Tuesday, February 21, 2012

ESRF X-rays illuminate lunar interior

Image of artificial moon rock sample, measuring about half
millimeter across, made with an electron microprobe at ambient
temperature after the experiment with X-rays. The fragmenta-
tion of the sample occurred when it was extracted from the
small diamond cylinder in which it had been melted under high
pressure and temperature [ESRF/Nature].
Does the Moon still have even a small, warm liquid core? The answer can only be apparent indirectly, behind its dance movements and the combined angular momentum of it juggled components; the Moon’s anisotropy. If, as investigators now claim, the Moon’s outer surface is still shrinking or, in some cases, stretching, other outward evidence of even a small warm and liquid core can only be discovered indirectly. Why, for example, is any evidence of volcanism on the Moon’s surface at least a billion years old?

A science team in the Netherlands claims to have discovered one answer, the natural buoyancy of molten but poorly mixed constituent materials closer to the Moon’s core. The world’s press is reporting their more subtle investigation, using X-rays, with headlines about future lunar volcanism, which contrasts with their own press release and it's secondary headline:

Deep lunar magma is too heavy to produce active volcanoes

"Scientists have now identified a likely reason for this peaceful surface life: 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 scientists 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 results are published in the Journal Nature Geosciences on 19 February 2012.

"The team was led by Mirjam van Kan Parker and Wim van Westrenen from VU University Amsterdam and comprised of scientists from the Universities of Paris 6/CNRS, Lyon 1/CNRS, Edinburgh, and the European Synchrotron Radiation Facility (ESRF) in Grenoble.

Exploded schematic of the high-pressure cell
assembly for the ESRF synchrotron  X-ray
experiment. The artificial moon rock samples
(orange) were placed inside the ring-shaped, natural
diamond sample holder (grey), and surrounded by
a large disk-shaped container (red) [ESRF/Nature].
"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 (European Synchrotron Radiation Facility) in Grenoble, 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”, says 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. “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”, concludes Wim van Westrenen."

Reference: Mirjam van Kan Parker, et al., Neutral buoyancy of titanium-rich melts in the deep lunar interior, Nature Geoscience advanced online publication, 19 February 2012


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