Taken by NASA's Lunar Reconnaissance Orbiter Camera (LROC), the images reveal "
wrinkle ridges"--curved hills and shallow trenches created by a lunar surface that is contracting as the moon loses heat and shrinks.
(These GRAIL data are also available on QuickMap and can be overlain over imagery that highlights the
wrinkle ridges, as seen on the bottom of the facing page.)
After first quarter the terminator crawls across vast Mare Imbrium, where you can spot not only
wrinkle ridges and small craters but lava-filled Plato (photo on previous page) and isolated peaks such as Pico and Piton.
To help gauge how the planet may have shrunk, the researchers looked at tectonic features, called lobate scarps and
wrinkle ridges, which result from interior cooling and surface compression.
The western half of Ancient Thebit is missing, but low-angle lighting reveals curved
wrinkle ridges that continue the arc of the missing rim.
Two kinds of features are seen only near the terminator: low
wrinkle ridges, winding for scores of miles across mare plains, and domes: low, blisterlike swellings, mostly near mare edges.
The occurrence of '
wrinkle ridges' within the depression proves that compressive forces as well as extensional forces would be needed to create graben.
For a long time these features were called "
wrinkle ridges," which described how we thought they formed.
Interestingly, the widespread volcanic plains near Mercury's north pole bear a disproportionate share of the
wrinkle ridges and thrust faults.
The Moon did experience plentiful volcanism between about 3.8 and 2.5 billion years ago, but most of it formed the flat lava plains we see as the maria, sporting only low
wrinkle ridges. The Moon also has some small-scale volcanic features--sinuous rilles, dark halo craters, and domes (July issue, page 20).
These mare ridges are also often called
wrinkle ridges because often they look like casual folds in a bed sheet.
Particularly notable in this lunar portrait are the rugged Caucaus Mountains on the western edge of Mare Serenitatis (at upper right), and the subtle
wrinkle ridges within the mare itself.