This image shows a double impact crater in Syria Planum, and probably formed when a binary asteroid pair (two asteroids closely orbiting each other, while also orbiting the Sun) struck the surface. The asteroids must have been about the same size, on the order of a few hundred meters across, to produce these craters.
How is it possible to say that the double crater is due to a binary asteroid, instead of two independent impacts? Neither crater shows signs of burial by ejecta from the other. More importantly, the ejecta (material thrown out of the craters) shows signs of interacting; the ridges extending to the southeast of the crater probably formed when ejecta from the craters collided in midair, causing more debris to pile up at certain points. This means that the impacts occurred within moments of each other.
Ejecta interaction features like this can also form in association with “secondary” craters (craters made by debris from other impacts, rather than by asteroids), since many secondary craters form at once. In this case, however, a binary asteroid is the likely cause. The very large size is one indicator (secondary craters are generally much smaller than the “primary” crater), and there is not an obvious nearby source crater. While secondary craters are common, binary impacts are expected to occur as well, since binary asteroid pairs are observed.
This crater pair is also of geologic interest since it exposes a cross-section of the local rocks. Thin, flat layers are visible in the upper walls. Since this region has seen extensive volcanic activity, these may be a mix of old lava flows and other volcanic debris. Exposures like this provide evidence for the extent and thickness of these deposits.
Written by: Colin Dundas (8 October 2008)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_009619_1630
Image: NASA/JPL/University of Arizona