The moon, our nearest celestial neighbor, still carries its share of mysteries, especially when it comes to the role of water in its past and present. Just last year a surprising series of studies indicated that the lunar surface is dusted with water molecules; another much-heralded experiment showed that at least some polar craters on the moon appear to be lined with water ice. Those stores of ice would be valuable resources to future moon visitors, who could count on a local source of water for an extended stay on the lunar surface.
But with the surface of the moon looking more watery all the time, a debate rages on over the influence of water dozens of kilometers below the surface, in the lunar mantle. The presence or absence of water in the mantle helps to constrain the moon's geologic history, stretching all the way back to the time that a planet-size object collided with Earth, producing a debris cloud that coalesced into the moon, as the prevailing view holds. When the lunar samples from the Apollo manned landings arrived on Earth, geologists noticed an interesting discrepancy between the two bodies—the lunar specimens appeared to be highly depleted in volatiles such as water. There are multiple explanations for why the moon and Earth should be so different in that regard, and with better understanding of the moon's composition may come greater knowledge of its formation and evolution.
In recent years, however, some studies have begun to challenge the idea that the moon is bone-dry. In particular, a 2008 paper in Nature showed that ancient volcanic specimens returned by Apollo astronauts contained traces of water and hinted at a surprisingly large indigenous water component in the lunar interior. (Scientific American is part of Nature Publishing Group.) The researchers concluded that much of the original water content in lunar magma had escaped as vapor during eruptions.
Now a new study, published online August 5 in Science, pushes back against the hydrated-moon hypothesis. Zachary Sharp, a geochemist at the University of New Mexico, and his colleagues measured the chemical composition of several Apollo samples and found that the chlorine content of the lunar rocks and soils implied that the moon's interior was largely bereft of water, as originally suspected.
On Earth, chlorine exists as two stable isotopes—atoms with different numbers of neutrons and hence different atomic weights—in a very predictable ratio: about three atoms of chlorine 35 for every atom of chlorine 37. But in the lunar specimens Sharp and his co-authors examined, the ratio between the two isotopes varied greatly between the samples, most of which were enriched in chlorine 37 relative to Earth.
The implication is that the processes that act to regulate the isotopic balance of chlorine on Earth are not present on the moon—and those processes depend in part on the presence of hydrogen, a component and key marker of water. During volcanic eruptions on Earth, Sharp explains, chlorine 35 escapes as vapor more readily than does its heavier counterpart, but chlorine 37 preferentially bonds with hydrogen to form hydrogen chloride, which also escapes as a vapor. "These two effects tend to cancel each other," Sharp says.
On a world without abundant hydrogen, chlorine 35 would still preferentially escape as vapor during molten rock flows, but the counterbalancing incorporation of chlorine 37 into escaping gas would not occur, leaving an anhydrous world relatively well-stocked with chlorine 37. That is just what the isotope ratios of the Apollo samples indicate—a general trend toward chlorine 37 enrichment.
The group also calculated the initial chlorine content of the lunar rocks, which allowed an estimation of the initial hydrogen levels based on the presumably small amounts of hydrogen chloride produced.
"Ultimately we came up with the only explanation consistent with all our observations and measurements," Sharp says, "which was that the hydrogen content in the lunar magmas—and hydrogen is really a proxy for water—had to have been very, very low." The inferred hydrogen content of the moon is roughly one ten-thousandth that of Earth, or possibly even lower. "That's pretty darn dry," Sharp says.
John Eiler, a geochemist at the California Institute of Technology, says he is impressed by the discovery of the unusual isotopic makeup of lunar chlorine but remains agnostic about what it means. "The chlorine contents and isotopic compositions of lunar materials are not a direct constraint on the hydrogen contents of those or related materials," Eiler says. The new work, then, "is actually a somewhat indirect and circuitous argument," he adds.
An author of the 2008 Nature paper showing a relatively large role for water in lunar history remains skeptical as well. "These guys have some very nice data on chlorine isotopes in the various lunar samples," says geochemist Erik Hauri of the Carnegie Institution for Science's Department of Terrestrial Magnetism in Washington, D.C., who adds that the new study presents "really kind of the first data of its kind to come from lunar rocks."
At the same time, Hauri says, "the thing that I have to take issue with is the water." The volcanic rocks examined by Sharp and his colleagues need not have always had a high ratio of chlorine to hydrogen, Hauri contends. For one thing, hydrogen is much less soluble in magma than chlorine is. "When the magma erupts, the first thing that degasses is water and hydrogen," he adds. In other words, the chlorine content of lunar magmas may primarily indicate processes that took place after any water and hydrogen originally present had already escaped.
The question of how much water—if any—lies within the moon's interior stretches back decades and is unlikely to be resolved in the near future, Sharp notes. "It's always the way science works—that controversies arise, and various people will make different measurements or come up with different ideas, and perhaps there will be a consensus on one idea," he adds. "I don't know if we'll come to a conclusion about this anytime soon."