James Kasting, from Pennsylvania State University and chair of the workshop’s organizing committee, was charged with summarizing up the workshop. Kasting began by explaining, from his perspective, what the National Academies of Sciences, Engineering, and Medicine wanted from the workshop. Desired outputs from this workshop were answers to the following two major questions:
- What are the key scientific and technological challenges in astrobiology as they relate to the search for life in the solar system and exoplanetary systems in the next decades?
- To what extent will current and planned NASA missions, such as the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope (JWST), the Wide Field Infrared Survey Telescope (WFIRST), Mars 2020, the Europa Clipper (and a possible Europa lander), international missions, ground-based telescopes, and other facilities, play in addressing the key questions relating to the search for life in the solar system and exoplanetary systems?
Kasting then summarized the sessions one by one and asked audience members to chime in with any additional remarks. In the first session, it was said that the search for biosignatures must also consider both the origin of life and the maintenance of life. John Baross said that hydrothermal vents may have played a role in the origin, or at least the early evolution, of life. Baross also said that serpentinization was a key process for providing the hydrogen, trace metals, and surface area needed for life.
Kasting then named free energy as an important consideration in the context of searching for life. Tori Hoehler said that this is the most fundamental requirement for life. Hoehler also said that life as we know it only uses redox chemistry (the transfer of electrons), as opposed to other sources of free energy. A member of the audience then clarified that light energy in photosynthesis is used, but the light energy just gets electrons moving. Life relies on transfer of electrons; light provides additional energy for these electrons in some forms of biosynthesis. Kasting then said that Eric Smith’s presentation stated that free energy gradients are also an essential factor in the metabolism-first origin of life theory.
Another key point was raised by Baross. He had stated that our understanding of the evolutionary relationships between extant organisms is still changing. Baross showed that the ribosomal RNA (rRNA) tree may contain only
two domains instead of three, with eukaryotes included in the Archaeal domain. This has implications as to the nature of the last common ancestor, which Baross said might have been a methanogen.
A member of the audience said that he would like to see a greater emphasis placed on understanding mineral catalysis, particularly for minerals composed of cations that are associated with some of the most ancient enzyme proteins (e.g., molybdenum and tungsten). A greater understanding of boron is also necessary, considering that some oceanic serpentinizing environments had a high concentration of boron. Related to that, the audience member thinks that the origin of life is the most fundamental question remaining in science. He wants to see scientists from every discipline working together to find the solution. He then made a prediction that an organism using a third energy source (not chemical or visible light) would be discovered on Earth, even if it was slow life, dividing once every tens of thousands of years. This prediction is the result of recent discoveries of Archaea growing on some of the lowest levels of energy sources ever imagined.
One important thing to worry about, an audience member said, is that complex order can depend on the details of the boundary conditions and on the substrate. Life, he said, fools us into thinking it can exist anywhere, but this is only because cells are protected by membranes, which have been tuned by a long period of Darwinian evolution. Because of membranes, a homogeneous biochemistry can exist in heterogeneous environments. If the details of the boundary conditions are important, understanding the organic geochemistry in the Hadean era and the earliest part of the Archean era should be a high priority.
Kasting said that one theme from this session was that Mars remains a possible abode for either extant or extinct life. John Grotzinger said that the Curiosity rover has provided new evidence for the repeated formation of long-lived lakes. There is also direct evidence from clays and magnetite that suggests that serpentinization-like reactions took place on Mars. Grotzinger thought that more small rovers were needed to explore a more diverse set of environments. Kasting then said that there is no consensus on how to explain the prolonged periods of warmth on early Mars.
Moving on to ocean worlds, Kasting repeated Kevin Hand in saying that the detection of life on an ocean world would almost certainly indicate an independent origin of life. Therefore, finding DNA there would imply a convergence towards a universal type of biology. Europa and Enceladus are particularly intriguing moons because of the presence of subsurface oceans. Many of the requirements for life are already present on ocean worlds: water, biologically important elements, water-rock interactions, and maybe even hydrothermal vents. Free energy is still an important consideration. Reduced materials could be provided by the mantle, while oxidants could be provided by crustal overturn. Kasting then wondered whether the free energy available was sufficient to create or sustain life on these icy ocean worlds.
Referring to Ellen Stofan’s presentation, Kasting said that NASA has multiple different pathways that it might pursue in order to detect biosignatures. Stofan thought that humans can play a key role in looking for life on Mars, such as enabling deep drilling. Others, however, have argued that human exploration should be delayed due to its cost or to avoid biological contamination until after it has been studied more thoroughly.
A workshop participant clarified that serpentinization itself has not been found on Mars, but rather, a sort of iron redox chemistry where you take reduced iron in an olivine and transfer it to magnetite. This releases hydrogen gas. The latter process is similar to serpentinization, but is not the same thing. She then emphasized and built upon two of Kasting’s other remarks on Mars. Not only has the Curiosity rover discovered long-lived lakes, it has also found evidence for even longer-lived groundwater. She then said that, with Curiosity’s discovery of boron, all of the necessary trace elements for life have now been found on the martian surface.
Bringing up Venus again, a workshop participant opined that an important question that needs to be answered is why Venus never had life (assuming it didn’t) despite having water. If life did exist, he wondered if it could have moved to Venus’s potentially habitable atmosphere. Kasting replied that Venus could have life in the clouds, but he thinks it unlikely. Kasting instead thinks that the best theory is that Venus went into a runaway greenhouse effect during the accretion phase and never came out of it.1 Kasting likes this because it gets rid of all the oxygen. However, other recent calculations have conversely claimed that there might have been water on the surface of Venus.
A member of the audience then wondered what could be done in the laboratory in terms of synthesizing biological molecules from prebiotic chemicals. For example, one type of RNA synthesis requires 1-molar dissolved phosphate. Another requires 25 milli-Molar (mM) dissolved borate. She does not know of any geological environments that are able to concentrate that much phosphate or borate. She then told the synthesizers to give her a scenario of a mineral exposed to an atmosphere with certain physical properties (e.g., temperature) and specified partial pressures of different molecules. She can plug these into well-established water-rock interaction-reaction programs to simulate weathering of the primary rock. These programs can give a rough idea of the ranges of solution chemistry as well as of any secondary minerals produced by the weathering process. Then the primary and secondary minerals are known, along with quantitative estimates of the ions. All the thermodynamic equilibria are then calculated for hundreds of minerals. Then, using this plausible range of ion concentrations, one could attempt a plausible synthesis experiment. Elements like boron will be found anywhere with a good detector, but one needs to also know its other properties, like its abundance and oxidation state.
Kasting then moved on to exoplanets. He said that the remote detection of biosignature gases has been studied much more intensively in recent years. In Vikki Meadows’ talk, she said that the combination of O2 and CH4 (or N2O) was still the best available remote biosignature. O2 by itself is ambiguous. However, O2 false positives could hopefully be identified from the planetary and environmental context of it. Thermodynamic disequilibrium by itself is not necessarily a biosignature because chemoautotrophic life (e.g., methanogens) would tend to drive an atmosphere towards equilibrium.
Describing William Bains’s talk, Kasting said that we should be aware that life on exoplanets might be “weird.” On a hydrogen-rich super-Earth, ammonia might be a possible biosignature, according to Bains. Humans cannot be too Earth-centric and focus only on life as we know it.
Kasting then said that the detection of life on exoplanets might start to become possible within the next few years. JWST, for example, might be able to do transit spectroscopy of an Earth-like star around an M dwarf. Nick Siegler had said in his talk that WFIRST will not find Earth-like planets, but it will find other non-transiting planets. It will also test either a space-based coronagraph or a starshade. Matteo Brogi in his talk said that, with the next generation of extremely large telescopes, Proxima Centauri b might be characterized from the ground.
Again opening it up to the audience, a participant in the conference agreed with an earlier point and said that the community had become sloppy with the term “biosignature.” He cautioned again to use the word “possible” or “potential” before the term “biosignature.”
Another member of the audience then suggested that the community move away from short, snappy descriptions of what a biosignature is. The entire environmental context must always be taken into account in order to avoid false positives. Furthermore, just the detection of a molecule is not enough evidence. The abundances are also important in order to discriminate its origin as being biological or abiological. Thermodynamic signatures, on the other hand, are more of a tool rather than a definitive thing. She said that the detection of thermodynamic disequilibrium only indicates the existence of a process that we still need to interpret and explain.
1 K. Hamano, Y. Abe, and H. Genda, 2013, Emergence of two types of terrestrial planet on solidification of magma ocean, Nature 497:607-610.
