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By NASA
Scientists have hypothesized since the 1960s that the Sun is a source of ingredients that form water on the Moon. When a stream of charged particles known as the solar wind smashes into the lunar surface, the idea goes, it triggers a chemical reaction that could make water molecules.
Now, in the most realistic lab simulation of this process yet, NASA-led researchers have confirmed this prediction.
The finding, researchers wrote in a March 17 paper in JGR Planets, has implications for NASA’s Artemis astronaut operations at the Moon’s South Pole. A critical resource for exploration, much of the water on the Moon is thought to be frozen in permanently shadowed regions at the poles.
“The exciting thing here is that with only lunar soil and a basic ingredient from the Sun, which is always spitting out hydrogen, there’s a possibility of creating water,” Li Hsia Yeo, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “That’s incredible to think about,” said Yeo, who led the study.
Solar wind flows constantly from the Sun. It’s made largely of protons, which are nuclei of hydrogen atoms that have lost their electrons. Traveling at more than one million miles per hour, the solar wind bathes the entire solar system. We see evidence of it on Earth when it lights up our sky in auroral light shows.
Computer-processed data of the solar wind from NASA’s STEREO spacecraft. Download here: https://svs.gsfc.nasa.gov/20278/ NASA/SwRI/Craig DeForest Most of the solar particles don’t reach the surface of Earth because our planet has a magnetic shield and an atmosphere to deflect them. But the Moon has no such protection. As computer models and lab experiments have shown, when protons smash into the Moon’s surface, which is made of a dusty and rocky material called regolith, they collide with electrons and recombine to form hydrogen atoms.
Then, the hydrogen atoms can migrate through the lunar surface and bond with the abundant oxygen atoms already present in minerals like silica to form hydroxyl (OH) molecules, a component of water, and water (H2O) molecules themselves.
Scientists have found evidence of both hydroxyl and water molecules in the Moon’s upper surface, just a few millimeters deep. These molecules leave behind a kind of chemical fingerprint — a noticeable dip in a wavy line on a graph that shows how light interacts with the regolith. With the current tools available, though, it is difficult to tell the difference between hydroxyl and water, so scientists use the term “water” to refer to either one or a mix of both molecules.
Many researchers think the solar wind is the main reason the molecules are there, though other sources like micrometeorite impacts could also help by creating heat and triggering chemical reactions.
In 2016, scientists discovered that water is released from the Moon during meteor showers. When a speck of comet debris strikes the moon, it vaporizes on impact, creating a shock wave in the lunar soil. With a sufficiently large impactor, this shock wave can breach the soil’s dry upper layer and release water molecules from a hydrated layer below. NASA’s LADEE spacecraft detected these water molecules as they entered the tenuous lunar atmosphere. NASA’s Goddard Space Flight Center Conceptual Image Lab Spacecraft measurements had already hinted that the solar wind is the primary driver of water, or its components, at the lunar surface. One key clue, confirmed by Yeo’s team’s experiment: the Moon’s water-related spectral signal changes over the course of the day.
In some regions, it’s stronger in the cooler morning and fades as the surface heats up, likely because water and hydrogen molecules move around or escape to space. As the surface cools again at night, the signal peaks again. This daily cycle points to an active source — most likely the solar wind—replenishing tiny amounts of water on the Moon each day.
To test whether this is true, Yeo and her colleague, Jason McLain, a research scientist at NASA Goddard, built a custom apparatus to examine Apollo lunar samples. In a first, the apparatus held all experiment components inside: a solar particle beam device, an airless chamber that simulated the Moon’s environment, and a molecule detector. Their invention allowed the researchers to avoid ever taking the sample out of the chamber — as other experiments did — and exposing it to contamination from the water in the air.
“It took a long time and many iterations to design the apparatus components and get them all to fit inside,” said McLain, “but it was worth it, because once we eliminated all possible sources of contamination, we learned that this decades-old idea about the solar wind turns out to be true.”
Using dust from two different samples picked up on the Moon by NASA’s Apollo 17 astronauts in 1972, Yeo and her colleagues first baked the samples to remove any possible water they could have picked up between air-tight storage in NASA’s space-sample curation facility at NASA’s Johnson Space Center in Houston and Goddard’s lab. Then, they used a tiny particle accelerator to bombard the dust with mock solar wind for several days — the equivalent of 80,000 years on the Moon, based on the high dose of the particles used.
They used a detector called a spectrometer to measure how much light the dust molecules reflected, which showed how the samples’ chemical makeup changed over time.
In the end, the team saw a drop in the light signal that bounced to their detector precisely at the point in the infrared region of the electromagnetic spectrum — near 3 microns — where water typically absorbs energy, leaving a telltale signature.
While they can’t conclusively say if their experiment made water molecules, the researchers reported in their study that the shape and width of the dip in the wavy line on their graph suggests that both hydroxyl and water were produced in the lunar samples.
By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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