Author: Eric Malikyte:

Since the 1970s it has been widely accepted that the moon most likely formed thanks to the collision of another planetary body in the solar system a really long time ago.

And while there are other theories for the formation of the moon (none of which hold much water…like the moon…hah), the question remains, if this really did happen, shouldn’t there be remnants of this ancient planet still on Earth?

Well, now, scientists think that this might actually be the case, and the remnants of this ancient alien world may actually lie deep within the Earth.

Theia v ProtoEarth

It’s generally accepted that our moon formed as a result of the ancient Earth colliding with an ancient protoplanet called Theia that was around the same size as Mars 4.5 billion years ago.

Artist impression of Proto Earth being impacted by Theia.

Though, there are plenty of pseudo-science peddlers out there that claim that it’s an alien spaceship or something.

Other theories include the Fission Theory, which proposes that the moon originated as part of the Earth and was cast off early in its history. The most compelling evidence for this theory is that the moon’s composition resembles the Earth’s mantle. The idea is that if the Earth was spinning rapidly enough, it could have cast off some of that material to form the moon, but other than what was just mentioned there’s no real evidence to support it.

The Capture Theory is the idea that the moon is a captured object, but this is extremely unlikely and can’t really explain the moon’s orbit the way it is today. The moon would have had to have been slowed down gradually before coming into the Earth’s orbit for this to be a likely scenario.

(Maybe it IS a spaceship?)

No, probably not.

The thing is the idea that the moon originally formed thanks to an impactor event has a lot of supporting evidence.

LLSVPs and Moon Debris

[Show image of the masses that cling to the Earth’s Core] This animation shows seismic data compiled and turned into a 3D image of our Earth’s interior. You’ll notice these masses seem to cling to our core.

These objects have puzzled scientists for quite some time, because when seismic waves pass through them, they also abruptly slow down, suggesting that they’re denser and chemically different from the rest of the material around them.

They’re also the largest “things” in the Earth’s mantle, according to Qian Yuan, a Ph.D student in geodynamics at Arizona State University.

Seismologists call these areas low-shear velocity provinces, or LLSVPs, and explanations for the formation of these “things” range from the idea that they simple crystallized out of the depths of the Earth’s primordial magma ocean, to the theory that they might be dense puddles of primitive mantle rock that survived the trauma of the impactor event that formed the moon.

Animation of LLSVPs based on the clustering analysis of Cottaar & Lekic (2016). The resolution of the clustering analysis results in the somewhat blocky depiction of the LLSVPs. Credit:
– Own work, CC BY-SA 4.0

But based on new isotopic evidence and computer modeling, Yuan believed that these “things” are actually the guts of the planet that impacted the Earth 4.5 billion years ago.

And that evidence comes from Iceland and Samoa. Seismic imaging has traced plumes of magma that feed volcanoes on both islands all the way down to these LLSVPs, suggesting that they’ve existed inside the Earth’s mantle since the impactor event that formed the moon.

Over the course of the last decade, researchers have discovered lavas on the two islands that contain an isotopic record of elements that only formed during the Earth’s first 100 million years, making them extremely valuable for scientists.

And adding to this data is a new model of the impactor event with Theia, which suggests that the event could have deposited dense rock deep into the Earth’s inner regions.

This impact theory was originally developed in the 1970s as a means to explain why the moon doesn’t have much of an iron core and why it’s so much dryer than the Earth.

In a cataclysmic impact like what is suggested to have happened with Theia, organic compounds and volatiles like water would most certainly have been vaporized upon impact and it’s very possible that some of this material escaped the Earth’s gravitational field during the event.

While Theia was originally thought to be around the size of Mars, more recent work from Yuan’s co-author, ASU Tempe astrophysicist Steve Desch, suggests that this protoplanet may have been almost as big as the Earth (although, it’s worth it to note that the Earth still would have been smaller than it is today without this material added to its mass).

The Moon rocks that were brought back from the Apollo missions have been extensively studied (when they’re not being gifted to foreign dignitaries at least) and what’s interesting is that when researchers measured the ratios of hydrogen to deuterium, a heavier hydrogen isotope that is a theoretical fuel source for nuclear fusion reactors, light hydrogen was far more abundant in the moon samples when compared to rocks we find here on Earth.

And this is actually really important because in order to be able to both capture and hold onto these quantities of light hydrogen, Theia would have had to have been huge.

In addition to this, this massive protoplanet would have been very dry, much like our moon.

Dead and Dry Theia

Theia, the dry, massive protoplanet, would have separated into layers with an iron-depleted core and iron-rich mantle, estimated to be around 2% to 3.5% denser than the Earth we know today.

