Abstract
F
This is in contrast to the specifics proposed by Fairén et al. (2017), which involve searching for martian life using spacecraft that carry Earth contamination at levels likely to confound the desired result—and potentially other future human objectives at Mars. Despite what Fairén et al. (2018) seem to believe, current planetary protection policies are not to blame for the priorities of the NASA Mars Exploration Program—or other space agencies' exploration programs—nor do arguments in favor of continued adherence to planetary protection policies, as they have been developed over decades by the international scientific community, “illustrate why we are not searching for life on Mars today and why we haven't done so during the last decades.”
Rather than revisiting the contentions of previous articles point-by-point, we here consider three decisions that need to be made when addressing the question, posed by Fairén et al., of how to detect martian life with future missions. These are
(1) Where should we look for martian life?
(2) What are the concerns associated with Earth contamination?
(3) Which missions should be sent, and when?
In reviewing how the Mars exploration community has evaluated these options over time, we illustrate the thought processes that have contributed to their development—which follow a train of logic that applies equally well to future missions.
1. Where Should We Look for Martian Life?
The NASA Mars Exploration Program, post-Viking, has not been actively searching for extant life on Mars because the program has effectively been following the roadmap laid out in An Exobiological Strategy for Mars Exploration from 1995 (Exobiology Program Office, 1995). One of the points emphasized in this strategy is that it doesn't make sense to go to Mars trying to search for extant life without having a specific, qualified target for such a search. Further, upon finding such a target, it's necessary to control against false-positive indications of life caused by Earth-sourced contamination (Space Studies Board, Task Group on Planetary Protection, 1992; Exobiology Program Office, 1995).
Specifically the Strategy states,
… It is evident that several hypothetical alternative niches for life on Mars have been suggested in the exobiological literature. As of this writing, however, these remain to be located and characterized. Thus, the initial thrust of the strategy for extant life on Mars must be to determine whether or not these environments actually exist. Only with the acquisition of this fundamental information will it be reasonable, from the point of view of extant biology, to probe such putative environments with landed instrumentation (Exobiology Program Office, 1995).
Until a likely place on Mars that may currently support life has been identified, searching for extant martian life is premature. If life was once widespread on Mars, and is not now, there should be much evidence for ancient martian life spread around the planet, and in less contaminable places than those that qualify as modern Special Regions. Hence, “searching for life on Mars” currently prioritizes looking for conditions that would preserve biomarkers expected to reflect ancient Mars, rather than present-day Mars, because we think we know where to look for them.
In fact, one of the authors of the Fairén et al. papers is a coauthor of a recent paper entitled “Critical Assessment of Analytical Techniques in the Search for Biomarkers on Mars: A Mummified Microbial Mat from Antarctica as a Best-Case Scenario” (Blanco et al., 2017). We agree with this approach. Detecting biomarkers of ancient (and possibly extinct, microbial-mat) life may, indeed, be the best way to find martian life.
Despite this, Fairén et al. (2018) seem to feel that the Space Studies Board advice to target locations with high preservation potential runs counter to “the actual priorities, goals, and desires of the Mars community”—but they do not cite references that identify even one specific location on Mars where a search for extant life is advocated as the primary mission goal. To justify this “community” statement, Fairén et al. (2018) listed the following references: McKay et al. (2013), Grossman (2013), Heldmann et al. (2014), Vago et al. (2015), King (2015), Levin and Straat (2016), Gordon and Sephton (2016), Smith et al. (2017), Xie et al. (2017), Niles et al. (2017), Ehlmann et al. (2017). These references generally relate to the concept of life detection on Mars, but the advocacy of these references for the assertion that a search for extant life is the highest priority for Mars exploration is decidedly weak. In contrast, one of the strongest reasons given for doing that search is related to ensuring the safety of future human explorers against a possible biological threat—exactly what planetary protection is intended to accomplish and what COSPAR policy specifies.
For a summary of how each of these references maps to a search for extant life on Mars, see Table 1.
Note that only 3 of the 11 papers (10 science papers and 1 news article) specifically support that goal, including support for the ExoMars mission, which is a Category IVb mission (sample train cleaned to the Viking standard), advocacy for a repeat of the Viking Labeled Release experiment by the principal investigator and coinvestigator of that experiment, and advocacy for an attempt to detect extant life as part of due diligence before humans have uncontrolled exposure to martian surface materials that could contain life.
