6
Future Directions: Report of Breakout Groups
John Grunsfeld, a former astronaut who until April 2016 served as associate administrator of NASA’s Science Mission Directorate (SMD), moderated the session on future directions for the search for alien life and the report-outs from the breakout groups that met for 2 hours earlier in the day. Grunsfeld started by explaining why he had asked the National Academies of Sciences, Engineering, and Medicine to organize this workshop. Grunsfeld wanted to know about our origins here on Earth and our place in the universe. He finds the question of whether we are alone, or if there is life beyond Earth, to be truly compelling—and a question which can be answered by scientific inquiry. In order to delve deeper into this question, he wanted to bring together scientists from many different disciplines and communities. Grunsfeld said that one takeaway message from this workshop was that the search for life in the universe would “make NASA great again.”
Grunsfeld then introduced the four breakout groups, each of which, while not offering consensus summaries, reported back their key points to the wider audience. These four breakout groups were as follows:
- In situ detection of life as we know it,
- In situ detection of life as we don’t know it,
- Remote detection of life as we know it, and
- Remote detection of life as we don’t know it.
The question each group had been given in order to guide them in their discussion was the following: How could targeted research over the next 5 to 10 years help advance the state of the art for life detection, including instrumentation and precursor research?
IN SITU DETECTION OF LIFE AS WE KNOW IT
Tanja Bosak of the Massachusetts Institute of Technology (MIT) and Nita Sahai of the University of Akron led the breakout group focusing on the in situ detection of life as we know it. Bosak began by saying that significant developments have been made in the field of DNA sequencing and analog environments. Current instrumentation, she said, could be used in upcoming missions on the in situ detection of life.
She then described some plans on the horizon: the Mars sample return mission, its associated analytical tools, and a holographic microscope. Planetary targets for the next generation of missions are also clear: Mars, Venus, and the icy ocean moons.
Most of the time in this breakout group was spent figuring out what they need in order to perform an in situ detection of life. A microscope was at the top of the list. There was also a strong emphasis on equipment to extract and handle samples. They would also like deep drills in order to test the material below what has currently been examined. Tools to assist in the capture and analysis of sprays (i.e., plumes) were also desired. Instruments that can perform clumped isotope analysis were also mentioned, although Bosak said that some already exist that could analyze the returned martian samples. She then gave a list of molecular types that an instrument should be able to detect: native fluorescents, adenosine tri-phosphate (ATP), dipicolinic acid (which is in spores), lipids, and more. These instruments should be cable of capillary microchip electrofluorosis and fluorescent immunoassay experiments. Any instrument sent into space must also be radiation hardened to prevent degradation, especially on longer missions. Other types of instruments that some members of the breakout group desired were an aerial sampling mission in Venus’s atmosphere and subsurface water detection on other bodies.
The breakout group also emphasized the necessity for precursor research to develop in situ detection of life. Of critical importance for life detection are positive and negative controls, a better understanding of false positives and false negatives, and how to detect different types of species and molecules related to life (and what they would even be).
Bosak then discussed the possible approaches to detecting life. The detection of life requires a spatially resolved analysis that combines multiple techniques. In a sample return mission, particularly from the outer solar system, the issue of how to preserve ices needs to be solved. Any sample return mission needs to have multiple laboratories independently analyze the samples as a hedge against false positives or negatives. Any mission focusing on the detection of life must also have collaboration between the instruments being flown. The discussion within the breakout group identified the need for funding to allow for such collaborations; currently, there is too much emphasis on competition among instrument teams. She also conveyed a desire that funding should be available for instrument development to increase the technology readiness level (TRL) and explore NASA Innovative Advanced Concepts (NIAC). Missions should also have greater mobility on the surfaces of extraterrestrial bodies. Conversely, they also want the ability to sample the same place at multiple times to test for temporal variations.
Sahai then summarized the wish list. Effectively, the breakout group wants to be able to quickly analyze a lot of samples and then triage them to pick out the specific samples that require more detailed analysis. A suite of biosignature analysis instruments would then extensively analyze the selected samples.
