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Cool image time! Today’s release of new captioned images from the high resolution camera on Mars Reconnaissance Orbiter (MRO) included a wonderful image of the melting carbon dioxide cap of Mars’s south pole. On the right is a cropped portion of the full image, showing what the MRO scientists nickname spiders, features that appear as the CO2 begins to turn into gas.
But these aren’t actual spiders. We call it “araneiform terrain,” to describe the spider-like radiating channels that form when carbon dioxide ice below the surface heats up and releases. This is an active seasonal process we don’t see on Earth. Like dry ice on Earth, the carbon dioxide ice on Mars sublimates as it warms (changes from solid to gas) and the gas becomes trapped below the surface.
Over time the trapped carbon dioxide gas builds in pressure and is eventually strong enough to break through the ice as a jet that erupts dust. The gas is released into the atmosphere and darker dust may be deposited around the vent or transported by winds to produce streaks. The loss of the sublimated carbon dioxide leaves behind these spider-like features etched into the surface.
The image above shows older spiders, formed during past seasonal events. If you click on the image you can see the full image, which shows darker spiders produced by this season’s cycle.