AAAS: American Association for the Advancement of Science
Abiotic: refers to the absence of life.
Ablation: the removal of material from the surface of an object by vaporization, chipping, or other erosive processes.
Accretion: the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disk. The model for Earth’s formation posits that clumps of minerals and gas and dust came together by gravitational attraction as they swirled around the young Sun. This “accretion” process caused the planet to gradually grow larger and larger.
Adenosine triphosphate (ATP): coenzyme used as an energy carrier in the cells of all known organisms.
Akilia meta-sedimentary rocks: Akilia is an island in southwestern Greenland that has a rock formation proposed to contain the oldest known sedimentary rocks on Earth (>3.85 Ga), and perhaps the oldest evidence of life on Earth.
Albedo: a measure of how much light a surface reflects.
Aldol reaction: when the enolate of an aldehyde or a ketone reacts at the α-carbon with the carbonyl of another molecule under basic or acidic conditions to obtain β-hydroxy aldehyde or ketone. “Aldol” is an abbreviation of aldehyde and alcohol.
Allometric scaling: Allometry is how characteristics of living creatures change with size. The term originally referred to the scaling relationship between the size of a body part and the size of the body as a whole, as both grow during development. However, the meaning of the term has been modified and expanded to refer to biological-scaling relationships in general. This includes morphological traits (e.g., the relationship between brain size and body size among adult humans), physiological traits (e.g., the relationship between metabolic rate and body size among mammal species), and ecological traits (e.g., the relationship between wing size and flight performance in birds).
ALMA: Atacama Large Millimeter Array
Amide bond (or peptide bond): a covalent bond between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, causing the release of a molecule of water (H2O); hence, the process is a dehydration synthesis reaction (or condensation reaction). The bonds that connect amino acids within a protein are amide bonds (or peptide bonds).
Anammox: an abbreviation for anaerobic ammonium oxidation, it is a microbial metabolism in which nitrite and ammonium are directly converted to dinitrogen (N2) gas.
Animals: multi-celled eukaryotes (i.e., the nucleus in their cells is enclosed in a membrane). Generally they are mobile and heterotrophic, meaning they must obtain organic molecules from the environment rather than create it from inorganic molecules (like autotrophic plants do).
Annotation: part of genome analysis in which gene sequences are identified by their particular biological functions.
Anthropogenic changes: changes that originate through human activity (chiefly relating to environmental pollution and pollutants).
Archaea: single-celled organisms that have traits of both bacteria and eukaryotes.
Archaean era: era that lasted from 4 billion to 2.5 billion years ago.
Astronomical forcing: describes how changes in Earth’s orbit—its eccentricity, axial tilt, and precession—affect climatic patterns. (See also, Milanković cycles.)
ATP: adenosine triphosphate
Authigenic: a mineral or sedimentary rock deposit that was generated in the location where it is found.
Autocatalytic reaction: when the reaction product is the same as the catalyst for that reaction.
Autotroph: an organism that “fixes” carbon, forming covalent bonds between inorganic molecules to form organic molecules. Plants and algae are photosynthetic autotrophs (or photoautotrophs), using sunlight as an energy source to drive the process.
Bacteriorhodopsin: a protein used by Archaea. It captures light energy and uses it to move protons across the membrane out of the cell.
Ballast: material that is used to provide stability to a structure.
Banded iron formations: alternating layers of iron-poor rock and iron-rich rock. The rock containing iron is often red in color due to oxidation.
Benthic: the ecological region at the lowest level of a body of water, including the sediment surface and some subsurface layers.
Biofilms: a group of microorganisms in which cells stick to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS), which can also trap and bind sediments.
Biogenic: something that is produced or brought about by living organisms.
Biological communities: groups of organisms whose composition and characteristics are determined by the environments they inhabit and the relation of the organisms to each other.
Biomarker: a substance that indicates the existence of a living organism, either currently living or long deceased.
Biomes: contiguous areas with similar climatic conditions, defined by the communities of plants, animals, and soil organisms that inhabit them. They are often referred to as “ecosystems.”
Biopolymers: a polymer (see below) that is produced by living organisms.
Biosignature: a detectable sign of life. This could include a molecule that would persist only thanks to the influence of life or a phenomenon that would occur only if life were present.
Blanks: Scientific controls act as comparison standards for an experiment. A “blank” does not contain any of the substance being tested, whereas other controls may contain a known amount of the substance.
Bolide: either a large comet or asteroid that impacts the Earth.
Brønsted acid: acid that donates a proton to a base, creating a reaction. (Related, Lewis acid.)
Brown dwarfs: stellar objects that are too low in mass to sustain hydrogen fusion in their cores.
Cambrian explosion: based on the fossil record, a huge array of diverse animal lifeforms appeared over a relatively short time period approximately 540 million years ago. When this was discovered, scientists labeled this sudden diversity the Cambrian explosion. We now know that animals had evolved before this time, so one explanation could be that they developed hard parts for the first time, leading to better preservation of fossils.
Capillary electrophoresis: an analytical technique that separates ions based on their electrophoretic mobility with the use of an applied voltage.
Carbonaceous chondrites: the most primitive types of meteorites. In addition to carbon, silicates, oxides, and sulfides, most contain water or minerals that have been altered in the presence of water and some contain organic compounds.
Carotenoids: organic pigments found in plants as well as in some photosynthetic bacteria and fungi.
Catabolism: the set of metabolic pathways that breaks down molecules into smaller units to release energy.
