A workshop to assess the science and technology of life detection techniques was organized by the Committee on the Origins and Evolution of Life (COEL) of the Board on Life Sciences (BLS) and the Space Studies Board (SSB). Topics discussed in the workshop included the search for extraterrestrial life in situ and in the laboratory, extant life and the signature of extinct life, and determination of the point of origin (terrestrial or not) of detected organisms. Areas not covered or covered only to the extent of their connection to the main topics included mechanisms of terrestrial contamination of other planetary bodies by spacecraft (although techniques to detect such contamination in spacecraft were covered), sterilization of spacecraft components, and quarantine of returned samples. These topics, especially the last, have been considered extensively in recent National Research Council (NRC) reports.
The workshop was designed around a series of four general questions, to be addressed in the papers presented by the participants:
How does one determine if living terrestrial organisms are on a spacecraft before launch?
How does one determine if there are living organisms in a returned sample?
How does one determine if living organisms have been present at some earlier epoch and have left fossil remnants behind in a returned sample?
How does one determine whether there are living organisms or fossils in samples examined robotically on another solar system body?
The nature of questions 2 through 4, as formulated, is such that a single, declarative answer to each is not possible given the great uncertainties that remain in our understanding of the possible range of chemistry and morphology that could constitute life. Question 1 can be answered more definitively because of our direct study of terrestrial organisms, but there remains intense debate over the level to which spacecraft sterilization should be achieved for missions to particular solar system bodies. For these reasons, the questions served primarily to frame the scope of the discussions that took place at the workshop and of the contributed papers.
The workshop opened with an introduction to the history and scope of the search for life to date. The next session enumerated current understanding of solar system targets for sample return (including meteorites and
interplanetary dust particles for which samples are available at present). The final two sessions dealt with techniques for detecting viable (including spore-forming) organisms and the signs of past life, respectively. Appendix B of this report gives the workshop agenda. Appendix C lists (and the enclosed CD-ROM contains) the set of papers written by the invited speakers at the workshop. The report itself presents introductory and concluding material written by COEL to relate the papers to the questions to be addressed. To facilitate discussion of the papers and workshop sessions, the introductory material is organized in parallel with the workshop sessions rather than the four questions listed above. The conclusions and recommendations (Chapter 5), however, are grouped in parallel with the questions themselves. The committee emphasizes that this is a workshop report, rather than a detailed strategy study, and so drawing very specific conclusions and recommendations is not appropriate.
CONCLUSIONS AND RECOMMENDATIONS
Detecting Organisms on a Spacecraft Prior to Launch: Preventing Forward Contamination
The most strikingly definitive result coming from the workshop concerns the dramatic improvement in laboratory techniques designed to detect terrestrial organisms, with principal application to spacecraft sterilization and hence forward planetary protection. As new techniques become available, they have to be incorporated into planetary protection protocols because current NASA protocols (based on culturing techniques) could miss up to 99 percent of microorganisms. Also, some techniques may not be suited to distinguishing between viable and nonviable organisms, a key issue in considering forward and back contamination.
Recommendations regarding specific sterilization techniques and levels of sterilization in order to avoid contamination of other planetary bodies were beyond the original purview of the workshop and this report. The main issue regarding sterilization from the point of view of the workshop is the ability to sample, poststerilization, the remaining level of terrestrial microorganisms to ensure that it is below the value required for a particular mission. Because all terrestrial organisms rely on the same basic biochemistry—specifically and most importantly, the RNA and DNA nucleic acid bases—amplification techniques to detect very small remnant levels of contamination are well understood.
The committee recommends that studies of future missions to astrobiologically interesting targets include explicit consideration of the types of sterilization for spacecraft systems, subsystems, and components and that sterilization costs be included in a realistic fashion. The committee recommends that special near-term emphasis be given to the issues of sample selection, spacecraft sample handling, and sample characterization. The committee also encourages further work to refine sterilization approaches to minimize impacts on mission costs and success.
Detection of Living Organisms in a Returned Sample
The committee is strongly encouraged by the multidisciplinary efforts to define the possible range of processes indicative of living organisms. Given the extreme difficulty (or impossibility) of inductively describing all possible living processes based on terrestrial biochemistry, no single approach, or even combination of approaches, will guarantee success with a given sample. Multiple approaches, both chemical (including isotopic and molecular) and microscopic, are key to the successful detection of life in a sample. Because of the rapid improvements in the technology for a variety of techniques, coupled with the realization that return of a sample from Mars (the highest-priority target in life detection) remains a decade away, it would be premature to recommend a particular technique or set of techniques at present.
The committee concludes that a number of very sensitive and specific techniques are available for detecting living or once-living organisms in a returned sample; however, these techniques depend on the organisms ' being composed of essentially terrestrial biopolymers. While other techniques exist for detecting a potentially broader suite of nonterrestrial-like (but carbon-based) organisms, their results will not be as definitive. Hence, multiple approaches will be required to establish the presence of life in a definitive fashion, unless such life happens to be essentially terrestrial in nature. There is a pressing need to develop
methods for the detection in single cells of evidence of metabolic activity and of specific macromolecules, including an analysis of their chemical structure and isotopic signature.
The committee recommends that a focused study be done in the near future to address the detection of microorganisms with varying degrees of nonterrestrial biochemistry, and the possible threat that such organisms might pose to terrestrial organisms.
