The primary problems with my original idea (described in Part II) for addressing the Lunar Farside Highlands Problem (described in Part I) are that the crust of the Moon formed long after the Earth had cooled down, and that the elements that form the Moon’s crust (that is, that distinguish it from the mantle) are actually refractories, not volatiles:  the heat of the Earth should actually concentrate them, not cause them to vaporize and flee to the farside.

GRAIL map of crustal thickness of the Moon, inferred from sensitive gravity measurements.

Calcium and aluminum are very concentrated in the crust of the Moon.  This is because when you add calcium and aluminum to silicates and then cool it, you get minerals like the anorthositic plagioclase feldspars, which are what found their way to the top of the magma ocean that covered the Moon after the giant impact and made the crust.  

And calcium and aluminum are two of the most refractory elements: they condense into solid form at very high temperatures.  In fact, CAI’s (calcium-aluminum rich inclusions) in meteorites are thought to be some of the most primordial materials in the Solar System, the first solids to form as the protoplanetary disk cooled.  

When Arpita showed us the vaporization temperatures of the various elements, we were at first disappointed, because it meant my idea wasn’t just wrong: it was exactly the opposite of how things should have gone.  But then we realized we could reverse the model, and we had a new explanation that actually made more sense.

Since the farside crust of the Moon is about twice as thick as the nearside crust, apparently the farside hemisphere has a lot more calcium and aluminum than the nearside, because that’s what its crust is made of.  But how does Earthshine accomplish this?

As the Moon was forming (and the Earth was re-accreting lots of material, too) lots of material was in vapor phase.  As material condensed it formed rocks that crashed into the proto-moon, partially remelting and revaporizing.  There was lots of stochastic, localized heating from collisions.  The Moon would have had a thick rock-vapor atmosphere. It was a messy, multi-phase, non-equilibrium mess.

And since the Earth was very hot, the whole mess had a very hard time cooling down.  In particular, the nearside of the Moon and the atmosphere there was very hot, and so was the disk that was forming the Moon.  But the outer part of the disk would be colder, the part of the disk in the shadow of the Moon would be colder, and the farside of the Moon was colder.  All other things being equal, a temperature gradient of this sort should lead to condensation fronts and gradients: as you move away from the Earth the refractories have an easier time condensing out of vapor phase, then much further away the volatiles can finally condense.  A temperature gradient should lead to a chemical gradient.  

And a chemical gradient is what we see on the Moon!  What’s more, the chemical gradient that is responsible for the crust looks like a condensation sequence, which is exactly the signature you expect from a temperature gradient.  There is also some early observational evidence to support a continuous differentiation mechanism versus stochastic deposits (Ohtake et al. 2012), which seems encouraging.

We identified three possible mechanisms that in this mess, refractories might preferentially find their way to the farside because of Earthshine.  

1) The outer part of the disk might preferentially “feed” the farside of the Moon.  If refractories were more commonly in solid phase in the outer parts of the disk, then they might preferentially collect there.

2) Impacts re-vaporize materials on the surface, as well as the impactor.  In the cold of the farside, refractories are likely to always remain solid, but on the hot nearside even refractories vaporize and mass deposition is equally inefficient for all species.  

3) The farside lunar atmosphere might form a cold trap for refractories.  Re-vaporized material enters the lunar atmosphere, then find their way to the cold trap and snow out.  Gaseous disk material entering the farside shadow would condense out Ca/Al rich grains might then be more likely to be deposited into the still-forming Moon.

If I had to bet, I’d bet on scenario 3 being the most important.

After the farside got preferentially polluted with refractories, the rest was deterministic: over the next thousands to millions of years, as the magma cooled into mantle and crust, the extra Ca an Al formed extra feldspars, which bound their way to the surface and formed an extra-thick crust.  And that’s why it looks so different:  all that extra armor on the back prevented impacts from penetrating down to the interior magma, so the maria couldn’t form there.

So, bottom line: 

1) The Moon has probably always had the same face towards the Earth, even during formation

2) The Moon’s formation was a messy business, with vapor phase likely to have been important

3) Earthshine was an important component of thermal energy budget of the post-giant-impact system, and should have produced a chemical gradient in the protolunar nebula and proto-lunar atmosphere

4) The present-day chemical dichotomy on the Moon looks an awful lot like the result of the condensation gradient one expects from Earthshine.

5) The lunar farside highlands are the result of a primordial chemical gradient caused by tidal locking and the temperature gradient caused by the hot post-impact Earth and, possibly, the shadow proto-Moon.

Here is Arpita’s paper (Roy, Wright & Sigurðsson 2014, ApJL 788, L42).  It is also at astro-ph/1406.2020.

The press release is here.

I’ve compressed this storyline a bit for clarity, and because my memory of the exact order and importance of various conversations have faded over the three years this project has spanned.  Also, there were many other people who helped shape our initially inchoate ideas into the coherent thesis of Arpita’s paper as we shopped this idea around; among them are Matija Ćuk, Bethany Ehlmann, Andrew Ingersoll, James Kasting, David Stevenson, Yuk Yung, Gary Glatzmaier, and Kevin Zahnle.  Arpita also contributed to this series of blog posts.