We have seen this scenario in science fiction a million times: Scientists uncover an alien organism previously buried under ice, or bring bugs from another celestial body to Earth. Before we know it, an apocalypse unveils: It was a deadly pathogen threatening to wipe out all life on Earth! This clichéd scenario is far removed from the probable reality, but the central questions stand: What would happen if we brought alien life to Earth? What should we do to prevent it, if anything, and how should we handle the possibility of contaminating another world with Earth life?

The question of planetary protection becomes especially timely when it comes to Mars. We want to discover whether it could have had life in the past, or even nowadays, and attempt that through missions like ESA/Roscosmos’ ExoMars 2020 rover and lander or NASA’s Mars 2020 rover. At the International Astronautical Congress in Guadalajara, SpaceX’s Elon Musk recently announced his plans of Mars colonization. We might argue whether they’re realistic in terms of finance, proposed timeline, providing the crew with sufficient safety, et cetera, but it’s inarguable that we’re witnessing something like a race to Mars. The moment when the first humans step onto the surface of the Red Planet may be fifteen or forty years away, but it will eventually happen. It will also be the moment planetary protection of Mars has essentially ended. Should we care?

Do astrobiologists dream of false positives?

Planetary protection is basically a set of rules to prevent inadvertent cross-contamination, be it bringing alien organisms on Earth, or, more pressingly, seeding other objects with Earth organisms. Why should we worry about that? Let’s imagine for a moment that there’s extant life on Mars. Forward contamination of the Red Planet with Earth life could potentially lead to its extinction—not because our organisms would prove to be some deadly disease for Martians, but more realistically by simply outcompeting them. This worst-case scenario, disastrous from a scientific, as well as ethical point of view, would depend on how widespread and sensitive that extant Martian life would be. There is, however, a very likely scenario involving false positives in life detection. Imagine that we send a costly robotic spacecraft to search for life on Mars. It returns exciting results: It detected fragments of nucleic acids, strange combinations of lipids, many amino acids present in proteins, chiral amino acids or sugars . . .

Each, and any of those, would present a compelling argument for life. It could also test regolith samples for signs of metabolic activity—and wow, there are some! It looks like we found life on Mars! After the initial excitement, doubts are raised: It transpired that the spacecraft’s sterilization had been flawed. Had it truly found Martian life, or simply detected what it had brought from Earth? Sequencing the DNA could reveal which of these possibilities is true, but the spacecraft in question isn’t equipped to do that. Another expensive mission must be mounted and even then, we still can’t be sure. If the organisms didn’t survive, and all we have are radiation-damaged fragments, a reliable result might be impossible. Even if the analysis shows traces of organisms known from Earth, can we be sure we didn’t overlook something else because our techniques are suited to picking up life with which we’re already familiar?

This is stuff of nightmares for many astrobiologists. The first spacecraft to ever land on Mars (and work for more than a few seconds, unlike the earlier Mars 3), the Viking landers, have been thoroughly sterilized. They were cleaned to minimize the bioburden first, and then encased in a bioshield and baked at 125 °C to diminish the chance that anything could survive on them even further. These were the first, and so far, the only, spacecraft with search for life as their primary mission. Three of their four biological experiments returned negative results, but one initially returned positive. However, as was found later, it could have been caused by the presence of perchlorates in Martian regolith. (And ironically enough, perchlorates could also be responsible for a part of the negative results if there had been organics on the sample sites.) The landers also found Mars to be a much colder and drier place than expected. Its thin atmosphere doesn’t allow for any pure liquid water to exist on the surface. Additionally, its harsh radiation environment and freezing temperatures would be unfriendly to life as we know it.

Connected biospheres?

For subsequent missions, whose aim was not to look for life, less severe planetary protection measures were chosen (baked spacecraft were no longer on the menu), but some found it unnecessary altogether. An interesting exchange played out in 2013 on the pages of Nature Geoscience, where Alberto Fairén and Dirk Schulze-Makuch argued for dropping the measures for most missions. If Mars could support Earth life, it’s already there, they argued. They didn’t only mean the probably unsterilized crash-landed spacecraft such as Soviet Union’s Mars 2 and 6 probes. Or the possibility that NASA’s Phoenix lander’s not-fully-sterilized legs stood in ice exposed by the lander’s retrorockets and may have touched liquid brines melted in the landing. Fairén and Schulze-Makuch point out that there is a good likelihood that life had been transferred between Earth and Mars naturally throughout their history by meteorites.

