Evidence for an explosive origin of central pit craters on Mars

Introduction: Central pit craters occur in over 1,000 impact structures on Mars in the lowto midlatitudes and exhibit a crater-in-crater configuration [13]. Central pits also occur in impact craters on icy satellites, including Ganymede and Callisto [1], but are seldom observed on other rocky planets, so an icy origin is inferred [4]. Central pit craters thus could provide a unique window into the Martian subsurface and the history of water at depth. Two principal models are proposed for central pit crater formation: explosive excavation [2,4-6], or drainage and collapse [7,8]. One way to test and distinguish between these hypotheses is to determine whether or not pit ejecta exist around central pits. Under an explosive origin scenario, material should be ejected and distributed around the pit. In the collapse scenario, significant amounts of material should not be ejected outside the rim, as most material travels gravitationally down into a cavity. An ejecta blanket might be manifested in several ways. First, the deposition of ejecta around a crater would build a topographically raised rim. Second, ejected material may have a different grain size and thus a different thermal inertia than material on the parent crater’s floor. In this study, we examine the morphology and thermophysical characteristics of central pit craters to test and distinguish between the two origin scenarios. Data and Methods: We conducted a survey to characterize the global population of central pits in impact craters ≥10 km in diameter and within ±60° of the equator using the Mars Odyssey Thermal Emission Imaging System (THEMIS) daytime infrared global mosaic [9]. Morphology of central pit craters in this study was assessed quantitatively using profiles from Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA) data [10], and qualitatively by shaded relief from THEMIS, Mars Reconnaissance Orbiter Context Camera (CTX) [11], and High Resolution Imaging Science Experiment (HiRISE) [12] images. Variations in thermal properties are observed using relative temperature maps from THEMIS nighttime infrared images [9]. Nighttime temperature differences are used as proxies for thermal inertia and rock abundance [1315]. MGS Thermal Emission Spectrometer (TES) albedos were used to characterize dust influences on pit thermal signatures [16]. Images and numerical data were viewed and plotted in JMARS [17]. Results: Elevation profiles were taken across several large (~50 km) central pit craters in JMARS using MOLA topography. Central pits often have rims slightly raised above the floors of their parent craters (Fig. 1). Fig. 1: An impact crater containing a central pit at 63.6°W, 17.6°S, viewed in a THEMIS daytime mosaic (left) and in a topographic profile from MOLA (right).