Glossary of Terms Used in the LDKB
The prevalence expresses the likelihood that a feature is produced by a particular source, such as life or a nonbiological process.
‘Prevalence’ was defined previously in epidemiology, where it indicates the proportion of a population that has a specific characteristic (e.g., a disease). Nominally, ‘prevalence’ in astrobiology applies across all biological systems and all environments, therefore it includes all occurrences of a feature on Earth and beyond. The implication is that an argument about general prevalence is essentially independent of environmental context but rather about the intrinsic aspects of a feature. However, some features are expected to be prevalent in some environments but unlikely in others. Therefore, it should be specified whether the prevalence of a feature is considered in the context of all environments or specific ones. The evaluation of prevalence within a specific environmental context is an evaluation of its ‘congruence.’ Posed as a question – ‘is a particular feature congruent with originating in the environment where it is observed?’ (Also, see definition of congruence.)
Features that accompanied the origins and earliest evolution of life (e.g., amino acids, lipids) might be more prevalent than features that arose later in evolution (e.g., lignin, chitin, collagen) and whose attributes were affected by later environmental changes. Since environments of habitable planets typically diverge over time, features arising earlier might be more prevalent than features arising later.
Signal strength is the prominence of a feature (e.g., its abundance, rate, structure, patterns, intensity, etc.) that provides evidence for the past or extant presence of life or a nonbiological process.
The prominence of a feature is determined by the balance between its rates of production and its rates of modification and/or destruction. For a biosignature, this balance is determined by its biological production, its survivability, and the patchiness of its occurrence in a target environment. The ‘target environment’ includes the environment prevailing during either feature production or its modification, degradation, and/or destruction. These environments may be different (separated by time and/or space).
An argument is an assertion that supports or contradicts a statement or theory that is put forward as a premise.
In the Life Detection Knowledge Base (LDKB), an argument refers to the diagnostic value of a given statement in supporting or contradicting biological or abiotic origins of a given feature. Arguments are drawn from scientific literature and are supplied by experts in the field. They are based on supporting evidence from the literature, which serve as the premises of the arguments. Since knowledge about life detection is highly diverse, this presentation helps users to absorb and evaluate knowledge in the form that is most useful for them in both planning and analyzing life detection missions and observations. In this respect, it is superior to other, commonly used presentations of knowledge, such as abstracts, summaries, keywords or key phrases.
An argument type addresses a key attribute of a feature that is essential for enabling its interpretation and that serves to classify arguments with a greater level of precision.
Examples of argument types within the LDKB include the following: Congruence, Patchiness, Production, and Survivability. See definitions of these terms in the Glossary.
A biosignature consists of substances, structures, patterns, processes, or ensembles of these features, that provide information about the current or former presence of life.
Observing different biosignatures provides evidence for the presence of life at a different level of confidence. Some biosignatures might be strongly indicative of life, whereas others might offer only weak support. Arguments and counterarguments regarding this matter provide the means to evaluate our confidence in a given biosignature. Since many individual features can be mimicked by abiotic processes, ensembles of substances, structures, patterns, or processes, it is typically required to consider a number of features to increase confidence in our interpretation.
Breadth of Utility
Breadth of utility expresses the extent to which the ability to characterize a feature also enhances the ability to achieve the broader scientific goals and objectives for investigating a target environment.
The attributes of a feature (its composition, structure, presence, or absence, etc.) may provide additional data that enhance the overall science return of a mission. For example, measurements of a biosignature’s chemical composition could also reveal geochemical details about past or present environmental processes and conditions. Observations of structural biosignatures could also help to characterize nonbiological geological processes and deposits.
Congruence represents the degree to which the attributes of a particular feature or its inferred source are consistent with its occurrence in a specific environment.
Congruence is prevalence evaluated within a specific environmental context. Making an argument about congruence is essentially stating to what extent a given feature is likely to be manifested in a specific setting. Congruence emphasizes the importance of the environmental context of a particular feature. A congruent feature might plausibly be produced in the environment in which it occurs, or it could have been produced under environmental conditions at the site that occurred sometime in the past. If neither of these options seem possible, the feature might have been produced elsewhere and then transported to the site. Examples of such transport processes on Mars include wind, meteorite impacts, and terrestrial contamination from spacecraft. When determining which features should be sought in a target environment, considerations of congruence can help to predict which features are more likely to occur there. Subsequently, after features and their environment have been characterized, considerations of congruence could contribute to the interpretation of the features.
Contamination is defined here as the addition of an Earth-sourced substance or object that potentially can mimic, alter or obscure a feature of interest, thereby potentially compromising the interpretation of that feature.
Some types of features (e.g., organic matter, gases, etc.) are more susceptible than others (e.g., rock fabrics, mineral assemblages, etc.) to being false positives due to Earth contamination. Contamination should be considered in connection with detection technologies, as they might be its source.
A criterion is a principle or standard by which something may be judged or decided. In the specific context of life detection, criteria are a set of standards against which to evaluate the utility of specific measurements in achieving specific life detection objectives.
