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Posted by: marsbug Apr 6 2011, 08:14 PM

HiRISE strikes again on the looking for water front: http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1537.pdf

Posted by: schaffman Apr 21 2011, 03:07 PM

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

Tom

Posted by: Drkskywxlt Apr 21 2011, 03:16 PM

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

Posted by: marsbug Apr 21 2011, 07:12 PM

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?

Posted by: Drkskywxlt Apr 22 2011, 12:06 AM

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.

Posted by: schaffman Apr 22 2011, 01:17 PM

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

Posted by: Drkskywxlt Apr 22 2011, 01:53 PM

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.

Posted by: schaffman Apr 22 2011, 03:47 PM

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

Posted by: Greg Hullender Apr 23 2011, 02:02 AM

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

Posted by: Fran Ontanaya Apr 23 2011, 12:18 PM

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.