Distributions and Characteristics of Martian Central Pit Craters

Introduction: Impact craters containing central pits are rare on dry bodies like the Moon but common on Mars and the icy moons of the outer solar system. Impact into and vaporization of subsurface volatiles is the probable cause of central pit formation [1, 2]. Vi-king-based studies of central pit craters on Mars found regional distributions of these craters, with an apparent preference for the outer rings of multiring basins [3]. However, image resolution hampered a complete analysis of central pit craters, particularly those at the smaller diameter range. Our current study utilizes THEMIS visible (VIS) and daytime infrared (IR) imagery to obtain a more complete survey of the distribution and characteristics of these unusual landform. Methodology: The 18 m/pixel resolution of THEMIS VIS greatly surpasses the ~40-100 m/pixel resolution of the Viking Orbiters, providing more detailed views of martian impact craters. However, THEMIS VIS is limited in its spatial coverage. THEMIS daytime IR, while lower in resolution (~100 m/pixel), covers much of the planet's surface. It also often provides clearer imagery than visible images in the presence of the prevalent atmospheric hazes. This is particularly the case for the higher latitudes, which were often plagued with clouds or thick hazes during the Viking missions and where few central pit craters were detected in the earlier study. Thus, the primary dataset for this analysis is THEMIS daytime IR with THEMIS VIS being used when available. We are perusing each THEMIS image frame in order of release date in search of central pit craters. When such a crater is identified, the latitude and longitude of its center is noted, its diameter is measured, the central pit's diameter is measured, the crater's preser-vational classification is determined [4], and its ejecta blanket, if present, is classified using the system described in [5]. Central pits are classified as floor pits if they occur on the crater floor (Figure 1) and summit pits if they occur on a central rise or central peak (Figure 2). Preliminary Results: To date, we have identified approximately 1500 central pit craters on Mars, many of which where not detected in the previous Viking-based analysis. Floor pits are approximately twice as common as summit pits (68% vs 32%). Some pits are rimless while others display a slightly raised rim. Diameter: Summit pit craters range in diameter from 7 to 40 km while those displaying a floor pit range from 6 to …