Chapter 13: Europa Voyager 2 flyby

Jupiter’s Moons (28 in all) 4 Galilean moons (each comparable with Earth’s moon); Io & Europa have thick rocky mantles surrounding iron/nickel cores & Europa has water/ice outer shell km thick Ganymede & Callisto deficient in rocky materials

Internal structure of the Galilean Moons: Densities of Moons decreases steadily with distance from Jupiter: Rocky mantles for Io and Europa to rock-ice mix of Callisto: 3500 kg / cubic metre down to 1900 kg / cubic metre Jovian system of Moons is much like a mini-Solar system in properties of its Moons.

Europa – 9.38 Jovian radii out, smooth ice- covered moon Resolution in a) is 5km, in d) it is 20m (taken by Galileo). - little cratering -> surface is being reworked…. By tidal flexing due to Jupiter? - Ice flows?

Europa’s ice covered ocean - possible life? Evidence for liquid ocean: Europa has a weak magnetic field! a) Frozen surface of Europa, “pulled apart terrain” - (as in sea ice) upwelling material filling gaps between separating ice sheets b) water has flooded section of the surface -> liquid ocean 100 km deep, beneath this surface?

Antarctica from space: - enormous mass of ice (Ross ice shelf) floats on the Antarctic ocean - what can be learned by comparing these images with those of Europa?

Properties of Europa: Reflectance spectra: surface composed mainly of water ice. Orange-brown markings indicate sulfur compounds - from volcanic activity of Io! Two major features of ice: i) splotches (chaotic terrain) – covering nearly ½ of surface – indicative of convection/upwelling and ii) lines (ice tectonics) – densely packed overlaid ridges, cracks, bands – indicative of plate motion/collision Paucity of craters -> resurfacing in a period of < 50Myr. Models: Ice at least 10 km thick, ocean up to 100 km deep…

Tidal force – the difference that gravity makes… (a) Gravitational force of the Moon at different points on Earth’s surface (b) Difference between Moon’s gravitational force at Earth’s centre and points on its surface – very weak! GET: surface distortion – the TIDES Side closest to Moon pulled away, side farthest away “left behind”

Tidal locking: eg. slowing the spin of the Earth Tidal force of Earth on Moon has locked Moon’s spin to same value as orbital period…..…..same face always to the Earth. Tidal force of the Moon on the Earth is slowing Earth’s spin by …1.5 milliseconds per century. How? Earth rotates – carrying bulge with it – offset with respect to Moon - Earth line: Moon’s gravity tugs back at the bulge, slowing rotation rate of Earth…. Earth’s spin slows and angular momentum transferred to Moon – which moves out.

Tidal heating in Io and Europa: Jupiter exerts very powerful gravitational force on inner moons. These are *not* tidally locked however! Io and Europa: prevented by perturbations from Io/Europa/Ganymede system in 1:2:4 resonance. Drives system into eccentric orbits -> can’t come into synchronous state -> bulge always faces Jupiter, but Io cannot keep same face towards Jupiter – so bulge constantly distorts surface -> huge constant tidal heating of the interior – which Tidal heating – from dissipation of energy in tidal bulge - drives the volcanism in Io (100 million megawatts) ….. And distortion of Europa and its ice cover (6 million megawatts)

Life on Europa? Mission concept: drill through ice of Europa to explore for possibility of life in ocean beneath surface.

Tidal heating of Europa Resurfacing/cracking -> exposure of oceanic water through crust to surface. Cracks open and close; tidal time-scale -> days Water/slush forced to surface, and then flows back… bringing back surface chemicals These include cometary material (organics), hydrogen peroxide (H 2 O 2 ), … Photosynthesis might be possible in open cracks Energy source for life… use hydrogen peroxide to oxidize reductants such as H, H 2 S,… chemosynthesis ** Also possible – hydrothermal based life from underwater volcanos? Life probably a very sparse biosphere by Earth standards..

The future… terrestrial planet finder(s): coronograph (left), interferometer (right)