Future astronauts headed to the moon may have an easier time finding water and digging up ice than had been thought.
In a paper published in Monday’s Nature Astronomy, a team of scientists used SOFIA, an infrared telescope mounted inside a 747 jumbo jet, to make observations that showed unambiguous evidence of water on parts of the moon where the sun shines.
“This discovery reveals that water might be distributed across the lunar surface and not limited to the cold shadowed places near the lunar poles,” Paul Hertz, the director of NASA’s astrophysics division, said during a news conference Monday.
Although that water could be difficult to collect by astronauts, another group of researchers on Monday said that in addition to big, frigid, deep and potentially treacherous craters in the moon’s polar regions, smaller and shallower depressions in the same areas may also be cold enough to hold onto water ice for millions, if not billions, of years.
These small water ice deposits could be a “real game changer,” Paul O. Hayne, a professor of astrophysical and planetary sciences at the University of Colorado, who led that research, said in an interview. “It could make it much more accessible to future astronauts and rover missions.”
The moon’s South Pole has become a desired destination for a number of robotic missions by NASA, China and other space programs.
Such ice might not only provide water for future astronauts to drink, but water molecules can also be broken apart into their constituent hydrogen and oxygen atoms. The oxygen would give the astronauts something to breathe. Hydrogen and oxygen can also be used as rocket propellant for trips home to Earth or even some day to Mars and beyond.
“Anytime we don’t need to pack water for our trip, we have an opportunity to take other useful items with us,” said Jacob Bleacher, chief exploration scientist for the NASA’s human exploration and operations directorate.
In the observations taken by NASA’s flying telescope, scientists were able to observe a wavelength of infrared light, at six microns, emitted by water molecules. Those emissions were seen in sunlit parts of the Clavius crater near the South Pole but not near the lunar Equator where temperatures get warmer.
Observations by spacecraft a decade ago had also suggested a more widespread distribution of water on the moon. Those measurements focused on a shorter, three-micron wavelength that was more ambiguous, unable to differentiate between a water molecule, which consists of two hydrogen atoms and one oxygen atom or hydroxyl, which has one hydrogen atom and one oxygen atom.
“Hydroxyl is actually the active ingredient in drain cleaners,” said Casey I. Honniball, a postdoctoral researcher at NASA’s Goddard Spaceflight Center in Greenbelt, Maryland, and lead author of the study that used SOFIA. “Hypothetically, if drain cleaner were on the moon, we could not tell the difference between the drain cleaner and water using the three-micron wavelength.”
The six-micron emissions are a “distinct chemical fingerprint” for water, Honniball said.
These observations cannot be performed from Earth’s surface because there is too much water in the lower atmosphere. Also, no lunar spacecraft, present or planned, has an instrument to examine this particular wavelength.
But SOFIA can. The aircraft, NASA’s Stratospheric Observatory for Infrared Astronomy, is a 747 with a sliding door that opens to allow a 106-inch, 17-ton telescope to peer into the night sky. But the observatory, a collaboration between NASA and the German Aerospace Center that has been in operation since 2010, is expensive, and the Obama and Trump administrations both sought to end the program. Each time, Congress has restored financing and SOFIA has continued flying.
Monday’s reported findings would not have been possible without it. At an altitude of some 45,000 feet, SOFIA rises above 99.9% of the water vapor in the atmosphere, said Naseem Rangwala, SOFIA’s project scientist.
The SOFIA results are in rough agreement with the earlier measurements and do not change the estimate of the amount of water on the moon. The concentration at Clavius is low — “roughly equivalent to a 12 ounce bottle of water within a cubic meter,” Honniball said.
“To be clear, it’s not puddles of water, but instead water molecules that are so spread apart that they do not form ice or liquid water,” Honniball said.
She said the water molecules may be formed by particles of the solar wind hitting the moon or by the impacts of micrometeorites on the lunar surface. What remains unclear is how the water persists there. The heat of sunlight would be expected to knock the molecules into space.
“We think the water is trapped within glass beads in the soil that form during these micrometeorite impacts,” Honniball said.
Also not known is how difficult it might be to extract the water.
“If it’s locked into the glass beads, as Casey mentioned, it may require more energy to extract it,” Bleacher said. “If the water is mixed up in the soil, it might be a little easier.”
Until now, the search for water ice on the moon has focused in the shadows of large polar craters, among the coldest places in the solar system, with temperatures dipping down to minus 400 degrees Fahrenheit. That is so cold that anything that lands there rarely leaves.
There, the evidence has not been in question. But landing in a big, cold, dark crater is potentially dangerous and not the easiest of places to work.
Hayne’s work found what they call micro cold traps — small patches of the moon that are not as big or as deep but can be just as cold.
Back in 2014, Hayne started wondering if there might be other parts of the moon that also remain frigid. The axis of the moon is tilted only 1.5 degrees and does not wobble much. That means the path of the sun through the lunar sky does not change, and even small, shallow craters near the poles can lie in permanent shadow.
“The sun never rises higher than 10 degrees in the sky,” Hayne said. “The sunlight is coming in at very grazing angles so they don’t need to be very deep at all. Just a fraction of a centimeter is sufficient.”
In addition, the rocks and soil on the moon do not easily conduct heat, and even a shadowed patch less than an inch across could still be cold enough to trap water.
“If we could prove that these tiny shadows exist, then those tiny shadows could also have very cold temperatures,” Hayne said.
Using images from NASA’s Lunar Reconnaissance Orbiter, which has been orbiting the moon since 2009, the scientists calculated that such micro cold traps could add up to 15,000 square miles of the lunar surface where ice could exist. They could not estimate the volume of water because they do not know how deep the water deposits extend.
The larger craters still constitute a large fraction of the permanently shadowed regions.
The accumulated layers of ice could record billions of years of the solar system’s history, almost like tree rings. “Where did Earth’s water come from?” Hayne said. “That’s still an open question.”
Knowledge of water on the moon “helps us understand the origins of Earth’s water,” he said.
NASA is scheduled to send a small robotic lander to the moon’s South Pole in 2022. An infrared camera on board built by a team led by Hayne will “be able to tell, to test our hypothesis directly for the first time,” he said.
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