NASA today announced the discovery of what appear to be signs of water recently flowing on the surface of Mars. In new photos of large Martian gullies taken by the Mars Global Surveyor not long before it expired, scientists noticed several instances of light-colored streaks that had not appeared in past photos of the same regions taken as recently as 2001. The light coloring is especially exciting considering that meteor impacts and Mars rovers tend to leave dark trails in the soil.

Although water on Mars isn’t necessarily a new discovery—geographical evidence suggests that water existed in ample quantities at some point in the planet’s past, and it is also found in the polar ice caps and in traces of atmospheric water vapor—the fact that it may still be actively flowing on the planet’s surface could indicate the presence of active subterranean aquifers, which may support microbial life. Some scientists are skeptical, observing that the streaks seen in the photographs may be avalanches of carbon dioxide or dust. NASA, though, remains optimistic and will continue to investigate the potential source of the water streams with its remaining spacecraft in the planet’s orbit, including the Mars Reconnaissance Orbiter. —John Mahoney