Wednesday, May 4, 2011

Anomalous Mounds on the floor of King

Odd-looking mounds on the floor of King crater. LROC Narrow Angle Camera (NAC) observation M115522928LE, LRO orbit 2158, December 15, 2009; incidence angle 75°; image is ~900 meters across. See the full width LROC Featured Image HERE [NASA/GSFC/Arizona State University]

James Ashley
LROC New System

What are these circular features?

These mounds are found within a large pool of now-solidified impact melt covering the floor of King crater (5.0°N, 120.5°E). North is up, and sunlight is coming from the east, which is how we can tell at a glance that these are positive-relief constructs, and not negative-relief impact craters (compare with some of the obvious craters in the image to convince yourself that this is true). There are many such mounds on the King crater floor. They are often circular, but occur in groups with irregular outlines as well.

One interpretation of the mounds is that the King impact melt remained hot long enough after accumulating as a pond, and partially crusting over during cooling, to still "ooze" here and there through holes or cracks in the fresh crust. The soft melt quickly cooled after reaching the surface and built itself up vertically while spreading out laterally at each point where this happened ... think of pressing a board with holes in it on a layer of wet mud or toothpaste.

While this explanation may sound similar in some ways to a volcanic process, this kind of "extrusion" differs from classic volcanism in a very fundamental way: The source in such a model is a shallow, surface emplacement of short-lived, molten material (the impact melt), as opposed to magma coming from the mantle or crustal magma chambers, which stay molten for much longer periods of time and tend to produce higher-pressure and more voluminous extrusions. Perhaps the weight of the cooling melt crust was enough to cause this (the board in our thought experiment). Perhaps another force assisted. Or perhaps the mounds formed in a way completely different from this scenario. Such are the problems of planetary science, and we can always rely on Nature to surprise us by presenting situations that do not lend themselves to easy interpretation or classification; hence the "anomalous" status for today's Featured Image.

A visit to King crater would help solve the mystery!

View the full LROC NAC frame HERE.

LROC WAC (Wide Angle Camera) monochrome mosaic Server zoom of the King crater region. 73 kilometer-wide King is a kind of splash pan. At some ancient moment, preserved here for billions of years, molten material was slung over the north rim, filling in the depression labeled King Y, and even as it cooled flowed back over the rim, carving a deep impression. King Y is also the location of a unique natural bridge, a discovery spotlighted by an LROC Featured Image in September 2010 [NASA/GSFC/Arizona State University].

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