THEMIS Image of the Day, August 10, 2018. Today’s VIS image shows a portion of Buvinda Vallis, a channel near the flank of Hecates Tholus. The closeness of the volcano suggests that the channel was eroded by a lava flow, perhaps in a couple of stages.
The double impact, however, shows a definite sequence of events. First, a smaller impact hit close to the channel, then a larger one followed almost immediately after. With the larger crater showing a complete, round bowl, the impacts were not simultaneous but separated by a brief interval, perhaps only a couple of seconds. (Had the impactors struck together, the wall between the two craters would have been straight.)
Double asteroids are fairly common in the Main Belt, next outward from Mars’ orbit. This pair must have had at least a little distance between them.