Saturn has a strange regular hexagon at one of its poles, believed to be the result of a band of hurricanes powered by auroral interactions. On Mars, the North Polar ice cap has a similar mystery – a spiral pattern of troughs cutting through the cap. Now Radar observations from the Shallow Subsurface Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter (MRO) have given a boost to an old theory as to why these patterns, known about since 1972, exist.
SHARAD was able to take readings from reflections of ground penetrating radar that could discern between ice and other materials as well as even layers in the ice. Over time a cross-section of the ice could be built up and compared with different models derived from the various competing theories – including catastrophic floods and solar heating with lateral transport of heat – of how this thing came about. The one that stood out as by far closest to what was seen was the wind driven one created by Alan Howard.
In this theory, Katabatic winds, ones that fall down slopes (in this case the top of the ice cap) meet pre-existing cracks and pass across (not through) them. As they do so, they transfer material from the upwind side to the downwind side. This has the effect of extending the crack, and indeed the Coriolis force (the fact that the planet Mars rotates over the course of the wind’s journey from the top of the cap, making it look like the wind is spiralling down) makes those extensions look like spirals.
Not only does the data from the radar suggest the cross section of the ice matches diagrams of the extended canyons drawn by Howard in his 1982 paper (as opposed to lateral transportation of heat, which requires wind flow through the canyons, having an equal effect on both sides), but they also suggest that the canyons have been slowly extending towards the poles and upward in elevation by around 600 metres in 2.5 million years. The deposited layers also suggest that ice flows on Mars are less frequent than on Earth, in contrast to the predictions of rival catastrophic flow theories.