A rare mineral in a Saharan meteorite points to the presence of significant amounts of ice not far beneath the moon’s surface, scientists say.
The meteorite, says Masahiro Kayama, a researcher from Tohoku University, Japan, is a chip from the moon that was blasted into space by an impact long ago. Scientists know it is of lunar origin, Kayama adds, because its chemistry, appearance, and isotope ratios match those of rocks brought back by Apollo astronauts.
They even know that it comes from a sprawling region known as Procellarum Terren, which spans much of the upper-left quadrant, as seen from Earth.
Such meteorites are rare – only 322 are currently known to exist, according to the official tally in the Meteoritical Bulletin Database – but they are important because many are from parts of the moon not visited by Apollo astronauts, says Melinda Hutson, curator of the Cascadia Meteorite Laboratory at Portland State University, in Oregon, US.
Kayama’s meteorite, named NWA 2727, fell to Earth about 17,000 years ago. But it wasn’t known to scientists until pieces of it were purchased from a Moroccan dealer in 2005. Now, Kayama and colleagues report in the journal Science Advances that it contains an unusual mineral called moganite.
Moganite is rare enough that it was unknown to science until 1984. It is similar to quartz, explains Kayama, but can only be formed from the evaporation of mineral-rich alkaline water. For moganite to have formed on the moon, he says, there had to have been an ice deposit not far beneath its surface, presumably brought in via an impact from a comet or water-rich asteroid.
Much of that water may have boiled off into space in the immediate aftermath of the impact, but some must have been trapped underground.
Then, something else, perhaps a subsequent impact, stirred things up, melting the ice and creating conditions for the formation of moganite. Yet another impact later blasted pieces of this rock off into space, from which some eventually made their way to Earth.
What makes this exciting, Kayama adds, is that if there was sufficient near-surface water in one region to form moganite, additional water may well still be lurking elsewhere in the form of other ice deposits. These could be anywhere from a few centimetres to a few hundred metres below the surface.
Other scientists are excited. Only a few days ago, for example, Terik Daly, a planetary scientist at Johns Hopkins University, Baltimore, Maryland, published an article in the same journal describing laboratory experiments showing how water from an impact like Kayama’s water-rich comet or asteroid could have been trapped indefinitely beneath the surface. “This paper picks up where my paper left off,” he says.
The study is also important for future lunar exploration and colonisation. Kayama’s team estimates that for moganite to form, the rocks below that part of the moon’s surface must have contained enough trapped ice to make up at least 0.6% of their weight. That doesn’t sound like much, but it’s equivalent to 18.8 litres of liquid per cubic metre.
That is approximately the amount of ice found at the moon’s South Pole by NASA’s 2009 LCROSS mission, which fired a giant projectile at it and observed the vapours released by the impact.
The projectile was aimed at one of the moon’s permanently shadowed polar “cold traps” where temperatures never rise above minus-240 degrees Celsius and traces of water vapour were expected to condense into ice deposits.
“The fact that the amounts of water implied by [the new] study align with findings from the LCROSS mission provides added confidence in the results,” says Daly.
Building a lunar base in such cold conditions would be difficult. Building a base in the more temperate climes of Procellarum Terren, however, would be far less challenging, and the moganite in NWA 2727 suggests that the resources might be there to make it easier than previously anticipated.
“Astronauts can get enough water [and from it, hydrogen and oxygen] for drinking, breathing, and fuel,” Kayama says.
Hutson agrees. Only a few days ago, scientists were bemoaning NASA’s decision to scrap plans for its Lunar Prospector mission, which was intended to send a rover to the moon in search of ice deposits. The new finding, she says, “will provide additional impetus for going back to the moon to search for water”.
