Mottled mound at Firsov

Low-angle incidence view of a curious mound on the floor of Firsov crater (51 km; 4.204°N, 112.697°E). 2.2 km field of view from LROC NAC observation M187506567R [NASA/GSFC/Arizona State University].

Hiroyuki Sato
LROC News System

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 meters 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.

46 km-wide field of view showing  the high-reflectance mound feature, near center of FriFirsovater, from LROC WAC monochrome (643 nm) observation M176892340CE, LRO orbit 11204, November 25, 2011; 62.51 incidence, resolution 58.62 meters from 43.41 km [NASA/GSFC/Arizona State University].

A number of previous Featured Image posts 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 examine that hypothesis.

The bright mound southeast of center on the floor of Firsov is not the only albedo "anomaly" in the vicinity of Firsov crater. This cycle of overlapping fields of view, juxtapositioning data ranging from LROC WAC-derived elevation models to Clementine UV-VIS color ratio maps from 1994, brings into stark relief the unnamed Copernican era crater northeast of Firsov, and also the dramatic patch of albedo swirls coincident with a locally intense crustal magnetism, photographed from orbit by the crew of Apollo 10. It seems distant and detached, but still these swirls are likely associated with the widely-scattered swirl fields farther to the west at Mare Marginis, on the opposite side of the Moon from the energetic basin-forming impact that formed Mare Orientale 3.1 billion years ago [NASA/GSFC/Arizona State University].

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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