The Astrophysical Context of Life (2005)

The National Aeronautics and Space Administration (NASA) Astrobiology Roadmap summarizes astrobiology in the following way:¹

Astrobiology thus addresses three fundamental questions:

• How does life begin and evolve?

• Does life exist elsewhere in the universe?

• What is the future of life on Earth and beyond?

Astrophysics provides the fundamental underpinnings for life: space and time. Astronomy has taught us that our universe is mostly space—lots of it. This space is filled with a vast number of galaxies, each with billions of stars and, accordingly, possible sites for the origin and evolution of life. Merely making a census of the possible sites for life in the vast space of our galaxy is a challenging task, rife with important unanswered questions: What sorts of environments are needed to seed life? What sorts

are needed to promote its success and its increasing complexity? Astronomy has also shown that our universe has time—lots of it here, too. Life has had a very large, but not infinite, time in which to become established, to grow, and to develop. The time kept by the clocks of orbiting planets marches smoothly onward, but the time marked by living organisms is roiled by turmoil and occasionally punctuated by catastrophic astronomical events. How does the enormous span of time, punctuated by drastic events, affect the origin and evolution of life?

Scientists also know from astrophysical study that the galactic environment has provided crucial ingredients for life. The big bang provided hydrogen for the eventual formation of water, in addition to helium and a smattering of light elements, such as lithium, that are of little consequence for life. All the elements needed for life as we know it—oxygen to complete the water molecule, indispensable carbon, phosphorus for nucleic acids and key metabolites, metal ions and transition elements to serve as catalysts in the chemistry of life and more—came from generations of stars that formed from the dilute interstellar gas. These stars evolved and forged heavier elements from the primordial hydrogen and helium and then expelled these critical elements back into space, sometimes with a gentle push and other times with a catastrophic explosion, to form the next generation of stars and planets, which were becoming ever more hospitable to the origin and evolution of life. In addition to playing a crucial role in the chemistry of life, the elements forged in stars provided natural radioactivity, which is, in part, responsible for the tectonic activity that shapes the Earth and which is one unavoidable source of mutation, the driving force of genetic evolution.

The galaxy that hosts all these processes has its own structure and composition, which will affect the conditions for the development and flourishing of life on a planet. It has denser regions near the center that tend to have more supernova explosions and greater concentrations of life-giving heavy elements. In the vicinity of the Sun, the Galaxy winds spiral arms that are the site of ongoing star formation. Far from the galactic center the abundance of heavy elements may be too dilute to support the growth of planets. The explosions of stars and other processes push around the interstellar gas, creating pockets of dense gas where new stars can form, along with large volumes of more dilute gas. The shock waves from supernova explosions create the bath of cosmic ray particles that suffuses the Galaxy.

Life may have formed in a warm tidal pool or in the seas of Hadean Earth, when massive bolide impacts were the rule. Astronomy has taught us, however, that complex molecules had already formed in the interstellar medium. Scientists know that complex chemistry transpires in interstellar space: some through gas-phase chemistry, some through catalysis on the surface of grains of various composition, some in quiescent environments, and some in environments ablaze with the intense ultraviolet light of clusters of massive stars. The limits of that complex interstellar chemistry are not yet known. Nor do we know the relevance of these interstellar processes to the chemical environment on the surface of a planet or to the origin of life.

Astronomy has yielded a rapidly expanding knowledge of bodies that are possible hosts for life—not only the planets and moons of our solar system but also, possibly, further afield, as extrasolar planets are discovered orbiting other stars. How do all these planets come to be as they are? What is their geology, what are their tectonic regimes? The full range of environments that might harbor at least microbial life is yet to be explored.

The astronomical context of life includes a bath of photons of electromagnetic radiation and energetic particles that affect life. Optical photons are an important source of energy. The radiation from Earth’s central star provides the bulk of the free energy necessary for the maintenance of its biosphere, through the intricate mechanisms of photosynthesis. Ultraviolet (UV) and visible light are likely to have played a role on prebiotic Earth through photochemical reactions. The total luminosity from host stars defines the classic habitable zone, the region around a star where a planet will be at the right surface temperature

to support liquid water. The light from host stars may be predominantly steady, but it is not always so, and variability in the form of flares is the norm for lower mass stars, which are in a constant state of flaring. The Sun was highly variable in the past and is sporadically so even now. This variability may play a role in how we think about the habitable zone, especially in the case of lower mass stars, where the energy of frequent flares may exceed the luminosity of the stellar photosphere. UV radiation affects biomolecular structure by, for instance, forming dimers that interfere with the ability of DNA to replicate. Ionizing radiation—x rays and gamma rays—produces free radicals that interfere with the biochemical processes in cells. The energetic particles generated by the Sun and those arriving as cosmic rays induce additional ionizations that interfere in these ways in exposed cells.

