China`s Chang e 4 lunar probe and Yutu 2 Lunar rover landing on the surface of the moon on January 3, 2019 with the sun in the background. 3D illustration

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Scientists have long debated the formation of our moon. While the majority of these lunar hypotheses have involved a large impact with a Mars-sized planet, the exact way in which these collisions took place — and the aftermath that followed — has long eluded us. A recent find published in Nature, however, has shed new light on this perennial problem.

The new paper summarizes data gathered by Yutu-2, a Chinese rover sent to the lunar surfaces last year. Its aim was to gather data on the largest impact crater present on the lunar surface — the South Pole-Aitken basin. This crater, which lies on the dark side of the Moon, stretches a total of 2,500 kilometers in diameter. This means that it covers about a quarter of the Moon’s surface — an expanse that researchers believe to be rich with evidence of the Moon’s ancient beginnings.

The size of the basin is only part of the reason for its value to scientists. Because the crater is so big, the impact that created it is thought to have generated so much power as to blow out bits and pieces of the lunar mantle beneath. And it’s this mantle sediment that they expected to yield geological clues about the Moon’s early formation.

Clues in the mantle

Knowing the composition of the Moon’s inner mantle can yield an abundance of relevant information as to how the Moon formed. Given that our friendly celestial satellite used to be a giant molten rock only a couple billion years ago (4.5 billion to be precise — the same as Earth), the mantle sediment can help show how the molten rock cooled and then settled into what it is today.

There are many models that predict the way in which this great molten sea settled. While each of these models yields slightly different predictions as to the exact composition of the Moon’s crust, mantle, and core, each is based on the premise that the more dense elements involved sunk to the core, while the less dense minerals floated to the surface.

These most recent findings from Yutu-2 help to tease apart exactly which of these many models is correct.

Moon Interior, A Cross Section Of The Moon'S Interior, Showing The Asymmetry In The Thickness Of The Crust Between The Near And Far Sides. (Photo By Encyclopaedia Britannica/UIG Via Getty Images)Moon Interior, A Cross Section Of The Moon'S Interior, Showing The Asymmetry In The Thickness Of The Crust Between The Near And Far Sides. (Photo By Encyclopaedia Britannica/UIG Via Getty Images)
Photo By Encyclopaedia Britannica/UIG Via Getty Images

Testing your minerals

To test the types of elements that existed at the top of this crater, the rover used infrared detection. More specifically, it analyzed the light frequencies given off by the different minerals in the basin. Researchers then compared these findings to samples gathered from other sites on the lunar surface.

Different minerals emit different wavelengths of the electromagnetic spectrum. The study of how these different wavelengths reflect from minerals is called infrared spectroscopy, and it’s these wavelengths of light that scientists analyze to determine the samples’ compositions.

What Yutu-2’s spectral analysis revealed about the mineral composition of the crater was that, lo and behold, it was made of a different set of minerals than samples gathered from other parts of the Moon. This is the first study to find such a difference.

The primary difference the rover found was that the minerals in the crater were 48 percent olivine and 42 percent low-calcium pyroxene (although the numbers varied slightly across region). Both of these minerals are high in iron and magnesium, which means that they would probably have sunk to the core during the earliest periods of lunar formation. They were, in other words, composed of dense mantle material that was likely flung to the surface when the impact occurred.

The other minerals they found were mostly of the crust we’ve found previously in other locations — lighter minerals that don’t hold much value other than to show their origin from the surface. What this data suggests is that most of the materials they sampled in the crater were of the mantle, and the rest were of the typical crust.

The data gathered from these finds can help lunar scientists calibrate their models for exactly how the Moon was formed post-collision.

Problems on the lunar front

There are some problems with the rover’s conclusions, however.

Because the Moon’s crust has a concentration of minerals that gives off a similar frequency to those in the Moon’s core, the data gathered by the rover might not be as accurate as it superficially appears. Plagioclase, for instance, a mineral we find abundantly in the crust, emits a similar infrared frequency to the heavier iron- and magnesium-rich minerals we found in the impact crater basin. To deal with these potential ambiguities, scientists would need to get better samples.

If the analyses are correct, however, the data would help to solidify certain hypotheses surrounding the Moon’s formation. While the data doesn’t currently establish one hypothesis or model over any other, it does help to direct our future progress toward uncovering the Moon’s tumultuous birth.