THE FORMATION OF PIT CRATERS WITHIN NOCTIS LABYRINTHUS, MARS

Pit craters are observed on a diverse range of planetary surfaces. They are circular, near circular, or elliptical depressions that are distinguished from impact craters by the lack of a raised rim. Aligned groups of pits tend to form linear crater chains, and chains may coalesce to form contiguous, trough-like structures. Many crater chains are situated within graben, long extensional structures characterized by a down-dropped floor bounded by two inward-dipping normal faults. Numerous mechanisms have been proposed for pit crater formation, including lava tube collapse, dike intrusion, drainage into extensional fractures, and dilational normal faulting.

Noctis Labyrinthus (NL) on Mars is a prominent feature situated between the Tharsis Rise and Valles Marineris, and features both extensive pit cratering and normal faulting. It is situated on a broad topographic rise that extends approximately 1,000 km east–west and 1,000 km north–south. Its unique placement between these two prominent landmarks on Mars means that understanding the formation of NL could provide useful insight into the evolution of this region. NL therefore provides a unique opportunity both to test leading formation hypotheses for pit craters and to understand the formational history of this part of Mars.

Here, we present preliminary results from a structural mapping project of NL at 1:100,000 scale. For pit craters within graben, we compared the displacement of the bounding faults with the depth of the pit craters using Mars Orbiter Laser Altimeter data, and find that the depth of NL pit craters is generally greater than the vertical topographic relief of the normal faults within which they are bounded. This finding may indicate a complicated structural relationship between the geometries and dips of the bounding normal faults and their associated pits. Moreover, the normal faults in this region appear organized into three distinct populations that may represent distinct stress regimes that have affected NL and the surrounding area over time, with the dominant direction oriented NE–SW. These observations, coupled with detailed structural analysis, will help us to continue to test major pit crater formation mechanisms, and improve our understanding of the NL region as a whole.