Tuesday, April 23, 2013

The peculiar domes of Stevinus

A distinctive positive-relief feature on the floor of Stevinus crater (32.760°S; 53.739°E). LROC Narrow Angle Camera (NAC) frame M113603383L, spacecraft orbit 1875, illumination is from the east, angle of incidence 57.67° field of view 1.9 km at 58 cm resolution from 55.68 km [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Today's image explores a portion of the Stevinus crater floor (southern hemisphere, nearside highlands). Here we see a topographic feature that can be found by the dozens throughout the area in many shapes and sizes. These mounded forms show positive relief upon a flat surface of ponded impact melt deposits (now solid).

Some are circular, while others show more irregular outlines. Some occur in clusters that appear to have coalesced, and others superpose one another. Some have smooth upper surfaces and others appear deflated with depressed central portions. The featured dome likely superposes an extension crack, indicating that it occurred after the crack formed.

What caused these peculiar mounds on the floor of Stevinus crater?

Some perspective on the peculiar dome of interest near center of this LROC Wide Angle Camera (WAC) monochrome (643nm) mosaic of three observations gathered in sequential orbits, November 20, 2011. Field of view roughly 40 km across. Angle of incidence 69.83° at 70 meters resolution from 50.66 km [NASA/GSFC/Arizona State University].
While not entirely clear without a better understanding of melt pond dynamics for still-molten deposits, we note that moderately viscous materials can behave in odd ways. The isolated occurrence of individual domes suggests molten behavior with each dome forming in-situ from a local source just beneath its position. Since the phenomenon is occurring in impact melt, we would be wrong to call this behavior volcanic. But something similar to volcanism in the sense that molten rock is locally "erupting" from an accumulated, still-hot deposit, might be appropriate for conceptual purposes. Perhaps isostatic readjustment of the crater floor "squeezes up" these blobs through holes or cracks in crust as the melt mass cools and thickens. This mechanism might explain why today's mound is centered over a fracture.

Reduced view of the LROC WAC mosaic from which the image immediately above it was taken shows the entirety of 71 km-wide Stevinus [NAXA/GSFC/Arizona State University].
Perhaps experiments with analog melts would be a good way to study impact melt behavior. Of course collecting samples is always recommended for any geologic study. What kinds of samples would be helpful for determining the solution to this mystery? Where should they be collected from and why?

Click HERE to see the full NAC frame. Other examples of odd features in impact melt deposits can be found with the Melt Fractures in Jackson Crater, Rippled Pond, and Anomalous Mounds on the King Crater Floor LROC Featured Image posts.

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