A new analysis of dust collected from the Moon suggests that water bound to the lunar surface may have come from the Sun.
Specifically, it could be the result of a bombardment of hydrogen ions from the solar wind, hitting the lunar surface, interacting with mineral oxides and bonding with dislodged oxygen. The result is water that could be hiding in the lunar regolith in significant amounts at mid and high latitudes.
This has implications for our understanding of the provenance and distribution of water on the Moon – and may even be relevant for our understanding of the origins of water on Earth.
The Moon looks like a fairly dry ball of dust, but recent studies have shown that there’s a lot more water up there than anyone ever imagined. Obviously, it doesn’t float in lakes and lagoons; it is bound to lunar regolith, perhaps hidden as ice in permanently shadowed craters, and sequestered in globules of volcanic glass.
This naturally leads to questions, such as how much water is up there exactly? How is it distributed? And where the hell does that come from? The last question probably has several answers.
Some of it could come from asteroid impacts. Some of the Earth. A possible source, however, is not the first thing that comes to mind when imagining cosmic rain clouds.
To be fair, the Sun isn’t exactly dripping with moisture, but its wind is certainly a reliable source of high-velocity hydrogen ions. Evidence that includes an analysis of lunar dirt from the Apollo missions has previously raised the strong possibility that the solar wind is responsible for at least some of the Moon’s ingredients for water.
Now, a team of researchers led by geochemists Yuchen Xu and Heng-Ci Tian from the Chinese Academy of Sciences have discovered the chemistry of grains recovered by the Chang’e-5 mission that further supports a solar source of lunar water.
They studied 17 grains: 7 olivine, 1 pyroxene, 4 plagioclase and 5 glass. These were all, unlike the low latitude samples collected by Apollo and Luna, from a mid-latitude region of the Moon, and collected from the youngest known lunar volcanic basalt, from the driest basalt basement.
Using Raman spectroscopy and energy dispersive X-ray spectroscopy, they studied the chemical composition of the edges of these grains – the 100-nanometer outer shell of the grain that is most exposed to weather conditions. spatial, and therefore the most altered in relation to the grain. interior.
The majority of these rims had a very high hydrogen concentration of 1116 to 2516 parts per million and very low deuterium/hydrogen isotope ratios. These reports are consistent with reports of these elements found in the solar wind, suggesting that the solar wind slammed into the Moon, depositing hydrogen on the lunar surface.
The water content derived from the solar wind present at the Chang’e-5 landing site, they found, should be around 46 parts per million. This is consistent with remote sensing measurements.
To determine if hydrogen could be retained in lunar minerals, the researchers then performed heating experiments on some of their grains. They found that after burial, the grains can effectively retain hydrogen.
Finally, the researchers conducted simulations on the conservation of hydrogen in the lunar soil at different temperatures. This revealed that temperature plays an important role in the implantation, migration and outgassing of hydrogen on the Moon. This implies that a significant amount of water derived from the solar wind could be retained at mid and high latitudes, where temperatures are cooler.
A model based on these findings suggests that the Moon’s polar regions could be much richer in water created by the solar wind – information that could be very useful for planning future lunar exploration missions.
“Polar lunar soils could hold more water than the Chang’e-5 samples,” says cosmochemist Yangting Lin of the Chinese Academy of Sciences.
“This discovery is of great importance for the future use of water resources on the Moon. In addition, thanks to the sorting and heating of the particles, it is relatively easy to exploit and use the water contained in the lunar ground.”
The research has been published in PNAS.