IN SITU DETECTION OF LIFE AS WE DON’T KNOW IT
Steven Benner of the Foundation for Applied Molecular Evolution and Mark Thiemens of the University of California, San Diego, led the breakout group on in situ detection of life as we don’t know it. Benner started out by saying that their breakout group first tried to figure how and where to look for life. Two camps arose, the people who want to “follow the water” and the people who want to “follow the carbon.”
The breakout group then tried to come up with a set of attributes they value in calling something “life.” One example is compartmentalization (or isolation) as a universal feature of life. For life as we know it, compartmentalization means a structured membrane. However, they decided that this was too constraining, so they chose to generalize the definition of compartmentalization to isolation such that Darwinism could still occur. Isolation is required both for controlling the flow of energy and for Darwinism. It allows for replication while preventing both parasitism and losing important biochemical molecules. Another feature of life is that it contains information. Other attributes life has are motion, the ability to use energy, and being in a community of life.
Benner said that there were three steps in detecting life: finding it, seeing it, and determining its composition. However, technical problems are associated with finding an isolated system, since we do not really know what could be allowed. As already mentioned, they decided to focus on looking for carbon and water as (likely) universal requirements for life. After finding life, they would want to see it do something. This can also be problematic. For example, Brownian motion of a particle could be easily deceiving. Benner said that maybe we would have to see
it do something more interesting, like dividing and replicating. He also worried that we might accidentally kill any life we found due to our ignorance of how to keep it alive, although that could also be used to check whether it was alive in the first place. Benner said that a good idea coming from the discussion would be to just take a sample of it and determine its entire molecular inventory.
Testing for life also depends on what kind of life it is—micro versus macro and extant versus extinct. Macro life would probably be pretty easy to find no matter what it was made of. Finding extant microbes might be possible. The biggest difficulty, of course, would be detecting extinct microbes.
The last topic Benner touched on was the possibility of searching for weird life on Earth. A potential type of compartmentalization, Benner said, was a mineral hole in a rock. There is no known life that goes into one of these holes and then gives up its membrane layers and chooses to instead use the hole itself as its compartment. We could search for that kind of life here on Earth, so maybe it should be done, especially considering it’s easier than doing the same experiment on Mars or another body.
They also discussed the genetic information molecule and the energy source. He said that everybody agreed that a thermal equilibrium is necessary. The energy used by known life is chemical or visible light. They therefore discussed abnormal sources of energy for life, such as ionizing gamma radiation. This type of life might also be extant on Earth, so he said that it would be best to search for it here first. Benner said that they agreed that a von Neumann automaton (capable of self-replication) should be classified as living or, at the very least, a biosignature.
Benner then finished by saying that any planet with life would likely have a biosphere filled with different species. Patterns and textures created by this biosphere could itself be a biosignature.
Audience Participation
A member of the breakout group then said that the list of general attributes required for life was formulated in the context of ancient life on a rocky planet. They then tried to expand the list to be inclusive of extant life, extinct life, and life on icy worlds too. She then went through the list. First they rejected the idea of a cellular morphology in favor of the compartmentalization or isolation morphology. They also wanted to eschew the idea of just organic matter being a biosignature, instead wanting to look for organic molecular compositions that indicate selectivity, patterns, and complexity. The mineralogy of the body is also important. The traditional mineralogy required for life, she said, needs to be broadened. For example, she said that iron metal or sulfate ions on Europa would be a potential biosignature. She called this “contextual chemistry and structures.” Biofabrics are another attribute of life, but on an icy world, one might expect cellular aggregates instead. One item on the list, isotopic knowledge, was controversial as to whether it was important in terms of determining biological processing. The breakout group also listed activity (in terms of metabolism, motility, and reproduction) and information carrying molecules. She said that this list is designed so that future missions could select for instruments than can address each of these points. Preferably, each point could be addressed by multiple instruments independently to ensure the validity of the results. Another audience member then agreed that missions should focus on these, even those missions searching for life as we know it.
