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Rings of Earth Formed the Moon

The Moon likely formed when a giant body collided with Earth, chomping a hungry bite out of it in a spectacular collision billions of years ago. That’s the abridged bedtime story, at least. There are still many pages missing from the Moon’s biography.

For instance, scientists are unsure about the size and speed of the colliding object, the percentage of debris that fell to earth, flew into space or formed the moon and how long it took the debris to condense toward a single, compact body that now floats in space 240,000 miles from Earth.

Although scientists have an idea about the beginning and end of the story of the Moon's origin, the steps in between are less understood. For example, a disk of material likely formed out of the scattered debris and wrapped itself around the Earth shortly after the collision. The structure of this disk is not known, but scientists think that it says a lot about why the Moon is one eightieth the mass of the Earth and how long it took the Moon to form, lending information about the satellite's chemical composition.


Caltech planetary scientist David Stevenson and planetary scientist graduate student Miki Nakajima offer the most detailed analysis of the type of disks that might have formed after the initial collision. They present their results in a paper recently accepted for publication in the science journal Icarus and can be found on the scientific paper repository arXiv.

The scenario in the video, one of multiple simulations that Stevenson and Nakajima are working on, is of a Mars-sized object colliding with pre-moon Earth. The color scale on the bottom right of the screen measures the objects’ entropy, which relates to temperature. You can see that after the collision -- as the dots change from red to yellow -- the temperature of Earth increases over time while the resulting disk remains relatively cool.

This is just one of the four scenarios that the authors consider in their published work. The other three are: A collision between a fast-spinning Earth and a small colliding object. A collision between two, equally massive objects that are each half the mass of the Earth. And a collision between two objects where the larger object is 3.5 times more massive than the smaller object.

Because the type and number of isotopes in Moon samples are similar to samples on Earth, scientists agree that the Moon likely formed from a disk of a mixture of material from Earth-parts and the disassembled colliding body. How much of the disk's material was a gas as opposed to liquid, however, is unclear.

From their models, the authors show that their different scenarios lead to disks with different amounts of gas. For example, the second scenario, with a fast-spinning Earth, produces a hot disk that consists of 80 to 90 percent gas whereas the first scenario leads to a disk that is about 20 percent gas.

Stevenson and Nakajima are the first to present the temperature profile of potential Moon-forming disks. So far, their results do not identify which, if any, of their scenarios leads to the right-sized Moon with isotopic similarity to Earth, Stevenson said. That will require additional calculations, which so far no one has attempted.

There are concerns with the traditional giant-impact theory of how the Moon formed. Last September, Stevenson co-organized a conference at The Royal Society, London on the “Origin of the moon” to further investigate the latest details and how they fit into the big picture. One new theory that came out of the conference was that Venus possibly gifted Earth the Moon.

“I think that what we’re actually confronted with here is the possibility that we haven’t found the right story,” Stevenson said. “It’s not a question of choosing the right story. It’s producing a different story, different from any currently out there.”

I'd like to think that if Martians were around shortly after the collision, they would have seen something akin to Saturn’s rings when they turned their sites on Earth.


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