A new discovery by a group of geologists at Arizona State University's School of Earth and Space Exploration shows that the moon has seen small but widespread eruptions of basaltic lava during the last 50 million years.
The discovery was announced in a paper published online Oct. 12 in Nature Geoscience.
Sarah Braden, a recent School of Earth and Space Exploration graduate, is the lead author of the study. Her colleagues include Julie Stopar, Samuel Lawrence and Mark Robinson, all researchers at the school, and Carolyn van der Bogert and Harald Hiesinger of the Westfälische Wilhelms-Universität Münster in Germany.
The team was able to identify 70 small volcanic features scattered across the moon's dark volcanic plains, or maria, according to an Arizona State University press release.
The features show as a combination of smooth, low, rounded mounds near patches of rough, blocky terrain. The scientists refer to these unusual areas as irregular mare patches.
"Finding previously unknown geologic features on the lunar surface is extremely exciting," says Braden, according to the release.
The features are too small to be seen from Earth, averaging less than a third of a mile across their largest dimension. One feature named Ina has been known for a while now, after being photographed from lunar orbit by Apollo 15 astronauts in the 1970s.
A number of studies indicated that Ina could be 10 million years or less, but only a few irregular mare patches were known and their significance was unclear, according to the release.
It wasn't until the scientists had high-resolution images showing the entire moon that they were able to realize the full extent and significance of the small lava features.
The images are a product of two Narrow Angle Cameras that form part of the Lunar Reconnaissance Orbiter Camera system, according to the release.
The ages of the irregular mare patch features come from studies of crater sizes and numbers within a given area by Braden, who was assisted by van der Bogert and Hiesinger.
The crater-counting dates are linked to laboratory ages provided by Apollo and Luna samples. Results show that instead of lunar volcanism stopping suddenly a billion years ago, it ended more gradually, continuing until less than 50 million years ago.
Activity at Ina ended about 33 million years ago, according to the release. At another irregular mare patch, Sosigenes, it stopped around 18 million years ago.
"The existence and young age of the irregular mare patches provides a new constraint for models of the lunar interior's thermal evolution," Braden says. "The lunar mantle had to remain hot enough for long enough to provide magma for the small-volume eruptions."
Robinson added that the discovery is hard to reconcile with what's currently believed about the temperature of the moon's interior.
"These young volcanic features are now prime targets for future exploration, both robotic and human," he said, according to the automaker.
The discovery gives the moon's volcanic history a new chapter.
"Our understanding of the moon is drastically changed by the evidence for volcanic eruptions at ages much younger than previously thought possible, and in multiple locations across the lunar maria," said Braden.
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