If there was once water on Mars, where did it go?
Mars is a small planet, so it has lost much of its atmosphere to space. Because there isn’t a thick atmosphere (and in particular, since there’s no ozone layer), ultraviolet light can penetrate all the way to the surface of Mars.
Ultraviolet light does a good job of breaking up water molecules into hydrogen and oxygen. Hydrogen, being very light, escapes into space. Oxygen reacts with surface minerals to create the characteristic rusted red color of Mars.
There was a recent thread on this. Most of the water was probably lost due to asteroid collisions early in Mars’ history blasting most of the atmoshpere into space, and the fact that Mars might not have started with as much water to begin with. Only about enough water to cover the whole planet to about 2 meters worth of water was liekly lost due to photolysis. The rest of the water is probably underground in permafrost.
Eh? Where’s that come from, the D/H ratio? Can I have a cite for the majority of Mars’ water loss being from impacts rather than photodissociation?
It’s a simple question, but it doesn’t have a simple answer. If you’re really interested, I can strongly recommend the book Mapping Mars by Oliver Morton, one of the best lay surveys of the red planet I’ve come across.
Part 3, “Water,” addresses this question specifically, and in some historical detail as the competing theories are winnowed down into what we consider our best modern guess. Even that’s being finessed, though, as new data from Spirit and Opportunity come back.
Seriously. It’s a great read.