CHRISTOPHER CRUISE: Alfred McEwen is lead investigator for the orbiter’s High Resolution Imaging Science Experiment. He also is a professor of planetary geology at the University of Arizona in Tucson. Professor McEwen says a flow of briny, or salty, water is the best explanation for the orbiter’s observations. Other spacecraft and Martian meteorites have shown that the Martian surface is salty.

The professor says the water on Mars may differ from that found on Earth. He says the Mars water might be more like a thick fluid. “Science” magazine published a report about the orbiter’s findings.

BOB DOUGHTY: Professor McEwen and his team believe that water probably flowed across ancient Mars. But whether water exists as a liquid on the Red Planet can still be debated. The iron oxide on Mars gives the planet its reddish color.

Philip Christensen is an expert in geological science at Arizona State University. He says scientists have known for years that ice existed on Mars.

PHILIP CHRISTENSEN: “We know Mars has a lot of ice. But this is the first time we have seen the potential for liquid water. It might be salty water. But it is still liquid. And I think that is the real key here. It is not that Mars does not have a lot of ice. But liquid water – certainly to an organism – is very, very very different than ice.”

Geology expert Lisa Pratt of Indiana University welcomes the research results. She says the discovery will help scientists plan future travels to look for life signs on present-day Mars.