Catalysis: to cause or accelerate; to drive forward. Catalysis is the increase in the rate of a chemical reaction of one or more reactants due to the participation of an additional substance called a catalyst.
Catalyst: substance that increases the rate of a chemical reaction of one or more reactants. Catalysts are not altered in the reaction they accelerate.
Chemical differentiation: as a planet cools from its molten state at formation, different elements sink or rise depending on their density, leading to distinct planetary layers. However, the chemical differentiation of the planet is not strictly tied to density. Because some elements are bound to other compatible elements, very dense elements can be found in the crust rather than the core, for instance.
Chemiosmosis: the movement of ions across a semi-permeable membrane from a solution of high concentration to one of low concentration; more specifically, it refers to the movement of protons used to generate ATP (adenosine triphosphate) in cellular respiration.
Chemoautotrophs: organisms that derive energy from chemical reactions of inorganic molecules in their environments. Most are bacteria or archaea that live in hostile environments such as deep sea vents.
Chemolithotrophs: organisms that obtain their energy from chemical reactions—such as the oxidation of reduced inorganic compounds like sulfide, ammonia, and hydrogen—and use carbon dioxide as a carbon source. They are also known as chemoautotrophs.
Chiral: a type of molecule that has a non-superposable mirror image, like right and left hands.
Chloroplasts: structures in plant and algal cells that conduct photosynthesis.
CHNOPS: the most common elements for life: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
Chromatography: the collective term for a set of laboratory techniques for the separation of mixtures.
Clade: a group consisting of an ancestor and all its descendants, a "branch" on the "Tree of Life." The ancestor may be an individual, a population, or even a species.
Cloning: the process of making genetically identical copies of DNA, cells, or entire organisms.
Co-solvent: a second solvent added to an original solvent, creating a mixture that has a greater ability to dissolve other substances.
Coalescence: the point of origin where all branches on the tree of life come together into a single common ancestor
Codon: a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis.
Cofactor: a non-protein chemical compound needed for a protein's biological activity. Cofactors are "helper molecules" for proteins (commonly enzymes), assisting in biochemical transformations.
Complex living systems: refers to a system with more parts, and more interactions among parts, than a less complex system.
Concretion: a compact mass within sedimentary rock, usually round or oval in shape, and usually formed from the same material as the rock plus the cementing mineral. Microbial activity can often be the trigger for the mineral cements, and such concretions can have fossils in the middle.
Condensation: a chemical reaction where molecules are joined together to form larger, more complex molecules, usually through the elimination of a simple molecule (usually water).
Confound: In statistics, a confound (or confounding variable) is an extraneous variable that correlates (directly or inversely) with both the dependent and independent variable.
Controls: Scientific controls act as comparison standards for an experiment. A “blank” does not contain any of the substance you are testing for, whereas other controls may contain a known amount of the substance.
Convection: the motion of a fluid (such as water or air) caused by heating, where the fluid carries energy with it as it moves away from the heat source.
Convergent: process whereby organisms not closely related, independently evolve similar traits as a result of having to adapt to similar environments.
Covalent bond: involves the sharing of electrons between atoms, creating a stable balance of attractive and repulsive forces.
Cryovolcano: colloquially known as an ice volcano, a cryovolcano erupts volatiles such as water, ammonia, or methane instead of molten rock.
Cyclic alkanes: hydrocarbon compounds.
Cytoplasmic homeostasis: the mechanisms and properties by which the cytoplasmic environment of a cell is regulated to maintain stable and relatively constant conditions.
D-sugars, L-amino acids: The L and D prefixes refer to the two types of chiral compositions.
D/H: the ratio of deuterium (2H) to hydrogen (H) in water.
Deep sequencing: when the same nucleotide is sequenced multiple times (in which the depth refers to the number of times it has been sequenced).
Denitrification: a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen (N2).
Detritus: particles of rock derived from preexisting rock through processes of weathering and erosion. Uraninites and pyrites readily weather when oxygen is present.
Diagenesis: the change of sedimentary rock during and after rock formation at temperatures and pressures less than what is required for the formation of metamorphic rocks but excludes surface alteration (weathering).
Diesters: a class of organic compounds, corresponding to inorganic salts, that form from an organic acid and an alcohol.
Disequilibrium: A system in equilibrium has properties that do not change over time. It is thought life on Earth depends on the energy of our planet being in disequilibrium (i.e., experiencing earthquakes, turbulence, heating, etc.)
DNA: deoxyribonucleic acid
Doubling time: the period of time required for a quantity to double in size or value.
Drift: refers to random fluctuations in gene variants within a population, which over time can lead to the loss of genes.
Dynamo: theory that describes how an interior rotating, convecting, and electrically conducting fluid generates a magnetic field. Earth’s dynamo (also called the “geodynamo”) is generated by molten iron circulating at Earth’s core.
Ecological niche: the space in which an organism or community lives, uses resources, competes with other life forms, and has other behaviors related to the specific aspects of that environment.
Electrophiles: positively charged species that are attracted to an electron-rich center.
Enantiomers: molecules that are chiral. A chiral molecule has a non-superposable mirror image, like right and left hands (thus, chiral molecules are often referred to as “handed”).
Encapsulation: when one molecule becomes contained within a larger molecule, such that it is prevented from contacting other molecules it otherwise would react with.
Endosymbiosis: when one organism lives inside another, each conveying benefits from the cooperative system. The endosymbiotic theory states that several key structures of eukaryotes originated through bacteria or other single-celled organisms being taken inside another cell.
Enzymatic activity: a measurement of how effectively an enzyme can catalyze a given reaction.
