Two probes circling the Red Planet have discovered evidence that water was once present in the northern hemisphere of Mars, a sign the planet's entire surface may have been habitable billions of years ago.

Lyot crater is one of nine sites in the Martian northern hemisphere with exposed clay-like minerals that formed in ancient watery environments. This image shows the locations of the minerals and the areas scouted by Mars Express' OMEGA instrument and MRO's CRISM spectrometer. Credit: NASA/ESA/JPL-Caltech/JHU-APL/IAS

The European Space Agency's Mars Express mission and NASA's Mars Reconnaissance Orbiter teamed up to make the discovery, which is the first definitive proof that all of Mars was affected by water, not just the southern hemisphere.

NASA announced the findings Thursday, and a report describing the research is published in the journal Science this week.

"We can now say that the planet was altered on a global scale by liquid water about four billion years ago," said John Carter of the University of Paris, the report's lead author.

According to NASA, spectrometers on Mars Express and MRO have mapped clay minerals at thousands of sites in the planet's southern highlands, confirming surface observations from the agency's rovers that Mars once hosted a wet, and perhaps habitable, environment.

But younger volcanic rock paved over the ancient surface material in the northern hemisphere, hiding the tell-tale clay minerals that must have formed in a watery environment.

The OMEGA spectrometer on Mars Express first detected the chemical signature of clays and phyllosilicates, both indicators that water once existed in those locations. The clays were exposed by more recent impact craters carved by asteroids and comets.

Scientists followed up on OMEGA's observations with the CRISM instrument on MRO, which verified the discovery with its high-resolution imager.

Artist's concept of MRO. Credit: NASA/JPL-Caltech

"We needed the better spatial resolution to confirm the identifications," Carter said. "The two instruments have different strengths, so there is a great advantage to using both."

CRISM studied 91 craters in the northern lowlands and found clays and phyllosilicates in at least nine of the sites.

The clay markers also give clues on the sequence of geologic events that shaped Mars, including a massive collision with a large object that slammed into the planet's northern hemisphere. The impact smashed down the northern hemisphere, leaving the region with a lower average elevation than southern Mars.

Researchers say evidence of water in the northern lowlands implies the planet-crushing impact occurred well before the end of the wet epoch in Martian history, narrowing the window of time in which Mars may have been habitable.

"That large impact would have eliminated any evidence for the surface environment in the north that preceded the impact," said Scott Murchie, CRISM principal investigator from the Johns Hopkins University Applied Physics Laboratory. "It must have happened well before the end of the wet period."