Our previous statement that “the Mars community is not convinced that a mission to attempt detection of extant martian life is a high priority” is well-supported by the recommendations of the most recent Planetary Science Decadal Survey (Space Studies Board, Committee on the Planetary Science Decadal Survey, 2012) on Planetary Science priorities through 2022. A mission to go to a Special Region and search for extant martian life, although espoused by Fairén et al. (2017, 2018), was not among the missions prioritized by the Space Studies Board in 2012. Note that, in the eyes of NASA and the US Congress, the Planetary Science Decadal Survey documents “the actual priorities, goals, and desires of the Mars community.”
It may be true, as Fairén et al. stated, that “the Mars community, including NASA, always points to life detection as a number-one priority in Mars exploration,” but it is important to note that “life detection” does not necessarily mean going to Special Regions to try to culture or sequence microbes. In fact, although we consider this evident lack of interest unfortunate, it would be even more unfortunate to permit access to Special Regions in the absence of appropriate planetary protection precautions.
2. What Are the Concerns Associated with Earth Contamination?
The current definition of Special Regions is based on the characteristics of possible contaminating Earth organisms, only (Rummel et al., 2014). But even granting that the best places to find signs of extant life on Mars may also be in Special Regions, the logic of introducing Earth organisms into the places where they are most likely to grow—and potentially obscure signals of martian life as a result—evades us.
Issues that are of most concern to mission planners include cost and technical capability—yet the Viking Program was successful in sending nearly sterile life-detection missions to Mars when that had never been done before. The development of technical capability certainly incurs costs—and estimates of the costs associated with performing full-system dry heat microbial reduction of a Mars lander have remained essentially constant at the equivalent of one large science instrument, as we cited previously (Rummel and Conley, 2017). On the one hand, it is not possible to say that this is inexpensive, given that recent large instruments (e.g., mass spectrometers) sent to Mars have cost over 100 million dollars. On the other hand, the total cost of recent Mars missions has run into several billions of dollars, so the fractional cost of planetary protection measures could be under 10%. Again, it becomes a question of priorities—if one really wanted to study a Mars Special Region, this would be an acceptable cost. However, because access to Special Regions was not identified as a high priority in Planetary Science Decadal Surveys, the investments necessary to reestablish those capabilities (linking back to the Viking experience) have not been made.
This does not mean that it is suddenly sensible to allow access to Special Regions by spacecraft that carry viable Earth organisms, as Fairén et al. propose, at least not without evaluating the potential consequences, and as a global community—not just the Mars community—deciding that we're willing to accept them.
In the context of introducing Earth contamination to Mars, Fairén et al. make the following three-point claim:
… the current robotic exploration of Mars will have little (if any) impact on potential martian biospheres or on our efforts for searching for active life on Mars, because (i) the microbial burden carried by unmanned robots is minimal and not renewable and, most importantly, known and identifiable; (ii) the martian surface is bactericidal in nature; and (iii) we know how to distinguish an Earth microorganism from potential martians.
As it turns out, the first two of these claims are already incorporated into the COSPAR Planetary Protection Policy—and the third is not supported by any available data.
2.1. Microbial burden on robotic spacecraft
It is thanks to planetary protection cleanliness requirements that the microbial burden on Mars robotic spacecraft is “minimal”—and also “not renewable” so long as the microbial passengers are not introduced into places where they can grow (the definition of Special Regions). However, current technologies to identify microbial populations carried on spacecraft are inadequate and would be even less well-developed if not required for planetary protection. A reasonable genetic inventory of microbial contaminants in the extremely oligotrophic environments of spacecraft assembly clean rooms may be achievable by using technologies developed for other fields but will require further advances in sample collection and analysis—developments that are currently being pursued by planetary protection researchers.
In contrast, the stringent control of diversity and quantity of microbial populations in spacecraft assembly clean rooms is not something other communities do—and is not something space agencies can do, or will ever do, without requirements being enforced by someone. This control is exactly what the COSPAR Planetary Protection Category IVc specifies to allow access to a Mars Special Region (Kminek and Rummel, 2015). Flying such a mission right away is fine with us—and we agree with Fairén et al. that Mars community advocacy, including appropriate investments in the necessary cleaning capabilities, is what is needed to get this sort of mission sent to that sort of place.
