Scientists are confident that water ice can be found at the Moon’s poles within permanently shadowed craters – craters that never see sunlight. However, observations show that water ice is present across much of the lunar surface, even during the day. This is a conundrum: Previous computer models predicted that any water ice that formed during the lunar night would burn off quickly as the Sun rose overhead.
“Spacecraft detected the possible presence of water on the dayside surface of the Moon over a decade ago, and this was confirmed by NASA’s Stratospheric Observatory for Infrared Astronomy [SOFIA] in 2020,” said Björn Davidsson, a scientist at NASA’s Jet Propulsion Laboratory in Southern California. “These findings seemed counterintuitive at first: water shouldn’t be able to survive in such a harsh environment. This raises intriguing questions about how volatiles, such as water ice, can survive on airless bodies and challenges our understanding of the lunar surface.”
In a new study, Davidsson and co-author Sona Hosseini, a research and instrument scientist at JPL, propose that shadows cast by the lunar surface’s “roughness” provide a safe haven for water ice, allowing it to form as surface frost far from the Moon’s poles. They also explain how the Moon’s exosphere (the tenuous gasses that act as a thin atmosphere) may play an important role in this puzzle.
The shadows cast by the roughness of the Moon’s surface create small cold spots for water ice to accumulate even during the harsh lunar daytime.
Water Traps and Frost Pockets
Many computer models simplify the lunar surface, making it flat and devoid of features. As a result, it is commonly assumed that the surface far from the poles heats up uniformly during lunar daytime, making it impossible for water ice to remain on the sunlit surface for an extended period of time.
So, how is water detected on the Moon beyond the permanently shadowed regions? One possible explanation for the detection is that water molecules are trapped inside a rock or the impact glass formed by the incredible heat and pressure of meteorite strikes. As this hypothesis suggests, water fused within these materials can remain on the surface even when heated by the Sun while producing the signal detected by SOFIA.
However, observations of the lunar surface show that the amount of water decreases before noon (when sunlight is at its strongest) and increases in the afternoon. This suggests that the water is moving from one location to another during the lunar day, which would be impossible if it was trapped inside a lunar rock or impact glass.
The computer model was revised by Davidsson and Hosseini to account for the surface roughness visible in images from the Apollo missions from 1969 to 1972, which show a lunar surface strewn with boulders and pockmarked with craters, creating plenty of shady areas even near noon. Davidsson and Hosseini explain how frost can form in small shadows and why the distribution of water changes throughout the day by incorporating surface roughness into their computer models.
Because there is no thick atmosphere to distribute heat around the surface, extremely cold, shaded areas with temperatures as low as minus 350 degrees Fahrenheit (minus 210 degrees Celsius) can be found next to hot, exposed to the Sun areas with temperatures as high as 240 degrees Fahrenheit (120 Celsius).
The surface frost that may accumulate in these cold, shaded areas is gradually exposed to sunlight and cycled into the Moon’s exosphere as the Sun moves through the lunar day. The water molecules then refreeze onto the surface, where they re-accumulate as frost in other cold, shaded locations.
“Frost is far more mobile than trapped water,” said Davidsson. “Therefore, this model provides a new mechanism that explains how water moves between the lunar surface and the thin lunar atmosphere.”
A Closer Look
While this is not the first study to consider surface roughness when calculating lunar surface temperatures, previous work did not consider how shadows would affect the ability of water molecules to remain on the surface as frost during the day. This new study is significant because it contributes to a better understanding of how lunar water enters and exits the Moon’s exosphere.
“Understanding water as a resource is critical for NASA and commercial endeavors for future human lunar exploration,” said Hosseini. “If water exists in the form of frost in the Moon’s sunlit regions, future explorers may use it as a source of fuel and drinking water. But first, we must determine how the exosphere and surface interact and what role this plays in the cycle.”
Hosseini is leading a team to develop ultra-miniature sensors to measure the faint signals from water ice in order to test this theory. The Heterodyne OH Lunar Miniaturized Spectrometer (HOLMS) is being developed for use on small stationary landers or autonomous rovers – such as JPL’s Autonomous Pop-Up Flat Folding Explorer Robot (A-PUFFER) – that may be sent to the Moon in the future to measure hydroxyl (a molecule that contains one hydrogen atom and an oxygen atom).
Hydroxyl, a molecular cousin of water (a molecule with two hydrogen atoms and one oxygen atom), can be used to estimate the amount of water in the exosphere. Because both water and hydroxyl can be produced by meteorite impacts and solar wind particles striking the lunar surface, measuring the presence of these molecules in the Moon’s exosphere can reveal how much water is produced while also revealing how it moves from place to place. However, time is of the essence when it comes to taking those measurements.
“The current lunar exploration by several nations and private companies suggests that significant artificial changes to the lunar environment will occur in the near future,” Hosseini said. “If this trend continues, we will miss out on learning about the natural lunar environment, particularly the water that circulates in the Moon’s pristine exosphere. As a result, the advanced development of ultra-compact, high-sensitivity instruments is critical and urgent.”
According to the researchers, this new study could help us better understand the role of shadows in the accumulation of water ice, and gas molecules beyond the Moon, such as on Mars or on particles in Saturn’s rings.