Goal 7: Determine how to recognize signatures of life on other worlds and on early Earth
Identify biosignatures that can reveal and characterize past or present life in ancient samples from Earth, extraterrestrial samples measured in situ, samples returned to Earth, remotely measured planetary atmospheres and surfaces, and other cosmic phenomena.
Our concepts of life and biosignatures are inextricably linked. To be useful for exploration, biosignatures must be defined in terms that can be measured and quantified. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of biomass and wastes; phenomena that can be sustained through self-replication and evolution. Habitable planets create nonbiological features that mimic biosignatures and therefore must be understood in order to clarify our interpretations. We must create a library of biosignatures and their nonbiological mimics of life as we know it. 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.
Background
Astrobiological exploration is founded upon the premise that signatures of life (biosignatures) encountered in space will be recognizable. A biosignature is an object, substance and/or pattern whose origin specifically requires a biological agent. The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological processes producing it. An example of such a biosignature might be complex organic molecules and/or structures whose formation is virtually unachievable in the absence of life. A potential biosignature is a feature that is consistent with biological processes and that, when it is encountered, challenges the researcher to attribute it either to inanimate or to biological processes. Such detection might compel investigators to gather more data before reaching a conclusion as to the presence or absence of life.
Catalogs of biosignatures must be developed that reflect fundamental and universal characteristics of life, and thus are not restricted solely to those attributes that represent local solutions to the challenges of survival. For example, certain examples of our biosphere's specific molecular machinery, e.g., DNA and proteins, might not necessarily be mimicked by other examples of life elsewhere in the cosmos. On the other hand, basic principles of biological evolution might indeed be universal.
However, not all of the universal attributes of life will be expressed in ancient planetary materials or detectable remotely (e.g., by astronomical methods). For example, the processes of biological evolution are highly diagnostic for life, but evidence of biological evolution might not be readily detected as such in a sample returned from Mars. However, better-preserved evidence of life might include complex structures that are often retained in aquatic sediments or can be preserved in large quantities in the environment. Thus, for example, categories of biosignatures can include the following: cellular and extracellular morphologies, biogenic fabrics in rocks, bio-organic molecular structures, chirality, biogenic minerals, biogenic stable isotope patterns in minerals and organic compounds, atmospheric gases, and remotely detectable features on planetary surfaces (photosynthetic pigments, etc.). On Earth, biosignatures also include those key minerals, atmospheric gases and crustal reservoirs of carbon, sulfur and other elements that collectively have recorded the enduring global impact of the utilization of free energy and the production of biomass and wastes. Oxygen-producing photosynthesis has simultaneously created large reservoirs of atmospheric oxygen, marine sulfates and sedimentary ferric iron and sulfates (its oxidized products), as well as large sedimentary reservoirs of biogenic organic matter and sulfides (its corresponding reduced products). Again, such features must be sufficiently complex and/or abundant so that they retain a diagnostic expression of some of life's universal attributes. Also, their formation by nonbiological processes should be highly improbable.
As more complex biological features eventually evolved, as evidenced by plants and animals, the associated biosignatures became easier to distinguish from the abiotic world. Human technology continues this trend, with the added benefit that it might be detected remotely. Thus, although technology is probably much more rare than life in the universe, its associated biosignatures perhaps enjoy a much higher "signal-to-noise" ratio. Accordingly, current methods should be further developed and novel methods should be identified for detecting electromagnetic radiation or other diagnostic artifacts that indicate remote technological civilizations.
Objective 7.1
Biosignatures to be sought in Solar System
materials
Learn how to recognize and interpret biosignatures which,
if identified in samples from ancient rocks on Earth or from other planets,
can help to detect and/or characterize ancient and/or present-day life.
Example Investigations
- Determine additional organic biomarkers that
will help to chart the presence and development of photosynthetic
microbiota in Precambrian rocks.
- Determine the features of sedimentary laminated
textures that uniquely require biological processes.
- Identify examples of chemical, mineralogical and stable isotopic biosignatures that can indicate the presence of subsurface biota (e.g., microbes living in aquifers), and that can be preserved in ancient rocks.
Objective 7.2
Biosignatures to be sought in nearby planetary
systems
Learn how to measure biosignatures that can reveal the existence of past or present life through remote observations.
Example Investigations
- Determine the nature and fate of reduced gases
that are produced by specific microbial ecosystems in an anoxic ("pre-oxygenated")
biosphere.
- Carry out laboratory, observational and modeling studies to separate false from true biosignatures (e.g., atmospheric O2 in a range of planetary environments [see Objective 1.2]).