NASA thinks it has found an explanation for the “spiders” it's spotted on Mars.
No, you haven't missed news of Martian arachnids. These spiders are a series of erosion channels that meet in a central pit and are common in regions near Mars' South Pole.
A decade's worth of data from The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) has now shown that spiders and the channels that comprise them are growing. As described by Ganna Portyankina of the University of Colorado, Boulder, lead author of the recent paper Present-day erosion of Martian polar terrain by the seasonal CO2 jets, clearly something is deepening and lengthening the troughs.
But what is that something? NASA expresses a fondness for the 2007 hypothesis that when spring comes to Mars carbon dioxide ice on the planet's surface warms. Ice on the bottom of ice sheets evaporates and “The trapped gas builds pressure until a crack forms in the ice sheet. Gas erupts out, and gas beneath the ice rushes toward the vent, picking up particles of sand and dust.” Repeat that a lot and you get a trough.
We know that there's ice around Mars' North Pole too. So why don't we see spiders there?
Portyankina and pals say the South Pole is less sandy than the North, so troughs are less likely to fill in again as wind blows sand about. Southern troughs therefore have a chance to lengthen, connect and form spiders.
Reg editorial guidelines frown upon large images in stories, so if you want to see this at work NASA has posted a lovely .GIF of MRO images here.
Spiders take a long time to grow: the paper supposes a spider forms over more than 1,000 Martian years (1,900-plus Earth years). The Register will happily follow up on this story after such a period has expired. ®