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NASA Says Mars Rocks Formed in a Salty Sea
NASA has made another announcement, live on NASA TV, regarding the discoveries of the Spirit and Opportunity rovers. They believe that the rocks examined by Opportunity were actually formed in water; that those rocks were actually sediments laid down in a shallow salty sea. They've already had outside scientists examine their data and those scientists concur with the conclusions. NASA has a story with explanations and some photos.
No, salt accumulates in the oceans from the erosion of surface soils and rocks, as the minerals wash into larger bodies of water. This may mean that Mars once had rain.
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It (probably) got there in the first place during Mars' formation, and perhaps later due to cometary bombardment.
As to why it was lost, crudely put: evaporation into outer space.
Molecules of volatile gasses, including water vapor, that waft into a planet's upper atmosphere occasionally reach escape velocity and are lost.
Why some gasses and not others? There are a bunch of factors at work:
Heavier gasses -- CO2, for example -- require more energy to get up to escape velocity. They statistically hang around longer.
Larger planets have higher escape velocities.
Planets farther from the Sun recieve less insolation, so there's less of a chance that a molecule will get kicked up to escape velocity.