The Moon is an almost completely still world, its eternal peace and quiet disrupted only by the occasional meteor or Apollo astronaut. But we now know there's plenty of magma inside the Moon. So why are there no volcanoes?
That's the question researchers at the European Synchrotron Radiation Facility (ESRF) set out to answer. They analyzed some of the rocks brought back from the Apollo missions to create artificial copies identical on the microscopic scale. They then melted these rocks so that their conditions matched those of the phenomenal pressures - over 45,000 bar - and temperatures - nearly 3,000 degrees Fahrenheit - that are found deep inside the Moon.
Their research builds off of recent analysis of Apollo seismometers by NASA scientists, who found that perhaps as much as 30% of the lunar mantle should be molten. On Earth, that amount of magma would find its way to the surface to create lots of volcanic eruptions, and yet the Moon appears to be completely dead volcanically. The only explanation is that something must be blocking or otherwise halting the magma's rise to the surface.
Now the researchers at the ESRF say they have the answer. Although they could only replicate the conditions of the lunar mantle on the tiniest of samples - and they needed the world's most powerful X-ray beam to analyze it - the researchers were able to determine the density of the rocks in the lunar mantle. Their computer simulations suggest most of the magma on the Moon is less dense than its surroundings, which means it should slowly bubble up to the surface like in Earth's volcanoes.
But there was one crucial exception. The Apollo 14 mission brought back titanium-rich glass that would produce magma that is at least as dense as the rocks around it. This type of magma would be unable to rise to the surface. The researchers believe that very early in the Moon's evolution, this titanium-rich magma formed on or near the lunar surface but then migrated vertically downwards, becoming heavy magma at the core-mantle boundary and effectively blocking the upward movement of any other possible magma. In a statement, researcher Wim van Westrenen explains this phenomenon, and how it might actually lead to a new lunar volcano far, far in the future:
"After descending, magma formed from these near-surface rocks, very rich in titanium, and accumulated at the bottom of the mantle – a bit like an upside-down volcano. Today, the Moon is still cooling down, as are the melts in its interior. In the distant future, the cooler and therefore solidifying melt will change in composition, likely making it less dense than its surroundings. This lighter magma could make its way again up to the surface forming an active volcano on the Moon – what a sight that would be! – but for the time being, this is just a hypothesis to stimulate more experiments."