We’ve been observing it since the dawn of time, but the moon still has surprises to spring. Far from being a cold, dead, inert body, the moon still has a molten core remaining from its formation about 4.5 billion years ago. As it continues to cool, the moon continues to contract, causing its crust to wrinkle.
That contraction, combined with gravitational stresses, is causing moonquakes — as big as 5 on the Richter scale, scientists from Maryland reported in Nature Geoscience this week.
All this supports the theory that the moon wasn’t born with the rest of the solar system, but was created when primordial Earth collided with another planet. The impact shattered the theoretical planet Theia, and the moon was created by accretion of Theian bits and bobs with gobs of material ejected from the traumatized Earth.
There was one snag with that theory: Most of the moon should consist of Theian debris, but rock brought back by astronauts was like Earth’s. Now American and Japanese scientists have resolved the conundrum without resorting to the word “coincidence.”
While we’re talking about moon rocks, the Chinese have found anomalous rock on the far side of the moon that they postulate originated in the mantle. But let’s start with its creation.
The solar system is hypothesized to have been created 4.6 billion years ago from a cloud of dust and gas that formed clumps in the frigid extremes of outer space. Probably the sun came first, surrounded by a disk of matter that accreted to form planets and at least some of their moons. It is the formation of the planets and moons that science has the most difficulty explaining. Not our moon, though: It is theorized to have been created later, by that giant impact. But there was a snag.
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When Theia collided with primordial, molten Earth around 4.5 billion years ago — at an angle, obviously — it shattered. Enough of early Earth survived to become the planet we know and love to abuse. The moon would have resulted mostly from the debris from Theia, which had been about the size of Mars, and material dislodged from here.
The snag had been that computer simulations predicted that most of the moon would consist of Theian rock. But rocks brought back by astronauts are like Terran rocks, explain Yale geophysicist Shun-ichiro Karato, and others, in Nature Geoscience.
Their eureka moment was the realization that the theoretical collision happened only about 50 million years after the Sun formed — a time when the proto-Earth coalescing from the nebula would have been completely molten. It would have been a whirling body of magma, but Theia was probably cold and solid.
So Karato and collaborators formulated a new model based on magmatic early Earth colliding with a giant hard rock. The collision would have heated the magma much more than it would have heated the Theian fragments. Heated magma would then have expanded in volume and burst into space.
“In our model, about 80 percent of the moon is made of proto-Earth materials,” said Karato, who has conducted extensive research on the chemical properties of proto-Earth magma. “In most of the previous models, about 80 percent of the moon is made of the impactor. This is a big difference.”
Billions of years later, both the Earth and moon retain hot cores. Add the stresses of gravity, and one upshot is quakes.
The Earth’s crust consists of continental plates floating on a sea of molten rock. Here, quakes result from the movement of the plates, or unevenness in the bedrock or fracking. The moon does not have continental plates or oil companies.
Yet the moon isn’t just tectonically active, it’s downright frisky — with 28 quakes recorded between 2 to 5 on the Richter scale from 1969 to 1977. The moon doesn’t look like a dried fruit to the naked eye, but NASA’s Lunar Reconnaissance Orbiter spacecraft took photos in 2010 that show thousands of thrust faults.
“These faults resemble small stair-shaped cliffs, or scarps, when seen from the lunar surface; each is roughly tens of yards high and a few miles long,” the team writes.
The moon also experiences deep quakes, up to 1,000 kilometers (620 miles) beneath the surface, and has a lot of them.
“We are not saying the moon has plate tectonics, rather that it is tectonically active,” Nicholas Schmerr, an assistant professor of geology at the University of Maryland, told Haaretz. “The moon is a single plate world. However, the crust contracts as it cools and, much like the shriveling skin on a grape that is drying into a raisin, shrinks and can crack.”
The lunar crust is much thicker than Earth’s, but its magmatic core has been detected using seismic wave analysis, by Dr. Renee Weber of NASA and others.
“The caveat is that the moon’s core is much smaller and cooler than Earth’s, with there likely being a great deal of sulfur in with the iron metal, which lets it stay molten at lower temperature,” Schmerr explains.
Analysis by Schmerr and others on the epicenter of the moonquakes found that a third of the quakes — eight — seem to have resulted from true tectonic activity along the thrust faults, rather than from asteroid impacts or rumblings deep within the moon’s interior.
Rumblings in the interior? “One of the main type of moonquakes detected by [the] Apollo [missions] were located deep within the lunar interior, approximately 800-1000 kilometers beneath the surface,” Schmerr says. “We’re not really sure what causes [these deep quakes], but Apollo detected thousands of them,” he adds.
Seek and ye shall find mantle
But what is that moon mantle like? The Apollo missions landed on the near side of the moon, but the Chinese made a historic probe landing on the far side — and they believe they found rocks from the mantle, which had been pushed to the surface by major impacts, as reported in Nature.
When a big meteor or other object strikes a planet, it can crack the crust, causing bits to fly out from the interior.
On this planet, we can get clues about the mantle composition from volcanoes, among other things. While the moon is turning out to be something other than a dead rock, its volcanoes are apparently long dormant by now, having apparently last erupted during the dinosaur age.
The thinking was to look close at impact craters big enough to have potentially caused mantle material to blast upward. The oldest and largest of the far side craters is a gigantic one named Von Kármán, which is in the South Pole-Aitken basin. Not only might moon mantle have reached the surface there: The huge crater also shows evidence that there had been lava flows in the distant past.
And it is there that the Yutu-2 rover tested rocks that weren’t like the crustal rocks.
Using spectrometer data, the scientists sought differences between surface rock and rock found in the crater — and, lo, they found low-calcium pyroxene and olivine minerals, which may originate from the upper mantle. The authors think these minerals came from below the South Pole-Aitken basin floor.
It should be stressed that these results are from spectrometer, not by possessing the far-side rock and analyzing it. In other words, it’s theoretical for now.
Forget Mars missions, which are probably a pipe dream anyway: Schmerr would dearly love humankind to go to the moon again. Maybe they could even tap the water found there — in the form of ice trapped in permanently shadowed pockets around the lunar poles, some of it apparently lying on the surface.
“We learned a lot from the Apollo missions, but they really only scratched the surface,” Schmerr says. “With a larger network of modern seismometers, we could make huge strides in our understanding of the moon’s geology. This provides some very promising low-hanging fruit for science on a future mission to the moon.”