Travel to Hawaii today and you’ll be able to witness spectacular “fire fountains” – giant bursts of liquid lava that can shoot hundreds of meters into the air. They are caused by volatiles: elements and compounds with low boiling points such as carbon dioxide, water and hydrogen. These turn into a gas as the lava rises and, when they expand, they cause the lava to erupt when it reaches the surface.
But that led to a problem for astrophysicists concerning the Moon. Samples returned by the Apollo 15 and 17 missions revealed tiny beads of volcanic glass on the lunar surface, a sure sign that the Moon had its own fire fountains long in the past, more than three billion years ago. The Moon, though, was thought to have been devoid of many volatiles including hydrogen and carbon. How, then, were these fountains able to form?
Researchers from Brown University and the Carnegie Institution for Science in Maryland now think they have an answer. Using a sophisticated ion probe technique, they took another look at the samples, and found that they once contained 44-64 parts per million of carbon. It is thought that this carbon combined with oxygen to form carbon monoxide (CO) gas under the lunar surface. As lava rose, the gas expanded, causing fire fountains on the Moon. When the lava erupted it cooled, forming the volcanic glass that can be seen today. The research was published in the journal Nature Geoscience.
"That suggests carbon was the driving process in [the Moon’s] early stages," said Brown University's Alberto Saal, a co-author on the research, in a statement.
The discovery was all the more difficult because any gas that was once on the Moon has now gone. So the team examined tiny dots of molten magma, known as melt inclusions, that were trapped in crystals of a mineral called olivine using the new and more accurate technique. These crystals trapped some of the carbon from that ancient magma.
"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 Erik Hauri from the Carnegie Institution for Science, a co-author on the study, in the statement. "It is really a remarkable achievement both scientifically and technically."
The discovery also adds further evidence to the theory that the Moon and Earth share a common origin, as the volatile reservoirs on the Moon mimic those on Earth. It’s thought that the Moon was formed by the resultant debris of a Mars-sized object striking Earth early in its life.
Image in text: Melt inclusions on a lunar sample. Saal Lab/Brow University.