When meteoroids come to call, they can really get under your skin.
Using data collected by NASA’s LADEE spacecraft, a team of researchers has found that space rocks bombarding the Moon might be slowly robbing it of a cache of water locked beneath its surface. The research, published today in the journal Nature Geoscience, presents new evidence for traces of water in lunar soil. However, with little atmosphere to shield it from bombardment, the Moon might be losing as much as 200 tons of water to the onslaught with each passing year.
Still, that’s no cause for alarm: Earth’s stalwart satellite is no stranger to being a cosmic punching bag. The Moon has probably been on this trajectory for billions of years—hinting at a lunar origin story that’s a lot wetter than scientists once thought.
“It’s exciting that this is potentially...exposing a deeper layer of the Moon,” says Eldar Noe Dobrea, a planetary scientist at the Planetary Science Institute who was not involved in the study. “If that’s the case, it constrains our understanding of the way the Moon formed.”
Only a decade ago, scientists believed the Moon to be bone dry. Although evidence to the contrary has accumulated over the past several years, many questions remain about the source and status of lunar water—and new measurements are difficult to make. With such a sparse atmosphere, the Moon can’t retain water on most of its rocky face, as any water that might well up to the surface would quickly be vaporized by sunlight.
In fact, the initial data beamed back from LADEE’s Neutral Mass Spectrometer (NMS) instrument, which characterized the composition of the Moon’s atmosphere, showed no water in the thin lunar air. But in January of 2014, study author Mehdi Benna, a planetary scientist at NASA’s Goddard Space Flight Center, noticed several unexpected “wiggles” in the measurements that “shouldn’t have been there.” Each one signified the fleeting presence of water in the air above the Moon’s gray, barren surface. (It’s possible that some of this was actually hydroxyl, a molecule composed of one hydrogen and one oxygen, that often reacts to form water.)
As Benna and his team would discover, these strange spikes in the data weren’t random. Each watery blip coincided, nearly without fail, with encounters between the Moon and meteoroid streams—dusty trails of cosmic dandruff left in the wakes of wayfaring comets.
Comets, and the meteoroids they shed, can deliver water to the planetary bodies they strike (a fraction of the Earth’s water might have come from such impacts). But that didn’t seem to account for what LADEE was observing on the Moon. “There was an order of magnitude more water being released than what [a meteoroid] could bring,” Benna says.
There was, however, a tantalizing alternative: that combative space rocks were punching through the Moon’s upper layers of soil—and excavating the treasure trove of water held below.
It was an odd theory, perhaps, but the matchup was uncanny. Between December 2013 and April 2014, the researchers identified 29 meteoroid streams in the Moon’s vicinity corresponding almost perfectly with spikes in their water data. At the peak of each shower, the Moon’s surface would be pummeled by a smattering of pebbles and rocks. Then, like clockwork, water appeared, making a brief cameo in the Moon’s atmosphere. The trends aligned so well that the team was even able to use the NMS’ readings to recreate the bombardment pattern of Geminids meteoroid stream, which the Moon encountered in December 2013, Benna says.
What’s more, it seems that, when it comes to water-mining meteoroids, size matters. Only those clocking in at 0.15 grams (think two grains of salt) or more seemed capable of liberating water. This threshold suggests the lunar surface is blanketed by a three-inch-thick veneer of super-dry soil. Lurking beneath this parched, protective layer, however, is water-laden dirt.
It’s not yet clear how much water lies in this subterranean stash. Based on the researchers’ calculations, it could be present at concentrations up to about 0.05 percent—admittedly, not much. And the Moon is likely losing more water than it gains, Benna says. It’s thought that there are only two modern sources of water on the Moon: comets, and a chemical reaction between hydrogen from solar wind and oxygen from lunar soil and rocks. Even combined, these processes aren’t enough to offset the water lost to rocky impacts.
“Early in my career, it seemed like these meteoroids were a large source of water [for the Moon],” says study author Dana Hurley, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. “But it seems like, if this hypothesis holds up, then meteoroids are actually a net loss to lunar water.”
Even so, it seems the Moon’s water supply is actually pretty stable—thanks in part to the buffer of dry soil, functioning like lunar plastic wrap. “Clearly, there’s still water to be lost,” Dobrea says. “If the Moon is losing water, it’s over geologic time scales. On human time scales, it’s inconsequential.”
In fact, it’s likely the Moon has already been very slowly depleting its reservoir for billions of years, perhaps since its earliest days. Whether it was hydrated at formation or inherited its water from a generous comet, however, remains a mystery. Finding the answer to that question could tell us a lot about how the Moon and other rocky inner Solar System bodies came into being, says Parvathy Prem, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory who has worked with Benna’s team, but did not contribute to the study.
“This is a really great, rigorous piece of investigative work,” says Faith Vilas, a planetary scientist at the Planetary Science Institute who was not involved in the study. “It expands everything that’s been said in the past...and plays up the idea that there’s more than one means by which water has been brought to the Moon.”
In the meantime, the fact that water is being regularly created and lost from the lunar surface could be “something that humans can exploit,” says Georgiana Kramer, a planetary scientist at the Planetary Science Institute who was not involved in the study. With the right materials and technology, she says, we could take advantage of these cycles to harvest oxygen or water from various locales across the Solar System.
Mining the Moon’s water is still a long way off. But when humans finally crack the lunar surface, they can do so with a hat tip to the meteoroids that came before them.
“When we look at the Moon at night, and think it’s peaceful and quiet, but it’s actually very active,” Benna says. “It’s a vibrant place where things are being captured and transported and released and sequestered. As much as we want to think of the Moon as desolate, it’s not.”