Deposits formed in Martian gullies during the past seven years suggest that liquid water exists on Mars today. An image taken by the Mars Global Surveyor spacecraft in 2005 shows a downhill track on the wall of a crater that was not present in the previous image of the crater, taken less than four years earlier. In subsequent views of the deposit, the sun's light is coming in at different angles, but the light-colored material remains, suggesting it is not a trick of the light or the result of dry erosion. Similarly, images of another crater from February 2004 show the beginnings of a second deposit, which has grown in subsequent images, according to a report in the December 8 edition of Science. Finding additional examples might be tricky, given that NASA lost contact with the Mars Global Surveyor last November after nearly 10 years of operation.
This article was originally published with the title "Free Flow on Mars" in Scientific American 296, 2, 26 (February 2007)