Chandrayaan-3 data confirm theory on the Moon's origin

Around 4.5 billion years ago, a Mars-sized planet called Theia came from the outer reaches of the solar system, or perhaps even beyond, and crashed into Earth, then still in its infancy. The impact broke off a part of the Earth. This chunk likely merged with bits of Theia and began orbiting the Earth. Thus, the Moon was formed.

The impact was such that the Moon was initially a swirling ball of molten magma, which scientists call the Lunar Magma Ocean (LMO). Over time, the angry magma cooled — and heavier minerals such as iron and magnesium sank into its mantle, while the lighter ones, comprising aluminium and silica, floated on the surface, forming the crust.

Indian lunar mission Chandrayaan-3's findings align with the LMO hypothesis, as a recent paper underlines. In 2023, the Indian Space Research Organisation (ISRO) sent a craft to the Moon. Chandrayaan-3's Moon lander and rover soft-landed on the Moon, around 500 km from the lunar south pole. Its seven payloads (one on the propulsion module, four on the lander Vikram and two on the rover Pragyan) have sent back around 55 GB of data, which ISRO has made available for global researchers.

Based on data collected by the Alpha Particle X-Ray Spectrometer (APXS) instrument on board Pragyan, the paper (bit.ly/moon-magnesium) says that the mission's finding endorses the LMO hypothesis. It also explains the effects of an impact that caused a crater — one of the largest in the solar system — on the Moon's southern tip. The South Pole-Aitken Basin, over 8 km deep, and 2,500 km wide, is believed to have been formed when an asteroid hit the Moon. "The... impact dislodged heavier minerals from the Moon's interior and strewed them on the lunar regolith. That explains the higher presence of magnesium around Shiv Shakti Point (Chandrayaan-3's landing site), as it is close to the basin's rim, around 350 km away," elaborates Anil Bhardwaj, Director of the Ahmedabad-based Physical Research Laboratory (PRL),  which developed the APXS payload.

The composition of the soil at the landing site is similar to that found around the lunar equator, which had earlier been tested. "We found 4-4.5% of magnesium minerals; around the equator, the composition is around 2%," says Santosh V. Vadawale, lead author of the paper. Given the location of the sampling, Vadawale says that they could confidently hypothesise that the higher magnesium content was because of the asteroid hitting the Moon.

APXS took 23 measurements, but as these were all within a distance of 103 metres, Vadawale believes that another sampling, at a similar latitude, will help either strengthen the present theory or suggest another hypothesis.

While this is PRL's primary paper from the APXS data, more analyses could emerge, as other researchers, too, are studying the data; for instance, analysing the presence of minor elements such as sulphur and nickel. The magnesium suite rocks are olivine and pyroxene. APXS records "slightly higher olivine than pyroxene". Vadawale says that this observation is in contrast to remote sensing observations, which show a higher level of pyroxene. "...the APXS measurements serve as the first ground truth in the solar polar highlands," the paper notes. "The data will help calibrate remote sensing instruments for future missions," Bhardwaj adds.

Ramananda Chakrabarti, Professor at the Centre for Earth Sciences, Indian Institute of Science, Bengaluru, says that the first ever in situ geochemical data from the lunar southern latitudes "validates our understanding of the Moon's evolution and provides additional insights into the dynamics of the lunar surface due to impact cratering". He, however, is "a bit surprised" that the geochemical data shows an almost equal contribution of crustal origin and sub-crustal origin rocks at the site. He feels that since the lunar soil is made of mechanically pulverised material, readings from an instrument as sophisticated as the APXS could be expected to vary slightly at different points, depending on which rock was present at that site. However, all 23 measurement sites show a similar geochemical distribution.

Is this a bias of the sampling, the grain size of the layer, the short range of the rover, or something else? The answers may come when Chandrayaan-4 brings back lunar soil.