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

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

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."

This special publication features the photography of British astro-imager Nik Szymanek and covers a range of photographic methods from basic to advanced. Beautiful pictures of the night sky can be obtained with a simple camera and tripod before tackling more difficult projects, such as guided astrophotography through the telescope and CCD imaging.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This new poster features some of the best pictures from NASA's amazing Mars Exploration Rovers Spirit and Opportunity.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE