Carbon said to be responsible for lunar fire fountains

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Researchers have been trying to understand what gases may have been responsible for fire fountain eruptions on Moon that caused formation of tiny beads of volcanic glass, which was found on its surface during Apollo missions.

Fire fountains are a common occurrence in Hawaii, but the key drivers of these eruptions are volatile gases that are available on Earth. Which gases would have made it possible for eruptions to occur on the moon is a mystery that hasn’t been solved until now.

Researchers from Brown University and the Carnegie Institution for Science claim to have identified the volatile gas responsible for the fire fountains on moon.

According to the team of scientists, lava associated with lunar fire fountains contained significant amounts of carbon. As it rose from the lunar depths, that carbon combined with oxygen to make 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.

“The question for many years was what gas produced these sorts of eruptions on the Moon,” said Alberto Saal, associate professor of Earth, environmental, and planetary sciences at Brown and corresponding author of the new research. “The gas is gone, so it hasn’t been easy to figure out.”

Saal and colleagues detected water in lunar volcanic beads after which they even found sulfur, chlorine and fluorine. Though it was apparent after the discovery of these elements on moon that the Earth’s natural satellite isn’t completely devoid of volatiles as once thought, but none of these were consistent with fire fountain eruptions. For example, if water had been the driving force, there should be mineralogical signatures in recovered samples. There are none.

So, Saal and his colleagues carefully analysed glass beads brought back to Earth from the Apollo 15 and 17 missions and 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.

Though the team had been able to detect other volatiles from the samples, measurement of carbon was something that remained elusive due to the high detection limits of the available analytical techniques. This is where an invention by Erik Hauri from Carnegie Institution for Science came to the rescue. Hauri had 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.”

Through their analysis researchers found that the samples contained initially 44 to 64 parts per million carbon. Having confirmed existence of carbon, researchers developed 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. Saal and colleagues changed several parameters to match the composition and conditions affecting lunar magma.

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.

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. Saal and his colleagues have shown previously that Earth and the Moon have similar concentrations of water and other volatiles. They have also shown that hydrogen isotope ratios from lunar samples are similar to that of Earth.

If volatile reservoirs on the Earth and Moon do indeed share a common source, it has implications for understanding the Moon’s origin. 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.