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The Moon is covered in craters and rocks, creating a surface “roughness” that casts shadows, as shown in this photograph from the Apollo 17 mission of 1972. These cold shadows can allow water ice to accumulate. in the form of frost even during the day. The detail area is highlighted in the following illustration. Credit: NASA
Shadows cast by the roughness of the Moon’s surface create small cold spots for water ice to accumulate even on a harsh lunar day.
Scientists are convinced that water ice can be found at the Moon’s poles inside permanently shaded craters – in other words, craters that never receive sunlight. But observations show that water ice is also present over much of the lunar surface, even during the day. It’s a conundrum: Previous computer models suggested that any water ice that forms during lunar night should quickly burn up as the Sun rises above us.
“More than a decade ago, a spacecraft detected the possible presence of water on the moon’s diurnal surface, and this has been confirmed by NASA’s Stratospheric Observatory for Infrared Astronomy. [SOFIA] in 2020, ”said Björn Davidsson, scientist at NASA’s Jet Propulsion Laboratory in Southern California. “These observations were, at first, counterintuitive: water should not survive in this harsh environment. This challenges our understanding of the lunar surface and raises intriguing questions about how volatile substances, like water ice, can survive on airless bodies. “
In a new study, Davidsson and co-author Sona Hosseini, researcher and instrument scientist at JPL, suggest that the shadows created by the “roughness” of the lunar surface serve as a refuge for water ice, allowing it to retreat. form as surface frost far from the poles of the moon. They also explain how the Moon’s exosphere (the fine gases that act like a thin atmosphere) can have an important role to play in this puzzle.
This illustration zooms in to the area of detail shown in the previous photo, showing how shadows allow water ice to survive on the sunlit lunar surface. When shadows move as the Sun passes overhead, the exposed frost lasts long enough to be detected by spacecraft. Credit: NASA / JPL-Caltech
Water traps and ice pockets
Many computer models simplify the lunar surface, making it flat and featureless. As a result, it is often assumed that the surface far from the poles warms evenly during the lunar day, which would prevent water ice from staying on the sunny surface for a long time.
So how come water is detected on the Moon beyond permanently shaded regions? One explanation for the detection is that water molecules can be trapped inside impact rock or glass created by the incredible heat and pressure of meteor impacts. Merged within these materials, as this hypothesis suggests, water can remain on the surface even heated by the Sun while creating the signal detected by SOFIA.
But a problem with this idea is that observations of the lunar surface show that the amount of water decreases before noon (when sunlight is at its peak) and increases in the afternoon. This indicates that water can move from one place to another during the lunar day, which would be impossible if it were trapped inside moon rock or impact glass.
Davidsson and Hosseini revised the computer model to take into account the surface roughness apparent in images from the Apollo missions from 1969 to 1972, which show a lunar surface strewn with boulders and pockmarked with craters, creating many gray areas even around midday. Taking this surface roughness into account in their computer models, Davidsson and Hosseini explain how it is possible for frost to form in small shadows and why the water distribution changes throughout the day.
Because there is no thick atmosphere to distribute heat around the surface, extremely cold, shady areas, where temperatures can drop to around minus 350 degrees Fahrenheit (minus 210 degrees Celsius), can be around hot areas exposed to the sun, where temperatures can also reach up to 240 Fahrenheit (120 Celsius).
As the Sun travels through the lunar day, the surface frost that can build up in these cold, shaded areas is slowly exposed to sunlight and recycled into the Moon’s exosphere. The water molecules then refreeze on the surface, re-accumulating as frost in other cold, shady places.
“Frost is much more mobile than trapped water,” Davidsson said. “Therefore, this model provides a new mechanism that explains how water moves between the lunar surface and the thin lunar atmosphere.”
One hypothesis is that water molecules are trapped in lunar matter (left). But a new study postulates that water molecules (right) remain as frost on the surface in cold shadows and move to other cold places via the thin exosphere. Credit: NASA / JPL-Caltech
To look closer
While this is not the first study to take surface roughness into account when calculating lunar surface temperatures, previous work did not consider how shadows would affect the capacity of water molecules. to remain on the surface during the day in the form of frost. This new study is important because it helps us better understand how lunar water is released and removed from the Moon’s exosphere.
“Understanding water as a resource is essential for NASA and commercial efforts for future human lunar exploration,” Hosseini said. “If water is available as frost in the sunny regions of the Moon, future explorers could use it as a resource for fuel and potable water. But first, we need to understand how the exosphere and the surface interact and what role this plays in the cycle.
To test this theory, Hosseini is leading a team to develop ultra-miniature sensors to measure weak signals from water ice. The Heterodyne OH Miniaturized Lunar Spectrometer (HOLMS) is under development for use on small stationary landers or autonomous rovers – like JPL’s Autonomous Folding Flatbed Explorer Robot (A-PUFFER), for example – which can be sent on the Moon in the future to make direct measurements of hydroxyl (a molecule that contains one atom of hydrogen and one atom of oxygen).
Hydroxyl, which is a molecular cousin of water (a molecule with of them hydrogen atoms and an oxygen atom), can serve as an indicator of how much water may be present in the exosphere. Water and hydroxyl could be created by meteor impacts and solar wind particles hitting the lunar surface, so measuring the presence of these molecules in the Moon’s exosphere can reveal how much water is created while by showing how it moves from place to place. But time is running out to take these measurements.
“Current lunar exploration by several nations and private companies indicates significant man-made changes in the lunar environment in the near future,” Hosseini said. “If this trend continues, we will lose the opportunity to understand the natural lunar environment, especially the water that circulates through the pristine exosphere of the Moon. Therefore, the advanced development of ultra-compact and high-sensitivity instruments is of critical and urgent importance.
The researchers point out that this new study could help us better understand the role that shadows play in the accumulation of water ice and gas molecules beyond the Moon, such as on Mars or even on the particles of the rings. of Saturn.
The study was published in the Monthly notices from the Royal Astronomical Society August 2, 2021.
Reference: “Implications of surface roughness in models of water desorption on the Moon” by Björn JR Davidsson and Sona Hosseini, August 2, 2021, Monthly notices from the Royal Astronomical Society.
DOI: 10.1093 / mnras / stab1360
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