As interest in lunar exploration resurges among governments, private companies and NGOs, the study authors wrote, it becomes crucial to understand how exploration may impact research opportunities. This knowledge can help inform the creation of planetary protection strategies for the lunar environment, as well as lunar missions designed to minimize impact on that environment - and the clues about our past it may contain.
The study appears in Journal of Geophysical Research: Planets, AGU's journal for original research in planetary science.
"We are trying to protect science and our investment in space," said Silvio Sinibaldi, the planetary protection officer at the European Space Agency and senior author on the study. The moon is a natural laboratory ripe for new discoveries, he said - but, paradoxically, "our activity can actually hinder scientific exploration."
At the moon's poles, craters cloaked in perpetual darkness (called permanently shadowed regions) hold ice which might contain materials delivered to the moon and Earth via comets and asteroids billions of years ago. Scientists hope those materials might include "prebiotic organic molecules" - key ingredients that, under the right conditions, may have combined to form the original building blocks of life, such as DNA. Finding those molecules in their original form could allow researchers to study how they gave rise to life on Earth.
"We know we have organic molecules in the solar system - in asteroids, for example," Sinibaldi said. "But how they came to perform specific functions like they do in biological matter is a gap we need to fill."
Earth's dynamic, ever-changing surface likely erased any trace of what those original molecules looked like long ago. The moon's surface, parts of which have remained relatively unaltered for billions of years, may preserve a better record - especially in the permanently shadowed regions, where molecules tend to accumulate due to cold temperatures that slow their movement. Unfortunately, that may also include molecules released by lunar spacecraft, potentially obscuring pristine evidence of life-originating materials.
Sinibaldi and Francisca Paiva, a physicist at Instituto Superior Tecnico and lead author of the study, built a computer model to simulate how that contamination might play out, using the European Space Agency's Argonaut mission as a case study. The simulations focused on how methane, the main organic compound released during combustion of Argonaut propellants, might spread across the lunar surface during a landing at the moon's South Pole. While previous studies had investigated how water molecules might move on the moon, none had done so for organic molecules like methane. The new model also accounted for how factors like solar wind and UV radiation would impact the methane's behavior.
"We were trying to model thousands of molecules and how they move, how they collide with one another, and how they interact with the surface," said Paiva, who was a master's student at KU Leuven and an intern at the European Space Agency during the research. "It required a lot of computational power. We had to run each simulation for days or weeks."
The model showed exhaust methane reaching the North Pole in under two lunar days. Within seven lunar days (almost 7 months on Earth), more than half of the total exhaust methane had been "cold trapped" at the frigid poles - 42% at the South Pole and 12% at the North.
"The timeframe was the biggest surprise," Sinibaldi said. "In a week, you could have distribution of molecules from the South to the North Pole."
That's partly because the moon has almost no atmosphere of other molecules to bump into. Impeded only by gravity, methane molecules on the moon bound freely across the landscape like bouncy balls across an empty room, energized by sunlight and slowed by cold.
"Their trajectories are basically ballistic," Paiva said. "They just hop around from one point to another." That's concerning, she explained, because it means there may be no foolproof landing sites anywhere. "We showed that molecules can travel across the whole moon. In the end, wherever you land, you will have contamination everywhere."
That doesn't mean there's nothing to be done to minimize contamination. Colder landing sites, Paiva noted, might still corral exhaust molecules better than warmer ones. There might also be ways around the contamination: Sinibaldi wants to study whether exhaust molecules might simply settle on the icy surfaces of PSRs, leaving material underneath unscathed for research.
Above all, the duo said, the results need confirmation from both additional simulations and real-life measurements on the moon. "I want to bring this discussion to mission teams, because, at the end of the day, it's not theoretical - it's a reality that we're going to go there," Sinibaldi said. "We will miss an opportunity if we don't have instruments on board to validate those models."
Paiva hopes to study whether molecules other than methane, including those in spacecraft hardware like paint and rubber, might also pose risks to research.
"We have laws regulating contamination of Earth environments like Antarctica and national parks," she said. "I think the moon is an environment as valuable as those."
Research Report:Can Spacecraft-Borne Contamination Compromise Our Understanding of Lunar Ice Chemistry?
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