Araneiform features on the surface of Mars, as imaged by the Mars Reconnaissance Orbiter in 2009. (NASA/JPL-Caltech/University of Arizona) ====================================================================== Mars has spiders unlike anything else in the Solar System. Between the ocher dunes, across the scars of impacts, long-legged shadows appear to scuttle across the dust. They aren't actual, living spiders. These tendrilled shapes that appear in satellite images of the red planet are made, like almost everything on Mars, from dust. They're known as araneiforms, small systems of dark troughs that appear only in the southern polar region of Mars in the planet's spring. Exactly how the araneiforms...