From what I have read, I understand that there is no known liquid water on mars. (this may very well be the fallacy of my thinking)
As we can all see in the current pictures, mars currently has an southern ice cap. From what I understand, this will disappear depending on it’s position in orbit around the sun (or change in season, if you will)
Certainly this ice would need to melt, creating liquid water, right? I suppose if could evaporate into the atmosphere (sublimation, right?) but how would it reappear as ice later - rain?
Read the book Worlds Without End by John S. Lewis. He’s a planetary scientist, and during the first part of the book he explains in great (and remarkably readable) detail the current thinking on how the bodies of the solar system formed. (The latter sections of the book are largely speculative; interesting, but not germane to this question.)
To simplify greatly, when the Great Disk of Dust began accreting into the solar system, the heavier elements (and materials resulting from the combination of said elements) fell more toward the center, while the lighter stuff stayed toward the outside. Hence, Mercury, the innermost planet, has a lot of iron, while Neptune and Uranus, the outermost big planets (disregarding Pluto, an odd case), contain a lot of helium.
Earth and Mars, being in the middle, have a mixture of heavy and light material, and they also wound up with a lot of the water. In the case of Earth, the body is large enough that its gravity and magnetic field protect its insulating atmosphere, which prevents water from boiling off in the vacuum. Mars, being smaller and less geologically active, cannot maintain an atmosphere, and its surface water dissipates, leaving only whatever is buried (protected by soil) or combined with other materials (e.g. frozen carbon dioxide, if I remember correctly; I’m a layman repeating what I’ve read as best as I can recall).
Incidentally, water is actually fairly common in the universe, from what we can tell. Comets are basically big snowballs, and the Oort Cloud and Kuiper Belt objects contain a lot of water ice. That’s why so much bad science fiction is so amusing; the lizard people in V, for example, didn’t need to come to Earth to steal our water: They could have mined a hundred times as much as we have from the crap orbiting outside the asteroid belt.
Mars has very low air pressure and very low humidity, and a temperature range averaging about 60°F (or 35°C) lower than Earth’s (and somewhat broader). Over most of the planet’s surface the pressure is too low to permit liquid standing water to exist – think of what happens to water spills in Phoenix in July, and extrapolate. Carbon dioxide would exhibit the same behavior on Earth if the temperature got low enough for condensation. I recall some years ago seeing a “sea level” line on a map of Mars, which was effectively the Hellas basin in the southern hemisphere – the area in which liquid water could exist when the temperature permitted.
(I am resisting the temptation to answer the OQ with "Well, when two hydrogen atoms and an oxygen atom like each other very much… ;))
