Fifty Shades of Darkness
A new explanation of the Moon’s early history is more confusing than illuminating.
A recent paper that purports to explain early lunar history has drawn some media attention, but mostly for reasons not related to its principal thesis. It illustrates the unfortunate confusion over common terms we use in describing the Moon’s two hemispheres – the near side (or hemisphere that constantly faces the Earth) and the far side (which always faces the opposite direction from Earth.) But this paper also introduces some brand new confusion, apparently just to keep the topic of space sexy and interesting.
It is incorrect to say “dark side” of the Moon when referring to the Moon’s far side. In pre-Space Age days, these two terms were often confused, with some believing that the far side was always in the dark. In fact, both hemispheres of the Moon receive equal amounts of solar illumination, just during opposite parts of the lunar day. The Moon rotates on its axis once every 29 days, so each hemisphere is in sunlight for two weeks. Daytime on the near side (when we see the lunar surface from Earth) is paralleled by nighttime on the far side. Thus, the far side is just as “dark” (or bright) as the near side, depending on the local time of day.
The use of the term “dark side” has cultural roots as evidenced in thesaurus lists showing “dark” as synonymous with “unknown.” Examples include the “Dark Ages” (which had just as many daylight hours as any other period of time) and “darkest Africa” (which was mostly unknown to people in 19th Century Europe). An amusing exchange occurred when the press office at Penn State University (where the scientific work was done) defended their use of the term “dark side” on this very basis, insistent that they used the term correctly.
But to the paper in question: The new work draws on some recent observations of planets in other star systems that orbit their primary at close distances. These objects experience extreme degrees of radiative heating from their local suns and thus, possess very hot surfaces. These conditions could result in these exoplanets possessing an atmosphere made up of ordinary rock-forming elements (such as aluminum) in vapor form. Different elements condense at different temperatures, so if one side of a planet were hotter than the other side, the sequence of condensation would be different for the two sides. Thus, if any of these exoplanets are in synchronous rotation around their suns (as the Moon is around the Earth), there might be a compositional difference between the star-facing side and the opposite side of the object.
This condition is postulated for the early Earth-Moon system by the authors of the new work. They contend that the Moon’s synchronous rotation was established immediately after the Moon formed, probably by a giant impact striking the Earth 4.5 billion years ago. The Moon was initially much closer to the Earth than it is now (slightly more than about 3 Earth radii, or about 20,000 km, compared to the current distance of 60 Earth radii, or 400,000 km). According to the authors (using the 20,000 km number, the closest distance possible – the Roche Limit – before tidal forces would rip the Moon apart), at this proximity the near side of the Moon received much more “Earthshine” (their term) than the far side. I note here that yet another term of confusion has now been brought into the discussion; the use of the term “earthshine” in this paper refers to the exposure of the lunar near side to thermal radiation from very hot surface of an early, molten Earth. They are not using this term in its understood meaning, in which the nighttime portion of the lunar near side is illuminated by sunlight reflected from the Earth’s surface.
The new study suggests that because the near side of the Moon was exposed to a constant view of the very hot, young Earth, the far side (a.-mistakenly-k.a. the “dark side”) would cool more quickly, thus becoming enriched in elements with high melting points, such as aluminum. According to the authors, this means that the lunar far side will be richer in this element and thus would form a thicker crust. The thicker crust would prevent extensive flooding by mare basalt lava, the dark smooth plains that partly cover significant fractions of the near side (which collectively make up the fanciful “Man in the Moon” pattern of light and dark we see with our naked eyes when we gaze up at the Moon). This convoluted sequence of events led to the bombastic headlines of many press stories, such as “Why the Moon’s Dark Side Has No Face.”
You may by now have begun to realize that there are two stories here. One is a bizarrely contrived reconstruction of the history of the early Moon, whose validity is not only mostly speculative but also largely untestable (something in common with most hypotheses dealing with the earliest stages of lunar evolution). The second one deals with the scientific illiteracy of many journalists, who regurgitate press release constructions and buzz words without any real analysis, understanding or thought. Thanks to this story, in addition to reinforcing the far side/dark side confusion, we now have yet another term of ambiguity – “Earthshine,” with a meaning that (until now) it never had.
The idea that the crust of the lunar far side is thicker than the near side is not new, indeed it was first discovered by gravity field mapping (done most recently and thoroughly by the GRAIL mission). What is new in this paper is the suggestion that the thicker crust is a consequence of near side heating by the radiant, molten Earth. In fact, we are still not even sure that the thicker crust on the far side is the cause of the paucity of mare flooding there. It is one of the explanations given for the near/far dichotomy, but not proven. The concept is based around the idea that all mare magmas originate at a similar depth inside the Moon and can only break through to the surface where the crust is thin. But such a postulate ignores the variety of depths of origin for the lavas, their compositional diversity, and their wide range in ages.
Most of the heat-producing radioactive elements are concentrated on the lunar near side, but strangely, only on the western portion of the near side, not an obvious consequence of the mechanism proposed by this paper (nor is this phenomenon addressed there). I don’t see why differing condensation times for the various elements would perforce lead to differing concentrations of the same elements on the two hemispheres; the authors apparently assume a well-mixed, hot silicate atmosphere in global equilibrium (for which there is no evidence). Finally, the Moon experienced an intense bombardment by asteroids and leftover debris from the accretion of the Solar System for more than 600 million years after formation, each impact redistributing vast quantities of material over whole hemispheres, not to mention re-orienting the spin axis (and possibly changing the rotation rate) of the Moon with each basin-forming event (a basin is a large impact crater, usually with a diameter greater than 300 km). All of these effects could significantly modify the model presented in this paper.
This is what happens when astrophysicists publish papers on lunar geology. Now if you will excuse me, I have a manuscript on galactic evolution that I plan to submit to the Journal of Geology.