REMOTE DETECTION OF LIFE AS WE KNOW IT
Vikki Meadows of the University of Washington and Sushil Atreya of the University of Michigan were the leaders of this breakout group. Edward Schwieterman of the University of California, Riverside, summarized the breakout session. He started with what we already know. He quoted Grunsfeld, “Atmospheric spectroscopy would show external observers that Earth is inhabited.” At a minimum, therefore, the goal should be having the ability to detect and recognize Earth life. Schwieterman said that the discussion fit into two categories: precursor science/ interpretation and technology/engineering.
There was consensus within the breakout group that more work needs to be done to determine the ratios of trace gases that can be biosignatures, but which can also be produced abiotically (e.g., CH4, O2, O3, and N2O). This requires knowledge of the environmental context to avoid false positives that could be caused by geochemical
or photochemical processes.1 A broader environmental context requires knowledge of the planetary architecture and correlations between different parameters, which could inform the interpretation of biosignatures. In atmospheres with a potential biosignature gas (e.g., O2), certain other gases could instead indicate an abiotic origin. Schwieterman also said that the feasibility of using isotopic measurements needs to be explored (e.g., 13C/12C and D/H ratios), including how to interpret the results. However, this would require very-high-resolution spectroscopy (R ~ 100,000). Biosignature gases might also have seasonal changes modulated by life, which could potentially be measured. Clouds and aerosols might obscure biosignatures, but they also provide information about the planet’s geophysical and atmospheric processes and the planet’s potential habitability.2 Another factor in a planet’s habitability is tectonic and volcanic activity. Sulfur gases, he said, could be used to infer these properties about the planet.3 Polarimetry could also be informative in terms of both biological chirality and scattering processes in the atmosphere.4,5
Schwieterman then listed a number of technological advancements that are required to enable or enhance the scientific return of future missions. The wavelength range is one of the most important properties of any instrument. It determines which molecules you can detect. Because many molecules have overlapping spectral lines and bands, multi-band measurements should be pursued, particularly if done alongside low-resolution spectroscopy. Detector technologies and telescope size limit the wavelength range accessible for each target (which is also a function of the angular separation between planet and star), so new technologies or larger telescopes would expand the number of observable targets.6 The noise sources, such as exozodiacal light, also vary as a function of wavelength. Improvements to cooling technology would keep thermal noise down in the near- and mid-infrared, enhancing the science return at these wavelengths.
Another technological advancement that could increase scientific return is high-resolution spectroscopy (R ~ 10,000-100,000), which is necessary for uniquely fingerprinting molecules (and especially isotopes) and their mixing ratios. Pushing high-resolution spectroscopy into space requires miniaturization of existing and developing technology. This will be difficult and expensive. A potential partial solution is to use high-resolution facilities on the ground and complement them with lower-resolution instruments in space.7
The group also suggested that more advanced technologies should also be pursued. A photon detector that can resolve energies in the ultraviolet, visible, and infrared regions of the spectrum could vastly reduce the noise and technical hurdles of data reduction. Coronagraph technology, currently under development, has a throughput problem. Only 1 to 3 percent of the total light gets through. This also needs to be improved. Lastly, the breakout group discussed the technical aspects of polarimetry.
Audience Participation
A member of the audience then asked whether they thought that Venus-like exoplanets could be observed in the search for life with near-future technologies. Schwieterman said in response that he hopes that the James Webb Space Telescope (JWST) will get some transit spectra of Venus-like worlds, considering that there is a (geometric
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1 S.D. Domagal-Goldman, A. Segura, M.W. Claire, T.D. Robinson, and V.S. Meadows, 2014, Abiotic ozone and oxygen in atmospheres similar to prebiotic Earth, The Astrophysical Journal 792:43.
2 G. Arney, S.D. Domagal-Goldman, V.S. Meadows, E.T. Wolf, E. Schwieterman, B. Charnay, M. Clare, E. Hébrard, and M.G. Trainer, 2016, The pale orange dot: The spectrum and habitability of hazy Archean Earth, Astrobiology 16:873.
3 L. Kaltenegger and D. Sasselov, 2010, Detecting planetary geochemical cycles on exoplanets: Atmospheric signatures and the case of SO2, The Astrophysical Journal 708:1162.
