In a new twist on the giant impact theory, a new idea posits that the Moon might have formed from the vaporized remains of Earth after an epic collision with another planet-sized body.
From all the time and effort humans have put into observing and studying the Moon, there is an awful lot we still don’t know about it, particularly when it comes to how it formed.
Most planetary scientists agree that our Moon was created when a planet-sized body hit Earth after it had almost completely formed. But they seem to disagree on nearly everything else. Now, a group of researchers has come up with an idea that upends that so-called “Big Splat” theory: If the giant impact first completely obliterated Earth, the Moon might have formed from our planet’s vaporized remains.
Big Splat, Big Problems
The longest-standing theory, developed in the 1970s, proposes that an object with the mass of Mars delivered a glancing blow to Earth, launching large amounts of rock into an orbiting ring that coalesced to form the Moon. Most of the Moon would have been made of material from the impactor’s mantle. The angle of the impact gave the Earth-Moon system its current angular momentum.
But over the years problems with the theory have emerged. For one, astronomers haven’t found any trace of the impactor’s chemical makeup. Instead, measurements of isotopic ratios of different elements — such as oxygen’s three isotopes, 16O, 17O and 18O — show that the Moon and Earth are made of exactly the same stuff. This is odd because all other solar system bodies with known isotopic ratios have their own distinct signatures.
Researchers have tried to come up with a mechanism that could have masked the impactor’s signature or mixed it with enough of Earth’s material so that they became indistinguishable. There are a lot of possible mechanisms: mixing during and after the impact, a more energetic impact that could have resulted in more of the Moon’s material coming from Earth, or multiple impacts rather than a single one.
A New Lunar Origin Story
In 2017 researchers Sarah Stewart (University of California, Davis) and her graduate student Simon Lock (Harvard University) went a step further and proposed a radical new approach. They developed computer models showing that when two planet-mass objects collide, one possible outcome is that they become a synestia, a mass of vaporized rock and metal that takes the shape of a giant donut connected to a metal-rich central bulge. The bulge is the surviving core of the planet. It’s connected to an outer torus made mostly of silicate rocks that spins rapidly and expands beyond the lunar orbit.
Lock says that many researchers have trouble understanding what a synestia even is. “Many think that the synestia is something that’s kind of a layer on top of a planet rather than thinking about is as the whole planet,” Lock says. “The planet is the synestia.”
A synestia might have acted as the ultimate mixer — the impactor and the impacted body would have achieved almost total chemical equilibrium. Now Lock and Stewart, along with other researchers, have studied what a terrestrial synestia might have looked like and how well their model fits some key observables of the Moon-Earth system. Their results have been published online on February 28th in the Journal of Geophysical Research.
In their model, the Moon forms within the orbiting torus of the synestia. As the rock vapor radiates heat and cools down, it begins to condense into droplets of liquid rock. Bits of solid rock, launched into orbit by the impact, act as seeds that accrete droplets, growing into moonlets that eventually merge together.
Eventually the synestia shrank under the lunar orbit, leaving behind a fully formed but still molten Moon.
Terrestrial Synestia: Pros & Cons
A key constraint to any lunar origin scenario is explaining why the Moon has so few volatile elements, such as oxygen and carbon dioxide, compared to Earth. The researchers have estimated that the torus of the synestia would have reached high temperature and pressure. As things cool down, the most easily vaporizable stuff remains in a gaseous phase longer, so less of it makes into the Moon.
The synestia model is also more relaxed in what it requires from the original impact. The Big Splat theory required a body with the mass of Mars that just grazed Earth, almost missing it, so that enough material would be launched into orbit. The synestia model is more flexible: As long as the impact releases the energy needed to create a synestia, it works with a wide variety of impactor sizes and angles. As a result, the chance increases that such a Moon-forming event would happen in the first place.
So if Moon-forming events are more likely, then why don’t we see more large Moons around terrestrial planets? Our sample size might just be too small, Lock answers. “We might have to wait until we can study exomoons to be able to know how common large Moons are,” Lock says.
So far, the synestia model has produced mixed reactions within the ranks of planetary scientists. Some of them welcome it as a potential fix for the limitations of the giant impact theory, but others remain skeptical.
“Many of us would like a more natural scenario that makes it more or less inevitable that the Moon will have essentially the same isotopic composition as the Earth,” said planetary scientist Jay Melosh during a recent conversation with Sky & Telescope.
In the future, the team plans to further refine certain aspects of the model that are currently poorly understood. “In some aspects this is almost a proof of concept,” Lock says. Improving their understanding of the complex gravitational interactions between the forming Moon and its surroundings, as well as the specifics of how the vaporized and liquid materials interact within the cooling torus are among their most immediate goals.