Dramatic images of dried-up floodplains and apparent riverbeds have most astronomers convinced that liquid water once gushed on Mars, perhaps supporting ancient life. But last summer NASA astronomers announced that water may still be flowing on the Red Planet.
The evidence comes from the Mars Reconnaissance Orbiter, which captured images of thin, dark streaks—quite likely soil—creeping down several slopes during Mars’s spring and summer (see sequence at right). Dry soil should not move that way, so a team led by Alfred McEwen, principal investigator for the probe’s camera, proposed that liquid water carried the soil downhill. In the journal Science, the scientists noted that water could flow sluggishly just beneath the surface and remain liquid owing to high concentrations of salt, which acts as antifreeze. The orbiter continues to scan for clues; later this year NASA’s Curiosity rover will begin studying Martian surface chemistry up close.