How did the Earth capture the moon in orbit. I see no other planet with a moon one sixth the mass of the planet. When asteroids and meteors get past us without going in orbit, I wonder what mechanism could account for our moon.
Current theory is that we didn’t capture the moon. It is the result of a violent collision early in the earth’s history. A large volume of material was bashed away from the earth, but not far enough to escape. It coalesced to form the moon.
It’s called the Impact Theory. While there are other theories, including that it was floating past and we captured it, this is one that is most accepted among astronomers.
And the third theory is that the Earth and the Moon both formed at the same time from the same cloud of dust and gas. In a way, this theory isn’t too far from the impact theory, since the condensation of planets would have been the result of thousands of such impacts as local concentrations of mass ate up the smaller bodies nearby. These impacts could have destroyed and reformed lots of early planets, it’s just that only those that survived are still around. We don’t know, for example, that Venus or Mercury or Mars existed with their current mass in their current orbits when the collision that formed the Earth-Moon system happened.
While I am at it, since the moon is pockmarked with craters, when is the last time it got hit by a meteor.
The moon is hit by meteoroids constantly. (A meteor refers strictly to the flash of light created by the meteoroid, which doesn’t happen on the moon.) It has no atmosphere to burn the small pieces of debris, so everything hits it and lands on the surface. As far as the last BIG impact, I’m not sure, I’d have to do some research.
How big does something have to be before you’re willing to call it a meteor? (technically a meteoroid) The Earth gets hit by dust-grain-sized meteoroids all the time- they produce shooting stars, and you’ll see at least a few every night if you’re in a dark site away from city lights. There’s no reason to think that those dust grains don’t hit the moon as well (though, of course, they wouldn’t produce the shooting-star effect, since the moon has no atmosphere to burn them up).
At Marshall’s Lunar Impact Monitoring website, they list 38 lunar impact candidates between November 2005 and February 2007. Their telescope is sensitive enough to pick up the flash from meteors massing 1/2 kilogram or more.
The problem with any capture theory is that anything moving in a solar orbit is almost intrinsically going to be at escape speed relative to another body; unless its velocity is just so with respect to a planetary body, it is going to fall inward and then swing away on a hyperbolic track (provided, of course, that it doesn’t impact the parent body) because the total amount of kinetic and gravitational potential energy is conserved. With a large parent and a very small potential moon it is just barely possible that it might fall into some kind of equilibrium which allows it to be captured; with something like the Earth and its oversized satellite, it’s essentially impossible unless they just happened to be in a coincidental orbit, and even then the mutual attraction and resultant instabilities would probably cause the Moon to be kicked out of orbit rather than captured.
We also know–thanks to those “useless” rocks brought back by the Moon missions–that the composition of the Moon is distinctly terrestrial, ergo, either the Moon was formed from the same protoplanetary mass as the Earth, or that it is a result of an impact which blew a substantial chunk of mass into an elliptical orbit, which subsequently condensed into our poetic junior sibling. The problem with joint condensation from the same primordal mass is that mechanically it doesn’t really work; the amount of orbital momentum in the Earth-Moon system should have prevented the Moon from coalescing. (Note that this is different from the Roche limit, which predicts the distance at which a solid body held together by mutual gravity would be torn apart by tides, but rather that the amount of momentum would prevent said mass from ever coalescing to begin with.) So, the impact formation hypothesis is considered to be most likely by the vast consensus of astronomers and planetographers, though a lot of the reasoning is via inference.
A recent recorded impact is this one. Major impacts are rare, probably even rarer than ones to the Earth, although the Moon isn’t protected by a large protective blanket of gas. The large craters and lava “seas” visible from Earth, however, were formed about 3Byr or more ago.
Lemur866, although you are correct that we don’t know the early state of the planets, and they could have been subject to massive impacts, we can be fairly confident that any massive impacts on the scale of the one that hypothetically created the Moon are rare. For one, Mercury and Mars are smallish worlds for which such an impact would have completely disintegrated both bodies, and in fact the impact hypothesis requires a mass that is roughly Mars-sized, which begs the question as to what happened to the rest of it; did it blow apart and the resultant fragments continue on their way out to the Oort Cloud or out of the system entirely? Were they absorbed by the massive outer gas giants? (The asteroid belt is manifestly insufficient for all of this, though its appearant high metallic composition might contain some residues from such an impact.) Although the surface of Mars is weathered by both more recent volcanic activity and ongoing climatic erosion, and Venus (what little we can observe of it) is constantly being eroded and reformed, Mercury is ancient and while heavily pockmarked shows no signs of a catastrophic impact post its initial formation and end of geological activity.
At least insofar as our very, very limited empirical dataset of planetary formation allows us to make any general statements, the Earth-Moon system appears to be rare. Most moons are dramatically less massive by many orders of magnitude than their primary bodies (The Moon is about 1% the mass of the Earth), allowing for capture to be a reasonable hypothesis, at least in some cases, though most of the moons of the Jovian and Saturnian systems are thought more likely to have been coevolved with their primaries. Many people have attempted to derive some signficance from this; that the Moon is responsible for the stability of the Earth’s inclination, that it protects the Earth from meteor impacts, that it has skimmed off the heavy reducing atmosphere that makes Venus an unlivable hellhole, that it has provided a rhythmic basis necessary for life, et cetera. None of these hypotheses extends beyond errant speculation, and for all we know it could be as common an arrangement as double stars, despite our attempts at modeling “typical” planetary formation and extrapolating from our single dataset.
Stranger
So, to qualify as a meteorite, does a meteoroid have to hit the Earth specifically? Or would it qualify if it hit the moon?
The Wikipedia article just says “A meteorite is a meteoroid or asteroid that survives its entry into the atmosphere and strikes the ground.” Clearly the moon has no atmosphere for the meteoroid to survive entry into (which means no meteors on the moon, I guess!), but it does have ground for the meteoroid to strike. Does that make them meteorites?
The Space.com article that Stranger linked to referred to the entire Moon strike event as a meteoroid.
I don’t know of any official terminology, but my inclination would be that once any meteoroid has landed on any solid body, it has officially become a meteorite.
The Discovery Channel often shows a great show about just this question. It goes to great lengths to explain all the alternatives and how one team ran the simulations that lead to the current theory of the big impact. Sorry I can’t remember the name of the show, but it was obvious enough that you will pick it up in a programming guide.
Although Stranger On A Train already mentioned the Moon’s mass, a more accurate figure is that the Earth has about 81 times the mass of the Moon.
gonzomax you stated the Moon has 1/6 the mass of the Earth. No, instead, it has 1/6 the surface gravity of that of the Earth.
Yes. One of the rovers currently on Mars recently spotted a meteorite there. But you probably can’t find too many on the Moon. Since it has no atmosphere to slow them down, meteoroids mostly vaporize upon hitting the surface.