These samples of basalt, an igneous rock made up of rapidly cooled lava, were collected from the near side of the moon by the Chang'e 5 mission and are the youngest samples collected on any lunar mission, making them an invaluable resource for those studying the geological history of the moon.

In order to get an estimate of how deep within the moon the Chang'e 5 lava came from, the team conducted high-pressure and high-temperature experiments on a synthetic lava with an identical composition. Previous work from Chinese scientists has determined that the lava erupted about 2 billion years ago and remote sensing from orbit has showed it erupted in an area with very high abundances of potassium, thorium and uranium on the surface, all of which are radioactive and produce heat. Scientists believe that, in large amounts, these elements generate enough heat to keep the moon hot near the surface, slowing the cooling process over time.

"Using our experimental results and thermal evolution calculations, 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.

These findings contradict the previous theory that the temperature of the moon's outer portions was too low to support melting of the shallow interior 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 the mantle closer to the surface cooled first as the surface of the moon gradually lost heat to space, and that younger lavas like the one collected by Chang'e 5 must have come from the deep mantle where the moon would still be hot. This theory was backed by data from seismometers placed during the Apollo moon landings, but these findings suggest that there were still pockets of shallow mantle hot enough to partially melt even late into the moon's 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 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 towards 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."

Research Report:A shallow mantle source for the Chang'e 5 lavas revealshow top-down heating prolonged lunar magmatism

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