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Setting the Stage: Astrobiological Possibilities on Mars
Half a century ago, Carl Sagan observed that the first life-detection experiment to be activated and operated successfully on the surface of Mars was the Viking camera. Had a Martian creature been observed to walk, slither, or fly across the field of view, then the question of Mars life would have been, immediately and forever, answered.
Since Viking, a great deal has been learned about Mars and about its surface environment, as well as some aspects of the subsurface. During that same period of time, scientists have learned more about the nature, varieties, and distribution of life on Earth, greatly expanding a sense of the limits of terrestrial life and the extrema under which terrestrial organisms, especially, but not exclusively, microorganisms, can survive and reproduce. While the weight of what is still not known about even Earth life far exceeds the weight of what is known, Earth life remains the only example of life in the universe about which anything at all is known. Consequently, Earth life forms nearly the entire foundation for our thinking about the potential of similar life elsewhere, as well as possibilities of different forms of life, morphologically and biochemically adapted to other environments.
The geological fossil evidence indicates that life (in the form of colonies of cellular organisms) was present on Earth at least as far back as 3.5 billion years before the present, 1 billion years after the formation of the solar system. Geochemical signatures, which are less distinct than the fossils, in some of the oldest extant rock formations on Earth signal the possible presence of life as early as several hundreds of millions of years prior to that. For the first several hundred million years after Earth’s formation, continuing bombardment of the planet (the tail-end of planetary accretion) is thought to have prevented life from gaining a persistent foothold until the bombardment slowed and the surface of planet finally cooled and remained so. The heavy bombardment period early in Earth’s history may have played a complex role in the origin of terrestrial life, both propelling and temporarily inhibiting the formation of life (Osinski et al. 2020).
Many critical uncertainties remain in our knowledge of terrestrial conditions during the first billion years of Earth’s existence, In addition, our current understanding is, at best, vague about the numerous steps and the timescales involved in the onset and development of life on Earth, from the advent of chemical evolution, involving self-replicating molecules and chemical cycles, to the full elaboration of the complex structures and processes that living cellular organisms instantiate. Nevertheless, the evidence tells us that microbial life has been a major feature of Earth’s environment throughout most of our planet’s history, and that microorganisms appeared within a mere few hundred million years after life first became a possibility, a very short time in comparison with the ages of the planetary bodies in our Solar System.
The rapidity, arguably translatable to imply ease, with which microorganic life seems to have taken hold on Earth suggests the possibility that life’s origin is not an especially difficult or slow climb, and thus it need not be thought a rare event. However, that is speculation. All knowledge and ideas about life are based on the single example of life as it is known, inhabiting one planet, non-randomly selected from among the uncounted myriad.
Humans, the self-styled pinnacle of Earth life, have displayed long and deep interest in the prevalence and nature of life elsewhere. Speculations about the distribution and nature of extraterrestrial life have
reverberated in human thought and writing for millennia.1 That interest is manifest today as a major component of NASA’s goals for exploration of the solar system and the universe. For the foreseeable future, the solar system is likely the only part of the universe that will be accessible to up-close and detailed exploration and investigation. In that respect, the planets and selected smaller bodies in the solar system stand out as targets to investigate for the possible presence of life.
Current thinking suggests that liquid water plays an essential and unique biochemical role in the structures and processes of living systems, although other solvents that are able to exist as a liquid over a range of temperatures and pressures relevant to planetary bodies may also support life (e.g., hydrocarbons, ammonia). Consequently, attention to investigating the possible current or past presence of life on or within solar system objects focuses strongly on those that contain significant and persistent amounts of liquid water. In that respect, and in respect of its proximity, relative ease of access and operations, and its semblance of similarity to Earth as a terrestrial planet, Mars has been a persistent focus in thinking about possibilities for the existence of micro-organic life, either today or in the past.
Conditions may have become conducive to the origin of indigenous life on Mars as early as, or even before, that occurred on Earth (Schulze-Makuch and Irwin 2018). Some have speculated that life arose first on Mars, and that Earth life arose after material ejected from Mars as a consequence of early giant impacts, and made their way to Earth as suggested by literature (Wetherill 1984; Melosh 1988). This is speculation, but it underscores the reality that understanding the origin and history of life in the solar system requires an accounting of the history of life, if any, on Mars.
To the date of this report, investigations of Mars have addressed, slightly, whether living organisms currently exist on that planet’s surface, and somewhat more thoroughly, whether present conditions on the surface of Mars are conducive to the survival of Earth organisms. Half a century ago, the Viking Landers conducted the so-called Labeled Release Experiment, involving experiments and measurements on surface materials specifically aimed at searching out metabolic activity of possible Martian surface organisms.
The predominant, though not unanimous, view among the scientific community is that the Viking landers found no evidence of living organisms on the Martian surface. The equivocal positive results of the Viking Labeled Release Experiment (Levin and Straat 2016), despite the contradictory evidence finding organic material to be absent at the landing site, have sustained a still-continuing discussion about the possibility of indigenous organisms at the surface of Mars. The associated residual ambiguity of the Viking results has provoked some to call for follow-up repetition of the Labeled Release Experiment.
