Enantiomers are pairs of the same molecule that are mirror images of each other. By convention, one enantiomer is called left-handed (L), and the other enantiomer is called right-handed (R). Life on Earth displays a preference toward specific enantiomers to build functional polymers, which can result in high levels of enantiomeric excesses(ee) in a sample (i.e., a higher relative abundance of one enantiomer vs another). Therefore, relative enantiomer abundances can serve as ‘fingerprints’ of biological activity.
Abiotic organic synthesis tends to form equal mixtures of enantiomers in the absence of an asymmetric chiral influence (i.e., a physical or chemical process that favors a specific enantiomer). However, abiotic organic mixtures, such as those found in carbonaceous chondrites, sometimes display enantiomeric imbalances, which point to non-biotic mechanisms of enantioselectivity. While the sources of these abiotically produced imbalances are largely unknown, they must be co [read more]nsidered when interpreting enantiomeric ratios in natural samples, because they could trigger a false positive result.
Under certain conditions, enantiomeric imbalances can disappear over time due to spontaneous racemization, a chemical process that converts one enantiomer to the other until an equal population of both are formed. The rates of spontaneous racemization depend on physical and chemical factors, such as temperature, environment and on the intrinsic reactivities of the chiral molecules. Occasionally, spontaneous racemization of certain chiral molecules can occur at rates similar to the slowest growth rates of some terrestrial organisms. Such spontaneous processes must be considered when interpreting enantiomeric ratios in natural samples, because they could trigger a false negative result.
In the natural sciences, enantiomeric ratios can be expressed as enantiomeric excess (ee), defined as
ee=A-B
where A and B are the mole fractions of the major and minor enantiomers, respectively, so that A + B = 1. Enantiomeric excess can also be presented as ee% = (A-B)100 or 100(A’-B’)/(A’+B’) where A’, B’ are in moles.
Patterns of isotopic abundances within and between molecules can serve as ‘fingerprints’ of biological activity – hence isotopic biosignatures. Several biologically important elements have two or more stable isotopes. Differences in the nuclear masses of isotopes have minimal effects upon the electron orbitals of atoms and molecules, but they do alter the vibrational energy states of polyatomic molecules. Chemical bonds having heavier isotopes have lower vibrational energy states than those with lighter isotopes; accordingly, bonds between lighter isotopes are somewhat weaker than bonds having a heavier isotope. Molecules having bonds with higher force constants exhibit greater differences in zero-point energies between the molecules’ lighter and heavier isotopes. Therefore, compounds having bonds with higher force constants tend to acquire greater abundances of heavier isotopes.
Fractionation at equilibrium. Coexisting molec
[read more]ules can achieve mutual chemical and isotopic equilibrium when the forward and backward reactions between them occur at equal rates. Among molecules at chemical equilibrium, molecules having bonds with greater force constants will exhibit greater abundances of heavier isotopes. Differences in isotopic abundances between equilibrated molecules generally decrease at higher temperatures. Where isotopic differences between equilibrated molecules can be achieved due purely to chemical and physical phenomena, they do not necessarily require biological processes and therefore are not definitive biosignatures. But in some circumstances, enzymatic processes can achieve equilibrium between molecules under conditions where equilibration would otherwise be extremely slow in the absence of life. In those cases, evidence of isotopic equilibrium could constitute a definitive biosignature. Examples of these are given in subsequent sections.
Kinetic isotope fractionation. Enzymes can alter reaction pathways and rates of reactions in ways that create biosignatures. The capacity of an enzyme to modulate the rate of a biochemical reaction arises from the molecular structure of its ‘reaction intermediate’ and the environment within the enzyme’s reaction center. These attributes create the isotopic fractionation expressed in the overall reaction. Differences in the reaction mechanisms employed by biological versus nonbiological processes can create differences in the isotopic compositions of their respective products. Such differences can form the basis for distinguishing between isotopic patterns of biosignatures versus patterns that arise from nonbiological processes.
A kinetically controlled reaction can arise under conditions when its rate is at least somewhat inhibited and the rate from reactant to product is greater than the rate of the reverse reaction. In this case, a reactant equilibrates with a higher energy (less stable) ‘reaction intermediate’ species that ultimately leads to the product. Because the reacting bonds in the ‘reaction intermediate’ are typically weaker (they have lower force constants) than the bonds in the reactant, the ‘reaction intermediate’ becomes enriched in lighter isotopes, relative to the reactant. Also, for a reactant having lighter isotopes, the difference between its vibrational energy state and the state of the ‘reaction intermediate’ is smaller than the corresponding difference for a reactant having heavier isotopes. This also favors the ‘reaction intermediate’ to become enriched in lighter isotopes relative to the reactant. Thus, the reaction product is enriched in lighter isotopes relative to the reactant.
Isotopic ratios expressed in ‘del’ notation. In the natural sciences, the stable isotopic composition of a sample is typically expressed as the ratio of the abundance of the heavier isotope over that of the lighter isotope, relative to a standard. Carbon isotopic abundances are represented as follows:
where δ13CPDBV is the difference in permil (parts per thousand) between a sample and the carbon isotopic standard (the Pee Dee Belemnite Vienna carbonate - PDBV).
Sulfur isotopic abundances are represented as follows:
δ34SCDT = ((34S/32S)sample/(34S/32S)CDT - 1)1000,
where δ34SCDT is the difference in permil (parts per thousand) between a sample and the sulfur isotopic standard (Cañon Diablo Troilite - CDT).
