This observations shows the outer (distal) margin of the ejecta of Bacolor Crater. Ejecta (material thrown out of the crater) often simply fades out, becoming thinner as one looks further from the crater.
However, in some cases the ejecta has a flow-like edge, which might be related to subsurface water or ice being incorporated into the ejecta.
Written by: HiRISE Science Team (7 July 2011)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_020305_2125
Image: NASA/JPL/University of Arizona