Distribution, Morphology and Structural Associations of Martian Pit Crater

Introduction: Pit craters are circular to elliptical collapse features that commonly occur in alignments (chains) and that coalesce into troughs [1], [2], [3], [4], [5]. We have begun systematically mapping the regional distribution of pit chains in the western hemisphere of Mars using the Mars Orbiter Camera (MOC) data. Pit chains are most prominent in the Alba Patera region and along the Valles Marineris. They are also found in Tharsis Montes, the Noctis Labyrinthus and in the cratered region west of Daedalia Planum. Pit chains display many features that are commonly associated with extensional fault systems, and are often found within grabens or transitioning into grabens, although they are also found apart from a graben system. They also occur in en echelon arrays and a few display curved tips similar to interaction at the tips of en echelon faults or extension fractures [6]. Hypotheses regarding the formation of pit chains all require formation and collapse of substantial cavities. The following mechanisms have been proposed for the formation of pit chains: (i) dike swarms [1], [2], [3], [4], [7]; (ii) collapsed lava tubes (http://photojournal.jpl.nasa.gov/catalog/PIA03836); or (iii) fissuring beneath loose material [8]. Of the numerous pit chains mapped during this research, few if any show direct evidence of volcanic eruption or the presence of a dike. The collapsed lava tube hypothesis fails to account for pit chains that cross multiple lava flows. Preliminary results indicate that most pit chains in the western hemisphere of Mars form over fissures associated with dilational normal faults. Methodology: We systematically mapped large regions of the western hemisphere of Mars using MOC wide angle and narrow angle images. The wide angle images have a resolution of 232 m per pixel and were used to map regional distribution and orientation of pit chains. In the study area approximately 1000 pit chains were mapped with an additional 270 possible pit chains. The MOC narrow angle images, with a resolution of 5 m per pixel, were used to study detailed pit morphology. Results: Distribution and orientation. Pit chains are numerous in the Alba Patera region, occurring mainly tangential to Alba Patera in a NE orientation on the eastern side and a NW orientation on the western side. South of Alba Patera these chains trend north-south following the general orientations of the graben system. Pit chains also occur in an east-west orientation along the Valles Marineris, typically coalescing into scalloped troughs. West of the Valles Marineris is the Noctis Labyrinthus, where pit chains are found in numerous orientations with interesting cross-cutting relationships. The pit chains in Noctis Labyrinthus also commonly appear to have coalesced into scalloped troughs. Pit chains were mapped around the Tharsis volcanoes of Ascraeus Mons, Pavonis Mons and Arsia Mons. Pit craters in this region were found in concentric rings on the flanks of the volcanoes and a smaller number were mapped radial to the volcanoes. To the west of Daedalia Planum, in a heavily cratered region, pit chains were mapped radial to several impact craters.