Addendum on Scaling Up Origins of Life Research
The Potential Ambiguities of Exoplanet Atmospheric Spectroscopy
Previously in Why We Need Big Science to Answer One of the Ultimate Questions I discussed a large-scale scientific research program to study the origins of life. Recently I had my attention drawn to several interesting papers that led me to think more about a comprehensive origins of life scientific research program.
Ciro Villa tweeted a link to Alien imposters: Planets with oxygen don’t necessarily have life, which led me to the paper, “Gas Phase Chemistry of Cool Exoplanet Atmospheres: Insight from Laboratory Simulations” by Chao He, Sarah M. Hörst, Nikole K. Lewis, Julianne I. Moses, Eliza M.-R. Kempton, Mark S. Marley, Caroline V. Morley, Jeff A. Valenti, and Véronique Vuitton. Abel Méndez tweeted a link to “Oxygen and life: a cautionary tale” by Paul Scott Anderson, which was an exposition of the results of the same paper.
Thinking about abiotic sources of oxygen, I searched and found, “Titania may produce abiotic oxygen atmospheres on habitable exoplanets” by Norio Narita, Takafumi Enomoto, Shigeyuki Masaoka, and Nobuhiko Kusakabe. This paper and the previous one demonstrate that there are multiple processes not involving life that could produce oxygen. Thus if oxygen is revealed in future exoplanet atmospheric spectroscopy, it may or may not be a biosignature. It will take more than the presence of oxygen alone to be a smoking gun for life.
I also found the article Don’t Let Oxygen Fool You: As a sign of life on other planets, oxygen may not be the smoking gun some of us thought it was by Dirk Schulze-Makuch, which is an exposition of the Narita, et al. paper. As Schulze-Makuch observes, not only may oxygen give a false positive if taken as a biosignature, but also, for a significant period of terrestrial history, biology on Earth did not make use of oxygen. We need biosignatures other than oxygen for anaerobic life.
There is a lot more that could be said about biosignatures and exoplanet atmospheric spectroscopy, which latter we can expect to have within the next few decades. Being able to determine the atmospheres of exoplanets will be an exciting time, but also a confusing time. As we can see from the case of oxygen, it can be produced by biotic or abiotic processes, so while the finding of oxygen in an exoplanet atmosphere would be exciting, and would probably be hyped by the popular press, scientists would already be well aware that this is not a smoking gun for life.
There are also abiotic mechanisms for the production of methane (cf. “Abiotic Production of Methane in Terrestrial Planets,” by Andrés Guzmán-Marmolejo, Antígona Segura, and Elva Escobar-Briones), although most of the methane on Earth is produced by biological processes. It is possible that the presence of one particular gas alone (like oxygen or methane) is not a reliable biosignature, but that a certain combination of gases (an atmosphere with both oxygen and methane), or a particular ratio of particular gases in its atmosphere, may be a reliable biosignature. I don’t know anything about biochemistry, so I am not competent to judge on this, but others who are competent to judge will no doubt be able to converge upon fairly reliable atmospheric compositions that constitute biosignatures.
When biology and the life sciences eventually become sufficiently advanced that we can routinely manufacture artificial life in the laboratory (even if we choose not to do so for moral or social reasons), this capacity would make it possible to run an elaborate scientific research program modeling potential forms of life on other worlds. Suppose we were to manufacture every viable form of life that is an obvious variation on the theme of life as we know it. By this I mean that if we take Earth’s self-replicating macro-molecule DNA and try every substitution that we can for the four chemical bases of DNA — adenine, cytosine, guanine, and thymine — and determine which of these are viable (i.e., those which are also self-replicating macro-molecules), then we would get variations on the theme of DNA, and possibly alternative forms of life, still easily recognizable as life for us.
Additionally, we would want to “back breed” Earth life to resemble the earliest life on Earth that we can recognize as life (hopefully by this time we will also have made progress in origins of life research and can be a bit more definite about the immediate precursors to life as we know it), and use the viable variations on the theme of early life to create a large (yet finite) number of models for early biota on a habitable planet (i.e., a habitable planet like Earth, a peer world).
This would be like running a scaled up version of the Miller-Urey experiment — again, several different variations on the basic theme — with each experiment populated with one of the variants of artificial “early” life that we have created (either by manufacturing them as artificial life or by back breeding contemporary life to approximate them). We then let these experiments run for a good long time until a relatively stable ecosystem emerges from these presumptive elements of an early biota. At this point we can determine the characteristic gases, and the combinations and proportion of these gases, in the respective atmospheres of these several experiments.
In a sense, an experiment like this would be like skipping a step between the Miller-Urey experiment and experiments with (artificially produced) proto-bionts. Certainly, if we were to conduct scaled up versions of the Miller-Urey experiment and we could bring the development of these experiments all the way to the point of abiotically generating life, then we would simply want to continue this experiment. However, if it proves to take too much time to get from abiotic precursors to self-replicating macro-molecules, we could begin with the simplest self-replicating macro-molecules and skip the step of forming self-replicating macro-molecules.
If both the initial atmospheric content and the artificial proto-biont (or, as it is sometimes called, eobiont) are treated as variables, I expect that there will still be many possible experimental permutations to be run, even when narrowly constrained by viability. Again, multiple experiments will reveal that some of these combinations are not viable, so these experiments would fail early on, and we would come to focus on the viable combinations of atmospheres and eobionts.
If a scientific research program of this kind were to be carried out on a grand scale , and over an extended period of time, we would have a pretty good empirical basis for determining the kind of life that can be revealed by exoplanet atmospheric spectroscopy. Long, long before we reach any of these other worlds with their other life, we will know a good deal about their natural history. It will be gratifying to see the details up close, but mostly we will not be surprised when we travel to alien worlds. We will have been studying them for decades, if not for hundreds of years, before we get to see them for ourselves.