Enzymes: metabolic catalysts responsible for many life processes, including DNA synthesis and food digestion. Most enzymes are proteins.
Epistasis: refers to genes at multiple locations whose effects interact, often through one activating, suppressing, or modifying the expression of another.
Equilibrium: a system whose properties do not change over time. It is thought that life on Earth depends on the energy of our planet being far from equilibrium, i.e., experiencing earthquakes, turbulence, heating, etc.
Eukaryote: an organism whose cells contain a nucleus and other structures that are enclosed within membranes. Animals, plants, and other complex life forms are eukaryotes.
Eusociality: defined by the following characteristics: cooperative care of offspring, overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. Often observed in insects (ants, wasps, bees), eusociality is distinguished from all other social systems because individuals of at least one group lose the ability to perform at least one behavior characteristic of individuals in another group.
Eutectic: a mixture of chemical compounds or elements that have a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients.
Evolutionary landscape: a construct to think about and visualize how evolution has affected different aspects of life (including genes, species, populations, etc.)
Exaptation: a shift in the function of a trait during evolution. For instance, a trait that evolved because it served one particular function subsequently comes to serve another function.
Exoplanet light curve: When exoplanets pass in front of the star they orbit (as seen from Earth), they block a portion of the starlight. Telescopes measuring this dip in light over the course of the planet’s orbit provide data points for a light curve (where the two variables on the graph are time and total numbers of photons). Fitting models to the light curve, various planetary characteristics can be determined.
Exoplanets: planets outside our Solar System (i.e., planets that orbit other stars).
Extremophiles: an organism that thrives in environments considered extreme or detrimental to most life on Earth.
Felsic: igneous rocks enriched in lighter elements (e.g., silicon, oxygen, aluminum, sodium, and potassium) that form feldspar and quartz.
Flagellum: a lash-like appendage protruding from certain cells. The primary role is locomotion but it also functions as a sense organ, being sensitive to chemicals and temperatures outside the cell.
FTIR: Fourier transform infrared spectroscopy is an efficient method for processing spectra data obtained using interferometers.
Ga: billion years
GC–MS: Gas chromatography–mass spectrometry combines the features of gas–liquid chromatography and mass spectrometry to identify different substances within a test sample.
Gene amplification (also known as gene duplication): a process in which multiple copies of a gene are produced. The result is an amplification of the phenotype, or expressed trait, associated with the gene.
Genome annotation: part of genome analysis in which gene sequences are identified by their particular biological functions.
Genotype: the complete set of genetic information of an individual.
Geochemical proxies: elemental, isotopic, and molecular properties of the rock that allow us to fingerprint a particular transient feature or process on Earth’s surface in the distant past that cannot be observed directly. These tracers include inherited, well-preserved chemical records of the composition and temperature of the ocean and the gas content of the ancient atmosphere, or evidence for a particular organism or metabolic process (such as oxygen-producing photosynthesis).
Geochronology: determining the age of rocks, sediments, and fossils by measuring features within these structures, such as radioactive isotopes.
Gibbs free energy: a measure of the maximum available work that can be derived from any system under conditions of constant temperature and pressure.
GOE: Great Oxidation Event
Greenhouse: energy from the Sun heats the Earth's surface, and some of that energy reflects back into space. However, atmospheric gases such as water vapor and carbon dioxide can act like the glass roof of a greenhouse, trapping the outgoing energy and causing the climate to be warmer than it would be otherwise.
Habitability: potential for an environment to support life, be it on planet-wide or microscopic scales.
Habitable niche: the space in which an organism or community lives, uses resources, competes with other life forms, and has other behaviors related to the specific aspects of that environment.
Habitable zone: traditionally defined as the orbital region around a star where water can exist as a liquid on an object’s surface.
Hadean: the first geologic eon on Earth, lasting from the planet’s formation 4.540 billion years ago to 4 billion years ago.
Halophiles: organisms that thrive in salty environments.
Handedness: refers to molecular chirality; a chiral molecule has a non-superposable mirror image, like right and left hands. A chiral object and its mirror image are called enantiomorphs or, when referring to molecules, enantiomers.
Heterogeneous: when a structure is diverse in character and content.
Heterotrophs: organisms that cannot synthesize their own food and rely on other organisms for nutrition (versus autotrophs that can produce food from their surroundings via photosynthesis (light) or chemosynthesis (chemicals).
HGT: horizontal gene transfer
Homochirality: A substance is homochiral if all the constituent units are molecules of the same chiral form (enantiomer). A chiral molecule has a non-superposable mirror image, like right and left hands.
Homology: the existence of shared ancestry between a pair of structures, or genes, in different species.
Hopanoids: compounds found in bacteria and other primitive organisms (but not in archaea).
Horizontal gene transfer (also known as lateral gene transfer): refers to the transfer of genes between organisms in a manner other than traditional reproduction. It contrasts with vertical transfer, the transmission of genes from the parental generation to offspring via sexual or asexual reproduction.
Hydrolysis: the cleavage of chemical bonds by the addition of water.
Hydrophobic molecules: non-polar molecules that cluster together in solution and do not interact with water molecules. The mixing of oil and water is the classic example of a hydrophobic interaction.
Hydrosphere: all the water on a planet—on the surface, in the air, and underground.
Hydrothermal: refers to the activity of heated water in a rocky planet’s crust.
Hypercycle: a new level of organization whereby self-replicative units are connected in a cyclic, autocatalytic manner.
Icehouse: another name for Ice Age, it is a period when Earth is broadly covered by sheets of ice and temperatures are low.
