A Hard Rain’s A-Gonna Fall on the Dark Side of the Moon I: The Lunar Farside Highlands Problem

Arpita Roy, Steinn Sigurdsson, and I just finished a long project in lunar geology theory.  It’s been a trip!  You’ll hear more about it in the near future, I think, so I’m leading up to it in one of my “slow bloggings.”  

It all started with a talk by Diana Valencia here at Penn State which, for reasons I’ll get into later got me thinking about the Lunar Farside Highlands problem.  

I remember, the first time I saw a globe of the Moon as a boy, being struck by how different the farside looks.  It was all mountains and craters, not at all like the frontside with its broad, dark seas of basalts (“maria”).  Why?  I also remember never being able to get a good answer to my question, and it’s bothered me ever since.

It turns out, when the Luna 3 spacecraft orbited the moon on Oct 7, 1959, it transmitted back to Earth the first images of the part of the Moon we never see.  It was to everyone’s surprise then that there were very few maria and lots of mountains.  The reason for this remains a major mystery in lunar formation, called the “Lunar Farside Highlands Problem”.

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First image of the farside of the Moon, compiled by the Soviet Luna 3 mission

The Moon is thought to have formed via a giant impact of the Earth by a Mars-sized impactor.  The idea is that this impact shaved a significant fraction of the Earth’s mantle off, and that and most of the impactor went on to form the Moon, very near the Earth.  Since then, the Moon has slowly drifted away due to tides, but the shared origin of the Earth and Moon explains their very similar compositions.  

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Artist’s renditions of the Moon-forming giant impact.  The material thrown off eventually formed a disk around the Earth that eventually formed the Moon.

I should note that recent studies have shown that the chemical and isotopic compositions of the Earth and Moon are nearly identical, not just close, which may actually be inconsistent with the Giant Impact Hypothesis (see Linda Elkins-Tanton’s letter in Nature Geoscience, wondering if we need to start all over again!)

Most models that seek to explain the lunar dichotomy take one of two approaches:  construct a “just-so” story about how stochastic processes just happened to give us a two-faced moon, or else start by assuming that the moon had some initial, inherent dichotomy (like a thicker farside crust) and then showing that the difference in appearance naturally follows from that.

So, for instance, if the farside crust is thicker, then very large impacts are less likely to pierce the crust and create the upwelling of mantle material that creates the maria on the Moon.  The nearside, having a thinner crust, thus has several maria, while the farside is mostly mountains.  This makes sense… but WHY does the farside have a thicker crust?

One thing to note is that the thickest part of the crust probably HAS to be pointed directly away from the Earth (or maybe directly at it).  This is because the crust has a lower density than the rest of the moon, so a dichotomy leads to a gravitational quadrupole that tides can act upon.  Aharonson, Goldreich, and Sari showed that the configuration we have today is the energy-minimum state (i.e., if you spun the moon up, it is most likely to eventually settle back down with this side facing us).  There is also a stable configuration where the other side faces us, but it is less likely.  Unless the Moon has been spun up my some giant impact, the side we see today has been the side that faces the Earth since it became tidally locked. 

But we’re still left wondering WHY the farside has a thicker crust in the first place. Suggestions usually invoke stochastic processes:  from asymmetric heating on the two hemispheres (but WHY is it asymmetric?) to asymmetric bombardment (maybe because the orbit of the Moon made impacts hit one side preferentially). 

A nifty paper by Jutzi and Asphaug suggested that maybe when the Moon formed, it had a companion moon that formed exactly opposite it in the same orbit.  This companion moon’s orbit would not have been stable, but it might have been just properly placed to make a very soft collision with the moon as they both orbited the Earth.  This would have plastered the companion moon against the side of the Moon, giving it a thicker crust there.  Then tides would have reoriented the Moon so that face would be the farside.  This “slap-on Moon” idea is really neat (you need a slow collision to make it work, which is what would happen to a companion moon, and the idea of a companion moon is totally plausible). 

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However, reflectance spectra from the SELENE lunar orbiter (nicknamed “Kaguya” by the Japanese, after the legendary Japanese moon princess!) seem to indicate that elemental abundances (specifically Mg to Fe ratio) seems to vary continuously from farside to nearside. Ohtake et al. argue that this suggests a continuous differentiation mechanism rather than a foreign source that was “slapped-on”, whether it be a companion moon or impact deposits.

OK, that’s enough for now.  Next time: how Diana got me thinking about this, and our approach to the problem. 

Images in these posts are linked to their source.  Parts of this post were written by Arpita Roy.  

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