WASHINGTON — When early astronomers gazed up at the moon, they mistook its dark patches for seas. By the time astronaut Neil Armstrong stepped onto the Sea of Tranquility in 1969, scientists knew it was nothing but terra firma.
Most researchers concluded that the moon was ‘‘bone dry,’’ a celestial desert devoid of water or ice. But in a set of papers published beginning in 2008, geochemist Erik Hauri helped demonstrate that water existed on the moon after all, and that the moon’s interior might contain as much water as the Mediterranean Sea.
Dr. Hauri, who was 52 when he died Sept. 5 at his home in North Potomac, Md., helped usher in a new era in our understanding of the moon, an astronomical object now known to have ice on its poles and water deep inside its mantle. He had cancer, said his wife, Tracy Hauri.
A longtime researcher at Washington’s Carnegie Institution for Science, Dr. Hauri was first recognized for his work with highly sensitive instruments called ion microprobes, which he pushed ‘‘to their absolute technical limits,’’ said Larry Nittler, a cosmochemist and Carnegie colleague.
Using techniques he developed in the 1990s, Dr. Hauri used the instruments to examine slivers of shards, portions of rock the width of a human hair or smaller. He detected trace amounts of elements such as hydrogen and carbon, down to a few parts per million, work that enabled him to obtain key insights on the Earth and moon.
A onetime marine biology student, Dr. Hauri began studying rocks after deciding marine animals were fickle and uncooperative. But he spent much of his career outside the lab, collecting volcanic samples from Hawaii, Iceland, Alaska, and Polynesia that shed light on the movement of elements and minerals deep inside the Earth.
He was focusing on water, which has a broad impact on volcanic eruptions and the movement of tectonic plates, when his friend Alberto Saal suggested they conduct measurements for hydrogen, water, and other volatile substances using the lunar samples of the Apollo program.
‘‘When people measured these moon rocks they never found anything,’’ said Saal, a Brown University geochemist who attended graduate school with Dr. Hauri at MIT and the Woods Hole Oceanographic Institution. ‘‘We had a good technique. Nothing had been done on hydrogen for a long time. We said, ‘Why not try?’ ’’
It took three years for the researchers to obtain their samples from NASA, which twice rejected their research proposal, Saal said. But when he and Dr. Hauri reviewed their findings, ‘‘It was like a bomb had exploded in our hands.’’
Their work centered on a thimble’s worth of orange-tinted soil, which included tiny beads of volcanic glass collected by astronaut and geologist Harrison Schmitt in 1972. Described by Dr. Hauri as geological ‘‘team capsules,’’ the beads were formed when the moon was relatively young, when lava was ejected from volcanoes and cooled so fast that it turned to glass before falling to the ground.
In a 2008 article in the scientific journal Nature, the scientists reported that some of the beads contained trace amounts of water, about 50 parts per million. Three years later, in an article in Science, Dr. Hauri and his colleagues reported finding far more — about 100 times more water than had previously been believed. Their research indicated that the mantle of the moon, a hot, dense region just below the surface, contained about as much water as the upper mantle of the Earth.
‘‘If you take our measurements and use them to estimate the water content of the interior of the moon, you arrive at a volume of water that’s equivalent to the Mediterranean Sea. Now that’s a fair bit of water,’’ Dr. Hauri told NPR in 2011.
Their findings also raised new questions about the origin and evolution of the moon. Scientists have long believed that the moon was formed through a massive collision some 4.5 billion years ago, when a Mars-sized object struck Earth, knocking off a chunk of material that coalesced to form our lunar neighbor. Under that theory, however, the heat of the collision was assumed to have vaporized any water.
‘‘Our models for formation of the terrestrial planets involve these types of large collisions, which led to a prevailing wisdom that all the terrestrial planets formed bone-dry,’’ said Richard Carlson, director of Carnegie’s terrestrial magnetism department. ‘‘A number of observations, particularly Erik’s detection of water on the moon,’’ have forced scientists ‘‘to consider more complicated, but likely more accurate, models.’’
Outside of the lab, Dr. Hauri played guitar — worship music at church, and progressive metal at home. He also became a self-taught luthier, making more than a dozen guitars and bass guitars by hand after finding himself unable to acquire a 12-string Fender Stratocaster in a store.
In addition to his wife of 31 years, the former Tracy Spears, he leaves three children, his father, a sister, and a brother.