Black swans from MARS ?

A mission to study samples from the red planet is unlikely to bring anything alive back to Earth. But nobody can put the risks at zero.

October 5, 2023

In 1679, Dutch explorers reached Australia and observed the first black swan known to Western civilization. This discovery became something of a scientific phenomenon, going against a deeply held cultural, and perhaps innate, belief that what has already happened, or what usually happens, is a guide to what will happen. Scottish philosopher David Hume, an early and profound influence on the development of the scientific method, formalized the argument against inductive inference, declaring in 1739 that it was illogical to rely on past observations to predict the future.

In personal and political decisions, of course, humans still rely heavily on past experience to guide future activity. Indeed, doing so is all but unavoidable in daily life. What usually happens, after all, is what usually happens. In space exploration, however, such an approach can be, at the least, problematic. Humans don’t necessarily know what is usual beyond Earth’s bounds. Or what black swans might be out there.


In the next decade, the US National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) plan to send robots to Mars to retrieve rocks and dust from the red planet for study on Earth. The multibillion-dollar question NASA’s Mars Sample Return (MSR) mission will attempt to answer is one that has intrigued scientists, writers, and the general public for centuries: Has there ever been life on Mars? If there are live organisms in the samples, NASA will be surprised. Several agency officials have suggested that it’s unlikely samples from Mars will contain anything that poses a risk to Earthlings. But one of the primary purposes of the mission is to see if life ever did exist on Mars, and so NASA must prepare for the possibility that something in the samples is … alive. Public interest in that possibility seems likely to become intense.

Once the spacecraft are launched, it will take about five years to collect and return the samples. NASA’s current plans are in flux, but the general intention is to launch a Sample Retrieval Lander in about 2028, arriving at Mars in about 2030 to pick up samples already collected and stashed in canisters by the Perseverance rover. A rocket on board the lander will cache the samples and transport them to Mars orbit. The samples’ small container will be collected by the ESA’s contribution, the Earth Return Orbiter, which will carry the samples back to Earth. The Orbiting Sample will land somewhere in the Utah desert in the early-to-mid-2030s. From there it will be transported to an as-yet-undesigned level 4 biosafety lab, the highest category of labs for studying pathogens and biological weapons.

Artist rendering of robotic vehicles involved in the Mars Sample Return mission. (NASA)
Artist rendering of robotic vehicles involved in the Mars Sample Return mission. (NASA)

NASA’s first target for a manned mission to another solar system body was the Moon. By 1969, NASA was 10 missions into the Apollo program, and that year Apollo 11 brought 842 pounds of moon rocks and dust back to Earth. Between 1961 and 1969, the agency had weathered serious turf wars as it attempted to arrive at a plan to prevent potential “backward contamination,” as it’s called—the release of extraterrestrial life forms on Earth. The agency has generally taken the attitude that there are no extraterrestrial life forms in the places NASA seeks to explore in the foreseeable future. And so far, none has been found in the moon rocks. NASA expects none from the first batch of Mars rocks. Moreover, NASA has historically worried more about so-called forward contamination as regards samples from elsewhere. That is, if the samples were exposed to an Earth environment before being studied, it would be difficult to determine whether any living microbes in the samples originated elsewhere or were simply Earth organisms that had contaminated the samples. This forward contamination is already the case with Mars meteorites that have been found on Earth.

Many science experts have reasoned that because Earth is frequently gifted with meteorites that originated on Mars (at least 175 have been identified), if they’re carrying life, we’ve already been exposed. And as far as anyone knows, nothing terrible has happened. “I think the risks [of finding extant life] are not very high given the history of contamination of Earth by Mars,” David Paige, a UCLA physicist and deputy principal investigator on the Perseverance rover, says. “It’s not like I stay awake at night worrying that the Mars sample is coming back, and the Andromeda Strain is going to ensue.”

There is no scientific consenus on evidence of life on Mars. But there is intriguing science.

