Ice deposits in craters scattered across the Moon's south pole may have more than one source, according to a study which could help identify the origin of these deposits, and in planning for future human exploration. The discovery of ice deposits on the south pole of Moon has helped to renew interest in exploring the lunar surface, but no one is sure exactly when or how that ice got there.
The study, published in the journal Icarus, suggests that while a majority of those deposits are likely billions of years old, some may be much more recent. Ariel Deutsch, a graduate student at Brown University in the US, said that constraining the ages of the deposits is important both for basic science and for future lunar explorers who might make use of that ice for fuel and other purposes.
"The ages of these deposits can potentially tell us something about the origin of the ice, which helps us understand the sources and distribution of water in the inner solar system," Deutsch said. "For exploration purposes, we need to understand the lateral and vertical distributions of these deposits to figure out how best to access them. These distributions evolve with time, so having an idea of the age is important," she said.
Using data from NASA's Lunar Reconnaissance Orbiter, the researchers, including Jim Head from Brown University, and Gregory Neumann from the NASA Goddard Space Flight Center, looked at the ages of the large craters in which evidence for south pole ice deposits was found.
To date the craters, researchers count the number of smaller craters that have accrued inside the larger ones. Scientists have an approximate idea of the pace of impacts over time, so counting craters can help establish the ages of terrains. The majority of the reported ice deposits are found within large craters formed about 3.1 billion years or longer ago, the study found.
Since the ice can't be any older than the crater, that puts an upper bound on the age of the ice, according to the researchers. Just because the crater is old doesn't mean that the ice within it is also that old too, but in this case there's reason to believe the ice is indeed old, they said.
The deposits have a patchy distribution across crater floors, which suggests that the ice has been battered by small meteorite impacts and other debris over a long period of time. If those reported ice deposits are indeed ancient, that could have significant implications in terms of exploration and potential resource utilisation, the researchers said.
"There have been models of bombardment through time showing that ice starts to concentrate with depth. So if you have a surface layer that's old, you'd expect more underneath," Deutsch said. While the majority of ice was in the ancient craters, the researchers also found evidence for ice in smaller craters that, judging by their sharp, well-defined features, appear to be quite fresh.
This suggests that some of the deposits on the south pole got there relatively recently, researchers said. "That was a surprise. There hadn't really been any observations of ice in younger cold traps before," Deutsch said.
If there are indeed deposits of different ages, the researchers said, that suggests they may also have different sources. Older ice could have been sourced from water-bearing comets and asteroids impacting the surface, or through volcanic activity that drew water from deep within the Moon.
However, there are not many big water-bearing impactors around in recent times, and volcanism is thought to have ceased on the Moon over a billion years ago, the researchers said. More recent ice deposits would require different sources -- perhaps bombardment from pea-sized micrometeorites or implantation by solar wind, they said. The best way to find out for sure, the researchers said, is to send spacecraft there to get some samples.