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Lunar water came from Earth

Posted: 10 May 2013

Born in fire, but not born dry - the Moon's interior contains substantial water stolen from Earth following the Moon's creation in the molten debris from a cataclysmic collision between Earth and another still-born planet, according to new analysis of lunar rock samples.

The Moon retained some of Earth's water when it formed. Image: NASA/JPL/USGS
The collision model is the prevailing theory of how our Moon was formed, some fifty million years after the formation of the Solar System. The impactor, a protoplanet that scientists have christened Theia, was utterly destroyed in the collision, while large swathes of Earth's mantle were ripped from our planet, settling into a ring of molten debris that encircled the battered Earth and gradually coalesced into the Moon over a 100 million years.

The expectation had been that any volatiles (easily evaporated materials) such as water and hydrogen would have boiled away in the heat of the impact. Initial studies of the samples of lunar rock brought back from the Moon by the Apollo astronauts suggested the rocks - which had come from the interior of the Moon, brought to the surface by volcanic activity - showed that they were bone dry, as expected. This, however, changed in 2011 when scientists led by Erik Hauri of the Carnegie Institution of Washington were able to measure tiny quantities of water in the rocks. Now, their follow-up studies have shown that this water has been present within the Moon since the beginning.

Hauri's team inspected so-called 'melt inclusions', which are tiny glass crystals of olivine brought to the surface through volcanic eruptions. Within these melt inclusions are volcanic gases, plus traces of water. To try and figure out where the volatiles originated from, the team measured the ratio of the abundances of deuterium (an isotope of hydrogen with an extra neutron) and normal hydrogen within the water molecules and found the ratio to be identical to that found in water in Earth's mantle. It has already been determined that Earth's water came from asteroids known as carbonaceous chondrites, which also share the same deuterium/hydrogen ratio, rather than comets which sport a different ratio, having formed in a different, more distant, region of the Solar System. Carbonaceous chondrites are some of the oldest solid materials in the Solar System, forming even before the planets did.

What this means is that not only was ancient Earth wet prior to the moon-forming collision, but that Earth then passed some of this water on to the Moon. Furthermore, water is not the only volatile thought to have made the trip from Earth to the Moon: chlorine in the Apollo samples is a perfect isotopic match for chlorine found on Earth, for example, while carbon, fluorine, lithium and sulphur are also suspected says Alberto Saal, a member of Hauri's team from Brown University.

So rather than being born dry, the Moon was born very wet and scientists are struggling to understand how. Perhaps the collision was less violent than had been presumed, meaning that many of the volatiles did not boil away, but this would need new models that describe the collision.

A sample of Moon rock brought back by Apollo 15 astronauts. Image: Arizona State University, Tom Story
"Before last year it was thought that most of the material in the Moon came from the impactor [Theia]," says Hauri. "[Scientists] are recognising some wrinkles in the physics of impacting bodies that can allow most of the Moon to be formed from material that came from the Earth in the aftermath of the giant impact. It is entirely possible that as we recognise more detail in the aspects of planet formation, it will become straightforward to make a prediction of how much water can survive such impacts."

It should be said that the Moon's interior water has a different origin to the relatively scant water on the lunar surface, manifest as ice in permanently-shadowed craters or as molecules hopping around the Moon following the morning terminator, which was likely deposited there by subsequent asteroid and comet impacts and has been discovered only in the past few years thanks to India's Chandrayaan-1 spacecraft and NASA's kamikaze LCROSS mission that crashed into a crater on the Moon. On the other hand, scientists have had the Apollo lunar rock samples in their hands for the past 40 years. Why is this only coming to light now?

"It really is the new technology," says Hauri, who used a NanoSIMS 50L ion probe, which is a mass spectrometer that allows scientists to measure elemental composition to high precision, to measure the very low water content and hydrogen isotopes. "Geochemists tried very hard during the Apollo era to measure water in the lunar [samples], but the amounts were largely below the limits of detection of the instruments at that time."

In some ways, the finding solidifies the impact model for the origin of the Moon, showing that our natural satellite was borne out of material ripped from Earth. On the other hand, it seems there are still some big details in the model that need to be worked out, fully explaining the presence of water in the Moon, before we can conclusively describe the process by which the Moon was formed.

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