Yuan’s model for Theia’s ultimate fate suggested that after the collision, the protoplanet’s core would have quickly merged with the Earth’s, and so long as the material in Theia’s mantle was around 1.5% to 3.5% denser than Earths, then it would have sunk into the core of our young planet, ending their cosmic journey around our core.

Citronade – Own work, CC BY-SA 4.0

A massive Theia could explain the scale of these LLSVPs. These “things” when added together have six times the mass of the moon.

And if these objects really are extraterrestrial, then only a protoplanet (well, really, at this point it would have been a planet if it was the size of the Earth) like the theoretical Theia could have delivered them.

Just imagine what that would have looked like on the ancient protoEarth? If there was any life on the Earth 4.5 billion years ago, the sight would have been absolutely apocalyptic.

Not everyone is happy with the evidence presented, though.

Cue the title card.

Do LLSVPs Even Exist At All?

There is some question as to whether or not these LLSVPs even exist. Some geoscientists suggest that the apparent size of these continent-sized “things” could be an illusion created by the models we make based off of seismic data. And it is important to note that these LLSVPs have been imaged using extremely low-velocity seismic waves, meaning that they’re extremely hard to detect.

Barbara Romanowicz, a seismologist at UC Berkeley, and Anne Davaille, a geophysicist at Paris-Saclay University, suggested in a study in Tectonics last year, that rather than reaching up 1000 kilometers, these “things” may rise only a few hundred before breaking off into branching plumes. She also says, “There may be holes in them [and] they may be a bundle of tubes.”

Harriet Lau, a geophysicist at UC Berkeley says that smaller or less monolithic LLSVPs would also be consistent with a forthcoming analysis that finds that these “things” are densest at the bottom.

This analysis relies on two different methods to measure the depths of our Earth. One uses GPS stations that help us analyze the way that the moon’s tidal pull stretches the Earth as well as seismometers that help us understand how Earth’s natural vibrations penetrate the deepest parts of the mantle.

Harriet Lau says that, “Perhaps the real story behind the density is the distribution depth.”

If these “things” are less massive than Yuan suggests, it could make the idea of a Theia that was nearly the size of the protoEarth extremely unlikely.

Jennifer Jenkins, a seismologist at Durham University says that, “[Yuan’s picture] is not inconsistent with what we know, but I’m not entirely convinced.”

This doesn’t mean that Yuan is wrong, mind you, it just means that scientific teams will have to test the ideas further, perhaps by looking for geochemical similarities between the island lavas that we find in Samoa and Iceland and rocks from our Moon’s mantle.

But there isn’t exactly a ready supply of moon mantle rocks to test right now, as none of the samples brought back during the Apollo program are unaltered mantle rocks.

However, that may change if scientists get their wish to obtain rocks from the center of the moon’s largest impact crater in its south pole.

And as a matter of fact, both NASA and China are in the process of planning robotic missions to the moon’s south pole, which will probably happen sometime this decade.

The south pole of the moon is also a leading candidate site for a future manned NASA mission to the moon.

But if these “things” really are remnants of ancient Theia, they might not be the only ones left over from the impactor event 4.5 billion years ago.

Seismologists are starting to spot smaller, ultradense pockets of material in the deepest portions of the mantle, some of them only a few kilometers across, and often these pockets of material are very close to these LLSVPs.

Jenkins suggests that perhaps these could be the sunken remnants of iron-rich cores from other mini-planets that the Earth might have swallowed.

And how morbid would it be if Theia was just one grave in a planetary cemetery?

That would make Earth pretty… EVIL…or metal. Whatever.


Photo and Video Credits:

Simulation of Moon Formation: Credits: Jacob Kegerreis/Durham University’

Second Simulation of Moon Formation: Credit: Sergio Ruiz-Bonilla at Durham University

Map of LSSVPs: Credit: Sanne.cottaar, Creative Commons

Pillars of the earth. Using data gathered from 253 earthquakes around the world, this visualization is the first global tomographic model constructed based on adjoint tomography, an iterative full-waveform inversion technique. Credit: David Pugmire, ORNL

Illustration: Modified from R.G. Trønnes (2010), Mineral. Petrol. 99, 243-261. (See larger image)

Figure 1: -1 % (red) and +1 % (blue) δVs isocontours of tomography model SEMUCB-WM1 (French & Romanowicz, 2014).

Japan Meteorological Agency 59 Formula optical electromagnetic seismometer: Credit: Apple2000, License: Creative Commons