With its lunar sample-return mission last month, China kicked off a new surge in visitors to the Moon. At least eight spacecraft from nations including Russia, India, China, Japan and the United States are set to touch down on the lunar surface in the next three years.
For the first time ever, several of the upcoming missions will explore some of the Moon's most scientifically intriguing, yet sensitive areas those at its poles. Researchers are excited about studying water that lies frozen in shadowed craters in these regions. But they're also worried that increased traffic to the Moon might contaminate the very ice they want to study.
The ice is important to scientists for various reasons. Some want to analyse pristine samples to unlock clues to how and when Earth and the Moon accumulated water billions of years ago. Others want to mine the ice as fuel for rockets at future lunar bases.
Explorers now face a complicated choice. Do they start digging right away, to work out the processes by which they'll mine the ice and convert it to fuel? Or do they proceed slowly, to carefully preserve the scientific record encoded in the ice? "Right now, we've got some scientists saying we can't go anywhere near it because we're going to ruin it," says Clive Neal, a geoscientist at the University of Notre Dame in Indiana. "And others say we need it, so we're just going to go for it."
These tensions need to be resolved soon especially as NASA plans to send a series of missions to the Moon's south pole, starting with robotic landers in 2022 and culminating a few years later with astronauts stepping onto the Moon for the first time since 1972.
Last week, a report from the influential US National Academies of Sciences, Engineering and Medicine (NASEM) argued that space agencies need to prioritize what science they want from the lunar poles in order to explore them effectively. The international Committee on Space Research (COSPAR), which outlines best practices for space exploration, is also evaluating the situation and will decide in the coming months whether to issue new guidance for spacecraft going to the Moon. NASA is waiting for COSPAR's decision, and will then probably update its own regulations on how to visit the Moon responsibly.
As Moon exploration ramps up, "we have an obligation to do no harm to future science investigations", says Lisa Pratt, the planetary-protection officer for NASA who is based at the agency's Washington DC headquarters. The question is, "How do we get this right?"
No spacecraft has ever directly probed the Moon's poles and the ice that lurks there. The only mission to get close was India's Vikram lander, which crashed about 600 kilometres from the lunar south pole in 2019 instead of touching down and studying the surface. China is planning a Chang'e-6 mission that might visit the Moon's south pole, potentially scooping up ice and rocks and returning them to Earth as early as 2023. It would be the successor to Chang'e-5, which collected rocks from the Moon's mid-latitudes last December. Japan and India have also been discussing a robotic mission to the lunar south pole, as have Russia and the European Space Agency (ESA).
And then there's NASA. Under US President Donald Trump, the agency has been preparing a suite of missions to the Moon that are focused on the poles. According to current plans, NASA would send two robotic landers to the south pole in 2022, followed by a larger robotic rover, called VIPER, in 2023. It would sink its one-metre-long-drill into the lunar dirt to mine for ice. As early as the following year, humans would arrive and begin exploring icy craters. One of their goals might be to collect ice and fly it, still frozen, back to laboratories on Earth for study, said a NASA report released last month.
The possibility of explorers contaminating lunar ice is a problem no one anticipated five decades ago, when Apollo astronauts became the first humans to walk on the lunar surface. At the time, researchers thought the Moon was bone dry. Only in the past decade or so have they realized that there is water in many places, including frozen in dark polar craters1. Scientists have even found water in at least one sunlit place on the Moon, contained in minerals in the otherwise dry dirt2.
All this water could have arrived on the Moon by means of water-rich asteroids or comets, or by the solar wind bombarding its surface. Some of it might have come from inside the Moon, spewed out in volcanic eruptions from a water-rich interior. Regardless of its source, the Moon's water holds crucial scientific information.
The ice inside the sunlight-deprived craters at the Moon's poles might have accumulated over billions of years. If so, it holds a record not only of the Moon's early history, but also of Earth's. The Moon probably formed when a giant object slammed into the newborn Earth some 4.5 billion years ago, kicking up debris that coalesced into the Moon and intimately linking their histories. On Earth, geological activity, including plate tectonics, has erased much of the record of the planet's early history. But the Moon has no such activity a perfect study subject.
"The history of the Moon's water provides a lot of clues to how the Solar System has evolved through time," says Ariel Deutsch, a planetary scientist at NASA's Ames Research Center in Moffett Field, California.
Because of the importance of the Moon's ice, many researchers are cautious about how to explore it. In particular, some have been examining the possible contaminating effects of rocket exhaust on the frozen caches.
Parvathy Prem, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and her colleagues last year simulated a medium-sized lander arriving at the Moon at 70º south a few hundred kilometres from the ice-filled craters of the south pole. The simulation showed that even though a rocket would not release much water, the water it does release would spread all around the Moon and stay there for some time3. Even after 2 lunar days 2 months on Earth some 30–40% of the rocket's water would still be present, mostly frozen on the night side of the Moon. "The main takeaway was, the water vapour really goes everywhere," says Prem. So the Moon's polar ice has already been contaminated by past explorers.
COSPAR, the international group, has been asking hundreds of planetary scientists how much they worry about lunar exploration potentially interfering with science at the poles. More than 70% who responded to a survey in 2020 said they were concerned that contamination could compromise the scientific record held within the Moon's ice, says Gerhard Kminek, the planetary-protection officer for ESA in Noordwijk, the Netherlands, and vice-chair of COSPAR's planetary-protection committee.
In a white paper submitted to NASA, 19 scientists, including Prem and Deutsch, propose what they call an 'origins-first' mission to a shadowed crater at one of the Moon's poles. The goal would be to collect reasonably pristine samples of ice before traffic to the Moon picks up, to help scientists determine exactly how the ice there accumulated over time. Such a mission would tell them exactly how precious the ice's scientific record is and whether mining activities should be postponed, says Esther Beltran, a space scientist at the University of Central Florida in Orlando and co-author of the paper.
How to build a Moon base
NASA doesn't currently have funds allocated for an origins-first mission, and continues to plan on sending multiple spacecraft to lunar polar regions. But the agency is listening to scientists who are concerned about getting it right and intends to move carefully, says Pratt."We need to balance the drive for resource utilization with the need for scientific discovery and knowledge," she says.
Meanwhile, if COSPAR adopts new guidelines for lunar exploration, NASA and the space agencies of other nations probably will, too. COSPAR's current guidelines ask nations to keep a list of all organic materials such as carbon composites, paints and adhesives aboard missions headed for the Moon. Having that kind of list helps alleviate concerns about contamination, says Kminek, because it tells scientists exactly what sort of human-made material has entered the Moon's environment. One possible change might be for future missions also to keep a list of the gases that they would potentially emit from their rockets or life-support systems. Relevant players, including the Chinese space agency and commercial companies such as SpaceX and Blue Origin, have been at the table with COSPAR to discuss these possible changes, says Kminek.
As these discussions continue, however, some scientists aren't too worried about contamination issues. Neal and others note that water vapour from rocket exhaust settles only as a thin layer on the topmost part of the Moon's surface so it wouldn't take much work to dig below it to reach undisturbed ice beneath. Last week's NASEM report also notes that the risk of contaminating buried ice is low. And Kevin Cannon, a planetary scientist at the Colorado School of Mines in Golden, thinks that the small amounts of contamination introduced by exploring the Moon's ice are far outweighed by the scientific advances of figuring out where and how all the ice is distributed. He has been mapping where the largest, most accessible caches of ice might be4 (see 'Lunar ice caches').
Others have put forward several ideas for protecting the lunar ice. One proposal is to