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A New Wrinftla on tha Moon.

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 features.

As the ground under Mare Frigoris shifts, it pushes up wrinkle ridges, which typically snake along the ground for several miles. The longest ones stretch about 250 miles. Tectonic pushing and pulling of the lunar crust also sculpt curved hills called lobate scarps and shallow trenches known as graben.

Geologists can date them by studying another common lunar feature: impact craters. The longer a surface is struck by meteors, the more debris gets flung up from the impacts and covers nearby terrain, altering the landscape in a process called "impact gardening."

Craters collect more debris the longer they are around. The smaller they are, the less time they take to fill. Those smaller than the size of a football field typically would fill to the brim in under 1,000,000,000 years. LROC images revealed crisp tectonic features like the wrinkle ridges that formed after--and cut through--small, unfilled craters. That allowed Williams and his coauthors to deduce that the ridges emerged within the past 1,000,000,000 years or so.

Studying seismic activity on the moon is not new. The Apollo astronauts brought several seismometers to the lunar surface, which recorded thousands of moonquakes between 1969-77. The vast majority occurred deep in the moon's interior; a smaller number were determined to be of shallow depth, occurring in the lunar crust.

A paper in Nature Geoscience (see accompanying article) takes another look at these shallow moonquakes and establishes connections to some very young surface features called lobate thrust fault scarps. This opens the door to looking for similar connections with young wrinkle ridges described in the Icarus study.

Scientists--including Williams--now hope to glean similar science from Mars. NASA's InSight lander recently detected what is likely its first marsquake, along with several other seismic signals. The way a quake's seismic waves travel inside a planet can tell geologists about how rocky bodies are layered. That, in turn, can deepen our understanding of how Earth, its moon, and Mars first formed.

--Andrew Good, Media Relations, Jet Propulsion Laboratory, Pasadena, Calif.
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Author:Good, Andrew
Publication:USA Today (Magazine)
Date:Jul 1, 2019
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