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"We discovered this polarity change in the data and created a detailed geophysical image of the subsurface up to a few hundred meters depth."

"Our paper is really providing the first geophysical evidence to see this electromagnetic permittivity changed from a small value for the paleoregolith to a large value for the lava flows," Zhu said. The scientists observed changes in polarity as the electromagnetic pulses traveled down through the dense lava rock and the paleoregolith, allowing the team to distinguish between the different layers. Zhu said his team developed a four-step data processing flow to enhance the signal and suppress noise in the data. Previous studies have examined the dataset, created when the Yutu rover sent electromagnetic pulses into the lunar underground and listened as they echoed back. This is the very unique contribution of this work." "Our findings provide a constraint on what happened between two and three billion years ago. "Lunar scientists count craters on the moon and use computer models to determine the rate that regolith is produced," Zhu said. Over time, the rock breaks down into dust and soil, called regolith, with repeated asteroid impacts and space weathering, only to be buried by subsequent lava flows, the scientists said. The moon has experienced volcanic activity throughout its history, depositing lava rock on the surface. The findings suggest the paleoregolith formed much faster than previous estimates of 6.5 feet per billion years, the scientists said. The researchers identified a thick layer of paleoregolith, roughly 16 to 30 feet, sandwiched between two layers of lava rock believed to be 2.3 and 3.6 billion years old. The team, led by Zhu, conducted new analysis of radar data collected by China's Chang'e 3 mission in 2013, which performed the first direct ground radar measurements on the moon. "These layers have been undisturbed since their formation and could be important records for determining early asteroid impact and volcanic history of the moon." "Using careful data processing, we found interesting new evidence that this buried layer, called paleoregolith, may be much thicker than previously expected," said Tieyuan Zhu, assistant professor of geophysics at Penn State.