On a ring origin of the equatorial ridge of Iapetus Options

[Guest_AlexBlackwell_*]

Wing Ip just had an interesting Iapetus-related paper published in GRL.

[ngunn]

Under the scenario proposed I would expect volatiles within the descending ring material to produce a temporary atmosphere around Japetus. Could this have provided sufficient drag to melt some of the infalling ices so that they fell as torrents of freezing rain or slush, solidifying pretty quickly on contact with the ground? I imagine this would produce a pretty hard ridge - a pile of 'cryolavas' rather than loose 'cryoregolith' - that would respond to subsequent (post-atmosphere) cratering similarly to the rest of the moon.

[ugordan]

Could this have provided sufficient drag to melt some of the infalling ices so that they fell as torrents of freezing rain or slush, solidifying pretty quickly on contact with the ground?

My guess is that due to Iapetus' weak gravity and thus low orbital velocity and also the fact it was probably spinning much more rapidly in the past, the impact velocity would be quite low as others have suggested. Probably too low for impact melting. If impact heating would be low, I'd guess the gentler drag through an atmosphere would heat up the particles even less, giving them time to cool. So once again, no significant melting. The stuff would fall down as it entered the "atmosphere", as dusty material.

[ngunn]

My guess is that due to Iapetus' weak gravity and thus low orbital velocity and also the fact it was probably spinning much more rapidly in the past, the impact velocity would be quite low as others have suggested. Probably too low for impact melting. If impact heating would be low, I'd guess the gentler drag through an atmosphere would heat up the particles even less, giving them time to cool. So once again, no significant melting. The stuff would fall down as it entered the "atmosphere", as dusty material.

I know these are just qualitative speculations with no hard numbers to back them up but I'm not convinced by this no-melting argument. There is no way of knowing the temperature or thickness of any temporary atmsphere formed and sustained by a catastrophic process like this. I would expect an atmosphere formed in this way to be hottest at the top and coolest at the surface (with possibly a very strong temperature gradient: note that Titan's upper atmosphere is remarkably warm even now). It would certainly have had a huge scale height due to the low gravity, probably extending out to the inner edge of the ring. Much would depend on the rate of infall of material. However it would surely have been significantly warmer than the current temperature of Japetus, conceivably warm enough to melt at least some volatiles even without the additional heat from friction. Add frictional heating and it's not hard to imagine a slushy equatorial blizzard growing the bulge layer upon layer, sort of stalagmite-fashion but by freezing rather than mineral precipitation.

[ugordan]

The point I was trying to make is the greatest temperature rise an impactor will get is a sudden surface impact so all kinetic energy is instantly converted into heat, with no time to radiate that heat away. A prolonged drag through an atmosphere will leave an object more time to lose the heat and thus remain cooler. I was implying no impact melting --> no atmospheric friction melting.
Wikipedia suggests Iapetus' orbital velocity is 430 m/s. I'm really not an expert on whether or not that is enought to vaporize/melt ice at cryogenic temperatures.

[ngunn]

A prolonged drag through an atmosphere will leave an object more time to lose the heat

Yes, but it would lose it's heat to the atmosphere which is warmed up thereby.

I'm really not an expert on whether or not that is enought to vaporize/melt ice at cryogenic temperatures.

A very simplistic calculation (using half m v squared and the room-temperature specific heat of water) suggests not. However we are not talking about pure water ice but probably a complex mixture/solution/clathrate material. You don't have to melt every grain to fluidise the material, only the more volatile constituents. Also we can't assume that a catastrophically created atmosphere would necessarily be at 'cryogenic temperatures'.