Or still provide water for the moon, and "alien visitors" are found on the lunar surface.

Are there "extraterrestrial visitors" on the lunar surface? Can an "alien visitor" provide water to the moon? Based on the ultra-high spatial resolution imagery and spectral data obtained by Chang'e-4, researchers from the National Space Science Center of the Chinese Academy of Sciences and other units have identified carbonaceous chondrite impactor residues within one million years on the lunar surface for the first time. Studies have shown that the impact of volatile carbonaceous asteroids may still provide water for the current moon, while there may be impactor residues in younger lunar surface material, such as the Chang'e-5 return sample. The research results were published in Nature astronomy.

"The direct analysis of impactor residues that may exist in the Chang'e-5 sample will provide an important reference for the evolutionary history of impactor composition and types in the Earth-Moon system, and is expected to further constrain the evolution of solar system orbital dynamics and improve understanding of the impact history of the inner solar system." On November 30, Liu Yang, a researcher at the National Space Science Center of the Chinese Academy of Sciences, told Science and Technology Daily.

Impact remnants of carbonaceous chondrite meteorites have been found for the first time on the lunar surface

The Chang'e-4 probe successfully landed at von Carmen Crater in the South Pole-Aiken Basin on the far side of the moon in January 2019, and its Yutu-2 rover subsequently carried out continuous inspection of the lunar surface, obtaining ultra-high-resolution lunar surface images and hyperspectral data.

Liu Yang said that in the process of lunar surface inspection, "Yutu-2" found a small fresh impact crater and made a detailed spectral detection of this impact crater.

Based on the imagery and spectral data of the crater, the research team found that the spectrum of the suspected "residue" in the center of the crater showed obvious characteristics from the typical lunar soil and rock fragments inside and outside the crater.

Liu Yang explained that under normal circumstances, the reflection spectrum of lunar surface rocks or lunar soil will show the characteristics of "redization", and the space weathering of the lunar surface will generally further exacerbate this reddishing feature, that is, the reflectance increases with the increase of wavelength; however, the spectrum of the suspected residue in the center of the impact crater shows the characteristic of "blueization", that is, the reflectivity decreases with the increase of wavelength.

As a result, the research team speculates that there may be some kind of foreign impactor material with blued spectral characteristics mixed into the impact crater. "Previous researchers have found carbonaceous chondrite meteorite fragments in the 'Apollo samples', but never directly observed the impact residues of carbonaceous chondrite meteorites through remote sensing on the lunar surface. This is the first time that impact remnants of carbonaceous chondrites have been directly observed on the lunar surface. Liu Yang pointed out.

In fact, of asteroids, only the spectra of carbonaceous asteroids have bluening characteristics. The researchers collected the spectra of a large number of carbonaceous chondrite meteorites, and after careful comparison, found that the residue did have a high degree of similarity with the carbonaceous meteorite spectra.

There may be water in the impactor that remains in the impact melt or residue

In order to confirm the results of the analysis, the research team conducted a detailed topography analysis of the impact crater.

"Based on the stereoscopic images obtained by the Yutu-2 panoramic camera, our personnel used photogrammetry methods to construct a high-precision digital elevation model covering the small impact crater and the nearby area." Liu Yang said.

The analysis suggests that the crater may be a primary impact crater, rather than a secondary impact crater formed by the sputtering body from the original impact hitting the lunar surface again.

To further qualify the results, the researchers also studied the impact crater using numerical simulation techniques. The results showed that a loose impactor with a diameter of 15 cm hitting the lunar surface at a speed of 15 km/s could form the morphological features of the observed small impact crater, with residues distributed in the center of the impact crater.

"In this way, we can confirm that the crater should have formed after the impact of a small carbonaceous meteorite." Liu Yang stressed.

The impact transport process is considered to be one of the major contributors to water ice on the lunar surface and in permanent shadow areas, while carbonaceous asteroids are relatively water-rich and volatiles in small bodies, and during the impact, the water they carry may be partially retained on the lunar surface.

Liu Yang said that previous high-speed impact simulation experiments on the ground have found that there may be water in the impactor that can be retained in the impact melt or residue.

Based on the crater degradation model, the research team estimated the age at which the crater was formed. The results suggest that the crater should have been formed within a million years.

What's more, similar carbonaceous meteorite residues may be very common on the lunar surface, and there will be a high probability that similar impact residues will be found in the Chang'e-5 sample.

Liu Yang said that using higher spatial resolution telemetry spectral data, it is possible to find more similar impact residues on the lunar surface in the future, thereby further deepening the study of the source of lunar water.

Wen/Science and Technology Daily reporter Lu Chengkuan

Editor/Fan Hui