Post by Dr. Bradley J. Thompson.
On Earth, caves are naturally formed subterranean chambers that form unique geologic and biologic environments. Although terrestrial caves are commonly formed in limestone where slightly acidic water has partially dissolved the host rock, caves can also form in ice or even in solidified lava (flowing lava will often form a roofed-over channel or tube that remains hollow once the lava cools and solidifies). Caves provided early humans with their first form of shelter, and the walls of some caves still record evidence of their presence in the form of cave paintings. From an environmental standpoint, caves provide near constant temperature and relative humidity year-round, and thus can serve as a refuge when conditions at the surface are too extreme.
A HiRISE image, targeting the bright dusty lava plain to the northeast of Arsia Mons, one of the four giant Tharsis volcanoes contained an intriguing dark spot (HiRISE Image PSP_003647_1745). The absence of a raised rim or ejecta led the team to conclude it was a pit. Dark pits on some of the Martian volcanoes have been speculated to be entrances into caves. This is one of seven pits that have been noted in this region (Cushing et al., 2008).
A second image was taken at a later date from the west. Since the picture was taken at about 2:30 p.m. local (Mars) time, the sun was also shining from the west (HiRISE PSP_004847_1745). We can now see the eastern wall of the pit catching the sunlight. The new image confirms that the pit is essentially a vertical shaft through the lava flows on the flank of the volcano. Such pits form on volcanoes in Hawaii and are called “pit craters.” They generally do not connect to long open caverns but are the result of deep underground collapse.
From the shadow of the rim cast onto the wall of the pit we can calculate that the pit is at least 178 meters (584 feet) deep and 150 x 157 meters (492 x 515 feet) across. Cushing et al. (2008) have stretched the data to show large boulders on the pit floor. Mass wasting/slope retreat acts to widen and degrade pit craters over time, decreasing their depth:width ratios. The high aspect ratio of this feature may point to more recent collapse (Cushing et al, 2008).
Cushing, G. E. et al., (2008). Continuing study of anomalous pit craters in the Tharsis region of Mars: New observations from HiRISE and THEMIS, Lunar and Planetary Science Conference, 38, abstract no. 2447. [Abstract]
Moore, G. W. and G. N. Sullivan (1978) Speleology: The Study of Caves (2nd edition), Zephyrus Press, Teaneck, NJ, 150 pp.
Schulze-Makuch, D. and others (2005) Scenarios for the evolution of life on Mars, Journal of Geophysical Research, 100(E12), E12S23, doi: 10.1029/2005JE002430. [Abstract]