Scientists have discovered that impact collisions on the Moon not only form craters, but also change the structure and mineral composition of its surface
The Moon acquired its characteristic appearance due to collisions with space objects, which created impact craters on its surface. However, the craters — this is not the only thing left after these events. Researchers have found that the intense pressure and heat of such processes also change 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, 1.73 kilograms of regolith collected during the Chinese mission Chang'e-5 were delivered to Earth. The area from which the samples were taken is named Oceanus Procellarum. One of the samples contained a new mineral, Changesite-(Y), as well as an interesting combination of silica minerals (SiO2, silica & silicon (IV) oxide).
Scientists from the Chinese Academy of Sciences in a work entitled «Matter 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 impact metamorphism in the rocks of the Moon. The change in temperature and pressure occurs very quickly and has its own characteristics, including the formation of polymorphic forms of silica, such as stishovite and seifertite. They are chemically identical to quartz, but have a different crystal structure.
One of the scientists, Wei Du, noted that although the Moon's surface is covered with many impact craters, high-pressure minerals are rarely found in the samples. He explained that this may be due to the instability of such minerals at high temperatures. Therefore, those that were formed during collisions could undergo a reverse transformation.
However, the silica fragment from the sample contains both stishovite and seifertite, which theoretically could only exist at much higher pressures than those to which the sample was subjected. Researchers believe that seifertite formed from α-cristobalite during compression, and then part of the sample turned into stishovite as a result of increased temperature.
In addition, in a sample from the Chang'e-5 mission, scientists discovered the mineral Changesite-(Y), which is a phosphate and is characterized by colorless transparent crystals. The researchers assessed the impact parameters that led to the formation of the sample: the peak pressure ranged from 11 to 40 GPa, and the duration of the impact — from 0.1 to 1.0 seconds. The researchers were also able to estimate the size of the resulting crater using models of the shock waves, which were approximately 3 to 32 kilometers wide, depending on the angle of impact.
The scientists noted that the regolith ejecta associated with the sample primarily originated from four impact craters, where Aristarchus crater — the youngest. Examination of the samples led to the conclusion that the silica fragment containing seifertite and stishovite may have formed as a result of the collision that led to the formation of the Aristarchus crater.
The results of the study will help to understand how the surface of the Moon is formed and how collisions affect its mineral composition.