Space/Science — HabitableZone

Space/Science » in reply to Now what? Another NASA "significant find" briefing coming up.

Why water (vs dry/gas flow)?
dtb — (archived)

Not to steal Nick's thunder, but as I read it the main evidence that this is liquid water is that the flow deposit is bright -- thus likely to contain ice and/or bright salts. The logic there is that dust avalanches typically show as dark streaks, consistent with soil disturbed by the rovers (or Viking scoops or Sojourner) being generally darker than undisturbed ground.

OK, sounds plausible.

Two thoughts:

1. Are we _certain_ it's not a dry, bright deposit? I suppose you could have a light colored layer of on the wall that is also easily erodable (evaporites?) and thus more likely to generate dust. No apparent evidence for that in the photo, though.

2. How much water would have to be present in a flow to leave a thin coating of ice?

Let's postulate a flow of mixed CO2 gas, dust, salts, and water from an exposed and violently sublimating clathrate or something. What fraction would have to be water in order to leave a noticable icy coating behind? I'd guess not all that much. That's more than zero but still this might be more like a damp icy fog / dirt cloud rolling down the hill than like a flowing spring. Would the water even have to unfreeze -- what about fine ice granules? What exactly happens to CO2/water clathrate when you heat it up under Martian conditions?

Then again, maybe it's more like a flowing spring.

Nick's convinced me to be skeptical of flowing water explanations for features, but I remain uncertain who I think is more likely to be right for these gullies.

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