A favorite argument from astrobiological skeptics is that the Moon and Mars simply cannot currently support life, so we have nothing to worry about from either meteorites or the MSR mission. “The surface of Mars today is an incredibly harsh environment,” says Lindsay Hays, program officer for NASA’s astrobiology program. It’s true--Mars’s thin atmosphere is mostly carbon dioxide; the surface receives heavy doses of gamma radiation; ground temperatures vary greatly, from perhaps 71 degrees Fahrenheit during the day to -146 at night; and the drastically low humidity is similar to the Atacama desert in Chile. While Hays herself is thrilled to be looking for life on Mars, she does not believe it is there now. “There’s a good scientific consensus that early on in Martian history it was probably fairly similar to early Earth history,” she says. “It’s possible there may have been something going on on Mars as well.”

Discounting the possibility of potentially dangerous life on Mars has deep roots. Compare Hays’s statement with this 1967 description of the runup to the Apollo 11 moon mission from an oral history of Aleck Bond, an aerospace engineer in the Manned Spaceflight Center (MSC) management at the time. Bond believed that “there is practically no likelihood of any harmful bacterial life in the lunar material because of the bombardment from the solar radiation, the intense ultraviolet content of sunlight, the proton bombardment … the hard vacuum, and extremes of hot and cold.”

But at least some of the power of this argument has been eroded by recent research.

The Perseverance rover has been collecting Martian soil samples throughout its journey on the red planet. If the mission proceeds as planned, 30 samples will eventually be sent to Earth. (NASA video)

Mars samples will be an order of magnitude more fraught than moon rocks, because Mars has in the past been far more hospitable to life as we know it than other solar system objects. No previous claim that life has been detected on Mars has withstood close scrutiny. In 1976 Gilbert Levin interpreted the results of his pioneering experiments on the Mars Viking lander as clear evidence of life, and believed it for the rest of his life, but few other scientists did. In 1994 a meteorite called ALH80001 that contained a string of nanoscale beadlike shapes closely resembling bacteria took the world by storm, but after much analysis and debate, the scientific community decided the evidence just wasn’t strong enough. Currently there is no scientific consensus that we have found direct evidence of life on, or from, Mars. But there is intriguing science.

On Mars, orbiting satellites and the rovers have found carbon compounds, signs of seasonal methane and liquid water, caves that could protect life from punishing radiation, and fatty acids of ambiguous origin. There is consensus that if microbial life does exist, it will probably be living farther below the surface than current Mars rovers can reach. Some of this may be cleared up by the ESA’s Rosalind Franklin Rover, scheduled to leave for Mars in 2028. It will be able to drill to a depth of two meters.

On Earth, since the discovery of the burgeoning biodiversity of “black smokers,” the towers of precipitated minerals springing from seawater’s contact with hot magma at mid-ocean ridges, microbiologists have demonstrated the astonishing survival of microbes in places no one previously imagined. Various bacteria and archaea can live in high temperatures, high acidity, low temperatures, high alkalinity, and intense radiation. Lakes beneath the ice of Antarctica teem with microbes. Rocks deep beneath the bottom of the ocean harbor them, as do various layers of Earth’s atmosphere.

<em>Deinococcus radiodurans</em> is a bacterium that can survive thousands of times more radiation exposure than a human. Microbiologists have demonstrated the astonishing survival of microbes in places no one previously imagined. (Image: Alice Dohnalkova, EMSL/DOE)
Deinococcus radiodurans is a bacterium that can survive thousands of times more radiation exposure than a human. Microbiologists have demonstrated the astonishing survival of microbes in places no one previously imagined. (Image: Alice Dohnalkova, EMSL/DOE)

Two recent studies suggest that microbes may be able to survive space journeys. Deinococcus radiodurans, an Earth bacterium, can resist higher doses of ionizing radiation than any other organism. Michael Daly of the Uniformed Services University of the Health Sciences and colleagues placed several types of bacteria, including D. radiodurans and the familiar Escherichia coli, along with a yeast species, in conditions simulating a ride on a meteor in space: extremely high radiation, severe desiccation, and deep cold. The researchers were astonished to find that the desiccation and cold applied separately actually helped the bacteria survive the radiation better, and in combination the effect was spectacular. D. radiodurans was able to repair the radiation damage to its DNA five times better with the combined effects of cold and aridity than with exposure to the radiation alone.

