North rim of the Cabeus crater, just after LCROSS crash-landed there.

NASA/GSFC/Arizona State University

The minute the astronauts of Apollo 16 safely splashed down in the Pacific Ocean on April 27, 1972, the only thing Larry Taylor could think about was getting his hands on another sample of moon rocks. Taylor, a planetary geochemist at Purdue University and self-described “lunatic,” had examined hundreds of rocks that astronauts had brought back since first landing on the moon three years earlier. He soon recognized that this batch was different. The rocks were rusty, which suggested the presence of a substance supremely important to scientists and space explorers alike: water. The buzz throughout the community of lunar-rock scientists was electric, but Taylor had doubts. Any traces of water were most likely contamination from Earth, he decided; through analyzing other samples, he had come to believe that the moon was dry.

Over the years, Taylor’s skepticism hardened into conventional wisdom. “I was very vehement against lunar water,” he says. “Sometimes the people with the biggest mouth win.”

Last October 9, conventional wisdom got ripped to shreds. On that day a small NASA rocket slammed into a 60-mile-wide crater named Cabeus, a permanently shadowed dent very close to the lunar south pole. The extreme heat of the collision caused grains of water ice and other substances that had been frozen for billions of years to vaporize. A larger spacecraft followed a few minutes later to sniff the vapor cloud and send measurements back to Earth before itself crashing into the lunar surface. Preliminary findings from the mission—called LCROSS, for Lunar Crater Observation and Sensing Satellite—show that at the impact site water may account for about 5 percent of the lunar crater’s soil by weight, says Anthony Colaprete, principal investigator. That is as dry as the driest deserts on Earth, yet still far wetter than most scientists expected. Says Taylor, “I had to eat my shorts.”


The findings have transformed our idea of the moon from a desiccated dead zone into a complex and lively world. Researchers are now involved in several projects that trace the origins of lunar water, perhaps all the way back to the fateful moment billions of years ago when the moon was formed. At the same time, new research indicates that the moon continually produces its own water through a strange mating ritual between hydrogen from the sun and oxygen on the ground. And stunning new spacecraft images show fault lines, volcanic domes, and solidified lava flows on the lunar surface, forcing a reevaluation of long-held beliefs about the moon’s early evolution. Four decades after Apollo, Earth’s nearest neighbor in space is still full of surprises.

According to the latest thinking, lunar water is derived from comets that struck the moon billions of years ago, when the solar system was young. The ice they carried would have vaporized from the impact, settling eventually in permanently shadowed craters near the north and south lunar poles, where the extreme cold (below –400 degrees Fahrenheit, according to recent observations) would have preserved it almost indefinitely. This extraordinary storage ability could help explain NASA’s detection in early March of 650 million tons’ worth of ice at the moon’s dark, cold north pole. LCROSS also supported this theory when it crashed into the south pole by uncovering, in addition to water, other elements that are abundant on comets: carbon dioxide, hydrogen sulfide, and methane.

Still, comets alone may not explain all of the water scientists have found on the moon. Taylor, now at the University of Tennessee at Knoxville, and a team of planetary scientists have proposed a stunning hypothesis: Some of the water is produced by processes taking place on the surface right now. The key evidence comes from the Indian lunar probe Chandrayaan-1 and data from the 1998 Lunar Prospector mission. Together they reveal what Taylor calls lunar “dew”: a smidgen of water distributed all over the moon’s surface. The probes detected a daily fluctuation in the strength of the water signal, suggesting that the moon somehow produces small amounts of water at dawn before sizzling temperatures burn it off during the two-week-long lunar day.

Taylor has some ideas about how the moon manufactures its water. Since the moon has virtually no atmosphere, high-energy hydrogen ions ejected from the sun continually bombard the surface and break chemical bonds in the rocks. The lunar surface also gets harsh treatment from meteorite impacts, ultraviolet rays, and other sources. The net result of all this punishment is, as Taylor puts it, an “unhappy” surface, where oxygen atoms in the rocks regularly break their chemical bonds. The oxygen would bond with the incoming hydrogen ions and form good old H2O. Scientists have not yet worked out all the details of this mechanism—Colaprete says it “needs to be teased out a little more”—but if it proves correct, the consequences could stretch far beyond the lunar surface. If such a process occurs on the moon, Taylor says, it might also take place on other seemingly dry, airless bodies, such as Mercury or the asteroids. Water may exist where we least expected it.

While Taylor focuses on the present, some of his colleagues are using the discovery of water to learn more about the moon’s distant past. Currently the leading theory of how the moon formed is the giant impact hypothesis, which proposes that an object the size of Mars slammed into the infant Earth 4.5 billion years ago and knocked off large, molten chunks. These chunks floated around for a while and glommed onto one another, forming the moon. According to computer models, such a collision would have created searingly high temperatures that would have vaporized all the water, which is a big reason why scientists were so certain that the moon was dry.

Alberto Saal, a geochemist at Brown University, believes that the moon has been wet almost from the beginning. In 2008 Saal analyzed glassy volcanic rocks from the Apollo missions to see what they would reveal about the moon’s interior. To his surprise, he discovered not only that the rocks contained water but that the concentrations were greatest in the center. The implication is that the water was within the original lava that formed those rocks and so must still be present in the moon’s interior. If the water had come from contamination after the lava hardened, the water concentration would have been greater on the outside. “It’s really changing the way we look at things,” says Jeff Taylor, an expert in lunar geology at the University of Hawaii. “We don’t even know how water got to Earth, so understanding the origins of lunar water is very interesting.”