We finally have a detailed map of water on the moon

Water is key for life here on Earth, and it will be key for humans to travel around the solar system as well. It’s a heavy resource to lug aboard a spacecraft, so it’s best to get it from your destination when possible. Thankfully, there’s already some water on the moon—and astronomers just got a better look at where it is exactly.

New observations from the SOFIA airborne observatory (which completed its final flight in September 2022) produced a detailed map of water molecules near the moon’s South Pole. These results, recently accepted to the Planetary Science Journal and presented at the annual Lunar and Planetary Science Conference last week, are answering a critical question for both geology and future human exploration: Where can we find water on the moon?

“We don’t really know the basics of where [the water] is, how much, or how it got there,” says Paul Hayne, a planetary scientist at the University of Colorado not affiliated with the new research.

[Related: Mysterious bright spots fuel debate over whether Mars holds liquid water]

NASA’s 2010 LCROSS mission first sparked interest in the southern end of the moon when its radar revealed frozen water stored in places where the sun’s light can’t reach, like the bottoms of craters. A slew of follow-up observations by India’s Chandrayaan probes added further evidence for lunar water, but there was a catch—what astronomers identified as possible water molecules (H₂O) could have been a different arrangement of hydrogen and oxygen called hydroxyl (OH). SOFIA, however, had the power to search for a wider range of molecular signatures, meaning it could scan for a surefire sign of water instead of something that could be confused for hydroxyl. 

“These observations with SOFIA are important because they definitively map the water molecules on the sunlit surface of the moon,” says NASA Lunar scientist Casey Honniball, co-author on the new study. An accurate map of the icy areas can help planetary scientists distinguish between different ways water moves across the lunar surface, and learn how the life-giving compound got there in the first place. 

“We see more water in shady places, where the surface temperature is colder,” says William T. Reach, director of SOFIA and lead author on the paper. This is similar to how ski slopes facing away from the sun retain more of their snow here on Earth.

Researchers are considering two main scenarios to explain the origins of lunar water: evaporating water from comets that crashed into the moon, or water trapped in volcanic minerals created long ago. The SOFIA data hasn’t helped them to narrow down the source yet. “These are observations, and they don’t come labeled with a nice, tidy explanation,” adds Reach.

Although his team is still figuring out the provenance of the observed water, detecting it at all could be a boon for future human space exploration. A confident claim of water on the south pole of the moon explains “why we are targeting these regions so intently for the next phase of human and robotic lunar exploration,” says UCLA planetary scientist Tyler Horvath, who was not involved in the project.

Unfortunately, SOFIA can’t continue mapping the moon’s water—the modified Boeing 747 and telescope are now retired to the Pima Air & Space Museum in Tucson, Arizona. “I hope these results help pave the way for another one of these airborne observatories to be developed in the near future,” says Horvath.

[Related: Saying goodbye to SOFIA, NASA’s 747 with a telescope]

Despite the project’s untimely end, SOFIA managed to complete a large number of observations of the moon—among other celestial targets—in its final flights. In fact, it produced so much data that scientists are still sorting through it all. SOFIA’s discoveries “will continue for years to come,” says Honniball, and could prepare teams for future missions, all tackling questions about H₂O. Some prime examples include CalTech’s Lunar Trailblazer orbiter launching later this year, NASA’s water-hunting Volatiles Investigating Polar Exploration Rover (VIPER), and of course, the US Artemis program, which aims to land humans on the satellite’s southern regions as early as 2025.

These upcoming projects also promise the tantalizing prospect of delivering lunar soil samples back to Earth, something that hasn’t happened (for Americans, at least) since the Apollo program. “In the lab, even a single grain is like a world of its own revealing stories about the history and evolution of the material on the moon,” says Reach. Actually working with samples of lunar ice in a hands-on experiment could finally determine what form water takes on the moon.

Until then, planetary scientists will keep working through SOFIA’s moon maps, squeezing out every last drop of information they can.