Very small pieces of volcanic glass found on the surface of Moon by Apollo 15 and 17 astronauts are the products of volatile-rich, fire-fountain eruptions. A group of researchers, led by Dr Alberto Saal of Brown University, has now identified the gas that drove those eruptions.

Fire fountains, a type of eruption that occurs frequently in Hawaii, require the presence of volatiles mixed in with the erupting lava.

“The question for years was what gas produced these eruptions on the Moon. The gas is gone, so it hasn’t been easy to figure out,” said Dr Saal, who is a co-author of a study published in the journal Nature Geoscience.

The study suggests that lunar lava associated with fire fountains contained significant amounts of carbon. As it rose from the lunar depths, that carbon combined with oxygen to make substantial amounts carbon monoxide gas.

That carbon monoxide was responsible for the fire fountains that sprayed volcanic glass over parts of the lunar surface.

For their study, Dr Saal and co-authors analyzed glass beads brought back to Earth from the Apollo 15 and Apollo 17 missions. In particular, they looked at samples that contained melt inclusions, tiny dots of molten magma that became trapped within crystals of olivine. The crystals trap gases present in the magma before they can escape.

Although other volatiles were previously detected in the lunar volcanic glasses and melt inclusions, the measurement of carbon remained elusive due to the high detection limits of the available analytical techniques.

The scientists analyzed the melt inclusions using secondary ion mass spectroscopy. They calculated that the samples contained initially 44 to 64 parts per million of carbon.

Having detected carbon, they devised a theoretical model of how gases would escape from lunar magma at various depths and pressures, calibrated from the results of high-pressure lab experiments. The model had long been used for Earth.

The scientists changed several parameters to match the composition and conditions affecting lunar magma.

The model showed that carbon, as it combines with oxygen to form carbon monoxide gas, would have degassed before other volatiles.

“Most of the carbon would have degassed deep under the surface. Other volatiles like hydrogen degassed later, when the magma was much closer to the surface and after the lava began breaking up into small globules. That suggests carbon was driving the process in its early stages,” Dr Saal said.

In addition to providing a potential answer to the lunar fire fountains mystery, the findings also serve as more evidence that some volatile reservoirs in the lunar interior share a common origin with reservoirs in the Earth.

The amount of carbon detected in the melt inclusions was found to be very similar to the amount of carbon found in basalts erupted at Earth’s mid-ocean ridges.

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Diane T. Wetzel et al. Carbon content and degassing history of the lunar volcanic glasses. Nature Geoscience, published online August 24, 2015; doi: 10.1038/ngeo2511