Pictures taken by the camera on NASA's Mars Global Surveyor suggest that rivulets of liquid water were flowing on the Red Planet within the past few years and may still be flowing today, welling up from beneath the Martian surface and streaming down gullies along the sloping walls of impact craters.

Two craters in particular caught the attention of NASA scientists. Both are in the planet's southern hemisphere, and both looked unremarkable at first glance. That changed when the Surveyor team reimaged the craters over several years and spotted two brightly streaked gullies with branched endings—a hallmark of flowing water—that were not visible in earlier pictures. The researchers speculate that the streaks formed when water bubbled up from a subsurface reservoir and ran down the gullies, leaving behind a pale-toned trail of sedimentation that is seen in the Surveyor snapshots as a bright line against a darker background. 

"We reimaged it several times at different sun angles to be sure it wasn't just a trick of different illumination conditions," says Ken Edgett, a member of the research team that made the discovery. More than just a geologic curiosity, finding water on Mars has major implications for the search for life, because the presence of H₂O greatly increases the odds that living organisms once thrived on the planet, and perhaps still inhabit it today. "We didn't expect this to happen in our lifetime, let alone our 10-year mission," Edgett says.

But is the liquid flowing down these gullies really water? Probably, say experts at NASA's Jet Propulsion Laboratory, but more evidence is needed to seal the case. "You can never be certain from orbit," Edgett says. "But this is the best evidence yet of liquid water being present on Mars right now."