Hi All

Something cut out a chunk of my post. I think I messed up my html *sigh*

...search for Prentice, A.J.R. and there are plenty of his articles available online.]

believes the whole moon is like that - a vast barren landscape of smooth ice. He dismisses the popular view the moon has lakes or seas of hydrocarbons like ethane, which are still liquid at Titan's temperatures.

[A summary of his recent paper on Phoebe and Titan...

At Saturn's orbit (Tn = 94 K, pn= 4.7 × 10-7 bar), we have rock (0.494), water ice (0.474), graphite (0.032) and mean density 1.52 g/cm^3. Structural models for Titan yield a mean density of 2.09 g/cm^3 (homogeneous case) and 1.93 g/cm^3 (differen-tiated). For the latter, C/MR^2 = 0.32. It is predicted that Titan has no internal ocean or induced magnetic field but it may possess a small native dipole field due to thermoremanent magnetization. Capture of Titan was achieved by gas drag within the proto-Saturnian envelope whose initial size was ~ 60 RSat. Titan should thus have the same appearance as Triton

...thus his comment "smooth icy surface". Also, without ammonia or methane to change the thermal properties, the ice will reach equilibrium with the core's heat output by undergoing solid-state convection rather than forming an internal ocean.]

But Huygens, which will parachute to the surface on January 15 after being released from Cassini on Christmas Day, should have "the last say" on the matter, he says.

"One per cent of a surface is barely a sample," he says. "My experience has been if something's going to happen, it's going to show up fairly early in the piece."

Tidbinbilla, the NASA deep-space tracking station in the ACT, will be receiving signals from Huygens (via Cassini) during the mini-probe's descent to Titan.

Titan's surface is not the only mystery. Why is Titan the only moon (notwithstanding a super-thin wisp of gas surrounding one of Neptune's moons) with an atmosphere? Where did it come from? Was it thicker in the past?

Prentice believes gases released from Titan's interior formed its air.

[Not since I last read his papers - the methane might, but last I read he thought the nitrogen was from ammonia ice that had pyrolysed on accretion by Titan during the gas-drag phase in the proto-Saturn cloud. Hence it is from a veneer of volatiles not interior ices.]

Others, including Ross Taylor from the Australian National University, suspect Titan's atmosphere may not be its own. Titan's neighbouring moon, Hyperion, suffers from a tumbling, chaotic rotation, the result of a long-ago cataclysmic collision with a comet or asteroid. Titan's gravity may have scooped up the debris - a readymade atmosphere.

[Impacting ammonia ice would probably dissociate: 2NH3 -> N2 + 3H2...]

And what will Titan's chemistry tell us about the origin of life? Prentice says that if Titan were in Earth's orbit, 150 million kilometres from the sun, instead of being 10 times further away, life could have arisen. But it's not.

"The initial Earth may have started out the same way with a nitrogen atmosphere, with methane as well," he says. "But everyone agrees that it's too cold (on Titan) for anything to happen."

[we shall see... ;-) ]

Adam