The ancient surface of the moon once spewed lava geysers. Now, scientists believe they know what drove those eruptions.

Fire fountains are a form of colcanic eruption which occurs often in Hawaii. They require the presence of volatile compounds mixed in with the erupting lava, turning into gas as the lavas rise from the depths.

Expansion of that gas causes lava to blast into the air once it reaches the surface, like when you take the lid off a shaken bottle of Coke. Corresponding authors Alberto Saal, associate professor of earth, environmental, and planetary sciences at Brown University, says:

“The question for many years was what gas produced these sorts of eruptions on the Moon. The gas is gone, so it hasn’t been easy to figure out."

The research, published in Nature Geoscience, proposes that lava linked with lunar fire fountains contained significant amounts of carbon.

As it rose from the lunar depths, carbon combined with oxygen created substantial amounts of carbon monoxide (CO) gas. That CO gas was responsible for the fire fountains that sprayed volcanic glass over parts of the lunar surface.

Saal and his colleagues carefully analyzed glass beads brought back to Earth from the Apollo 15 and 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.

Even though 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.

NanoSIMS Ion Probe

Erik Hauri from Carnegie Institution for Science developed a state-of-the-art ion probe technique reducing the detection limits of carbon by two orders of magnitude. That allows a measurement of as low as 0.1 part per million.

“This breakthrough depended on the ability of Carnegie’s NanoSIMS ion probe to measure incredibly low levels of carbon, on objects that are the diameter of a human hair,” said Hauri. “It is really a remarkable achievement both scientifically and technically."

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

Having detected carbon, the researchers 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 model showed that carbon, as it combines with oxygen to form CO gas, would have degassed before other volatiles.

“Most of the carbon would have degassed deep under the surface,” Saal said. “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."

In addition to providing a potential answer to longstanding questions surrounding lunar fire fountains, the findings also serve as more evidence that some volatile reservoirs in the Moon’s interior share a common origin with reservoirs in the Earth, the researchers say.

Scientists believe the Moon formed when Earth was hit by a Mars-size object very early in its history. Debris from that impact accreted to form the Moon.

“The volatile evidence suggests that either some of Earth’s volatiles survived that impact and were included in the accretion of the Moon or that volatiles were delivered to both the Earth and Moon at the same time from a common source—perhaps a bombardment of primitive meteorites,” Saal said.

Diane T. Wetzel, Erik H. Hauri, Alberto E. Saal & Malcolm J. Rutherford Carbon content and degassing history of the lunar volcanic glasses Nature Geoscience (2015) doi:10.1038/ngeo2511

Illustration: gnuckx

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