For decades, astronomers have speculated that there might be water on the moon. In recent years this speculation has been confirmed that one orbiting satellite after another discovered water ice around the southern polar region of the moon. In this part of the lunar surface known as the South Pole Aitken Basin, the water ice can persist due to the many permanently shaded craters that are there.
So far, however, the scientists assumed that moon water can only be found in permanently shaded craters. Thanks to NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), water was observed for the first time on the sunlit side of the moon. This discovery suggests that water can be distributed over the entire surface of the moon, rather than limited to the dark corners.
The study describing its results was recently published in the journal Nature Astronomy. The study was led by Casey Honniball, a NASA postdoctoral fellow at the University of Hawaii Institute of Geophysics and Planetary Sciences, and included members from the Space Science Institute (SSI), Georgia Institute of Technology, and the Johns Hopkins University Applied Physics Laboratory (JHUAPL) ) and NASA's Goddard Space Flight Center.
SOFIA is essentially a modified Boeing 747SP aircraft fitted with a 2.7 meter infrared telescope. With a service cap of 11,600 to 13,700 meters (38,000 to 45,000 feet), SOFIA can fly over 99% of the earth's infrared blocking atmosphere and use the infrared CAmera for the SOFIA telescope (FORCAST) to search for otherwise faint objects.
When Dr. Honnibal and her colleagues observed the moon with SOFIA, they noticed the presence of water molecules (H2O) in the second largest crater visible from Earth. This is known as the Clavius Crater, which is located in the southern lunar hemisphere and is 231 km in diameter and 3.5 km in depth.
As Paul Hertz, director of the Department of Astrophysics in NASA's Directorate of Science Mission (SMD), said in a recent NASA statement:
“We had evidence that H2O – the well-known water we know – might 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 interesting questions about resources relevant to space exploration. "
Previous observations had found traces of hydrogen in this crater, but could not determine whether it was water or hydroxyl (OH). This chemical is created when molecular oxygen in the moon's regolith combines with hydrogen atoms, which are the result of charged particles (protons) emanating from the sun (solar wind) and picking up electrons on their way through space.
Elevation data of the moon showing the South Pole Aitken Basin. Photo credit: NASA / GSFC / University of Arizona
With the FORCAST telescope, Honnibal and her team were able to make this determination because the hydrogen they detected had a specific wavelength that only applies to water molecules (6.1 micrometers). As Honnibal explained:
“Before the SOFIA observations, we knew that there was some kind of hydration. But we didn't know how many, if any, water molecules actually were – how we drink every day – or rather drain cleaners … Without a thick atmosphere, water on the sunlit lunar surface should simply be lost into space. But somehow we see it. Something creates the water and something has to catch it there. "
The data obtained from SOFIA showed water concentrations of 100 to 412 parts per million (ppm) in a single cubic meter of soil spread over the surface of the moon. This is roughly equivalent to a 350 ml bottle and is about 100 times less water than the Sahara desert. Despite this small amount of water, the discovery is of great importance as it raises new questions about the origin of moon water and its persistence.
In addition, this find has an impact on lunar exploration, especially in long-term missions and lunar habitats. As part of the Artemis project, NASA would like to set up a "sustainable lunar exploration" program that includes a base in the southern polar region. The presence of water ice not only ensures the supply of drinking water, but can also be used to make propellants.
"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."
A close-up of the SOFIA telescope and primary mirror. Photo credit: NASA / Tom Tschida
This find is also significant because it represents the culmination of decades of research. When the Apollo astronauts first landed on the moon, it was believed that the moon was completely dry. Interestingly, it was the rocks that returned these missions that provided the first evidence of moon water, although these were rejected due to the contamination at the time.
Another interesting takeaway from this latest find is the fact that it is not the type of research that the SOFIA mission normally does. Normally, SOFIA's altitude observations are controlled by a camera that tracks stars so that the controllers can keep the telescope fixed on the target. These include faint objects (like red dwarfs and black holes) or distant objects like star clusters and galaxies.
The moon, which is neither faint nor distant, usually fills the entire field of view of the guide camera. After a test observation in August 2018, NASA scientists were convinced it was worth trying. Naseem Rangwala, SOFIA project scientist at NASA's Ames Research Center, said:
"It was indeed 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. It is incredible that this discovery emerged from a test. Now that we know we can do this, we are planning more flights to make more observations. "
SOFIA in flight with the telescope exposed. Image: NASA / Jim Ross
Of course, there are a number of unanswered questions that scientists have yet to grapple with. For starters, the question arises whether or not water deposits on the sunlit parts of the moon are accessible. There is also the lingering mystery of where lunar water comes from (whether it was indigenously produced or stored), how it accumulates and persists in sunlit regions, and how it is transported across the moon.
In the near future, NASA hopes to do follow-up observations with SOFIA to collect more data and (hopefully) answer these questions. In particular, they will look for additional water sources in sunlit locations and in different phases of the moon. This data will also inform future missions like NASA's Volatile Investigating Polar Exploration Rover (VIPER) and help create the first water source maps for human exploration.
Further reading: NASA, natural astronomy