The last time we had boots on the moon’s surface was almost 45 years ago with Apollo 17. And the samples gathered during this mission are helping shape what we know about the moon decades later.
For the longest time, scientists assumed the moon’s innards were mostly dry. Those assumptions were challenged in 2008 when a group of scientists detected trace amounts of water in volcanic glass beads astronauts brought back to Earth from the Apollo 15 and 17 missions. In 2011, scientists revealed the beads hold similar amounts of water as some basalts (volcanic rocks) on Earth. That suggests the moon’s interior could be home to a significant amount of water.
“The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise ‘dry’ mantle,” says Ralph Millikin. He’s the lead author of the new research and an associate professor in Brown’s Department of Earth, Environmental and Planetary Sciences.
Since we’re not putting humans on the moon anytime soon, it’s a question for satellites to answer. Armed with spectrometers, scientists measure the light that bounces off the moon’s surface. Depending on which wavelengths of light are absorbed or reflected, scientists can get a handle on what minerals are present.
For this study, scientists used data from the Moon Mineralogy Mapper aboard India’s now silent Chandrayaan-1 lunar orbiter. The orbiter is still circling the moon, but contact was suddenly lost ten months into its 2-year mission.
Sounds easy, but there was one major hurdle. The moon’s surface heats up as the sun hits it, especially where the rich volcanic rock deposits are located. And the thermally emitted radiation just happens to be at the same wavelengths that scientists use to look for water.
Milliken and his fellow researchers needed to account for the radiation to see how much water is there. Using measurements of samples they already have from the moon along with a detailed temperature profile of certain areas on the moon’s surface, the scientists were able to correct for the thermally emitted radiation.
What they found could have big implications for companies wanting to mine the moon. Evidence of water sits in almost all of the large pyroclastic deposits that have already been mapped on the moon’s surface. That includes deposits where the volcanic glass beads came from during the Apollo 15 and 17 missions.
The big news here is evidence of water is spread across the moon’s surface, according to Milliken. The Apollo 15 and 17 missions weren’t just lucky spots. “Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle,” says Milliken.
Ok, but how much water are we talking? The volcanic beads only have about 0.05% water by weight. But the deposits they came from are vast. Big enough that water could be potentially extracted.
For lunar exploration, this could be a game changer. Co-author Shuai Li explains, “Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative.”
Evidence of water also raises the question of how it got there. Right now, we believe the Moon formed after an object about the size of Mars slammed into Earth long ago. Scientists assumed water wouldn’t have survived the intense heat from the impact. This study suggests it did. Either the water weathered the impact somehow, or water-rich asteroids and comets brought it to the Moon.
“The exact origin of water in the lunar interior is still a big question,” says Li.