Molten magma may have been present on the moon’s surface longer than previously thought, new study finds
A new study from the University of Florida analyzing moon rock samples from a Chinese lunar exploration mission is rewriting our understanding of how the moon is cooled.
Stephen Elardo Ph.D., an assistant professor of geology at the UF College of Liberal Arts and Sciences, has found that lava on the far side of the moon likely cooled much later than previously thought, contradicting previous theories on how and when the moon’s layers formed.
The samples of basalt — an igneous rock formed from rapidly cooled lava — were gathered by China’s Chang’e 5 mission and are the first samples collected from the far side of the moon, as well as the youngest obtained on any lunar mission, making them an invaluable resource for those studying the moon’s geological history.
“Using radioactive dating, we put together a simple model showing that an enrichment in radioactive elements would have kept the Moon's upper mantle hundreds of degrees hotter than it would have been otherwise, even at 2 billion years ago,” explained Elardo.
In order to get an estimate of when these samples cooled into basalt, the team tested their chemical composition. Similar tests on previous lunar mantle samples found traces of elements like potassium, thorium and uranium and phosphorus, all of which produce heat in high concentrations.
Scientists believe that, in large amounts, these elements generate enough heat to keep magma molten near the Earth's surface, slowing the cooling process over time.
These findings contradict the previous theory that the surface’s temperature was too low to support molten magma by that time and may challenge the hypothesis about how the moon cooled. Prior to this study, the generally accepted theory was that the moon cooled from the top down. It was presumed that magma closer to the surface cooled first as the surface of the moon gradually lost heat to space, and that as you descended closer to the mantle you would find basalt rocks that cooled more recently than the basalt at the crust.
This theory was backed by data from seismometers placed during the first Apollo moon landing, but these findings suggest that there were still pockets of surface-level magma even late into the cooling process.
“Lunar magmatism, which is the record of volcanic activity on the moon, gives us a direct window into the composition of the Moon's mantle, which is where lavas/magmas ultimately come from,” said Elardo. “We don't have any direct samples of the Moon's mantle like we do for Earth, so our window into the composition of the mantle comes indirectly from its lavas.”
Establishing a detailed timeline of the moon’s evolution represents a critical step toward understanding how other celestial bodies form and grow. Processes like cooling and geological layer formation are key steps in the “life cycles” of other moons and small planets. As our closest neighbor in the solar system, the moon offers us our best chance of learning about these processes.
“My hope is that this study will lead to more work in lunar geodynamics, which is a field that uses complex computer simulations to model how planetary interiors move, flow and cool through time,” said Elardo. “This is an area, at least for the moon, where there's a lot of uncertainty, and my hope is that this study helps to give that community another important data point for future models.”