4 W.B. Sparks, J.H. Hough, L. Kolokolova, T.A. Germer, F. Chen, S. DasSarma, P. DasSarma, F.T. Robb, N. Manset, I.N. Reid, F.D. Macchetto, and W. Martin, 2009, Circular polarization in scattered light as a possible biomarker, Journal of Quantitative Spectroscopy and Radiative Transfer 110:1771.
5 J. Takahashi, Y. Itoh, H. Akitaya, A. Okazaki, K. Kawabata, Y. Oasa, and M. Isogai, 2013, Phase variation of Earthshine polarization spectra, Publications of the Astronomical Society of Japan 65:38.
6 C.C. Stark, A. Roberge, A. Mandell, M. Clampin, S.D. Domagal-Goldman, M.W. McElwain, and K.R. Stapelfeldt, 2015, Lower limits on aperture size for an exoearth detecting coronagraphic mission, The Astrophysical Journal 808:149.
7 M. Brogi, M. Line, J. Bean, J.-M. Désert, and H. Schwarz, 2016, A framework to combine low- and high-resolution spectroscopy for the atmospheres of transiting exoplanets, submitted to The Astrophysical Journal Letters, arXiv preprint, arXiv: 1612.07008.
transit probability) bias towards finding them compared to Earth-like worlds. A direct image of a Venus-like world, however, would be more difficult due to the small angular separation and inner working angle constraints. Meadows then also said that a potential super-Venus (Venus-like world, but with a larger radius), GJ 1132 b, will be one of the first targets of JWST. Schwieterman said that one challenge with the spectroscopy of Venus-like worlds will be the small scale height (H = (kT)/(μg)) due to the high mean molecular weight (μ) of the atmosphere. The scale height determines the magnitude of the transit features, which would be about 20 times smaller for a CO2-dominated atmosphere (μ = 44 g/mol) than an H2-dominated atmosphere (μ = 2 g/mol). The most promising molecular bands for characterizing Venus-like worlds are the 4.3 and 15 micron CO2 bands, which are in the range of the JWST Near-Infrared Spectrograph and Mid-Infrared Instrument, respectively.
REMOTE DETECTION OF LIFE AS WE DON’T KNOW IT
William Bains of MIT and John Baross of the University of Washington led the breakout group to discuss the remote detection of life as we don’t know it, otherwise known as “weird life.” Bains said that the group started out discussing just how weird to get, deciding against the extremely weird life, such as life made out of neutronium or interstellar clouds. They instead used the National Research Council (NRC) report The Limits of Organic Life in Planetary Systems,8 which has become known as the “Weird Life Report,” as their framework. They thus focused on carbon- and water-based life. Two new ideas since that report, Bains said, were a discussion of energy sources as a precursor to life and a growing realization of the importance of the statistical background information on whether a signature is biological or abiological.
Instead of looking at the mechanism of how biosignatures are created, the group decided to look for inputs and outputs. In that sense, there was a question of whether life’s possible weirdness was even relevant. A lot of what the group would want to look for to discover weird life would be equally valid as a sign of normal life as well, with a few exceptions.
However, weird life allows for an expansion of the definition of “habitable.” The habitable zone could be much wider and weirder. Examples include a planet outside the conventional habitable zone with a large greenhouse effect from H2, an ultra-cold ocean world (e.g., an ocean composed of water plus ammonia and salt), and a very hot world with a few habitable locations (e.g., the clouds of Venus). This wider range of planets allows for a wider range of the planetary system’s possible architecture and evolution. However, Bains said that the search for life doesn’t have to happen only on other bodies. Earth itself could harbor weird life.
Bains then outlined some research goals suggested by the group. One broad category was to move away from looking for just an Earth-like world in an Earth-like orbit around a Sun-like star, but to look for other combinations of planets and environments that could support life. It is not enough to just identify geochemistry of these alternative types of planets. Inputs, outputs, and rates of production also need to be modeled to check for detectability.
