Mars is a weird and wonderful place. Not only is it the only planet populated entirely by robots, but it also has a fascinating geological history. An image captured in 2011 has been making the rounds after it appeared on the NASA science blog and Astronomy Photo of the Day, and many have been wondering what’s up with the giant hole on Mars. As it turns out, we only have part of the story.
NASA spotted this unusual surface feature in 2011 thanks to the HiRISE instrument aboard the Mars Reconnaissance Orbiter (MRO). The image reveals a large hole in the center of an even larger crater on the slopes of the Pavonis Mons volcano. Follow-up examinations showed the hole was about 35 meters (114 feet) in diameter. The interior shadow angle suggests the floor of the cavity is 28 meters (92 feet) below the surface.
Scientists believe the hole is a lava tube skylight — it’s not the only one on Mars or even in that region, but it’s still a unique example of one. We have lava skylights here on Earth, and the formation of skylights on Mars is probably much the same. When lava flows solidify on the surface, the lava inside underground tubes can drain away, leaving an empty cavern. Over time, the roof can collapse to create a skylight.
The collapsed pit around the skylight is harder to characterize. It doesn’t match what we’d expect from an impact crater, but it could have formed from soil falling into the collapsed tube. A digital terrain map helped NASA estimate how much material would have fallen into the cavern. Those calculations show that the rubble pile inside is at least 62 meters tall, suggesting the original floor of the tube was at least 90 meters (295 feet) deep prior to the collapse. It’s quite a mystery, but it’s one we might be able to solve with future exploration of Mars.
Scientists are interested in lava tubes and skylights on Mars because they could provide safe harbor from dangerous radiation on the surface. That makes them good candidates to contain Martian life, if any still exists on the planet. Human explorers could also use the tubes as shelter or the start of a colony. NASA has experimented with cave-optimized rovers that could one day go to Mars to traverse these tunnels.
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