The researchers estimated the “ionizing radiation survival limits of … DNA-based life-forms to be hundreds of millions of years of background radiation while buried in the Martian subsurface. [Emphasis added.] Our findings imply that … backward contamination is a possibility if life ever existed on Mars.”

Another experiment by Daly and colleagues found that when D. radiodurans and other species of bacteria and yeasts were exposed to lower-dose chronic radiation, as opposed to short, high doses of radiation, the D. radiodurans cells protected not only others of their species but also much of the local population of more sensitive species. The larger the population, the more likely this was to occur. Thus the size, diversity, and radiation resistance of any microbe population in a NASA canister might be a crucial factor in its ability to survive space travel.

If Mars life uses the same chemical scaffolding that Earth life does, we will be able to identify it. But it may use some other system to metabolize and reproduce that we haven’t imagined or tested.

In either case, biosafety expert Rocco Casagrande does not expect any Mars life to be dangerous on Earth. “Pathogenicity and the ability to infect an organism like us is a highly evolved trait,” he says. The risk of pathogens escaping from biolabs studying Earth microbes is “way, way more serious than space microbes.” So is the risk of Earth pathogens diffusing from melting permafrost, as anthrax and influenza are already doing, for example, University of Cambridge researchers have asserted. But nobody can put the potential risk of samples from Mars at zero.

Quarantined in a modified Airstream trailer after returning from the Moon on July 24, 1969, the Apollo 11 astronauts exchange "a-OK" signs with President Nixon.
Quarantined in a modified Airstream trailer after returning from the Moon on July 24, 1969, the Apollo 11 astronauts exchange "a-OK" signs with President Nixon.

In the past, NASA’s skepticism about extraterrestrial life led it to procrastinate on microbiological design issues. According to NASA’s own history of the Lunar Receiving Laboratory, Apollo 11 might not have included any kind of protection against potential moon microbes if NASA had been left to its own devices. At the time, the agency was largely peopled by engineers whose main concern was assuring the safety of astronauts in space and their survival on reentry to Earth. Early in mission planning, the Apollo 11 astronauts were expected to bring back samples only to satisfy geologists, not biologists.

NASA’s Manned Spaceflight Center in Houston, Texas, now known as the Johnson Space Center, was charged with designing and building a Lunar Receiving Laboratory. Although some NASA experts expressed concern about backward contamination during early planning, management ignored them, and it was mainly through external pressure from academic scientists, the National Academies of Science, the Public Health Service, the US Army and other major federal power centers that NASA incorporated backward contamination risk into its plans and designs.

A 1967 NASA diagram showing the role of the Lunar Receiving Laboratory in the quarantine process.

One of the most severe problems in this process was the fact that engineers and scientists did not get along. According to the lab’s history, “Many scientists … believed that personnel with engineering backgrounds were not able to consider the issues that would be of greatest concerns to the scientific community.”

On the other hand, one of the lab’s managers “observed that ‘the scientific community places a very heavy degree of importance on the possession of a Ph.D.—and preferably a Ph.D. from the same major professor that I got mine from.’”

Bond characterized the lack of respect physical scientists had for biologists: “As many of the [Earth scientists] did not believe that back contamination was a serious threat, they argued that ‘obtaining scientifically priceless samples from the Moon and then using a significant portion of them for injections into mice or soil or wheat seedlings seemed absurd....’”

NASA was under intense pressure to finish the Lunar Receiving Laboratory so Apollo 11 could meet its deadline for launch. Because MSC management had disregarded contamination risks at first, controls over those risks entered the planning process when it was well along. Construction started on the building before the design was completed. By the time the painful process came to fruition in 1969, the lab had been transformed from a small clean room to a large facility with clean rooms equipped with glove boxes, a radiation detecting room, and living space to quarantine returning astronauts.

NASA’s planetary protection officer, Nick Benardini, insists that, unlike during Apollo 11, NASA’s current culture is collaborative with external stakeholders.

Asked if today’s NASA engineers listen to the biologists, after a pause Benardini laughed, saying, “We might have some very rich conversations, but the main thrust is making sure we have the right intent for public safety.”

An illustration from a 1981 NASA report on the feasibility of an orbiting quarantine facility for analyzing Mars samples.