Immunoassay: a test that measures the presence or concentration of a macromolecule in a solution through the use of an antibody or immunoglobulin.
In silico: studies performed as part of a computer simulation.
In situ: Latin for “in place.” In biology, it means examining a phenomenon exactly where it occurs without moving it to another medium.
In vitro: Studies that are done in a laboratory environment using test tubes, petri dishes, etc.
In vitro assays: scientific tests performed in a test tube or other sterile container, rather than in a living organism. The test measures the activity of a drug on a sample of organic tissue.
In vivo: studies in which the effects of various biological entities are tested on whole, living organisms, as opposed to partial or dead organisms.
Interferometer: many telescopes or mirrors linked together. The greater area of the linked telescopes provides a higher resolution of astronomical targets than what a single telescope can observe.
Isomerization: the process by which one molecule is transformed into another molecule through the rearrangement of atoms.
Isomers: molecules with the same molecular formula but different chemical structures.
Isoprenoids: lipids found in all living things.
Isotope: a version of an element with different numbers of neutrons in the atom. This affects the mass of the element. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon (with mass numbers 12, 13, and 14.)
Isotopic fractionation: the enrichment of one isotope—an element that varies in the number of neutrons in the atom—relative to another isotope. This usually occurs with the same element, such as the isotopes Carbon-14, Carbon-13, and Carbon-12.
Kinetics: the rate at which chemical reactions occur as well as the energy required for reactions to proceed.
L and S amino acids: the “L” and “S” refer to the configuration of the amino acid molecule. In the “L” form, the hydroxyl group is on the left side of the molecule. (Amino acids in proteins are all the “L” form.) “S” indicates that the molecular form is arranged counterclockwise.
L-amino acids, D-sugars: The L and D prefixes refer to the two types of chiral compositions.
Lab-on-a-chip: a device that integrates one or several laboratory functions on a single electronic circuit chip only millimeters to a few square centimeters in size.
Lamination: thin sedimentary layers, typically one centimeter or thinner.
Laser-induced fluorescence: a spectroscopic method used for studying the structure of molecules, detection of selective species, and flow visualization and measurements. The species to be examined is excited with a laser, and then will de-excite and emit light at a wavelength longer than the excitation wavelength.
Latent heat: a type of energy released or absorbed in the atmosphere. Latent heat is related to changes in phase between liquids, gases, and solids (as opposed to sensible heat, which is related to changes in temperature of a gas or object with no change in phase.)
Lateral gene transfer: see horizontal gene transfer.
Lewis acid: when this acid reacts with a base, it doesn’t involve the transfer of a proton, but instead accepts a pair of electrons. (Related, Brønsted acid)
Ligation: the covalent linking of two ends of DNA or RNA molecules, usually with the enzymes DNA ligase or RNA ligase.
Linear polymer: a polymer where the molecules form long chains, without branches or crosslinked structures.
Lithosphere: the outermost shell of a rocky planet. On Earth, the lithosphere is the crust and the relatively elastic portion of the upper mantle.
LUCA: stands for last universal common ancestor.
M-dwarf stars: also known as red dwarfs, M-dwarf stars are smaller, cooler, and less luminous than our Sun.
Macromolecules: large molecules (polymers) composed of many repeated subunits (monomers).
Macroorganisms: lifeforms large enough to be seen with an unaided eye; i.e., without a microscope.
Mafic: rocks that are igneous silicate characterized by a more magnesian and ferrous (iron with a +2 oxidation state) composition, with fewer alkali elements.
Mantle: of a rocky planet, the region between the crust and the outer core. Earth’s mantle is a silicate solid that, over geological time, acts like a viscous liquid.
Mass spectrometry: an analytical technique that produces spectra of the masses of the atoms or molecules comprising a sample of material.
Mass-independent isotope fractionation: refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes.
Mesozoic oceanic anoxic events: episodes of widespread oxygen deficiency in the deep ocean on timescales of hundreds of thousands of years, during the Mesozoic era (252 to 66 million years ago).
Messenger RNA: see mRNA.
Metabolism: a set of chemical reactions that all life needs to grow, reproduce, maintain structure, and respond to the environment. Metabolic pathways involve one chemical being transformed into another chemical.
Metabolite: a product necessary for metabolism or a product of metabolism. The term usually refers to small molecules.
Metagenomics: the study of genetic material recovered directly from environmental samples.
Metallicity: in astronomy, refers to an object's composition of chemical elements other than hydrogen or helium.
Metallomics: the comprehensive analysis of metal and metalloid species within a cell or tissue type.
Metamorphic: rock that has been transformed by heat or pressure.
Metaphytes: multi-cellular plants.
Metasedimentary rocks: sedimentary rocks that have been altered by metamorphism.
Metatranscriptomics: the study of actively expressed genes within a multi-species community over space and time.
Metazoans: multi-cellular animals.
Methanogenesis: the formation of methane by microbes.
Methanotrophy: the process whereby microbial organisms are able to metabolize methane (with and without oxygen) as a source of carbon and energy.
Micelle: an aggregation of molecules in water that have both polar or charged groups and nonpolar regions (amphiphilic molecules).
Milanković cycles: predictable, periodic variations in Earth’s orbital properties—on timescales of tens to hundreds of thousands of years—linked specifically to the tilt and wobble of Earth’s axis and the shape of Earth’s orbit around the Sun. These parameters impact the flux of solar radiation to Earth’s surface and, thus, climate. (See also, astronomical forcing.)
