For a long time the Moon was considered completely dry. Analysis of samples recovered by the Apollo missions showed only traces of water and it was believed that these traces were due to contamination of the Earth. Over the past two decades, new analyzes, observations from spacecraft missions, and theoretical models have shown that initial assessment was incorrect.
Since then, “water” has been detected within the minerals of lunar rocks. Water ice has also been found to mix with lunar dust grains in cold regions and permanently in the shade near the lunar poles.
But scientists weren’t sure how much “molecular water” was, which is made up of two parts hydrogen and one part oxygen (H₂O). Now, two new studies published in Nature Astronomy provide an answer, while also giving an idea of how and where to extract it.
Water and more water
The term “water” is not only used for molecular water, but also for hydrogen (H) and hydroxyl (OH) detections. Although astronauts could combine H and OH to form molecular water on the lunar surface, it is important to know in what form these compounds are initially present. That’s because this will have an impact on their stability and location under lunar surface conditions, and the effort required to extract them. Molecular water, if present as ice, would be easier to extract than the hydroxyl locked up in rocks.
The presence of water on the Moon is scientifically interesting. Their layout and shape can help address some deep questions. For example, how did water and other volatile substances get into the inner Solar System in the first place? Was it produced there or carried by asteroids or meteorites? Knowing more about the specific compound could help us find out.
Understanding how much water there is and its location is also incredibly useful for planning human missions to the Moon and beyond. Water represents a key resource that can be used for life support purposes, but it can also be broken down into its constituent elements and put to other uses. Oxygen could replenish air supplies and be used in simple chemical reactions on the lunar surface to extract other useful resources from regolith (soil made up of tiny grains). Water could also be used as rocket fuel in the form of liquid hydrogen and liquid oxygen.
This means that the Moon has great potential to become a refueling base for space missions to the inner Solar System or beyond. Its lower gravity and lack of atmosphere means it would require less fuel to launch from there than from Earth. When space agencies talk about resource utilization in situ on the Moon, water is at the center of their plans, which makes the new findings very exciting.
A new investigation
Instruments aboard several spacecraft have previously measured the reflectivity (light broken down by wavelength) of the Moon. These detect the light coming from a surface to measure how much energy it reflects at a specific wavelength. This will differ depending on the composition of the surface. Because it has water, the Moon’s surface absorbs light at wavelengths of 3 µm (0.000003 meters).
However, absorptions at this wavelength cannot distinguish between molecular water and hydroxylic compounds.
Using the NASA / DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope, which flies at 13 kilometers, the team that produced one of the new papers observed sunlit sections of the Moon’s surface at wavelengths of 5 -8 𝜇m.
The H₂O results in a characteristic peak in the spectrum at 6𝜇m, and by comparing an area near the equatorial as a baseline (which is believed to have almost no water) with an area near the south pole, this study reports the first observations unequivocal water in ambient conditions on the lunar surface in an abundance of 100 to 400 parts per million.
This is several orders of magnitude too large for most of the water to adsorb on the surfaces of the regolith grains. Instead, the authors suggest that the water they have observed must be encased within a glass formed by tiny meteorites that impact and melt the already hydrated grains of regolith. Alternatively, it could be present in gaps between grain boundaries, which would facilitate its removal. Knowing the exact place where this water is found would be of great interest to future explorers, as it would determine the processes and energy required to extract it.
Fortunately, the other paper used new theoretical models, based on temperature data and higher-resolution images from the Lunar Reconnaissance Orbiter, to refine predictions of where the right conditions exist for molecular water to be trapped like ice.
Previous research has already shown that such kilometer-wide “cold traps” exist in permanently shaded areas near the poles where there may be water ice. However, the evidence from an orbiting spacecraft was inconclusive about whether it was molecular water or hydroxyl. The new study finds that there are also abundant small cold traps where conditions allow water ice to accumulate, on the centimeter or decimeter scale. In fact, these traps should be hundreds or thousands of times more numerous than the largest cold traps.
The team estimates that 0.1% of the total lunar surface is cool enough to trap water in the form of ice, and that most of these ice traps are found at high latitudes (> 80 °). This is particularly close to the lunar south pole, reducing the choice of future landing sites that are more likely to find trapped icy water. However, it is important to realize that the two studies investigated areas at different latitudes (55 ° -75 ° S vs.> 80 ° S) and therefore cannot be directly compared.
However, these latest discoveries further improve our understanding of the history of water in our closest neighbor. They will undoubtedly reinforce the plans to return to the Moon. Instruments such as the European Space Agency’s (PROSPECT payload on the Moon 27) will be able to perform measurements on the Moon to “check on the ground” these tantalizing glimpses of the vast amount of information that has yet to be discovered.
James Mortimer, Postdoctoral researcher in Planetary Science and Exploration, The Open University y Mahesh Anand, Professor of Planetary Science and Exploration, The Open University
Este artículo fue publicado originalmente en The Conversation. Lea el original.
