Wednesday, November 28, 2012

Lassell D Ejecta

A variety of effects are still visible from this recent impact in Mare Nubium (14.60°S; 10.26°W). LROC Narrow Angle Camera (NAC) frame M111660844L, LRO orbit 1589, October 31, 2009; illumination from the east, north is up, from full field of view, approximately 1 kilometer wide, 51 cm per pixel resolution from 49.13 km altitude [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

The complex geologic process of impact cratering often results in a diverse medley of landforms and other surface features. The more nuanced of these are best observed in fresh craters because the subtlest attributes of impacts are those most easily removed by space weathering. Lassell D crater (2 km diameter) has been described as "one of the freshest craters on the Moon" (Muller, et al., 1986). 

In the proximal (nearby) ejecta blanket we see a hummocky, streaked surface with dune-like forms, ribbon-shaped lobes, and an eye-catching admixture of low- and high-reflectance soils. Immediately following the high-energy of impact, advancing walls of ejecta hugged the ground and moved like a dry tsunami across this region.

The west interior and ejecta blanket of Lassell D. The area detailed in the LROC Featured Image is on the crater's eastern flank, outside the field of view above, capturing the rough, young crater's sharp features on an earlier pass. The 5 km field of view above is from a mosaic of the left and right frames of LROC NAC M135257059, spacecraft orbit 5066, July 31, 2010; incidence angle 59.64° at 50 cm resolution from 46.63 km [NASA/GSFC/Arizona State University].
The crenulations are the result of mechanical interactions of the moving debris with pre-existing topography. As the wave of rock and dust is arrested by this resistance, some portions of the debris continue flowing while others slow and stop moving. The result is a wavy landform, a cross-section of which might reveal how the lobes partially rode up and over each other, hence the descriptive term "imbricated deceleration lobes."

LROC Wide Angle Camera (WAC) mosaic as context, from original image 118 km-wide field of view, resampled with added contrast to, perhaps unnecessarily, bring out from the background the subtle fresh and widespread ray system of Lassell D [NASA/GSFC/Arizona State University].
As regolith redevelops and matures over the tens of millions of years to come, these features will gradually diminish. Which features would disappear first and why? Examine the full NAC frame HERE. Additional examples of fresh impact features can be found in Kamarov, Icarus, and The Lavish Lobes of Necho R.

Lassell D's affect on the Lassell Massif (above and below), to the east. The massif and crater group, a spectral "Red Spot," is speculated to be intrusions of silicate-rich lava characterized by a higher viscosity than the Moon's far more common pyroclastic domes. The feature shows a much lower iron-oxide concentration than the surrounding basalt plains and marks the southwest border of a high thorium signature. LROC WAC observation M129350040C (604nm) [NASA/GSFC/Arizona State University].
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].

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