The particle and electromagnetic radiation environments may also set bounds on where in the Galaxy life is feasible. As outlined above, the Galaxy is not an intrinsically quiescent environment; rather, it presents a fundamentally disturbed ecology for the stars and planets it hosts. Novas and supernovas, winds and radiation from massive stars, and even occasional catastrophic gamma-ray bursts are all part of the natural astronomical environment in the Galaxy. Along with explosive events come cosmic rays that alter atmospheric chemistry, may affect cloud cover, and provide an additional source of mutation. The flux of cosmic rays inevitably fluctuates significantly since the sources are stochastic, and screening by the heliopause will be modulated as the Sun moves through a range of interstellar gas and attendant ram pressures. The same will be true of any star hosting planets and moons that might cradle life.

Scientists look to astronomy to find other hosts for life. The search for extrasolar planets has been a resounding success, but the search for terrestrial planets is in its earliest planning phases. Developing missions like NASA’s Terrestrial Planet Finder and the European Space Agency’s Darwin will open new vistas in this quest. There is a great surge of interest in defining and understanding biomarkers that might reveal nascent life on another planet.

One of the principal benefits of bringing an astronomical perspective to the study of astrobiology is the effect on the conceptual framework in which astrobiological research is pursued. Of course we know Earth best, and it is the only site for life that we know. An astronomical perspective encourages us to keep the bigger picture in mind. Common questions for all astrobiologists should be these: How would this apply on another planet? How would this be different? How would it be the same? The differences might be vast, or they might be tiny, but they will surely exist, and by keeping that in mind we will more aggressively push the frontiers of our knowledge and thinking.

Another conceptual area is characterized by the question, What level of disturbance is good for the formation and evolution of life? Too much disturbance is surely inimical to the creation of life, its survival, and its evolution. Equally true, it seems, is that life would evolve more slowly to greater complexity in a relatively quiescent environment driven only by unrepaired replication errors. On the macroscopic scale, there is evidence that life would evolve more rapidly to greater complexity in a varied environment. So, a key conceptual issue, driven at least in part by the recognition that the Galaxy does present an unavoidably disturbed ecology, is the level of disturbance that is good for evolution.

In this study, the committee attempts to address some of these issues for the benefit of the astrobiological community and for the funding agencies—NASA, the National Science Foundation (NSF), the Department of Energy (DOE), and the National Institutes of Health (NIH)—that are relevant to this enterprise. The purpose is to foster the interaction of other disciplines with astronomy and vice versa, and to aid planning, research, and funding. The committee attempts to identify what gaps there might yet be in the study of the astrophysical context of life after the funding of the latest round of NASA Astrobiology Institute (NAI) teams. It comments on relevant space missions where appropriate and

attempts to identify topics, especially in areas that are not now receiving much attention and that are therefore ripe for interdisciplinary work, with astronomy as a key component.

Life on Earth exists in an astronomical environment. Because astrophysical research and perspectives play an important role in astrobiological research, the Committee on the Origins and Evolution of Life was charged with investigating how to augment and integrate the activity of astronomy and astrophysics in the intellectual enterprise of astrobiology, in NASA’s astrobiology program, and in relevant programs in other federal agencies.

The specific tasks of this study were as follows:

1. Outline current astronomical research relevant to astrobiology.

2. Define important areas that are relatively understudied and hence in need of more attention and support.

3. Address the means to integrate astrophysical research into the astrobiology enterprise.

4. Identify areas where there can be especially fruitful collaboration among astrophysicists, biologists, chemists, biochemists, planetary geologists, and planetary scientists that will serve the goals of astrobiological research.

5. Identify areas of astronomy that are likely to remain remote from the astrobiological enterprise.

6. Suggest areas where ongoing research sponsored by NSF, DOE, and NIH can augment NASA support of astrobiological research and education in a manner that specifically complements the astronomical interconnection with other disciplines.

7. Where applicable, point out relevance to NASA missions.

To further these goals, the committee summarizes current astrophysical research within the overall astrobiological enterprise (Chapters 2 and 3) and identifies areas that it thinks are understudied and deserving of greater effort (Chapter 4).