Matt_dk writes "Scientists are now able to explain why Mars' residual southern ice cap is misplaced, thanks to data from ESA's Mars Express spacecraft (the same probe running the 'Mars Webcam'). It turns out the martian weather system is to blame. And so is the largest impact crater on Mars — even though it is nowhere near the south pole. Like Earth, Mars has frozen polar caps, but unlike Earth, these caps are made of carbon dioxide ice as well as water ice. During the southern hemisphere's summer, much of the ice cap sublimates, a process in which the ice turns straight back into gas, leaving behind what is known as the residual polar cap. The mystery was that while the winter cap is symmetrical about the south pole, the residual cap was offset, and scientists couldn't figure out why."