MODIFICATION OF MARTIAN IMPACT CRATERS: ALAN HOWARD'S LEGACY

The very first Mariner spacecraft images showed that instead of the famous canals or obvious evidence of life, the surface of Mars was covered by impact craters. Although scientists were surprised (and somewhat disappointed) from this discovery, one of the very first geomorphic observations made about Mars was that its impact craters were different from those found on the Moon. Many martian craters lack a raised rim, obvious ejecta deposits, and a central pit or peak suggesting that they were somehow modified in the past. Because of the importance of aeolian and volcanic processes in shaping most of the modern landscape, many investigators suggested that martian craters had been buried by either lava or airfall deposits. However, such aggradational deposits typically have a finite thickness, meaning that while a lava flow might be able to innudate a crater of a particular diameter up to the crater rim crest, all smaller craters would be eradicated while the rims of all larger diameter craters would still rise above the surrounding lava flow. Together with Alan Howard, it was shown that only erosional processes were capable of explaining the size range of modified craters as well as the preserved morphologic stages of modification. By applying Alan’s drainage basin evolution model [Howard, A.D., Water Resources Research, 30(7), 226-2285] to photoclinometrically-derived topographic profiles of modified impact craters, the process responsible for crater modification became clear : it had rained on early Mars. This was the smoking gun that forever changed the paradigm from a cold, dry early Mars to one that was warm and wet. And although climate modelers still seem unable to explain how warm and wet conditions were possible under faint young Sun conditions, the geology is now clear, and the idea that there was precipitation and surface runoff in the past has been repeatedly vindicated by both advanced orbital and lander spacecraft observations.