DIFFERENTIATING LAVA TUBE SKYLIGHTS FROM PIT CRATERS; A STUDY OF THE CAVE-LIKE STRUCTURES ON ARSIA MONS, MARS

We propose the cave-like structures recently identified on Mars are actually pit craters, rather than "cave skylights" as they were initially described. Our conclusions are drawn by comparing the structural characteristics of Earth-analogue pit craters to the Arsia Mons features. On Earth, differentiating lava tube skylights from pit craters in the 100-250 m width range using remote sensing imagery is often easy and accurate. Several relevant factors include width, depth, shape and mutiplicity. The widest recorded terrestrial lava tube cave has a passage width of about 50 m and most are much narrower; confluent collapse of adjacent passages theoretically might reach widths of 100 m but such confluent sinks lack the circularity of pit craters. Further, whereas all documented terrestrial pit craters in the 100-250 m range are shaft features more than 30 m in depth, the deepest known terrestrial lava tube cave is less than 30 m deep and their skylights and other associated collapse features are even shallower. Pit craters rarely occur in closely spaced groups, and each pit crater in such groups maintains its characteristic near-circular shape from a nadir-viewing perspective, and retains a shaft-like interior structure. In contrast, skylights commonly occur together with other, readily identifiable types of lava tube collapse features (e.g., irregularly shaped orifices, orifices elongated in the direction of flow, multiple orifices outlining complex patterns of braided lava tubes, and trench segments). Like terrestrial pit craters, the cave-like features on Arsia Mons, Mars (a) are spread out across the northern flank of this shield volcano, (b) are circular in nature, and (c) appear to be deep shafts based on the results of photoclinometry routines. While the Arsia Mons features are clearly the most compelling evidence for cave-like structures on the Red Planet, we conclude these features are pit craters and likely do not afford access to subterranean Mars.