be very well hidden (like on the bottom of the Antarctic or Greenland i Even though it's Sci-Fi, I almost like to believe that human beings move from planet to planet, using up local resources and destroying them. While some repliers have noted the lack of realism, I find the psychological aspect interesting. Why do you want such a story? Wouldn't a story where humanity was a constructive influence on the universe be better even if a tad boring? no, venus and mars will never become habitable, venus will not become cooler, nor mars wetter. most planetary bodies in the universe cannot support life. It is fine for humans to use the empty unihabitable things in any way they want, nothing of value will be lost. Not that I disagree that water was the cause; However, all fluid is made of matter and can briefly suspend particles of other more dense matter thus providing the capability to form deposits and layering if said fluid is in motion. The Martian atmosphere is known to have Dust Storms -- I put it to you that these Dust Storms are such suspensions of matter having varied densities, and that the dust is, in fact, relocated. I believe that Mars was not always a solid rock because it shows evidence of volcanism -- Magma is also a fluid / matter suspension and is thus capable of forming layers of material. Unless we observe the actual layers and their material compositions we will not know