Billions of years ago, Earth's moon formed vast basins called "mare" (pronounced mar-ay). Scientists long have assumed these basins were dead, still places where the last geologic activity occurred long before dinosaurs roamed Earth, but a survey of more than 12,000 images reveals that at least one lunar mare has been cracking and shifting as much as other parts of the moon and even may be doing so today. The study adds to a growing understanding that the moon is an actively changing world.
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. The features are described in a study published in Icarus, and led by Nathan Williams, postdoctoral researcher at NASA's Jet Propulsion Laboratory.
Previous research has found similar surface features in the moon's highlands, but wrinkle ridges never have been seen in basins before now. For this study, Williams and his coauthors focused on a region near the moon's north pole, called Mare Frigoris, or the Cold Sea.
The study estimates that some of the ridges emerged in the last 1,000,000,000 years, while others may be no older than 40,000,000 years. That is relatively fresh in geologic terms; previous studies have estimated these basins all stopped contracting about 1,200,000,000 years ago.
Both Earth and its moon experience what is known as tectonics, processes that push up mountains, rip apart land masses, and create quakes. On Earth, these processes occur constantly as the planet's mantle causes pieces of crust, called plates, to shift against one another. The moon does not have tectonic plates; instead, its tectonic action occurs as the moon slowly loses heat from when it was formed nearly 4,500,000,000 years ago. The heat loss causes its interior to shrink, crinkling the surface and creating distinctive features like those identified in the study.
'The moon is still quaking and shaking from its own internal processes," says Williams. "It's been losing heat over billions of years, shrinking and becoming denser."
The effect is similar to a car tire in winter: as the temperature drops, air inside the tire contracts and creates a squishier surface.
The moon's tectonic action especially is visible in Mare Frigoris. By poring over more than 12,000 images taken by LROC, Williams and his coauthors identified thousands of tectonically created...