How on earth did ice get on the Moon's south pole? No one knows.

Recently, scientists discovered ice deposits in craters all over the Moon's south pole. This has replenished scientific interest in examining the lunar surface in its southern area again.

Scientists have an inkling that most of these deposits are probably billions of years old. A few might be more recent. One study published in the journal Icarus indicates the age of the ice deposits.

Ariel Deutsch, a graduate student in Brown University's Department of Earth, Environmental and Planetary Sciences and the study's lead author, explained that it is important to constrain the ages of the deposits. It helps to give an insight into basic science and also helps future lunar explorers interested in using the 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."

Deutsch collaborated with Jim Head, a professor at Brown, and Gregory Neumann from the NASA Goddard Space Flight Center. He leveraged data from NASA's Lunar Reconnaissance Orbiter, which has been revolving around the Moon since 2009. The scientists examined how far back the large craters with evidence for south pole ice deposits could be calculated. The scientists also looked at the smaller craters within the bigger ones. As they seemed to have a rather accurate idea of impact over time, counting the craters could identify the age of the ground.

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 limit on the age of the ice. The ice deposits seem to be old, as they are rather patchily distributed across the floors of the craters. This indicates that the ice has been impacted by micrometeorites and other kinds of debris over time.

Ice on moon
The Lunar Reconnaissance Orbiter captured views of the Lunar South Pole. NASA/Scientific Visualization Studio

"There have been models of bombardment through time showing that ice starts to concentrate with depth," Deutsch said. "So if you have a surface layer that's old, you'd expect more underneath."

There was also evidence for ice in smaller craters, which, seem to be sharper and more well-defined. Hence, they seem to be fresh.

"That was a surprise," Deutsch said. "There hadn't really been any observations of ice in younger cold traps before."

As the deposits had different ages, their sources were also different. The ice that was older might have got sourced from water-bearing comets and asteroids on the surface, or through volcanic activity, which drew out water from inside the depths of the Moon. However, as there are not too many large liquid impactors recently. No volcanoes have been erupting on the moon recently, and have probably stopped a billion years ago. Hence, recent ice deposits would require other sources, perhaps even tiny micrometeorites.

There are some hopes of getting some relevant samples, as NASA's Artemis program is looking to put humans on the Moon by 2024.

"When we think about sending humans back to the Moon for long-term exploration, we need to know what resources are there that we can count on, and we currently don't know," Jim Head said. "Studies like this one help us make predictions about where we need to go to answer those questions." The article was published in Sciencedirect.com.