Lunar Reconnaissance Orbiter Camera

Mottled Mound

M187506567R thumb
Low-incidence angle image (~9°, near noon) of a curious mound on the floor of Firsov crater. Image width is 2.2 km [LROC NAC M187506567RNASA/GSFC/Arizona State University].


Firsov is a 51-km diameter crater located in the farside highlands, approximately 240 km east of King crater. The depth of Firsov's floor from the rim crest is an impressive 4.5 km (that’s 2.5 times the depth of the Grand Canyon in Arizona!). The bright (highly reflective) mound on the crater floor is about 200 m in height, and 2.5 km in diameter, and really catches your eye. The central portion of the crater floor is relatively flat, suggesting that it at least partially consists of a long-solidified pool of impact-melt; the mound is located within this melt pond deposit.

M187506567 context
Context view of Firsov crater in LROC WAC monochrome mosaic (100 m/pix) overlain with color coded elevation from the LROC WAC stereo DTM (GLD100, Scholten et al., 2012). The footprint of NAC M187506567R (blue box) and the location of opening image (yellow arrow) are indicated [NASA/GSFC/Arizona State University].

A number of previous Featured Images explored the origins of mounds occurring inside impact craters. Hypotheses include volcanic eruptions, impact debris, and the squeeze-ups of impact melt. Today's Featured Image highlights the degradation of these mounds, instead of their origin. The low-incidence angle of the top image (~9°) highlights differences in albedo on the mound top, what causes these bright patches? Perhaps, as the mound surface degrades over time, the high-reflectance materials are exposed unevenly, for example, due to a bumpy surface morphology, where local, topographically high portions are exposed faster and newly exposed material is immature (and thus brighter). Alternatively, the mound may be constructed from non-uniform materials and/or compositions that exhibit a range of reflectivities. However, scientists believe that during impacts any compositional differences within the target are homogenized in melt deposits. This mound would be a great place to check that hypothesis!

Related posts:

Shiny Mound

Kagami-mochi on the Moon!

Pancakes in a melt pond

Donut Holes

The Domes of Stevinus Crater

That's a Relief

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