Wednesday, February 1, 2012

LROC Fractures in Ohm's Melt Pond

A thin layer of regolith covers fractures around 18 meters wide in the Ohm crater impact melt pond. LROC Narrow Angle Camera (NAC) observation M166582212R, orbit 9683, July 29, 2011; incidence angle 58.41° at a resolution of 0.63 meters per pixel from 58.48 km, image field of view is 793 meters across. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].
Lillian Ostrach
LROC News System
Impact melt deposits are some of the most distinctive geologic features on the Moon, probably because no two impact melt deposit morphologies are alike! Studying the morphology of an impact melt pond can reveal much. For example, if the melt deposits are thick enough to cover ejecta blocks on the crater floor, the surface will most likely be quite smooth. However, in many cases on the Moon, the impact melt pond is not of uniform thickness, nor is it uniformly distributed within the crater. The surface features of the impact melt reflect whether the distribution of melt is uneven in a crater or not thick enough to cover the blocks and boulders on the crater floor. If this is the case, the impact melt may appear hummocky, have mounds exposed, and may even be extensively fractured

Such is the case in today's Featured Image, highlighting a portion of the impact melt solidified within Ohm crater (18.309°N, 246.265°E), near the crater's eastern wall.

Field of view in LROC Featured Image (yellow box) in context of nearly the full width of the M166582212R LROC NAC frame. Tour the variety of features in the high resolution image linked at the end of this article [NASA/GSFC/Arizona State University].
LROC Wide Angle Camera (WAC) monochrome mosaic of Ohm crater (18.309°N, 246.265°E, ~63 km diameter). Ohm's impact melt pond is not evenly distributed about the crater floor, appearing thicker in some areas (western portion of the crater) than other, more hummocky (eastern). Asterisk notes the location of the opening image [NASA/GSFC/Arizona State University].
Although difficult to discern in the LROC NAC opening image (but observed in the full frame), there is a topographic bulge in the lower-left, flattening out toward the upper right. This slight difference in topography, probably caused by the proximity to the crater wall, was responsible for the different fracture patterns. As the melt cooled, it experienced stress that promoted crack formation. Cracks, and other types of structural failure, follow the local stress fields and the path of least resistance, which explains why there are at least two different orientations of cracks visible in the opening image. The melt on top of the bulge cracked to relieve the stress of the cooling melt as it stretched over the mound to form in a southeast-northwest direction. The second set of cracks formed in a northeast-southwest direction perpendicular to the first set and most likely result from the stress of cooling so near to the crater wall. However, additional observations and mapping may help better determine the stress fields interacting to create the complicated sets of fractures within this melt pond.

Ohm resides in the wide transitional zone between the below global mean elevations characterizing the Moon's nearside and the disproportionate heights of the farside, north-northeast of South Pole Aitken basin. In this simulated low orbit view from the NASA ILIADS application Ohm is seen from the south (LROC WAC 100 meter Global Mosaic on LOLA laser altimetry digital elevation model, v.2). The decline in elevation along the horizon, from west to east, is authentic [NASA/GSFC/ILIADS/Arizona State University].
How many orientations of cracks can you find in the full LROC NAC frame? Do you see mounds or obstacles that may influence the formation of these cracks in the melt?

Related Posts:
Melt and more melt
Impact melt tongue
A Definite Sail Rock
Big Ohm Boulder
Farside Delights

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