The moon is slowly shrinking over time, causing wrinkles in its crust and its earthquakes, according to photos taken by the NASA Lunar Reconnaissance Orbiter.
Unlike the Earth, the moon has no tectonic plates. Instead, while the inside of the moon has cooled in the last few hundred million years, it has caused the surface to ripple as it shrinks. Unlike the flexible skin of a grape when it shrinks in a grape, the fragile crust of the moon breaks. This creates cliffs of steps called thrust faults as part of the crust is pushed up and over another narrow part of the crust.
Now there are thousands of cliffs scattered on the lunar surface, on average a few miles and tens of meters high. The orbiter has taken photos of over 3,500 of these since 2009. In 1972, the Apollo 17 astronauts Eugene Cernan and Harrison Schmitt had to trace one of these cliffs, the fault of the Lee-Lincoln fault, zigzagging the lunar rover on it.
Today the moon is 50 meters more "thin" due to this process. And as it shrinks, the moon actively produces earthquakes along the faults. The researchers reviewed the seismic data they had from the moon to compare with the images collected by the orbiter.
The data of seismometers placed on the moon during the Apollo 11, 12, 14, 15 and 16 missions revealed 28 earthquakes recorded between 1969 and 1977. The researchers compared the position of the epicentres for those earthquakes with the orbiter image of the faults. At least eight of the earthquakes occurred due to activity along the faults. This excludes the possibility of impacts of asteroids or noises coming from inside the moon.
This means that the seismometers of the Apollo recorded the narrowing of the moon, the researchers said. The study of the seismic data of Apollo and the analysis of over 12,000 photos of the orbiter were published Monday in the journal Nature Geoscience.
"It is truly extraordinary to see how data from nearly 50 years ago and the mission [orbiter] were combined to advance our understanding of the Moon as they suggest where future missions to study the internal processes of the Moon should go," he said. John Keller in a statement, author of the study and scientist of the Lunar Reconnaissance Orbiter project of NASA's Goddard Space Flight Center.
The researchers believe that earthquakes still occur on the Moon, which means that it is actively changing.
"Our analysis gives the first evidence that these faults are still active and probably produce earthquakes today as the Moon continues to cool and shrink gradually," said Thomas Watters, senior scientist at the Center for Earth and Planetary Studies at the Smithsonian National Air and Space Museum of Washington. "Some of these earthquakes can be quite strong, around five on the Richter scale."
Some of the earthquakes also occurred at a point in the orbit of the Moon when it was farther from the Earth, indicating that the stress due to the tide of the Earth's gravity could have contributed to stress on the lunar crust.
"Active tectonics is not often seen anywhere except Earth, so it is very exciting to think that these defects can still produce earthquakes," said Nicholas Schmerr in a statement, author of studies and assistant professor of geology at the University of Maryland. Schmerr designed the algorithm that re-analyzed Apollo's data.
The researchers have noticed other evidence in the photos of the orbiter of landslides and boulders on the bottom of luminous spots, signaling the recent activity. Over time, the lunar surface darkens due to weathering and radiation, so bright spots are areas where recent activities have exposed areas on the lunar surface.
"For me, these findings underscore that we must return to the moon," said Schmerr. "We have learned a lot from the Apollo missions, but in reality they have only scratched the surface: with a wider network of modern seismometers, we could make enormous strides in our understanding of the geology of the moon, providing some very promising low-impact fruits for science in a future mission to the moon. "
"Establishing a new network of seismometers on the lunar surface should be a priority for the human exploration of the Moon, both to learn more about the interior of the Moon and to determine how much of dangerous presence of lunar earthquakes," he said. Renee Weber, co-author of the study and the planetary seismologist at NASA's Marshall Space Flight Center, in a statement.
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