NASA culture contains contradictions. According to Stephen Johnson’s history of systems engineering at NASA, the agency’s culture has swung back and forth between structuring preventive checks and balances on one side and cost- and bureaucracy-cutting practices on the other. Safety costs more, but as it decreases failures, overconfidence rises, leading to a belief that costs can be cut again, according to Johnson.

The cost-cutting phase of the early 1980s switched back toward safety after the televised, in-flight disintegration of the Space Shuttle Challenger in 1986. This did not last long; somewhat paradoxically, the Challenger disaster was cited by NASA as an example of too much concern for safety: “Following the Challenger accident in 1986, initial processing activities at Kennedy Space Center were overly conservative in order to maintain safety.” By the early 1990s, however, NASA’s mantra flipped again, becoming “faster, better, cheaper” until new unmanned missions led to a string of embarrassing failures.

NASA has a poor record of keeping track of extraterrestrial materials.

In 1992, the Mars Observer was launched, bearing eight instruments to measure meteorology, topography, infrared emissions, atmospheric dust, surface radiation, and other aspects of Mars. NASA lost communication with the Observer while it was approaching Mars orbit. A 1994 report concluded that fuel had leaked through an improperly designed valve, causing an explosion, which prevented communication. The Observer’s fate is unknown.

In 1996 the Mars Climate Orbiter duly arrived at Mars to study atmospheric dust and water vapor. Again, communication was lost during the spacecraft’s orbit insertion maneuver. An investigation determined that the Orbiter must have disintegrated in Mars’ atmosphere because the NASA team was using metric units, but the spacecraft manufacturer, Lockheed Martin, sent commands in inches and feet.

The Mars Polar Lander, launched in 1999 and equipped with a light detection and ranging (LIDAR) instrument to study atmospheric aerosols, a microphone, a weather station, and other instruments, disappeared on approach to Mars’s south pole. Yet again, communication failed after the Lander entered the atmosphere. The Lander’s software mistook the deployment of its legs for actual touchdown, turned off its engines at the wrong time, and crashed.

(NASA / Goddard Space Flight Center)

After these failures, in 2000 NASA reviewed all its spacecraft missions scheduled to launch that year, submitting each to a “Red Team” review, which is a final scrutiny of a mission to find errors before launch. The new policy was not completely effective. The 2004 Genesis probe, returning from a voyage to collect solar wind particles, crashed in Utah, spilling its cargo on the desert because Lockheed Martin had installed the probe’s accelerometers backwards.

Human errors clearly play a large role in space mishaps, and they can easily propagate through a system without correction. A NASA report analyzing the crash of the Genesis probe found that a mistake in the design of a set of centimeter-sized gravity switches, or “G-switches.” Each G-switch included a plunger that was supposed to depress a spring. When the spring reached complete compression, it was supposed to close a circuit, which would have told the capsule it had entered the atmosphere and triggered the release of the first parachute. But the sensors had been designed backwards and were never tested. The report identified the following errors:

  • the design process inverted the G-switch sensor design;
  • the design review process did not detect the design error;
  • the verification process did not detect the design error; and
  • the Red Team review process did not uncover the failure in the verification process.

NASA also has a poor record of keeping track of extraterrestrial materials once they arrive on Earth. The agency’s inspector general reported in 2011 that NASA had lent out 26,000 of its 163,000 extraterrestrial samples. Some of these went to scientists for study, but others served as political swag, as when President Richard Nixon handed out moon fragments to all 50 US governors and numerous international leaders. By 2011, 517 samples had been lost or stolen. A little over half were eventually recovered. One fragment, given to the governor of West Virginia, was found in the home of a retired dentist.

Clearly, for the Mars samples, NASA needs to improve on past performance.

The Perseverance rover has collected and cached 38 samples from Jezero Crater in hermetically sealed canisters.

The crater contained a lake of liquid water and possibly the "key building blocks of life" about 2.5 billion years ago, a time when microbial life on Earth was already thriving.

NASA plans to isolate the samples from the machines and people that will handle them all the way from pickup on Mars to delivery to Earth. Theoretically this will prevent both forward and backward contamination.

