Life on Giant Planets & Their Moons

1. Life on Giant Planets & Their Moons Stephen Eikenberry 27 March 2013 AST 2037

2. much larger than terrestrial planets • not solid - gaseous • all have rings • all have many moons The Jovian Planets Jupiter Jupiter, Saturn, Uranus, Neptune

3. Jupiter • Named after the most powerful Roman god • third-brightest object in the night sky (after the Moon and Venus) • Atmospheric bands are very different than inner planets • Many moons – four largest called Galilean Moons

4. Saturn • Named after the father of Jupiter in Greco-Roman mythology • Almost twice Jupiter’s distance from the Sun • Similar banded atmosphere • Uniform butterscotch hue • Many moons • Spectacular ring system

5. Uranus • Discovered by William Herschel in 1781 • Named after father of Saturn • Barely visible to naked eye • Featureless atmosphere • Deviations in the expected orbit of Uranus pointed to the possibility of another planet influencing its motion

6. Neptune • There had to be another planet influencing Uranus • 1845 - John Adams determined the planet’s mass and orbit • 10 months later - Urbain Leverrier, independently came up with the same result • 1846 - Johann Galle found the new planet Neptune • Cannot be seen with naked eye • “Bluish” Jupiter atmosphere

7. Space Craft Exploration of Jovian Planets • Voyager 1and 2 left Earth in 1977 • reached Jupiter in March and July of 1979 • Used Jupiter’s strong gravity to send them on to Saturn - gravity assist • Voyager 2 used Saturn’s gravity to propel it to Uranus and then on to Neptune • Studied planetary magnetic fields and analyzed multi-wavelength radiation • Both are now headed out into interstellar space!

8. Space Craft Exploration of Jovian Planets • Galileo - launched in 1989 and reached Jupiter in December 1995 • Gravity assists from Venus and Earth • Two components: atmospheric probe and orbiter • Probe descended into Jupiter’s atmosphere • Orbiter went through moon system • Cassini mission to Saturn • Studying Saturn’s moon Titan in much the same way as Galileo studied Jupiter

9. Jovian Planet Properties • Most of their mass is Hydrogen and Helium – light elements = low densities • High surface gravity allows their atmospheres to retain these light elements • Dense compact core at the center • But, NO SOLID SURFACE – gaseous atmosphere becomes denser (eventually liquid) at core • Differential Rotation – outer regions rotate slower than inner regions

10. Jovian planets - axis tilt and magnetic fields • Uranus has the most inclined rotational axis - extreme seasons! • All appear to have strong magnetic fields - rapid rotation and liquid conductive cores or mantles

11. Jupiter’s Atmosphere • Two main features: colored bands and Great Red Spot • molecular hydrogen – 86% • helium – 14% • small amounts of methane, ammonia, and water vapor • Darker colored belts lie atop downward moving convective cells • Lighter zones are above upward moving cells • Belts are low-pressure, Zones are high pressure • As on Earth, wind moves from high to low • But rotation causes wind patterns to move East/West along equator • Temperature difference between bands is main reason for color difference

12. Jupiter’s Atmosphere • Haze lies at the upper edge of the troposphere • Thin layer of white ammonia clouds – 125 – 150 K • Colored clouds below that • Warmer - 200 K • clouds are mostly droplets or crystals of ammonium hydrosulfide • At deeper levels, clouds of water ice or water vapor The Galileo probe survived for about an hour before being crushed at this altitude.

13. Weather on Jupiter • Main weather feature – Great Red Spot! • swirling hurricane winds • has lasted over 300 years! • diameter twice that of Earth • rotates with planet’s interior • the spot appears to be confined and powered by the zonal flow Smaller storms look like white ovals (this one is over 40 years old) Why do the storms last so long? On Earth, hurricanes lose power when then come upon land No continents on Jupiter – nothing to stop them once they start

14. Saturn’s Atmosphere • molecular hydrogen 92.4% • helium 7.4% • traces of methane and ammonia • Layer of haze • Troposphere contains 3 cloud layers • Overall temperature is cooler than Jupiter • Atmosphere thickness is 3 times that of Jupiter (caused by lower surface gravity on Saturn) • Thicker clouds result in less varied visible colors • ammonia ice • ammonium hydrosulfide ice • water ice

15. Weather on Saturn • Computer enhanced image shows bands, oval storm systems, and turbulent flow patterns like those seen on Jupiter

16. Atmospheres of Uranus and Neptune • molecular hydrogen 84% • helium 14% • methane 2% (Uranus) 3% (Neptune) Abundance of methane gives these planets their blue color Methane absorbs longer wavelength light (red) and reflects short wavelength light (blue)