Another research goal is to further explore energy capture, specifically the relationship between photon flux, energy per photon, plausible photon capture mechanisms and efficiencies, and oxidants and reductants available in the environment. This relates to looking for a “blip” in the data at certain wavelengths. The terrestrial “red edge” is just one example of such a blip. Any dips, edges, peaks, or other blips need to be examined in the search for life. A biological origin of a weird blip might be ruled out in this way. Revisiting early Earth could be a useful exercise to explore whether different photosynthetic or energy capture processes were used. Bains then again emphasized that there could be a type of life here on Earth using a radically different source of energy, such as thermal, magnetic, or mechanical energy (although previous sessions had been skeptical about these).
An obvious sign of life would be any sort of technosignature, a biosignature that requires technology, such as gases that are very unlikely to be formed naturally. Other indications of life would be large-scale differences from what is expected. Bains gave an example of a Mars-sized planet in a Mars-like orbit, but with the climate of Los Angeles. Even more bizarre examples of technosignatures include rearranging planetary systems, Dyson spheres, Alderson disks, von Neumann probes, and machine civilizations.
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8 National Research Council, 2007, The Limits of Organic Life in Planetary Systems, Washington, D.C.: The National Academies Press.
Audience Participation
A member of the audience then said that hot Jupiters are weird and wondered if that could be evidence of an advanced civilization. Bains said that the idea was interesting, but that the planetary migration people would consider natural (i.e., non-technological) explanations for planetary migration to be more plausible. Another audience member then emphasized that they do not have a “life between the gaps” approach to weird life. He compared their approach to how climatologists proved not just that climate change is happening, but that it’s anthropogenic. Basically, they proved that it was anthropogenic by showing that robust, trustworthy models of the global climate could only represent reality when anthropogenic-induced warming was included. He said that the astronomical and astrobiological communities need a similar set of robust, trustworthy models that only invoke biological processes when all other explanations are insufficient to explain the data.
Another commenter then brought up Venus again. He said that we still cannot explain how Venus became the way it is today, suggesting that maybe life caused the changes.
GENERAL DISCUSSION
Grunsfeld continued as the moderator for the general discussion portion. (The text in this section is not necessarily in chronological order. Comments have been moved out of chronological order to improve flow and preserve continuity of thought.)
Microscopy and Cellular Morphology
Grunsfeld started off the general discussion by bringing up microscopy, specifically on Mars. The Curiosity rover has a microscopic camera called the Mars Hand Lens Imager (MAHLI).
Two discussion items the first audience member brought up came from the NRC report Signs of Life9 (2002), which identified microscopy as a technology that needed further development. Microscopy at the <1 micron level still needs to be developed, she said. The other discussion item was to move beyond single-purpose instruments and instead move toward instruments that combine multiple experiments for biosignatures or that allow for the chemical analysis of specific samples identified through microscopy. Another commenter quipped that that’s why we just need to send astronauts there to do analysis in situ or at least in a nearby analytical laboratory.
Another member of the audience talked about imaging new places in greater detail. Every time we have done so, we have discovered amazing new and unexpected things. Only once we see these new things can we start theorizing about them and developing new experiments. He also said that imaging is important for public support. He thought that the Apollo image of Earth taken from the Moon was especially powerful. Furthermore, he said, continuing to image new things, big or small, is important for maintaining public support. Grunsfeld specifically noted the Hubble Space Telescope’s role in this. Then another audience member brought up microscopy in the same sense, noting that high-resolution spectroscopy has allowed for the imaging of single molecules, and saying that efforts should be focused on making more powerful microscopes too.
A workshop participant said that, following the claim being made that the martian meteorite Allan Hills 84001 contained signs of martian life, there has been a fear of using, talking about, or searching for morphology as a biosignature on Mars. She thought that this is why microscopy has not been used enough and worries that people will continue to avoid using morphology as a sign of life. Another audience member agreed and was puzzled why microscopes haven’t been used. Not only can they increase scientific information, but he said that the public would love to see beautiful, microscopic images. Then another conference participant agreed with the need for microscopes, but said it needed to be combined with chemical analyses of the same samples.
