Water was positive on Mars several years ago. What is the holdup?
"Not all water is fit to drink," said Andrew Knoll, a member of the rover science team who is a biologist at Harvard University, Cambridge, Mass...and...
...the roles and fate of water on a planet whose most striking difference from Earth is a scarcity of water.Mars has water, but Mars has very little atmosphere. Liquid water can't exist except during transitory events (such as impact).