Leonard David's INSIDE OUTER SPACE

Shackleton Crater, the floor of which is permanently shadowed from the Sun, appears to be home to deposits of water ice. A new study sheds light on how old these and other deposits on the Moon’s south pole might be.
Credit: NASA/GSFC/Arizona State University

A new study sheds light on how old deposits of water ice at the Moon’s south pole might be.

Observations are mounting regarding ice deposits in craters scattered across the Moon’s south pole, also dubbed as “cold traps.” However, when or how that ice got there remains puzzling.

Credit: NASA

A new 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.

Ice as a resource

Ariel Deutsch, a graduate student in Brown University’s Department of Earth, Environmental and Planetary Sciences and the study’s lead author, says that constraining the ages of the deposits is important both for basic science and for future lunar explorers who might process that ice for oxygen, water, rocket fuel, and other purposes.

For the study, Deutsch worked with Jim Head, a professor at Brown, and Gregory Neumann from the NASA Goddard Space Flight Center.

“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,” Head said. “Studies like this one help us make predictions about where we need to go to answer those questions,” he said in a Brown University statement.

Lunar Reconnaissance Orbiter flies over Shackleton crater near the lunar south pole in this computer rendering.
Credit: NASA’s Scientific Visualization Studio

Orbiter data

“For exploration purposes,” Deutsch adds in the university statement, “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.”

Using data from NASA’s Lunar Reconnaissance Orbiter (LRO) — which has been orbiting the Moon since 2009 — large craters were assessed in which evidence for south pole ice deposits was found.

To age-date the craters, the number of smaller craters were counted 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.

Lunar South Pole, 4 peaks are identified which are illuminated more than 80% of the time.
Credit: JAXA

Aging the ice

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. 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.

The deposits have a patchy distribution across crater floors. This suggests that the ice has been battered by micrometeorite impacts and other debris over a long period of time.

Newly developed extraction technique for the Moon, thermal mining, makes use of mirrors to exploit sun-shy, water ice-laden polar craters.
Credit: School of Mines/Dreyer, Williams, Sowers

Significant implications

If those reported ice deposits are indeed ancient, that could have significant implications in terms of exploration and potential resource utilization.

“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.”

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. That suggests that some of the deposits on the south pole got there relatively recently.

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

If there are indeed deposits of different ages, the researchers say, that suggests they may also have different sources.

Exploration of south pole crater. Water ice-rich resource ready for processing?
Credit: NASA

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.

But there aren’t many big water-bearing impactors around in recent times, and volcanism is thought to have ceased on the Moon over a billion years ago. So more recent ice deposits would require different sources — perhaps bombardment from pea-sized micrometeorites or implantation by solar wind.

NASA’s Artemis program – back to the Moon by female/male astronaut crew in 2024.
Credit: NASA

Next steps

The best way to find out for sure, the researchers say, is to send spacecraft there to get some samples.

NASA’s Artemis program aims to put humans on the Moon by 2024, and plans to fly numerous precursor missions with robotic spacecraft in the meantime.

Head says studies like this recently completed work will help to shape those future missions.

The new research in Icarus – “Analyzing the ages of south polar craters on the Moon: Implications for the sources and evolution of surface water ice” – is available here:

https://www.sciencedirect.com/science/article/abs/pii/S0019103519303689?via%3Dihub