We have not yet found firm evidence for liquid water on the surface of Mars. In general the conditions at the surface are such that water is either frozen or in the vapor form. Now recently there's been discoveries of dark stains on the sides of hill slopes that scientists think might be liquid watermarks. It will be very interesting to see if that really proves to be liquid water.
Joy Crisp
Mars Exploration Rover Mission Project Scientist
Jet Propulsion Laboratory