There are over 130 known meteorites of Martian origin on Earth—and think about how many we had missed because they had been wiped out by erosion, buried underneath layers of soil, subducted or landed in the oceans in the preceding eons, or they are still here but simply haven’t been found by scientists yet. Estimates based on the probability of a planet taking a hit and trajectories of the escaping debris show that meteorite exchange between Earth and Mars was probably very frequent, and although there would be many more Martian meteorites here on Earth deeper in the gravity well, there would still be plenty of Earth material on Mars. But could anything survive so violent an event such an asteroid impact, then thousands of years in space, and as a bonus landing on another planet—going through the atmosphere, crashing on the surface?

It seems that something could. Many bacteria, and specifically their spores, can withstand shock events sending them through abrupt temperature changes and acceleration, irradiation, exposure to vacuum—all of the above. Some of the model species from these experiments, such as Bacillus subtilis, are commonly found in spacecraft assembly rooms and on spacecraft surfaces. The EXPOSE facility aboard the ISS hosted several experiments attempting to mimic the conditions of being hurled into space or on Mars. And while being directly exposed to cosmic radiation kills most spores, just a feeble protection like another layer of spores, a tiny crack in the surface or a little dust improves the odds of survival substantially. Inside larger rocks, they could probably wait out millennia quite comfortably. And while we can’t keep a test running for thousands of years, nature can do that for us: Viable bacteria and archea were extracted from Siberian permafrost, Antarctic glaciers, amber and salt crystals after not just many thousands, but sometimes even millions of years. The current record is held by a strain of Bacillus recovered from over 250-million-years-old salt crystals. And while it’s not the same as spending the time inside a rock in space, it’s a more than powerful testimonial of life’s resilience.

Any War of The Worlds might have played about hundreds of millions of years before humans came on the scene. And if it can’t survive there, there is no point in planetary protection of Mars.

With great risk comes great responsibility

With all this in mind, could the protective measures, which could add millions of dollars or euros to the required budget and years to the development of a life-searching mission, be obsolete? NASA planetary protection officer Catharine Conley with her colleague John Rummel were quick to respond to Fairén and Schulze-Makuch on the pages of the same journal. They emphasize that even after just millions of years’ isolation, any transplanted Earth life would be of great scientific significance, and any scenario playing out after introduction of the current species is impossible to predict—just look at the many surprising invasive species in our own ecosystems. (Also, if Martian life was related to Earth life, it would be even harder to discern spacecraft-bound contamination from pre-existent life, so it could be viewed as a reason for even more stringent sterilization.)

To protect Mars even further, so-called “special regions” had been proposed over ten years ago, and the definition is gradually revised—usually every two years. At first, we thought that only polar regions had enough water reservoirs in the ice caps and permafrost; but buried glaciers may be present near the equator, and newer discoveries such as the recurring slope lineae (RSLs) thought to contain some liquid water (although newest observations cast a shade upon that claim), show us essentially all of Mars might end up treated as a “special region.” The latest suggestion to update the definition, by Petra Rettberg and colleagues, succinctly points out that we need to address the “known unknowns”: Could water vapor instead of liquid water suffice for an organism to reproduce itself? Could only one terrestrial organism spread on Mars, even under good conditions? Do multi-species colonies have a better chance at surviving and reproducing? The questions certainly don’t end there.

Some answers might be brought by ESA’s ExoMars rover, currently scheduled to launch in early 2020. The rover will adhere to similar measures such as the Viking landers; in fact, some of its components are undergoing “baking” already. It will be, after all, the first mission after the Vikings aiming to search for possible extant life. But it won’t visit the special regions. This is where planetary protection could easily become something of a Catch 22. We want to search for life in places where it might be feasible and we need data from there to work on more informed planetary protection measures. But getting a spacecraft sterile enough to be allowed to go there would be difficult and especially for financial reasons, agencies might be wary.

Yet all planetary protection of Mars is essentially over when humans first step onto its surface. No matter how well we sterilize our equipment or spacesuits, we’re walking consorcia of countless species—beside our human cells, we carry some protozoans from different groups, and lots of bacteria and archea. Especially if we land somewhere with lots of sunlight for solar panels and ice for fuel manufacture, we’ll be on the best way to contaminate Mars (or possibly ourselves too?). Our window of opportunity to study the pristine Red Planet may be shortening.

Beyond Mars

As Fairén and Schulze-Makuch note, planetary protection of Mars might be unwarranted. But what about other celestial bodies where life may prevail? Take Venus, for example. Its hellish surface is a sterilization procedure of its own; nothing could possibly survive there. But up in the cloud layer around 50 km high, the conditions would be much more favorable to life as we know it. No planetary protection is required for Venus missions under the current rules. Yet seeding its cloud layer with photosynthetic microorganisms tolerant to high acidity and low water availability (which thrive in extreme conditions on Earth), would not be unthinkable. Some scientists, such as Dirk Schulze-Makuch or Charles Cockell, are considering the possibility of local life in the cloud layer.

Though Venus presents a potential abode of life, albeit perhaps unlikely due to its turbulent history, it’s not a place that draws astrobiologists’ hopes most. Inner oceans of several icy moons of our solar system, especially Jupiter’s Europa and Saturn’s Enceladus, represent places of high interest. Covered by thick icy crusts, these environments could have been isolated from each other and any other potentially life-bearing places. Europa’s extreme radiation conditions on its surface are a sterilizing procedure in itself, but Enceladus’ surface is, if you don’t mind the cold and lack of atmosphere, quite balmy in comparison. Future missions to the icy moons will have to weigh the risk of getting contaminants under the surface. Besides, many more moons or dwarf planets might host subsurface oceans (most recent claims include Saturn’s moon Dione). Some, like Ganymede’s, will be locked between a thick ice crust above and a thick layer of high-pressure ice beneath. Is it really warranted to use planetary protection measures there?

And what about places very different from Earth, but considered as potential—if unlikely—abodes of life? One such place is Saturn’s moon Titan. ESA’s Huygens probe, which had landed there in 2005, was assembled in a clean room, but not specifically sterilized. Titan, the only moon in our system with a substantial atmosphere, has proven to be even more interesting than previously thought, and there are several hypotheses of potential biochemistries that could work in its methane lakes. Needless to say, if there was life, it would be very different from ours. Such “alien” alien life would actually be good news for planetary protection, because the risks of outcompeting would be much lower.

The 1967 Outer Space Treaty prohibits harmful contamination of space. But what would happen if someone were to violate it? First of all, not all states have signed the treaty, though the vast majority have. Any violation would probably be handled by the United Nations, but legal skirmishes over what constitutes “harmful contamination” and political battles could render the law toothless. Handling actions not by state institutions but by private companies would be even more problematic, and space law experts continue writing theses on how we should include firms or individuals in space treaties. In the meantime, an invasion could begin.

We don’t know what bringing life to Mars would do. We cannot be sure how extensive and how successful (if at all) any natural exchange had been. With some luck, we might form some more accurate guesses after several more robotic missions, and then tighten, relax, or drop planetary protection measures on the Red Planet.

Maybe we’ll eventually find out that Earth is all alone in the solar system as a place hosting life. Maybe we’ll discover the opposite and find local life on Mars, the icy moons, or elsewhere. But we’ll have to go there to find out which is true. In doing that, we should bear in mind the risk of contaminating other worlds, and try to minimize it at least until we know reasonably well that they’re lifeless.

Further reading

• NASA Planetary Protection Office’s website
• When Biospheres Collide (by Michael Meltzer, free ebook detailing the history and working of planetary protection, 2015)
• “The Planetary Police” (by Eric Hand, in Nature 459, 308-309; 2009)
• “Microbial Stowaways to Mars Identified” (by Jyoti Madhusoodanan, in Nature News; 19 May 2014)
• “Why Hunting for Life in Martian Water Will Be a Tricky Task” (by Lee Billings, in Nature News/Scientific American; 28 September 2015)
• “Martians Might Be Real. That Makes Mars Exploration Way More Complicated” (by Kevin Carey, in Wired; 7 August 2016)
• “Mars Contamination Fear Could Divert Curiosity Rover” (by Alexandra Witze, in Nature News; 7 September 2016)

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