Chang’e-4 Lander and its Rover Have Turned up new Mysteries on the Moon’s far side. The Moon’s Mantle Blasted Onto the Surface?

On January 3rd, 2019, the Chinese National Space Administration (CNSA) successfully landed their Chang’e-4 mission on the far side of the Moon. This mission represents a major milestone for China, being the fourth lander-rover mission to be sent to the Moon, and the first mission in history to land on the “dark side of the Moon”. And what it manages to uncover there is sure to excite and inspire scientists for many years to come.

For example, the mission’s Yutu-2 (Jade Rabbit-2) rover made an impressive find that may confirm a theory about lunar impacts. After collecting spectral data from the moon’s largest crater (the South Pole-Aitken Basin) the Chang’e-4 mission team from the Chinese Academy of Sciences (CAS) concluded that the impact that created the Basin turned up material from deep within the Moon’s mantle. This finding could offer new insight into how the Moon evolved over the course of billions of years.

A paper that describes the team’s findings was recently published in the scientific journal Nature. The team was led by Li Chunlai, a professor of the National Astronomical Observatories of Chinese Academy of Sciences (NAOC), and consisted of multiple members from the CAS’ Key Laboratories of Lunar and Deep Space Exploration and Space Active Opto-Electronics Technology.

Image of the Yutu-2 rover moving away from the Chang’e-4 mission’s landing zone. Credit: CNSA

For about 60 years, robotic spacecraft and even a handful of crewed missions have been exploring the lunar surface. Based on the data they collected, a theory emerged in the 1970s that early in its history, the Moon’s surface was covered by an ocean of magma. As the surface began to cool and solidify, lighter minerals (like basalt) floated to the top while heavier elements (olivine and pyroxene) sank deeper to the core.

In keeping with this theory, scientists also speculated that impacts by asteroids, meteors and space junk would crack through the crust and kick up heavier material from the mantle. As Li explained in a recent CAS press statement:

“Understanding the composition of the lunar mantel is critical for testing whether a magma ocean ever existed, as postulated. It also helps advance our understanding of the thermal and magmatic evolution of the Moon.”

Located at the Moon’s south pole, the SPA basin is the largest, oldest, and deepest known crater on the Moon. Measuring about 2,500 km (930 mi) in diameter and 13 km (8.1 mi) deep, it is believed to have formed as a result of a massive impact that took place 3.9 billion years ago. This corresponds to the Late Heavy Bombardment period, when the planets of the inner Solar System were struck by a disproportionately large number of asteroids.

Image captured by Chang’e-4 showed the landscape near the landing site. Credit: NAOC/CNSA

To test this theory, the mission team collected spectral data samples from the flat stretches of the SPA basin, as well as from smaller and deeper impact craters within it. What they expected to find was a wealth of mantle material on the flat stretches, but were instead surprised to find mere traces of olivine. On Earth, this rock-forming mineral is a primary component of the upper mantle.

On its own, this finding could be seen as an indication that predictions about the composition of the lunar mantle have been incorrect. However, samples taken from deeper impacts revealed higher concentrations of olivine, which presented a real conundrum. A possible explanation, according to Li, is that the mantle consists of equal parts olivine and pyroxene, rather than being dominated by one.

In order to confirm these findings, the Chang’e-4 will need to explore the area around its landing site and gather more spectral data to get a better understanding of its geology. What it reveals may inevitably cause scientists to reassess their theories about the composition of the lunar mantle, not to mention the Moon’s geological history.

In addition, an improved understanding of the Moon’s evolution may provide a window into the evolution of Earth and the other terrestrial planets as well. Not only is the surface of the Moon very well preserved compared to Earth’s (owing to the absence of an atmosphere, weather patterns or geological activity), but the predominant theory is that the Earth and Moon formed from the same basic materials.

These and other theories about how our Solar System and its celestial bodies came to be will be tested thanks to Chang’e-4 and its brave Yutu-2 rover. Among them, whether or not terrestrial lifeforms can live on the Moon for extended periods of time! Stay tuned…

Further Reading: Chinese Academy of Sciences, Nature