Mid-sized Martian Basins: a Window into Early Martian History

Introduction: Studies of the compensation state and evolution of lunar impact basins using gravity and topography data have revealed much about the early thermal history of the Moon [e.g. 1, 2, 3]. While Mars has had a far more complex history, it is likely that the ancient martian impact basins also have much to tell us about the early thermal evolution of the planet. To address this, here we use gravity and topography data in conjunction with a viscoelastic model to better understand the mechanical evolution of martian impact basins between 275 and 1000 km in diameter. In particular, we are interested in the role of vis-cous relaxation of impact basins, as this relates directly to the planet's thermal state. Previous work [3] has demonstrated that viscous relaxation is likely to have been an important process on the early Moon. In that there is ample evidence of high heat flux [e.g., 5] and high interior water content [e.g., 6] early in martian history, viscous relaxation may have played an even greater role on Mars. Indeed, this may account for the anomalously low Moho relief reported beneath several mid-sized basins [4] and the abundance of quasi-circular depressions (QCDs) on Mars [7]. As on the Moon, the main characteristics expected of relaxed basins on Mars are low-amplitude surface and Moho topography. Unlike the Moon, Mars has been extensively reworked by surface processes such as aeolian and fluvial erosion and sedimentary deposi-tion. Though the surface expression of a basin may be severely degraded by these processes, Moho relief is more resistant to surface modification. Indeed, the example of Utopia – which appears to have been filled with at least 10 km of material early in martian history [8] – shows that burial tends to produce large gravity anomalies. Martian Basins: From the Barlow crater database (available at http://webgis.wr.usgs.gov/), we compiled a list of 25 craters and their locations in the 275-1000 km diameter range. Using MOLA topography data, we measured the depths (d) of the craters and constructed azimuthally-averaged topographic profiles. We then extracted the amplitude of Moho relief (h M) from the most recent spherical harmonic crustal thickness model