Bengaluru: A sea of hot, molten rock or magma once existed under the lunar surface, findings of Chandrayaan-3 have revealed, and its analysis of matter deep below the Moon has opened up new avenues for studying the surface while even potentially identifying safe landing spots.
The data has backed up the Lunar Magma Ocean theory, which argues that large amounts of magma cooled to form the Moon’s surface around 4.2 billion years ago.
Exactly a year after India’s historic Chandrayaan-3 touched down on the lunar south pole region – the first ever lander to do so – scientists from the Physical Research Laboratory (PRL), Ahmedabad, have released the initial batch of data obtained from the third Moon mission.
The lander-rover duo went where no craft had ever gone, and the rover analysed the very first samples from the Moon’s southern polar region, which faces the Earth.
The findings revealed evidence through chemical remnants in the rocks of a former ocean of magma, adding to proof presented from other equatorial and northern hemisphere samples from the Moon, which has similar composition in terms of minerals and chemical makeup, despite the geographical distance.
The readings were taken by the Pragyan rover, which travelled 103 metres on the lunar surface, and made 23 on spot or in-situ analyses at various locations to study the lunar soil (regolith) composition.
“The samples are consistent with those from other regions and we are able to add one more piece of evidence to confirm the past presence of a lunar magma ocean,” explained lead author and high-energy astrophysicist Santosh Vadawale.
The study further revealed a mixture of magnesium and olivine compounds that typically exist deep within the Moon at a depth of nearly 100 kilometres. The team concluded these materials came from a very large impact crater over 2,500-km wide, in the south pole.
“The sample we analysed was very unique for its composition. It had material from the surface, from below the surface, and from deep within the Moon,” explained co-author and lunar geologist Neeraj Srivastava. “The geological context of this area makes it a special case, as it also contains material mixed in from the large crater in the South Pole Aitken (SPA) basin.”
The findings were published in the journal Nature Wednesday.
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What Chandrayaan-3 data found
The most commonly accepted theory of the formation of the Moon is that of a Mars-sized ancient planet called Theia colliding into ancient Earth around 4.5 billion years ago, when planets were just forming.
This is called the giant impact hypothesis, and according to it, Theia collided either head-on or partially into Earth, melting both bodies either partially or the Moon fully. This resulted in a bunch of rocks orbiting the broken Earth like Saturn’s rings. Eventually, these rocks would have gathered (accretion) together, coalescing to form the Moon as we see it today.
This theory is supported by analysis of the lunar and Earth’s crust, which also share some material.
“During the process of accretion, heat is generated, which itself would have either fully or partially melted the forming Moon, leading to a magma ocean,” explained Vadawale.
The presence of a magma ocean on the Moon is all but accepted; however, experts are still divided on whether there was a global magma ocean or a partial one. The findings currently lean more towards global, although there isn’t enough data to disprove the partial ocean hypothesis either.
“We were the first to land in the Moon’s southern pole and analyse the sample here in the highland regions, and they are almost similar to those studied by Apollo 16 (USA) and Luna 20 (former USSR) thousands of kilometres away. So there is strong support for the hypothesis,” Vadawale said.
Mega South Pole-Aitken basin
The Moon sports a giant-impact crater right at its south pole, and this is one of the largest-impact craters in the solar system. It is named after the south pole on one side of it, and the Aitken basin on the other, and spans a width of 2,500 km. It is the oldest and the deepest crater as well, measuring a surface depth of 10 km, and having formed over 4.2 billion years ago, when the present-day Moon itself was relatively new.
The “Shiv Shakti” point, where the Vikram lander came down, is located about 350 km from the rim of the basin, and the soil samples here contain olivine, which is typically found very deep as it sinks to the bottom of an astronomical body due to its weight.
This material is thought to have been ejected from the mantle of the Moon over 100 km deep, after the SPA impact event occurred. The spewed material or ejecta was spread out for hundreds of km, and mixed with the regolith, which also constantly undergoes weathering due to exposure to space.
“The rim of such a crater is the ideal place to study soil (regolith) material, as these places have ejecta coming from nearly a 100 km below, shallower material from about 10 km or so depth, as well as surface material,” explained Srivastava. “It would be wonderful to have such a sample on Earth for analysis, it can help reveal a lot about the evolution of the Moon.”
Pragyan’s work
The Pragyan rover rolled out of Vikram on 23 August 2023 and after soaking in sunlight into its batteries, it began traversing the region, moving a total of 103 metres on the surface and performing in-situ analysis.
The analyses of regolith samples for this paper was performed by the Alpha Particle X-ray Spectrometer or the APXS instrument on board the rover.
These observations provided the first measurements revealing uniformity in large regions in the regolith in the southern highlands. They also were the first in-situ measurements of chemical composition of high-latitude lunar highlands or elevated regions closer to the poles, which are also exciting due to the presence of water.
The rover went to sleep permanently after it depleted its solar power, and the 14-day long lunar night set in. However, the site of the rover was specifically chosen for the unexplored nature of the lunar south pole, the local presence of water, and unique geological features. On the other side of the Moon, China’s Chang’e missions are also performing similar in-situ sample analyses.
As analysis of data from Chandrayaan-3 carries on, India’s space agency ISRO and PRL are also working on the Chandrayaan-4 mission, which is expected to land in the south pole in 2028 and bring samples back to Earth.
(Edited by Tikli Basu)
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