The Spirit rover, incredibly still working on Mars over 1000 days after its arrival, has made another important discovery, spotting what appears to be a "bomb sag." This may not sound terribly interesting, but to a geologist, this is heady stuff. A bomb sag is a kind of material formed in volcanic explosions on Earth. Rock is ejected up by the explosion and then falls into soft deposits, deforming as it lands. Spirit has snapped just such an object, preserved in layered rocks on the lower slopes of a plateau called Home Plate. Also spotted by Spirit are signs in the surrounding rock of of tiny spherical particles that look like accretionary lapilli. These are coagulated bits of ash that typically rain down after a volcanic explosion, so the evidence suggests that this was a genuine volcanic explosion, rather than the fallout from a meteor strike. What has the scientists interested is that they are seeing lots of evidence from photographs snapped from orbit that this sort of feature is very common, so it looks like Mars had a very violent past. Additional evidence places water at the scene as well. There is for instance a great deal of chlorine evident, which points toward the presence of a briny fluid, and it looks like the bomb sag landed with a splat. The fact that the material it sits on is basalt is also telling. Basalt is not normally associated with explosions, except when it meets water and you then get a steam-driven blast.