In December 2020, the Chang'e-5 probe made headlines by delivering 1.73 kilograms of lunar regolith from the Oceanus Procellarum, a vast lunar mare. This marked the first instance of lunar sample return since the Soviet Union's Luna 24 mission in 1976. Among the treasures retrieved was Changesite-(Y), a new mineral, alongside a complex assortment of silica minerals, heralding a new chapter in lunar research.
The analysis of these samples, conducted by researchers from the Chinese Academy of Sciences and documented in the AIP Publishing journal "Matter and Radiation at Extremes," delves into the unique composition of the Chang'e-5 regolith. By comparing it with samples from other lunar and Martian missions, the study unveils the dynamic processes shaping the lunar surface.
Celestial impacts on the moon, involving asteroids and comets, generate extreme conditions, leading to impact metamorphism. This process, characterized by swift changes in temperature and pressure, results in the formation of high-pressure minerals with distinct crystalline structures. Stishovite and seifertite, silica polymorphs found in the Chang'e-5 samples, are prime examples, chemically identical to quartz but markedly different in their crystal makeup.
"Although tens of thousands of impact craters blanket the lunar surface, high-pressure minerals like stishovite and seifertite are rarities in lunar samples," explained Wei Du, one of the study's authors. This scarcity is attributed to the instability of these minerals at elevated temperatures, suggesting a retrograde metamorphism process post-impact.
The presence of both stishovite and seifertite in a single silica fragment within the Chang'e-5 samples posed a puzzle, as these minerals typically form under vastly different pressure conditions. The research team's findings suggest a sequential transformation process, with seifertite forming from a-cristobalite under compression, and stishovite emerging during the heating phase that follows.
This intricate dance of minerals reveals not only the conditions during the impact but also the potential origins of the samples. The researchers' analysis points to the Aristarchus crater as the likely source of the silica fragment, deducing this from the minerals' sensitivity to thermal metamorphism and the crater's relative youth compared to other lunar features.
The Chang'e-5 mission's success underscores the power of modern analytical techniques in unraveling the history of celestial bodies. The discovery of Changesite-(Y), a colorless, transparent phosphate mineral, alongside the detailed study of silica polymorphs, provides invaluable clues to understanding the moon's geological past.
By estimating the impact pressures and durations that formed the sample, the study offers a glimpse into the violent events that have shaped the lunar surface. These insights, drawn from just a handful of regolith, highlight the moon's dynamic history and the ongoing quest to decipher its secrets through the lens of space exploration.
As we continue to analyze the bounty returned by Chang'e-5, the mission stands as a testament to the enduring fascination with our closest celestial neighbor and the relentless pursuit of knowledge that drives humanity's exploration of the cosmos.
Research Report:High-pressure minerals and new lunar mineral Changesite-(Y) in Chang'e-5 regolith
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