Universe Today
Where did the water we believe is on the Moon come from? Most scientists think they know the answer - from the solar wind. They believed the hydrogen atoms that make up the solar wind bombarded the lunar surface, which is made up primarily of silica. When that hydrogen hits the oxygen atoms in that silica, the oxygen is sometimes released and freed to bond with the incoming hydrogen, which in some cases creates water. But no one has ever attempted to replicate that process to prove its feasibility. A new paper by Li Hsia Yeo and their colleagues at NASA's Goddard Space Flight Center describes the first experimental evidence of that reaction.
To perform this experiment on Earth, the authors needed two things: something equivalent to the solar wind and something comparable to lunar regolith. The solar wind is comprised of protons—basically hydrogen atoms with their electrons stripped off. Mimicking this on Earth proved tricky, as they had to build a custom miniaturized particle accelerator to simulate the solar wind.
While it might seem technically simpler, their next task was certainly more administratively challenging—obtaining a sample of actual Moon regolith from the Apollo missions. Dust collected during Apollo 17's final lunar trip had already been packed in an airtight storage container since the 1970s, but the authors went ahead and baked the sample again just to make sure there was no water present.
Fraser discusses the best use of the Moon.
Once the samples were obtained and the accelerator was set up, the final piece of the experimental puzzle was a spectrometer, which could show the presence of water. When ready, they blasted the sample with enough simulated "solar wind" to be the equivalent of about 80,000 years on the lunar surface. During that time, they watched for infrared dips around 3um, precisely what they saw, representing a tell-tale water sign.
However, it is also a tell-tale sign of hydroxyl (OH), which has the same spectral profile as water and is also one of the potential by-products of the solar wind hitting the lunar regolith. As a press release announcing the finding states, "they can't conclusively say if their experiment made water molecules." However, finding even hydroxyl molecules is a step in the right direction.
One other data point lends credence to the continual replenishment of water via the solar wind, instead of more sporadic replenishment from sources such as micrometeoroids - the spectrographic signal of water seems to vary with time. It is strong in the morning, decreases throughout the lunar "day", and then increases again over the lunar "night". The most obvious explanation for this cycle is that some water burns off during the day, being exposed to the Sun. Over the two-week-long lunar night, the amount of water starts to build back up again, as would be expected if it is created by an external force such as the solar wind, and not being stripped away right away.
New data from LADEE shows how water might show up on the Moon after a micrometeoroid impact.
Credit - NASA's Goddard Space Flight Center Conceptual Image Lab
Ultimately, this experiment lends more credibility to the idea that lunar water is created by the interaction between the lunar regolith and the solar wind and that the solar wind itself is always slightly replenishing the amount of water available on the Moon. Artemis astronauts, or whoever winds up back on the lunar surface, will undoubtedly be happy for that, no matter how that water got there.
Learn More:
NASA - Can Solar Wind Make Water on Moon? NASA Experiment Shows Maybe
L.H. Yeo et al - Hydroxylation and Hydrogen Diffusion in Lunar Samples: Spectral Measurements During Proton Irradiation
UT - The Solar Wind is Creating Water on the Surface of the Moon
UT - The Moon's Atmosphere Comes from Space Weathering