At the Biosignatures of Extant Life on Ocean Worlds Workshop earlier in the year, one participant from that workshop said that everybody just wanted to see a cell. However, morphology (i.e., looking like a cell) was not enough. They also needed to know its chemical and/or molecular composition, its structure, and what it does.
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9 National Research Council, Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques, The National Academies Press, Washington, D.C., 2002.
A member of the audience then continued, saying that there is much more you can do with microscopy than just taking a monochrome image of a cell. Simple additions can detect things like proteins, lipids, saccharides, auto-fluorescence, chemotaxis, and index of refraction. Then he said that the microscope should input samples into a chemical analysis instrument.
Agreeing with the previous commenter, another workshop participant said that it was a good first step. However, all microbial morphology is dominated by the physical forces at the micro scale. This means that cells are almost always spheres or tubes. Additional information beyond just morphology must be extracted to conclude whether it is life.
In more exotic environments, such as below ground, another audience member said that cells can have a variety of very complex shapes (e.g., chrysanthemum or polygonal). Surface microbes, she said, might have environmental selection effects pushing them to be spheres or tubes, but more protected microbes could have a more diverse set of shapes. She then agreed with earlier comments for the need of an instrument capable of doing a variety of analyses on the same sample either simultaneously or one immediately after the other. Another workshop participant agreed. He said that we need to consider that cells could have irregular shapes and could be very sparse. What is really needed, he said, was an algorithmic search that could scan the entire field and look for repeated shapes that it could then make available to the instruments.
A microbiologist in the audience who has worked on microscopy in extremely dilute environments (and self-proclaimed “Debbie Downer”) said that a cell being 1 micron large was optimistic. She said that they’re usually about 0.1 to 0.2 microns or could even be smaller in oligotrophic environments. She also said that the pretty visualizations of cells on Earth have already been filtered and stained. Cells aren’t visible with a regular microscope. She also said that morphology is difficult to determine when it is just a dot of light. If the cell is in water, there is a chance that the cells could be filtered out. She studied life in the Vostok ice core and had to filter at least 100 ml—and often more, up to a full liter—of water to be able to count them with a microscope and extract their DNA. Even looking for things like auto-fluorescence is not a strong biosignature as minerals can exhibit the same property. In sediments on Earth, they have abandoned using microscopes in favor of using rigorous extraction techniques instead. She did agree that you would need to do a chemical analysis experiment on the same sample of material that was put under a microscope. However, she said that this would be very difficult to do, which is why she thinks we need astronauts on Mars to do it.
Weird Life
Shifting to the topic of weird life, another participant in the workshop wondered about the possibility of life on Titan. He said that there has been discussion about life using acetylene and hydrogen as nutrients and existing in hydrocarbon fluids instead of water. Another audience member said that it would necessarily be very cold life. Therefore, getting complex molecules dissolved into a solution would be almost impossible. The diversity of molecules would be limited only to small molecules. The solubility just isn’t there. Another member of the audience continued this discussion, saying that there was an entire group dedicated to potential life on Titan. For example, the solubility of argon in methane at 95 K is extremely low. He said that in the report Signs of Life10 a liquid solvent was deemed to be required for life as we know it. Life in the gas phase has issues of gravitational collapse versus dispersion, while life in the solid phase would be very slow because of its metabolism. He said that there could be life in the Oort Cloud living on the occasional photon, but it would be slow. Going back to Titan, he said that practically nothing dissolves in its surface hydrocarbon lakes, but he did say that its subsurface water-ammonia ocean would be more favorable for solubility and therefore life. In general, however, cryosolvents are bad for life because they’re cold. The audience member posing the initial question about life on Titan then asked whether a water ocean with 5 to 10 percent ammonia was problematic. The other audience member then responded that it would not be a problem. A strong acid or base deprotonates thymine and guanine so that they can no longer bind to adenine or cytosine, respectively. Ammonia, however, is not that basic, having a pH of only about 10 to 12.
