An extremely dark feature on Mars is probably just a pit – not the entrance to a deep cavern that future astronauts could call home, a new image reveals.
The 150- by 157-metre feature was first noticed in an image taken by NASA's Mars Reconnaissance Orbiter on 5 May 2007 using a camera called the High Resolution Imaging Science Experiment (HiRISE).
Viewed from directly overhead, the dark spot showed no evidence of walls or a floor, leading some HiRISE scientists to suspect it was the opening to a cavern (scroll down for image).
That would have been exciting, since caves might be good places to search for life, as they offer protection from intense ultraviolet radiation from the Sun. Caves could also provide shelter for any future human visitors to Mars.
The new image, however, suggests the feature is just a vertical shaft cutting into the surface. Taken on 8 August from a different angle, the image reveals a wall on the feature's eastern
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Dark pits on some of the Martian volcanoes have been speculated to be entrances into caves. A previous HiRISE image, looking essentially straight down, saw only darkness in this pit. This time the pit was imaged 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. We can now see the eastern wall of the pit catching the sunlight.

This confirms that this pit is essentially a vertical shaft cut through the lava flows on the flank of the volcano. Such pits form on similar 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 78 meters (255 feet) deep. The pit is 150 x 157 meters (492 x 515 feet) across.