The Effect of Impacts on the Early Martian Climate

Introduction: The first images returned by the Mariner 7 spacecraft of the Martian surface showed a landscape heavily scared by impacts. Mariner 9 imaging revealed geomorphic features including valley networks and outflow channels that suggest liquid water once flowed at the surface of Mars. Further evidence for water erosion and surface modification has come from the Viking Spacecraft, Mars Pathfinder, Mars Global Surveyor's (MGS) Mars Obiter Camera (MOC), and Mars Odyssey’s THEMIS instrument. In addition to network channels, this evidence includes apparent paleolake beds, fluvial fans and sedimentary layers. The estimated erosion rates necessary to explain the observed surface morphologies (Craddock and Maxwell, 1993; Golombek and Bridges, 2000) present a conundrum. The rates of erosion appear to be highest when the early sun was fainter and only 75% as luminous as it is today. All of this evidence points to a very different climate than what exists on Mars today. The most popular paradigm for the formation of the valley networks is that Mars had at one time a warm (Taverage > 273), wetter and stable climate. Possible warming mechanisms have included increased surface pressures (Pollack et al, 1987), carbon dioxide clouds (Forget and Pierrehumbert, 1997) and trace geenhouse gasses. Yet to date climate models have not been able to produce a continuously warm and wet early Mars (Haberle, 1998). The rates of erosion appear to correlate with the rate at which Mars was impacted (Carr and Waenke, 1992) thus an alternate possibility is transient warm and wet conditions initiated by large impacts. It is widely accepted that even relatively small impacts (~10 km) have altered the past climate of Earth to such an extent as to cause mass extinctions (Toon et al., 1997). Mars has been impacted with a similar distribution of objects. The impact record at Mars is preserved in the abundance of observable craters on it surface. Impact induced climate change must have occurred on Mars.