Neil DeGrasse Tyson, the new Carl Sagan, in his recent series on Natl Geo “Cosmos” stated that our moon was captured by gravity by our planet. He said it was one of the many boulders at the creation of our solar system. However, on a different TV production on the Science channel. it was stated that the moon was created from the Earth after another planet struck the earth and tore off a big chunk of the Earth. it said that this was confirmed as moon rocks are identical in composition to Earth rocks. So which is it?
I don’t actually remember moon formation in that Cosmos version. I though the Theia collisionhypothesis was the current lead candidate.
Duckster’s last cite is the reason stated by Tyson on Cosmos. From reading the cites, it appears that the weight of the evidence supports the Science channel’s version: That a collision with another body, called Theia, caused a portion of the Earth to separate and form our present moon. Theia, if it survived would be our second moon, but much smaller.
Both theories are popular beliefs. However, I seem to recall reading recently that the “captured moon theory” is presently in vogue over the “moon formed from junk carved out of the earth by high impact” theory.
Not sure if you can answer this, Barbitu8, but for what it’s worth: I never bought this. If this is so, then where does one notice a significant piece of missing earth? If under the sea, do we find evidence of a huge crater under the ocean from which the moon was carved?
The Theia collision is thought to have occurred ~4 billion years ago when Earth was basically a molten sphere of rock. You’re not about to find any sort of impact crater after that amount of time - consider that the oldest large scale rock formations are dated to only 2.5 to 3 billion years ago.
That theory was in vogue in the past, and perhaps when Tyson filmed his series, but Duckster’s links show that new evidence this year leads to the collision theory.
As to the missing chunk of Earth, the planet would have been bigger before the collision. Natural tectonic forces, erosion, etc. would have not made this evident. Or a crater could exist under a deep portion of a sea, or perhaps the crater discovered off Mexico, attributed to a large meteor, could be it.
This would have happened around 4.5 billion years ago, and totally wrecked the proto-Earth, which would have reformed into a sphere via gravity. (And is still the mainstream theory.) If you had a method of cutting the Earth in two equal halves and separating them, those two halves would reform into two smaller spheres.
(I’m not sure off-the-cuff how many pieces the Earth could be separated into with each big enough to reform into a sphere via it’s own gravity. Dozens, at least. Every known spherical rock in the solar system other than Venus could be gathered together inside of the Earth, if it were a hollow shell.)
Really, the most implausible part is that it was called Theia. I doubt it even had a name.
It does now.
A rough rule of thumb that scientists use for impacts is that the crater formed is around 10 times the diameter of the impacting body. Traditionally the K-T (yes, I know I should call it the K-Pg, but I don’t want to) impactor is still estimated to have been around 6 miles in diameter, but this figure was given when the Chicxulub crater (the one off the Yucatan Peninsula) was thought to be smaller. With the crater now being measured to be at least 110 miles in diameter, IMHO it is time to upgrade the size estimate for the impactor into the 10 mile range, but I don’t know how the experts feel about that. The Chicxulub crater is a tiny pinprick compared to the damage that Theia would have made (going by the 10x size rule of thumb, the “crater” would have been larger than the diameter of the Earth.)
Are you sure about this? Spheres don’t pack especially tightly.
Yep, I did it myself. I like playing around with CGI programs, but lack much actual talent at it. Mapping textures to spheres is dead simple, though, so I created to-scale models of all the planets, “official” dwarf planets, and major moons of the solar system. I then used rigid body physics simulation to fill a hollow shell of the Earth. They fit. Here is a video I created.
Think of it this way: the impact was so large that every solid piece of the Earth was either smashed or liquefied. The solid crust is a pretty small portion of the planet if you think about it, so in a collision at this scale, thinking of the Earth as a liquid droplet of molten iron is a better image than thinking of us as a rocky body.
The “splash” from this collision was thrown out around the Earth and then 1) fell back to the Earth, which formed a new solid crust from the liquid magma, 2) rejoined with other pieces in a stable orbit to form the moon or 3) was ejected from Earth orbit altogether.
(On a total hijack: option 3 is kind of an exciting thought. It’s almost certain that there are other pieces of the collision somewhere in our solar system which might give us some fascinating information about the early Earth. Given sci-fi advances in technology, maybe we could find them some day.)
Neil DeGrasse Tyson is not exactly wrong. The Moon was captured by the Earth, using an extreme form of lithobraking that irrevocably mixed both objects together.
I’m not so sure that’s certain. Anything tossed out of Earth’s gravity would have ended up orbiting the Sun in an orbit that crosses that of the Earth. So most of it would eventually be “recaptured” by either the Earth or the Moon. Of the stuff that doesn’t, a lot would probably hit some other planet like Venus.
However, it is possible. There could be some fragments that got into some kind of resonant orbit with the Earth, such as that of Cruithne, so that the orbits never intersected. So I’d look for bits of early Earth among that group of objects (there’s a half dozen or so known asteroids in horseshoe or other resonant orbits).
Interesting, I’d have said the opposite, and the moon seems to be composed of material similar to Earth’s crust rather than the usual proportions of planetary material, supporting the idea the Earth received a glancing blow from another, possibly Mars-sized planet.
Perhaps you heard about Neptune’s moon Triton? It orbits backwards relative to most other solar system objects and is thought to have been captured.
He’s certainly not coming across as particularly Saganesque here (as others have noted). Current thinking is definitely coming down on the side of the Giant Impact.
Here’s an article from last year that cites additional evidence for this.
Darren Garrison, you missed at least one dwarf planet, the first of them to be discovered: Ceres. And depending on your standards for “spherical”, one or two other asteroids might qualify, too. Not all of the dwarf planets are in the outer solar system.
They’d still all fit in pretty easily, though.