There seems to be significantly more water on the moon than expected - and it may even be created thereby solar radiation.
Scientists have been working for years to uncover the secrets of moon water. Now two new studies published in Nature Astronomy confirm that water can be found practically everywhere on the surface of the moon.
One of the studies reports the first clear evidence of water molecules that adhere to or are trapped in the sand grains of the lunar soil in the sunny areas of the surface.
The second study modeled small areas on the moon that are permanently in shadow.
It found that around 40,000 square kilometers - an area that is about twice the size of Saxony-Anhalt - are cold enough to accommodate ice. That is about 20 percent more than originally assumed.
The scientists investigated the forms in which the water occurs on the moon's surface and where it is located.
In this way, they hope to better understand the moon's mysterious water cycle. Unlike on Earth, where water circulates in rivers and precipitation, water formation on the moon could be more exotic: for example, through hydrogen in the solar wind, which reacts with oxygen on the surface, or through icy meteorites that hit the moon. Moon water could also migrate from sunny regions to shady areas.
But the exact movements of this water and the possible transition from sunny to shady zones remain a mystery. "We still have a lot of work to do before we understand whether the two are even connected," says Jessica Sunshine, a planetary scientist at the University of Maryland who was not on either of the two study teams. But the new research "suggests a much more complex process than we previously thought"
This work is also vital for future humans flying to the moon or beyond. This includes NASA's upcoming Artemis mission, which will bring the first woman and next man to the moon.
If there are actually usable amounts of water and ice, it might be possible to mine this resource for use in fuel. This could reduce the burden that future space travelers will have to carry with them on their ventures beyond the earth.
The new insights into the moon water are part of the slowly changing image of our companion in the sky. The moon was once thought to be a desolate, parched landscape.
However, it is increasingly emerging as a dynamic world with complex sources for many forms of water.
"It was a slow revolution," says Paul Hayne, a planetary researcher at the University of Colorado Boulder and lead author of the study on the shadow zones. "But it was a revolution."
A world full of water
The moon is a world of extreme heat and cold. Daytime temperatures near the lunar equator can rise to a scorching 121 ° C, while at night they drop to a frosty minus -133 ° C. And without a thick protective atmosphere, evaporated water can quickly escape into space.
However, to the delight of the scientists, faint traces of water seem to remain on the sunlit moon surface. Instruments on three spaceships confirmed the discovery, announced in 2009, but there was a catch: the analysis failed to tell the difference between water and hydroxyl.
The researchers looked for signatures of water on the lunar surface in the infrared range. Much like visible light split up by a prism, "infrared has its own rainbow even if we can't see it," says Casey Honniball, a postdoctoral researcher at NASA's Goddard Space Flight Center and lead author of the study on the Water molecules.
These earlier analyzes focused on part of the infrared spectrum where both water and hydroxyl glow. By choosing a different section of the spectrum, Honniball and her colleagues were finally able to spot the H2O.
Photos from NASA: Earth and moon as seen by space probes
"To be honest, I don't know why anyone didn't think of it before - it's a brilliant idea," says Sunshine, who worked on the discovery of water signals on the moon in 2009.
For the new study, Honniball and her colleagues collected data during a flight from SOFIA (Stratospheric Observatory for Infrared Astronomy) in 2018. The infrared telescope is mounted on a jumbo jet. A few months later, Honniball was processing the data from her couch when the water signal appeared. “I think I was screaming,” she says.
Water is available, but it is probably scarce: its concentration is probably only around 350 grams per cubic meter of lunar soil.
It's a hundred times drier than the Sahara, notes Honniball. However, she emphasizes that more work is needed to check water levels. The current estimates are ultimately based only on a single, temporally and locally limited observation.
But for Sunshine, it's a long-awaited confirmation. “It's very satisfying,” she says. "I am very grateful that you took on the work."
Ice in the moon shadow
The second study focused on the shaded areas of the lunar surface. Scientists have long suspected that in permanently shady areas, water could be present as ice in huge craters.
That was confirmed in October 2009 when NASA crashed part of the LCROSS spacecraft in a shady area near the south pole of the moon. She saw signs of ice in the impact cloud.
Researchers have since mapped these large frosty regions. But after looking at high-resolution images of the lunar surface from NASA's Lunar Reconnaissance Orbiter, the new study team realized that shady ice zones could be more common than expected, even in very small areas. Every time they enlarged the picture, they saw more of it: “It goes all the way down into the shadows,” says Hayne.
Using temperature and shadow models of the moon, the team found that ice can form even in tiny areas the size of an ant.
These tiny shadows can be just as cold as their larger counterparts, notes Hayne. The moon's atmosphere is so thin that it doesn't equalize surface temperatures, so a boiling hotspot can be right next to an icy cold area.
The new research suggests that the area covered by cold, shady zones is about 20 percent larger than previously estimated.
If all of these zones are full of frost, the amount of ice would be billions of pounds of water, says Hayne. But how many of these zones actually contain ice is still an open question.
Meteorites and moon missions of the future
Together, the studies could help scientists understand how the lunar water cycle works. The water on the moon comes from a few different sources. In part, it could reach the moon with meteorites that collide with the surface.
Some of the water is also likely to be formed when hydrogen from the solar wind reacts with surface oxygen to form hydroxyl. The heat of the sun or the impact of micrometeorites could cause hydroxyl molecules to collide, which then form H2O, explains Honniball.
The heat from impact objects such as micrometeorites could also melt part of the rock surface and evaporate any water nearby.
As the melt cools to glass, it could trap the water vapors - and that could be the reason for the watery signal that Honniball and her team discovered.
But how and where exactly the water moves on the surface remains unknown. Meteorites could release a small amount of the liquid from the surface.
The sun could also play a role in the movement of the water, as the signal for water and hydroxyl weaken when the lunar day heat peaks, Sunshine says. “But is it really lost or does it migrate to some shadow zone?” She asks. "These little cold traps could help us understand that."
Scientists still have a lot to learn about moon water, but some answers may already be on the way: In 2022, NASA plans to send the Volatile Investigating Polar Exploration Rover (VIPER) to the moon's south pole to look for water ice.
Further clues should come from the Lunar Compact Infrared Imaging System (L-CIRIS), which is also slated for a mission in 2022, says Hayne.
Scientists have speculated about the existence of water on the moon since at least the sixties. But in the coming years, we should finally develop a complete picture of where moon water is hiding - and whether future researchers can use it.
Source: National Geographic
