Many lunar deposits contain surprisingly high amounts of titanium; some samples contain up to 18 percent TiO2 by weight. While this fact was interesting to future miners, it also attracted the attention of planetary scientists, who for more than fifty years could not explain how these rocks formed, let alone reached the surface of the Moon. Now the mystery is probably solved.

When the Apollo astronauts returned from the moon with 380 kilograms (840 pounds) of rocks, some samples looked familiar to geologists. Others do not, but only because they require extraterrestrial conditions, such as the absence of water or air, to create them. However, some lunar items did not fit the obvious explanations. For example, it turns out that the volcanic basalts that form the lunar “seas” are much more diverse in composition than their terrestrial counterparts, with a hundredfold difference in titanium concentrations.

Titanium-rich basalt has remained a mystery ever since. Orbital maps showing that these rocks are quite common further deepened the mystery. Now, a team led by Professor Tim Elliott of the University of Bristol has come close to recreating the mysterious basalt in the laboratory, providing an explanation for how it might have formed on the Moon.

“The origin of volcanic lunar rocks is a fascinating story that describes an ‘avalanche’ of planetary-scale, unstable crystalline debris formed by the cooling of a primordial magma ocean,” Elliott said in a statement. said. “The key limiting factor in this epic story is the presence of a type of magma unique to the Moon, but explaining how this type of magma reaches the surface for sampling by space missions has been a difficult problem. It’s great that this dilemma has been solved.”

The problem was that basalt not only had a high titanium content, but also had a low density compared to most similar rocks on Earth. This lightness contributed to large-scale eruptions 3.5 billion years ago, before the Moon ceased to be volcanically active; But geologists have struggled to explain how and why this combination only appears on the Moon.

Some geologists have suggested that titanium-rich basalts come from materials known as ilmenite cumulates found in the lunar mantle. However, the partially melted cumulates in the laboratory do not match the basalts in question. Additionally, the product is so dense that it is unlikely to reach the surface.

Elliott and colleagues showed that when ilmenite-containing cumulates reacted with the common minerals olivine and orthopyroxene, the melted product matched the titanium-rich basalts that people were trying to explain. The coincidence extends to its low intensity, which makes sense for large-scale explosions.

“Although this model does not fully reflect the interaction between the melt and the solid body of the Moon, we suggest that titanium-rich magmas erupting on the Moon’s surface may have been produced by partial melting of ilmenite-containing cumulates. The melts undergo significant changes in their elemental and isotopic elements.” composition resulting from the jet stream in the lunar mantle,” Elliott and co-authors write. “Therefore, the jet stream may be a critical process that reduces melt density and allows high titanium melts to erupt on the lunar surface.”