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For most of the space age, the moon was considered a world without water. In recent years, however, an ongoing teardrop discovery has shown that at least parts of the moon – such as the large permanently shaded craters at its poles – contain significant water deposits. This week, two new studies published in Nature Astronomy open the tap a little more to the prospect of an unexpectedly watery moon.
The timing is good for NASA and other space agencies that are planning ambitious human lunar exploration and even settlement missions. After all, where there is water, there can be life, even if that life still requires space suits and radiation resistant habitats.
A possible sign of solar heated water
The first new smell of lunar water emerged from data collected by NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA). This modified Boeing 747SP jet provides its 2.7-meter telescope with greater than 99% view of the atmosphere’s obscuring water vapor, a unique capability that allows agile infrared observations without the use of space structures.
In late August 2018, a team led by Casey Honniball, a member of NASA’s postdoctoral program at the agency’s Goddard Space Flight Center and a researcher at the University of Hawaii at Manoa, used infrared instruments aboard SOFIA to study the lunar surface illuminated by the sun. The observations, lasting only 10 minutes, focused on a region in high southern latitudes near the large lunar crater Clavius, and revealed strong infrared emission at a wavelength of six microns (µm) from the crater and from the surrounding landscape. Heated by the sun, something on the lunar surface was re-emitting the absorbed radiation just as molecular water would – simple H2O.
NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) is shown in flight with the sliding door on its 17-ton infrared telescope fully open. Credit: NASA and Jim Ross
“We are not aware of any other reasonable material for the Moon that exhibits a single 6 µm spectral characteristic other than H2O,” report Honniball and his fellow researchers in their new paper. The authors suggest that the purported water is most likely stored in natural volcanic glass or sandwiched between microscopic grains of rock dust. Both scenarios could provide shielding from extreme temperatures and near-vacuum conditions on the lunar surface, allowing water to persist. As to how it got there, no one is sure, but the main explanation is that the water may have formed from the free oxygen and hydrogen released from the moon rocks by the impacts of micrometeorites.
Using SOFIA is a new and unique approach to lunar science, Honniball says, but it’s not the first time Earth observations have revealed a six-micron emission from the moon. Balloon observations by astronomers GR Hunt and JW Salisbury showed the spectral feature, he says. But Hunt and Salisbury did not mention it in their paper on that research, published in 1969. Instead, they focused on characterizing minerals on the lunar surface. “Maybe they just didn’t know they had made a great discovery,” speculates Honniball.
A glass half full
Honniball and his colleagues have already received additional SOFIA time for follow-up observations. “We hope to map most of the moon to characterize the behavior of the water,” he says. “Does it vary on the lunar surface with the time of day and the lunar latitude? This will help us understand its sources and where it resides. “
And that, in turn, could tell the world how useful this newfound water could one day prove to be. The extraction will be simple if the water exists mainly on the surface of the rock grains: it will be enough to collect the lunar soil and subject it to moderate heating. If, however, water is stuck in the glass, the material must be dissolved to release the water for collection – a much more energy-hungry process.
“At the moment we don’t have a good idea if the water we see with SOFIA is enough to make the glass worth melting,” says Honniball. “However, if we find that the abundances are high enough, this may be a more feasible option than extracting water ice in permanently shaded regions, which are extreme environments and difficult to work with.”
Jack Schmitt, a geologist who, as a member of the Apollo 17 crew, remains the only professional scientist to have walked on the moon, says SOFIA measurement may not reveal true molecular water but something more fragile and transient. “The question I would like to ask,” says Schmitt, “is whether the SOFIA data can be related to the possible weak bond of solar wind hydrogen with oxygen on the surface of the silicate and mineral glass grains in the regolith rather than being actual. molecular water? “
A product of such reactions could be hydroxyl, a molecule just a hydrogen atom running out of water. Honniball, however, states that the six-micron emission seen by SOFIA is inconsistent with hydroxyl.
Regardless of the substance behind the SOFIA signal, Schmitt notes that the basic chemistry should allow moisture to be squeezed out of even dry lunar material. “Heating the hydrogen-containing regolith to several hundred degrees would cause the hydrogen to react with the oxygen in the silicates to produce water almost anywhere on the moon,” he says.
Small shadows, immense possibilities
Another paper published alongside the SOFIA study in Nature Astronomy highlights an increase in the distribution of permanently shaded areas on the moon – places without sunlight known as cold traps – where extremely low temperatures could freeze and sequester water essentially indefinitely. allowing it to accumulate. in significant deposits over geological time.
Oblique view of the sunny edge of Shackleton, a large, deep crater near the lunar south pole with a permanently shaded interior that is home to substantial water ice. Credits: NASA, GSFC and Arizona State University
Scientists have studied these lunar regions for decades for their water-welcoming potential, but previous work has focused on large cold traps inside huge craters at the poles of the moon. Conversely, this latest result extends the size range of the considered cold traps up to one centimeter in diameter. By analyzing high-resolution images from NASA’s Lunar Reconnaissance Orbiter, a team led by the University of Colorado Boulder planetary scientist Paul Hayne found that such “micro” cold traps are far more widespread than large, well-studied ones in the vicinity of lunar poles. The new accounting increases the total land area with the ability to trap water to around 40,000 square kilometers, a pan-lunar region that, taken together, would be twice the size of Wales.
“Recently discovered micro cold traps are the most numerous on the moon, thousands of times more abundant than previously mapped cold traps,” says Hayne. “If they are all full of ice, this could be a substantial amount, perhaps more than a billion kilograms of water.”
Hayne adds, however, that in situ sampling by robots or astronauts is necessary to correctly assess their actual ice content. “The really exciting thing about micro cold traps is that they are much more accessible, which could allow for more efficient extraction and use for both scientific and exploratory purposes,” he says. Indeed, this proliferation of tiny potential ice reservoirs could be much more accessible to future missions, Hayne says, because they exist in areas where an astronaut bathed in sunlight could comfortably and safely use a tool to reach a dangerously cold shadow. to dig ice.
For now, to further judge the value of micro cold traps, Hayne and his colleagues will use a high-tech camera dubbed the Lunar Compact Infrared Imaging System, which will travel to the moon with the first southern polar lander mission of NASA’s Commercial Lunar Payload Program. services as early as 2022. The camera will take close-up images of micro cold traps for the first time and measure their temperature.
Ground Truthing
On the one hand, SOFIA and micro cold trap studies have welcomed the news. However, the big picture remains the same, says Ian Crawford, a moon expert at Birkbeck, University of London.
Clearly, he says, the more easily accessible water is on the moon, the greater the opportunities for its on-site extraction and use to support immediate exploration efforts. Eventually, the development of lunar water as a resource could trigger an entire extraterrestrial economy where the substance would become a profitable raw material for rocket fuel and other valuable consumables. For now, however, “basic truth measurements are urgently needed to confirm the inferences made on the basis of remote sensing measurements,” Crawford says.
Angel Abbud-Madrid, director of the Center for Space Resources at the Colorado School of Mines in Golden, Colorado, also points to direct measurements as the next most important step to follow from new discoveries. “What is now needed is to touch the lunar surface and gather detailed information about the terrain,” he says. “Confirmation not only of the existence of water ice, but of its morphology, concentration, distribution and abundance is a must to proceed with the exploration and use plans of existing resources.”
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