Deposits of water inside the moon are from the same source as the water found on Earth, according to surprising new research that raises questions about how, exactly, lunar formation occurred 4.5 billion years ago.

In a NASA-funded study published in Science, a team of experts evaluated extremely small quantities of water inside volcanic glass samples brought back from the Moon. Their goal was to zero in on hydrogen and its heavier isotope, deuterium: water molecules have different ratios of deuterium to hydrogen, depending on where they originated in the solar system. "Think of this as looking for a fingerprint for water's origin," study author Alberto Saal, Ph.D., an associate professor of Geological Sciences at Brown University, told The Verge. "This ratio is that fingerprint."

Earlier research suggested that the deuterium-hydrogen ratio from Moon water samples was quite high, Saal said. Such a ratio indicated to experts that the Moon's water was derived from collisions with comets. This latest study, however, calls that idea into question: using "the best samples we have so far" from the Moon, researchers found that the deuterium-hydrogen ratio appears to be quite low. In fact, it's about the same as the ratio from water found in carbonaceous condrites — meteorites thought to have supplied Earth with its own water supply. "What this tells us is that water on Earth shares the same source as water on the Moon," Saal said. "And not from comets."

That's interesting enough on its own, but the research is more significant in the context of the Moon's origins. Right now, experts posit that the Moon was formed during a massive impact that essentially broke off a piece of our planet. Such an impact, however, would have generated enough heat to vaporize any water. "How did the hydrogen survive? Honestly, we don't know," Saal said. "There's a problem with this theory."

And this isn't the first recent study to poke holes in the longstanding idea of a "Big Impact" triggering the Moon's formation: Earlier this year, scientists discovered significant quantities of water deep inside ancient Moon rock samples, also suggesting that the Moon has harbored water since it was formed.

Despite the ongoing uncertainty, there's no doubt that scientists will continue to unravel the mystery: NASA in 2017 will deploy a water-hunting rover designed, in part, to analyze isotopes in recovered water samples.