The Chinese lunar rover Yutu—which means “jade rabbit”—is unveiling the first new “ground truth” from the Earth’s celestial neighbor in close to 40 years.
Since the American Apollo (1969-1972) missions, American observation of the moon is primarily achieved from orbit. However, such distance poses problems when attempting to interpret the moon’s regolith layer.
In 2013, China’s Chang’e-3 mission touched down on the moon, landing in the northern part of the Mare Imbrium, an impact basin filled by lava flows. The regolith layer was thin and unmixed with debris, which meant it resembled the composition of underlying bedrock.
Analyzation of the surrounding area has revealed the area’s basalts differ from the basalts returned to Earth from the Apollo and Russia’s Luna (1970-1976) missions.
“The diversity tells us that the moon’s upper mantle is much less uniform in composition than Earth’s,” said Prof. Bradley J. Jolliff, of St. Louis’ Washington Univ. “And correlating chemistry with age, we can see how the moon’s volcanism changed over time.”
The university worked with Shandong Univ. for one of two partnerships that resulted in the study recently published in Nature Communications.
According to the researchers, the sampled basalts from the Apollo and Luna missions formed during a period of peak volcanism, which occurred between 3 and 4 billion years ago. Where Chang’e-3 landed, the basalts are younger than 3 billion-years-old.
Additionally, the Apollo and Luna basalts were either high or low in their titanium content. Yutu, via the alpha-particle X-ray spectrometer and the near-infrared hyperspectral imager, revealed the basalts it sampled were intermediate in titanium content, and rich in iron.
“The variable titanium distribution on the lunar surface suggests that the moon’s interior was not homogenized,” said Jolliff. “We’re still trying to figure out exactly how this happened. Possibly there were big impacts during the magma ocean stage that disrupted the mantle’s formation.”
The titanium was in a mineral called ilmenite, which researchers believe didn’t crystallize until about 5% of the moon’s original melt remained.
Prior to that, two magnesium-and iron-rich minerals (olivine and pyroxene) crystallized, followed by plagioclase feldspar. The latter floated to the magma’s surface and the former sunk due to their respective densities.
Researchers said this separation process led to formation of the moon’s mantle and crust.
The landing site was also rich in olivine, something the researchers were first baffled by due to its differing crystallization time from ilmenite. Yutu, however, observed the olivine was iron-rich, meaning it formed later. Magnesium-rich olivine crystallizes earlier.
The scientists hypothesize that when the moon’s magma ocean crystallized, iron-rich pyroxene and ilmenite, “which formed late and at the crust-mantle boundary, might have begun to sink, and early-formed magnesium-rich olivine might have begun to rise,” according to Washington Univ. “As this occurred, the two minerals might have mixed and hybridized.”
