The Formation of the Moon May Have Stretched the Earth Into a Potato
The question of how the Moon formed has fascinated astronomers and geologists for decades. Currently, the prevailing theory is that the Earth was struck by an approximately Mars-sized impactor very early in its life. This impact catastrophically changed our planet, re-liquefying the surface and hurling an enormous cloud of debris into orbit. Eventually, that debris coalesced into the Moon.
We’ve known for a long time that the newly formed Earth-Moon system was very different from the one we have today. The Moon would have been orbiting just 8,000 miles above the surface of the Earth, while the Earth day might have just been 2.5 hours long. A new paper by Simon Lock, a planetary scientist with the California Institute of Technology, looked at what the consequences for the Earth would have been during this tumultuous time period and found something interesting. At 8,000 miles away, the gravitational pull of the Moon on the Earth would have been significantly increased, stretching our planet from a sphere-ish shape into something considerably more like a potato.
According to Lock, the oldest-known rocks on Earth — 4.4-billion-year-old zircons, embedded in younger rock structures — must have formed within 100M years of the Moon impact in order for the timelines to fit. Modern subduction processes produce zircons, but these processes are not expected to have been active on Earth so near to its formation (among other differences, the very young Earth had a much higher internal temperature than our planet does today). Lock and his co-author, P.D. Asimow, write:
As the Moon receded from Earth, the planet’s spin period increased and its shape changed dramatically, becoming roughly spherical within a few 10s Myrs. Such a rapid and substantial change in shape had significant consequences for Earth’s crust, atmosphere and interior.
A number of geologic features on Earth make sense if we assume this kind of process. Pulling the Earth into a potato-like shape and imparting so much energy into it would have liquified the surface crust and stretched the planet, with variations of mass and gravity by as much as 10 percent over the surface of Potato World. Over time, as the Moon moved away, Earth’s rotational energy slowed, and the planet began to revert back to a more normal shape.
Theia’s early impact on Earth may have driven the dynamic processes required to form zircons so early in our planetary history. Ironically, this means that yes, the early Earth was significantly more flat than the current version. (Flat Earthers, don’t get excited. It wasn’t a disc, and it wasn’t a dome, and it certainly hasn’t looked that way for ~4.4 billion years).
According to National Geographic, the early Earth would have had an enormous equatorial bulge of solid rock, rather like what happens when you try wearing a pair of pants two sizes too small. Instead of the tides rising and lowering the surface of the ocean, the Moon actually raised a massive bulge of rock. This offset caused the Moon to begin moving away from the Earth, while the rate of spin on Earth began to slow. As it slowed, the crust tore apart at the poles, releasing magma on the surface, while the tremendous band of elevated rock at the equator began to collide with itself, creating a great deal of mountain-building in a very short period of time. Virtually all of this crust is gone now, sunk deep into the mantle, but there are still a handful of cratons on the planet that date back to roughly this time period.
But that’s where things get interesting. By forcing an enormous amount of surface rock into the mantle, the inner part of the Earth was handed considerably more compelling kinds of rocks to make new rocks out of. This, according to Lock, may have played a major part in the geological development of Earth. It’s not capable to completely parse how different the initial planet was, partly because the core contained about 3x more heat than it does today and the rock was much more liquid.
One of the things that set Earth apart from other planets and moons we’re aware of is that the Earth has plate tectonics. The rocky planets and moons of the solar system have what’s known as “stagnant lid tectonics” in which the principal release of stored energy is through volcanism, rather than the movement of interlocking plates.
During this time period, the Earth may have followed a stagnant lid tectonic system, just as Venus is believed to do today. On a world with too much heat, the rocks are sufficiently plastic to melt back together rather than forming the irregular plates that “float” on a surface of magma, far beneath the planet’s surface. There are some theories that plate tectonics on Earth may be a fairly recent development of the past 1-2 billion years, and that we moved from one system to the other once the radiogenic heat in the Earth’s core had cooled enough to allow the tectonic plates to break and remain broken, rather than always melting back together.
The implications of this paper are that the Moon may have an even more transformative impact on Earth’s geology, driving complex mineral formation in the crust and mantle. The tremendous heat and energy of Theia represent a profound “remixing” effect. One theory about the rise of life on Earth, in fact, posits that the Earth-Moon system is responsible for the abundance of life we see around us for various reasons, including its impact on the length of our day or the complex geological transformation it wrought on our entire planet.
Sorry, however, for the potato.
Feature image by NASA