Kasting stated that NASA has been looking for extraterrestrial life for at least 40 years. According to Ben Clark, the Viking life detection experiments did not give us the knowledge we hoped they would, but they still resulted in a lot of useful information. Clark also said that the Viking results provide a cautionary tale about thinking about false positives carefully before announcing any results.
In his talk, Gary Ruvkun explained that looking for DNA-based life is easy. All we need to do, according to Ruvkun, is to send an Oxford Nanopore machine to Mars. Since we already have a large database of DNA for Earth life, discriminating between terrestrial life and alien life should be easy. However, extracting the DNA and preparing the sample might be difficult. Additionally, it can be difficult to discriminate one microbe from another one. Another issue is that convergent evolution might complicate the interpretation.
Kasting then mentioned Steve Benner’s “paradox of life” from his talk, the idea that molecular systems required for life could not have arisen without life. Kasting sees this as a challenge to the metabolism-first hypothesis, which says that some molecular systems arose abiotically. Benner had also said that life in liquid water would be based on biopolymers with a backbone of repeating charges that would be easy to detect even if it was not DNA-based. According to Benner, dry land may have been a requirement to allow for the formation of borate evaporites to build up a high enough concentration of boron needed for life. If that logic is correct, then water worlds might be a bad place to search for (RNA-based) life.
A workshop participant said that there are many orders of magnitude separating the selectivity of what is possible at the functional group level in synthetic chemistry versus what is actually used in our biosphere. The big question, he said, is figuring out what mechanisms produce these selection effects and whether they all require a Darwinian dynamic and the emergence of individuality.
Benner then made an offer that he said he has made many times before. He thinks that the metabolism-first model lacks sufficient actionable substance, meaning that they cannot actually test it. However, he offered to synthesize and ship (for free) any necessary molecules to anybody with a specific, actionable chemical system in which a metabolism-first model might operate.
Another participant said that the statement that water worlds might not be good places to search for life is too strong. The water worlds, he said, may have all the ingredients we’re looking for. Enceladus has both organic material and lots of water. Although Enceladus doesn’t have dry land, it does have a rocky core. The boundary layer may provide a template and mineral interactions that could allow for life. He thinks that these are still promising places. Another member of the audience then responded, saying that the water problem applies to RNA and other molecules that have bonds that are thermodynamically unstable in water. Dry land, he said, is the way to make these bonds stable. Therefore, water worlds would not be a good place to search for RNA-based life. If there were a genetic biopolymer that was not very sensitive to water hydrolysis, a water world could be a great place for that kind of life. However, if you are going to have a genetic biopolymer that is sensitive to water hydrolysis, there has to be a compartmentalization mechanism that can dehydrate a small volume. He said that it is a tough problem for which he does not have a solution.
A participant at the workshop then said that, when considering the origins of life on other planets, the environmental context has to include more than just the planet. It has to include the entire planetary system and the star. For example, a water world could host life that was originally created on another world with dry land and then shipped over via impact.
In the interests of time, Kasting then briefly went through the instrumentation chapter. Morgan Cable described missions and instruments that could allow for the detection of life in plumes. Shawn Domagal-Goldman said that the Habitable Exoplanet Imaging Mission (HabEx) or the Large UV/Optical/Infrared Surveyor (LUVOIR) could
potentially detect life on exoplanets. Jen Eigenbrode detailed missions and instruments designed to detect organic materials on Mars.
A member of the audience then said that the one thing that they cannot constrain about martian organic geochemistry is what the ionizing radiation does to organics at the surface, both at the time when it was first put into the ground and at the time that it became exposed to radiation. However, life could have also contributed to this. She then said that the one thing we know about how radiation affects organics is that the surface material is the worst place to look for life’s potential contribution to the supply of organics. To answer this, she thinks we need to excavate or drill deeper than previous missions ever have.
Kasting then concluded by saying that there are many questions remaining concerning the detection and interpretation of biosignatures. He then hoped that the astrobiological community would organize itself ahead of the two upcoming decadal surveys facilitated by the National Academies (astronomy and astrophysics and planetary science) in order to provide them with a coherent set of principles and suggestions.
Michael Moloney, the director of Space and Aeronautics at the National Academies, then thanked the committee, the workshop participants, and the members of the Space Studies Board staff for a great two days of discussions. Moloney then closed the workshop.