It turns out David Bowie was right: there really are spiders on Mars!
These ones, though, aren’t eight-legged creepy-crawlies, but rather a geological phenomenon.
Spiders on Mars are really a geological phenomenon called ‘araneiform terrain’, if you want to get all technical about it.
All the same, look at pictures of the Martian surface by the Mars Reconnaissance Orbiter or Mars Express missions.
These little black bobs do indeed look like dozens of spiders crawling around.
What causes spiders on Mars
The 'spiders' on Mars aren't really spiders, of course.
Spiders on Mars are a seasonal feature related to the composition of Mars’s atmosphere, which is 95% carbon dioxide (CO2).
During the Martian winter, layers of frozen carbon dioxide build up on the ground.
Come spring, sunlight begins to thaw this ice from the bottom upwards, so big clouds of CO2 gas form below the surface of the ice.
Eventually, once the top-level ice ‘crust’ has thinned to a metre or so in thickness, these gas clouds burst free.
Gas and dirt erupts through the ice in huge plumes, like water in a geyser or lava from a volcano.
As the plumes subside, the dust and dirt they’ve dragged up from the surface below falls back down to the ground.
This forms the distinctive spider-like marks that you can see in these pictures.
The individual ‘spiders’ on Mars measure anywhere from 45m to 1km (150 to 3,280 feet) across.
Discovery
From the point of view of Earth-bound astronomers, these spiders are a relatively new phenomenon, having only been observed for the first time in the past few years as higher-resolution images of the Red Planet have come within our reach.
They can found all over the Martian surface, but are particularly common in the region around a large structure that’s been dubbed Inca City because its high ridges, which form squares and rectangles, resemble the Incan ruins that archaelogists have discovered here on Earth.
Why ‘spiders’ should be more common in this region, though, is currently unknown – as are the mechanisms behind Inca City’s unusual topography generally.
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