Searching for the Moon’s Mantle
We’ve studied and examined the Apollo samples of the lunar maria (pronounced MAR-ee-uh) for thirty years but despite the thorough search of these collections, we have never found a sample of the deep mantle from which these lavas were formed. How might such a deeply seated rock find its way to the surface? Large basin-creating impacts on the Moon might have dug through tens of kilometers of crust (down into its deepest layers) to excavate samples of the mantle. Another occurrence of mantle samples is as small chunks of rock included in lava. Fragments of rock may be ripped away during the ascent of the dense, liquid magma and become inclusions in the solidified lava flows; these “stranger” rocks are called xenoliths. Despite an exhaustive search of the Apollo samples, no samples of the mantle have been found, either as a fragment of basin ejecta or as a xenolith in the mare basalts.
If no one has ever seen it, how do we know what the lunar mantle looks like? The properties and composition of planetary interiors are inferred by indirect evidence. Seismometers left on the surface by Apollo crews measured the velocity of seismic waves inside the Moon, an indirect measure of the density of the deep interior. The density of the mantle is high enough so that common surface rocks cannot make up a significant portion of it; the rocks must contain large amounts of the minerals olivine and pyroxene. In addition, the mantle rocks were partially melted to make the mare basalts that cover the surface in places. The chemical composition of these lavas show they were made by melting a rock rich in magnesium and iron. Finally, xenoliths of the Earth’s mantle are sometimes found entrained in lavas – these pieces are made up of the olivine-pyroxene rock peridotite (after the mineral olivine (the gem form is peridot) that makes up most of it.) So the idea that the rocks of the mantle are olivine-rich is a well-grounded concept for which we have abundant independent evidence.
Now a new scientific paper concludes that fragments of the mantle (the dense magnesium- and iron-rich portion of the Moon that lies below 70-100 km depth) are exposed on the surface, brought up from depth by the impact of giant asteroids 4 billion years ago. Such a finding would indeed be significant, as geologists always seek rocks from the deep interior to aid our understanding of the Moon’s structure and composition. Data from the orbiting Japanese Kaguya mission shows that olivine is present in the surface deposits of some lunar craters. But how do they go from this observation to the interpretation of lunar mantle? Basically, they mapped out the occurrences of these olivine deposits and found that many of them occur within the rims of large impact basins. Based on models produced from gravity mapping, the crust of the Moon is thought to be thin here and the mantle is close to the surface. Thus, these large impact basins could have excavated chunks of the mantle, throwing them out onto the surface of the Moon.
Why is the mineral olivine important? Olivine is a silicate mineral rich in magnesium and iron; it forms one of the basic, silicate building blocks of the rocky planets. In magma (liquid rock), olivine crystallizes first and its composition is a key indicator of the composition of the magma. Current prevailing wisdom is that the early Moon was largely molten (the “magma ocean” phase); whether it was completely molten or merely liquid in its outer portion is uncertain, but in such a huge system of liquid rock, olivine is the earliest mineral that crystallizes. Being dense, crystallizing olivine would sink in the liquid magma, slowly accumulating deep in the Moon. As the entire Moon solidified, these “cumulate” layers of olivine and other iron-rich minerals would make up the mantle. Later, the mantle partially re-melted, creating liquids that erupted onto the surface as basalt lava and formed the dark lowland plains - the maria.
So how does the Kaguya interpretation hold up on examination? Estimates of crustal thickness are of the current Moon, after the basins formed. There is no particular reason to suppose that a given basin-forming impact occurred in terrain of thin crust – the crust is thin here because the basin formed. True enough, some of these impact features are very large – the Imbrium basin, a large crater on the western near side, is well over 1000 km in diameter, large enough to have punched through the thickest sections of the crust, one would think. Indeed, Imbrium is one of the sites that the Kaguya team propose as having excavated mantle. So they are finding these areas in the places where one might expect to find them.
Olivine is a very common mineral and abundant in the lunar crust. A curious fact is that olivine grains in lunar highland rocks tend to have high amounts of calcium, a minor element but a key diagnostic of the crystallization environment. In Earth rocks, olivine formed at depth has very low concentrations of calcium. My colleague, the late Graham Ryder concluded that the olivine crystals in dunite (a rock made up almost completely of olivine) from the Apollo 17 site – a sample proposed as a piece of the lunar mantle – likely came from the accumulation of crystals at a depth of only a few kilometers, far shallower than the tens of kilometers depth to the mantle.
Because the Kaguya spectral mapper is detecting only the presence of olivine, we cannot distinguish between pure olivine and olivine crystallized with plagioclase, what lunar scientists call troctolite. Troctolite is common in the Apollo highland samples, but is a relatively rare rock on Earth. It consists of (more or less) equal parts olivine and plagioclase, a calcium- and sodium-rich silicate mineral. Troctolites make up some of the most deeply derived rocks found in the Apollo collections, but all studied to date seem to be of crustal, not mantle, provenance. There is no objective evidence that the olivine seen by Kaguya is not derived from troctolites and/or dunites of crustal (not mantle) origin.
The long held desire of lunar scientists to sample deeper levels of the Moon is understandable, but we must proceed cautiously. Just as a sample return mission to the floor of the largest basin on the Moon is no guarantee that we will obtain the rocks needed to answer questions about early cratering history, the new finding of abundant olivine on the Moon does not mean that pieces of the mantle are lying on the surface, awaiting collection by some future mission. The Kaguya findings are intriguing and very interesting, but not definitive evidence for the presence of mantle fragments on the lunar surface.