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IMAGE: Infrared (top left) and other images from NASA’s Lunar Reconnaissance Orbiter revealed strange, bare places where the omnipresent moon dust is missing. The spots suggest an active tectonic process …
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Credit: NASA
PROVIDENCE, R.I. [Brown University] – Researchers have discovered a system of ridges that extend along the near side of the Moon with newly exposed rocks. The ridges could be evidence of active lunar tectonic processes, the researchers say, possibly echoing a long-ago impact that nearly shattered the Moon.
“There is an assumption that the Moon is long dead, but we continue to discover that this is not the case,” said Peter Schultz, professor in the Department of Earth, Environmental and Planetary Sciences at Brown University and co-author of the research, which is published in the newspaper geology. “Based on this document, it appears that the Moon may still be cracking and cracking, potentially today, and we can see the evidence on these ridges.”
Most of the Moon’s surface is covered by a regolith, a layer of crushed rock dust created by the constant bombardment of small meteorites and other impactors. Regolith-free areas where the Moon’s bedrock is exposed are very rare. But Adomas Valantinas, a graduate student at the University of Bern who led the research while a visiting scholar at Brown, used data from NASA’s Lunar Reconnaissance Orbiter (LRO) to detect strange naked places in and around the lunar mary, the big dark spots on the near side of the moon.
“The exposed blocks on the surface have a relatively short lifespan because regolith accumulation occurs constantly,” Schultz said. “So when we see them, there must be some explanation of how and why they were exposed in certain places.”
For the study, Valantinas used the LRO Diviner instrument, which measures the temperature of the lunar surface. Just as concrete-covered cities on Earth retain more heat than the countryside, the bedrock and block layers on the Moon stay warmer during the lunar night than regolith-covered surfaces. Using Diviner’s nocturnal observations, Valantinas discovered more than 500 patches of exposed bedrock on narrow ridges following a pattern across the moon-side Maria.
Schultz says some ridges covered with exposed bedrock have been seen before. But those ridges were at the edges of the old lava-filled impact basins and could be explained by the continuous fall in response to the weight caused by the lava fill. But this new study found that the most active ridges are related to a mysterious system of tectonic features (ridges and faults) on the near lunar side, unrelated to lava-filled basins and other young faults running through the highlands.
“The distribution we found here asks for a different explanation,” said Schultz.
Valantinas and Schultz mapped all the exposures revealed in the Diviner data and found an interesting correlation. In 2014, NASA’s GRAIL mission found a network of ancient cracks in the Moon’s crust. Those cracks became channels through which magma flowed to the Moon’s surface to form deep intrusions. Valantinas and Schultz demonstrated that the block ridges seemed to align perfectly with the deep intrusions revealed by GRAIL.
“It’s almost a one-to-one correlation,” said Schultz. “That makes us think that what we are seeing is a continuous process driven by things that happen inside the Moon.”
Schultz and Valantinas suggest that the ridges above these ancient intrusions still rise upward. Upward movement breaks the surface and allows the regolith to drain into cracks and gaps, leaving the blocks exposed. Because the bare spots on the Moon get covered fairly quickly, this cracking must be fairly recent, possibly even current. They refer to what they have found as ANTS, for Nearby Active Tectonic System.
Researchers believe that ANTS was launched billions of years ago with a giant impact on the far side of the Moon. In previous studies, Schultz and a coworker proposed this impact, which formed the 1,500-mile South Pole Aitken Basin, shattering the interior on the opposite side, the near-Earth side. Magma then filled these cracks and monitored the pattern of levees detected on the GRAIL mission. The blocky ridges that comprise the ANTS now track ongoing adjustments throughout these old weaknesses.
“This looks like the ridges responded to something that happened 4.3 billion years ago,” said Schultz. “Giant impacts have lasting effects. The Moon has a long memory. What we are seeing today on the surface is testimony to its long memory and the secrets it still holds.”
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