An artist's concept of the Mars Express orbiter near the Red Planet. Photo: ESA

"The latest evidence that liquid water has flowed on Mars very recently, makes Mars Express even more relevant," says Agustin Chicarro, Mars Express Project Scientist. "Water may have flowed tens of thousands or a million years ago - that's still recent in geological terms. Or it may even be flowing now. Either way, this is very important."

The evidence is in the form of images taken with the camera on board NASA's Mars Global Surveyor (MGS), the only spacecraft now in orbit around the red planet. The images show surprisingly young features, most probably created by the seepage of groundwater on crater and valley slopes. Most of the features are at high latitudes in the southern hemisphere. They suggest that liquid water may still exist much closer to the surface of Mars than anyone thought possible, given the thin atmosphere and low temperatures on the planet today.

The evidence raises the stakes over two questions that Mars Express will set out to answer. Where is the water that flowed so copiously on the red planet early in its history? And is there life on Mars?

It now seems likely that much of the water remains in underground aquifers and may even be seeping to the surface in a few places today. If so, what allows the water to remain liquid on the surface long enough to carve the features seen by MGS? Are we witnessing evidence of the type of internal heating that gives rise to hot springs on Earth? Or are conditions at the surface sometimes different from what we expect?

"The water must be at less than 1 km depth and possibly as little as 100 m. This means that the radar (MARSIS) on board Mars Express should be able to detect it quite easily," says Jean-Loup Bertaux Principal Investigator for Mars Express's SPICAM experiment. The 1 km depth is well within the radar's capability of mapping underground water to a depth of 5 km.

Channeled aprons in a small crater within Newton crater. Photo: NASA/JPL/MSSS

"The discovery opens up again the door to life on Mars and adds impetus to Beagle 2, Mars Express's lander whose job is to look for signs of present and extinct life," says Chicarro. "On Earth, we know of all the incredible variety of organisms that survive in underground water, especially around hot springs. It could now be that this sort of environment exists on Mars."

Several Mars Express instruments will also be re-examining their list of interesting sites for special observation. "We'll be thinking about the most efficient way to operate our instrument to analyse as many of these water features as possible," says Gerhard Neukum, Principal Investigator for the High Resolution Stereo Camera (HRSC) on Mars Express. "The HRSC will be able to get images with as high a resolution as the camera on MGS, but we'll be able to point the camera more accurately and will be able to investigate the topography and morphology in great detail."

"Until these latest MGS images, my working hypothesis had been that Mars has come to rest, but it now seems that there's more activity than we'd expected, "says Neukum.