Thanks for the info and link, Alex. This is another tantalizing piece of evidence for past water in large quantities, though I'm sure there will be alternate explanations proposed. I have a hard time imagining the explosive CO2/clathrate floods in Nick's model behaving as Ivanov and Head describe, and I don't think White Mars accomodates large liquid oceans. It's hard waiting for Odyssey to arrive; I really hope it makes it in one piece and returns some more definitive evidence.

I find it interesting that the channels described end at the same elevation, even though they are of different ages, indicating either that the hypothesized ocean was around for a long time or that it formed repeatedly and to the same levels.