April 30, 2025

Scientists of the Physical Research Laboratory (PRL), Ahmedabad have analysed the concentrations of volatile elements measured at Shiv Shakti statio near the South Polar Region, using the Alpha Particle X-ray Spectrometer (APXS) onboard the Pragyan rover of the Chandrayaan-3 mission. A comparison of the APXS measured abundances with the other available data revealed anomalous depletion in sodium and potassium, but enrichment in sulfur in the soils at the highland landing site. This study published in the journal Nature Communications Earth and Environment has revealed potential presence of primitive lunar mantle materials at the landing site, which was excavated during formation of the South Pole-Aitken (SPA) basin 4.3 Ga ago and redistributed by subsequent impacts on the SPA basin ejecta. The primitive mantle contributed the excess sulfur, which got mixed up with the materials at the landing site. The low levels of sodium and potassium at the landing site suggests that the KREEP (potassium, rare earth elements, and phosphorus) might not have existed at the place and time of SPA basin formation. This new finding makes the Chandrayaan-3 landing site a promising site to access primitive mantle samples, which is otherwise lacking in the existing lunar collections.

On 23 August 2023, the Chandrayaan-3 mission marks the first successful landing in the South Polar Region of the Moon. The Alpha Particle X-ray Spectrometer (APXS) aboard Pragyan rover directly measured the elemental composition of the Moon’s surface at unexplored location, i.e., Shiv Shakti statio located at 69.37° S, 32.32° E in the southern high-latitude highlands of the nearside of the Moon. The APXS measurement of major elements supported the lunar magma ocean (LMO) hypothesis. It also provided clues for the presence of lower crust and/or upper mantle materials at the landing site (Vadawale et al., Nature, 2024; https://doi.org/10.1038/s41586-024-07870-7).

The APXS also measured abundances of volatile elements, including sodium, potassium, and sulfur in the highland soils, and reported varied concentrations ranging from 700-2800 ppm, 300-400 ppm, and 900-1400 ppm, respectively (Fig. 1). Scientists from the Physical Research Laboratory (PRL), Ahmedabad, have made a detailed analysis of the abundances of these volatile elements. Interestingly, the concentrations of sodium and potassium at the Chandrayaan-3 landing site was found to be much lower compared that found in the soil samples from lunar highlands in earlier missions (Apollo 16 and Luna 20), while the concentration of sulfur was found to be 300-500 ppm higher than in soils from Apollo 16 and Luna 20 missions. The anomalous differences in the concentrations of these volatile elements make it important to investigate the probable source that led to their enrichment or depletion at the Chandrayaan-3 landing site.

On the Moon, the Type I Carbonaceous chondrite (CC) meteorites crashing on the surface can add approximately 400-1000 ppm of sulfur to the soil (Fig. 2). But, this is still insufficient to explain the APXS measured excess of 200-400 ppm sulfur. Moreover, the surface temperature observed at the landing site ranges between 250 and 300 K, which is much higher than the volatility temperature of sulfur (Fig. 2). In this case, the cold-trapping mechanism of surface enrichment of sulfur for geologically long periods is not possible at the landing site. Therefore, there has to be another source of sulfur that increased its concentration at the landing site.

During the end of the lunar magma ocean (LMO) crystallization stages, the residual molten layer became enriched in a mineral called Troilite (FeS). The PRL scientists have proposed that the South Pole-Aitken (SPA) basin impact event at 4.3 Ga excavated this FeS from the sulfur-enriched primitive lunar mantle, while the KREEP layer was still in the process of formation (Fig. 2). 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.

To summarize, a comparison of volatile elemental abundance data from Chandrayaan-3 soil relative to previous missions, suggests much lower amounts of sodium and potassium, but higher levels of sulfur. Such a pattern indicates that the soil at the landing site include primitive mantle material excavated by the SPA basin impact, billions of years ago. That makes the Chandrayaan-3 landing site a promising place for future missions to collect samples, particularly to study the early evolution of the Moon.

The APXS experiment was designed and developed by Physical Research Laboratory, Ahmedabad, a unit of Department of Space, Government of India. A large group of scientists and engineers from various ISRO centers has contributed in the successful planning, implementation, and execution of the Chandrayaan-3 mission.