NASA’s Opportunity rover has found definitive evidence that Mars was once “drenched” with liquid water, mission scientists announced on Tuesday.

The finding ends decades of speculation on whether the red planet, which is now cold and desert-like, was wet in the past.

The Opportunity rover has been studying the rocks at its Martian landing site for several weeks. Bit by bit, the evidence has been mounting that large quantities of liquid water acted on the rocks sometime in the planet’s past.

“The last puzzle piece fell into place a few days ago,” said Steve Squyres, the lead scientist for the twin rovers mission. “We have concluded that the rocks here were once soaked in liquid water.”

The clincher was the detection of high concentrations of sulfates and other minerals that typically form in water. On earth, rocks only acquire such high concentrations of these minerals by forming in water or being exposed to large quantities of water after formation.

Dr. Squyres outlined an even more exciting possibility–that liquid water may still exist today tens or hundreds of meters below the Martian surface. “You might have to dig pretty deep…[but] it’s definitely a possibility,” he said.

NASA landed the two identical rovers on opposite sides of Mars earlier this year. The rover named Spirit landed in a large crater named Gusev on January 4. Opportunity landed in an area called Meridiani Planum on January 25.

The immediate landing areas at the two sites are quite different. Opportunity happened to land inside a crater, where rocks bearing telltale signs of water were exposed. Spirit landed in a flat area, where the rocks appear to be of mostly volcanic origin.

The European Space Agency’s Beagle 2 rover which was scheduled to land on Mars on December 25, 2003, but it failed to send a signal back to Earth confirming the landing and mission controllers have been unable to contact it since.

NASA’s twin rovers have been driving around, taking pictures, digging in the soil, and grinding and examining rocks since they arrived.

In addition to the evidence for ancient water uncovered by opportunity, the two rovers have achieved several scientific firsts, surpassing NASA’s Mars Pathfinder rover, which landed in 1997 and which is the only other rover to have driven on Mars.

Opportunity made a movie of the Martian sunset, which helped scientists measure the amount of dust in the atmosphere. It has also been occasionally peering up at the sky above it as ESA’s Mars Express orbiter passes overhead and looks down, providing valuable simultaneous measurements of the atmosphere from different perspectives. Spirit took the first pictures to show individual grains of Martian sand.

One of the few glitches so far in the mission was a temporary communications outage with Spirit on January 22. The rover’s computer was resetting itself dozens of times each day. Engineers traced the problem to Spirit’s overloaded flash memory, a type of hardware which is also used in digital cameras and other electronic devices. After engineers reformatted the flash memory, the rover returned to functioning normally.

Spirit should arrive at the edge of a 150-metre-wide crater named Bonneville in a few days, after driving several hundred metres from its landing site. Opportunity is heading towards a similar-sized crater named Endurance, but won’t reach it for a few more weeks. Scientists are excited to see the inside of the craters, because it will allow them to examine rocks deep beneath the surface, which may offer clues to a much earlier time in Mars’ past.

“Stay tuned-there’s some very good stuff in the week or two ahead of us,” said Dr. Squyres.