Earth May Once Have Had Two Moons

AaronW writes "According to a story at space.com, Earth may once have had two moons. The smaller moon, estimated to be 750 miles (1200km) wide and only 4% of the mass of the larger moon, crashed into the far side of the larger moon which caused the features we see today on the moon. The surface of the far side of the moon is quite different than the side facing the earth, having a different composition and a much rougher terrain."

Comments

Question for those more knowledgable than I

[elrous0]

I plead ignorance on the topic, but this explanation strikes me as a little far-fetched considering that it seems more reasonable to me to conclude that the far side of the moon being so different might more reasonably have something to do with the fact that it's permanently facing away from earth. Wouldn't the far side get somewhat more meteor impacts and somewhat more exposure to cosmic radiation, for example? It would seem to me that the earth-facing side would be at least somewhat shielded by earth, compared to the far side--and that over a very long period of time this could make for a difference in geology. For that matter, wouldn't the gravitational field of the earth also have some effect on lunar geology over extremely long periods of time (effecting the two sides somewhat differently), much as the lunar field effects earth's oceans in the very short term?

Perhaps someone more familiar with lunar geology than your humble narrator could explain why these differences are thought to be unrelated to its orientation to earth, and need to be explained instead by a hypothesis as radical as a moon impact.

Re:Question for those more knowledgable than I

[arisvega]

Hard to say without more quantitative analysis- this moon hasn't always had a 'far side'. There where times where its rotation was not tidaly locked with Earth -i.e. it has not been always showing Earth the same face, this is something that needs time to happen.

Re:Question for those more knowledgable than I

[Mindcontrolled]

Also pleading ignorance, as I am a biochemist and a moon is a couple of magnitudes away from the scale of things I am used to deal with - from basic physics, though, I don't see how facing/not facing the earth would make a geological difference. Why it should come from another moon impacting there and not from a collision with any old asteroid seems a bit far fetched for me.

Re:Question for those more knowledgable than I

[ArsenneLupin]

it has not been always showing Earth the same face

Yes, that's why, in the olden days, the moon used to be called "full frontal nudity"...

Re:Question for those more knowledgable than I

[bberens]

When you consider the gravitational field rather than line of sight then the "shadow" that Earth casts is really quite large.

Re:Question for those more knowledgable than I

[gcnaddict]

The explanation given by the paper is that it would've been far more likely for a trojan satellite (one which shares the orbit of our known Moon with the Earth) to have gently crashed into the Moon at a rate of just a few, perhaps one or two, miles per second, which is a collission speed so gentle as to be in absolutely miniscule ranges of probability with an asteroid impact. The net effect of such a slow impact wouldn't be a crater; rather, it would be roughly the same as mashing a clump of dirt on a bigger ball of clay.

As for the far-side bit, the moon wasn't always tidally locked. Tidal locking happens with lots of time.

Re:Question for those more knowledgable than I

[tysonedwards]

The bottom line is that we saw from the Apollo missions and tests conducted on the few Moon rocks that were brought back that the Moon has been tidally locked in it's current orientation for nearly 3.4 billion years (or at least that is what NASA was claiming back in 2005, they may have changed their estimates since). This is assuming that the accretion process and the cooling process to create the Moon did not occur naturally in a tidally locked configuration, which computer models have shown that if an object roughly the size of Mars had collided with the Earth while it was still molten nearly 4.6 billion years ago that the Moon would form in a tidally locked configuration after it accretes.

Considering that current models show that the Moon formed 4.5 billion years ago, the current theory is that for the majority of it's life, the sides that do not directly face the Earth (the edges and the far side as viewed from Earth) are statistically more likely to have been hit than the side facing us, as our gravitational pull would serve to deflect many of those asteroids towards us instead of the near side of the Moon.

The bottom line is that a 750 mile object of 4% the mass of the Moon, colliding at the far side of the Moon (as suggested in this article) would have had a very noticeable effect on the Moon's orbital trajectory around the Earth, providing an eccentric elliptical orbit, make it non-tidally locked, and most importantly would be sending the Moon on a spiral towards us, rather than away from us as we are currently seeing.

Re:Question for those more knowledgable than I

[dgatwood]

Assuming a random distribution of meteors from every direction, the shadow of the earth isn't that impressive.

First, your initial assumption is begging the question, IMO; the statement that the distribution is random is equivalent to the statement that impacts will be equally distributed, therefore you're saying that impacts will be equally distributed because they are equally distributed.

