Analysis of LRO data shows that permanently shadowed craters near the moon's south pole have accumulated ice for at least 1.5 billion years, but Haworth Crater stands out as a more promising target than Schilton for future missions.
More than half a century after the last manned landing, humanity is once again in the race to the moon – just last week NASA's Artemis II mission launched, and American, Russian and Chinese plans are already in the works to establish permanent bases on the moon. Unlike the Apollo missions that landed at six different destinations, in the 21st century everyone is looking to the moon's south pole. About a century ago, the hypothesis that the moon's poles contain ice deposits was first proposed, and a few indirect measurements made in the last twenty years have supported it. In space, ice is a sought-after resource that can be used to extract water for drinking and agriculture, separate it into jet fuel for deep-space travel, and even learn about the history of celestial bodies.
Now, scientists at the Weizmann Institute of Science and their partners in the United States are revealing new scientific evidence that ice has been gradually accumulating on the Moon for at least a billion and a half years. The new study, whose findings are published today in the scientific journal Nature Astronomy, has identified ancient "cold traps" on the lunar surface and is marking them as preferred targets for future missions.
Unlike the Earth, which tilts on its side, so that the position of the sun in the sky changes throughout the year, the Moon almost does not tilt and the sun is always above its equator. If you stand at one of the poles of the Moon, you see the sun moving in a monthly cycle along the horizon, and does not rise and set as on Earth. Therefore, solar radiation is unable to illuminate and heat deep craters surrounded by steep walls at the poles, and they are called permanently shaded areas. This was not always the case, in the past the Moon tilted more on its side, but in recent billions of years it has been straightening out. In 2023, scientists found that as its angle of inclination decreased, more craters near its poles became permanently shaded and cooled. They were able to calculate when each crater lost sunlight, and to deduce the "age" of each shaded area.
In the new study, Prof. Oded Aharonson from the Department of Earth and Planetary Sciences at the institute and his research partners – Prof. Paul Hein from the University of Colorado at Boulder and Dr. Norbert Schergoffer from the Planetary Science Institute in Honolulu – whether there is a relationship between the age of a shadowed region and the proportion of the area covered by ice within it. Ice reflects more ultraviolet light at certain wavelengths compared to the rocky layer on the moon's surface, which allows us to speculate where it is located. The advantage of ultraviolet light is that it comes not only from the sun, but also from stars in the night sky and manages to reach the shadowed regions. The scientists used data collected from an instrument that absorbs weak ultraviolet light and is located aboard NASA's LRO probe, which has been orbiting the moon and mapping it since 2009.
"Finding liquid, usable water outside Earth is one of the most important challenges in astronomy."
"We found that the earlier an area became shaded in the past, the more of it was covered in ice," explains Prof. Aaronson. "This trend began at least 1.5 billion years ago and ice accumulation has continued for the past hundred million years. This suggests that the ice accumulating on the moon comes from a nearly continuous source or sources, and that it did not arrive there in a single event of a large comet impact."
For ice not only to appear on the lunar surface but also to be preserved for hundreds of millions or even billions of years without evaporating, extremely low temperatures are required, on the order of minus 160 degrees Celsius. Areas where such temperatures prevail year-round are called cold traps. Many of the permanently shaded areas are also cold traps, but not all, since crater walls can radiate heat inward. In order to identify the best areas to search for ice on the moon, scientists used geometric formulas to calculate which permanently shaded areas are also cold traps and when throughout history they became so.
"The longer an area was a cold trap, the more ice it accumulated," explains Prof. Aaronson. "In most cases, a crater became shaded and a cold trap at the same time - but not always. An example of this is Shackleton Crater, an area that has been shaded for about 3.5 billion years and was considered a promising target for searching for ice, but we found that it only became a cold trap about 500 million years ago. To identify targets for a future mission, we searched for the oldest cold traps, and found several large traps that are more than 3.3 billion years old near the south pole of the moon."
These findings are of great importance at this time, as finding and sampling ice is at the top of the list of goals for the manned missions in NASA's Artemis program, which is expected to land astronauts at the south pole of the moon. In NASA's vision, establishing a permanent camp on the moon in the coming years is supposed to serve as preparation, and perhaps also as a transit station, for manned missions to Mars. "Sample ice will be the golden proof of its existence," says Prof. Aaronson. "It will make it possible to examine how the chemical composition of water on the moon and on the surface of the Earth is similar or different, and will reveal whether and how manned missions to the moon could use ice."
This research is a starting point for future studies and missions that will focus on the oldest cold traps, which are promising targets for searching for ice – primarily a lunar crater known as Haworth. “A spacecraft that lands on the lunar surface will be able to collect a lot of information about the ice stored there,” says Prof. Hein. “Also, using rover vehicles, it will be possible to approach it and even sample it.”
The source of moon water
Although they have not yet determined the source of the moon's water, the scientists have built a simple mathematical model that allows them to explore different possibilities. According to the model, the amount of ice on the moon's surface is affected by three processes: water supply, evaporation, and mixing - a process in which movements of the moon's soil and rocks change the distribution of ice and bury it below the surface. The fact that little ice has been detected in young ice traps and that the ice accumulates slowly over hundreds of millions of years has led to the assessment that the rates of water supply and loss on the moon are relatively rapid, like a faucet filling a perforated bucket.
One explanation the scientists offer in the article for the source of the moon's water is that volatile water is present in its core and rises to the surface due to volcanic activity. Another possibility is that the source is the solar wind – a stream of hydrogen atoms capable of participating in chemical reactions and creating water on the moon's surface. A third possibility is that it is still asteroid and comet impacts, but not a single event but multiple events that occur every few million years.
"Finding liquid, usable water outside Earth is one of the most important challenges in astronomy," says Prof. Aaronson. "The planned missions to the Moon may allow us to decipher the origin of the water on its surface, but also much more. As Earth's natural satellite, the Moon is an excellent laboratory for learning about the history of our planet and the water on it. We will also be able to learn about the composition and distribution of water that may await us on more distant stars and moons that we have not yet visited."
