Not Your Average Crater

An asymmetric impact crater (14.349°S, 350.977°E) on the Lassell Massif revealing low reflectance material. NAC frame M152939732L, illumination is from the northwest, north is up, image is ~820 m wide [NASA/GSFC/Arizona State University].

This week's Featured Images focus collectively on a region of the Moon known to present a ruddy discoloration through the eyepiece of ground-based telescopes (and also directly to the eyes of Apollo Command Module pilots). Known colloquially as "lunar red spots," such areas include the Gruithuisen Domes, Mons Hansteen, and the Helmet region. The red spot under observation this week is the Lassell Massif. This fascinating feature is a highstanding region of the Lassell Complex, located within the Alphonsus A basin in northeastern Mare Nubium, and consists of a rugged (though somewhat muted), hilly terrain, and several large, steep-walled depressions.


This wider field image from the same NAC frame shows an adjacent impact (northwest quadrant) to have excavated similar dark materials in the region; white square is the Featured Image location. Image is ~2.2 km wide [NASA/GSFC/Arizona State University].

Today's Featured Image shows a striking impact crater in the northern hills of the massif. The crater exhibits an asymmetrical ejecta pattern, suggesting it to be the result of an oblique impact. The crater outline and ejecta distribution may also have been influenced by the uneven local topography. More geologically intriguing than this crater's shape, however, are the intermixed high and low reflectance materials excavated by the impact. 

WAC mosaic context image. The red square indicates the location of today's Featured Image, north is up; image is ~67 km wide [NASA/GSFC/Arizona State University].

The low reflectance materials (here seen as blocky) are suspected to be pyroclastic in origin, which speaks to the volcanic history of the Moon. Lassell crater itself (lower right in context image above) was described as being a source of pyroclastic materials since telescopic mapping efforts of the 1960s and '70s. The Lassell Massif is arguably close enough to this (and potentially other mare sources) to have received a share of these materials on the summits of its hills. Alternatively the central depressions on the massif may themselves be volcanic vents.

While most lunar volcanism produced basaltic rocks, recent evidence is suggesting that Lassell Massif and other red spot areas are silica-rich, iron-poor volcanic deposits -- perhaps similar to the kind of volcanism we see with the Long Valley Caldera on Earth. While darker deposits tend to indicate an iron-rich material, their presence among silica-rich materials (which might seem like a geologic contradiction) may actually be suggestive of complex volcanism, where a variety of volcanic deposits are possible, and for which the Long Valley volcanics remain a good example. 

What are the high reflectance materials seen distributed outward from the crater? Possibly we are seeing ancient highland anorthositic material excavated from beneath an overlying volcanic deposit or deposits. What a fascinating spot for a future explorer!

The next two Featured Image posts will include a brief discussion of this type of volcanism on the Moon, and explore some of the clues scientists are using to probe the Lassell Massif area. See the full NAC frame below. Other examples of pyroclastic deposits include DMD Excavations, Pyroclastic Trails, and Pyroclastic Excavation

Published by James Ashley on 12 March 2013