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.