Thursday, September 1, 2011

Leathery vs. Smoother surface composition

Leathery surface and a transition to a smoother surface at a change in grade on the north slope of 4200 meter high Mons Bradley, a 31 km wide promontory of the 978 km long nearside Montes Apenninus chain. LROC Narrow Angle Camera (NAC) observation M116309931R, LRO orbit 2274, December 24, 2009; resolution 0.73 meters/pixel, field of view 876 meters, incidence angle 84° with sunlight from the southwest. View the full-size LROC Featured Image HERE. [NASA/GSFC/Arizona State University].

Hiroyuki Sato
LROC News System

Today's Featured Image displays a portion of Mons Bradley (highland materials), about 150 km southwest of the Apollo 15 landing site. The southern half of this image has a leathery, rough texture, while the northern half shows a relatively smooth surface peppered with small impact craters. The leathery texture occurs on the northern slope of Mons Bradley, and makes a gradual transition to the smoother surface instead of an abrupt contact.

A 1200 x 2400 meter segment of NAC frame M116309931R, centered on the area within the LROC Featured Image, August 31, 2011. The downward slope of Mons Bradley becomes less steep at the elevation coincident with the change in surface composition [NASA/GSFC/Arizona State University].

What is the origin of these two contrasting surfaces? The upper smoother area may be an old ejecta blanket covering a pre-existing leathery surface. In that case, the smooth area is younger. If so, why are there more craters on the smooth surface? The surface regolith (unconsolidated rock debris comprising the first few meters of most of the lunar surface) might have been removed in the southern area, taking its store of impact features along with it. But what mechanism could remove regolith over such a broad area?

Mons Bradley, looking south from a vantage 20 km over Hadley Rille. The reduction in grade, and the transition from a "leathery" surface to a smoother composition, is at the elevation above center in this image, a stitching of two LROC Wide Angle Camera (WAC) observation from LRO orbits 4533 & 4534, June 20, 2010 overlaying the lunar Digital Elevation Model included with the Google Earth application. View a larger version of the image HERE.

Lunar Orbiter Laser Altimeter (LOLA) data reveal an almost uniform slope all the way down from the top of Mons Bradley until this leather/smooth texture boundary, at which point the slope then shallows. This may be suggesting that the origin of the leathery texture is disturbed regolith - perhaps the result of slow creep down the Mons Bradley slope. In this case, the leathery surface would be younger than the smoother surface. What other clues could be looked for to explain the origins, and determine the relative age, of these interesting rock units?

LROC WAC monochrome mosaic 100 m/pixel around Mons Bradley (centered on 22.41°N, 1.03°E). Blue box and white star indicate the locations of NAC frame M116309931R and the LROC Featured Image released August 31, 2011. View the larger rendition HERE [NASA/GSFC/Arizona State University].

Explore the mysterious leathery and smooth surfaces at Mons Bradley in the full NAC frame!

No comments: