The dream of landing humans on Mars stretches back more than a century, and NASA hopes to finally achieve this ambitious spaceflight goal within the next few decades. As inspiring as it would be for astronauts to leave footprints on another planet, however, scientists worry about a scenario in which Martian microbes (assuming they exist) manage to hitch a ride back to Earth, where they break loose—a threat known as “backward contamination.”
NASA made it clear that “safeguarding the Earth from potential back[ward] contamination is the highest planetary protection priority in Mars exploration,” in a NASA Interim Directive (NID) released last week that reforms the agency’s previous policies on this issue. As for what the agency is worried about, the NID mentions “extraterrestrial life and bioactive molecules” such as prions, which are pathogenic misfolded proteins that cause neurological disease in humans and animals.
NASA’s two new NIDs, which address Moon missions as well as human exploration on Mars, build on several decades’ worth of research about interplanetary contamination and implement some of the recommendations from a report issued by the Planetary Protection Independent Review Board last October.
Nobody knows if Mars hosts life, though most scientists think that the Martian surface is probably too cold, dry, and irradiated to be inhabited. However, Mars had some of the right conditions to support life billions of years ago, and indigenous Martians, perhaps in the form of bacteria, may persist deep under its icecaps or in subterranean water reservoirs. Even the most remote chance of bringing alien life, or any extraterrestrial substances that could cause harm, back to Earth is a cause for serious concern.
“Why wouldn’t we go ahead and be cautious, given that we only have this one biosphere and we like it here?” said John Rummel, a former NASA Planetary Protection Officer and Senior Scientist for Astrobiology, who is now a principal partner of Friday Harbor Partners, LLC, in a call.
“We don’t know anything about potential pathogens on Mars, only that we think they’re unlikely,” he added. Unlikely, but still worth planning for.
Planetary protection is written into the Outer Space Treaty of 1967, the foundational document of international space law. The treaty prohibits the pollution of alien worlds with Earth organisms, which is known as “forward contamination,” as well as “adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter,” referring to backward contamination.
The threat of forward contamination has been a contentious issue for Mars missions dating all the way back to the Viking landers, which became the first probes to operate on the planet’s surface in the 1970s. Several landers and rovers have reached the terrain of Mars since then, all of which underwent sterilization before they were launched to space.
That said, Earth is home to countless extremophiles, which are microbes that can endure intense conditions. It's possible that some of these hardy bugs could survive the trip to Mars and contaminate any alien ecosystems that might exist on the planet. For this reason, robotic missions have not been permitted to land in so-called Special Regions of Mars, which are areas that are more likely to support life, be it Martian or Earthling.
“The idea about Special Regions, in terms of keeping them off-limits, is so that if you have some biological potential there, you don’t trample all over it with Earth organisms and lose track,” explained Rummel. “You don’t want to go ahead and destroy the life that you’re looking for.”
NASA’s plan to send humans to Mars required reforms to these planetary protection protocols, because you can’t sterilize a living body that’s packed with microbes. Moreover, astronauts on Mars may need to land close to Special Regions in order to generate their own water, and other survival materials.
Alberto Fairén, an astrobiologist at Cornell University, has advocated for updating planetary protection protocols in advance of any human missions to avoid forward contamination “before it is too late,” in the words of a 2017 Astrobiology paper that he led.
“I think the community is considering seriously the problem that, once you have humans on Mars, everything is compromised from a biological point of view, and determining whether any traces of life are Martians or Earthlings would be extremely difficult,” said Fairén in an email. “I'm very happy to see that these concerns about exploring Mars astrobiologically, before sending astronauts, are very seriously addressed by several colleagues.”
The new directive acknowledges that some degree of forward contamination from human missions is inevitable on Mars, noting that “it will not be possible for all human-associated processes and mission operations to be conducted within entirely closed systems.” As a result, NASA intends to close numerous “knowledge gaps” that will limit the risk of forward contamination, rather than eliminate it entirely. It’s harder to prepare for the avoidance of backward contamination, because the very notion of extant Martian life is one gigantic knowledge gap.
The threat that speculative Martian microbes pose to us may depend on how closely they are related to Earth life. Fairén is skeptical that Mars life would be communicable to Earthlings because it would simply be too alien to interact with our living world.
“It is true that we are basically speculating, but from a purely academic point of view about evolution, applying the basic principles we know that guide the way nature functions on Earth, I simply don't see how microorganisms evolving separately during billions of years could have the molecular mechanisms to interact with each other,” he said.
”We are seeing this basic concept at play right now with the coronavirus: it can infect us because it has the molecular ‘keys’ to enter our cells, inherited from very, very similar ‘keys’ developed during long periods of time in other mammals (whether bats or pangolins),” he added. “Conversely, I don't see how a Martian microbe living today, evolved totally separately from the Earth's biosphere, could have developed the ‘keys’ to enter terrestrial forms of life. Or vice versa. For me, this just doesn't make sense.”
On the other hand, Earth and Mars have been exchanging meteorites for billions of years, so it’s possible that similar lifeforms may have emerged on both worlds as a result of these interactions. The upshot is that we just don’t know, which means planetary protection protocols will likely continue to advise a careful approach, including extensive surveys of any region of Mars selected to serve as a base for astronauts.
“NASA will determine if it is necessary to conduct a precursor in situ experiment at a location close to the human mission landing or operating sites to characterize any organic constituents that are present,” the new NID notes. “The measurement should be on airborne materials and on materials from the surface and down to a depth to which astronauts may be exposed, and to establish a baseline scientific understanding.”
While the directive focuses on human missions to Mars, it also reiterates NASA’s commitment to “preclude backward contamination of Earth by extraterrestrial life and bioactive molecules” in samples that are robotically returned from other worlds to Earth. This is especially relevant given that NASA’s Perseverance rover, which is due for launch to Mars in a few weeks, is tasked with drilling out Martian samples to be collected by a future robotic probe, which will send them back to Earth.
Even before the advent of the interplanetary spaceflight, people wrestled with the threat of biological cross-contamination between Earth and other bodies. Take the famous twist at the end of H.G. Wells’ novel The War of the Worlds, in which the bloodthirsty Martians that invade Earth are ultimately wiped out by our planet’s own microscopic pathogens.
If humans aim to make the opposite journey, from Earth to Mars, we should be prepared for the possibility, however unlikely, that Martian microbes could similarly infect or harm astronauts, as well as any Earth ecosystems they reach.
“You don’t want these sorts of things to be discovered when your tourist flight wants to come home and some of the passengers start to die off,” Rummel said. “You don’t want to keep everybody from coming home because a couple of people die in a way that’s considered mysterious. You want to have that covered.”
“It’s great that NASA’s doing this,” he added, referring to the new NID. “I think in terms of setting the path for the future, they’ve done the right thing for Mars.”