A new model – “The Big Splat” – explains the strange asymmetry of the moon | Hacker News Reader

A new model – “The Big Splat” – explains the strange asymmetry of the moon

nautil.us

While trying to find more information on this new model/theory, I found another one:

http://news.psu.edu/story/317841/2014/06/09/research/55-year...

Which purports that the difference was created due to tidal locking and a temperature gradient in the early formative years of the Earth and Moon.

"Because the moon’s rotation is perfectly synchronized with its revolution..."

i'm not sure how i feel about this definition of "rotation". i would say that "rotation" in the celestial body context would always refer to rotation about an axis passing through the object. while "revolution" is rotation about an axis external to the object.

So no problem then. I would substitute "orbit" for revolution in your definitions, but either way you get the same thing - it rotates once per orbit/revolution.

I think that's exactly what it means in this case also - the period of rotation on its own axis is exactly matching the period of revolution of its center of mass around the Earth. So that's why the same it is always presenting the same face towards Earth. Did you understand this differently?

That is the type of theory that when you hear it sounds so plausible that it must be true.

>> That is the type of theory that when you hear it sounds so plausible that it must be true.

But when 2 bodies of that size come together, and one of them splats onto the other, there is going to be a major change in rotation rate. There is no reason given for the resulting rotation rate to match the orbital period. I find that bothersome.

There is a potential reason:

There is "drag" resulting from tides whenever a body is rotating faster or slower than the orbital period. (First approximation: actually there are weird things like orbital resonances. Mercury for example.)

This drag becomes much more prevalent when the body is asymmetrical - say, when a large body just hit it. (Again: first approximation.)

As such, one could conceivably say that the moon was rotating faster than it is now, and the impact slowed it down enough that the drag in the meantime was enough to tidally lock it to Earth sometime between the impact and now. (Or slower, and sped up. Same thing.)

Although I don't know nearly enough to know how large the "window" would be of rotation rates that would tidally lock in the amount of time between the impact and now.

The thing that is a little hard to account for is the synchronized rotation: how could the splat happen in exactly such a way that the rugged far side remains out of Earth's view. Say that before the splat, the original Moon is already synchronized so that it faces the Earth with one side at all times.

The popular hypothesis that the Moon fragmented away from the Earth does contain an explanation for both phenomena: the synchronous rotation, as well as the different characteristics of the far side.

Since the Moon is small and has no plate tectonic activity, it would tend to "record" a permanent geological snapshot of the "gash" on the side where it was separated from the larger object. The larger object with plate tectonic activity would "heal" over many eons, since that activity basically re-surfaces it.

The Wikipedia article [http://en.wikipedia.org/wiki/Giant_impact_hypothesis] does not say, under Difficulties, anything about the difference in characteristics between near and far side being specifically a problem. There are difficulties stemming from the composition of the Moon as such.

Reply on news.ycombinator.com