Our Moon Was Likely Covered in a Magma Ocean Long Ago, and New Data From India’s Lunar Rover Supports That Theory

Roughly 4.5 billion years ago, a Mars-sized celestial body slammed into our home planet. The catastrophic collision vaporized the impactor, known as Theia, and tore a chunk out of the proto-Earth. These planetary fragments went into orbit and mixed together, eventually coalescing to form the moon.

According to this widely accepted lunar origin story, known as the impact theory, the newborn moon dissipated its leftover energy into heat, which melted its surface into an ocean of magma that remained for millions of years.

Now, new lunar soil composition measurements collected by India’s Chandrayaan-3 mission add more evidence that the moon was once covered in a magma ocean. The findings were published in the journal Nature last week.

“The theory of early evolution of the moon becomes much more robust in the light of our observations,” Santosh Vadawale, an X-ray astronomer and lead author of the study, tells BBC News’ Georgina Rannard.

Last year, Chandrayaan-3’s Vikram lander touched down near the moon’s south pole and released a rover called Pragyan. Over the course of ten days, Pragyan collected a plethora of data, including the chemical composition of the lunar regolith—or the loose surface material that covers the solid rock beneath—with an instrument called an alpha-particle X-ray spectrometer (APXS).

Back on Earth, the team of Indian astronomers analyzed data on the 23 regolith samples studied by the APXS to understand the soil’s mineral composition. These were the first soil composition measurements ever registered from near the moon’s south pole, and the scientists found that all of them included ferroan anorthosite, a type of white rock.

That finding supports the idea that the moon was once enveloped by magma. “A key prediction of the lunar magma ocean hypothesis is the presence of a largely anorthositic crust,” Vadawale tells Popular Science’s Tom Hawking. This is because during the moon’s magma ocean phase, dense rocks—like magnesium-rich mafic rocks—would have sunk inward, while lighter rocks—like ferroan anorthosite—would have floated to the top.

The new lunar south pole samples align with findings from previous samples collected near the moon’s equator during the Apollo 11 mission, which also contained ferroan anorthosite. And they match orbital data from the earlier Chandrayaan-1 and Chandrayaan-2 missions—a consistency that makes scientists trust the orbital data sets more, per the Conversation’s Vivian Lam.

“It’s sort of what we expected to be there based on orbital data, but the ground truth is always really good to get,” Lindy Elkins-Tanton, a planetary scientist at Arizona State University who was not involved in the study, tells Nature News’ Michael Marshall.

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However, the team also found magnesium in the regolith, even though magnesium-rich rocks were expected to have sunk lower in the crust. They suggest the impact of a meteor crash—likely responsible for the giant ancient impact crater called South Pole-Aitken basin—may have dug up denser rocks from deep within the moon’s mantle.

Pragyan’s measurements also revealed a higher level of olivine than pyroxene, both of which are magnesium- and iron-bearing minerals, per Popular Science. This was unexpected, given that other samples from around the South Pole-Aitken basin have shown the opposite—more pyroxene than olivine—and scientists aren’t sure why Pragyan’s observations were different.

Mahesh Anand, a planetary scientist at the Open University in England who was not involved in the study, tells Nature News that scientists will need more lunar samples brought to Earth to understand this difference—a formidable goal of the next Chandrayaan mission.

Margherita Bassi | READ MORE

Margherita Bassi is a trilingual storyteller and freelance journalist with a particular interest in ancient history, astronomy and human interest stories.