New study complicates theory that ancient impact pierced Moon's crust

The largest and oldest impact crater of the moon probably does not contain minerals below the lunar crust, which complicates the theory that an ancient, massive event would have crossed the lunar crust during the formation of the crater, reveals a new study.

A study published earlier this year analyzed how lunar materials reflect light in order to determine that a basin forming formation forming an ancient massive crater, the South-Aitken Pole Basin, caused mineral breakage from the bottom of the lunar mantle. If the mantle materials broke the lunar crust, their study could provide important clues to the history of the moon.

Now, new search in AGU newspaper Geophysical Research Letters re-examined the same data, acquired by the Chinese spacecraft rover Chang & # 39; E 4, which landed in the crater in January 2019. The new study reveals that the crater crust consists mainly of a common mineral of the lunar crust not detected in previous analyzes. The new findings suggest that the bottom of the basin may not have exposed lunar mantle material, as previously reported.

"We do not see the mantle materials at the landing site as expected," said Hao Zhang, a scientist in planetary sciences at China's University of Geosciences in Wuhan, China, and co-author. of the new study.

The new study complicates theories on the formation of the largest and oldest crater on the moon, enriching the body of knowledge about the history of the moon.

Meetings of the South Pole-Aitken Basin

The South-Aitken Pole Basin is considered one of the largest craters in the solar system and the oldest in the Moon. The basin has a diameter of 2500 km and a depth of about 13 km. The basin is on the far side of the Moon, the enigmatic area facing the Earth. It was intact until the landing of Chang & # 39; E 4 in the crater in January 2019.

Although scientists have not radiometrically dated the age of the basin, some estimates would place it at 4.2 billion years.

The scientists hypothesized that the formation event of the South Pole-Aitken basin had broken the lunar crust, because of the current depth of the basin. Crustal topographic maps estimate that the crust extends only 30 kilometers below the crater, while the rest of the lunar crust has an average thickness of 40 kilometers (25 miles).

The moon was once covered with molten magma. Over time, they have cooled and split into layers of crust and mantle characterized by many features, including their mineral composition. Clinopyroxene, orthopyroxene and olivine are all minerals associated with the lunar mantle. They sometimes appear on the surface of the moon, but high concentrations in a given region may indicate that the mantle has already perforated the crust.

Test of the crustal composition

Spectroscopy studies how matter interacts with light. Minerals absorb specific wavelengths of light and color, giving them unique signatures. Astrophysicists perform different types of spectroscopy to determine the composition and concentration of different materials on planetary bodies and their regions, based on these unique signatures.

Previous search published in May in the journal Nature found concentrations of clinopyroxene, orthopyroxene and olivine in the crater – amounts high enough to apparently confirm the theory that the mantle had already crossed the crust. the Nature This study analyzed ground spectroscopic data from Chang & # 39; E 4 and processed them using a variety of functions. This process allowed them to identify the mineral mathematically best suited to the spectral compositions of each.

Zhang and his colleagues also analyzed the spectroscopic data acquired by the Chang & E's 4 rover instruments after landing the probe in the crater. They used a technique that compared the documented reflections of light and the color of the lunar surface with a database of known minerals. The database took into account the particle size of minerals, the way they interact with light and their response to alteration of space – changes to the soil surface caused by the 39 solar irradiation and the bombardment of tiny particles that the surface of the moon undergoes.

This different process allowed the researchers to detect and measure the amount of plagioclase in the crater. Plagioclase is a mineral created from lava in cooling. It is also one of the most common rocks on the surface of the moon. The results showed that plagioclase accounted for 56 to 72% of the crater composition, making it the major mineral. The high concentration of plagioclase suggests that the lunar crust was not pierced by an old impact.

The new study also revealed that the landing site on the crater had concentrations of 9 to 28% orthopyroxene, 4 to 19% clinopyroxene and 2 to 12% olivine. Although all three minerals are in the basin, they are not present in sufficient quantities to prove an impact event once the crust has broken, according to the study's authors.

The new study complicates the certainty of previous results and emphasizes the need for further research on the lunar surface of the hidden side, according to Zhang.

This story is reissued with the permission of AGU Blogs (http://blogs.agu.org), a community of blogs on Earth and Space Sciences, hosted by the American Geophysical Union. Read the original story here.