London: Using powerful X-rays to measure lunar rock densities, a team of scientists have found why our Moon, contrary to Earth, has no active volcanoes, and traces of its past volcanic activity.
The team led by Mirjam van Kan Parker and Wim van Westrenen from VU University Amsterdam suggest that the hot, molten rock in the Moon’s deep interior could be so dense that it is simply too heavy to rise to the surface like a bubble in water.
For their experiments, the team produced microscopic copies of moon rock collected by the Apollo missions and melted them at the extremely high pressures and temperatures found inside the Moon. They then measured their densities with powerful X-rays.
The team also comprised scientists from the Universities of Paris 6/CNRS, Lyon 1/CNRS, Edinburgh, and the European Synchrotron Radiation Facility (ESRF) in Grenoble.
Last year, NASA scientists claim that the deepest parts of the lunar mantle, bordering on the small metallic core, are partially molten, by up to 30 per cent.
In the Earth, such bodies of magma tend to move towards the surface leading to volcanic eruptions. If the deep interior of the Moon contains so much magma, why don’t we see spectacular volcanic eruptions at its surface?
The driving force for vertical movement of magma is the density difference between the magma and the surrounding solid material, making the liquid magma move slowly upwards like a bubble. The lighter the liquid magma is, the more violent the upward movement will be.
To determine the density of lunar magma, Wim van Westrenen and his colleagues synthesised moon rock in their laboratory in Amsterdam, using the composition derived from Apollo samples as their “recipe”.
The pressures and temperatures close to the core of the Moon are more than 45,000 bar and about 1500 degrees. It is possible to generate these extreme conditions with small samples, heating them with a high electric current while squashing them in a press.
By measuring the attenuation of a powerful synchrotron X-ray beam at the ESRF traversing the sample both solid and molten, the density at high pressure and high temperature could be measured.
“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. Without a bright beam of X-rays, you cannot measure these density variations”, said Mohamed Mezouar from the ESRF.
The measurements at the ESRF were combined with computer simulations to calculate the magma density at any location in the Moon.
Nearly all the lunar magmas were found to be less dense than their solid surroundings, similar to the situation on Earth. There is one important exception: small droplets of titanium-rich glass first found in Apollo 14 mission samples produce liquid magma as dense as the rocks found in the deepest parts of the lunar mantle today. This magma would not move towards the surface.
Such titanium-rich magma can only be formed by melting titanium rich solid rocks. Previous experiments have shown that such rocks were formed soon after the formation of the Moon at shallow levels, close to the surface.
How did they get deep into the mantle? The scientists conclude that large vertical movements must have occurred early in the history of the Moon, during which titanium-rich rocks descended from near the surface all the way to the core-mantle boundary.
The results are published in the Journal Nature Geosciences.