Study Explains Why Lunar Craters Are Bigger On the Near Side

An anonymous reader writes "A new study of asteroid craters on the moon has uncovered some big differences in the composition of the crust on the two sides of the moon. 'While massive impact basins pockmark the moon's near side, its far side contains considerably smaller basins. The discrepancy in crater distribution has puzzled scientists for decades. To investigate what may have caused this difference, the team obtained data from NASA's twin GRAIL probes, which orbited the moon from January to December 2012. During its mission, the probes circled the moon, making measurements of its gravity. Zuber and her colleagues used this data to generate a highly detailed map of the moon's crust, showing areas where the crust thickens and thins; in general, the group observed that the moon's near side has a thinner crust than its far side.'"

Comments

earlier paper links

[Trepidity]

The paper this press release is about doesn't seem to be online, but two papers from the past few months analyzing this GRAIL data (with some of the same authors) are available:

"Gravity Field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission", the initial report of the observations

"The Crust of the Moon as Seen by GRAIL", reconstructing the crust thickness and composition from the observations

Not really solving the puzzle.

[Hans Lehmann]

OK, there are larger craters because the crust is thinner on this side, but why is the crust thinner on this side? Mere happenstance, or is it caused by orbital mechanics or some other reason?

Re:Not really solving the puzzle.

[DMUTPeregrine]

Probably some relation to orbital mechanics, Luna likely became tidally locked to the Earth before it had fully cooled. The tidal forces on Luna would have affected how the materials settled. I'm not an expert on this by any means, so I can't calculate what exactly would happen.

Re:Not really solving the puzzle.

[InfiniteLoopCounter]

First that theory assumes the the moon became instantly tidally locked from near the moment of its creation, which seems highly unlikely for a body born of an impact, followed by re-impact. (The debris impacts on the far side would occur more often, because the near side would not be shielded by the earth, but that works ONLY once the proto-moon is tidally locked.).

I'm not sure there is much in the way of evidence for exactly when the moon became tidally locked.

This is the current leading theory. Yes, it is very recent, but in the video I linked to you'll see lots of famous physicsts that you should recognize.

It's been a while since I was in physics studying this kind of thing, but it seems to me that since it is a smaller body and formed around the earth at a much smaller distance from the earth and then moved out, that there would be only a handful of parameters that would determine how long a tidal lock would take. First would be the small mass of the moon and smaller iron core, which would lead to faster tidal lock than say a planet around a star. Second would be the distance from the Earth (smaller the distance the faster it would occur). Thirdly, the impact between the 2 'proto-moons' would directly influence the rotation rate and axis of rotation of the moon (although from a disk around the earth, the eccentricty with respect the earth would be minimal). Since the moon was very close to the earth around its formation, and it formed in orbit around the earth, I would assume that a tidal lock would have occured very soon after its formation. Probably somewhere there is a simulation to show this.

Here's an article that explains why the composition is likely to be different for more fluid materials upon the theorised collision of the 2 'proto-moons'. This explains in principal that the less solid objects were drawn to the near side (as per the original article linked to in the summary), leaving a thinner crust on the near side. The only new interesting take-home is that the period of bombardment of comets and asteroids (due to the stabilization of the orbits of Jupiter, Saturn, Neptune, and Uranus throwing them towards the inner solar system) was likely slightly less violent than predicted before.

Thin crust, thick crust

[mrbester]

Anybody else unaccountably hungry?

I thought this was kind of obvious

[localroger]

If the Moon was created by a glancing collision between the Earth and a Mar-sized protoplanet, which seems to be the going theory nowadays, then the Moon was created in Low Earth Orbit during the very heavy bombardment phase of the LHB. Once the Moon became tidally locked -- which would have happened pretty quickly at such proximity -- Nearside was shielded from most further bombardment by the Earth. So Nearside is kind of a fossile from the heaviest epoch of the LHB, while Farside continued to get pelted as the big stuff was swept up, and finally got the fine dusting of the last scraps evening it out. It would also have continued to accumulate crud, which Nearside wouldn't, thus the thicker Farside crust.

Nevermind that...

[excelsior_gr]

Are there any news of a monolith?