Two gigantic chunks of material lurking deep under the surface of Earth might be remnants of an alien world called Theia that violently collided with our infant planet in an ancient impact that created the Moon.
That sentence may sound like the synopsis for a sci-fi epic, but it’s actually a new hypothesis proposed by scientists in a presentation at the 52nd Lunar and Planetary Science Conference 2021, which was held virtually last week.
A team led by Qian Yuan, a PhD student studying geological sciences at Arizona State University, suggests that two mysteriously dense zones located more than 1,000 miles under Earth’s surface are “left-over Theia mantle materials,” according to the presentation summary.
Yuan and his colleagues back up this trippy idea with geodynamical models as well as evidence collected about these subterranean zones, which are known as large low-shear-velocity provinces (LLSVPs), or more informally as the “blobs.” Located under the Pacific Ocean and West Africa, these continent-sized LLSVPs are hundreds of miles high and thousands of miles across, making them the most massive formations in Earth’s interior.
“You could say that these are the biggest and largest meteorites, if they are mostly Theia’s mantle,” said Yuan in a call. “It’s very cool.”
Yuan first started thinking about the possibility that the blobs might have been extraterrestrially punched into Earth’s interior during a class about the so-called “giant impact hypothesis” taught by Mikhail Zolotov, a planetary geophysicist at ASU.
This hypothesis proposes that the Moon was formed in the fallout of a catastrophic collision some 4.5 billion years ago. Earth had barely been born before another planet crashed into it, spewing enormous volumes of planetary debris into space that eventually accumulated into the radiant natural satellite that graces our skies today. Scientists have generally estimated that this bygone planet, named Theia after the mother of a Greek lunar goddess, was about the size of Mars, but some models suggest it was both smaller and bigger than that scale.
When Zolotov mentioned that one of the hypothesis’ weaknesses was that no clearly identifiable pieces of Theia have ever been found, Yuan had a sudden flash of insight.
“That really struck me, because at the time I was studying LLSVPs,” he said. “I knew that people were arguing that LLSVPs could be even older than the age of the Moon.”
“When I got back to my office, I just did a very easy calculation where I compared the size of the LLSVPs to the mantle of Mars, because Theia was believed to be very similar to Mars,” Yuan continued. “The total mass of the Moon, together with the LLSVPs, is almost perfectly matched with Mars’ mantle.”
This initial calculation suggested the scale of the LLSVPs was roughly in line with an ancient impact origin, though Theia would need to have been bigger than Mars because a lot of this mantle material would be lost to space in the aftermath of the crash.
“You will lose some material during the impact, for sure,” Yuan said. “You cannot transfer Theia’s mantle whole.”
Still, the result convinced Yuan that he was on the right track, though he thought it was “kind of crazy,” he said. But after discussing the hypothesis with his advisor and other experts on the giant impact hypothesis, he began to accumulate more evidence that these subterranean blobs really might have come from outer space.
For instance, Yuan came across a 2019 study in Geochemistry, led by ASU astrophysicist Steve Desch, that includes an in-depth analysis of Moon rocks returned to Earth from the Apollo missions. The research suggested that Theia was about four times larger than Mars, with low levels of moisture and an iron-rich mantle that was about two to three percent denser than Earth’s mantle.
As it happens, the LLSVPs are also estimated to be about two to three percent more dense than the surrounding mantle, providing another intriguing hint that they could be pieces of Theia. When Desch heard about Yuan’s hypothesis, his interest was piqued and he co-authored the new research that was presented at the conference last week.
It is mind-boggling to imagine that there are enormous scraps of an alien world deep under our feet, injected during Earth’s infancy by a disastrous world-smashing event that created the Moon. Right now, this all remains hypothetical, but Yuan is optimistic that there are ways to build on the existing evidence and perhaps prove that the LLSVPs have an extraterrestrial origin.
For instance, if scientists could get their hands on pristine samples of the Moon’s mantle, perhaps retrieved from the exposed crater in the lunar South Pole, they might be able to compare its composition to the makeup of the LLSVPs. Though these blobs are located far too deep inside our planet to be directly sampled, their composition can be inferred from seismic studies of their densities and analysis of volcanic islands that are linked to these deep zones via mantle plumes.
“If people can find some chemical relationship between the lunar mantle materials with the LLSVPs, it may suggest they have the same ancestor: Theia,” Yuan said. “Otherwise, these two materials shouldn’t have any relationship.”
“The LLSVPs are thought to be very old, and have noble gas isotopes,” he continued. “If the mantle rocks of the Moon also have similar noble gas isotopes, I think that’s the final task for our hypothesis.”