Simulated degradation of lunar impact craters and a new method for age dating farside mare deposits

With the advent of Clementine data it is now possible to determine the lithology and extent of geologic materials on the Moon, particularly the farside mare deposits. However, traditional crater counting techniques do not provide reliable age estimates of these materials owing to their small surface areas, To support such studies, we present a model for estimating their age by analyzing the morphometry of degraded craters 1-3 km in diameter. A photoclinometric model was adapted for use with monoscopic 0.750-mu m ultraviolet-visible and high-resolution images where we extracted the topography of fresh craters. A two-dimensional computer model simulating linear diffusional creep was applied to fresh craters at a variety of diameters. The resulting profiles were then compared to photoclinometric profiles of degraded craters of known ages for calibration. Application of the resulting model to degraded craters in the mare deposit of the central Apollo basin (-36.5 degrees latitude, 208.0 degrees longitude) indicates that this material was emplaced during the early Imbrian period (similar to 3.85 Ga). By calculating the amount of material eroded from each of the degraded craters observed in this unit, the average erosion rate is estimated to be 2.0+/-0.1 x 10(-7) mm/yr on the Moon since the Imbrian. The estimated amount of material eroded during any given period suggests that the erosion rate has decreased with time, implying that the flux of larger impactors has as well.