You would actually expect the majority of meteor impacts in an orbital system to be caused by a fast-moving planet or moon in orbit catching up a slower-moving piece of debris in orbit. Meteor impacts, statistically speaking, tend to come overwhelmingly from the leading side of the planet, and are usually pretty close to falling parallel to the direction of orbit. In the case of moon impacts, that includes both the orbit of the Earth around the sun and the orbit of the moon around the Earth, with the caveat that the latter is largely uninteresting because the speed is so small relative to the former. Thus, there should, in theory, be slightly more impacts on the side of the moon in the direction of its travel, but the difference should be relatively tiny, probably to the point of being undetectable.

Second, it's not the shadow of the planet, but rather the shadow of the gravity well that is significant when it comes to impacts. Earth's gravity well has an effect that extends way beyond its physical surface. So basically, the question becomes one of whether its gravity well is more likely to pull a random hunk of rock into such a position that it would hit the moon or pull it into such a position that it would miss the moon. This question can be answered fairly simply: it will make it less likely to hit the side of the moon that faces Earth and more likely to hit the other side. Earth's gravity pulls random objects towards it. Therefore, if one side of the moon is always facing Earth, it is pulling objects away from that side, and pulling objects towards the opposite side.

This leaves a couple of questions:

1. Is Earth's gravity enough to make up for that disparity?

2. Are the maria disproportionately weighted towards the near side of the moon due to gravity in some way?

If the answer to both of those questions is no, then we can start to consider other curious concepts like a second moon.

Re:Question for those more knowledgable than I

[martin-boundary]

Thus, there should, in theory, be slightly more impacts on the side of the moon in the direction of its travel, but the difference should be relatively tiny, probably to the point of being undetectable.

Either this explanation works, or it doesn't. If it does, then slightly more is true: the density of impacts on the near side of the moon should increase as you move away from the closest point facing the earth.

That's trivial to see from your picture. When the moon is directly in front of the earth's trajectory, the impacts can only hit the far side. But when the moon is located to the side (ie momentarily parallel to the earth's trajectory) then only half its far side gets impacts, and half its front side.

So your theory is testable: look at a picture of the front side of the moon, and estimate the density of impacts as a radial function from the center point of the disk. It should be increasing as you go towards the edge, and it should be radially symmetric. The same is true for the back if you have a picture of it. It should be maximum at the farthest point from the earth.

If that picture doesn't look like that, then your explanation is too simple to fit the facts.

Earth may once *have* had two moons.

[chaboud]

Serious guys. You learn read book now. Thanksbye.

2 moons?

[margeman2k3]

That's no moon...

Re:Speak Proper Yoda, Please

[YodasEvilTwin]

You mean "Two Moons, Earth May Once Have Had" or "Had Two Moons Once, Earth May Have." Yoda often places the predicate (minus the helper verb or all verbs) before the subject -- "Lost a planet, Master Obi-Wan has" -- or otherwise rearranges phrases. He doesn't jumble words around randomly.

Re:Some folks will have trouble accepting this

[CannonballHead]

So a story based on apparently a single study (unproven) that "suggests" that the earth MAY have had (in other words, this *could* be a plausible explanation for why our one moon looks like this). And you already *have* accepted it while criticizing those who won't?

The study hasn't even, as far as I can tell, been reviewed or in some way shown to be plausible, however that works with studies on essentially history.

Gravity not visible surface

[perpenso]

I don't think you can use the observable earth as an indication of the protection offered. I would expect that the earth's "gravity well" (sorry, a more proper term escapes me at the moment) would be more relevant. That said I am quite suspicious of your overall approach. It seems to be commonly accepted that Jupiter provides the earth with significant protection, now consider the percentage of the sky hemisphere that jupiter occupies.

oh stop it.

[mevets]

You don't need to look to something so esoteric to find missing facts in sacred texts. For example, I don't recall much about micro-organisms in the various bits I've read; however there is a fair amount about various hygienic practices.

Is there really any need to nit-pick at peoples beliefs? Nobody's beliefs are literally right anyways.

Final Fantasy IV

[MoldySpore]

No. The second Moon was the home to the Lunarians. The Lunarians are a race of beings from a world destroyed which became the asteroid belt, and are identified by a moon-shape crest on their foreheads. They created this artificial moon, resting until a time they believe their kind can co-exist with humans. But the whole second moon thing got screwed up for everyone, because after Cecil whooped Zeromus' ass, the Lunarians decided to throw that moon into gear and get the hell away from us.

Thanks for nothing, Lunarians.

I blame Uranus

[Maintenance Goof]

Seems that we regularly 'discover' that Uranus has another moon or two every now and then. http://en.wikipedia.org/wiki/Moons_of_Uranus I think we know who the culprit is. I would advise you to be a bit more responsible for what Uranus does. Don't think we haven't noticed. Bathing might help.

Re:earth may once had?

[blair1q]

Have, or have not. There is no can has.