The ‘Big Whack,’ as I’ve heard it called, is the impact of a planetary embryo of perhaps Mars-size (or larger) that is thought to have struck the Earth during the latter era of planet formation. Or we can call it the ‘Giant Impact,’ as Arizona State scientists did in a presentation at the virtual Lunar and Planetary Science Conference recently concluded. Whatever the name, the event offers a model for the formation of the Moon, one that explains the latter’s small iron core and the anomalous high degree of angular momentum of the Earth-Moon system.
The impact of the protoplanet called Theia would have been a fearsome thing, blasting pieces of both worlds into space that later coalesced into the Moon. Think When Worlds Collide, the 1933 science fiction novel written by Philip Wylie and Edwin Balmer, whose cover is irresistible and thus must be reproduced here. Better known, of course, is the 1951 film of the same name, produced by George Pal. Neither has anything to do with the Moon but vast objects running into each other offer possibilities Hollywood was bound to seize at some point.
The new work on the Moon’s formation is less photogenic, but it draws on a detectable feature within the mantle of the Earth, so-called Large Low Shear Velocity Provinces (LLSVPs), which have been confirmed through seismic wave detections. The slowing of seismic waves that encounter LLSVPs is an indication that the material they’re made of is denser than the rest of the mantle, and indeed they seem to rest on the rim of the outer core, which is itself telling.
The work is complicated by the lack of agreement on the size of Theia and questions about its composition, as a description in the Lunar and Planetary Science Conference materials (LPI Contrib. No. 2548) makes clear:
…one of the most critical issues related to this scenario is that no evidence has been found for the existence of the hypothesized planetary embryo Theia. This is in part because of its widely debated size ranging from 0.1~0.45 Earth mass (M?) [5] and enstatite to carbonaceous chondrite composition [6]–[9]. Moreover, whereas it is mostly agreed that the core of Theia promptly merged with the proto-Earth core shortly after the impact [3], what fraction of and how the Theia mantle was preserved into the Earth mantle remain elusive. This post-impact process is not only responsible for the initial thermal and compositional structures of the Earth, but also significantly affects Earth’s long-term chemical evolution.
Image: The Giant Impact hypothesis for the origin of the LLSVPs. Credit: Li et al.
Did intact pieces of Theia’s mantle survive inside the Earth? The Arizona State study indicates that some of these materials account for the Large Low Shear Velocity Provinces, sinking to the bottom of Earth’s mantle in the process. The authors use numerical modeling experiments to support the idea, working with mantle materials from both Theia and the Earth as key components and producing models where the Theia materials sink and survive.
The key here is the density of the Theia mantle, allowing the material to survive mixing by convection within Earth’s mantle as long as it is denser than the material around it. The analysis suggests that the Theia materials were rich in iron and several percent denser, allowing them to sink to Earth’s lowest mantle and accumulate into the LLSVPs. The initial thickness of the Theia mantle materials in this model is in a range between 350 kilometers and 500 kilometers, existing as chunks of a protoplanet in the form of LLSVPs beneath Africa and the Pacific Ocean.
For more, see Yuan et al., “Giant Impact Origin for the Large Low Shear Velocity Provinces,” 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548), available here. The paper is in process at Geophysical Research Letters.
The average density of the Earth is anomalously high compared to the nearby planets:
Densities of the 4 rocky planets
Mercury – 5.43
Venus – 5.24
Earth – 5.51 Moon – 3.34
Mars – 3.93
The Moon’s density is anomalously low compared to the density of Earth’s mantle and closer to the density of Mars’ mantle, and much closer to asteroidal carbonaceous chondrites. Is it p0ssible that Theia’s core was added to both the Earth’s core and mixed with the Earth’s mantle, while the CC material composed the Moon. IOW, is it possible that the diagram of the collision where some of Theia’s mantle is heavier than the Earth’s mantle and sinks towards the Earth’s core incorrect?
Here is a GIF of the anomalies, could it be material just been squeezed out of the core as it crystallises.
https://en.wikipedia.org/wiki/Large_low-shear-velocity_provinces#/media/File:LLSVP.gif
The event may explain the Fermi Paradox
Alex Tolley, the Earth’s mantle actually has a lower 3.3 g/cm, the crust has an even lower density: 2.83 g/cm3 https://en.wikipedia.org/wiki/Continental_crust#:~:text=The%20average%20density%20of%20continental,about%202.9%20g%2Fcm3.
The idea is that most the moons crust is made of anorthosite or plagioclase feldspar which is the same composition of Earth’s mantle, the idea used to support the giant impact hypothesis. Moons and Planets, William J. Kaufmann.
Excuse me the mistake. It’s Planets and Moons by William J. Kaufmann. 1979.
The context of the big whack is whatever characteristics it created, like these “blobs” in the mantle and the like, is how the characteristics resulted in other characteristics such as a magnetic field and plate tectonics. Earth has a very thin crust compared to Venus. Is it possible that the big whack is responsible for thinning Earth’s crust such that plate tectonics became possible? Is the big whack responsible for the core rotating at a higher rate as well as metal concentrations such that the Earth has a decent magnetic field?
My suspicion is that the big whack altered the Earth sufficiently that it is very unusual compared to other plants its size and location in habitable zone such it makea our Earth a “rare Earth”.
I mean, how often do “big whacks” occur in planetary formation? Probably not often.
That’s an interesting idea, Abelard Linsey. The size of the planet is important since a smaller world like Mars and the Moon have thicker crusts since they cooled of faster without plate tectonics. I recall reading somewhere in articles online about a theory that the oceans help lubricate the plates and cool the crust off. The placement of the continents cool off the crust as Pangaea slit up.
Venus might have had plate tectonics is the distant past before it’s oceans evaporated. Maybe someday we will have the technology to drill for core samples or land a seismograph there to help determine the structure of the inside of Venus. Without plate movement and slippage there might not be strong Earthquakes or Venusquakes there so we could explode a bomb on the surface or crash a small asteroid there to make a quake so the seismograph could detect the Venusquake.
I also remember a paper that we got scraped by a massive planet 700 million years ago…North America I think. We might have been a water world.