Scientists believe they've discovered why the moon has no active volcanoes, despite containing plenty of liquid magma.
The answer, they say, is that the magma is so dense that it's simply too heavy to bubble to the surface.
The European scientists copied the composition of moon rocks collected by the Apollo missions, and melted them at the extremely high pressures and temperatures found inside the moon. They then measured their densities using X-rays.
"We had to use the most brilliant X-ray beam in the world for this experiment because the magma sample is so tiny and confined in a massive, highly absorbing container," says Mohamed Mezouar from the European Synchrotron Radiation Facility (ESRF) in Grenoble.
"Without a bright beam of X-rays, you cannot measure these density variations."
The measurements were combined with computer simulations to calculate the magma density at any location in the moon.
And nearly all the lunar magmas were found to be less dense than their solid surroundings, similar to the situation on Earth.
However, there was one important exception: small droplets of titanium-rich glass first found in Apollo 14 mission samples produced liquid magma as dense as the rocks found in the deepest parts of the lunar mantle today.
This magma would be too heavy to move towards the surface, says the team.
Previous research has shown that titanium-rich rocks were formed soon after the formation of the Moon at shallow levels, close to the surface.
It now seems that, early in the history of the moon, such titanium-rich rocks descended from near the surface all the way to the core-mantle boundary.
"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," says Wim van Westrenen from VU University Amsterdam.
"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."