Monday, March 4, 2024

Lunar sample, delivered by a Chinese mission in 2020, contains minerals that provide keys to unraveling the history of the Moon

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New crater on Oceanus Procellarum. Source: NASA/GSFC / Arizona State University

The Moon acquired its characteristic appearance due to collisions with cosmic objects, which created impact craters on its surface. However, craters are not the only thing left after these events. Researchers have discovered that the intense pressure and high temperatures of such processes also alter the mineral composition and structure of the Moon’s surface. Analysis of these changes helps to learn a lot about the past of the Earth’s satellite.

Recently was delivered to Earth 1,73 kilograms of regolith, collected during the Chinese mission Chang’e-5. The area from which the samples were taken is called Oceanus Procellarum. One of the samples contained the new mineral Changesite-(Y), as well as an interesting combination of silicate minerals (SiO2, silica — silicon oxide (IV)).

Scientists from the Chinese Academy of Sciences, in a work titled «Materials and radiation in extreme conditions», compared the composition of the sample with other samples of lunar and Martian regolith.

 

Collisions with asteroids and comets at high speeds cause shock metamorphism in the Moon’s rocks. Changes in temperature and pressure occur very rapidly and have their own characteristics, including the formation of polymorphic forms of chertite, such as stisthoite and zeyfertite. They are chemically identical to quartz, but have a different crystalline structure.

One of the scientists, Wei Du, noted that, although the Moon’s surface is covered with numerous impact craters, high-pressure minerals are rarely found in the samples. He discovered that this may be related to the instability of such minerals at high temperatures. Therefore, those that were formed during the collision could undergo reverse transformation.

However, a fragment of chernozem from the specimen contains statisite and zephyrite, which can theoretically exist only at much higher pressures than those at which the specimen was subducted. Researchers believe that zeifertite was formed from α-cristobalite in the process of compression, and then part of the sample transformed into solidite as a result of increasing temperature.

In addition, in the samples of the Chang’e-5 mission, scientists discovered the mineral Changesite-(Y), which is a phosphate and is characterized by colorless transparent crystals. Researchers evaluated the parameters of the shock impact, which led to the formation of the sample: the peak pressure was from 11 to 40 GPa, and the duration of the impact — from 0,1 to 1,0 seconds. Researchers were also able to estimate the size of the crater formed using shock wave models, which ranged from approximately 3 to 32 kilometers in width, depending on the angle of impact.

Scientists have noted that the regolith emissions associated with the sample mainly originate from four impact craters, with the crater Aristarchus being the youngest. Measurement of samples allowed to conclude that a fragment of chernozem, containing zeyfertite and staishite, was probably formed as a result of the collision, which led to the formation of the crater Aristarkh.

The results of the research will help understand how the Moon’s surface is formed and how collisions affect its mineral composition.

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