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Tuesday, April 16, 2024

How Do You Prove There’s Ice on the Moon? With a Lunar Flashlight

NASA is serious about getting people back to the moon. That means it’s time to find out where all the water is. 

This weekend, just as the Orion capsule was returning from its first moon voyage, the agency launched a briefcase-sized satellite called Lunar Flashlight that will search for frost in the shadowed regions of the lunar South Pole and begin to build a rough map of where it can be found. It’s a step toward understanding where that water came from, and whether it’s a renewable resource that astronauts could drink or convert into rocket fuel for the trip back to Earth. If the moon has enough water, it could become a pit stop for exploring even deeper into the solar system

Lunar Flashlight is about “learning to live and work off-planet, and really establishing that we can work in a remote location without umbilical cords—without the constant need to resupply everything,” says Barbara Cohen, a NASA Goddard Space Flight Center planetary scientist and the mission’s principal investigator. 

The moon, like Earth, formed in an extremely dry environment, so any ice that’s present today had to have gotten there later. Most likely, it came from comets and asteroids bombarding the surface, leaving behind small amounts of H2O in their impact craters. Outgassing from active volcanoes may have also deposited water onto the lunar regolith. Some researchers even think that the solar wind—streams of hydrogen ions that flare from the sun—could be interacting with oxygen in the moon’s soil to create water. If water ends up in a permanently shadowed region of the moon, it could get trapped there forever as ice. (Cohen likens it to the way snow lingers in the shade of tall skyscrapers during the winter, with no heat to melt it away.) That makes the lunar South Pole, a basin on the far side of the moon that hasn’t seen a lick of sunlight in 2 billion years, an ideal place to look. The temperature there is a frigid -400 degrees Fahrenheit—colder than the surface of Pluto, and just a tad warmer than absolute zero. 

There’s plenty of evidence to suggest that the South Pole’s surface is frosted: Both satellite and ground-based data collected over the past 30 years are “consistent with water ice,” Cohen says. “There’s really no other plausible explanation.” Still, some researchers are hesitant to conclude that it’s ice because measurements from different experiments don’t completely match up with each other, and some also disagree with theoretical models. “More data is helpful,” Cohen says.

Enter Lunar Flashlight. As it flies by the South Pole, the satellite’s reflectometer will beam four lasers—each tuned to a different wavelength of near-infrared light—into the shadowed depths of each crater, then count how many photons reflect back up from the surface. Two of those lasers have wavelengths that can only be absorbed by water ice, so if the satellite records less reflected light than expected, that’s a smoking gun. The team will also be able to figure out how frosted the moon’s surface is by the amount of laser light that gets absorbed. 

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“For me, this is a dream measurement,” says David Paige, a planetary scientist at the University of California, Los Angeles who has been part of the mission since its inception nearly a decade ago. “It’s an opportunity to make a real rapid advancement with such a small spacecraft.” Once the team has mapped out the distribution of surface ice at the South Pole, they can use it to guide future landers, rovers, and eventually humans to places where they can collect frosty samples. 

Today’s astronauts are stuck having to pack their water with them. It’s heavy and incompressible, which means it’s expensive to launch and takes up space that could be used to fit more scientific instruments. “Lunar Flashlight might be the key that unlocks the door to longer-lived, even more ambitious missions,” says Johns Hopkins University planetary scientist Parvathy Prem, who is not involved in the project. 

Lunar ice is also scientifically interesting, Prem says, because it may be preserving an ancient record of how water arrived in the Earth-moon system. Someday, frozen samples from the moon could be transported to our own planet and analyzed for molecular fingerprints that reveal the ice’s origins. The presence of carbon, for instance, would suggest that water arrived from asteroids or comets. Sulfur would mean it came from volcanoes. Hydroxyl, a molecule containing the same ingredients as water, would make the solar wind responsible. Any of these findings could hint that the moon had—or still has—its own water cycle, a series of steps by which H2O flows between the lunar interior, surface, and atmosphere. 

It will take three months for Lunar Flashlight to reach the moon, taking a roundabout journey to conserve the limited fuel it can carry. Once there, the spacecraft will settle into an odd, oval-shaped trajectory for the same reason, skimming as close as 10 kilometers above the South Pole’s surface for just a few minutes in its six-and-a-half-day orbit. Paige, who leads the mission’s science operations center, thinks they’ll be ready to start taking data next April and expects the team will operate Lunar Flashlight for at least four months after it reaches orbit, until—like most lunar satellites—it eventually crashes into the moon. He anticipates that first results will be released by the end of 2023. 

Paige notes that last week marked the 50th anniversary of Apollo 17, the last time humans set foot on lunar soil. Since then, he says, scientists have learned so much more about what the moon can reveal about our cosmological past, and what resources it may offer for our interstellar future. “The push to go to the moon is very exciting,” Paige says, and the Lunar Flashlight is an important contribution to that effort—“no matter what we discover.” 

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