2.2. Biocidal factors on Mars
Information we have gained about the martian surface environment does include evidence of challenging environmental factors, such as perchlorates, peroxides, and UV radiation that are biocidal to many Earth microbes. However, there is no justification for Fairén et al. to conclude, regarding a robotic rover, that “after the interplanetary trip and just one single sol on the surface of Mars receiving direct sunlight” a spacecraft “will be as biologically clean (and maybe even more) as the Viking probes were when they left Earth.”
This statement is invalidated both by the fact that some microbes will ride inside a nonsterilized rover and by data from earlier Mars missions and experimental studies.
In fact, what we currently know about Earth microbes suggests that some microbes that are relatively common in spacecraft assembly clean rooms (e.g., SAFR-032) are also resistant to the biocidal factors that have been identified on Mars (cf. Schuerger et al., 2006, Nicholson et al., 2012). Further, even susceptible Earth microbes will not be affected if they are not exposed to the relevant biocidal factors—for example, not every external surface on a Mars lander is exposed to sunlight, but all may be covered by a UV-protective dust layer.
As stated by Schuerger et al. (2006) “the presence of UV resistant microbes on spacecraft surfaces rapidly covered in dust during landing operations, and non-Sun-exposed surfaces of spacecraft remain concerns that must continue to be addressed through adequate spacecraft sanitizing procedures prior to launch.”
This potential for survival is something that needs to be appreciated, especially if a search for martian life can have any hope of success. For example, in Box 1 of Fairén et al. (2018), the authors state that planetary protection maintains “some accepted popular concepts [that] are either outdated or simply wrong,” although those concepts are basic points related to biological cross contamination and to the capacities of adventitious pathogens. We were entertained by the mental image of jungle parrots flying around on Mars, but parrots are no more poorly suited to survive there than in some microbe-supporting Earth environments, such as Yellowstone's Grand Prismatic Spring.
In the context of understanding the possible advent and survival of more hardy Earth organisms on Mars, it is curious that Fairén et al. (2018) ignored another of their coauthor's papers (Goordial et al., 2016), which states
Dry permafrost as observed in University Valley is rare on Earth, likely only occurring in the McMurdo Dry Valleys, but is commonplace in the northern polar regions of Mars at the Phoenix landing site (Levy et al., 2009). Thus, our results have implications for our understanding of the cold limits of life in terrestrial environments, with potential implications for habitability models of Mars near surface permafrost and other icy worlds.
2.3. Distinguishing Earth and martian microbes
Fairén et al. (2018) stated that “We are clever enough to recognize martians, if they exist.” This demonstrates impressive confidence, given that the basic composition of martian life remains totally unknown. They go on to note that much of what they say about identifying martian life based on DNA sequences is valid “if (a big if) martian life is genetically similar to Earth's.” True! But in the absence of evidence for each specific characteristic we might predict about martian life, it is permissible (and even advisable) to consider alternative scenarios as well as the preferred one.
In line with our general skepticism about relying on martian life to use DNA, we are also concerned by scenarios in which identifying a DNA-bearing martian organism would not be simple—for example, the continuing discovery of Earth organisms that root near the base of the 16/18S RNA tree, and also the potential for lateral gene transfer to alter the anticipated placement of putative martian organisms on a general relatedness tree of life.
Fairén et al. did go on to cite other strategies for detecting martian life that do not involve detecting nucleic acids—but many of the biosignatures proposed for use in these other methods would be even more easily confused or obscured by Earth microbial contamination than is DNA. In this context, the suggestion of Fairén et al. that “Relaxing the forward contamination rules and allowing a dedicated search for life on Mars now” would almost guarantee that we do find life on Mars—but not necessarily martian life.
Their further suggestion that this “would actually assist in understanding the actual risks of returning samples from Mars in the future, as we will have a better idea whether there is life on Mars or not” is entirely incorrect. It could be very damaging to future Mars exploration, if we do detect indications of life on Mars but can't be confident that these are not martian life, because the noise of Earth contamination is so high. In the context of bringing martian material to Earth, a false-negative result is a much greater risk to the safety of Earth than a false positive—for this reason, if some sort of life has been detected on Mars, we have to assume that martian life is present until it can be demonstrated otherwise. This would not be an easy task…
Fairén and his coauthors contend that “using one argument on one side of the issue and then the opposite argument on other side of the same issue, however it fits, is not acceptable in a scientific debate.” We are not primarily engaging in a “debate,”—our objective is to establish appropriate parameters for ensuring the effectiveness of a search for martian life, and associated questions of Earth safety assurance. As such, some arguments do cut both ways.
