When Apollo 15 astronauts David Scott and James Irwin scooped up a patch of oddly shimmering green soil near Hadley Rille in 1971, neither could have known the tiny glass beads packed inside their sample bag would, decades later, help overturn one of the Moon’s most stubborn scientific assumptions: that its interior was bone dry.
It would take more than 35 years, a new generation of laboratory instruments and several follow-up studies before scientists confirmed what those green flecks were hiding. Researchers led by geologist Alberto Saal of Brown University, working with the Carnegie Institution for Science and Case Western Reserve University, detected the first unambiguous trace of water locked inside the volcanic glass beads, a finding published in Nature in July 2008 that suggested water had survived the Moon’s violent birth rather than being entirely vaporised, as the long-dominant giant-impact theory of lunar formation had implied.
How Did Apollo 15 Astronauts Spot the Green Glass in the First Place?
The discovery began with a moment of doubt on the lunar surface itself. Spotting the unusual tint in the regolith near the Apennine Mountains, Irwin was recorded over the radio checking with Scott whether the colour was real or a trick of the light, asking “sure it’s green and not just white albedo again?” according to the Apollo mission transcript held in Brown University’s account of the landing.
Convinced the tint was genuine, the astronauts collected the beads along with the rest of their haul, unaware they had gathered what would become one of the most consequential samples of the entire Apollo programme.
What Did Scientists Initially Believe About the Moon’s Interior?
For decades, the prevailing view held that the Moon’s interior had been almost completely stripped of water and other volatile elements during its formation. Planetary scientist Darby Dyar of Mount Holyoke College, who has studied lunar glass beads since graduate school, told the American Geophysical Union’s publication Eos that the instruments available when the Apollo samples first returned “indicated…that there was absolutely no water on the Moon.” That consensus held firm for nearly four decades.
How Was Water Finally Detected Inside the Beads?
The breakthrough came down to sensitivity. Carnegie Institution geochemist Erik Hauri developed a refined secondary ion mass spectrometry technique capable of detecting far smaller traces of water than earlier instruments could manage, allowing Saal’s team to identify water signatures inside the Apollo 15 green glass and Apollo 17 orange glass that previous analyses had missed entirely.
A follow-up study in 2011 examined tiny crystalline melt inclusions trapped within the beads and found they held roughly as much water as some basalts found on Earth, according to Brown University’s own account of the research.
What Did Later Studies Reveal About the Scale of Lunar Water?
The Apollo samples represented just two landing sites, leaving open the question of whether they were unusual outliers or evidence of a genuinely water-rich mantle. A 2017 study led by Brown’s Ralph Milliken and Shuai Li of the University of Hawaii combined laboratory data from the Apollo samples with orbital measurements from India’s Chandrayaan-1 spacecraft, examining volcanic deposits across the lunar surface that had never been physically sampled.
“The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet,” Milliken said in a statement released by the university.
Why Does This Discovery Matter for Understanding the Moon’s Origin?
The implications reach beyond a single rock sample. Jim Head, a Brown University professor of geological sciences who helped select the Apollo 15 landing site, said the water findings “absolutely revolutionised our thinking about how the Moon formed.”
A Moon born from a giant impact between Earth and a Mars-sized body was long assumed to have lost virtually all its volatiles to the heat of that collision; evidence of indigenous water complicates that picture and raises fresh questions about how much of Earth’s own water might trace back to a shared origin with its satellite.
What Questions Remain About Water Deep Inside the Moon?
Even with mounting evidence, scientists caution the full picture remains incomplete. Dyar noted that research into the Moon’s interior has moved quickly in recent years, observing “we’re suddenly revolutionising our idea about what the interior of the Moon looks like.”
Estimates of the total volume involved vary widely: one geologist quoted by National Geographic, Francis McCubbin, calculated that all the water locked in the lunar interior, if extracted, could cover the Moon’s surface in an ocean roughly a yard deep. Researchers continue to debate whether the Apollo 15 and 17 sites are representative of the wider mantle or reflect localised, unusually wet pockets, a question that additional sample-return missions may eventually help settle.
