Atypical pit craters on Mars: New insights from THEMIS, CTX, and HiRISE observations

Abstract

More than 100 pit craters in the Tharsis region of Mars exhibit morphologies, diameters, and thermal behaviors that diverge from the much larger bowl-shaped pit craters that occur in most regions across Mars. These Atypical Pit Craters (APCs) generally have sharp and distinct rims, vertical or overhanging walls that extend down to their floors, surface diameters of~50-350 m, and high depth to diameter (d/D) ratios that are usually greater than 0.3 (which is an upper range value for impacts and bowl-shaped pit craters) and can exceed values of 1.8. Observations by the Mars Odyssey Thermal Emission Imaging System (THEMIS) show that APC floor temperatures are warmer at night and fluctuate with much lower diurnal amplitudes than nearby surfaces or adjacent bowl-shaped pit craters. Kīlauea volcano, Hawai'i, hosts pit craters that formed through subsurface collapse into active volcanic dikes, resulting in pits that can appear morphologically analogous to either APCs or bowl-shaped pit craters. Partially drained dikes are sometimes exposed within the lower walls and floors of these terrestrial APC analogs and can form extensive cave systems with unique microclimates. Similar caves in Martian pit craters are of great interest for astrobiology. This study uses new observations by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE) and Context Camera to refine previous work where seven APCs were described from lower resolution THEMIS visible wavelength observations. Here we identify locations of 115 APCs, map their distribution across the Tharsis region, characterize their internal morphologies with high-resolution observations, and discuss possible formation mechanisms.

Martian cave air-movement via Helmholtz resonance

“…The work of Cushing et al (2015) and Cushing (2012) has identified numerous locations on both Elysium and Tharsis which appear to be collapsed lava tube ceilings. The results of such collapse structures resemble craters (Fig.…”

“…Using the work of Cushing et al (2015), we have identified five examples of pit craters with large depth to (Cushing et al, 2015). In A) the debris is transported away and in B) the debris accumulates at the base of the skylight.…”

“…Martian cave formation processes are hypothesized to include mass-wasting (e.g., undercutting of scarps), soluble rock dissolution, tectonism and volcanic processes (e.g., lava tubes). The research of Cushing et al (2007aCushing et al ( ,b, 2012Cushing et al ( , 2015 and others indicates that Martian caves exist in the form of lava tubes as well as drained basaltic dikes. The presence of extensive volcanic provinces suggests that lava tubes and dikes may be relatively common.…”

“…Fig. 1 shows the two most noteworthy volcanic provinces: Elysium Montes and Tharsis Montes and the approximate locations of several pit craters identified by Cushing et al (2015).…”

“…2. We can then approximate a pressure difference as the passageway cross-sectional area A moves along a small distance y: Cushing et al (2015), are indicated by black circles in panel A. Fig.…”