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

2. 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

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

4. Formation of the Galilean Moons
The inner parts of the nebula were hotter than the outer
The outer satellites (Ganymede and Callisto) formed from rock and ice
Much of the energy that powers the large moons of the solar system comes from tidal heating
Need close, elliptical orbit

5. The Interiors of the Galilean Moons

6. 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

7. 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

8. 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

9. Gravity and Energy
An object of mass (m) launched off of the surface of a planet has kinetic energy:
When it reaches its maximum height (h) it has potential energy equal to
Where g is the acceleration of gravity for the planet
where M is the mass of the planet, R is the radius of the planet and G = 6.67X10-11 (in units of m3 kg-1 s-2)
KE = PE
Note that all distances are in meters and masses are in kilograms

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

11. 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?

12. Models for the Interior of Europa

13. 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?

14. 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

15. Callisto It has experienced the least tidal heating
Fairly uniform mixture of ice and rock
Covered with craters
Large impact basin Valhalla
impact occurred about 4 billion years ago

16. Titan Only moon with an atmosphere Why does Titan have an atmosphere?
Most distant planet sized solid body
Only moon with an atmosphere
Why does Titan have an atmosphere?
Titan is cold enough so that the gas in the atmosphere is slow moving

17. Titan’s Atmosphere
200 km deep with a surface pressure of 1.5 atmospheres
Much of the ammonia was disassociated by UV light from the Sun and the hydrogen escaped
90% of the atmosphere is N2

18. 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 lakes
Titan is very cold (95 K=-288 F) and so it would be difficult for life to form

19. Triton
Triton is in a decaying, highly inclined (23 degrees), retrograde orbit
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

20. 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

21. Next Time
Read Chapter 14.3

22. 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

23. Summary: Io and Europa Io Europa
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

24. Summary: Ganymede and Callisto
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

25. Summary: Titan and Triton
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