Paucity of <200-m-Diameter Craters in Vastitas Borealis, Mars, and Implications for Geomorphic Processes

Introduction: Size-frequency statistics of impact craters in two considered Phoenix landing sites [1] within the Vastitas Borealis in the northern plains of Mars show a turn-down in the number of craters with diameters < 200 m relative to the distribution expected from published crater production functions. In this work, I describe the geologic implications of the apparent removal of these small craters. Background: The images examined lie entirely in the Early Amazonian Vastitas Borealis unit Abvi., which is hypothesized to be composed of sediments possibly deposited by large outflows from sources in southern highland regions and subsequently reworked [2]. The relative age of this unit compared to others was determined based on the area-normalized frequency of 5-16 km impact craters [3]. The Vastitas Borealis are overprinted by patterned ground and polygonal terrains presumed to have formed from thermal cycling of ice-rich sediments [4]. Indeed, analysis of neutron spectrometer data from the Mars Odyssey Gamma Ray Spectrometer and Neutron Spectrometer package have been interpreted to be indicative of the presence of at least 25-40% water-ice by mass over this region [4, 5]. Several workers have argued for a history of burial and exhumation of impact craters in the Vastitas Borealis. A population of northern plains “stealth” circular depressions of diameters 16-75 km are interpreted as impact craters buried by Vastitas Borealis materials [6]. Depth-diameter ratios of northern plains impact craters of diameters 6-216 km which show a population of craters with interior deposits raised above the surrounding plains have been argued to indicate the past burial of these craters by ice-rich deposits and subsequent partial exhumation [7]. The goal of this work is to describe the morphologies and size-frequency relationships of sub-km-diameter craterforms. Since the frequency of impact craters is expected to increase with decreasing diameters [8], and since smaller features are more susceptible to modification by surface processes, such an analysis should provide insights into surficial modification processes. Methodology: Craterforms visible in Mars Orbiter Camera (MOC) narrow angle images covering two of the proposed Phoenix landing sites were located and cataloged according to morphology. Location 1, which lies within Phoenix landing region B [1], is a box covering latitudes 66.9-68.0 N and longitudes 126.7133.3 E. Location 2, which lies within Phoenix landing region C [1], is a box covering latitudes 69.370.6 N and longitudes 76.2-83.7 E. MOC images utilized in this study have resolutions of 1.7-12 m/pixel. MOC images which exhibit low contrast (often due to low incidence angles, atmospheric dust, or haze) or low signal-to-noise are excluded from the analysis. MOC images in this analysis included 29 MOC images covering 1253 km over Location 1 and 22 MOC images covering 900 km over Location 2. To be counted, craterforms had to exhibit either a defined rim or a discernible slope-break around a circular depression. Three points were identified on each craterform's rim and used to calculate its central location and diameter. Additionally, each crater was categorized by its plan-view morphology. The qualitative morphologic categories are as follows: bowl-shaped craters, partially filled (Fig. 1a), ghost craters (circular features showing no apparent topographic depression in its interior and often only identifiable by a rim remnant), mounded (a crater with a rim which clearly rises above the surrounding terrain) (Fig. 1b), and a miscellaneous category for craters which do not fit into any of the above categories.