Early consideration of alternative scenarios that could confound interpretation of results is what contingency planning is all about.
3. Which Missions Should Be Sent, and When?
We do genuinely agree with the position of Fairén et al. “that we need to resume a dedicated robotic search for life on Mars as soon as possible, before manned missions reach the planet and it becomes too late.” We dispute, however, their contention that planetary protection cleanliness requirements are somehow to blame for the delay in sending life-detection missions to Mars. We also disagree with the proposal that cleanliness requirements should be relaxed on near-term robotic missions—rather, improvements in our understanding of environments on Mars and the capabilities of Earth organisms highlight the continued need to ensure that planetary protection precautions are effective.
As noted by Fairén et al., robotic missions will carry with them only the bioburden that they have when they leave Earth, but some will survive the trip. For a mission not going to a Special Region and not searching for extant life, many will remain alive inside spacecraft components for decades or more. The surface of Mars will kill many, but not all of them, and depending on the final resting place of the robot (in an old impact crater, rolled down a slope, sitting in the shadow of a large rock), some may have a chance to escape captivity and find a better opportunity. A handful could survive, and if lodged near martian ice, they might one day do even better than that.
Note that the spread of Earth contamination on Mars is not dependent on the original amount introduced. Taking a dirty spacecraft to a location specifically defined as a place where Earth organisms can survive could likely lead to the “discovery” of a contaminating Earth organism, and the potential mis-identification of it as a martian … by others … who could then claim that NASA is hiding the discovery of life on Mars. Such a mission should not be flown—because such a “detection” would make the possibility of a biological threat to human explorers of more concern than it needs to be.
By the time human missions are sent, the goal should be that additional information about both Earth microbes and martian environments will be made available from robotic missions to facilitate human mission goals and provide for the appropriate tailoring of planetary protection requirements.
Human missions, by design, will keep associated microbes alive as long as habitats are powered, and many will reproduce in great profusion. The humans, however, will need to contain and constrain microbial growth and reproduction, both to ensure proper operation of life support systems and because the humans and the microbes will be competing for water. It is true that “some degree of forward contamination associated with human astronaut explorers is inevitable” (Conley and Rummel, 2010), but for human exploration missions, the spread of those microbes into Mars Special Regions can be minimized, if not avoided completely, by appropriate operational and technological precautions. We do not dwell on the possibility of human missions arriving on the surface of Mars prior to an opportunity to conduct an appropriately clean mission to some Special Region on that planet, because we judge the likelihood of that event to be small.
The short paper by Ehlmann et al. (2017) noted the need for “evidence that extant life is not widespread in martian surface materials” and the opportunity for the “continued characterization of martian surface materials to determine whether extant life is present” as important opportunities to address critical knowledge gaps about Mars. We contend that it would be ethically incorrect to send a human mission without understanding the resultant potential for exposing people (tourists?) to martian materials that may contain living organisms. Regulatory frameworks for licensing space missions exist, and treaty obligations apply even to countries that may not have implemented the appropriate national laws. It would be surprising if human missions to Mars were authorized to launch, prior to consideration of these sorts of concerns.
For multiple reasons, we reject the proposal by Fairén et al. that we should cut corners and attempt to fly robotic missions that could only equivocally resolve questions about martian life, before humans arrive on the planet. Nonetheless, we believe that a Mars life-detection mission is essential to the health and safety of future humans on Mars and that this effort needs to be conducted in the most effective possible way, without sacrificing its essential credibility in the name of expediency.
We strongly encourage continued discussion of these issues, both within the Mars exploration community and also in an expanded global community of citizens who both pay for and could be affected by such efforts.
Footnotes
Acknowledgments
The work described here was supported by the NASA Planetary Sciences Division and the Office of Planetary Protection. Thanks, too, to Sherry Cady and the staff of Astrobiology for their assistance with this paper and for the Forum of which it is a part.