Two kissing craters revealed in a new image from Mars shows evidence of past glacial activity, according to the European Space Agency.
The hourglass-shaped pair were found at the eastern edge of Hellas Basin - roughly 38° South latitude and 104° East.
The impact craters lie near a mountain and scientists suspect a glacier accumulated at the base at some time in the past. If so, ice first flowed into the upper, smaller crater, which measures about 9 kilometres across.
The glacier then continued its flow downhill into the lower crater, which is about 17 km in diameter. Stripes in the craters probably indicate the direction of glacial flow from one crater to another.
"There are many features similar to this on Mars, though this is most certainly one of the largest and most dramatic," says John Mustard, a geologist at Brown University, Rhode Island, US.
Geological features like this on the Red Planet have been attributed to ice or glaciers since the 1970s.
"I believe this, and other evidence, is very convincing in showing that glaciers are probably on Mars," Mustard says. "Are they active today? Well, that's another story, but certainly they were active in the recent geologic past."
The picture was taken by the High Resolution Stereo Camera on Europe's Mars Express orbiter.