The current scenario has Perseverance meeting up with the Sample Retrieval Lander which will be carrying a Mars Ascent Vehicle (MAV) and two helicopters like those already in use on Mars. The helicopters are there as backup in case something goes wrong during the canister transfer.

The MAV is a rocket with a cargo space containing a basketball-sized receptacle called the Orbiting Sample (OS) equipped with chambers for the canisters resembling those in a honeycomb.

NASA expects there to be windblown dust on the sample exteriors, which it will sterilize with ultraviolet light.

The OS will seal itself, after which the MAV will close up and launch itself into Mars orbit, where it will disgorge the OS.

The European part of the mission, the Earth Return Orbiter, will come up behind the OS, swallow it, and carry the OS back to Earth, where the Orbiter will eject it for re-entry and landing.

The OS will be protected by a conical heat shield but no parachute as it enters Earth’s atmosphere and lands in the Utah Test and Training Range about 80 miles west of Salt Lake City.

Technicians will put the OS in a bag and the bag into a case which they will sterilize with heat or chemicals. From there the case will be transported in a vault on a flatbed truck to a nearby, purpose-built containment facility designed along the lines of a Biosafety Lab 4 (BSL-4), the highest degree of containment for studying pathogens and the weaponization of microbes. NASA is confident that it can execute this elaborate choreography—the most complex robotic space project yet.

The proposed biolab will have to combine the traits of a standard high-containment laboratory with those of facilities protecting extraterrestrial material from exposure to Earth conditions (called “pristine” labs). BSL-4 labs are built so that the air pressure is lower in interior rooms and in glove boxes where pathogens or potential biological weapons are studied than it is in other rooms or outside the building, which prevents microbes from escaping.

But in order to protect extraterrestrial material from Earthly contamination, the reverse must also apply. Higher air pressure inside a room containing Mars rocks will keep Earth microbes from getting in. The lab would also need to ensure that the samples are not degraded or contaminated when studied for biological purposes and thereby ruined for study by other scientists like planetary geologists and geochemists. Some potential hybrid designs for the Sample Receiving Facility include an outer layer with negative air pressure surrounding an inner space with positive air pressure. Between the two spaces would be a third, double-walled isolator containing a high purity inert gas such as argon.

This slide from an assessment of "strategies for meeting planetary protection requirements" depicts the negative and positive pressure environment needed to isolate materials in a future Mars Sample Receiving facility.

The MSR lab will join the rapidly increasing number of BSL-4 labs around the world, now numbering 51, with 18 more planned, mostly in Asia. Their safety is questionable. As the Bulletin has highlighted, the oversight and regulation of BSL labs both in the United States and globally is haphazard and falling behind advances in biotechnology. A 2022 study by the King’s College London School of Security Studies reported 71 incidents involving the release of infectious pathogens either intentionally or accidentally between 1975 and 2016—a rate of almost two a year.

NASA’s public-facing Planetary Protection Group information does not claim any specific expertise in biosafety or security. Its molecular biology lab at the Jet Propulsion Laboratory is a BSL-2 and appears to be focused primarily on forward contamination and sterilization of outgoing spacecraft and instruments. It is not clear how involved pathogen experts—such as those at the Centers for Disease Control and Prevention—will be in the MSR mission. Of the developing MSR biolab design, Benardini says, “I don’t think there’s major technological hurdles. It’s not an everyday thing we do or an everyday facility we would see, but it’s not infeasible to imagine or build such a system. It just takes work.”

The Planetary Protection Group's expertise centers on sterilization of space-faring hardware.

NASA’s bland confidence in its own competence and the low odds of finding life on Mars are not shared by the popular culture, which has adopted a decidedly different attitude toward extraterrestrial life. The public imagination bloomed during the Apollo missions, generating memorable novels and improbable films and television series galore. Most of these involved encounters with vastly advanced humanoids, but a 1959 film, Angry Red Planet, featured cruel amoeba-like microorganisms grown to excessive size. Arthropods (Alien) and reptiles (the V series, innumerable Star Trek episodes) are also popular models for evil aliens. The V series creatively combined reptilian aliens with “red-dust bacteria.” In the 2013 film The Last Days on Mars, astronauts find living microbes on Mars that turn crew members into zombies.

