Concept art of a lunar base. Image: NASA
Humans have dreamed about living on the Moon for centuries, but this ambitious vision hinges on the amount of water that lies hidden in the lunar surface. As an essential resource for life support, rocket fuel, and power generators, the water supply on the Moon directly correlates to the odds of establishing a long-term human presence there.
Now, scientists have announced the first unambiguous evidence of water on sunlit surfaces of the Moon and concluded that lunar water ice “may be more widely distributed and accessible as a resource for future missions than previously thought,” according to a major research package published in Nature Astronomy.
“Today we are announcing that, for the first time, water has been confirmed to be present on a sunlit surface of the Moon,” said Paul Hertz, astrophysics division director at NASA Headquarters in Washington DC, during a NASA teleconference on Monday.
“This is exciting because the expectation is that any water present on a sunlit surface of the Moon would not survive the lunar day,” continued Hertz. “The discovery reveals that water might be distributed across the lunar surface and not limited to the cold shadowed places near the lunar poles, where we have previously discovered water ice.”
The important discoveries are outlined in two studies that respectively report the unmistakable signature of lunar molecular water and reveal an expanded map of shadowy “cold traps” at the Moon’s poles that may contain reservoirs of water ice.
Decades of previous research already strongly supports the existence of water on the Moon, but this pair of studies reveals new insights about the whereabouts, properties, and possible extent of this valuable resource.
The results suggest that some 40,000 square kilometers of the Moon’s surface may contain water ice, about twice what was previously estimated. For reference, the Moon’s total surface area is about 38 million kilometers, so the vast majority of its terrain is bone-dry as a result of searing temperatures during the two-week-long lunar day.
Scientists led by Casey I. Honniball, a NASA postdoctoral program fellow at the agency’s Goddard Space Flight Center, captured the signal of molecular water using an airborne telescope called the Stratospheric Observatory for Infrared Astronomy (SOFIA).
Honniball and her colleagues note that three spacecraft have already detected “widespread hydration” on the Moon, but that it was not clear whether the source was water or other hydroxyl (OH) compounds, according to their study, which is entitled, “Molecular water detected on the sunlit Moon by SOFIA.”
To distinguish between those possibilities, the team searched the lunar south pole for signs of a special spectral signal, called the 6-micron emission feature, that is unique to water. During a flight on August 31, 2018, SOFIA was able to pick up clear readings of the signal in and around a large crater called Clavius at the south pole.
“We find that the distribution of water over the small latitude range is a result of local geology and is probably not a global phenomenon,” Honniball and her colleagues said in the study.
“We suggest that a majority of the water we detect must be stored within glasses or in voids between grains sheltered from the harsh lunar environment, allowing the water to remain on the lunar surface,” the team added.
The detection is particularly interesting because it shows that water can survive on parts of the sunlit Moon. Honniball and her colleagues estimated that this water is present in abundances ranging from 100 to 400 parts per million, which is about equal to a 12-ounce bottle of water within a cubic meter of lunar soil.
“To be clear, this is not puddles of water but instead water molecules that are so spread apart that they do not form ice or liquid water,” Honniball said during the telecon.
In addition to these tiny water oases, the Moon’s south pole contains an array of small patches of water ice, according to another study entitled “Micro cold traps on the Moon.”
A team led by Paul Hayne, a planetary scientist at the University of Colorado Boulder, used data collected by NASA’s Lunar Reconnaissance Orbiter to map out regions of the lunar poles that never receive direct sunlight, due to a combination of the Moon’s axis and the shadows cast by its rough terrain.
Past missions have revealed that large deposits of water ice may be frozen into huge shaded craters at the poles, but Hayne and his colleagues expanded the map to account for ice locked away in smaller cold traps, down to the size of a penny. The team’s models suggest that the most numerous cold traps on the Moon measure just one centimeter across, suggesting there could be twice as much water ice at the lunar poles than previously thought.
“Small-scale shadows in the polar regions, which we term ‘micro cold traps’, substantially augment the cold-trapping area of the Moon, and may also influence the transport and sequestration of water,” the team said in the study.
“The abundance of small-scale cold traps implies that future missions exploring for ice may more easily target and access one of these potential reservoirs,” the researchers added. “If water is found in micro cold traps, the sheer number and topographic accessibility of these locales would facilitate future human and robotic exploration of the Moon.”
Together, the two new studies led by Honniball and Haynes represent a breakthrough in our understanding of the Moon’s water supply, which has direct implications for humanity’s long-time vision of moving our species to the lunar surface.
The new research is especially topical given that NASA plans to land humans on the Moon in the 2020s and use lunar resources as part of its Artemis program, prompting thorny discussions about legal and ethical extraction of materials on the Moon. Now that we have a better estimate of the Moon’s untapped water, it is more urgent than ever that the international community establishes standards for lunar exploration and resource extraction.
Update: This article has been updated with comments from a NASA teleconference about Honniball’s research.