Miller–Urey experiment: originally simulated the conditions thought (at the time) to represent the atmosphere of the early Earth and tested for the production of molecules that may have played a role in the origin of life. More than 20 types of amino acids were produced in this experiment. It is also known as a “spark discharge” experiment because the energy for chemical reactions is often provided by the spark between two high-voltage electrodes. The results of this experiment were published in 1953.
Mineral skeletons: During the Cambrian period (540–485 million years ago), animals first developed skeletons made from minerals such as calcite, magnesian calcite, aragonite, apatite, and opal.
Mitochondrion: a structure within cells that generates most of the cell's supply of energy in the form of ATP (adenosine triphosphate). In addition, mitochondria are involved in cellular signaling, differentiation, control, growth, and death.
Mixing ratio: the amount of one component of a mixture, relative to the total amount of all other components. For example, the mass of water vapor to the mass of dry air.
Modularity: a property of system architecture in which complex structures or functions are divided into simpler and weakly linked components. Modularity has been observed in all model systems and can be studied at nearly every scale of organization (molecular interactions up to the whole organism).
Moiety: a well-defined part of a larger molecule.
Moist greenhouse: a hot climate that is caused by an abundance of water vapor in the atmosphere of a planet.
Molecular clouds: Sometimes called stellar nurseries because they are where stars are born, molecular clouds are collections of gas that are dense enough to permit the formation of molecules (most typically hydrogen [H2]).
Molecular emission features: on the spectrum of frequencies of electromagnetic radiation, these are emitted by atoms or molecules as they transition from a higher-energy to a lower-energy state. Because each element's emission spectrum is unique, spectroscopy can be used to identify them.
Monomer: a molecule that can form a chemical bond with other molecules that are similar or identical to form a polymer. For example, amino acids are the monomers of proteins.
mRNA (messenger RNA): conveys genetic information from DNA to the ribosome.
NAD+: Nicotinamide adenine dinucleotide (NAD+) is a coenzyme found in all cells. Also NADH.
NAI: NASA Astrobiology Institute
NC10: a recently discovered phyla that is only represented by recovered DNA sequences; no member of this phyla has yet been cultured. The presence of NC10 seems to be associated with nitrite- and methane-rich freshwater environments.
Network analysis: study of graphs as a representation of either symmetric relations or, more generally, of asymmetric relations between discrete objects.
Neutral drift: evolutionary change at the molecular level that is caused by random drift of neutral mutant alleles rather than by natural selection.
Neutrality: At the molecular level, random genetic mutations can occur that have no effect (i.e., are neutral).
NMR: Nuclear magnetic resonance spectroscopy exploits the magnetic properties of certain atomic nuclei to determine the physical and chemical properties of atoms or the molecules in which they are contained.
Non-covalent bonds (or non-covalent interactions): weak interactions between ions, molecules, and parts of molecules. These bonds are essential for determining the three-dimensional shape of large molecules (e.g., proteins) but are weak enough to be continually broken and reformed at room temperature. Biologically important non-covalent bonds are 10 to 100 times weaker than covalent bonds.
NSF: National Science Foundation
Nucleic acids, proteins, carbohydrates, and lipids are all essential macromolecules of life.:
Nucleobase: see Watson-Crick nucleobases.
Nucleophile: a reactant that provides a pair of electrons to form a new covalent bond.
Nucleoside triphosphates: used as a source of chemical energy to drive a variety of biochemical reactions, they also serve as the activated precursors of DNA and RNA synthesis. (Nucleotides may have one, two, or three phosphate groups covalently linked at the 5' hydroxyl of ribose. These are referred to as nucleoside mono-, di-, and triphosphates, respectively.)
Nucleotide: composed of a nucleobase (adenine, thymine, cytosine, guanine, or uracil), a fivecarbon sugar, and one or more phosphate groups. Nucleotides serve as subunits (monomers) of nucleic acids like DNA or RNA.
Nuclide: an atom or ion that is characterized by the contents of the nucleus.
Obligate: a necessary quality, e.g., obligate aerobes need oxygen in order to survive.
Obliquity: In astronomy, obliquity (or axial tilt) is the angle between an object’s rotational axis and its orbital axis; i.e., the angle between its equatorial plane and orbital plane.
Oligomer: a molecular complex that consists of several monomer units, typically larger than dimers or trimers, but smaller than polymers. For example, a DNA molecule 10 nucleotides in length is considered an oligomer, whereas a gene-length molecule is considered a polymer.
Oligonucleotides: short, single-stranded DNA or RNA molecules.
Oligopeptides: peptides that have between 2 and 20 amino acids.
Oligosaccharide: a polymer containing a small number (typically 3 to 9) of simple sugars (monosaccharides).
One-pot reaction: in chemistry, a strategy to improve the efficiency of a chemical reaction, whereby a reactant is subjected to successive chemical reactions in just one reactor without isolation or purification between reactions.
Orbital eccentricity: the amount by which an orbit deviates from a perfect circle.
Organelle: a specialized subunit within a cell that has a specific function; it is usually separately enclosed within its own lipid bilayer.
Organic: molecules that contain carbon atoms. The distinction from “inorganic” is historically arbitrary; some carbon-containing molecules such as carbides, or simple carbon-oxygen combinations (like CO2), are considered inorganic.
Orthogonality: refers to the possibility of two or more supramolecular, often non-covalent interactions being compatible, reversibly forming without interference from the other.
Overprint: (also known as imprint) development or superposition of structures on original formations. Examples include contamination, thermal breakdown of organic matter, and extreme metamorphism.