But the über-tale about space germs is Michael Crichton’s 1969 novel The Andromeda Strain and the 1971 and 2008 movies of the same name, in which an extraterrestrial microbe escapes from a crashed satellite and scientists barely avert a humanity-erasing pandemic. Crichton’s innovation was to combine fear of extraterrestrial conquest with unease about biological weapons laboratories (even though the risk of one of these things is not like the other).

NASA experts insist that the international Committee on Space Research (COSPAR) and NASA’s internal policies require the utmost precautions to prevent the escape and spread of any Mars material on Earth, and that the MSR mission is expected to conform to COSPAR’s most stringent standards. Caitlyn Singam, a bioengineering doctoral candidate at the University of Maryland with degrees in biology and systems engineering, does not think COSPAR’s planetary protection protocols are up to date. Singam presented her COSPAR critique at the 2022 International Astronautical Congress.

“Microscopic life tends to be a lot more variable than macroscopic in terms of viability,” Singam says. “That doesn’t get incorporated into planetary protection strategies. My take is you’re underestimating microbes. They are the wild cards … they’re always going to surprise you in how adaptable they are. They haven’t survived this long by not being flexible.” They reproduce and mutate frequently, she adds, and they’re able to share genes directly by horizontal transfer, which “allows them to adapt as a population over a short period of time.”

There is only a small cohort of outright objectors to the MSR mission, centered around the International Consortium Against Mars Sample Return (ICAMSR). Barry DiGregorio is ICAMSR’s founder and a former Honorary Research Fellow with the Buckingham Centre for Astrobiology in the United Kingdom, which was established by the late Sir Fred Hoyle and is now led by astronomer Chandra Wickramasinghe.

DiGregorio says NASA’s biggest error has already occurred: the decision to bring back samples at all.

DiGregorio cites historical statements by astrophysicist Carl Sagan, pioneering microbial geneticist Joshua Lederberg, and evolutionary microbiologist Carl Woese expressing concern about bringing Mars to Earth. Beyond that, he thinks the moment of re-entry and landing of samples presents the greatest risk.

“This cargo won’t even have a parachute,” says DiGregorio. “They’re going to hope it comes through the atmosphere, and it won’t be knocked off course by space debris.”

A March 2022 drop test of the Earth Entry System  was considered a success. NASA is confident it can execute the elaborate choreography required to bring Mars samples safely down to Earth. (NASA)
A March 2022 drop test of the Earth Entry System was considered a success. NASA is confident it can execute the elaborate choreography required to bring Mars samples safely down to Earth. (NASA)

However remote, the possibility that dangerous alien life will be imported to Earth spooks the public, especially because fear of future contagion tends to spike after pandemics. Public comments about the Environmental Protection Agency’s required Programmatic Environmental Impact Statement (PEIS) trended against the MSR.

“NASA (and its partner ESA) seem willing to put the curiosity of a handful of space scientists above the rights of the billions of humans on Earth to live safely and without contagion,” one contributor wrote.

DiGregorio’s position about the MRS mission is very close to a statement about climate change by the statistician and financier Nassim Taleb, who has written extensively about black swans. Taleb and three colleagues published a letter in 2015 saying in part, “We have only one planet. This fact radically constrains the kinds of risks that are appropriate to take at a large scale. Even a risk with a very low probability becomes unacceptable when it affects all of us—there is no reversing mistakes of that magnitude.”

Given that the current rovers and sophisticated instruments in Mars orbit have operated very well, it may seem that we could just continue investigating from a distance. DiGregorio favors building a space station or a moon base to deal with Mars samples. But NASA and many academic scientists contend that nothing can replace observation with instruments that are not easily adapted to space conditions. According to the Planetary Society, “[C]ertain questions can only be answered by tools that are too large, heavy, and power-hungry to fly on spacecraft.”

Hays agrees. “Any time you make an instrument [for robotic spacecraft or landers], you have to work to condense it and reduce power consumption,” Hays says, so “on a mission you’re going to lose some resolution.”

