Conclusions and Recommendations
The statutory goals of the National Aeronautics and Space Administration (NASA)—sending missions to the solar system, understanding the origin and distribution of life in the galaxy, and contributing to the scientific development of the United States—remain. However, recent long-term planning has come to focus on human missions, first to the Moon, then to Mars, and then beyond. Human missions have both strengths and weaknesses. It is clear that an astronaut can improvise and explore in a fashion not possible for even the most sophisticated robot, including exploration for living systems. Balancing that advantage, in addition to the much greater cost per unit of data obtained, is the fact that human exploration is concomitant with human contamination. No discovery that we can make in our exploration of the solar system would have a greater impact on our view of our position in the cosmos or be more inspiring than the discovery of an alien life form, even a primitive one.
At the same time, it is clear that nothing would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it either because of the consequences of contamination or because of the lack of proper tools and scientific preparation.
The committee’s investigation makes clear that life is possible in forms different from those on Earth. Different specific biomolecules may be considered highly likely in extraterrestrial life. Different architectures at the microscopic and macroscopic levels must also be considered likely. Advocates of replicator-first theories hold that—with reasonable interactions between minerals, polar refractory solvents, and organic species—the spontaneous emergence of genetic biopolymers may be expected. Other scientists feel that the chances of such an event are infinitesimal but that other circumstances may suffice for the origin of life. Whichever group is correct, the likelihood of encountering some form of life in subsurface Mars and sub-ice Europa appears high. Life-detection strategies should be redesigned to maximize the possibility of successful detection of life by seeking intermediates common to the two theories.
Furthermore, life should be considered possible in aqueous environments that are extreme in their solute content, in their acidity or alkalinity, and in their temperature range, especially with ammonia as an antifreeze in low-temperature water-ammonia eutectics. The committee sees no reason to exclude the possibility of life in environments as diverse as the aerosols above Venus and the water-ammonia eutectics of Titan. It seems that life is less likely in more exotic solvents—such as liquid dinitrogen, liquid methane, and supercritical dihydrogen—but this conclusion is based on few data.
Given the importance of developing the understanding and capability to detect and recognize possible life forms in other planetary environments, the committee offers recommendations for three lines of research: labora-
tory studies, field studies, and space studies. The first two are clearly appropriate for both NASA and the National Science Foundation (NSF), and the third is likely to be solely in the purview of NASA.
Laboratory studies are needed that will help to elucidate the origin of life, especially studies that use information from NASA missions, the inventory of organic materials in the cosmos, and interactions between organic materials and minerals set in a planetary context. There is a need for basic research to understand interactions of organic and inorganic species in exotic solvents, including water under extreme conditions (as found on Venus, Mars, Europa, Enceladus, and elsewhere), water-ammonia eutectics at low temperatures (as possible on Titan), and liquid cryosolvents (as found on Triton and elsewhere). A need also exists for synthetic biology that constructs and studies molecular systems capable of Darwinian evolution that are different from standard DNA and RNA, especially those designed to improve understanding of the chemical possibilities that support Darwinian evolution. Such studies should include the search for self-sustaining energy-driven metabolic cycles. The committee’s recommendations to NASA and NSF for specific kinds of laboratory studies are as follows.
Origin-of-life studies, including prebiotic-chemistry and directed-evolution studies that address physiologies different from those of known organisms. Such alternate physiologies can include novel metabolisms and growth in extreme conditions that are not found on Earth but are found on other planets and moons. Some examples are growth in media with low ratios of water to organic solvents, the substitution of arsenic for phosphate, the use of carbon-silicon polymers, and the use of mineral catalysis instead of enzyme catalysis.
Further studies of chirality, particularly studies focused on the hypothesis that specific environmental conditions can favor chiral selection, or on an alternative model that life with L-amino acids and D-sugars is better “fit,” from an evolutionary perspective, to evolve into complex organisms.
Work to understand the environmental characteristics that can affect the ability of organisms to fractionate key elements, including not only carbon but also sulfur, nitrogen, iron, molybdenum, nickel, and tungsten. An understanding of how life fractionates transition elements could provide an essential marker for past and present life and insight into their metabolic potential. Even weird carbon life will use elements for energy, such structural polymers as proteins or protein analogues, and oxidation and reduction reactions.
Many of the environments on Earth that have analogous environments on other planets and moons have not been adequately sampled. They include the deep subseafloor crust, the deep oligotrophic ocean, and the upper atmosphere. Therefore, the committee places particular emphasis on the search for organisms that have novel metabolisms that use novel energy sources, including black-body radiation or sources thought to be ancient but ubiquitous on other rocky planets. They include organisms that use hydrogen and sulfur as energy sources and iron (FeIII) as an electron acceptor. The focus should be on microbial ecosystems that do not depend on photosynthesis. There is also an opportunity to look at submarine hydrothermal vent environments as primordial and to design experiments to search for organisms that have relic genes and metabolisms that can reveal something about early life. The committee’s recommendations to NASA and NSF for field studies are as follows:
A search for remnants of an RNA world in extant extremophiles that are deeply rooted in the phylogenetic tree of life. They could include RNA genes that, unlike the common retrotransposons found in eukaryotes that are just “selfish genes,” may have some function in the cell. The search should also include viruses from hyperthermophilic archaeans that have already been shown, in the study at Yellowstone, to be unlike anything that has been seen before and that have characteristics of all three domains of life.
