Even as the new Curiosity Mars Rover is making its way toward Mars for an August landing, one of the old (smaller) rovers, Opportunity, is still making discoveries after its almost 8 years on the surface of the red planet. Last week, scientists announced the discovery of a small vein of the mineral "calcium sulfate" -- most likely the form we call gypsum on Earth. Found in Endeavor Crater on Mars, this mineral deposit could not exist unless Mars had flowing water in the past.
This discovery cements (pardon the pun *) our understanding that billions of years ago, when Mars had not yet lost its thick atmosphere, there were streams, lakes, and underground flows of water on the planet. A number of other minerals that require water have been found by earlier rovers and probes. We also see large-scale areas that look like former river channels, flood plains, and lakes.
Today, Mars is drier than the worst desert on Earth and its air is horribly thin, because little Mars -- with its weaker gravity -- could not hold on to its air. But ancient Mars could have been much more Earth-like than Mars today. This is why astronomers continue to hold out hope that some sort of primitive life might have gotten started there long ago and that our exploring machines might find fossils or chemical indications of such a "second genesis" on another planet.
for more on this discovery.
Also, see my short article on Mars in general at: http://www.pbs.org/seeinginthedark/astronomy-topics/mars.html
(* The pun: gypsum is used in plaster of Paris and as a form of cement)