Really! A streak on the wall of a Martian crater has been found in Mars Global Surveyor satellite images that wasn't there four years earlier.
Water ice and water vapor are known to have existed below the surface of Mars in the relatively recent past, and water ice has been seen at the poles, but this is a pretty clear sign that water still flows on the surface of Mars. If water is present, that would raise the possibility of life.
The before-and-after pictures show what appear to be deposits of materials left by bursts of water flowing down the sides of the gullies.