Icarus 197, 452–457 (2008)

A curious elongated crater in the northern lowlands of Mars may mark the final resting place of a lost moonlet. A related crater a short distance away and 'butterfly wings' of ejecta to either side show that the crater was formed by the larger of two objects following the same, shallow trajectory.

According to modelling by John Chappelow and Robert Herrick at the University of Alaska Fairbanks, the distance to the secondary crater makes it improbable that this was the impact of an asteroid that split up in the atmosphere. And the alignment of the crater and its secondary makes it unlikely to have been a double asteroid. A small moon brought down by tidal drag and fractured in the atmosphere is, they argue, the most likely source.