Life in the Terrestrial Planet Region:

1. Life in the Terrestrial Planet Region: X Terrible Extremes of Temperature, No Atmosphere, UV, Cosmic Rays, Little or No Volatiles, No Liquids • MERCURY • MOON • VENUS • MARS X Terrible Extremes of Temperature, No Atmosphere, UV, Cosmic Rays, Little or No Volatiles, No Liquids X Very High Temperatures, No or Little Water Young Surface  No Fossil Record ? Evidence for Liquid Water in Past Possible Environments for Life to Survive? Volatiles and Water Present Now

2. Moving Outwards

3. Gas and Ice Giant Planets:Prospects for Life? • Ingredients for organic chemistry • Atmospheric layers with roughly Earth-like Temperature & Pressure; • Butno solid surfaces (except ice crystals) and no liquid water (except very deep in Uranus and Neptune), • Violent winds and convective turbulence would mix any life-bearing gas quickly over extremes of temperasture & pressure. • Although the sunlight is very weak, internal heat is available. • They are very difficult to explore!

4. Life on Jupiter? • All the basic molecular ingredients are present in the atmospheres • But convection will ultimately (and quickly) bring any organic molecules down to regions where T~ 700 C (over 1200 F!) where they will be destroyed Sagan & Salpeter “speculations” “floaters” “hunters”

5. Io Europa Ganymede Callisto Life on Moons of Planets? For instance, the Galilean Satellites of Jupiter

6. Medium & large moons • Enough self-gravity to be spherical • Are, or were, geologically active. • Have substantial amounts of ice. • Roughly circular, equatorial orbits in same direction as planet rotation.

7. Small moons • Far more numerous than the medium and large moons. • Not enough gravity to be spherical: “potato-shaped” • Captured asteroids, so orbits do not follow patterns. • Orbits can be tilted, elliptical, and even backwards! • No atmospheres or liquid water – not suitable for life

8. Focus on the large & medium sized moons

9. Sizes and orbits

10. What about…- a source of energy (Sun is too faint)?- presence of liquid water?

11. Tidal Forces Because the gravitational force decreases with (distance)2, the attractive force experienced by one object (e.g., the Earth) due to the gravitational field of a second object (e.g., the Moon) varies with position (closest parts attracted most strongly). Tidal forces are difference forces.

13. Synchronous Rotation • …is when the rotation period of a moon, planet, or star equals its orbital period about another object. • Tidal friction on the Moon (caused by Earth) has slowed its rotation down to a period of one month. • The Moon now rotates synchronously. • We always see the same side of the Moon. • Tidal friction on the Moon has ceased since its tidal bulges are always aligned with Earth.

14. All of the large moons in the solar system are in synchronous rotation.

15. Tidal Heating in Jovian Moons The four inner moons of Jupiter - Io, Europa, and Ganymede - all show evidence of geological activity - indicators of molten interiors. The heat source is tidal heating. Moons have elliptical orbit and synchronous rotation - one side always faces Jupiter • as Ganymede completes one orbit, Europa completes exactly two orbits, and Io completes exactly four orbits - moons periodically line up - causes orbital ellipticity. • tidal bulges are constantly being flexed in different directions - generates friction inside

17. Effects of Tidal Interactions • Rotation • Rotation of moons become synchronized with their orbits. • They keep the same face toward the planet. • The rotation of the planet is slowed down. • Orbits • Orbits of moons mostly evolve outward. • Internal “Tidal Heating” • Eccentric orbits lead to periodic flexing of the moon’s shape which heats the interior. • Orbital resonances with other moons can maintain eccentric orbits and tidal heating.

18. Io Europa Ganymede Callisto Jupiter’s Galilean Satellite’s

19. Io

20. Io’s Volcanoes Io is the most volcanically active world in the solar system.

21. Jupiter’s tidal forces flex Io like a ball of silly putty. • - friction generates heat • - interior of Io is molten • Volcanoes erupt frequently. • - sulfur in the lava accounts for yellow color • surface ice vaporizes and jets away • Thin atmosphere made up mainly of sulfur dioxide, produced by volcanic activity and temporarily retained by the moon’s gravity. • Evidence of tectonics and impact cratering is covered. Io

22. Volcanic Plumes

23. Lava fountain - active lava hot enough to cause "bleeding" in Galileo's camera - overloading of camera by the brightness of the target Newly erupted hot lava flow. Dark, "L"-shaped lava flow marks the location of the November 1999 eruption.

24. Gas and Dust Plume A broad plume of gas and dust about 80 km high above a lava flow

25. Europa

26. Jupiter’s Europa • Has similar but weaker tidal heating, • Has a young cracked water ice crust perhaps only a few kilometers thick, and • May have a warm ocean of liquid water below the crust. • Could there be life?

27. Europa Icy surface - “fresh” - almost no craters

28. Ocean under the ice? • 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 (< 11 km on Earth)

29. Surface of Jupiter’s Moon: :Europa Fractures in Floating Ice Icebergs

30. Evidence of a Subsurface ocean Jumbled crust with icebergs and surface cracks with double-ridged pattern - caused by tidal flexing of thick layer of ice on top of liquid ocean of water.

31. Europa’s interior also warmed by tidal heating Salty - Europa has a magnetic field

32. Sub-Crust Ocean . . . . Hydrogen-Carbon compounds likely: Amino acids Life in the Ocean? First New Ocean Since Balboa

33. Missions to Europa http://www.jpl.nasa.gov/europaorbiter/

34. Ganymede

35. Ganymede • Largest moon in the solar system • Clear evidence of geological activity • Tidal heating expected - but is it enough?

36. Ganymede Wrinkles due to tectonic movement in ice crust in (distant) past - possible water deep below?

37. 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

39. Callisto

40. Callisto • “Classic” cratered iceball. • No tidal heating - no orbital resonances. • But it has magnetic field !?

41. Scarp close up Callisto Possible water deep?

42. 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

44. Life on Galilean Moons? X Very active volcanically. Hostile environment • Io • Europa • Ganymede • Callisto ? Subsurface saline ocean, hydrothermal vents? ? Subsurface saline ocean? hydrothermal vents? ? ? Subsurface saline ocean?