Lunar Oceanus Procellarum Created by Supervolcano, not Impact

Oceanus Procellarum, a vast dark patch visible on the western edge of the Moon's near side, has long been a source of mystery for planetary scientists. Some have suggested that the "ocean of storms" is part of a giant basin formed by an asteroid impact early in the Moon's history. But new research published today in Nature deals a pretty big blow to the impact theory.

The new study, based on data from NASA's GRAIL mission, found a series of linear gravitational anomalies forming a giant rectangle, nearly 1,600 miles across, running beneath the Procellarum region. Those anomalies appear to be the remnants of ancient rifts in the Moon's crust, say the authors of the new study. The rifts provided a vast "magma plumbing system" that flooded the region with volcanic lava between 3 and 4 billion years ago. That giant flux of lava solidified to form the dark basalts we see from Earth.

It's the shape of the underlying gravity anomalies that cast doubt on impact hypothesis, said Jim Head, the Louis and Elizabeth Scherck Distinguished Professor of Geological Sciences at Brown and one of the authors of the new paper.

"Instead of a central circular gravity anomaly like all other impact basins, at Procellarum we see these linear features forming this huge rectangle," Head said. "This shape argues strongly for an internal origin and suggests internal forces."

The research team, led by Jeffrey Andrews-Hanna of the Colorado School of Mines, suggests a new hypothesis for just what those internal forces may have been. The process, the researchers believe, was driven by the geochemical composition of the Moon's crust in the Procellarum region.

Early in its history, the Moon is believed to have been entirely covered in molten magma, which slowly cooled to form the crust. However, the Procellarum region is known to have a high concentration of uranium, thorium, and potassium — radioactive elements that produce heat. The researchers believe those elements may have caused Procellarum to cool and solidify after the rest of the crust had already cooled. When Procellarum did finally cool, it shrank and pulled away from the surrounding crust, forming the giant rifts seen in the new data. Magma flowed into those rifts and flooded the region.

"We think this is a really good, testable alternative to the impact basin theory," said Head. "Everything we see suggests that internal forces were critical in the formation of Procellarum."

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Large Lunar Subsurface Volcanic Structures Uncovered by Grail

Subsurface structures of large volcanic complexes on the nearside of the Moon: A view from GRAIL gravity

Authors:

Huang et al

Abstract:

The lunar nearside large volcanic complexes, such as the Rümker Hills, Aristarchus Plateau, and Marius Hills are likely sites of intense and sustained magmatic activity. These volcanic complexes, recently proposed to be shield volcanoes, are generally located at regionally high elevations and some feature relatively well-localized positive gravity anomalies. Applying localized spectral analyses on high-resolution gravity data obtained from the Gravity Recovery and Interior Laboratory (GRAIL) mission and topography data returned from the Lunar Reconnaissance Orbiter (LRO) spacecraft, we study the subsurface structures of these volcanic complexes. The gravity signal is predicted using a thin elastic lithospheric model that considers both surface and subsurface loads. Best-fit crustal and load densities show that the topographic highs of Rümker Hills, Marius Hills, Gardner and Kepler are mainly composed of material that has a density of more than 2850 kg m-3, which is consistent with that of emplaced igneous rocks. Both the Aristarchus Plateau and Hortensius have relatively lower crustal and surface load densities, with mean values around 2550 kg m-3, which are well consistent with the average bulk density of the lunar highland crust. These results, together with evidence of multiple volcanic edifices on the surface, suggest that the shallow crusts of the Rümker Hills, Marius Hills, Gardner and Kepler are mainly composed of dense intrusive/extrusive magmatic units, and those of the Aristarchus Plateau and Hortensius are mainly composed of low density materials with only small amounts of superimposed volcanic material. To further constrain the subsurface structures beneath these volcanic complexes, we analyze the Bouguer gravity anomalies for these regions. Results show that dense materials that might be solidified intrusions exist beneath Rümker Hills, Marius Hills, Gardner and Prinz, but no substantial dense materials have been detected beneath the Aristarchus Plateau, Hortensius, Kepler or Cauchy. The different subsurface structures among these large volcanic complexes suggest that the volcanism style at the lunar nearside is different from each other, even for those formed at the same geological time.