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For decades, astronomers have speculated that there may be water on the Moon. In recent years, this speculation confirmed one orbiting satellite after another detected water ice around the south polar region of the Moon. Within this part of the lunar surface, known as the Aitken South Pole Basin, water ice can persist due to the many permanently shadowed craters found there.
But until now, scientists were operating under the assumption that lunar water could only be found in permanently shaded craters. But thanks to NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), water has been observed on the sunlit side of the Moon for the first time. This discovery indicates that water can be distributed over the entire lunar surface and is not limited to dark corners.
The study describing their findings appeared recently in the journal Astronomy of nature. The study was led by Casey Honniball, a NASA postdoctoral fellow at the University of Hawaii Institute of Geophysics and Planetology, and included members of the Space Sciences Institute (SSI), Georgia Institute of Technology, Physics Laboratory Johns Hopkins University Applied (JHUAPL). ) and NASA’s Goddard Space Flight Center.
SOFIA is essentially a modified Boeing 747SP aircraft equipped with a 2.7-meter (106-inch) infrared telescope. With a service ceiling of 11,600 to 13,700 meters (38,000 to 45,000 feet), SOFIA can fly above 99% of Earth’s infrared blocking atmosphere and search for objects that would otherwise be faint using its Infrared Object Camera. Weak for the SOFIA Telescope (FORCAST).
When Dr. Honnibal and her colleagues looked at the Moon with SOFIA, what they noticed was the presence of water molecules (H2O) in the second largest crater visible from Earth. This is known as Clavius Crater, which is located in the southern hemisphere of the Moon and is 231 km (143.5 miles) in diameter and 3.5 km (2 miles) deep.
As Paul Hertz, director of the Astrophysics Division of NASA’s Science Mission Directorate (SMD), said in a recent NASA statement:
“We had indications that H2Or, the familiar water we know, could be present on the sunlit side of the Moon. Now we know it’s there. This discovery challenges our understanding of the lunar surface and raises intriguing questions about resources relevant to deep space exploration. “
While previous observations had detected traces of hydrogen in this crater, they could not determine whether it was due to water or hydroxyl (OH). This chemical is formed when molecular oxygen in the lunar regolith joins with hydrogen atoms, which are the result of charged particles (protons) emanating from the Sun (solar wind) that collect electrons as they travel through space.
Using the FORCAST telescope, Honnibal and his team were able to make that determination, since the hydrogen they detected had a specific wavelength that is unique to water molecules (6.1 microns). As Honnibal explained:
“Before SOFIA’s observations, we knew there was some kind of hydration. But we didn’t know how much, if any, were actually water molecules, as we drink every day, or something more like a drain cleaner … Without a thick atmosphere, the water on the lunar surface illuminated by the sun should be lost in space. Yet somehow we are seeing it. Something is generating the water and something must be trapping it there. “
Data obtained by SOFIA revealed water concentrations of 100 to 412 parts per million (ppm) in a single cubic meter of soil scattered on the lunar surface. This is the equivalent of approximately a 350 ml (12 oz) bottle and is approximately 100 times less water than that found in the Sahara desert. Despite this small amount of water, the discovery is highly significant as it raises new questions about the origins of lunar water and how it can persist.
Furthermore, this finding has implications for lunar exploration, particularly when it comes to long-duration missions and lunar habitats. As part of Project Artemis, NASA is looking to establish a “sustainable lunar exploration” program that will include a base around the south polar region. The presence of water ice will not only ensure the supply of drinking water, it could also be used to make propellants.
“Water is a valuable resource, both for scientific purposes and for the use of our explorers,” said Jacob Bleacher, chief exploration scientist with NASA’s Human Exploration and Operations Mission Directorate. “If we can use the resources of the Moon, then we can transport less water and more equipment to help enable new scientific discoveries.”
This finding is also significant in the way it represents the culmination of decades of research. When Apollo astronauts first landed on the Moon, the Moon was believed to be completely dry. Interestingly, it was the rocks that these missions returned that provided the first hints of lunar water, although they were discarded as a result of contamination at the time.
Another interesting conclusion from this latest finding is the fact that it is not the kind of research that the SOFIA mission usually does. Typically, SOFIA’s high-altitude observations are guided by a camera that tracks the stars, allowing controllers to keep the telescope fixed on their target. These include objects that are faint (such as red dwarfs and black holes) or distant objects, such as star clusters and galaxies.
The Moon, which is neither faint nor distant, would normally fill the entire field of view of the guide camera. After conducting a test observation in August 2018, NASA scientists were convinced it was worth a try. Said Naseem Rangwala, SOFIA project scientist at NASA Ames Research Center:
“In fact, it was the first time SOFIA had looked at the Moon and we weren’t even completely sure if we would get reliable data, but the questions about the water on the Moon forced us to try. It’s amazing that this discovery came out of what was essentially a test, and now that we know we can do this, we are planning more flights to make more observations.
Naturally, there are a number of unanswered questions that scientists have yet to grapple with. To begin with, there is the question of whether the water reservoirs in the sunlit parts of the Moon are accessible or not. There is also the ongoing mystery about the origins of lunar water (whether it was indigenously produced or deposited), how it can accumulate and persist in sunlit regions, and how it is transported across the Moon.
In the near future, NASA hopes to conduct follow-up observations with SOFIA to collect more data and (hopefully) answer these questions. In particular, they will look for additional sources of water in places illuminated by the sun and during different phases of the moon. This data will also inform future missions like NASA’s Volatile Polar Research Rover (VIPER) and help create the first water source maps for human exploration.
Further reading: NASA, Nature astronomy