17. Weather on Uranus and Neptune • Uranus • Few clouds in the cold upper atmosphere – featureless • Upper layer of haze blocks out the lower, warmer clouds • Neptune • Upper atmosphere is slightly warmer than Uranus (despite its further distance from Sun) • More visible features (thinner haze, less dense clouds lie higher) • Storms – Great Dark Spot • Seen in 1989 – gone in 1994

18. Internal Structures – models that fit the data Metallic hydrogen is like liquid metal Uranus/Neptune Saturn Jupiter • Increasing temperature and pressure deeper in core • Jupiter bulges at radius (7% larger) • Saturn less assymetric – larger core – same basic overall structure on a smaller scale • Uranus/Neptune have a high density “slush” below cloud level - compressed water clouds

19. Internal Heating • Primordial Heat • Generation of Heat • Effect of internal heating - raises the temperature of the interior and atmosphere to higher values than expected from the Sun’s heating alone -Jupiter’s heat source results from strong heating during formation by the collapse of material onto the core -Saturn generates some heat due to the gravitational contraction of helium gas

20. Life on the Giant Planets? • We see lots of “life chemicals”! (Water, methane, etc.) • Lightning observed too • Similar to early Earth atmosphere (?) • Expect complex hydrocarbons (Urey-Miller) (?) • At some depth, have warm temps (~300K) • But … these temps at high pressure • Methane is not a sign of life here – just a sign of LOTS of free hydrogen, some carbon, relatively little oxygen • No solid surface – no oceans, no tidal pools, no clay matrix – in short, no (Earth-like) places for life formation

21. Life on the Giant Planets? • What about gas matrix life? (i.e. no solid surface, but life in the air) • Wind speed turbulence problem – try to put together a house of cards outdoors in a hurricane • That’s easier than forming life in Jupiter’s atmosphere! • Convection • Causes any chemicals from warm lower layers to rise • This gives exposure to solar UV radiation, which breaks it down

22. Does this mean NO life here? • Nope • Floater possibility • But, this is pure speculation

23. Moons • Almost all moons in the Solar System orbit the Jovian planets • Jupiter’s 4 major moons are the Galilean satellites: Io, Europa, Callisto, and Ganymede • Jupiter has 100’s of smaller moons

24. Io • Innermost Galilean satellite • Reddish color • Smooth “young” surface • Extreme volcanic activity • Interior tidally heated by Jupiter (& Europa)

25. Io – Tidal Heating

26. Europa • Second Galilean satellite • Whitish, highly-reflective color • Smooth surface with patterns similar to ice caps on Earth • Young surface • May be a liquid “waterworld” with ice crust

27. Life on Europa? • Possible view of Europan ocean • Note thermal segregation

28. Life on Europa? • Europa “seabed” may resemble “black smoker” environment on Earth (!) • Probably little O2 in the water (where would it come from?) • But anaerobic bacteria are the basis for life in Earth vents (even if larger life O2-dependent) • NASA developing preliminary plans for a drilling/submarine exploration mission

29. Callisto & Ganymede • Rock/ice moons • Evidence for some “glacial” flows

30. Saturn’s Titan • Largest moon • “Smooth” surface due to thick atmosphere (!) • Infrared images show evidence for “continents” beneath

31. Titan’s Surface Lakes of liquid hydrocarbon imaged from orbit via Cassini; T ~85K (probably ethane/methane) Huygens lander mission in 2006

32. Titan’s Surface Lakes of liquid hydrocarbon imaged from orbit via Cassini Huygens lander mission in 2006

33. Life on Titan? • Solid/liquid phase environment similar to Earth’s surface • Totally different chemicals and temp ranges; will not be H2O-based like on Earth • But … still a possibility

34. Neptune’s Triton • Largest Neptunian moon • Retrograde orbit • Orbital tilt 20-degrees • Could indicate KBO origin ??

35. Other moons

36. Enceladus • Saturn moon • Water here too?

37. Enceladus • Saturn moon • Water here too?

38. Summary • The Giant Planets lack solid surfaces and have extreme wind speed, turbulence and convection in their atmospheres • Still .. we cannot rule out life there (floaters?) • Moons such as Europa and Enceladus may have liquid water oceans with Earth-like temperatures and geothermal vents; these may be promising places to search for life • The moon Titan has a thick atmosphere and (apparently) lakes/rivers of liquid hydrocarbons; temps are COLD, but we can imagine some low-temperature, slow-reaction life developing here too (?)