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

Formation of the Moon
Artwork of a Mars-sized object colliding into the Earth early in solar system history. Many planetary scientists believe that an impact such as this threw off the debris which eventually formed the Moon, but the scenario has its problems.
Getty Images / Lynette Cook

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.

The structure of a planet, a planet with a disk and a synestia, all of the same mass.
Simon Lock and Sarah Stewart

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.

Terrestrial Synestia (art)
This artist’s concept shows the hot, molten moon emerging from a synestia, a giant spinning donut of vaporized rock that formed when planet-sized objects collided. The synestia is in the process of condensing to form the Earth. The illustration of the synestia is based on a NASA artist's rendering of a protoplanetary disk.
Sarah Stewart

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.


Big Splat


Image of Spacivity


March 1, 2018 at 10:28 pm

My thoughts on the moon and Earth is this:
The moon is actually the core of a comet that did bump into the earth and shed the remainder of ice onto the earth. The ice cooled down the planet, the steam formed the atmosphere and the melted ice formed oceans and lakes all over the world. The core bounced off but the moon continues to stay with our orbit due to spacivity, which is, spacial gravity forms that are different from gravity on the planets.
It's obvious that the atmosphere began very warm when life began to form on Earth and larger more adapted animals, like dinosaurs, developed before humans and the animal life that earth has now.
The ricochet also pushed the earth out to the orbit it has now approximately 93 million miles from the sun.

What do you think?

You must be logged in to post a comment.

Image of

March 3, 2018 at 1:24 pm

TOM!! .. I really like your explanation, but what do you think about the fact that the COMET would not have the same stuff but the moon and earth are!
from article: "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..."

great job though!

You must be logged in to post a comment.

Image of Rod


March 1, 2018 at 11:17 pm

"So if Moon-forming events are more likely, then why don’t we see more large Moons around terrestrial planets?" Some good points in this report the proto-earth in synestia stage. Was Venus in a synestia stage, short lived and where is its moon? Computer models can be fun but models like this have many moving parts 🙂

You must be logged in to post a comment.

Image of Anthony Barreiro

Anthony Barreiro

March 2, 2018 at 1:18 pm

Is there any evidence for or against this hypothesis here on the Earth?

You must be logged in to post a comment.

Image of John Schnupp

John Schnupp

March 3, 2018 at 4:34 am

How would this hold up to Occam's Razor (the simpler the explanation the more likely it is better than a complex one) ?

I think the synestia model is much more simple and likely to occur than the big splat. In the big splat there are too many specifics, mass of the planet, mass of the impactor, angle of impact, all required to be just right for the creation of the Moon. The synestia model is much more simple in that it only requires the impact to be large enough to create a synestia. A synestia is not brand new, never before heard of physics.

While the synestia model as laid out in the article might not be 100% correct, it is a much simpler and elegant solution.

John Schnupp, N3CNL
1995 XLH 1200 +106,400
Georgia, VT
44.7675°N, 73.1592°W

You must be logged in to post a comment.

Image of Rod


March 3, 2018 at 7:50 am

From what I have been able to dig up on the synestia phase or stage of the proto-earth, it is very short lived, perhaps 300 years or so. The synestia is dependent too upon large impact(s) to form. The proto-Moon would likely be similar configuration as the giant impact model after formation. Perhaps 23,000 km distance from the proto-earth and an orbital period about 0.5 day with mean velocity about or a bit over 7800 mph. Likely 3-4 earth radii distance initially from the proto-earth. Is there any evidence for initial conditions like this for the Earth-Moon system? Do we have a proto-earth rock to sample today? I take all these computer simulations of the past with a bit of salt too 🙂

You must be logged in to post a comment.

Image of

March 3, 2018 at 1:29 pm

but the Sinestia is around the earth or the moon ? or both?
the article shows the moon in sinestia but the pic shows the earth in sinestia ...
For sure I am the one wrong: just wondering what I am missing.
thank yoU!

You must be logged in to post a comment.

Image of Aquinas


July 3, 2018 at 12:40 pm

The synestia is around the earth's core because initially the matter distribution of the synestia is nearly cylindrically symmetric around earth. The moon condenses out of some of that orbiting material in the synestia. Most of the rest of the synestia falls back to earth.

You must be logged in to post a comment.

Image of Rod


March 6, 2018 at 9:37 am

My observation about the synestia phase of the proto-earth and origin of the Moon by separating from the proto-earth. Q: Are we returning to the fission theory for the Moon, proposed by George Darwin?

You must be logged in to post a comment.

Image of Aquinas


July 3, 2018 at 12:37 pm

I like the synestia model for explaining the similarity of earth and moon, but I also like as least as well the nuclear explosion model proposed by van Westrenend et al. in 2009 & 2013 because it is consistent with known georeactors on earth, the high density of earth's core, the abundance of heavy metals in the Canadian Shield, and because existing georeactors in earth could power earth's long-lived magnetic field and plate tectonics.

You must be logged in to post a comment.

You must be logged in to post a comment.