India's Chandrayaan-3 mission has provided the first measurements of the composition of the soil near the moon's south pole 1. The minerals found provide further evidence that the lunar surface was completely melted shortly after the moon was formed.

The Chandrayaan-3 Vikram lander landed on the surface on August 23, 2023. He deployed a rover called Pragyan, which collected data ranging from temperature to seismological measurements over 10 days.

Pragyan also studied the chemical composition of regolith: the fine material that covers much of the lunar surface. The rover stopped 23 times and deployed an instrument called an Alpha Particle X-ray Spectrometer (APXS).

Santosh Vadawale, an X-ray astronomer at the Physical Research Laboratory in Ahmedabad, India, and his colleagues analyzed the radiation data collected by the APXS and used this information to identify the elements in the regolith and their relative abundances, which in turn revealed the mineral composition of the soil. The team found that all 23 samples were composed primarily of ferroanorthosite, a common mineral on the Moon. The results were announced todayNaturepublished.

"It's kind of what we expected based on orbital data, but it's always good to get the real truth," says Lindy Elkins-Tanton, a planetary scientist at Arizona State University in Tempe.

Previous landers achieved similar results. However, the samples of Chandrayaan-3 are the first from the subpolar region: previous landers visited equatorial and mid-latitudinal zones. Together, this suggests that the composition of the regolith is uniform across the entire lunar surface.

Vadawale says this is direct confirmation that the lunar surface was a molten magma sea immediately after it formed. The lunar magma sea theory was first proposed by two independent groups in 1970 after analyzing rocks collected from the Apollo 11 landing in 1969.

Origin of the moon

The best model for the Moon's origin suggests that the newly formed Earth was hit by a large impactor called Theia, which vaporized the planet's surface and ejected a large amount of material into orbit. The dispersed material quickly clumped together and formed the moon. This impact theory explains why lunar rocks have an isotopic composition similar to that on Earth.

The material that formed the moon had a lot of energy that needed to be dissipated. This escaped in the form of heat and melted the surface of the young moon into a sea of ​​magma. Dense mafic rocks rich in metals such as magnesium sank into the moon's interior. Lighter rocks, including anorthosite, floated upward, forming highlands similar to those visited by Chandrayaan-3.

“It further supports the lunar magma sea hypothesis,” says Mahesh Anand, a planetary scientist at the Open University in Milton Keynes, UK.

Vadawale and his colleagues found that their samples contained elevated amounts of magnesium compared to the levels of calcium. This suggests that deeper mafic materials were mixed into the regolith.

The researchers attribute this to the events that formed an enormous impact crater called the South Pole-Aitken Basin, the rim of which is 350 kilometers from the Chandrayaan-3 landing site. “When you create an impact crater that big, it's supposed to excavate some deeper materials,” Vadawale says, because the impactor penetrates deep into the crust. This deeper, magnesium-rich material would have been distributed over a large area, slightly altering the composition of the regolith that Pragyan studied.

One problem with this idea, however, is that the South Pole-Aitken Basin appears to be dominated by a mineral called pyroxene, which doesn't quite fit with Pragyan's data, Anand says. Clarification will likely require returning samples to Earth, he says.

The next Chandrayaan mission, which is in early stages of development, intends to do just that.

“For me, this is a story about the Success of Indian space program says Elkins-Tanton.