Oxidation: the loss of electrons or an increase in degree of oxidation. Oxidation is usually combined with reduction—the gain of electrons or a decrease in the degree of oxidation—in socalled “redox” reactions that change the oxidation state of atoms or molecules.
Panspermia: the hypothesis that life exists throughout the Universe and is distributed by meteors, asteroids, and comets. For Earth, it presumes that life originated elsewhere and was transported here.
PCR: The polymerase chain reaction is a biochemical technology used to amplify a single or a few copies of a piece of DNA, generating thousands to millions of copies of a particular DNA sequence.
Peptide bond: see amide bond.
Peptides: short chains of amino acid monomers linked by peptide (amide) bonds. Peptides are distinguished from proteins on the basis of size, and contain 70 or fewer amino acids.
Perihelion: the point of an orbit when the object (planet, comet, asteroid) is closest to its star.
Petrography: the detailed study of rock features to determine their origin, distribution, and structure.
Phase space: represents all possible states of a system, with each possible state corresponding to one unique point in the phase space.
Phenotype: the composite of an organism’s observable traits (body shape, behavior, etc.). A phenotype results from the expression of an organism's genes as well as the influence of environmental factors.
Phosphodiester bond: a group of strong covalent bonds between a phosphate group and two 5-carbon ring carbohydrates (pentoses) over two ester bonds. Phosphodiester bonds are central to most life on Earth, as they make up the backbone of the strands of DNA.
Photolysis: when a chemical compound is broken down by light.
Photometry: the measurement of the brightness or intensity of light, as perceived by the human eye.
Photosynthesis: the process of converting energy from sunlight into chemical energy. In oxygenic photosynthesis, water (H2O) is split, carbon dioxide (CO2) is fixed to make a sugar, and oxygen is released as a waste product.
Phototrophs: organisms that capture photons to acquire energy, using the energy from light to carry out various cellular metabolic processes.
Phylogenetic tree: a diagram that shows the evolutionary relationships of different species based on their genes.
Phylogenetics: the study of evolutionary relationships among groups of organisms, which are discovered through molecular sequencing data and morphological data matrices.
Physicochemical: relating to physics and chemistry.
PI: Principal Investigator
Piezophiles: an organism that can grow and reproduce at high pressures.
pKa: a measurement of the strength of an acid. The larger the value of pKa, the weaker the acid. For a given chemical group, the pKa is the solution pH at which one half of these groups will be protonated and one half will be ionized (i.e., lost a proton to solution).
Planetesimals: when cosmic dust grains in protoplanetary disks collide and stick together, planetesimals form. When the bodies reach sizes of approximately one kilometer, they can attract each other through their mutual gravity, aiding further growth into moon-sized protoplanets.
Plate tectonics: the movement of the outer layer of our planet (the lithosphere) that is broken into several plates. The movement is driven by geological processes in the layer below (i.e., in the mantle).
Platinum-group elements: the six elements platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), and ruthenium (Ru). They are the rarest metals on Earth but five of them are more plentiful in meteorites and can therefore indicate when a crater was made by an impact.
PLFA: Phospholipid-derived fatty acids are used in microbial ecology as chemotaxonomic markers of bacteria and other organisms.
Polar solvent: a liquid with molecules that have a slight electrical charge.
Polyamide: a macromolecule with repeating units linked by amide bonds. Proteins are polyamides.
Polyanion: an anion (negatively charged ion) that has more than one negative charge.
Polymer: a large molecule (or “macromolecule”) composed of many repeated subunits (monomers). A protein, for instance, is made up of many amino acid monomers.
Polynucleotide: a polymer or chain of nucleotides. DNA is a pair of polynucleotides. RNA is also a polynucleotide.
Polypeptide: a long, continuous, and unbranched peptide chain.
Prebiotic: refers to chemistry or processes that eventually led to the origin of life.
Precambrian: this time covers the vast bulk of Earth's history, starting with the planet's creation about 4.5 billion years ago and ending with the emergence of complex, multi-cellular life forms during the Cambrian period, 540 million years ago.
Primers for gene amplification: A scientific technique called polymerase chain reaction (PCR) combines primers (short DNA fragments) and DNA polymerases (enzymes that build DNA from nucleotides) to generate thousands to millions of copies of a DNA sequence, thereby “amplifying” the genes.
Prion: an infectious agent composed of a misfolded protein; it acts as a template, causing existing, properly folded proteins to also misfold.
Prokaryotes: a group of organisms whose cells lack a membrane-bound nucleus.
Proteome: refers to the complete set of proteins within an individual.
Proteomics: the large-scale study of proteins, particularly their structures and functions.
Protobiopolymers: polymers that may have preceded RNA, proteins, and/or DNA.
Protocells: the evolutionary precursors to the first cells and thought to be self-ordered spherical collections of lipids.
Protoplanetary disk: a flattened dense cloud of gas and dust rotating around a newly formed star. Protoplanetary discs are thought to evolve into planetary systems. Gravity causes dust and gas to clump together to form protoplanets. Protoplanets eventually accumulate enough material to become planets.
Psychrophiles: an organism that can grow and reproduce at low temperatures.
Racemic: a mixture with equal amounts of left- and right-handed enantiomers (chiral molecules).
Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission, to x-rays and gamma rays at the short wavelength end.:
Radiative transfer: the physical phenomenon of energy transfer in the form of electromagnetic radiation.
Radiogenic: formed by radioactive decay.
Raman spectroscopy: provides information about molecular vibrations that can be used for sample identification. The technique involves shining a light source (laser) on a sample and detecting the scattered light.
