Why Does that Martian Crater have a Hole?

Discovery News.Aug. 30, 2011 -- On the slopes of the vast Martian shield volcano Pavonis Mons, a rather odd-looking crater resides. Originally spotted by NASA's Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) earlier this year, mission managers decided to zoom in on the suspect feature using the awesome power of the MRO's High Resolution Imaging Science Experiment (HiRISE) camera. Indeed, as HiRISE has confirmed, this is one very odd-looking crater.

From a distance, the crater appeared to have a black spot in its center, looking almost like the bulls eye on a dartboard. But on closer scrutiny the spot turned out to be the collapsed roof of an underground cavern. Similar features have been seen on Mars and the moon before and are commonly referred to as "skylights." However, this skylight is unique in that it formed at the base of a larger crater.

This Pavonis Mons skylight is approximately 35 meters (115 feet) in diameter, and by using the shadow on the cavern's floor as a guide, HiRISE scientists have estimated that the feature is around 20 meters (65 feet) deep, according to the mission's website.

Skylights often form in volcanic regions where ancient lava flows created subsurface lava tubes. When these lava tubes are evacuated, and tube sides hardened by solidified lava, caverns may form. Should the roofs of these caverns collapse, skylights may form.

These features are often eyed as potential candidates for future manned missions to Mars and the moon as they provide access to naturally-occurring sub-surface caves that astronauts could use as shelter for permanent habitats.

The skylight shown here will likely keep scientists guessing for some time as to how it formed at the base of a crater. Is the crater an ancient impact feature that breached the roof of an underlying cavern? Or is it simply an erosion feature where surface material has slumped and drained into the skylight?

Obviously piquing the interesting of HiRISE scientists, the feature will be revisited during an orbital pass later this year so a second high-resolution photo can be snapped. Combining both photos will create a stereo image, hopefully unraveling what caused this strange feature.

-- by Ian O'Neill

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