Buy the Cavern on Mars space photo.
High quality HiRISE picture, slide, or Duratrans backlit transparency. NASA photograph PSP-003647-1745
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Click to see selection as Astronomy Picture of the Day (APOD) - May 28, 2007
Candidate Cavern Entrance Northeast of Arsia Mons.
This image shows a very dark spot (about the size of a football field) on an otherwise bright dusty lava plain to the northeast of Arsia Mons, one of the four giant Tharsis volcanoes.
This is not an impact crater as it lacks a raised rim or ejecta. What's amazing is that we cannot see any detail in the shadow!
The HiRISE camera is very sensitive and we can see details in almost any shadow on Mars, but not here. We also cannot see the deep walls of the pit. The best interpretation is that this is a collapse pit into a cavern or at least a pit with overhanging walls. We cannot see the walls because they are either perfectly vertical and extremely dark or, more likely, overhanging.
The pit must be very deep to prevent detection of the floor from skylight, which is quite bright on Mars.
Release date: May 23, 2007
Credit: Image courtesy NASA/JPL/University of Arizona