To the extent possible, reasonable efforts (defined through carefully deliberated scientific strategies) should be made to assess the potential for extant life on other planetary surfaces in situ, using robotic missions. Some of the approaches available for the detection of living organisms are available in miniaturized form and are potentially space qualifiable for an in situ life detection mission. The results will markedly increase confidence about the risk factors associated with a given sample that could be returned to Earth for further study and will provide scientific evidence to further justify the expense of a return mission. Since life (or past life) will concentrate in habitats that provide suitable nutrients and chemical or physical conditions, its distribution on any planetary body will be patchy and of varying local abundance. Appropriate site selection for sample return is critical and will determine the amount of sample required for testing and the need for possible sample concentration.
Multiple measurements with different techniques will be required to perform triage on a set of field samples at a given landing site, so as to select the most promising samples for in situ or returned life detection. This recommendation holds as well for selection of sites for in situ life detection (extant or extinct), as noted below.
Determination of the Past Presence of Living Organisms in a Sample
The committee concludes that the search strategy for evidence of extinct life must include the identification of suitable landing sites, the selection of the appropriate rock types, and multiple analytical techniques that, in the aggregate, are capable of distinguishing between abiogenic and biogenic signatures. The assessment of extinct biosignatures will likely require a sample-return mission to carry out the sophisticated set of measurements needed to make this determination.
The most vigorous debate at the workshop centered on interpretation of potential signatures of life in samples available today in the laboratory and, in particular, in the SNC meteorite ALH84001, which is generally accepted to have originally been a part of the martian crust. Important disagreements exist within the community over the interpretation of properties of this meteorite in terms of their biological significance, and at least some of the disagreements are the result of a lack of repeat analysis of a particular sample or phase by multiple groups. The committee recommends that any plans for analysis of returned extraterrestrial samples include a provision for repeat analyses of a subset of the same material, preferably by different teams. The committee encourages early development and testing of appropriate protocols using existing samples of high astrobiological interest (e.g., ALH84001).
The committee recommends that attention be given to understanding thoroughly the rates and nature of degradation of biosignatures in planetary environments. Theoretical and experimental studies should be supplemented with comparative analysis of putative samples of extraterrestrial biomarkers (e.g., ALH84001), with a specific eye to better understanding the issue of degradation of signatures of past life. Additionally, the identification and development of new and possibly universal biosignature approaches should be an active area of study.
In contrast with the detection of extraterrestrial life, analysis of extraterrestrial organic material is a mature field with a number of important results based on direct analysis of meteorites (including SNCs, which are likely martian) and cosmic dust. Remote sensing analysis of organic molecules in various bodies in the outer solar system and in molecular clouds has provided a foundation for understanding the distribution and abiotic evolution of carbon-bearing material. The absence of detectable organic molecules at the surface of Mars played an important role in the interpretation of the Viking life detection experiments. Detection of simple organic molecules (e.g., methane) has been accomplished for the atmospheres of very cool brown dwarf stars and will play a key role in the protocol for the eventual remote spectroscopic assessment of the habitability of extrasolar planets. Continued
increases in sensitivity and in the diagnostic value of techniques to detect organic molecules in extraterrestrial samples, particularly in situ, will be an important part of the overall effort to assess the existence of past or present life in the solar system. The committee concludes that it is crucial to continue the development of techniques to detect and analyze in situ organic chemical systems of either biotic or abiotic origin, with the goal of increasing the techniques' sensitivity and diagnostic capability.
Detection of Life, Extant or Extinct, Examined Robotically on Another Solar System Body
In situ life detection will require commitment to a small subset of available techniques because spacecraft resources will always be constrained, at least for the foreseeable future. Hence, a specific set of hypotheses regarding the samples to be analyzed must be made prior to launch; this intrinsically decreases the likelihood of successfully detecting life, because such hypotheses are invariably based on Earth-centric assumptions. On the other hand, in situ analysis is not subject to the concern of back contamination of Earth; hence sample handling is, in that respect, greatly simplified. Whether and to what extent attempts to detect life in situ will be made prior to return of a sample to Earth is an unresolved issue in NASA's Mars Exploration Program. Potential confusion of the results by terrestrial contaminants is a particular concern for in situ studies, because of the limited number and types of tests that can be done. Accurate knowledge of the prelaunch level of terrestrial contamination and a method of tagging terrestrial organisms would maximize the chances of an interpretable result.
Because of the continuing rapid improvements in technology, it is not appropriate at this time to recommend a specific set of techniques for in situ life detection, but in situ life detection will require commitment to a small set of potential techniques with significant lead time to ensure that they can be space qualified. The committee encourages continued efforts to develop innovative and miniaturizable techniques for in situ life detection. It must be stressed that selecting the combination of techniques for in situ life detection is dependent as well on the physical and chemical characteristics of the sampling site on a particular planetary body.
Appropriate site selection is crucial to maximizing the chances of finding evidence for extant or extinct life in samples either analyzed in situ or collected for return to Earth. While this point seems obvious, the committee notes that over the history of Mars exploration the engineering constraints associated with safe operations usually have conflicted with reaching the most scientifically interesting sites. For Mars, this means that landing site selection cannot be based primarily on issues of spacecraft safety. Furthermore, proper site selection will require a series of missions including orbital reconnaissance followed by exploration of selected sites by landed vehicles. An informed and continuing dialogue between scientists engaged in life detection and mission planners is essential if astrobiologically interesting samples from Mars are to be obtained.