The Moon's Two Sides Look So Different Thanks To 4.5 Billion-Year-Old Physics (forbes.com)

StartsWithABang writes: 4.5 billion years ago, a giant object collided with our proto-Earth, kicking up debris that eventually coalesced into the Moon. While the near side contains dark maria and lunar lowlands, the far side is almost exclusive heavily cratered, high-mountainous regions. This was a mystery for a long time, but it appears that heating from the hot, young Earth caused a chemical and crustal difference between the two faces.

Comments

I could be missing something

[ILongForDarkness]

but couldn't the far side have craters and the near side few because something big, like I don't know the earth, blocks one side and not the other?

Re:I could be missing something

[Michael Woodhams]

As seen from the moon, the Earth is only about two degrees across, so the proportion of projectiles blocked by it would be miniscule. Even that small effect is reduced (possibly beyond zero) by 'gravitational focusing': projectiles which come towards the moon from the direction of the Earth which would otherwise have missed can be deflected by Earth's gravity such that they hit. (And this happens more often than projectiles that would have hit being deflected so they miss.)

Here is a paper I found on gravitational focusing.

Re:I could be missing something

[rasmusbr]

Another interesting aside is that many have tried to explain gravity by postulating that the universe is full of tiny particles that fly about randomly in all directions and that gravity works because bodies block the particles from hitting one another.This is sometimes called the screening theory of gravity.

If you make some reasonable assumptions you will find that two nearby bodies would block particles from hitting one another, creating forces that follow the inverse square law...

These theories also predict that planets will de-orbit and crash into their stars, and that moons will similarly crash into their plants. But hey, no theory is perfect.

Re:I could be missing something

[Dog-Cow]

Last time I checked, English was a natural language, not a technical language. As long as the meaning is understood, the expression is valid. In English, it's perfectly valid to refer to reciprocal relationships from either direction. If something is twice as far, it is half as close. If something is half as far, it is twice as close.

The moon could have been artificially created.

[Anonymous Coward]

Another, more plausible explanation is that the moon was actually an ancient artificial space station, placed in a very specific orbit around the earth. Over time, debris has collected on it, giving it its current appearance. The fact that it always faces the earth, and orbits the earth once every month is very intriguing. This isn't something we'd expect to happen were the moon a natural creation. That suggests that it likely was created as an artificial space station which has since become, for lack of a better term, dusty over a long span of time.

Re:The moon could have been artificially created.

[Anonymous Coward]

That's no space station. It's a moon!

Re:The moon could have been artificially created.

[ClickOnThis]

The fact that it always faces the earth,

Nothing remarkable about that. It's due to tidal locking and is quite common.

and orbits the earth once every month is very intriguing.

Yea, and Lou Gehrig died of Lou Gehrig's disease. [cue creepy music]

Re: The moon could have been artificially created.

[JDevers]

Damned, you would think he had to see that coming.

Re:How could the Earth heat it?

[techno-vampire]

I think that you misunderstood the article. It's not that the hot side faced the Earth, it's that the heat of the Earth (around 2700 Kelvins) kept that side of the Moon (orbiting considerably closer than it does now) hot longer than the side facing away.

Those _are_ some old physics.

[berchca]

I wish we still had physics like that today... these new-fangled ones just don't coalesce like they used to.

Re:How could the Earth heat it?

[Michael Woodhams]

1 billion years ago the Earth had 100 to 1,000 foot tides as the Moon and the Earth were much closer

My initial response is "I don't think so." My second response is to calculate, so here goes:
Current distance to moon = 384,400 km = 4 x 10^8m
Current rate of increase in distance to moon = 3.8 cm/year = 4 x 10^-2 m/year.
If this rate were constant over a billion (10^9) years, then a billion years ago the distance to the moon was 4 x 10^-2m/year*10^9year = 4 x 10^7 m closer, or 10% closer. Tidal effect strengths are inverse-cube in distance, so a billion years ago, lunar tides would have been about 30% larger than now.

This doesn't come close to "100 to 1000 foot tides."

Re:Please StopWithABang

[jfdavis668]

The link could have gone straight to the Penn State news article Forbes was reporting on: http://news.psu.edu/story/3178...

Re:How could the Earth heat it?

[Namarrgon]

Good, but the rate of increase in distance to the moon isn't constant (it was faster in the past), and it's thought that the moon formed at a distance of only about 20 to 30 thousand kilometers.

By your maths, 4B years ago would've put the moon at 60% its current distance, but at formation it is more likely to have been only 6% of current distance. Assuming similar mass to today, 60% closer implies more like 4.6x the current tidal force - but 6% distance might be 4,600x stronger forces (probably more, given that the distance to the Earth's surface was even closer). How this translates into actual tidal sizes is left as an exercise for someone who knows more than I do.

Of course back then there probably wasn't much water around, given terrestrial temperatures in the thousands of degrees, but there may have been some impressive magma tides instead.

Re:Please StopWithABang

[thegarbz]

But then StartsWithABank won't be able to feed his family. This isn't a Forbes problem, it's a Slashdot not standing up to submitters who submit every single one of their lame stories as front page news.

Stop linking to Medium

[thegarbz]

Well I guess Slashdot did finally stop linking to Medium. Though it looks like StartsWithABang just moved his personal blog to Forbes and now reposts everything he writes from there.