in the equatorial plane of Jupiter, and in the same direction ! => formed from a rotating disk of gas and dust like a miniature solar system Formation contd. ! Most smaller Jovian moons are most likely captured asteroids and comets. ! The largest Jovian moon that appears to have been captured is Triton, the moon of Neptune – it orbits backwards relative to its planet’s rotation Composition Planetary objects fall into several groups: ◦ Giant planets with radii greater than 10,000 km and low densities ◦ Terrestrial planets and terrestrial-like bodies ◦ Icy satellites ◦ Small asteroids ! Jovian moons typically made of ice and rock. Outer Solar System cold enough to allow ices to condense along with metal and rock. ! Average densities of Jovian moons << lower than Earth (water ice). Variations as a function of temperature within Solar System ! Water ice condensed easily at temperatures near Jupiter. Methane and other ices condensed only at colder temperatures – farther away from the Sun. ! Jupiter’s moon contains significant quantities of water ice, but no other ices. ! ◦ Jupiter’s moons show a decrease in density with distance from Jupiter – formed of a cloud of gas that was hotter in the center than in its outer regions. ! Moons of more distant planets contain higher proportion of water ice compared to rocks, and contain other ices in addition to water ice (methane). Orbital periods ! ! ! Io Europa Ganymede 1.77 days 3.55 days 7.15 days 1:2:4 ratio – not a coincidence. Reason why Europa and perhaps Ganymede are good prospects for life. This induces tidal heating! Tides - Earth ! ! ! A feature of oceans (but solid material has small tides too). Two high and two low tides per day. Tides are due to the gravitational pull being stronger on side of Earth closest to it (Sun causes smaller tides) Earth-Moon gravity keeps them orbiting each other. But side of Earth closest to Moon has slightly stronger pull to Moon => bulges towards it. Other side has weaker pull => bulges away compared to rest of Earth. The Earth spins once a day while the bulge always points towards and away from the Moon => high and low tides. ! We always see the same face of the Moon. ◦ This means: period of orbit = period of spin = tidal locking ! Tidal bulge of Earth is ‘leading’ because its rotates fast. ◦ The Moon will pull it back wards, slowing rotation of the Earth. ◦ Bulges of Earth will pull on the Moon, into a larger orbit! ! Similarly Earth causes bulges on the Moon ! Once the Moon is slowed down sufficiently, it will be locked into position Tides – Jupiter ! Jupiter rotates in 9.8 hours. ◦ => Tidal bulges raised by its moons always lead the moons. ◦ Just as with our Moon, Galilean satellites are moving outwards with time. ◦ The solid material in the bulges of the satellites will be ‘squeezed’ and ‘kneeded’ => source of heat Suppose that there were only one big moon around Jupiter. ! Tidal effects would push moon away, and quickly make that moon’s rotation period equal to its orbit ◦ it would always put the same face to Jupiter ! Then, there would be no ‘squeezing and kneading’ of the moon, hence no heat produced. ! Io, Europa, and Ganymede cold and dead because they are too small to hold onto any significant amount of their heat of formation? ! We have several Moons, not only one ◦ Orbital resonances between Io, Europa and Ganymede ! Sequence of orbits for 1 period for Ganymede, 2 periods for Europa and 4 periods of Io. ◦ Regular gravitational kicks at the same phase – resonance ◦ Orbits get eccentric, so can’t keep the same face to Jupiter all the time ◦ Tidal forces ‘squeeze’ the planets and heat them up (mostly Io and Europa) Tidal heating of the Jovian Moons ! ! Normally the moons would loose their internal heat as they cool off, but the tidal forces creates internal heat Io is the most volcanically active body in the solar system! ! More than 80 currently active volcanoes lasting months-years ! Ejecta speed 1000 km/s Each volcano ejects about 10,000 tons/s ! ! ! ! ! Io’s red colors from different sulfur compounds and molten silicon rock Yellow colors are from final stable configuration of S in a ring (S8, ordinary sulfur) Frozen SO2 snowflakes are white Time between photos a few months ! Plume on left rises to 260 km (150 mi) above surface. Blue due to scattered sunlight. Voyager 1 & Galileo Geyser function (Earth) ! Surface water trickle down until heated by rocks (heated by magma, geothermally) ! Convection then brings water upwards, in one or more 'tubes' in rocks • The tubes are narrow: cooling by convection cannot occur • Upper cooler water pressure downwards on warmer water (pressurecooker), causing a rapid heating process • When boiling occurs, the water expands and flashes out Geysers on Io ! ! ! ! Surface gravity lower than on Earth, no atmospheric pressure ‘Old faithful’ on Io: 37km plumes No water: sulfur and sulfur dioxide Fast transitions from liquid => gas => sulfurous snowflakes ! Activity causes surface to slowly change over the years ! Surface completely lacks impact craters Io lacks water and extreme volcanic activity makes is improbable for hosting life. ! Voyager 2 (1979) Galileo (1996) Europa ! ! Surface and structure very different from Io Spectroscopic observations from Earth indicated Europa’s surface is almost pure frozen water, probably covers an ocean ! Europa is the smoothest body in the Solar System ◦ Almost no craters, no mountains ◦ Young surface reprocessed by geological activity