Moon's darkest craters may now reveal how ice accumulated over a billion years at the South Pole

A race to build a permanent base on the Moon is underway, and after the successful Artemis II mission, we are a step closer to that dream. Space agencies, including NASA, have also zeroed in on a location for a base: the Moon’s South Pole. The reason behind it is that the lunar South Pole has ice deposits, and a recent study, published in Nature Astronomy, has found new evidence to bolster what we know further. The study, led by researchers at the Weizmann Institute of Science, shows that ice has been accumulating on the Moon’s South Pole for at least 1.5 billion years. According to them, the spots that contain ice have been identified as ancient ‘cold traps’ and prime targets for future lunar missions.   

Ice is an indispensable resource for colonizing the Moon. It can be converted into potable water and water for irrigation. Water can also be split to make rocket fuel that can help explore space beyond the Moon, and it can be used to shed light on the history of celestial bodies. Earth’s axial tilt causes the Sun’s position to change throughout the year. The Moon has almost no axial tilt, making the Sun always appear approximately above its equator. If you stand at one of the lunar poles, you would see the Sun close to the horizon as it completes a monthly cycle. It doesn’t rise and set as it does on Earth. This prevents sunlight from reaching and warming the deep craters at the lunar poles, which are designated as “permanently shadowed regions.” 

But things were different in the distant past when the Moon had a much greater axial tilt, something which it gradually lost over billions of years. In 2023, researchers found that the Moon’s tilt decreased, permanently shadowing and cooling more and more craters near the poles. They determined the age of each permanently shadowed region by calculating when each crater received its last sunlight. This finding intrigued Prof. Oded Aharonson of Weizmann’s Earth and Planetary Science Department, Prof. Paul Hayne at the University of Colorado Boulder, and Dr. Norbert Schörghofer of the Planetary Science Institute in Honolulu, and they set out to find a link between the age of a permanently shadowed region and the proportion of its area blanketed by ice. 

It is possible to locate ice since it reflects more ultraviolet light at certain wavelengths than the Moon’s rocky surface. Ultraviolet light from the Sun and distant stars floods the Moon and even its shadowed areas. Based on the data captured by a laser altimeter and an ultraviolet-sensitive device aboard NASA’s Lunar Reconnaissance Orbiter (LRO), which has been orbiting and mapping the Moon since 2009, the researchers behind the present study developed a 3-D animation and found ice-rich craters in the South Pole. But some of them were also found to be lacking ice. One such crater is the Shackleton Crater closest to the lunar south pole, which remained too warm for much of lunar history to collect ice. In contrast, the Haworth Crater (the one with the greatest expected ice coverage) has been collecting ice for billions of years. This crater has been identified as a prime location for future crewed missions.

“We found that the earlier a region became shadowed, the larger the area that was able to accumulate ice,” said Aharonson in a statement. “This trend began at least 1.5 billion years ago and has continued even over the past 100 million years. This suggests that ice has been building up on the Moon from a nearly continuous source – or sources – rather than through a single event such as a large comet impact,” he added. Permanently shadowed craters alone, however, are not enough to trap ice. For ice to persist for hundreds of millions or billions of years without evaporating, temperatures should hover around minus 160 degrees Celsius. Regions that are capable of maintaining such low temperatures year-round are known as cold traps. Many of the shadowed regions are cold traps. However, some cold traps fail to maintain this temperature because surrounding walls radiate heat into the crater. 

“The longer a given region has been a cold trap, the more ice it has accumulated,” Aharonson explained. “In most cases, a crater became shadowed and turned into a cold trap at the same time – but not always. For example, Shackleton Crater has been shadowed for about 3.5 billion years and was considered a promising site in the search for lunar ice. We discovered, however, that it only became a cold trap around 500 million years ago. To identify targets for future missions, we searched for the oldest cold traps and found several extensive ones more than 3.3 billion years old near the Moon’s South Pole,” he added.

But, how do ice deposits form on the Moon’s South Pole? For an answer, the researchers turned to a mathematical model.  The model reveals that the amount of ice on the Moon’s surface depends on three processes: water supply, evaporation, and impact gardening, a process in which ice is redistributed and buried beneath the lunar surface. In addition, the researchers found that younger cold traps have little ice, combined with the slow accumulation of ice over hundreds of millions of years. This led the researchers to suggest that both water supply and water loss on the Moon occur at relatively rapid rates. This is akin to a faucet filling a leaking bucket. Another possibility is that volatile water from the Moon’s interior reaches the surface through volcanic activity. 

Solar wind can also supply a stream of hydrogen atoms that trigger chemical reactions on the lunar surface to form water. A third option is asteroid and comet impacts, which could have contributed to the formation of water and then ice via multiple impacts occurring every few million years. “Finding water beyond Earth in liquid and usable form is one of the most important challenges in astronomy,” Aharonson said. “Planned lunar missions may help us determine the origin of water on the Moon – but they could also teach us much more. As Earth’s natural satellite, the Moon is an excellent laboratory for studying the history of our planet and its water. Moreover, we may gain insights into the composition and distribution of water that could be waiting for us on more distant planets and moons we have yet to visit,” he added. These findings are really exciting since NASA’s long-term vision includes establishing a permanent lunar base to serve as preparation – and possibly a transit station – for future manned missions to Mars. 

More on Starlust 

Scientists send microscopic worms to space to aid future Moon missions