Goal 1: Understand the nature and distribution of habitable environments in the Universe
Determine the potential for habitable planets beyond the Solar System, and characterize those that are observable.
A planet or planetary satellite is habitable if it can sustain life that originates there or if it sustains life that is carried to the object. The Astrobiology program seeks to expand our understanding of the most fundamental environmental requirements for habitability. However, in the near term, we must proceed with our current concepts regarding the requirements for habitability. That is, habitable environments must provide extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism. Habitability is not necessarily associated with a single specific environment; it can embrace a suite of environments that communicate through exchange of materials. The processes by which crucial biologically useful chemicals are carried to a planet and change its level of habitability can be explored through the fields of prebiotic chemistry and chemical evolution. A major long-range goal for astrobiology is to recognize habitability beyond the Solar System, independent of the presence of life, or to recognize habitability by detecting the presence of life (see Goal 7: Biosignatures).
Background
Research in astrobiology supports NASA's Origins theme in its attempt to search for habitable or inhabited environments beyond the Solar System. Humans have pondered for millenia whether other inhabitable worlds exist. Now, for the first time, they have an opportunity to look and see. Of course it is not possible to examine the ~1010 Earth-like planets that simple statistical models predict to exist in our galaxy, much less the ~1021 such planets expected to be in the universe. Still, it should be possible to determine whether terrestrial planets are indeed as common as predicted above, whether a substantial fraction of them show signs of habitability, and whether an appreciable fraction of these show biosignatures.
A key difference between the search for life in the Solar System and the search in external planetary systems is that, within the Solar System, interplanetary transfer of viable microbes seems a plausible process, and therefore the discovery of life elsewhere in the Solar System seems plausible. While this is indeed of extraordinary interest, it may not cast light on whether it is easy or difficult for life to begin. On the other hand, the fact that dispersion times between stars are ~ 105 to 106 times longer than for dispersion within the Solar System makes independent origination of life-forms outside the Solar System more probable.
The research objectives under this goal address three key questions. First, do terrestrial planets and large satellites tend to form in a state where they are likely to become habitable, or do habitable environments emerge only after a sequence of less probable events? Second, how frequently do habitable environments arise on solid planets, including large satellites? Third, what are the specific signs of habitability and habitation, and how do such signs change with the circumstances of the planet (e.g., mass, distance from its star, history and relative abundance of volatile compounds)? To address these questions effectively, we must investigate how habitable planetary systems form and evolve (Goals 1 and 2), and we must understand the ultimate environmental limits of life (Goal 5).
Much of this effort focuses upon the presence or absence of liquid water in bulk form. Water is made from the two most abundant chemically reactive elements in the universe, and it is the necessary ingredient for Earth's type of life. Liquid water has played an intimate, if not fully understood, role in the origin and development of life on Earth. Water contributes to the dynamic properties of an Earth-sized planet, permitting convection within the planetary crust that might be essential to supporting Earth-like life by creating local chemical disequilibria that provide energy for life. Water maintains a strong polar-nonpolar dichotomy with certain organic substances. This dichotomy has allowed life on Earth to form independent stable cellular structures. Thus the primary focus of Goal 1 is concerned with planets having a liquid water boundary layer, although the focus may expand to include other planets or satellites as astrobiology matures as a discipline.
There is also a focus—though not exclusive—on molecular oxygen and ozone as biosignatures (see also Goal 7), and therefore on dealing with the interface between the understanding of the geological and biological aspects of oxygen, and the details of the spectral features that can be observed and interpreted remotely. Oxygen is a very common element that has provided Earth with its most distinctive biosignature. The chemical state of an Earth-like planet, as well as the geological activity that delivers reduced species to the surface environment, will cause virtually all of the molecular oxygen to be consumed unless it is produced rapidly (e.g., by oxygen-producing photosynthesis). Also, the relatively modest ultraviolet fluxes of many stars prevent rapid production of oxygen from photo-dissociation of water. These factors will help to prevent the possibility of false positive detections of oxygen biosignatures.
The challenge of remotely detecting life on a planet that has not developed a biogenic source of oxygen is fraught with unknowns. What chemical species and spectral signatures should be sought? What metabolic processes might be operating? How does one guard against a false positive detection? Research that is guided both by our knowledge of Earth's early biosphere (i.e., before the rise of an oxygenated atmosphere) and by studies of alternative biological systems can help address these questions and provide guidance to astronomers seeking evidence of life elsewhere (Goal 7).
Objective 1.1
Models of formation and evolution of habitable
planets
Investigate how solid planets form, acquire liquid water and other volatile species and organic compounds, and how processes in planetary systems and galaxies affect their environments and habitability. Use theoretical and observational studies of the formation and evolution of planetary systems and their habitable zones to predict where water-dependent life is likely to be found in such systems.
Example investigations
- Study the relationship between stellar metallicity and planet formation. Determine if there is a galactic habitable zone.
- Model the origin of planetary systems, especially water delivery to and loss from terrestrial-like planets of various size and mass. Determine how water loss affects climate, surface and interior processes, and how these changes affect habitability.
- Develop comprehensive models of the environments of terrestrial-like planets to investigate the evolution of habitability.
Objective 1.2
Indirect and direct astronomical
observations of extrasolar habitable planets
Conduct astronomical, theoretical, and laboratory spectroscopic investigations to support planning for and interpretation of data from missions to detect and characterize extrasolar planets.
Example investigations
- Investigate novel methods for detecting and
characterizing extrasolar planets, particularly those that might lead
to an improved understanding of the frequency of habitable Earth-like
planets.
- Use atmospheric models to understand the range
of planetary conditions that can be determined from low resolution,
full-disk spectra at visible, near-IR, and thermal wavelengths. Use
data on Venus, Earth, and Mars to validate these models.
- Model a variety of biosignatures, including the ozone 9.7 micron band and oxygen A band signatures and their variations over Earth's geological and biological history (also relevant to Objective 7.2).