The mystery of the radar holes on Mars
There is a region on Mars the size of Scandinavia that has no radar returns. But what at first sounds like an exciting secret has a natural cause: a meter-thick layer of dust and small particles absorbs every radar signal. When a team of scientists at the National Radio Astronomy Observatory in New Mexico spotted the black spot on their radar maps of Mars in 1991, they dubbed it Ste alth, after the famous military aircraft. One of the researchers, Bryan J. Butler, couldn't let go of the mystery surrounding these radar holes. He probed Mercury and Saturn's moon Titan with radar beams, but kept dwelling on the ste alth region. Finally, together with colleagues, he tackled the problem with a different method, which ultimately brought success: he compared the radar images with high-resolution photos from the Viking mission from the 1970s (Journal of Geophysical Research).
Then, Ste alth stretches 2000 kilometers along the Martian equator, west of and leeward to Arsia Mons and Pavonis Mons, two massive extinct volcanoes crowning the imposing Tharsis massif. According to the researchers, ste alth consists of a five-meter-thick layer of loose dust and small particles that were ejected during a violent eruption of the volcanoes and got stuck in a crack in the ground. It is possible that this eruption only took place about 700 million years ago and thus represents the last twitch of the largest volcanic area in our solar system.
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