The Astrophysical Context of Life (2005)

One issue that has been faced historically by NASA’s Exobiology program, and is now being faced by various astrobiology programs, is where to draw the line between astrobiology and other areas of astronomy and astrophysics. This question has no obvious, unambiguous answer. The presence of life in the universe depends on many different factors, especially the existence of the biogenic elements (including, but not limited to, carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, and iron) and of planets where these elements can combine in a warm, sufficiently stable environment to form life. The existence of warm planets requires the coexistence of stars and star-forming nebulas. The existence of these objects requires the existence of galaxies, until ultimately the existence of everything depends on the occurrence of the big bang, some 14 billion years ago. Thus, if one wishes to take a broad view of astrobiology, such fields as star formation and cosmology should be included.

There is an obvious danger in trying to be this inclusive. NASA’s astrobiology program is supported by substantial, but finite, amounts of funding. If one defines astrobiology too broadly, then this funding will be spread over such a wide range of disciplines that its impact on the core issues surrounding the topic of life in the universe will be hopelessly diluted. In exobiology, this issue came to a head more than a decade ago, when the program started receiving large numbers of proposals from observational astrochemists who were studying the distribution of carbon in the universe. Carbon is a key element for biochemistry, it was argued, so it is important to understand where and in what chemical forms it is found. Indeed, the discovery of organic molecules in interstellar clouds by radio astronomers in the 1960s was of indisputable significance for astrobiology. It is certainly conceivable that some or all of the key compounds required for the origin of life were formed in low-temperature gas-phase reactions or by ultraviolet-driven chemistry occurring in the icy mantles of interstellar dust grains. It is also true that much of the carbon in interstellar space consists of poorly characterized, polycyclic aromatic hydrocarbons that are important absorbers of stellar radiation at certain wavelengths but that probably have little or nothing to do with the origin of life.

How does one define the boundary between astrobiology and astrochemistry in these examples? In the Exobiology program, it was decided that the proposals needed to make a clear connection between the carbon in interstellar space and its delivery to Earth in the form of cometary or meteoritic material. More generally, the parts of astronomy that are most relevant to astrobiology are those that might directly influence either the origin or later evolution of life. They include many different subjects and should be viewed not as being narrow but rather as being life-focused. By applying this principle in a reasonable manner, it should be possible to define the parts of astronomy that ought to be included in astrobiological research at the critical step when funding decisions are made. Intellectually, the sweep should be broader in order to encompass areas that, with work and insight, can be brought directly into the astrobiological enterprise.

Finding. Funding for astrobiology is limited, and the boundaries of the field are unclear; there is a risk that not all funds will go toward research topics that are justifiably “astrobiology.”

Recommendation. In funding decisions, NASA and other funding agencies should regard astronomical research as astrobiology if it is life-focused in plausible ways.

With these points of view in mind, the committee summarizes currently funded astronomical research at the NASA Astrobiology Institute (NAI) and in other NASA-funded astrobiology programs.

There is significant astronomical content in the research proposed or being done by the present NAI nodes; however, there is some potential redundancy since many subjects are treated or studied at multiple centers. The committee identified seven specific subject areas that are the focus of more than one NAI node:

• Planet formation. Planetary formation is being investigated at seven NAI nodes: NASA Goddard Space Flight Center (GSFC), Penn State Astrobiology Research Center (PSARC), Carnegie Institution of Washington (CIW), the University of Colorado at Boulder (CU-Boulder), the University of Washington (UW), the University of Arizona (UA), and NASA Ames Research Center (ARC). PSARC is investigating how stellar metallicity (elements with atomic weight greater than that of helium) affects planet formation and the possibility of planets around white dwarfs. CIW is modeling planetary formation and works on the detection of extrasolar planets. CU-Boulder is proposing to study the evolution of protoplanetary disks into planets. UA will study planetary formation through observations of circumstellar disks, while ARC is studying planetary formation in the context of planet habitability. GSFC will simulate interstellar clouds and protoplanetary chemistry.

