The Moons of the Gas Giants

1. The Moons of the Gas Giants Astronomy 311 Professor Lee Carkner Lecture 20

2. Why do the rings of Uranus and Neptune look dark? • They are made of small silicate particles • They receive very little light from the sun • They only radiate light at ultraviolet wavelengths • They are covered with carbon soot • The rings are blocked by a screen of dust

3. The blue and white colors we see in the atmospheres’ of Uranus and Neptune are due to, • gaseous helium and water clouds • ammonia clouds and sulfur clouds • gaseous methane and methane clouds • gaseous methane and carbon dioxide clouds • the fact that light is sometimes absorbed and sometimes scattered off ice crystals in the atmosphere

4. Jupiter’s Lovers • Ganymede was Jupiter’s cup bearer • Titan is so named because Saturn was the king of the Titans • Triton was Neptune’s son

5. The Galilean Moons • The 1st, 3rd, 4th and 6th largest moons in the solar system • Made from rock and ice

6. Orbits of the Galilean Moons • 1 rotation on its axis per 1 revolution around Jupiter • This is due to the small separation between the inner moons

7. Formation of the Galilean Moons • The inner parts of the nebula were hotter than the outer • The inner satellites (Io and Europa) formed mostly from rock • The inner 3 satellites experienced tidal heating and differentiated into a rocky core and an icy crust • Much of the energy that powers the large moons of the solar system comes from tidal heating

8. The Interiors of the Galilean Moons

9. Galileo Visits the Galilean Moons • Made several passes by each moon • Galileo refused to die and kept taking data up to Sept 2003 deliberate crash into Jupiter • Galileo has achieved imaging resolutions up to 50 times better than Voyager

10. Io • Io has an elliptical orbit, so the tidal forces on it vary with time • The hot interior produces massive volcanism and a differentiated interior • Io has a iron core surrounded by a molten rocky mantle

11. Volcanism on Io • Io has no impact craters • Volcanoes produce plumes of material that extend up to 280 km above the surface • Volcanoes can be very long lived • Some have been observed for 20 years

12. Io’s Plasma Torus • Ions are atoms that have lost an electron giving them a net electrical charge • As Jupiter rotates its changing magnetic field produces an electrical current through the torus and interior of Io

13. Europa • Under the ice is water or warm fluid ice • Tidal heating produces the internal energy • Tidal flex may also crack the surface

14. Evidence for Warm Oceans on Europa • Galileo has imaged faults where the ice has pulled apart and water as flowed up • Galileo magnetometer measurements indicate that Europa has a variable magnetic field • On Earth simple life forms evolved under water at warm deep ocean vents • Could something similar have happened on Europa?

15. Models for the Interior of Europa

16. Ganymede • Ganymede is the largest moon in the solar system • Slightly larger than Mercury • But, Ganymede shows evidence for surface alteration • Did it once have a more eccentric orbit?

17. The Surface of Ganymede • Old dark terrain • Dark due to the ice being covered with dust from meteoroid impacts • New bright terrain • Bright due to fracturing of the icy surface

18. Callisto • It has experienced the least tidal heating • Callisto shows few signs of interior or surface activity • Large impact basin Valhalla • 3000 km in diameter • impact occurred about 4 billion years ago

19. Titan • Most distant planet sized solid body • Thick atmosphere make the surface impossible to see • Titan is large enough to have a reasonably strong gravitational field

20. Titan’s Atmosphere • Titan has a thicker atmosphere than the Earth • Titan’s atmosphere may have originally been composed of ammonia (NH3) and methane (CH4) • 90% of the atmosphere is N2 • Much of the rest is methane

21. Chemicals in Titan’s Atmosphere • Titan’s atmosphere also contains hydrocarbons (composed of H and C) and polymers (long chains of H, N and C) • Methane can rain from atmosphere and form streams and maybe lakes • Titan is very cold (95 K=-288 F) and so it would be difficult for life to form

22. Triton • It is probably a captured Kuiper belt object • Triton shows evidence of geologic activity • Few craters • Plumes of outgassing material • When Triton was first captured it was probably in a highly eccentric orbit which resulted in tidal heating

23. Triton’s Atmosphere • Triton is very cold (37 K) and thus nitrogen is mostly frozen on the surface • A little bit of nitrogen evaporates to produce the atmosphere

24. Next Time • Read Chapter 14.3

25. Summary • The six large moons of the gas giants resemble the terrestrial planets of the inner solar system • They can have volcanoes, atmospheres, and evidence of resurfacing • In general they are cold and have rocky interiors and icy exteriors • Some produce internal energy through tidal heating • Europa and Titan may possibly have the conditions for life to exist

26. Summary: Io and Europa • Io • Strong tidal heating produces massive volcanism • Volcanism produces powerful outgassed plumes, sulfurous surface and plasma torus of ions • Europa • Icy surface shows evidence for water flowing up from interior • May have a warm subsurface ocean due to tidal heating

27. Summary: Ganymede and Callisto • Ganymede • Shows both old dark terrain and bright new terrain • Must have had more internal heat to drive geologic activity in the past • Callisto • No tidal heating results in no differentiation • Fairly uniform mixture of icy and rock with many craters

28. Summary: Titan and Triton • Titan • Large size and low temperatures results in an thick atmosphere • Atmosphere composed of nitrogen, methane, hydrocarbons and polymers • Triton • Has a decaying, inclined retrograde orbit • Thin atmosphere and surface activity