The moon’s surface is covered by ancient lava flows that are often different from those found on the earth. While volcanic rocks on the earth rarely contain more than 2% titanium dioxide (TiO2), some lunar basalts — common volcanic rocks — carry up to 18%, a fact that planetary scientists have struggled to explain for decades.
A new study by researchers from IIT-Kharagpur and the Physical Research Laboratory (PRL), Ahmedabad, published in Geochimica et Cosmochimica Acta, has now offered an experimental account of how these titanium-rich basalts could have formed.
The study’s authors were Himela Moitra, Sujoy Ghosh, Tamalkanti Mukherjee, Saibal Gupta, and Kuljeet Kaur Marhas.
Cameras on landers
The Chandrayaan-4 mission, which the Indian Space Research Organisation (ISRO) has planned for 2028, aims to collect rock samples from the moon and return them to the earth, making the choice of landing site critical. The study’s findings could help inform that decision.
Prof. Ghosh, one of the lead authors and associate professor at IIT-Kharagpur, said, “Regions near the lunar south pole, such as those being evaluated for Chandrayaan-4, including areas near Shiv Shakti region, have been studied in detail using data from Chandrayaan-2, NASA’s Lunar Reconnaissance Orbiter, and other missions. What our work adds is a deep interior perspective.”
According to the study’s first author Himela Moitra, “High-resolution microscopic cameras on landers can help identify minerals in lunar rocks, while instruments such as X-ray fluorescence and X-ray diffraction can determine their chemical composition before collection.”
“Spectroscopic tools such as Raman and visible-near infrared spectroscopy can help confirm the mineral phases in rocks before they are collected. Similar instruments have already been successfully used in Mars missions,” Tamalkanti Mukherjee, a PhD student at IIT Kharagpur and co-author of the study, added.
The European Space Agency is also planning to launch its Lunar Volatile and Mineralogy Mapping Orbiter mission in 2028 to map the distribution of water and ilmenite on the moon.
Too high or too low
Roughly 4.3 billion years ago, the moon was still cooling from a global ocean of molten rock. In the process, olivine and orthopyroxene crystallised first, then plagioclase, which floated up to form the moon’s pale crust. The last to crystallise was a dense, iron- and titanium-rich layer containing minerals called clinopyroxene, ilmenite, and fayalitic olivine. Scientists call this the ilmenite-bearing cumulate (IBC) layer.
The IBC layer was too dense to stay put. Gravity pulled it downwards through the less dense, magnesium-rich mantle in a process called cumulate overturn. As it sank into the hotter regions of the lunar interior, the IBC layer began to melt. The titanium-rich partial melts it produced are widely thought to be the source of the moon’s titanium-rich basalts — but the exact mechanism has remained contested.
When researchers previously tried to melt IBC rocks in the lab, the resulting liquids didn’t match the basalts on the moon’s surface: they either didn’t have enough magnesium or were too dense to rise and erupt as lava. The authors of the new study set out to find the missing link.
They used a piston-cylinder apparatus at IIT Kharagpur, capable of exerting pressures up to 3 gigapascals (GPa) of pressure — equivalent to that under 700 km deep inside the moon — and temperatures of 1,500 °C.
The team designed two sets of experiments. In one set, they placed a thin layer of a synthetic IBC layer above a layer of San Carlos olivine, a mineral on the earth that is a good proxy for the moon’s magnesium-rich mantle, inside a capsule and subjected it to pressures of 1-3 GPa and temperatures of 1,075-1,500 °C. This setup mimicked the place where a sinking IBC layer comes in contact with the mantle. In the other kind of experiments, the team blended the two materials together before subjecting them to similar conditions, simulating a chemical interaction during a slow descent or ascent.
‘Significant progress’
The results of the tests suggested that basalts high in titanium were created in a complex process involving both reactions and mixing.
The first kind of experiments generated melts containing 9-19% titanium dioxide but they were stubbornly low in magnesium oxide, which is the same discrepancy older studies had run into. The mixed experiments produced basalts that were too high in magnesium and too low in titanium, on the other hand.
“Indian laboratories, including those at IIT Kharagpur, PRL Ahmedabad, and other ISRO centres, have made significant progress in recent years,” Prof. Ghosh said. “Our study demonstrates that high-pressure experimental work relevant to planetary interiors can now be carried out entirely within India, marking an important step toward building indigenous capability in planetary science.”
When the team simulated a combination of these processes and outcomes on a computer, they found that some molten rocks could have risen directly and erupted with moderate amounts of titanium. Those rocks very rich in titanium could have however become stuck deep inside the moon. Later, fresh magma rising from below could have mixed with these trapped pockets and the combined molten mass could have erupted as lava rich in titanium.
Repository of melts
Per the study, this two-stage model could successfully reproduce the observed magnesium, titanium, silicon, and iron contents of the moon’s high-titanium basalts, but underestimated aluminium oxide and calcium oxide.
The model could also explain why volcanic activity high in titanium continued throughout the moon’s geological history rather than being confined to its earliest period: because the natural satellite had a repository of titanium-rich melts in its interior for billions of years, waiting for the right conditions to bring them to the surface.
mukunth.v@thehindu.co.in
Published – March 24, 2026 08:10 am IST