A search for organisms with novel metabolic and bioenergetic pathways, particularly pathways involved in carbon dioxide and carbon monoxide reduction and methane oxidation coupled with electron acceptors other than oxygen. Regardless of how weird carbon life might be, there will be a “unity of metabolism” in which all
organisms use the same carbon and energy sources. There will be a finite number of ways to use the carbon and energy sources, and science is not close to knowing this limit.
A search for organisms that derive some of their catalytic activity from minerals rather than protein enzymes, including organisms that combine mineral and protein catalysis.
A search for organisms from environments that are limited in key nutrients, including phosphorus and iron, and determination of whether they can substitute other elements, such as arsenic, for phosphorus. This effort would involve a search for adenosine tri-arsenate instead of adenosine triphosphate and for DNA with arsenic instead of phosphorus.
A search for life that can extract essential nutrients—such as phosphorus, iron, and other metals—from rocks, such as pyrites and apatite.
A search for anomalous gene sequences in conserved genes, particularly DNA- and RNA-modifying genes. The anomalous sequence in the Nanoarchium 16S rRNA gene may indicate that there are others in other extremophiles, and these may have some significance in the origin of genes and the RNA world.
Study of the resistance of microorganisms that form biofilms on minerals to the harsh conditions of interplanetary transport.
A search for life that stores its heredity in chemicals other than nucleic acids.
The laboratory and field studies that are recommended above are not expensive by any metric; progress can be made with consistent NASA support at the $20 million annual level. That is a small fraction of the cost of a single launch of a space shuttle or of the contribution over the years to the space station, and it is a nearly negligible fraction of the cost of a human mission to Mars. Laboratory and field studies are a necessary component of such a mission. The results obtained from such studies not only will provide an answer to the question, Why go to Mars? but also will be needed to prevent a human landing on Mars from vitiating a key discovery that might have the greatest of effects for science and society.
The committee’s specific recommendations to NASA for space studies are as follows:
Programs that combine the exploration of potential metabolic cycles with the synthetic biology of unnatural nucleic acid analogues and their building blocks and that use the results to guide the design of instruments. This may be one of the principal ways in which ground-based research in astrobiology can inform NASA missions of exploration.
Astrobiology measurements that can potentially distinguish between life on Mars (and possibly other bodies) that arrived via material ejected from Earth (or vice versa) and life that emerged on another body independently of life on Earth. The scientific and societal effects of discovering a second genesis of life, as opposed to discovering another branch on the same tree of life, cannot be overstated.
Inclusion in missions planned for Mars of instruments that detect lighter atoms, simple organic functional groups, and organic carbon to help distinguish between “replicator-first” and “metabolism-first” theories of the origin of life by identifying organic mixtures that differ sharply in composition from the nearby random collections identified in meteorites. Similar considerations should guide inclusion of small-organic-molecule detectors that could function on the surfaces of Europa, Enceladus, and Titan.
Consideration, in view of the discovery of evidence of liquid water-ammonia eutectics on Titan and active water geysers on Saturn’s moon Enceladus, of whether the planned missions to the solar system should be reordered to permit returning to Titan or Enceladus earlier than is now scheduled. The discovery of evidence of liquid water-ammonia eutectics on Titan provides a context for the potential for polar fluids outside what is normally regarded as the “habitable zone.” The stay of the Cassini-Huygens mission on the surface of Titan was unfortunately brief; but this moon of Saturn is the locale that is arguably likely to support exotic life.
Finally, the committee calls attention to the importance of using remote sensing to detect and characterize extrasolar planets that could support alternative carbon-based life. In addition to looking for evidence of water in
such places, it will be valuable to look for evidence of plate tectonics, because that would indicate hydrothermal activity and the abiotic production of chemical energy and carbon sources that can support life. Studies of atmospheres of extrasolar planets will benefit from access to models of atmospheric conditions on planets that never have evolved oxygenic photosynthesis and that remain anaerobic. Earth did not accumulate oxygen during the first roughly 3 billion years, and it did not form an ozone layer until about 1.5 billion years ago. There is considerable emphasis on looking for contemporary Earth atmospheres that have oxygen and an ozone layer, but there should also be models of atmospheres with different anaerobic microbial ecosystems, atmospheres that might parallel the different stages in the evolution of Earth’s atmospheres over 4 billion years, and atmospheric conditions that could indicate the presence of a tectonically active planet.