The Image of the Week for 20th February 2012 featured exploding boulders, discovered by Dr Anthony Cook, in Schiller crater which is located in the southwestern region of the Moon, south of Oceanus Procellarum at coordinates 51.8 S / 40.0 W. This crater is interesting in its own right apart from the exploding boulders!
Schiller crater is one of the most uniquely shaped craters on the Moon and its formation is still a bit of a mystery. It is elongated as if it had been stretched lengthwise at some point but was probably created by a grazing (oblique) impact or is a secondary impact crater. At least one article claims that it was created by a multiple impact, i.e. the impacting object broke up just before hitting the Moon (see Formation of Irregular Craters on the Moon below).
The crater is approximately 180 km in length and 70 km wide.
The image below is an LROC WAC Global 100 meter mosaic draped over the laser altimetry (LOLA) digital elevation model as seen from an imaginary point 65 kilometers over the elongated crater’s southeast.
Credit: NASA/GFSC/Arizona State University
An interesting article about boulders and boulder tracks around the central peak complex of Schiller crater. Some worthwhile images in the article.
LROC: A Recent Journey
Geologic Studies of the Schiller-Schickard Region of the Moon – PDF
Formation of Green Coffee Cup Shaped Irregular Craters on the Moon – PDF
We are used to seeing boulder tracks on Moon Zoo and often come across (or actively go hunting for!) the boulder that caused them. Usually we find something like these large intact boulders having come to rest at the end of their tracks.
highlighted by placidstorm and kodemunkey
Moon Zoo team member Dr Anthony Cook recently sent me this picture of two boulder tracks in Schiller crater:
In this case the boulders are far from intact and appear to have “exploded” at the end of their journeys. What might have caused these boulders to fracture and fragment? One theory Tony suggested was that due to being under tension the boulders might have fractured before they rolled down the slope and that the movement further weakened them. Then over time the extreme temperature variations between lunar day and night could have fragmented the weakened rocks resulting in the appearance we see in the image.
I’m a bit puzzled though why the one on the top left has rock debris so far away from the centre. The boulder that looks like a skull rock on the bottom right has debris a lot closer to it, that could simply be explained by bits falling off as one would expect from the explanation above.
An alternative theory is that the boulders did roll down the hill intact, but were of sufficient size, area and age to be impacted by later meteorites, and these high velocity impacts split the rocks into many pieces. However, as Tony points out, the chances of this happening to two large rocks next to each other seem a bit remote.
Here is the NAC image M109502471L and the LROC article “A Recent Journey.”
In order to study this process in more detail we need more examples. So if you find any exploded (or partly exploded) boulders please post them on the forum here.
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:
Other views can be found in M168399883RC and M170754606LC”