Emily makes the case that the impacting body was a tiny Martian moonlet that fragmented on entry in the atmosphere.
[right, "A mosaic of three images captured by the THEMIS instrument on Mars Odyssey in its daytime mode covers a very unusual elongated crater located north of Acheron Fossae on Mars. Source images: V13947003, V30669005, and V12100004. Image scale is 37.7 m/pixel; the crater is 10 kilometers long but only 7.5 wide. Credit: NASA / JPL / ASU / mosaic by Emily Lakdawalla"]