Thursday, April 29, 2010

Wrinkle Ridges in Mare Tranquilitatis

Lunar wrinkle ridges can be hundreds of kilometers long, tens of kilometers wide, and hundreds of meters high. Read more in Wilhelms, 1987. They can have a sinuous or linear appearance with asymmetric cross-sections. In other words, the feature that you see could easily be made up of a smaller ridge superposed on a broad rise.

Close up of a northwest trending wrinkle ridge in the high-Ti basaltic lava plains of Mare Tranquillitatis. The bright areas along the steepest parts of the ridge are places where less mature subsurface materials have been exposed by small impacts or fracturing of the bedrock as the original mare surface buckled. Image is 1.25 km wide Credit: NASA/Goddard/Arizona State University.

Lunar wrinkle ridges are found in the mare basalt deposits that filled the giant impact basins on the Moon. Frequently, wrinkle ridges are oriented concentric to large impact basins. Loading of the basin floor with multiple eruptions of mare basalts causes the formation of wrinkle ridges in the basin center and graben, or trenches, along the mare margins (see below). The tremendous weight of the newer basalt layers then causes the center of the basin to sag. As the basin center sags, the basalts slide inward, causing compression that results in folding and faulting of the basalt. The sagging of the basin under the weight of the basalt also causes the opening of graben along the edges of the basalt layers.

Long wrinkle ridges in Mare Tranquillitatis in LRO Wide Angle Camera (WAC) image M117345275M. The section of wrinkle ridge shown in the top image is in the center of this WAC view. Arago crater (lower left), is 26 km in diameter. Credit: NASA/Goddard/Arizona State University.

Idealized cross-section through an impact basin that filled with three episodes of mare basalt eruptions. Loading of the basin floor with mare basalts causes the formation of wrinkle ridges in the basin center and graben along the mare margins. Credit: Hiesinger, 1999, PhD dissertation

We also know, thanks to the samples collected by the Apollo 11 astronauts, that the mare basalts in this region are rich in titanium. Study of the Apollo samples has shown it is relatively straightforward to extract resources (especially oxygen) from titanium-rich lunar soils.

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