About 4.5 billion years ago, a Mars-sized protoplanet called Theia was having a very bad day. Its orbit put it on a collision course with another, larger protoplanet, and the two collided with such force that Theia was effectively obliterated. To be fair, the other protoplanet didn’t fare much better: the force of the impact shot much of its volume into space. Some of this material rained back to the surface, while the rest mixed with the remains of Theia and eventually merged into a single satellite: our moon.
This scenario represents ours best understanding of how the moon was formed. A new paper published on August 21 in Nature provides new evidence in support of this theory, but also raises questions about the fine details of the models we constructed to simulate the collision. The article is based on data collected by the Chandrayaan-3 missionwhose Pragyan rover collected the first samples of regolith from a high-latitude region of the moon.
As Santosh Vadawale, the lead author of the article, explains Popular scienceAn important aspect of the theory is the ‘lunar magma-ocean hypothesis’. The energy released by the cataclysmic impact “would [have] melted the outer few hundred kilometers of the moon.” This would have meant that the newly formed moon was completely covered in magma: a global magma ocean, so hot and deep that it took at least tens of millions of years to cool and solidify into rock.
If the moon’s surface had been liquid for millions of years, we would expect relatively light minerals to have floated to the surface, while heavier minerals would have sunk to the bottom. (Think of how a mixture of oil and water will eventually separate, leaving the oil floating on the water.) Vadawale says that in geological terms we expect the moon’s surface to be composed largely of minerals called anorthosites: “An important prediction of the lunar magma-ocean hypothesis is the presence of a largely anorthositic crust.”
This prediction was first put to the test by the Apollo missions, whose samples showed that the moon’s surface did indeed exist largely anorthositic. Since then several other missions have taken samples from areas at the equator and mid-latitude, but until the arrival of Chandrayaan-3, areas closer to the poles had remained unexplored.
“The areas at high latitudes… have experienced impact cratering more often due to their older age,” Vadawale explains. “This makes it challenging to identify safe landing zones of sufficient size, which is likely why [early] landings took place in relatively safe Mare regions. However, the importance of landing closer to the poles has been known for some time, and … the number of attempts to land at high latitudes has increased.”
Chandrayaan-3, according to Vadawale, represents the first fully successful landing in such a region. Its success enabled the deployment of a rover vehicle that took samples of nearby soil, allowing researchers to examine its composition and compare it with that of lower areas. Vadawale says the terrain composition was largely as expected: “The regolith in this region is predominantly… similar to equatorial highlands. This provides further support for the lunar magma-ocean hypothesis.”
One surprise, however, was the presence of a relatively large amount of olivine, a relatively heavy magnesium-based mineral. Vadawale explains that the discovery of this mineral is not remarkable in itself: “While very early models of LMO suggested a crust made of pure anorthosite, further evolution of the model suggests that the crust… [contain] certain amounts of magnesium and iron-containing minerals [like] olivine and pyroxene.” Such heavy minerals can also be ejected from below the surface by large meteor impacts – and Chandrayaan-3’s landing site is close to the South Pole’s Aitken Basin, an immense basin that is the moon’s largest, oldest and deepest impact crater.
So it is not the presence of olivine that was unexpected; it was the amount of olivine present that came as a surprise, and in particular the ratio of olivine to another heavy magnesium-based mineral called pyroxene. Other samples contained more pyroxene than olivine; however, the samples taken by Pragyan contained more olivine than pyroxene. As the paper notes, “This is a new finding and is at odds with other soils in the lunar highlands (from the repository of returned samples and lunar meteorites).”
Why? No one knows – not yet. But it is potentially very important, because it has the potential to further refine models of how exactly the moon formed. “The explanation for why olivine is slightly higher than pyroxene,” says Vadawale, “is a very important finding because it has the potential to constrain several LMO models.” However, he warns against jumping to conclusions: “More specific details can only be obtained on the basis of further modeling.”
