Explanation:
In a close-up
from the
HiRISE instrument
onboard the Mars Reconnaissance Orbiter,
this mysterious dark pit, about 150 meters across,
lies on the north slope of ancient
martian volcano
Arsia Mons.
Lacking raised rims and other impact crater characteristics, this pit
and others like it were originally
identified in visible
light and infrared images from the Mars Odyssey and
Mars Global Surveyor spacecraft.
While the visible light images showed only
darkness within, infrared
thermal signatures indicated that the
openings penetrated deep under the martian surface and perhaps were
skylights to underground caverns.
In this later image, the pit wall is partially
illuminated by sunlight and seen to be nearly vertical,
though the bottom, at least 78 meters below, is still not visible.
The dark martian pits are thought to
be related to
collapse pits in the lava flow,
similar to Hawaiian volcano
pit craters.
