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?
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.