This category includes visual observable characteristics, arrangements, and spatial relationships of a visual feature that aid in assessing biogenic / abiogenic origin, irrespective of geochemistry or scale. This content was initially developed from geologic phenomena expected on a rocky planet, though measurement parameters (MPs) should apply to any visually observable target.
Measurement parameters (MPs) include fundamental attributes of biogenic features and abiotic mimics, plus key emergent features of structures (i.e., stromatolites, microbially induced sedimentary structures) that result from microbe-mineral interactions. Emergent features have attributes that individual components of those features do not have. MPs considered here are size distribution, spatial distribution, shape/form, orientation, and texture / fabric of features of interest. These measurable parameters were adapted from classic geological terminologies and designed to be useful and “agnostic” for a v [read more]isual unknown feature.
The description of structural (visual) attributes included here avoids the use of terms based on (bio)genetic classifications.
Often, companion LDKB entries are included in the “Chemistry” category with a link provided in the relevant “Structure” argument to describe their full value as potential biosignatures.
The distribution of a feature’s dimensions / magnitude.
For example, for laminations, this can refer to laminae thickness distribution, measured in cross-section parallel to its growth direction. For tunnels, this can refer to tunnel diameter distributions and length ranges.
Spatial distribution is the proportion of a 2D surface covered by a feature within an area of interest. Of interest are patterns on surfaces and the spatial frequency (including repetitions) of distinctive features that intersect the surface or its component grains (allochthonous, autochthonous), minerals (detrital, chemical), mineraloids (non-X-ray crystalline), and other components (e.g., dispersed organics, fossils, etc.).
For example, in basalt, tunnels can be distributed along fractures, starting at a void, and providing an opening for water and / or microorganisms to enter the rock.
Shape / form is the geometric shape, morphology, 2D or 3D structure, habit / crystal habit, surface relief, or surface topography.
For example, spherical cells, also called cocci, represent a morphological variety of unicellular organisms and are distinguished from other cellular morphologies, such as rods and filaments. Upon burial and compaction, coccoidal microfossils may undergo deformation to resemble ruptured or deflated spheres (e.g. raisin-like), or may become flattened compressions. Abiotic processes commonly create spherical morphologies, too (i.e. "blueberries" on Mars) and other botryoidal mineral habits.
Orientation is the position of a feature relative to its surroundings or another parameter (e.g., “Orientation/lower border of view area”). Orientation includes linear arrangements of features, minerals, cell-sized objects, and 3D morphological spatial arrangements across scales.
One example is tunnels in igneous and metamorphic rock, which can be positioned relative to fractures, vesicles (bubbles), minerals, or other tunnels. An example argument for a biogenic prevalence of the tunnels is that cells reside at the margin of the vesicle and mine the glass for energy and material, since tunnel orientation allows the cells to continually encounter fresh glass. An argument for abiotic prevalence of the tunnels is that tunnels around olivine could follow lines of stress created by the differential thermal shrinkage of olivine and glass. An additional example includes filaments in a mat, which can have a vertical or horizontal orientation relative to the defined surface.
Texture describes the relationships of a rock’s components to one another at the microstructural scale. A rock’s texture records its origin and post-depositional (i.e., taphonomic) histories. For a sedimentary rock deposited in the presence of a microbial community, these components include grains, mineral crystals, aqueous precipitates, fossils, organic remains, and other rock fragments. Fabric refers to the spatial arrangement and geometric patterns of the visual components, such as grains, filaments, laminae, of a sedimentary rock that display some level of directionality. A rock fabric can be described by the orientation and pattern produced by single or multiple components of a sediment. Fabric is the relative orientation of multiple features (i.e., grains, filaments, laminae), including the appearance of the components of features (e.g., spatial and geometric pattern formed from all materials present).
For example, stromatolites (laminated geobiological structures) can dis [read more]play distinctive fabrics and textures (such as laminated fabrics oriented parallel to the growth surface of a sedimentary deposit). The combination of those component parts may reveal biogenic attributes and visual patterns that can be diagnostic of various types of microbial input.
This biosignature category includes visual characteristics, arrangements, and spatial relationships of physical features – irrespective of geochemistry or scale – that can be used for life detection in sedimentary rocks and other types of geological deposits affected by water-rock interactions on rocky planets. To assess whether a structural feature originated from or has components of a biological origin, or whether its appearance was produced without any biological input, we present arguments that describe its measurable characteristics (referred to as measurement parameters), supported by evidence of examples from different types of environments. This content is based on peer-reviewed research of geobiological and geological phenomena preserved in the geological record throughout Earth history that could be found on other potentiall
[read more]y habitable rocky planets that might have hosted microbial life.
Measurement parameters (MPs) are fundamental attributes of biogenic structures and abiotic mimics (size and spatial distribution, shape and form, orientation) that result from microbe-mineral interactions, plus key features (fabric and texture) that are essential characteristics of emergent features like stromatolites and microbially induced sedimentary structures (MISS). Emergent features have attributes that the individual components of such features do not have. The MPs considered here were adapted from classical geological terminologies and designed to be “agnostic” for a visual unknown feature so as to enhance its usefulness in life-detection evaluations.
Often, companion LDKB entries are included in a particular “Chemistry” category with a link provided in the relevant “Structure” argument to describe their full value as potential biosignatures.