HiRISE strikes again on the looking for water front: LPSC 2011, spectral evidence for liquid water on Mars.

Interesting. I wonder whether the brines could be a source for the gypum detected in the polar dunes.

Tom

Alfred McEwan stood up during that talk to specifically say that he, speaking for the HIRISE team, does NOT agree with their conclusions.

Thats good, a claim like that needs to be challenged (without passing judgment on it myself - challenging it is just scientific process at work). Do you recall any of the specifics of McEwans critisicm?

Unfortunately, no. He wasn't denying that they were seeing SOMETHING, but just said it wasn't liquid water. Don't recall what he thought it was.

One thing the abstract doesn't overtly mention is that the area showing the purported location of the spectral signals for brines (Fig. 2) is located in the crater Richardson, near the south pole. The abstract just says "polar region." The discussion of the Phoenix results at the beginning leads one to conclude the authors are talking about the north polar region. The elevations around Richardson crater are about 1 km above datum in contrast to the several km below datum for the north polar region. It seems to me that the lower atmospheric pressures in the south polar regions make these locations a less likely place for liquid brines than in the north. No?

Tom

Perhaps...does the salt in the water also allow it to stay liquid at low pressures as it lowers the freezing point? For pure water that's certainly true.

I think adding salts decreases the vapor pressure of pure water, which increases its boiling temperature. The water boils when its vapor pressure exceeds atmospheric pressure. So, briny liquids should also be more stable at higher elevations on Mars than pure water, all else being equal. Still, I’d expect the stability field of the same liquid to be wider at lower elevations, making its presence more likely in the low elevation north than the south. Aslo the presence of abundant water ice is better documented for the north polar region.

As an additional note, the authors’ Fig. 2 is from images taken in the southern spring (Ls 210°–220°) when the seasonal CO2 cap is rapidly sublimating. I think CO2 can interfere with the spectral signature of water. (It’s interesting that CO2 ice is not included in their Fig. 1 for comparison.)

All in all, based on this layman’s opinion, McEwen is right to be skeptical.

Tom

I think the question about the salts' effect on boiling/melting points has been asked here before. If I remember right, the answer was that these are sulfate salts, and the effect is much smaller than for the sort of salts we're used to thinking about.

--Greg

It doesn't necessarily have to be exposed to the atmosphere. A thin crust of ice could hold enough pressure to reach the triple point.