Finding and sampling the Moon’s ancient interior mantle — one of the science drivers for sending robotic spacecraft and future NASA astronauts to the Moon’s South Pole Aitken basin — is just as likely achievable at similar deep impact basins scattered around the lunar surface.
At least that’s the view reached by planetary scientists who have been analyzing the most recent data from NASA’s Gravity Recovery And Interior Laboratory (GRAIL) and its Lunar Reconnaissance Orbiter (LRO) missions as well as from Japan’s SELENE (Kaguya) lunar orbiter.
The consensus is that the lunar crust is actually thinner than previously thought.
If so, this would have made it easier for large impactors of the sort that carved out the near side’s Crisium impact basin, the far side’s Moscoviense impact basin and the Aitken basin to also have excavated some of the Moon’s geologically-compelling early mantle.
It’s this interior mantle that lunar scientists need to sample in order to help nail down the details of how the Moon formed more than 4 billion years ago.
If indeed, lunar mantle can be found at multiple sites on or near the Moon’s surface, it would give future lunar base planners more options in where they might aim for a future scientific outpost.
“Should this really be true, it would provide an opportunity to possibly sample the first extraterrestrial mantle material in future lunar missions,” Katarina Miljkovic, planetary scientist and a research fellow at Australia’s Curtin University, told me.
Such mantle is thought to be primarily made up of the minerals olivine and pyroxene, with olivine likely playing an important role in the lunar mantle’s upper tiers.
“By sampling this mantle,” said Miljkovic, “we’d know for sure what is its chemical composition and would be extremely valuable in terms of understanding how the Moon formed.”
To date, most detections of olivine at the surface are at the near side’s Crisium impact basin and the far side’s Moscoviense basin. However, there are some indications of olivine in the Serenitatis, Nectaris, Humorum, and Humboldtianum near side basins as well as the Schrödingeron basin on the lunar far side.
Sampling a range of mantle compositions would allow us to test models for crystallization and composition of the Moon’s original molten magma ocean, Jeff Taylor, a planetary scientist at the University of Hawaii, told me. But he says the real problem is that pure mantle rock might not be present.
“Besides being melted it might be mixed with crustal material, giving us a mixed composition,” said Taylor. “That is not uninteresting, but makes the job of deciphering lunar mantle dynamics and composition difficult.”
Taylor notes that the orange pyroclastic glass found by the Apollo 17 astronauts at the Taurus Littrow Valley did ultimately come from the mantle. But, he says, it did so by partial melting at depths of hundreds of kilometers and subsequently erupting onto the lunar surface.
The orange glass magma provides information about the mineralogy at depth and the abundances of important trace elements, says Taylor. But he says he doubts that any of that colorful, pyroclastic surface glass is examples of excavated mantle rock.
As for determining at what depth real excavated mantle might be found within the lunar basins themselves, Miljkovic says she and colleagues are now comparing spectral mapping with numerical impact modeling that indicate how lunar basins could have formed.
She says this should provide a constraint on the depth of the material exposed within the basins’ centers.