SAD LOVE QUOTES: Lunar hot spot: Volcanoes found on far side of the moon offer tantalising clues to its thermal history

Tuesday, July 26, 2011

Lunar hot spot: Volcanoes found on far side of the moon offer tantalising clues to its thermal history

Last updated at 11:30 AM on 26th July 2011

New images have revealed a curious 'hot spot' on the far side of the moon to be a small volcanic province created by the upwelling of silicic magma, according to scientists.

The unusual location of the province, and the surprising composition of the lava that formed it, offer tantalising clues to the moon's thermal history, they said.

The hot spot is a concentration of a radioactive element thorium sitting between the very large and ancient impact craters Compton and Belkovich that was first detected by Lunar Prospector's gamma-ray spectrometer in 1998.

Volcano district: A map of the abundance of thorium on the moon

Volcano district: This map of the abundance of thorium on the moon shows that most of this radioactive element is concentrated in a region on its near side (left). But there is also a small hot spot called the Compton-Belkovich Thorium Anomaly (labelled C-B) on the side of the moon that faces away from Earth

The Compton-Belkovich Thorium Anomaly, as it is called, appears as a bullseye when the spectrometer data are projected onto a map, with the highest thorium concentration at its centre.

Recent observations, made with the powerful Lunar Reconnaissance Orbiter (LRO) optical cameras, have allowed scientists to distinguish volcanic features in terrain at the centre of the bullseye.

High-resolution 3D models of the terrain and information from the LRO Diviner instrument have revealed geological features diagnostic not just of volcanism but also of much rarer silicic volcanism.

The volcanic province's very existence will force scientists to modify ideas about the moon's volcanic history, said lead researcher Bradley Jolliff, of Washington University in St. Louis.

He said: 'To find evidence of this unusual composition located where it is, and appearing to be relatively recent volcanic activity is a fundamentally new result and will make us think again about the moon's thermal and volcanic evolution.'

The study is published in the journal Nature Geoscience.

Computerised image of the volcanic terrain: At the centre of the province is an irregular depression that might well be a caldera and at its edges are domes with features that suggest they were formed by the intrusion of high-viscosity silicic lava, a type of lava rare on the moon

Lunar volcanism is very different from terrestrial volcanism because the moon is a small body that cooled quickly and never developed rock-recycling plate tectonics like those on our planet.

The moon, thought to have been created when a Mars-size body slammed into Earth about 4.5billion years ago, was originally a hellish world covered by a roiling ocean of molten rock some 400kms deep.

But because the moon was small and had no atmosphere, the magma ocean cooled quickly, within perhaps 100million years.

Eventually lighter minerals such as feldspar crystallised out of the magma and floated to the top to create huge masses of feldspathic rock that formed the lunar highlands. Denser iron- and magnesium-rich minerals sank when they crystallised, forming the upper part of the moon's mantle.

Cameras aboard the Lunar Reconnaissance Orbiter, launched in 2009, show that the centre of the Compton-Belkovich Thorium Anomaly was relatively reflective in visible light compared to its surroundings

The differentiation of the crust and mantle was followed by a wave of volcanic activity between about three to four billion years ago, when basaltic lavas erupted on the lunar surface, filling old impact craters and other low spots to form the lunar mare.

One of the mysteries of lunar volcanism is the unequal distribution of these flood basalts. Nearly a third of the moon's near side is covered by ancient flood basalts but its far side, where the crustal rocks are thicker, has much less.

Moreover, almost all of the volcanism on the moon is basaltic rather than silicic, enriched in minerals containing the elements iron and magnesium rather than the elements silicon and aluminium.

Earth's continental crust, which reflects active geological processes such as subduction, magma intrusion and mountain building, includes many rocks whose compositions are intermediate between basalt and silica-rich rocks like granite, which are common on Earth.

On the moon, on the other hand, there are many basaltic rocks and only a small fraction of granite. Rocks of intermediate composition are all but missing.