Just how wet is the Moon's interior?
Scientists had thought our Moon to be mostly free of water up until around a decade ago. However, studies are starting to trickle in that suggest our satellite may be much wetter than we realized. The latest example has drawn on satellite data to uncover volcanic deposits spread out across its surface that contain surprising amounts of trapped water.
Things changed for our presumedly parched Moon back in 2008, when trace amounts of water were detected in samples brought back by the Apollo astronauts. There were concerns that the water had entered those samples on their return to our planet, but further inspection has since revealed that they were just as rich in water as some basalts here on Earth.
"The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise 'dry' mantle," said Ralph Milliken, lead author of the new research and an associate professor in Brown University's Department of Earth, Environmental and Planetary Sciences.
In 2013, NASA's Moon Mineralogy Mapper, an imaging spectrometer that flew aboard India's Chandrayaan-1 lunar orbiter, returned data revealing water locked in mineral grains on the surface of the Moon. And now relying on data from that same instrument and a new thermal correction technique, Milliken and his team have unearthed yet more evidence that the Moon is water-rich.
The new thermal correction technique overcomes one of the previous limitations of using orbital instruments to measure water in volcanic deposits on the Moon, which present as glassy beads formed by magma eruptions coming from its interior. Normally, taking these measurements involves bouncing light off the surface and seeing which wavelengths are reflected back and which are not, which can give an indication of the minerals and compounds within.
But because the surface of the Moon becomes warmer over the course of the day, the readings gathered by the spectrometer can be confused by the emitted thermal radiation, which happens to occur at the same wavelength as those that represent water. The researchers were able to correct for this, however, by building a detailed temperature profile of the areas of interest and combining it with data from the Apollo samples.
"That thermally emitted radiation happens at the same wavelengths that we need to use to look for water," Milliken said. "So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component."
Using this method, the scientists found evidence of water in nearly all large volcanic deposits to have been mapped on the Moon's surface, including those near the Apollo landing sites. The amount of water inside these volcanic beads is relatively small, only around 0.05 percent of their weight, but the deposits are large and the water could potentially be extracted by future lunar explorers.
"The distribution of these water-rich deposits is the key thing," Milliken says. "They're spread across the surface, which tells us that the water found in the Apollo samples isn't a one-off. Lunar pyroclastics (volcanic deposits) seem to be universally water-rich, which suggests the same may be true of the mantle."
If indeed the Moon's interior is wet, it makes for some interesting discussion around its formation. It is thought that the Moon was formed when a planet-sized object smashed into the Earth during the early days of our solar system. Any of the hydrogen needed to form water would be unlikely to survive such an impact, which is a key reason that the Moon was assumed to be so dry for so long.
That water was transported to the Moon by ancient asteroids soon after its formation is one possible explanation for its newly discovered wetness. Last year, scientists working with data from unmanned lunar missions published a paper arguing that a class of water-rich asteroid delivered as much as 80 percent of the Moon's water, with comets also making a contribution. The topic is still very much subject to debate.
"The growing evidence for water inside the Moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified," Li said. "The exact origin of water in the lunar interior is still a big question."
The research was published in the journal Nature Geoscience.
Source: Brown University