The Pragyan rover's wheels penetrated several centimeters into the lunar soil, allowing the rover's APXS instrument to analyze the composition of the lunar regolith beneath the upper surface layer. Credit: ISRO
4.5 billion years ago, amid the chaos of the early solar system, the young Earth was rocked by a cataclysmic collision with another Mars-sized planet. The debris hurled into space eventually formed a sphere of molten rock, a world covered by a global ocean of magma that then cooled and solidified, forming an airless companion to Earth in our night sky.
This is how scientists believe the Moon formed, and the scenario became even clearer today thanks to the release of evidence of its primordial magma ocean collected a year ago by India's lunar rover.
The rover's analysis of rock composition at the landing site is the first of its kind from the Moon's polar regions, expanding the body of evidence to other areas of the Moon. Previous analyses of the Moon's surface composition were limited to equatorial and mid-latitude regions.
The results are also a sign of India's continued advances in lunar exploration. The details were published on August 21 by an all-Indian team in the journal Nature.
Going south
The composition of the lunar surface has been investigated by the US Surveyor robotic missions and by laboratory analyses of lunar samples returned by the Apollo, Russian Luna and Chinese Chang'e 3 missions. Analyses of each of these sites showed compositions that were broadly similar, suggesting they all came from a single magma reservoir. But data from the polar regions were needed to bolster the theory that the Moon's magma ocean extended across its entire surface.
India's Vikram lander landed on August 23, 2023, alone 390 miles (630 kilometers)) from the lunar south pole. The lander then deployed a rover, named Pragyan (Sanskrit for “wisdom”), capable of analyzing the chemical composition of the lunar surface near the landing site. In the 10 days after landing, the Pragyan rover covered a distance of 340 feet (103 meters).
To analyze the composition of a rock sample, the rover’s Alpha Particle and X-ray Spectrometer (APXS) instrument irradiated samples with alpha particles and X-rays emitted from an internal radioactive source. By measuring the energies of the scattered alpha particles and the X-ray fluorescence of the target sample, researchers were able to determine a sample’s elemental composition. The APXS instrument’s scope can penetrate no deeper than a millimeter into the surface, but by studying soil churned up by the Pragyan rover’s wheels, mission engineers were able to analyze samples several centimeters deep.
The landing site, now called Shiv Shakti Point (a Hindu term for the male and female natural forces that maintain balance in the universe), turned out to be remarkably uniform in composition: analysis of 23 sites around Vikram's landing site consistently revealed the presence of ferrous anorthosite.
Anorthosite is a rock composed of a single mineral. In the case of lunar anorthosite, this is a lightweight silica mineral that is thought to have floated to the surface from a global magma ocean that existed when the Moon formed about 4.6 billion years ago. Ferrous anorthosite, such as that found at Point Shiv Shakti, also contains a percentage of iron, hence the name ferrous for the ferrous mineral it contains. Denser rock types, such as magnesium- and iron-rich olivine and pyroxene, a mineral found in igneous rocks formed by cooling magma, sank into this global magma ocean and formed the Moon's mantle, leaving the lighter anorthosite on the surface.
The uniform presence of anorthosite at Point Shiv Shakti is a key and exciting find because its composition closely matches the silicon, aluminum, calcium, iron, and magnesium percentages of anorthosites found at the Apollo 16 and Luna 20 sites, 1,120 miles and 1,400 miles to the north (1,800 and 2,250 km), respectively. This distant data point extends the reach of proven anorthosite deposits, strongly implying that the same event occurred at all locations. This chain of evidence further strengthens the theory that the lightweight mineral is the solidified upper layer of a once-global magma ocean.
Considering the impacts
However, not all of the material at the Shiv Shakti spot comes from this solid upper layer. APXS data also show elevated levels of magnesium and calcium, implying that material from deeper layers of the lunar mantle has been mixed in.
This mixing was likely the result of meteorites impacting the region. The area lies just 350 kilometers (217 miles) from the edge of the South Pole-Aitkin Basin, the largest known impact basin in the solar system. According to models, the resulting layer of ejecta from the South Pole-Aitkin Basin is between 1.1 and 2.1 kilometers (0.7 and 1.3 miles) deep at the Vikram landing site. Impacts that created the nearby Manzinus and Boguslawsky craters further churned up the ejecta layer, and the Vikram landing site also sits atop an ancient debris streak spewed from Schomberger Crater, 200 kilometers (125 miles) to the south.
The detection of minerals in the deepest layers of the lunar crust shows that mantle material excavated by large basin impacts 4 billion years ago may be spread across the entire surface of the Moon. It also shows how important it is for future lunar explorers to include trained geologists who can recognize the different types of rocks on the Moon's surface.
Understanding our Moon and how it formed also helps us better understand the history of our solar system. One thing is certain: results like these from Vikram show that progress in this task is no longer just the province of the superpowers of the mid-20th century.
