October 26, 2020 PUBLICATION 20-105
This image shows the moon's Clavius crater with an image showing the water trapped there in the lunar floor, as well as an image from NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) that found sunlit moon water. Credits: NASA / Daniel Rutter
NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) has for the first time confirmed water on the moon's sunlit surface. This discovery suggests that water can be dispersed across the lunar surface and not limited to cold, shady locations.
SOFIA has discovered water molecules (H2O) in the Clavius crater, one of the largest craters in the southern lunar hemisphere that is visible from Earth. Previous observations of the lunar surface revealed some form of hydrogen, but could not distinguish between water and its close chemical cousin, hydroxyl (OH). Data from this location shows that water in concentrations of 100 to 412 ppm – roughly equivalent to a 12-ounce bottle of water – is trapped in one cubic meter of earth spread across the lunar surface. The results will be published in the latest edition of Nature Astronomy.
"We had evidence that H2O – the familiar water we know – might exist on the sunlit side of the moon," said Paul Hertz, director of the astrophysics division in the directorate of scientific missions at NASA headquarters in Washington. “Now we know it's there. This discovery challenges our understanding of the lunar surface and raises interesting questions about resources relevant to space exploration. "
For comparison: the Sahara has 100 times as much water as SOFIA in the lunar soil. Despite the small quantities, the discovery raises new questions about how water is created and how it stays on the rough, airless surface of the moon.
Water is a precious resource in space and an essential part of life as we know it. Whether the SOFIA water found is easily accessible for use as a resource remains to be determined. As part of NASA's Artemis program, the agency is committed to learning all about the presence of water on the moon before sending the first woman and next man to the surface of the moon in 2024 and building a sustainable human presence there by the end of the year Decade.
SOFIA's results build on years of previous research investigating the presence of water on the moon. When the Apollo astronauts first returned from the moon in 1969, it was considered completely dry. Orbital and impactor missions over the past 20 years, such as the NASA satellite for observing and recording lunar craters, confirmed ice in permanently shadowed craters around the poles of the moon. In the meantime, several spaceships – including the Cassini mission and the Deep Impact comet mission, as well as the Chandrayaan-1 mission of the Indian space research organization – and NASA's ground-based infrared telescope facility looked broadly over the lunar surface and found evidence of hydration in sunnier regions . However, these missions were unable to definitively differentiate the form in which they existed – either H2O or OH.
"Before the SOFIA observations, we knew there was some type of hydration," said Casey Honniball, lead author who published the results of her thesis at the University of Hawaii at Mānoa, Honolulu. "But we didn't know how many, if any, were actually water molecules – the way we drink every day – or something like drain cleaner."
Scientists using NASA's telescope on an airplane, the Stratospheric Observatory for Infrared Astronomy, first discovered water on a sunlit lunar surface. SOFIA is a modified Boeing 747SP aircraft that enables astronomers to study the Solar System and beyond in ways that ground-based telescopes cannot. Molecular water, H2O, was found in Clavius Crater, one of the largest craters visible from Earth in the southern lunar hemisphere. This discovery suggests that water can be dispersed across the lunar surface and not limited to cold, shady locations. Credits: NASA / Ames Research Center
SOFIA offered a new way of looking at the moon. This modified Boeing 747SP jetliner with a telescope that is 106 inches in diameter flies at altitudes up to 45,000 feet and reaches over 99% of the water vapor in Earth's atmosphere for a clearer view of the infrared universe. With its infrared CAmera for weak objects for the SOFIA telescope (FORCAST), SOFIA was able to record the specific wavelength of 6.1 micrometers for water molecules and discovered a relatively surprising concentration in the sunny Clavius crater.
"Without a thick atmosphere, water on the sunlit lunar surface should only be lost to space," said Honniball, who is now a postdoctoral fellow at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "But somehow we see it. Something creates the water and something has to catch it there."
Several forces could play a role in the delivery or production of this water. Micrometeorites that rain on the lunar surface and transport small amounts of water could deposit the water on the lunar surface upon impact. Another possibility is that there is a two-step process in which the sun's solar wind delivers hydrogen to the lunar surface and causes a chemical reaction with oxygenated minerals in the soil to form hydroxyl. In the meantime, radiation from bombarding micrometeorites could convert this hydroxyl group into water.
How the water is then stored and accumulated also raises some interesting questions. The water could become trapped in tiny bead-like structures in the ground that are formed from the high heat generated by micrometeorite impacts. Another possibility is for the water to be hidden between grains of the lunar soil and protected from sunlight – potentially making it a little more accessible than water that is enclosed in pearl-like structures.
For a mission designed to look at distant, dark objects like black holes, star clusters and galaxies, SOFIA's spotlight on Earth's closest and brightest neighbors was a departure from normal business operations. Telescope operators typically use a guide camera to track stars while keeping the telescope steady on its target. However, the moon is so close and bright that it fills the entire field of view of the guide camera. With no stars visible, it was unclear whether the telescope could reliably track the moon. To determine this, the operators decided in August 2018 to conduct a test observation.
"It was, in fact, the first time SOFIA had looked at the moon, and we weren't even entirely sure we would get reliable data, but questions about moon water forced us to try," said Naseem Rangwala, SOFIA's project Scientists at NASA's Ames Research Center in California's Silicon Valley. "It's incredible that this discovery came out of a test. Now that we know we can do this, we are planning more flights to get more observations."
The follow-up flights of SOFIA will search for water in additional sunlit locations and in different phases of the moon in order to learn more about how the water is produced, stored and moved across the moon. The data will complement the work of future lunar missions such as NASA's Volatile Investigating Polar Exploration Rover (VIPER) to produce the first water resource maps of the moon for future human space exploration.
In the same issue of Natural Astronomy, scientists published an article using theoretical models and data from NASA's Lunar Reconnaissance Orbiter, suggesting that water in tiny shadows that keep temperatures below freezing trapped over more moon than currently expected could be. You can find the results here.
"Water is a valuable resource, both for scientific purposes and for use by our explorers," said Jacob Bleacher, senior exploration scientist with NASA's directorate of human exploration and operations. "If we can use the resources on the moon, we can take less water and more equipment with us to enable new scientific discoveries."
SOFIA is a joint project between NASA and the German Aerospace Center. Ames manages the SOFIA program, science and mission operations in collaboration with the Universities Space Research Association based in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft is serviced and operated by NASA's Armstrong Flight Research Center Building 703 in Palmdale, California.
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