Phys.org's Study suggests Earth and Moon not identical oxygen twins says

They found that the oxygen isotopic composition varied depending on the type of rock tested. This may be due to the degree of mixing between the molten Moon and vapor atmosphere following the impact. Oxygen isotopes from samples taken from the deep lunar mantle were the most different to oxygen isotopes from Earth

"This data suggests that the deep lunar mantle may have experienced the least mixing and is most representative of the impactor Theia," said Sharp. "Based on the results from our isotopic analysis, Theia would have an origin farther out from the Sun relative to Earth and shows that Theia's distinct oxygen isotope composition was not completely lost through homogenization during the giant impact."

and links to Erick J. Cano et al, Distinct oxygen isotope compositions of the Earth and Moon, Nature Geoscience (2020) which is paywalled. The abstract says:

Here we present high-precision oxygen isotope analyses of a range of lunar lithologies and show that the Earth and Moon in fact have distinctly different oxygen isotope compositions.

but "lithologies" sounds like it refers to lithosphere which doesn't sound like it includes the "deep mantle".

Question: How exactly do they take deep mantle samples from the Moon and get them to Earth? How deep does one have to go to reach the deep mantle? From what I remember the Apollo astronauts either simply walked around and picked rocks up, or hammered off some bits, or occasionally did some hand-drilling. They loaded about 100 kg up each time and flew them back to Earth made possible by throwing life-support equipment out the door as well as other less necessary items before takeoff. The deep mantle sounds pretty-much out of reach


1 Answer 1


There are three ways samples of the mantle could arrive on the surface of the Moon.

  1. Drilling deep holes into the mantle, take a sample and send to the surface.
  2. As ejecta from a large deep impact crater, particularly where the crust is thin.
  3. During volcanic eruptions, the upward moving magma tears of chunks of mantle from the walls of the volcanic "vent" and ejects pieces of mantle embedded in the resulting lava as xenoliths.

We know the Apollo astronauts only drilled down about 20 cm into the regolith, so there are no drilled samples of the Moon's mantle. This means that the only way we might have a sample of the mantle is by serendipitously picking up a sample from the surface after it was ejected by the other two methods.

The Chinese rover Yutu, on the far side of the Moon, in the South Pole Aitken Basin

... which was formed by a celestial collision over four billion years ago. With a diameter of 2,500 km and a depth of about 13 km, the basin is the moon’s oldest and largest impact crater. Very large impact craters can potentially penetrate through the crust and enable probes to sample the lunar mantle.

... the lunar soil at the landing area contained a large amount of olivine, low-calcium pyroxene and a small trace of high-calcium pyroxene, which are very likely from the lunar mantle.

Notice the term "very likely" near the end of that quote.

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.

Concerning the Apollo samples,

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.

... Data from the orbiting Japanese Kaguya mission shows that olivine is present in the surface deposits of some lunar craters.

... 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.

... 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.

... 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.

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.

In short, the sample tested most likely wasn't from the mantle but because of mineralogical similarities it is being used as an analogue for a mantle sample.

  • $\begingroup$ Of the three sites where planetary science questions are generally considered on-topic (astro, space and earth), it looks like I chose well, thank you! $\endgroup$
    – uhoh
    Mar 11, 2020 at 5:38

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