Re:Why is it always water? (Score 3)

As a planetary geologist (and co-author of the Nature article in question), we suspect water as the fluid involved partly because of the large amount of water in the polar ice caps, the small amount of water in the atmosphere, and the general abundance of water in the solar system and universe (hydrogen is the most common element, and oxygen is up there too). Plus, the climate conditions on Mars are pretty close to allowing liquid water, so it is reasonable to suspect it could have been liquid under past climates.