Standardization is essential in supporting an apples-to-apples comparison among diverse biosignatures. A well-developed literature on decision making and evaluative methodologies indicates that suitable criteria should be: (i) universal (equivalently applicable) across all features to which they would be applied, (ii) complete, as a set, in capturing all aspects of a feature that are relevant in assessing its value in the context of a life detection strategy, and (iii) disjoint (non-overlapping) to the greatest extent possible. Prevalence and Feature Strength are examples of CLD criteria used to evaluate biosignatures. The criteria used in LDKB have been developed via workshops that engaged the broad life detection community.
The environment is the physical, chemical and historical context surrounding a particular locality and the features that exist there.
Environmental attributes of a locality include its temperature, pressure, radiation, the reservoirs and fluxes of solids and other chemicals, and the local history of these attributes. Environments can be defined at a different level of resolution, from highly local to global (e.g., Mars). Local definitions are advantageous in describing homogeneous conditions in a given environment. However, the number of arguments related to local environments might be small, which makes their evaluation difficult. Further, a large number of environments might make it difficult to search for them in LDKB. Although classifications based on global environments do not suffer from the same disadvantages, they might be associated with highly heterogeneous conditions and involve many arguments of different nature, making their evaluation difficult. In LDKB, an intermediate level of resolution is used. The environments at this level were selected via community input. Extensive studies in other fields demonstrated that intermediate levels of classification are usually most informative.
A piece of evidence is information indicating whether an argument is true or valid.
Evidence can consist of observations, measurements or models that address a feature’s prevalence (e.g., its congruence), and signal strength (e.g., its production, patchiness, and survivability).
Patchiness expresses the degree to which the distribution of a feature is heterogeneous in space and time within its environmental context.
The abundance of a feature might differ substantially across the landscape where it has developed. Features could occur intermittently across spatial scales ranging from cm to km. Since local environments have changed over time, the abundances of substances, structures and activity might be patchy with respect to the deposition, modification and preservation of features in a stratigraphic sedimentary rock sequence. A patchy distribution would clearly negatively impact search in a target environment.
Production as applied here indicates the rate at which a feature is created in a target environment.
The production of a feature is determined by the process(es) that create it and the modulation of those processes by their environments. Temperature, pressure, chemical conditions, and the availability of useful energy can determine the production of a feature. High production rate is usually a desirable characteristic of a given biosignature. It is essential that the production rate exceeds or at least balances the destruction rate. Otherwise, observing a given biosignature might be very challenging or even impossible.
The survivability of a feature indicates the extent to which it persists against processes that can modify or destroy the feature.
Assessments of survivability must take into account both the intrinsic properties of a feature as well as the array of environmental processes that can modify or destroy the feature. Both physical and chemical processes can affect survivability. The survivability of various details of a feature can vary substantially. Specific details that are more diagnostic of a particular source that created a feature also tend to have lower survivability. However, although partial remnants of a particular feature might be less diagnostic of its origin, they still merit attention because, for example, their detection in the target environment hints that better-preserved features might exist nearby. Considerations of both intrinsic details of features and the destructive processes in an environment can influence the selection of exploration targets and measurement capabilities to investigate them.
Terms Specific to Structures
In geology, a lamination is a small-scale sequence of fine layers (laminae; singular: lamina) that occurs in sedimentary deposits. Laminae can have both biological and abiotic origin, and thus they are an important feature for life detection. Laminae derived from inorganic processes in sedimentary rocks are often seen as planar structures, formed through gravitational deposition, whereas lamina of biologic origin are often undulatory and bordering a substrate rock.
Filamentous material that serves as scaffolding and nucleation site for subsequent accretion.
Wavy layers, which may be of uniform or variable width along axes and between bands.
Small, submillimeter grains, generally of uniform size. (Calcite is the compositional implication; description is limited to carbonates.)
Laminated organo-sedimentary structure formed underwater (usually at or near the water's surface) by the influence of microbial activity.
A secondary mineral feature that forms in caves, i.e., after the initial cave has formed.
A mineral texture or habit in which the mineral has a globular external form resembling a cluster of grapes.
A distinctly bedded, or broadly lenticular body of rock composed mainly of the remains of sedentary organisms (such as shell beds, crinoid beds, or coral beds) — compare bioherm.
A mound of material laid down by sedentary marine organisms, especially a coral reef (note: also implies composition).
The study of how organisms decay and become fossilized or preserved in the paleontological record (adjective; taphonomic).
The disturbance of sedimentary deposits by living organisms.
The height of a structure at the time of growth (e.g., stromatolites in outcrop may have heights of several meters, yet at the time of growth, the height of the structure above the surrounding sediment may only be a few millimeters. Synoptic profile refers to the macromorphologies of a stromatolite viewed in cross-section (i.e., the shape of a planar cut oriented parallel to the growth direction of the structure).
Materials and minerals that do not have an orderly and repeating arrangement of their constituent atoms, while crystalline minerals do.