Rarified: atmospheres that are less dense and compressed. Due to gravitation, Earth’s atmosphere at higher elevations is more rarified than the atmosphere closer to the ground.
Reagent: a substance that is added to a system to create a chemical reaction.
Recalcitrant: compounds that are resistant to being broken down through chemical processes.
Red beds: sedimentary rocks typically consisting of sandstone, siltstone, and shale. They are predominantly red in color due to the presence of iron oxides.
Redox: Reduction and oxidation, or “redox” reactions, are chemical reactions in which the oxidation state of atoms or molecules change. Oxidation is the loss of electrons, or an increase in degree of oxidation. Reduction is the gain of electrons, or a decrease in the degree of oxidation.
Redox couple: a reducing species and its corresponding oxidized form, e.g., Fe2+/Fe3+. See Redox.
Redox state: a measure of the balance of redox, or reduction-oxidation, reactions in a system. These reactions are characterized by a transfer of electrons, which is often facilitated by microbiological activity.
Reduced gas: a gas with a low oxidation number (or “high reduction”), which is usually hydrogenrich. Strongly reduced gases include methane, ammonia, and hydrogen sulfide. Such gases are strongly associated with the origin of life.
Reduction: the gain of electrons, or a decrease in the degree of oxidation. Reduction is usually combined with oxidation—the loss of electrons, or an increase in degree of oxidation—in so-called “redox” reactions which change the oxidation state of atoms or molecules.
Refractory: materials that remain a solid until exposed to high temperatures. (The opposite term is “volatile.”)
Regio-specific chemical reaction: when one structural isomer is produced exclusively, when other isomers are also theoretically possible.
Replication: the process of generating identical copies of a DNA molecule. This occurs in all forms of life and is the basis for biological inheritance.
Respiration pathways: on a cellular level, respiration refers to the biochemical pathway by which cells release energy from the chemical bonds of food molecules and then provide that energy for the essential processes of life.
Respiratory oxygen: Just as animals need oxygen in the atmosphere to breathe, sea creatures need dissolved (or respiratory) oxygen in the ocean. The amount of dissolved oxygen in the ocean depends on temperature and salinity.
Ribosomal gene sequences: Ribosomes link amino acids together, in the order determined by messenger RNA, to build proteins. A major part of ribosomes is ribosomal RNA (rRNA). All organisms have rRNA, and the gene sequences are of ancient origin, so it is used to determine evolutionary relationships.
Ribosome: a large and complex molecular machine found within all living cells that serves as the primary site of protein synthesis (translation).
Riboswitches: ribonucleic acid sequences encoded within messenger RNA that perform a regulatory function, directly affecting the function of genes.
RNA: ribonucleic acid
RNA polymerase: an enzyme necessary for constructing RNA chains using DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses.
RNA world: hypothesis that proposes that self-replicating ribonucleic acid (RNA) molecules were precursors to current life, which is based on deoxyribonucleic acid (DNA), RNA, and proteins.
Runaway greenhouse: when the atmosphere is such that sunlight cannot escape back into space and temperatures continue to rise until the oceans boil away.
Runaway state: a system beyond a certain “tipping point,” where feedbacks push a system so far in one direction there is no going back to the previous state. The most commonly cited example of this effect is a “runaway greenhouse” climate, where atmospheric carbon dioxide and other greenhouse gases raise the global temperature and cause other events to occur that also raise the global temperature (e.g., ice sheet disintegration and melting of methane hydrates).
S and L amino acids: the “S” and “L” refer to the configuration of the amino acid molecule. In the “L” form, the hydroxyl group is on the left side of the molecule. (Amino acids in proteins are all the “L” form.) “S” indicates the molecular form is arranged counterclockwise. Sedimentary rocks, rocks that form by the deposition of sediment, usually in lakes or other bodies of water.
Sedimentation: the deposition of sediment, usually in lakes or other bodies of water, forming sedimentary rocks.
Semi-major axis: that part of a planet’s orbital ellipse that is the radius of the orbit at the two most distant points.
Sensible heat: a type of energy released or absorbed in the atmosphere. Sensible heat is related to changes in temperature of a gas or object with no change in phase (as opposed to latent heat, which is related to changes in phase between liquids, gases, and solids).
Serpentinization: a reaction between water and mantle material (usually mafic rock), whereby water is added to the mineral structure of the rock and heat and hydrogen (H2) are released.
SETI: Search for Extraterrestrial Intelligence
Siderophile: elements, such as iridium or gold, that tend to bond with metallic iron; they are “ironloving” elements.
Solar flare: a sudden, rapid, and intense variation in brightness of the Sun (or other star). This occurs when magnetic energy that has built up in the star’s atmosphere is suddenly released.
Solid-state convection: a complicated phenomenon in the Earth’s mantle that causes various tectonic activities, especially magmatism and plate tectonics, and makes the mantle evolve over geologic time.
Solubilization: to bring a substance into solution, i.e., dissolve in a liquid state.
Solvation (also known as dissolution): the interaction of a solute with a solvent, which leads to stabilization of the solute in the solution, such as sodium in water.
Spectra: refers to the range of frequencies of electromagnetic radiation. Humans see the visible light portion of the electromagnetic spectrum.
Sputtering: occurs when a substance with high moisture is heated, resulting in explosive loss of material.
Stellar flare: see solar flare.
STEM: science, technology, engineering, and mathematics
Stereochemistry: the study of the relative spatial arrangement of atoms that form the structure of molecules. An important branch of stereochemistry is the study of chiral molecules. Stereochemistry is also known as 3D chemistry because the prefix "stereo" means “threedimensionality.”
