Unusual Deposits on Moon’s Near Side Suggest Possibility of ‘Unique Volcanic Process’
Three University of Maryland researchers helped unravel a lunar mystery hiding in plain sight.
In a new analysis of data collected remotely by NASA’s Moon Mineralogy Mapper, University of Maryland researchers revealed that the central region of the moon’s near side has been hiding a Utah-sized secret in plain sight. The Sinus Aestuum dark mantle deposit—home to the glass and mineral remnants of volcanic eruptions billions of years ago—is unlike anything else found on the moon.
“At the center of the lunar near side, you can see it,” study co-author Jessica Sunshine, an astronomy and geology professor at UMD, said of the deposit. “It’s big, it’s dark—we just didn't know it was different, even though we've been staring at it for millennia."
Published in the Journal of Geophysical Research: Planets, the UMD team’s research confirmed that, unlike other materials formed by volcanic eruptions on the moon, the Sinus Aestuum deposit is extremely rich in a mineral called spinel. These deposits also cover a much larger area than previously thought, with UMD researchers extending the deposit’s known range by several hundred kilometers.
“It’s over four times larger than the next largest explosive volcanic deposit on the moon,” said the study’s lead author, UMD geology Ph.D. student Cosmo Sikes. “It has a mineralogy you don't see anywhere else on the lunar surface, and because of that, it's telling us that some unique volcanic process has occurred.”
The research team, which also includes Geology Associate Professor Megan Newcombe, is unsure of the exact volcanic mechanism that created these deposits. However, their circular distribution suggests they could have come from a large eruption in a centrally located source.
While eruptions were common in the moon’s early history, the unusual composition of these deposits raises new questions about volcanism on the moon. To create an Earth-based equivalent of the Sinus Aestuum deposits, Sikes is now synthesizing spinels in Newcombe’s Planetary Volcanism Laboratory, which has furnaces that can reach 2,900 degrees Fahrenheit and mimic the conditions of a lunar volcano.
Through this process, the researchers hope to learn whether certain features of the moon’s surface can be explained by a particular spinel composition.
“This may reveal the formation mechanism of the spinel because different spinel compositions are thought to form in different ways,” Newcombe explained. “For example, spinels containing a lot of aluminum may have formed when magma reacted with the lunar crust during its journey to the surface.”
Studying deposits like the ones found in the Sinus Aestuum region enables researchers to work backward, helping them piece together volcanic processes that happened millions or billions of years ago. This same approach could be used to explain how volcanism functions in a wide range of planetary environments.
“We see volcanism happening under certain conditions here on Earth, but on the moon, the gravity is different, the heat is different and the chemistry is different,” Sunshine said. “So anytime we can figure out how a planetary surface produces volcanism in its context, it can tell us more about how volcanism works. And it's almost always more complex than we thought.”
Sikes, who is writing his dissertation on the Sinus Aestuum deposit, looks forward to furthering scientists’ understanding of the moon’s volcanic history.
“Finding something that hasn't been found before is exciting,” Sikes said, “and it will give us an idea of the volcanic processes that shaped the lunar surface.”
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Their paper, “Investigations of the Sinus Aestuum DMD: An Anomalously Large and Compositionally Distinct Lunar Pyroclastic Deposit,” was published in the Journal of Geophysical Research: Planets on August 27, 2025.
This research was supported by the Future Investigators in NASA Earth and Space Science and Technology program (Grant No. 80NSSC24K0453) and the Geophysical Exploration of the Dynamics and Evolution of the Solar System program (Grant No. 80NSSC19M0216). This article does not necessarily reflect the views of these organizations.