This is a major factor for the biological instruments. Bacteria can be studied with optical microscopes, but viruses are a different case, says Ken Stedman, co-chair of NASA’s Virus Focus Group and a biology professor at Portland State University. To search for viruses in Mars samples, Stedman says, “I would want to use a transmission electron microscope. The really good ones are 20 feet tall and cost $20 million”—far too heavy and expensive to adapt to remote robotics—and in any case his investigations will likely take place in an academic setting; NASA is not planning to equip the lab with such instruments. At present the lab is conceived of as a depot where the samples will be assessed for safety and catalogued, not a laboratory investigating specific scientific questions. The idea is that once safety is determined, samples will be loaned to other laboratories with specialized equipment.

Some believe NASA’s biggest error has already occurred: the decision to bring back samples at all.

There is another problem with assessing the risk of the MSR mission, and it is embedded in the philosophical assumptions and cognitive tools humans use to analyze risk. For decades researchers and managers making risk estimates have been accustomed to disregarding the outliers—such as the five percent of the spectrum at either extreme of a probability bell curve—exactly because these outcomes are so unlikely. Few science organizations want to expend effort and money on low-probability events—even if they have high consequences should they occur. It is very tempting to chop off the outliers.

But for extreme cases, Casagrande, who says he works only on existential risks that could end civilization, thinks “it does make sense to look at the one-in-a-thousand or once-in-a-lifetime [chance].” Still, he adds, “if you’re going to take a civilization-destroying risk, there should be some evidence that it could happen.”

 Here the black swan rears its graceful head, a volume of Hume under its wing.

Taleb introduced the idea of the black swan in finance and economics in 2007. Writing in the New York Times, he listed the key traits of a black swan event as follows: “First, it is an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility. Second, it carries an extreme impact. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurrence after the fact, making it explainable and predictable.”

Life on Mars fits this description neatly.

Chester Everline, a systems engineer at the Jet Propulsion Laboratory, said it another way in a public comment to the MSR PEIS. “The repeated appearance of statements pertaining to the Martian surface being too inhospitable for life to survive there today [leaves] the impression (perhaps unintentional) that from an environmental risk perspective there is very, very, likely nothing that will be returned from Mars capable of adversely impacting Earth's biosphere,” he wrote. “However, the basis for such statements is a conclusion drawn from a consensus” rather than empirical evidence. He added, “Given that we know essentially nothing about how Martian material may interact with Earth life we can neither rule it out nor completely protect our planet from such a potentiality.”

Asked how NASA’s preparations for low-probability, high-consequence events are progressing, Benardini says, “In our probabilistic risk assessments, we have a lot of that complexity built in. What is the spectrum of care-abouts, what should we be focusing our energy on, from a systems-engineering and safety perspective?”

NASA is leaning rather heavily on “a conclusion drawn from consensus.” It has a reason: that’s the only thing it can do without scrapping the mission. In claiming to look only for past life, NASA is hedging its bets. It can have its cake and eat it too by finding past but not present life on Mars.

A little cognitive dissonance is inevitable when trying to parse NASA’s insistence that it is looking for extraterrestrial life but doesn’t really expect to find any, and that it is taking all precautions, but they’re probably not necessary. This can be explained, says Paige, the UCLA physicist, by non-scientific issues.

“The key question is,” says Paige, “is this worth the amount of money we’re putting into it? And are there any risks? The concern by the administrators was if we hype this is as if we’re looking for life on Mars, and we don’t find it,” the program will lose its justification. “Whereas if we go for this potentially less consequential [goal], like looking for past life, then the program goes on.”

The MSR mission is likely to undergo more revision; very little of it is set in stone. And it is suffering from the same ills as another high-cost NASA effort, the James Webb Space Telescope (JWST). First proposed in 1996, the Webb took 25 years to arrive in orbit, at a cost of nearly $10 billion. The MSR’S total cost is approaching that, and in July the Senate Appropriations Committee handed NASA an ultimatum: Keep the mission within budget, or it could be canceled. A further blow came in September when an Independent Review Board declared “[T]here is currently no credible, congruent technical, nor properly margined schedule, cost, and technical baseline that can be accomplished with the likely available funding.” Further, the report noted, although the mission is a noble one, “The organizational arrangement greatly amplifies cultural differences and dynamics,” indicating that NASA has yet to resolve the turf wars that have plagued Mars missions since Apollo.

