What happens when two protoplanets collide? One theory suggests this is how Earth’s moon was formed. In 1946, Canadian geologist Reginald Daly suggested that during Earth’s formation, a Mars-sized protoplanet collided with Earth. The timing of the collision was later estimated to be roughly 4.51 billion years ago. This “giant-impact” or “big splash” ejected materials that later coalesced to form the moon, circling Earth on the same orbit as Earth revolves around the sun. The hypothesized protoplanet was later named Theia after a Greek titan who gave birth to the moon goddess Selene.
Several lines of evidence support the giant-impact idea. In particular, rock samples brought back from the moon on the Apollo in 1971 showed stable isotope ratios identical with those of Earth’s rocks, suggesting a common origin.
If Theia did hit Earth, what happened to its materials afterward? Researcher Qian Yuan and colleagues tried to answer this question in a recent paper published in Nature. Their work suggested that the large low velocity provinces sitting in the Earth’s lower mantle and atop the liquid outer core are relics of Theia’s mantle materials.
The LLVPs are two continent-sized blobs of dense, hot materials: the Pacific LLVP and the African LLVP. Both are considered roots of mantle plumes and hotspots concentrated in the Pacific and African plates, respectively. The scientific community has no direct access to the LLVPs, but they are recognized via seismic tomography of the Earth’s interior.
The origin of LLVPs is an enigma. One idea is that they formed from deep-sinking, iron-rich oceanic slabs at subduction trenches; in other words, they are “subducted slab graveyards” sitting at the core-mantle boundary.
Yuan, a postdoctoral fellow at Caltech, began connecting LLVPs with Theia during a geochemistry class at Arizona State University.
“Professor Mikhail Zolotov expressed his puzzlement over why the community had not found any evidence supporting the existence of Theia,” Yuan said.
That got Yuan’s wheels turning.
Why are there only two LLVPs?
Yuan commented, “The current morphology of LLVPs is the consequence of long-term interaction with the mantle convection currents. It is likely that there were three or four in the early Earth, but it ended with two LLVPs in later geologic history.”
Yuan’s team ran numerical simulations of the Earth-Theia impact based on the dynamic and thermal conditions of these two protoplanets at that time. Their simulation depicts the emergence of the LLVPs after the giant impact. If true, this scenario suggests Theia’s inner materials have become part of the Earth’s body as LLVPs.
Theia’s Next Chapter
In May, Yuan and colleagues published an article in Geophysical Research Letters, arguing that soon after the LLVPs formed, mantle plumes arising from these overheated core-mantle provinces impinged and melted parts of the Earth’s early crust, initiating the first subduction. This would mean plume tectonics predated and triggered plate tectonics. This idea is not new. Taras Gerya of ETH in Switzerland has long studied the plume-induced subduction, but Yuan and colleagues’ new work considers hot buoyant LLVPs to be the root cause of the mantle plumes that induced the earliest subduction in Earth’s history. They illustrated this process using numerical modeling and simulations.
When did the first subduction happen? The oldest minerals found in Earth’s rocks are zircon crystals dating back to 4.3 billion years ago, indicating that the Earth’s crust formed less than 200 million years after Earth’s formation. If Yuan and colleagues’ work is correct, it appears that solid Earth’s segregation into the core-mantle-crust structure and the mantle convection currents and crustal subduction all occurred over a short interval, geologically speaking.
A Lasting Impact
This work is key to understanding Earth and the moon’s origins, but investigating the Earth’s deep interior is also important to understand the crustal processes that help shape natural resources and habitats. For example, plume-induced subduction has been suggested for the Caribbean plate subduction in Late Cretaceous times following the formation of Caribbean Large Igneous Province 100 million years ago.
Additionally, many ocean island basalts contain deep-Earth elements and volatiles. If the LLVPs are indeed relics of Theia’s mantle, it also means, according to Yuan, that “these primordial signatures like helium, xenon and neon could originate from Theia’s mantle.” That would give material evidence for Theia. But, if a different theory not involving Theia can be demonstrated for the moon’s origin, we will need to reconsider the origin of the LLVPs.