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CellBioGuy comments on "Stupid" questions thread - Less Wrong Discussion
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It not just NaCl, its lots of minerals that get deposited as the water they were dissolved in goes away - they're called 'evaporites'. They can be hard to see if they are very old if they get covered with other substances, and mars has had a long time for wind to blow teeny sediments everywhere. Rock spectroscopy is also not nearly as straightforward as that of gases.
One of the things found by recent rovers is indeed minerals that are only laid down in moist environments. See http://www.giss.nasa.gov/research/briefs/gornitz_07/ , http://onlinelibrary.wiley.com/doi/10.1002/gj.1326/abstract .
As for amounts of salinity... Mars probably never had quite as much water as Earth had and it may have gone away quickly. The deepest parts of the apparent Northern ocean probably only had a few hundred meters at most. That also means less evaporites. Additionally a lot of the other areas where water seemed to flow (especially away from the Northern lowlands) seem to have come from massive eruptions of ground-water that evaporated quickly after a gigantic flood rather than a long period of standing water.
Thank you!
So, basically (3) was almost completely wrong, and (1) missed the fact that “ocean” doesn’t mean quite the same thing everywhere.
Could you explain (2) a little bit? I see in Earth seawater there’s about 15 times more NaCl by mass than other solutes. Is there an obvious reason for that, and is that Earth-specific?
I honestly don't know much about relative salinities of terrestrial versus Martian prospective oceans. I do know however that everywhere that's been closely sampled so far by rovers and landers has had lots of perchlorate (Cl O4) salts in the soil, sometimes up to 0.5% of the mass. This can form when chloride salts react with surrounding minerals under the influence of ultraviolet light... nobody is terribly confident yet about what actually happened there to make them given that these results are new since the Phoenix lander and Spirit and Opportunity, but it's certainly interesting and suggestive.
I also think I should add that there is some evidence that a good chunk of Mars's water went underground - the topography of just about everything within ~30 or 40 degrees of the poles is indicative of crater walls slumping from shifting permafrost and there seems to be plenty of solid water in or under the soil there. The oceans may not have only dried up so long ago, they may have sunk downwards simultaneously.