Strange Martian feature not a 'bottomless' cave after all

 作者:郜毂货     |      日期:2019-02-27 10:04:08
By David Shiga (Image: NASA/JPL/University of Arizona) (Image: NASA/JPL/University of Arizona) 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 side. It is not clear how deep the pit is, because its floor has still not been seen. But the HiRISE team says it must be at least 78 metres deep. Similar “pit craters” form on the flanks of volcanoes in Hawaii when lava deep underground drains away, causing the overlying rock to collapse downward, forming a well. Although this particular feature did not turn out to be a cave entrance,