• Biomarkers. The Virtual Planetary Laboratory (VPL) at NAI is exploring the possibility of detecting biomarkers (especially ozone) on planets around F, G, K, and M stars. The astronomical environment and the spectral characteristics and variability of the host star are important for this work. ARC will assess the prospects of survival of biospheres and the strategies to detect them. The Search for Extraterrestrial Intelligence Institute (SETI) looks for novel biosignatures—namely, signs of extraterrestrial technology.

• Planetary habitability. Astronomical influence on planetary habitability is one of the most widely studied topics at the NAI. The University of California at Los Angeles (UCLA) is studying the links between orbital dynamics, impact histories, and geological evolution and the effect of these links on planetary habitability. ARC is estimating how the delivery of volatiles, impacts, and orbital eccentricities may affect habitability. VPL is using planetary models and time-dependent stellar spectra to investigate surface-incident ultraviolet flux and planetary habitability for different parent star/planetary atmosphere combinations. SETI is using observational data and analyses to define the habitability of Europa and planets orbiting cool M stars. UW and PSARC also have a research component in this area. UA will investigate giant planet atmospheres.

• Planet detection. Extrasolar planetary detection has already been studied at both UCLA and CIW. They propose to continue this effort. UCLA will include planetary detection in nearby clusters of young stars.

• Bombardment. Life-related consequences of impacts are a focus of research by several nodes. UA proposes to connect giant impacts through circumstellar dust disk evolution. UCLA proposes a geological exploration of the Bellingshausen Sea, the only known site of an asteroid impact into a deep-ocean basin, to understand the processes and environmental effects of an oceanic event of this scale. UW investigates conditions for habitability and notes that periodic catastrophic events, including bolide impacts, may be necessary to create and maintain the high variability of habitable conditions that results in increased biodiversity and biocomplexity. Impacts were the topic of a workshop and are the subject of the Impact Focus Group.

• Water. The exogenous delivery of water to earthlike planets is being investigated by five NAI nodes: GSFC, CIW, the University of Hawaii (UH), ARC, and the University of California at Berkeley (UCB). CIW proposes research on the water in martian meteorites and its deuterium/hydrogen ratio. UH will observe the abundance and distribution of matter in the interstellar medium, circumstellar disks, and icy outer-solar-system bodies. GSFC will investigate the role of icy planetesimals in the delivery of water. ARC will consider delivery of water and the conditions to preserve it. UCB will explore the history of water on Mars.

• Mars. UCB will undertake studies that directly inform the selection of optimal sites for future Mars exploration and will design strategies for remote geomicrobiological investigation. ARC will try to define biosignatures for a martian biosphere. For example, the signature of methane has been detected by ground-based telescopes.¹

Of the nodes that propose work on astronomy, ARC, CIW, GSFC, SETI, and UA propose work that connects astronomy and some other discipline. UCLA, CU-Boulder, UH, and UW propose astronomical research (primarily planet formation) in which the astrophysics is substantially independent of the other components of the proposed work. The work being done by PSARC and VPL is beginning to make the connection with the astronomical environment an explicit part of the proposed work. It is not clear that UCB does so. Four nodes focus on nonastronomical aspects of astrobiology: Indiana University, the Marine Biological Laboratory at Woods Hole, Michigan State University, and the NASA Jet Propulsion Laboratory.

NASA has also supported astronomy research relevant to the astrobiology program under other programs:

• Exobiology

• Origins

• Cosmochemistry

• NASA Specialized Center of Research and Training (NSCORT)

• Terrestrial Planet Finder

The Exobiology program is a long-standing one that now serves as the major source of funding for single-principal-investigator (PI) astrobiology research associated with the broader astrobiological program, including grants to members of NAI teams. Within the Exobiology program, the committee identified 27 proposals funded since 1999 that deal with astronomical topics. Most of these focus on chemistry. Interstellar chemistry (organic and inorganic), atmospheric chemistry, and impact chemistry were represented. Delivery of material to planets and extrasolar planets was a minor focus (2 of 27 proposals).