One could say that the Webb telescope was worth the money and trouble because it is working superbly, and there is still hope the MSR can work, too, if properly executed. More and more biological and geochemical research suggests that life on Mars—at least life resembling that on Earth, and even extant—is a possibility. But the swan reminds us that what has happened on Earth is not a foolproof guide to what has happened on Mars, and our remote investigations can’t provide conclusive evidence. Assuming the mission is still deemed worthwhile, it seems the only choice is to proceed with extreme caution.

Former NASA planetary protection officer John Rummel told Scientific American in 2022, “People have to have some kind of respect for the unknown. If you have that respect, then you can do a credible job, and the public is well-served by your caution.”

NASA should not be forced to cut corners or permitted to disregard the real chance of a black swan from Mars. NASA’s record shows what can result when that occurs—human errors spike, and the historically dominant specialties—namely engineering, physics, and astronomy—tend to allow biological knowledge and risks to slip to the bottom of the priority pile. In this case, that laxity is the real peril, because when humans are cavalier toward—or oblivious to—black swans, they are unlikely to react appropriately upon their discovery or even recognize one from the extraterrestrial equivalent of Australia. 


VALERIE BROWN is a science writer based in Oregon. She has covered environmental health, climate, nuclear waste, and microbiology, among other subjects, receiving an explanatory journalism award from the Society of Environmental Journalists for an article about epigenetics. In her spare time she writes songs and novels.

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9 months ago

Superb article. This effort needs to be killed. Not one scientist of any stripe can quantify the risk. I’m all for Mars exploration, but let’s do it there, on Mars. We may have to delay the science until the required technology arrives, but that’s ok.

Thomas D.
9 months ago

Excellent article. As someone who submitted an extensive comment in the PEIS process against the current plan for Mars Sample Return, I greatly appreciate increased exposure and discussion of the unknown risks to our only biosphere from the possible accidental release of Mars microorganisms. Millions and billions of people will undoubtedly oppose this plan as they learn of it. My concerns are not just the very low risk of an infectious Mars pathogen, but also include Mars microorganisms which are merely invasive and could cause unquantifiable consequences to our biosphere. NASA, ESA, and the Chinese National Space Agency should send… Read more »

M Otis Beard
M Otis Beard
9 months ago

The ability of Earth organisms to do us harm is a matter of long, long co-evolution; as such, it seems tremendously unlikely that any organism that evolved elsewhere would pose any danger. Mars-evolved organisms may not even use DNA/RNA. On the other hand, we have zero idea what sort of strategies or adaptive abilities an organism not evolved on Earth might have that would surprise us. On the third hand (stop staring at me), there is some possibility that life here and on Mars originated in the same place. . . but given that if this is the case, Terran… Read more »

9 months ago

NASA does not need to bring the samples back to earth directly, and it’s a perfect justification for updates to the space station. As the designer of two BSL4 labs in the northeast, and the first truly compliant BL4 in Pennsylvania, no matter how thorough you are, the risk is not Zero. (Even the alpha/beta transport vessels illustrated above have a known point of failure at the “ring of concern”.) And of course, humans are the main problem.

9 months ago

Mars is already on earth in the form of meteorites (360 separate falls/finds by last count in MetBull). Clearly the long journey in space and extreme heat upon entry would have done much to sterilize any Martians that hitched a ride, but the example of D. radiodurans does suggest that the risk of contamination is nonzero, human involvement or not!

9 months ago

You give WAY too much credit to NASA wrt protecting against potential moon microbes upon the return of Apollo 11. Michael Collins – the third astronaut in the crew – wrote in his autobiography “Carrying the Fire” about the farcical results of NASA’s plans. It was laughable.

8 months ago

How can they say there’s no risk and use as evidence the fact that meteors from Mars have fallen on earth and ‘nothing happened?’ We’ll be bringing back samples packed in their specially made boxes, treated with kid gloves…. If there’s anything living in those rocks, we’ll make sure to give it the best chance at survival. Brings to mind the recent scifi film, “LIFE.”