Tuesday, March 12, 2013

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, LRO orbit 7672, February 21, 2011; incidence angle 17.58° from the northwest, resolution 0.49 meters per pixel over a field of view approximately 820 meters across, from 39.65 km [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

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.

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.

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].
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.

WAC mosaic context image. The white box outlines the area shown in the second image following above, field of view roughly 67 kilometers across [NASA/GSFC/Arizona State University].
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? Likely we are seeing ancient highland anorthositic material excavated from beneath an overlying volcanic deposit(s). What a fascinating spot for a future explorer!

From a 2010 demonstration, animation of separate LROC WAC observations of the geologically interesting Lassell Massif and crater group east of Lassell D, showing the latter's fresh ray system intruding from the west. This is more easily discerned under a high Sun while topography is easier to view under a mid-morning Sun in the east-northeast. The bright, widespread ejecta streamers from Lassell D alternates with a visible chevron affect by the Lassell D pressure front [NASA/GSFC/Arizona State University].
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 HERE. Other examples of pyroclastic deposits include DMD Excavations, Pyroclastic Trails, and Pyroclastic Excavation.

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