A new void in the melt?
One of the many types of features we are looking out for on Moon Zoo are the Lava tube skylights – ceiling collapses in lava tubes in regions which have been subjected to lunar volcanism.
|Marius Hills Lava Tube Sky Light –65 metres wide
||~||Mare Ingenii Tube Sky Light –130 metres wide
These pits or caves would provide ready made shelters for any future manned missions. There’s more information in these LROC news articles: Marius Hills Pit – Lava Tube Skylight? and How Common are Mare Pit Craters?.
Forum member JFincannon started discovering similar looking features in non-volcanic regions and called them “collapsed voids.” These appear to be holes in impact melt possibly as a result of the melt cooling rapidly and cracking. However the regular round morphology of these “voids” still has us puzzled. There are more details in JFincannon’s blog post Potential Caves and Sink Holes in Copernicus Crater
Here are some of his Copernicus finds:
There was much discussion on the forum about how these voids might have formed but their origins are still unresolved. However, they are clearly not craters. They do not display typical crater morphology, having flat, boulder-filled bottoms and very distinctive sharp “rims” without the familiar crater walls and without any ejecta.
Impact melt can crack in spectacular ways as this LROC article featuring Giordano Bruno shows: Fragmented Impact Melt. JFincannon referred to another source describing collapsed features in melt ponds: Lunar Caves in Mare deposits imaged by the LROC Narrow Angle Cameras which stated that:
“Collapse features over probable lava tubes within mare (skylights) may provide points of ingress to larger “trunk” cave passages. Collapse features over areas of melt pond drainage suggest additional sublunarean voids. Both types of cave offer intriguing exploration and habitation opportunities.”
We thought JFincannon’s latest potential impact melt void was similarly intriguing and worth highlighting. It is much smaller than previous examples at around 24m diameter and is situated at latitude 72.468 : longitude -31.393 in Philolaus Crater. As JFincannon points out, this far north the Sun never gets higher than 18 degrees above the horizon. This means the bottom of any pits are unlikely to be illuminated making visual confirmation impossible. It also makes spotting them at all very challenging as shadows at this latitude are very long and black. However, this latest candidate does bear all the hallmarks of a potential void which JFincannon describes as:
“… a sharply dark area surrounded by a lighter, grayer one. In these images, the crater-like feature has a steep enough inner slope to brighten this Sun facing side, while the darkness does not seem to be due to a hill or raised crater rim. Also, other craters around it seem shallower. So it could be a deep small crater or a pit.”
Here it is: