There is plenty of evidence that water formed vast oceans in the distant past on Mars, carving valleys and other features that are clearly apparent on its surface.
However, the possibility of liquid water on the red planet's surface today is perplexing, because it cannot survive there due to sub-zero temperatures and the thin Martian atmosphere. As such, the possibility of flowing water on the surface of Mars has been highly debated.
For instance, last year researchers suggested globs of salty water were seen on the legs of NASA's Phoenix Mars Lander. But others on the same team countered that the spots may have been frost instead.
There was no doubt that water ice was present at near the Phoenix lander – which set down in the Martian arctic. In fact, water ice has been found on Mars at the planet's poles, inside some craters and sitting beneath the surface across vast swaths of the Martian mid-latitude regions. But questions still remain on when water last flowed on the red planet's surface.
"There's been no smoking gun evidence yet as to whether liquid water has been on the surface of Mars in the recent past," Meyer said.
One of the main problems about proving whether or not water still flows on the surface of Mars "is that where we have seen gullies also tends to be where we don't want to land spacecraft," Meyer explained.
"The hope is that orbiter missions can catch something in the act involving liquid water near the surface today," he said. "We do suspect there's liquid water in the subsurface — it's just depends on how deep you have to go."
Europe's Mars Express spacecraft has been probing for hidden pockets of ice and liquid water using ground-penetrating radar. NASA's powerful Mars Reconnaissance Orbiter has also been searching for evidence of Mars' watery past from orbit.