According to NASA, Mars was once sopping wet.
From the article: “Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry,” said Dr. Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for the science instruments on Opportunity and its twin, Spirit. “We’ve been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion,” he said.
Dr. James Garvin, lead scientist for Mars and lunar exploration at NASA Headquarters, Washington, said, “NASA launched the Mars Exploration Rover mission specifically to check whether at least one part of Mars ever had a persistently wet environment that could possibly have been hospitable to life. Today we have strong evidence for an exciting answer: Yes.”