Sunday, July 3, 2011

Zebra Stripes

Boulders decorate the slopes of this relatively small but richly complex wrinkle ridge in equatorial Oceanus Procellarum. LROC Narrow Angle Camera (NAC) observation M135507772R, LRO orbit 5103, August 3, 2010; solar illumination incidence angle 56°, resolution is half a meter per pixel in a field of view roughly 800 meters across. See the detailed, full-sized LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

This portion of a wrinkle ridge in southern Oceanus Procellarum, located at about 2.18°N, 48.55°W, has many boulders gathered on its slopes. These boulders are eroding out of the wrinkle ridge.

How can we tell? In this image there are no fresh impact craters that could have thrown the boulders all over the wrinkle ridge. Even if the boulders did come from a far away impact crater, we would expect the boulders to be distributed more randomly. Instead the boulders neatly line the ridge. What is the erosive mechanism? There is no wind or rain on the Moon. Most likely micrometeorites are slowly blasting loose regolith particles, leaving behind bedrock that was fractured by the faulting that resulted in the ridge formation. Many other wrinkle ridges observed by the LROC NAC also have boulders most likely caused by wrinkle ridge erosion.

The "zebra-striped" wrinkle ridge highlighted as the LROC Featured Image, June 29, 2011, is truly located in Procellarum's Rub' Al Khali, an apparently smooth "empty quarter" even when the vast basin's interior is view from Earth at low solar illumination angles. The subject feature (arrow) lies about 200 km northwest of the Flamsteed P "ghost crater" landing site for Surveyor 1, or perhaps better, at the southwestern range of Kepler's tenuous rays. Eight months before the NAC Featured Image further up was captured, dense and distinct families of secondary craters and the sparse anatomy of the area were readily swept up in LROC Wide Angle Camera (WAC) monochrome (643 nm) observation M117813262ME. The image cropped above shows roughly a 40 by 80 kilometer area straddling the equator, and the immediate basin floor ranges between 1750 and 1950 meters below global mean elevation [NASA/GSFC/Arizona State University].

Wrinkle ridges are fascinating tectonic features found in nearly all the lunar mare. They commonly have a distinct broad, low-relief arch with a more steeply-sloped ridge superposed on the arch. One theory of wrinkle ridge formation is described as a simple chain of events and physical forces. Scientists think that wrinkle ridges form due to the forces created when large amounts of mare basalt erupt on top of existing rock. Basalt is much denser than the anorthositic crust on which the mare basalts are deposited.

As the basalt fills in low areas in the crust, the increased weight causes sagging in the crust. As the crust sags and changes shape, the forces due to the shape change act on the basalt above the crust. The basalt deposit ends up compressed. The stress in the basalt due to the compression produces faulting within the basalt. When a fault forms in the mare, the effect we see on the surface is a wrinkle ridge. When you think about it, the wrinkle ridges in the basalt are caused by the basalt itself!

Explore the entire NAC frame!

Related Posts:
Wrinkle ridge in Oceanus Procellarum
Wrinkled Planet
Forked wrinkle ridge

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