Within the Origins program, the intent of which was to fund astronomical research, the committee identified 54 proposals funded in the last year with relevance to astrobiology. In addition, this program funded two meetings on topics relevant to astrobiology. The main funded topics included (1) meteorite chemistry/organics, (2) observation and detection of extrasolar planets, (3) dynamics and modeling of extrasolar planets, and (4) spectroscopy and modeling of planetary atmospheres. One study of impact and bombardment is being undertaken.

Origins was a program entirely separate from the NAI and the Exobiology programs in terms of management and funding. The goals of the 2003 Origins Roadmap overlapped considerably with those of the Astrobiology Roadmap. The issue of communicating work in the Origins program to the core astrobiology program was raised in the Life in the Universe report. With the new NASA vision for space exploration, the issue of which research comes within the rubric of the vision takes on new import and will probably have an impact on budgets. A draft of the present report noted that the old administrative themes (Origins, Structure and Evolution of the Universe, Solar System Exploration, Mars Exploration, and the Sun-Earth Connection) might no longer serve the current missions of NASA and suggested that the theme structure, and the role of astrobiology within it, be reconsidered to achieve greater clarity of mission, programs, and budget priorities. In June 2004, the theme structure was abolished. At this writing, it is not clear what structure will replace it within the new Science Mission Directorate or how the funding and administration of astrobiology programs will be handled. The committee urges NASA to ensure that research and training in astrobiology play an integral role in the new structure.

In the last round of selections by the Cosmochemistry program, in 2002, 60 proposals were selected. The majority of these were relevant to astrobiology. Among the topics proposed were meteorite mineralogy, lunar and martian rock studies, interplanetary dust, presolar grain inclusions, and issues of planetary differentiation. Few of the studies seem to be placed in a broader astronomical context.

The NSCORT program was reviewed in Life in the Universe. The program has been in existence for some time, but only two centers are now relevant to astrobiology, the University of California at San Diego (UCSD) and Rensselaer Polytechnic Institute (RPI). They host consortia where the collaborators are, typically, colocated and focus on graduate education. The UCSD effort in exobiology has as its research themes plausible chemistry under plausible prebiotic conditions, Earth-based synthesis and

inputs from space, the nature of the first genetic material, and RNA/DNA/protein evolution under laboratory conditions. The second of these topics is manifestly related to astronomy. The New York Center for Studies on the Origin of Life, at RPI, has in its purview the astrophysical topics of interstellar chemistry, chemistry and physics in the solar nebula, and planetary habitability, as well as topics in biology, chemistry, and Earth science. In the current structure, NSCORTs focus on graduate training, but in a rather restricted range of areas. Graduate training does occur within the context of many of the NAI teams, but most of the focus and associated budgets is aimed at public outreach and education, not graduate education per se. NASA should consider what mix of NAI and NSCORT funding will best address the critical issue of interdisciplinary graduate training.

The committee senses in this summary of currently funded research a tendency to support previously well-defined, accepted areas of research. There is much to be done, so funding of different groups to do related work from independent points of view has great merit. Since the focus of work is in only seven areas in the NAI, there is a hint that research that seeks to broaden the bounds of the astronomy/ astrobiology interface is not promoted in the current funding structure. It is difficult to assess whether this redundancy gave any cause for concern in the evaluation of the new and renewed NAI nodes. In the Exobiology program, where there should be more flexibility to promote innovative, if somewhat risky, ideas, the focus seems even tighter, on interstellar chemistry. The Origins science was substantially single-PI research and so might also show more range, but it tends to overlap strongly with the major themes of the NAI. To a certain extent this represents the “sweet spot” of current research, but may also indicate a “bandwagon” effect and a certain conservatism on the part of referees and funding agencies.

Finding: Review of current astronomically oriented research shows that work is concentrated in relatively few areas, more so in the Exobiology program than at the NAI.

