looking for Mars

[calimac]

When I was learning science in school, the origin of the Moon was considered a mystery. There were various theories, but none held a consensus. More recently, there appears to have been a definite turn towards a particular theory, because I see it reported as settled fact often, for instance in the 2/28 New Yorker article on earth-grazing asteroids, by Tad Friend:

"Along the way, a collision with a Mars-size planet broke loose the mass that became the moon."

They always say "a Mars-size planet," and I wonder: what is this Mars-sized planet? Where did it come from? Where did it go? (Since the Moon itself evidently broke off from the Earth.) Were there a lot of rogue planets wandering around in the early days? What happened to them all? I know there are a lot of asteroids outside the asteroid belt, but if they were broken-up planets, wouldn't they be more clustered? Something's missing from this story, and the online sources I've seen are not helpful.

Comments

[barondave.livejournal.com]

Sort of like whenever a media person is describing a foreign country, they say something like, "... about the size of Wisconsin".

Still, what they're talking about is The Great Impact Hypothesis which says (and I'm wildly paraphrasing) that in the early days of the solar system, there were a great number of planet-sized bodies whizzing around. And they collided, stabilizing into our current 9 8 planets, with a bunch of smaller bodies in less clean orbits. This is why, among other things, all the planets have different axial tilts. One of the later collisions, when the Earth was solid, was with a large object that had enough oomph to blow off the moon (and leave residue). They calculate "Mars size" by the necessary mass it would take to accomplish such.

Veilikovsky is either grinning broadly from heaven or turning over in his grave.

[kalimac.livejournal.com]

OK, but if these bodies kept crashing into each other, then why aren't there more moons like this in the solar system (or are there? surely Jupiter's Galilean moons didn't form this way), and, especially if they didn't all form large moons, then where did all these rogue bodies go?

[barondave.livejournal.com]

As I understand this comparatively new theory (and again, I may not have all the details down), either

a) early enough in the process (which lasted several hundred million years) the newly forming masses combined to form the planets,

b) they were close enough to the sun that they fell in rather than hit the orbiting mass (hence, no moons for Mercury or Venus),

c) were absorbed into the gas giants,

d) became moons,

e) destroyed each other, leading to the asteroid belt, or

f) whizzed out of the ecliptic, helping to form the Kuiper Belt and the comets are remnants.

We're talking about the large, "Mars sized", objects, not the various smaller meteors which clearly impacted the Earth later (much to the chagrin of the dinosaurs) or created craters on our moon, etc. At some point (around 4.2 billion years ago, by this theory), the larger objects coalesced into more-or-less our present Solar System.

[kalimac.livejournal.com]

This would still remain more credible if the Moon were a captured rogue planet, not the result of a crash, but apparently its composition doesn't support that theory. But if the Moon is mostly made of material from the Earth, and if the rogue planet was Mars-sized, which is a heck of a lot bigger than the Moon, then I still wonder: where did it go?

[mithriltabby]

It’s still right here. The body that’s now the Earth was previously of a mass equal to (Earth + Moon – Mars); then the Mars-sized body hits, and it’s momentarily of mass (Earth + Moon); then the impact energy of being smacked causes a Moon-sized chunk of the planet to come off the other side.

looking for Mars

[igor-junior.livejournal.com]

The hypothesized object is called by some Theia and formed at one of the Lagrange points 60 degrees ahead or behind Earth in Earth's orbit. Once the mass gets above a certain size the Lagrange point is no longer stable. The object had a low velocity collision with Earth (low velocity by astronomical standards).

The reason it is thought the Moon resulted from a collision is that the Earth and Moon have the exact same ratio of Oxygen isotopes which does not match the ratio on Mars or any meteorite. So the Moon must be made of material from Earth.

There is a good Wikipedia entry on the theory, the theory has its problems but it has no good rival.

Coincidentally, Kepler has found a planetary system with just such a pair of planets in the same orbit, one 60 degrees behind the other.

-Richard

[kalimac.livejournal.com]

Unfortunately, that really doesn't parse with what I've read elsewhere, including the new comment from igor_junior below. The reason we know the Moon comes from Earth and is not a captured body is because its composition matches Earth's. It therefore can't be a rogue planet that came in.

But if the "Mars-sized planet" was absorbed by Earth even as Earth spit off the Moon from the other side, then the Earth today is about 10% Mars-sized planet, and its own composition would not match the composition of the Moon whose composition is supposed to be the same as the Earth's.

Or is the Moon also about 10% Mars-sized planet? That doesn't fit your description.

Or did the Mars-sized planet have the same composition as the Earth? If that's possible, then the Moon could also be a captured body of the same composition, and the entire proof that it came from the Earth disappears.