Stereoisomers: isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution) but that differ only in the three-dimensional orientations of their atoms in space.
Stereospecificity: the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants.
Sterols: organic molecules found in plants, animals, and fungi.
Stratigraphy: a branch of geology which studies rock layers (strata) and layering (stratification).
Stromatolites: layered structures formed by the trapping, binding, and cementation of sedimentary grains by microbial mats, especially cyanobacteria. Stromatolites provide the most ancient records of life on Earth, with fossil remains that date from more than 3.5 billion years ago.
Subduction: the process in plate tectonics where one plate slips under another and melts into the mantle.
Substrates: substances that an organism can consume and metabolize for growth.
Super Earth: a planet generally defined as having a mass between 2 and 10 times that of our planet.
Supercontinent: a collection of most of the world’s land mass into a single continent. Due to tectonic activity, supercontinents have assembled and then broken apart several times throughout Earth’s history. The most recent supercontinent (Pangea) existed 300 million years ago.
Superfamilies: large groups of closely related proteins or other molecules.
Symbiotic: a close association between two or more organisms of different species. This association is often, though not necessarily, beneficial for each member.
Synthetase: an enzyme that catalyzes the linking together of two molecules, usually using the energy derived from the concurrent splitting off of a pyrophosphate group from a triphosphate.
Syntrophic: communities where one organism relies on another for nourishment (or they each rely on the other). This type of symbiosis is important for key biochemical transactions on Earth, such as the microbially mediated cycling of carbon.
Taxa: groups of organisms that are genetically related or otherwise have characteristics that differentiate them from other groups.
Tectonism (tectonics): see plate tectonics.
Template: a structure that acts as a “mold,” directing the pattern of a second structure. In biology, this usually refers to a strand of DNA directing the creation of a complementary DNA strand.
Terrestrial: planets that are solid (as opposed to gaseous planets like Jupiter) and, like Earth, are usually composed of silicate rocks.
Thermodynamic disequilibria: chemical systems that undergo reactions in which the overall number of particular molecules in the system changes.
Thermodynamic stability: a system in its lowest energy state, or in chemical equilibrium with its environment. Chemical reactions may occur, but the overall number of molecules in their particular form remains the same.
Thermodynamics: the energy of a system (often heat/temperature) or the process by which reactions occur.
Ti in zircon thermometry: measuring the titanium content in zircon crystals can indicate the temperature under which the crystals formed.
Tidal force: the gravitational influence one body has on another, where the side nearest is affected more strongly than the far side. The tidal force exerted by our Moon causes tides in Earth’s ocean.
Tidal heat: a gravitational effect where a less massive object orbits a more massive one (such as the moon Europa orbiting the gas giant planet Jupiter). In non-circular (eccentric) orbits, the less massive body becomes distorted or “squeezed,” and this change in shape causes friction between the rocks, generating internal heating.
Tidal locking: when an orbiting body always faces the same side to the body it orbits.
Trace element isotope systematics: the process of calculating the radioactive decay of isotopes in order to determine the date when something was alive (for organisms) or when it formed (for features such as impact craters).
Transduction: in biophysics, the conveyance of energy from one electron (a donor) to another (a receptor) at the same time that the class of energy changes.
Transit: when a celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen from the observer’s vantage point.
Translation: the process in which ribosomes “read” messenger RNA base pairs in order to assemble the amino acid chains that form proteins.
Tree of Life: a way to describe the evolutionary relationships of all life on Earth. Early diagrams of the interconnected nature of life, such as from Charles Darwin’s On the Origin of Species, appeared tree-like, with branches depicting the divergence of species.
tRNA: an RNA molecule that serves as the physical link between the nucleotide sequence of nucleic acids (DNA and RNA) and the amino acid sequence of proteins. Aminoacyl-tRNA is tRNA to which its cognated amino acid is chemically bonded (charged). The aa-tRNA deliver the amino acid to the ribosome for incorporation into the polypeptide chain that is being produced.
Trophism: refers to feeding and nutrition; e.g., chemotrophy, phototrophy, etc.
Vertical inheritance: the transmission of genes from the parental generation to offspring via sexual or asexual reproduction.
Vesicle: an assembly of lipid molecules, such as a cell membrane.
Viscosity: a measure of a fluid's resistance to flowing. Whether a fluid has a high or low viscosity depends on the internal friction caused by the fluid’s molecular structure.
Volatile: a substance that easily evaporates at normal temperatures.
Watson-Crick nucleobases: the heterocycles (i.e., ring structures made of two elements) guanine, cytosine, adenine, and thymine that form the base pairs between the two polymers of a DNA double helix.
Weathering: the process by which rocks are broken down by chemical reactions. Different types of chemical weathering caused by exposure to water, oxygen, carbon dioxide, and acids can alter the minerals found in rocks.
White dwarf: a stellar remnant of high density composed mainly of electron-degenerate matter. A white dwarf has the mass of our Sun but the volume of Earth. Our Sun will become a white dwarf after it goes through its red giant phase.
Xenobiotic: a form of biology that is not familiar to science or is not found in nature, i.e., synthetic biological systems and biochemistries that differ from the DNA/RNA/20 amino acid system.
Xerophiles: an organism that can grow and reproduce in conditions with a low availability of water.
δ13C: an isotopic signature, a measure of the ratio of stable Carbon 13 to Carbon 12 isotopes, in parts per thousand.