A new study by the Physical Research Laboratory (PRL), Ahmedabad, has found the potential presence of primitive lunar mantle materials, which is likely to have been formed during the formation of the South Pole-Aitken (SPA) basin 4.3 billion years ago, at the Chandrayaan-3 landing site.
SPA basin is one of the Moon’s largest and oldest impact features in the solar system. The Chandrayaan-3 landing site is located 350kms from the basin.
Researchers said the findings, published in Nature Communications Earth and Environment on Wednesday, could aid in the study of the early evolution of the Moon.
Analysing the concentrations of volatiles (chemical elements and compounds) measured by the Alpha Particle X-ray Spectrometer (APXS), an instrument on-board the Pragyan rover at Shiv Shakti statio near the South Polar Region, the 12-member team found high levels of sulphur at the Chandrayaan-3 highland landing site in the range of 900-1400 ppm, which was 300-500 ppm (parts per million) higher than in soil samples from lunar highlands from in Apollo 16 and Luna 20 missions.
However, the levels of sodium and potassium from the Chandrayaan-3 landing site was found to be much lower at 700-2800ppm and 300-400 ppm respectively as compared to the earlier missions. The APXS experiment was designed and developed by PRL, a unit of the department of space.
Researchers said the differences in the concentrations of these volatile elements as compared to those found in Apollo 16 and Luna 20 missions make it important to investigate the probable source that led to their enrichment or depletion at the Chandrayaan-3 landing site.
Explaining how the team arrived at the present conclusion, Rishitosh K Sinha, lead author of the study said that on the Moon, around 400-1000 ppm of sulphur can come from Type I carbonaceous chondrite (CC) meteorites crashing on the lunar surface.
“However, this is still less by 200-400 ppm than what APXS measured at the landing site. Moreover, the surface temperatures at the landing site, which is located at 70 degrees south in the Southern Polar region, is much higher for sulphur to condense in the plume as compared to if the site was closer (85-90 degrees) to the South Pole,” said Sinha. Closer to the South Pole, where surface temperatures are lower, sulphur can condense into solid form.
Therefore, said Sinha, there had to be another source of sulphur that increased its concentration at the landing site. “The other possible source for excess sulphur therefore would be the primitive lunar mantle material that would have thrown up during the SPA basin formation.”
He further explained that the low levels of sodium and potassium at the Chandrayaan-3 landing site as opposed to the Apollo 16 and Luna 20 missions suggests that potassium, rare earth elements, and phosphorus (called KREEP) might not have existed at the place and time of SPA basin formation. “This new finding therefore makes the Chandrayaan-3 landing site a promising site to access primitive mantle samples, which is otherwise lacking in the existing lunar collections,” said Sinha.
Sinha said during the end of the lunar magma ocean (LMO) crystallisation stages, when the Moon’s initial molten state solidified, the residual molten layer became enriched in a mineral called Troilite (FeS). “We have proposed that the SPA basin impact event excavated this FeS from the sulphur-enriched primitive lunar mantle, while the KREEP layer was still in the process of formation. The subsequent impacts on the SPA basin ejecta stirred things up, mixing sulfur-rich materials from the SPA basin with the nearby material at the landing site,” he said.
On August 23, 2023, the Chandrayaan-3 mission made the first successful landing in the South Polar region of the Moon following which APXS directly measured the elemental composition of the Moon’s surface at Shiv Shakti statio, an unexplored location, in the southern high-latitude highlands of the nearside of the Moon.
Last year, a study published in Nature by the same group had found evidence of high magnesium in the soil at the same landing site that could have originated from the deeper layers of the Moon. It provided clues for the presence of lower crust and/or upper mantle materials at the landing site. “High magnesium also comes from the mantle. With evidence of high concentrations of sulphur now, the present study becomes robust since it complements the previous study.”
Anil Bhardwaj, Director PRL, also a co-author in this study, said that finding primitive mantle material is important because Apollo and Luna missions only comprised collections of lunar samples. “We really don’t have samples from the lunar mantle. These samples are crucial to understand how the Moon was formed, what connection does it have with the Earth, presence and proportion of volatile elements in the lunar interior and how it evolved over time,” said Bhardwaj.
Commenting on the significance of the study, Rajesh VJ, professor, department of earth and space sciences at the Indian Institute of Space Science and Technology, Thiruvananthapuram, who was not involved with the study, said, the primitive mantle material from any planetary bodies is significant, as it provides vital information about the chemical composition of the early materials (rocks/minerals) present towards the deeper part (especially the mantle), before it underwent any sort of petrological modifications.
Hence, these materials are used by geoscientists to understand the origin and early evolution of a planetary body. “The discovery of sulphur-rich primitive mantle materials provides a rare opportunity for planetary scientists to conduct detailed investigations of the early evolution of the Moon. It provides the scientific community with more vital information about the composition of the lunar interior and its volatile contents,” he said.
