This is one of the many new impact sites that have been dated using repeat imagery of the same spot on Mars. A dark spot was seen in an image from the Context camera on the Mars Reconnaissance Orbiter, a spot that was not present in previous observations from other missions. When HiRISE followed up with this image, an impact origin was confirmed. In this case, it wasn't just one crater that had formed, but more than thirty.

A cluster of craters was created because the asteroid or comet colliding with Mars broke up in the atmosphere. Even though the Martian atmosphere is very thin compared to Earth's, the impacting objects are traveling at such high speeds that the force of friction is very large. Some of that frictional force goes into slowing the object down. If the impactor is weak, perhaps made of porous material or already fractured by previous impacts in the asteroid belt, it can also break apart as it hurtles through the atmosphere. Each of the fragments collides with the surface at very high velocity, forming separate craters.

The largest crater in the cluster is only approximately 4.5 meters (about 15 feet) across, and the smallest ones are so tiny, they can't even be seen with HiRISE resolution. However, the dark streaks emanating from them can be seen. These streaks are either ejecta (material that was excavated during the impact) or areas where lighter-colored dust has been removed, exposing the darker layer underneath. The dark streaks meandering through the scene are probably not related to the impact. They are more likely the tracks of dust devils, which similarly remove lighter-colored dust to leave darker trails in their wake.

This particular impact was probably quite oblique - in other words, the object came in at a steep angle to the vertical. One clue that indicates an oblique impact is the asymmetry of the ejecta rays. This asymmetry can also be used to estimate the direction from which the asteroid or comet came. The ejecta rays have a distinctive pattern, also seen in experimental impacts, with more ejecta toward the east and west, and very little to the north. The cluster itself is also elongated along the same orientation, strengthening the argument for an oblique impact in that direction.


Written by: Ingrid Daubar  (9 June 2008)