A lunar sample returned by China’s 2020 lunar mission contained minerals that provided clues to their origins. The Earth’s moon acquired the appearance of Swiss cheese due to the fact that celestial bodies crashed into its surface, creating impact craters. But craters aren’t the only thing left behind; The intense pressure and heat of such an impact also affects the rocks and dust known as regolith that cover the lunar surface, changing its mineral composition and structure. Analysis of the resulting minerals gives modern researchers clues about the Moon’s past.

Analysis of Chang’e-5 lunar samples

China’s Chang’e-5, the first mission to return lunar samples since the Soviet Luna 24 in 1976, delivered 1.73 kilograms of regolith from the Oceanus Procellarum aircraft, named for its enormous size. The sample landed from Chang’e-5 (CE-5) in late 2020 and contained a strange combination of siliceous minerals as well as the new mineral Changesite-(Y).

In AIP Publishing Magazine Matter and Radiation at Extreme Points Researchers from the Chinese Academy of Sciences compared the composition of the CE-5 material with other lunar and Martian regolith samples. They investigated the possible causes and origins of the lunar sample’s unique composition.

Impact metamorphism and lunar minerals

Asteroids and comets collide with the Moon at extreme speeds, causing impact (impact) metamorphism in lunar rocks. This change in temperature and pressure is rapid and has distinctive characteristics, including the formation of silica polymorphs such as stischovite and seyfertite, which are chemically identical to quartz but have different crystal structures.

“Although the lunar surface is covered with tens of thousands of impact craters, high-pressure minerals are rare in lunar samples,” author Wei Du said. “One possible explanation for this is that most high-pressure minerals are unstable at high temperatures. Therefore, those formed during the impact may undergo regression.”

However, the silica fraction in sample CE-5 contains both stischovite and seyfertite; these are minerals that theoretically coexist only at much higher pressures than those to which the sample is exposed. The authors determined that seyfertite was present as a phase between stishovite and α-cristobalite, a third silica polymorph also found in the sample.

“In other words, seyfertite may form from α-cristobalite during the compression process, and part of the sample will transform to stishovite during the subsequent temperature raising process,” Du said.

Significance of Chang’e-5’s findings

The mission also returned a new lunar mineral, Changesite-(Y), a phosphate mineral characterized by colorless, transparent columnar crystals.

The researchers estimated the peak pressure (11-40 GPa) and impact duration (0.1-1.0 seconds) of the impact that formed the sample. Combining this information with shock wave models, they estimated that the resulting crater was between 3 and 32 kilometers wide, depending on the angle of impact. Remote sensing shows that the remote eruption in the CE-5 regolith originated mainly from four impact craters, with the Aristarchus crater being the youngest among the four remote craters. Because seyfertite and stischovite were easily disrupted by thermal metamorphism, they concluded that the silica fragment probably came from the impact that formed Aristarchus Crater.

This exemplary return mission demonstrated the power of modern analysis and how it can help elucidate the history of celestial bodies.