A member of the audience then brought up a 2016 study by Kan et al. in which proteins from an Icelandic bacterium were used to coax microbes into producing a carbon-silicon bond in a process called directed evolu-
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10 NRC, Signs of Life, 2002.
tion.11 He then asked whether it was feasible that there could be silicon-based life. If the answer is yes, he asked if there is a way to model or predict biosignatures of silicon-based life. A member of the audience then responded by saying that you first need to decide whether we are carbon-based life. Elaborating, he said that a peptide’s backbone is C-C-N-C-C-N-C-C-N and that DNA has chains of O-C-C-C-O-P. More elements than just carbon are needed. He then said that the Kan et al. study set up a high-energy process that allowed for the C-Si bond to form. The fact that the result was a C-Si bond makes it difficult to say whether it is a carbon- or silicon-based life. He said that the study was still a spectacular result though. If, however, you wanted to make silicon-based life with a backbone of repeating silicon atoms, these molecules are already known. However, because the d orbitals of silicon conduct electricity, these molecules are quite unusual. This kind of silicon-based life is unconvincing. However, he thinks that a Si-C-C-O kind of life would be productive. The Kan et al. (2016) paper is a very reasonable way to get this C-Si bond. The problem with silicon-based life is how much silicon is present. The more silicon in life, the more the life deviates from the natural chemical reactivity we’re familiar with. He then said that Earth life isn’t really carbon-based; the interesting things life does are actually done by the nitrogen and oxygen associated with the carbon.
The key point of Kan et al. (2016), another audience member said, was that it used Darwinian evolution. The selection criteria were set artificially, but the rest of it was Darwinian mutation and selection. It shows that evolution can produce a C-Si bond given the right environment. Another conference participant replied saying that the experiment provided an evolutionary pressure on an enzyme that already could create C-Si bonds (but extremely inefficiently) and evolved it to make it much more efficient (although still pretty inefficient). A key point, he said, was that the starting materials were artificially selected. They fed the bacteria silanes instead of silicates. It is a reaction that happens on its own, and they took an enzyme that already catalyzed it and made it more efficient. Enzymes exist that can handle more than C-Si bonds. Other enzymes can handle carbon-fluorine bonds, carbon-iodine bonds, and even one that makes cyanide-bromine bonds. However, it needs an evolutionary pressure to create these bonds. The carbon-fluorine bond, for example, creates toxic molecules in plants that kill animals that eat them. If there were an environment where a C-Si bond was advantageous and no other sort of bond was, he absolutely thinks that there would be life making C-Si bonds there.
The Term “Biosignature”
A participant in the workshop then said that it is imperative that the community continues and increases communication with Congress, the public, and the world about astrobiology. As such, she feels that the word “biosignature” has been used in too many different and ambiguous ways. It’s usually referred to as a possible signature of life by scientists, not a definite signature of life. However, the public and policymakers interpret “biosignature” as being a definite sign of life. In this light, she thinks that the word “biosignature” should be abandoned and replaced by something that is clearer.
This same issue was covered about 10 years ago, according to one workshop participant. There, they decided on two terms: “biosignature” and “potential signature.” A biosignature is a definite sign of life, while a potential biosignature is a feature of interest that demands further investigation. He said that he thinks it would be a mistake to completely abandon the term “biosignature.”
The “Exoplanet Biosignatures Workshop Without Walls” hosted by the Nexus for Exoplanet System Science (NExSS) and the NASA Astrobiology Institute coined the term “biohint,” according to one audience member who had gone to both workshops. He feels that these terms address this issue appropriately. The general sense of the assembled audience was that more thinking is needed to come up with a term that conveys the uncertainty of “potential biosignature” for the interested public.
An audience member then finished the general discussion with a thought that it would be a great idea to design an app like the Star Trek tricorder to educate people on what biosignatures/biohints were. The general public could use it to learn about potential habitable environments or look at trace gases and minerals in the context of potential biosignatures.
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11 S.B.J. Kan, R.D. Lewsis, K. Chen, and F.H. Arnold, 2016, Directed evolution of cytochrome c for carbon-silicon bond formation: Bringing silicon to life, Science 354:1048.