Recommendation: The committee recommends that NASA continue to ensure that an appropriate diversity of topics is included within the astrophysics component of astrobiology and that its support be coordinated with funding through other relevant programs.

Even where astrophysical research is supported in the NAI, there is rather little current evidence for integrated activity. This may change with the new NAI teams, which have a more explicit astronomical imperative, but the goal of integration needs to be encouraged and monitored. There are research areas where interactivity is explicitly defined. ARC proposes to do laboratory and computational definition of molecules of biological significance followed by astronomical searches for them. GSFC proposes to analyze complex organics formed in grain-catalyzed reactions and radiation-processed ices and then compare the results with astronomical observations. UA proposes to determine signatures of prebiotic compounds and then search for them in space. Nevertheless, the current astronomy supported by the extant NAI teams consists substantially of observations of star formation and protostellar disks. While this is clearly astronomy, it is not clear that the work being done would pass the filter defined above in the context of the Exobiology program—namely, that astronomy that is most relevant to astrobiology is astronomy that directly influenced either the origin or later evolution and sustainability of life. Perhaps the question should be this: Would the astronomy nominally supported by current NAI nodes have passed muster as “astrobiology” in the Exobiology program? NASA should request that NAI annual reports include information on interdisciplinary work, specifically the integration of astronomical

research with other disciplines. NAI teams, in particular, should make an explicit effort to promote joint research between astronomers and researchers in other disciplines. Evidence for this would consist of astronomers writing papers with biologists or geologists or at least citing that literature (and vice versa) and dual degree programs.

Recommendation: The committee recommends that NASA develop metrics to evaluate the degree to which truly interdisciplinary work involving astronomy and astrophysics is being done in the current NAI nodes.

The Terrestrial Planet Finder (TPF)² mission and Darwin,³ the parallel program of the European Space Agency, will search for terrestrial planets and attempt to detect evidence of life. The successful planning and execution of the NASA TPF mission will require a great deal of interdisciplinary input from astrobiologists working with astrophysicist colleagues. As a specific example, stellar astrophysicists viewing their research objects through the lenses of astrobiology must now try to understand potential TPF target stars as the astrophysical environment and energy source for potential life-sustaining planets, not just as stars.

TPF will require the attention of the astronomical community in at least three ways: work on improving star lists; work on improving predicted signals from various types of planets in various evolutionary or temporary states; and work on developing techniques and concepts for planet detection and characterization. The astronomical context will be important to learn about the atmospheric lifetime or practical detectability of a biomarker in a given astrophysical environment—that is, the scenario under which TPF will operate. Biosignatures can be generated in part by radiation from the parent star, as happens when ozone is generated from molecular oxygen. Biomarkers are also detectable in either stellar reflected light in the visible or as a function of planetary atmospheric temperature structure in the mid-infrared. (This planetary temperature structure is also a function of chemical composition of the atmosphere, all of which—chemical composition and temperature structure—is driven by the spectral energy distribution and variability of the host star.) The astronomical perspective forces us to consider the nature and detectability of biosignatures for host stars that are unlike our own Sun, and it requires strong interdisciplinary collaboration between stellar astronomers, planetary atmospheric physicists, atmospheric chemists, and spectroscopists.

The specific point that large departures from equilibrium are likely to be driven by biological processes was made by Lovelock.⁴ It is based on the idea that enzymes are highly selective, whereas inorganic catalysts are not. Thus we may identify a biomarker long before we identify the process that gave rise to it. The Astrobiology Roadmap also notes as follows: “A strategy is needed for recognizing novel biosignatures. This strategy ultimately should accommodate a diversity of habitable conditions, biota and technologies in the universe that probably exceeds the diversity observed on Earth.” The opportunity for interdisciplinary work in these areas is great.

At least two teams in NAI (VPL and UA) are already working on TPF-related problems; however, the main resources associated with the mission(s) come from mission funds, not astrobiology resources.