Altogether, Mars, 45 years after the earliest in situ investigations of that planet, remains an object of intense astrobiological interest. Consequently, in the further exploration and possible exploitation of Mars, care must continue to be taken to preserve the ability of astrobiological explorations and measurements to answer major questions pertinent to understanding the planetary origin and history of life in the solar system.
As delineated in Chapter 1, starting about 70 years ago, the world scientific community, anticipating the imminent advent of space exploration, grappled with the challenge of controlling and mitigating the risks of biological cross contamination among planetary bodies caused by space travel between the planets, whether robotic or (eventually) peopled. Significant risks have been understood to exist in both directions, each calling for carefully planned and executed planetary protection measures:
Backward contamination, involving the return and release on Earth of extraterrestrial organisms capable of surviving and proliferating to the extent of disrupting terrestrial ecosystems or introducing new diseases to people, animals or plant life.
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1 Epicurus, in Letter to Herodotus (on physics), ca. 300 BCE: “There are infinite worlds both like and unlike this world of ours. For the atoms, being infinite in number . . . are borne out far into space . . . and have not been used up, either on one world or a limited number of worlds. So that there nowhere exists an obstacle to the infinite.”
Forward contamination, involving transport to, and release upon, another planetary environment of terrestrial organisms2 capable of proliferating in that environment to the extent of interfering with and confounding the search for, and identification of, indigenous organisms.
This report is concerned with questions of controlling the deposition and proliferation of contaminating Earth organisms brought to Mars in the course of exploration and other activities (i.e., forward contamination). Here, the committee defines proliferation as growth of a microbial population that has the potential to escape containment imposed by existing environmental conditions or engineered constraints (e.g., inhospitable surface conditions, limited melt zone beneath a lander, enclosure within a human-made structure). The focus of the committee is not contamination per se (i.e., the mere transfer of organisms to the Martian environment) but rather contamination that has the potential to proliferate and interfere with subsequent exploratory searches for, and, if found, characterization of, indigenous life on Mars.
Planetary protection policies were developed for the two specific purposes. One of the purposes was to protect planets against forward biological contamination, well enough, and long enough, to allow scientific investigations a reasonable likelihood of being able to establish whether life had originated, or existed, on any planets in the solar system other than Earth. The consequent requirement was to reasonably control potential confoundment by Earth organisms carried on spacecraft. The principal hazard has, from the beginning, been considered to derive from the possibility that certain transported organisms might somehow find a supportive refuge, survive, thrive, and proliferate in the alien environment, possibly leading to widespread, even planet-wide, in the extreme, colonization, thus inhibiting the ability to ascertain whether the organisms were indigenous or were merely Earthly contaminants. For example, finding mold growing on the insulation within a spacecraft lander that had been sitting decades on the surface of Mars would be interesting; but, in isolation, it would compellingly be interpretable as Earthly contamination. It would not confound the search for indigenous Mars life.
The search for life on Mars could be confounded if one or more viable terrestrial organisms was transported to a subsurface cave system on Mars with conditions allowing the organisms to survive, grow, and proliferate. Moreover, if that cave system were to be connected to similar subterranean systems, hydrologically or by atmospheric transport of droplets, that could engender a situation even more confounding to the search for indigenous Mars life.
Considerations such as these define and shape the forward contamination hazards on Mars and, at the same time, provide the degrees of freedom that can allow safe exploration and possible development on Mars. The committee developed its report sensitive to both the risks, the opportunities and the goals, and recognizing that planetary protection measures are not intended to inhibit any activity in extraterrestrial environments, but rather to foster such activity while preserving, to the greatest extent reasonable, the prospect that important scientific goals can be realized.
If there was confidence that any indigenous Mars life would be differentiable, inherently, from Earth life, the challenge would be simpler. In that case, any Earthly cellular contaminants found on Mars would, upon sufficiently close examination, confidently be interpretable as such, and would not confuse the search for indigenous Martian organisms. However, there does not currently exist a broad scientific consensus around any intrinsic approach to reliably and definitively distinguish indigenous extraterrestrial (cellular) life from terrestrial contamination. The application of planetary protection protocols aimed at controlling earthly contamination to low limits remains the method of choice to minimize confounding searches for indigenous life on Mars and in other extraterrestrial environments. Were indigenous organisms to be found on Mars, the differences and similarities would become singular targets of investigation, focused on the relationships between the two populations, involving such possibilities as independent origin, natural (i.e., impact-ejecta) transfer between the two planets, or even questions of possible convergent evolution. Consequently, the search for life on Mars cannot rely on the ability to distinguish Martian life from terrestrial organisms brought to the planet.
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2 The committee uses the term “terrestrial” as meaning “originating from Earth.”
Finding 1: The discovery of indigenous life on Mars would be a signal event in the development of human knowledge, with widespread impact and implications. Preserving unambiguous separation or distinguishability of terrestrial organisms from indigenous Martian organisms, by application of planetary protection protocols, or by other scientifically accepted means, is essential to realizing NASA’s solar system exploration goals and addressing profound questions that have long preoccupied humans.
