Moon Rock Mineral Hints at Watery Lunar Past

A mineral discovered in a volcanic rock from the moon has been found to contain elements that may point to the moon’s watery past, a new study suggests.

Scientists found the mineral apatite in a type of lunar volcanic rock called basalt, which contained the same so-called volatile elements as those found in common igneous rocks on Earth. The moon rock was collected by Apollo 14 astronauts in 1971.

Volatile elements, which include hydrogen, chlorine and sulfur, are those that most easily escape from a rock when it is heated. The presence of these volatile elements in rocks can provide insight into the crust and atmosphere of a planet or moon.
“Apatite is a mineral that can incorporate water into its structure, but it can also grow in a water-free environment,” Jeremy Boyce, the study’s lead researcher at the California Institute of Technology in Pasadena, Calif., told in an e-mail interview. “The amount of water in the apatite is – at least in part – a function of the water content in the surrounding magma from which the apatite grew.”

In other words, apatite that grew in the presence of water will contain water, whereas apatite that grew in a water-free environment will not. So, by comparing the volatile elements in the moon’s apatite to those on Earth – which formed in water for sure – scientists can determine if the lunar mineral also formed in water, which could shine a light on the present and past composition of the moon.

What this means for the moon

“Measuring water content in lunar minerals is one way of estimating the volatile content of the moon,” Boyce said. “Water (made of volatile elements hydrogen and oxygen) is the most common volatile species on Earth, and is also essential for life as we know it, so it is often what people refer to when speaking of volatiles.”

The research is detailed in the July 22 issue of the journal Nature and suggests that at least some lunar minerals are as rich in volatile elements as their terrestrial counterparts.

The existence of water on the moon had long been speculated until last year, when scientists confirmed the presence of water molecules on the moon’s surface and water ice in shadowed lunar craters.

Based on the prevailing theory of its formation, the moon is commonly thought to contain fewer volatile elements than the Earth.

“The most popular theory on the formation of the moon is that the Earth was hit by a “Mars-sized” object, and that the Earth and moon are made of the materials that survived the impact,” Boyce said. “This impact caused these materials to be heated, and some of the volatile elements were lost to space.”

Yet, this view has recently been challenged by a report of the presence of small quantities of volatile elements, and fluorine, in tiny mineral droplets of lunar volcanic glass called spherules.

This was interpreted as evidence of significant water content in the moon’s pre-eruptive magma – a conclusion that relied on modeling the diffusion of volatiles out of the glasses during the eruption.

A volatile-rich environment?

The new study conducted by Boyce and his colleagues provides additional evidence, from a separate source, for a more volatile-rich moon.

The researchers measured concentrations of water, chlorine and sulfur in a lunar apatite and compared it to the hundreds of similar analyses that have been made of apatites from Earth. The study produced some surprising results.

“We found that the volatile content of lunar apatite was indistinguishable from that of the population of terrestrial apatites,” Boyce said. “This was unexpected because the Earth is ‘wet’ and the moon is ‘dry.'”

The findings appear to suggest similarities in the Earth and moon’s geo-chemical history.

“These measurements tell us that conditions existed in lunar rocks that could create apatite crystals that are very similar to those apatite crystals that grew from ‘wet’ magmas on Earth,” Boyce said. “So we know the moon has at least a little bit of water and these other volatile elements. But how much exactly is a matter of some debate.”

But, since the apatite could have been altered by eruption, the researchers cannot definitely infer the concentrations of these elements in the parent magma. Still, this study will give scientists a better understanding of the volatile budget of lunar apatites, said Boyce.

Further research could also enable researchers to make measurements within a larger context.

“We need to work on relating the volatile content of the apatite to the moon as a whole,” Boyce said. “There are several steps required in order to make that connection, and we don’t know enough to make a robust estimate of the water content of the whole moon.”

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