Two craters on Mars proves that there were glaciers in the past on the Red Planet. The craters touch and show a flow line from one, the upper one, downhill into the lower crater. According to the New Scientist, 
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.