NASA discovery: the moon is shrinking

Long cliffs and slopes recently discovered on the surface of the Moon indicate that it began a process of global contraction in recent (geological) time, and may still be contracting today. This is according to teams that analyzed images taken by the LRO spacecraft

Long cliffs and slopes recently discovered on the surface of the Moon indicate that it began a process of global contraction in recent (geological) time, and may still be contracting today. This is evident from the work of teams that analyzed images taken by the LRO (Lunar Reconnaissance Orbiter) spacecraft. The results provide important clues as to the current geology of the Moon and its tectonic development.

The Moon was formed in a chaotic environment of heavy bombardment by asteroids and meteors. These collisions, along with the decay of radioactive elements, made the moon hot. The moon grew colder over the years. And scientists believed for a long time that the moon shrunk as it cooled, especially in its early history. However, the new study reveals relatively late tectonic activity related to the continued cooling and shrinking of the moon's surface.

"We estimate that the formations known as lobate scarps (lobe slopes) were formed less than a billion years ago and may be younger, even as little as 25 million years old," said Dr. Thomas Waters of the Center for Planetary and Earth Studies at the Smithsonian Air and Space Museum in Washington. . Even if it is a lot of time in human terms, a billion years represents only 4% of the current age of the moon - which is over XNUMX billion years old.

"Based on the size of the formation, we estimate that the distance between the center of the moon and its surface has shrunk by about 100 meters," said Waters, lead author of the scientific paper that appeared in the journal Science on August 20.

"These exciting results underscore the importance of global observations for understanding global processes," said Dr. John Keller, LRO Deputy Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. As the LRO mission continues into its new phase, with an emphasis on scientific measurements, our ability to create an inventory of lunar geologic features will be a powerful tool for understanding the history of the Moon and the Solar System.”

The cliffs that are a component of the formation are relatively small. The largest of them is about 100 meters high and it extends for several kilometers, but their typical length is small and their height is up to several tens of meters. The team believes that the cliffs are among the freshest formations on the moon. Partly because they cut small craters. Since the universe is constantly bombarded by meteors, it is likely that formations such as small craters (less than 400 meters in diameter) are young formations because they are destroyed by new impacts and do not survive long. Therefore, if small craters were disturbed by such wrinkles, it means that the wrinkle was created after the crater and is therefore even younger than the crater. Impressive evidence is that in the large craters, which are likely to be ancient, these cliffs did not appear. These cliffs appear crisp and relatively unworn.

Several lobate scarps formations on the Moon were discovered during the Apollo missions with analysis of high-resolution images from the panoramic cameras mounted on the Apollo 15, 16, and 17 landers. However, on these missions the spacecraft orbited over regions near the lunar equator, and were only able to photograph about -20 percent of the moon's surface, so researchers cannot be sure that the cliffs were not the result of local activity around the equator. LRO team members discovered 14 new previously undiscovered cliffs, seven of which are at high latitudes (above 60 degrees). This discovery confirmed that this is a global phenomenon and the shrinking of the moon provides the most likely explanation for their wide distribution.

As the moon contracted, its mantle and crustal surface were forced to react, creating fault lines where sections of the crust cracked and pushed over each other. Many of these cliffs or folds are semi-circular or earlobe shaped, giving rise to the name lobate scarps. Scientists aren't sure why they look like this, but maybe it happens like this: It might be the way lunar dust (regolith) expresses fracture lines, Waters says.

Ear lobe slopes are also found on other worlds in the solar system, including the planet Mercury where they are much larger. "The Lobate scarps on the planet Hema are the size of mountains - a kilometer and a half high and they extend over hundreds of kilometers. says Waters. These massive slopes lead scientists to conclude that the planet Mercury was completely molten when it formed. If this is true, Mercury is expected to contract more as it cools and thus create larger slopes, compared to a world that was perhaps partially molten with a relatively small core. Our moon had a volume less than one-third that of Mercury, but because these types of contours on the moon's surface are much smaller, the team believes that the moon contracted at a much slower rate than Mercury.

Because these slopes are so young, the conclusion is that the Moon has recently cooled and contracted. Seismometers placed on the moon by the Apollo missions recorded lunar tremors. While most are attributed to meteorite impacts, tides caused by Earth's gravity, and temperature changes between day and night, it is possible that some of these tremors may be associated with these slopes, Waters says.

The team plans to compare the photos of the slopes taken by Apollo's panoramic cameras with the new LRO images to see if they have changed at all in the decades since, which might indicate recent activity.

While Earth's tides likely aren't strong enough to create these slopes, they may contribute to their appearance, and possibly influence their direction, Waters says. Over the next few years, the team hopes to use LRO's high-resolution narrow-angle cameras (NACs) to build a highly detailed global map of the Moon. As part of the process, the team members will be able to identify additional slopes and examine whether there is a preferred direction or other features that may be caused by the gravitational force of the Earth.

"The resolution of NACs' ultraviolet images is changing how we look at the Moon," said Dr. Mark Robinson of the School of Earth and Space Studies at the University of Arizona in Tampa, Arizona, a collaborator on the current study and a researcher working with the LRO camera. Not only have we identified many slopes than were previously known, we also see in better detail the slopes from the Apollo photographs."

to the notice of the researchers

On the same topic on the science website:

• Selenology - the geology of the moon (article from 1969)
• Titan's contractions give birth to mountains