There is a fundamental tension in space exploration that has created ongoing debates for decades. By creating the infrastructure we need to explore other worlds, we damage them in some way, making them either less scientifically interesting or less “pristine,” which some would argue, in itself, is a bad thing. A new paper available in JGR Planets, from Francisca Paiva, a physicist at Instituto Superior Técnico, and Silvio Sinibaldi, the European Space Agency’s (ESA’s) planetary protection officer, argues that, in the Moon’s case at least, the problem is even worse than we originally thought.
Their paper looks at how methane, one of the primary exhaust gases used in the descent and launch of landers on the Moon, is spread across its surface. In particular, it checks how this organic compound might be collected in Permanently Shadowed Regions (PSRs) that are believed to house pristine ice from the formation of the solar system, which could provide insights into the prebiotic molecules that were common in the solar system before the development of life on Earth.
The authors developed a model that would track how methane would migrate from the landing site of a lander, such as the Argonaut, ESA’s workhorse lunar lander, planned for a 2031 launch in support of the Artemis program. They found that, no matter where the lander touched down, a significant amount (more than 50%) of the methane produced during the descent would wind up in a PSR.
Fraser talks about the ice hiding in the Moon’s PSRs.
That is a problem. Methane would confound scientific studies of organic molecules in those otherwise pristine ices. Every time a scientist would take a core sample and see a methane signal, they would have to question whether it came from ancient, prebiotic chemistry or simply from the rocket that brought their instrumentation there.
Notably, even landings at one pole sent methane the entire way across the lunar surface to reside in PSRs at the opposite pole. For example, for a landing at the south pole, about 42% of the methane created during the descent ended up in PSRs at the south pole, while 12% of it ended up in PSRs in the north. Essentially, it doesn’t matter where you land on the lunar surface, you would be infecting everywhere with organic compounds that could potentially confuse future biological researchers.
Perhaps even more shockingly, this process was fast. They found the median time for a methane molecule to go from the Lunar south pole to the North Pole was only 32 Earth days. The simulation the paper is based on, which resulted in over half of the methane released eventually residing there, only ran for 7 lunar days – approximately 7 Earth months.
NASA video describing how water molecules move about in the PSRs after meteor impacts. Credit – NASA Goddard YouTube Channel
It seems like this would easily be solved by simply stripping the top sample of ice from any cores that future researchers would take in the PSRs. However, lunar ice is much more porous than that on Earth, and the methane captured in the PSRs could filter down into the sub-structure, making it indistinguishable from that which was placed there eons ago. In addition, the lunar surface is constantly being pelted with micrometeoroids in a process known as “gardening”. These would churn up the ice itself, making it hard to distinguish past from present in terms of the depth of the ice – unlike on Earth where ice from lower depths in a core is typically older.
So why are these organic molecules so attracted to PSRs that over half of them end up residing there? PSRs are notoriously cold, with temperatures reaching down into the double digit Kelvin range. At such cold temperatures, the molecules themselves, which might have been on a ballistic trajectory from the rocket launch, slow down – often to a point where they no longer have enough energy to leave the PSR.
There are some processes that could eliminate the methane on its way to the PSRs. Once it hits the lunar surface again after settling down via gravity from its lunar trajectory, one of three things would happen to the molecule. Either it would be destroyed by UV radiation from the Sun, swept along by the solar wind, or remain trapped where it is through a combination of cold temperatures and physical forces binding it to the regolith. PSRs are by far the most common area for the third scenario to happen, making them the most likely “collectors” of methane molecules – exactly where we don’t want them to be.
The LCROSS mission intentionally rammed a projectile into a PSR on the lunar surface to detect water – this video shows what it saw. Credit – NASA Video YouTube Channel
The authors argue that the current Committee on Space Research (COSPAR) rules for planetary protection need to be rethought in light of these simulated results. One of the current highest levels for lunar exploration is category IIb, for missions specifically targeting PSRs. In that case, you must provide COSPAR with a list of organic materials onboard the lander. However, it doesn’t do much to actually protect the sites intended for study, let alone those on the literal opposite side of this small world.
Lunar exploration programs are still ramping up, so we still have some time to get a cohesive planetary protection protocol together in time to save these pristine, unique places in the solar system. Whether or not all of the players in the new lunar race – governments, NGOs, and private companies – will agree to those protocols is a different matter. But papers like this are what would be needed to convince them to.
Learn More:
AGU – Lunar spacecraft exhaust could obscure clues to origins of life
F.S. Paiva & S. Sinibaldi – Can Spacecraft-Borne Contamination Compromise Our Understanding of Lunar Ice Chemistry?
UT – The Moon’s Southern Ice is Relatively Young
UT – One Crater on the Moon is Filled with Ice and Gas that Came from a Comet Impact