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Earth's moon as observed in visible light (left), topography (center, where red is high and blue is low), and the GRAIL gravity gradients (right). The Procellarum region is a broad region of low topography covered in dark mare basalt. The gravity gradients reveal a giant rectangular pattern of structures surrounding the region. (Credit: NASA/GSFC/JPL/Colorado School of Mines/MIT)


Lava, not asteroid, explains dark patch on the moon

A dark patch on the moon known as Oceanus Procellarum, or “Ocean of Storms,” was likely not caused by an asteroid strike, as previously thought. Scientists now say ancient volcanoes likely formed the region.

Researchers detected a series of linear gravitational anomalies that form a giant rectangle, nearly 1,600 miles across, running beneath the region. Those anomalies appear to be the remnants of ancient rifts in the moon’s crust.

full moon
The full moon as seen from the Earth, with the Procellarum border structures—the gravitational anomalies—superimposed in red. (Credit: Kopernik Observatory/NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center)

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 the impact hypothesis, says Jim Head, professor of geological sciences at Brown University and one of the authors of the new paper published in the journal Nature.

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

Other dark patches

Most scientists believe that the moon was covered in molten magma early in its history. That magma 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,” Head says. “Everything we see suggests that internal forces were critical in the formation of Procellarum.”

The familiar face of the moon’s near side is dominated by the lunar maria, the dark patches etched across the surface. Most of the large circular features—like Mare Serenitatis (Sea of Serenity) and Mare Imbrium (Sea of Rains)—are likely impact basins that later filled with volcanic lava, which eventually cooled to form the dark basalts.

Samples gathered during the Apollo missions, and data gathered by subsequent unmanned missions, helped to confirm that idea.

GRAIL spacecraft

But the origin of Oceanus Procellarum remained up in the air, largely because it simply doesn’t look like the known basins. It is shaped a bit like a horseshoe, while the other basins are round.

Procellarum also lacks surrounding mountains and radial grooves scoured by impact ejecta, both telltale signs of an impact basin.

Still, the idea that Procellarum was indeed formed by an impact surfaced in the mid-1970s. Proponents of the impact theory argued that Procellarum looked different simply because it was much older than the other basins. Because of its age, the telltale mountains and grooves had been eroded away, and debris had partially filled the basin’s midsection, giving it the horseshoe shape.

Finally settling the debate required a mission like NASA’s GRAIL, Head says. The mission is led by Maria Zuber, who earned her PhD at Brown and is now vice president of research at Massachusetts Instititue of Technolog. The twin GRAIL spacecraft, which orbited the Moon in 2012, made detailed maps of the Moon’s gravity. Those maps have revealed important details about the Moon’s subsurface crust.

“It’s like putting the crust under an X-ray,” Head says. “We can go into the subsurface and see what’s there. And when you’re looking at what could be very old features, that’s what you have to do because signatures at the surface become degraded over time.”

Apollo 15 mission

Apollo 15 Commander David R. Scott, a visiting professor in the department of Earth, environmental, and planetary sciences at Brown, explored the Hadley-Apennine region at the edge of the Imbrium basin in 1971.

“Orbiting the moon following our surface exploration, it was very clear that Oceanus Procellarum differed in many ways from the circular maria in terms of its volcanic and tectonic activity,” he says. “After so many years of puzzling over this, GRAIL has now provided the data to show why it is so distinctly different.”

Source: Brown University

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