The High-Resolution Imaging Science Experiment, or HiRISE, is a 20-inch (0.5-m) f/24 telescope — the largest ever flown to another world — that serves as the primary camera aboard NASA's Mars Reconnaissance Orbiter. Its job is to look down on the Red Planet and record the ruddy landscape with unprecedented detail — typically just 1 foot (0.3 m).
The camera continues to click away, and on January 10th it spotted a remarkable double crater in the Icaria Fossae region. As HiRISE principal investigator Alfred McEwen (University of Arizona) points out, whatever excavated these conjoined pits, each about 1 mile (1½ km) across, "must have separated into two distinct pieces prior to impact in order for two craters to be recognizable." There's no way to tell if the paired projectiles were from an asteroid or a comet, but the two pieces must have been equally massive to make equally sized craters.
The impactors wouldn't necessarily have hit at precisely the same moment: for example, even striking at the relatively slow collision speed of 10 miles (15 km) per second, they could have trailed one another slightly. However, the near-perfect symmetry, including the splash of debris extending out of the frame toward the top and bottom, argues for a side-by-side delivery.
Planetary scientists now realize that the solar system's smallest bodies come in a wild assortment of shapes. It's not too much of a stretch to imagine that lumpy bodies like Comet 103P/Hartley 2 or asteroid 25143 Itokawa could have created this feature. McEwen's team has also identified the remnants of a triple crater elsewhere in the HiRISE archive.
Double craters aren't especially rare, even on Earth. The side-by-side Clearwater Lakes in northern Québec aren't quite equal in size, but they're big &mdash they must have made a real mess of things when they formed some 290 million years ago.
Of course, HiRISE is spotting far more than just curious craters. Check out this gallery of amazing shots taken from its first 4½ years of operation.