Definitions

Definitions

The definitions below were compiled by members of the NSCORT Journal Club and organized and edited by Dr. Nicolle Zellner.

Accretion - the buildup of particles into larger particles by adhesion or gravitational forces; in the case of the early Solar System, this consisted of clumps becoming planetesimals, which then attracted more and larger rock bodies, eventually becoming planets.

Acetate - CH₃CO_2-(aq.) as a free ion; CH₃CO₂ as a 'fragment' in a molecule.

Acidophile - an organism that preferentially grows at a pH below 5.4 (acidic conditions).

Aerobe - an organism that utilizes molecular oxygen as a terminal electron acceptor in aerobic respiration.

Albedo - a measure of an object's reflecting power; a complete reflection gives an albedo of 1.

Aliphatic hydrocarbon - see hydrocarbon.

Alkaliphile - an organism that is capable of growing at a pH up to 11.5 (basic conditions).

Amino acid - the fundamental monomer unit for proteins; R-CH(NH₂)CO₂H.

Amorphous - a low-temperature state of ice in which the molecules are held together by hydrogen (H) bonds and do not have enough energy to assume a crystalline form; a non-crystalline solid phase.

Anaerobe - an organism that does not employ oxygen as a terminal electron acceptor.

Analyte - the compound, ion, or radical of interest in a system that is being examined.

Anion - a negatively charged ion (eg. Cl^-_(aq.)).

Aromatic hydrocarbon - see hydrocarbon.

Asteroid - minor planet; one of several thousand very small members of the Solar System that revolve around the Sun, generally between the orbits of Mars and Jupiter.

Asthenosphere - ultramafic layer of earth lying below the lithosphere; marked by low seismic velocities, suggesting that it is partly molten.

Astronomical unit - the average distance between the Earth and the Sun; 93 million miles, 150 million km, 8.3 light minutes.

Autotroph - an organism that can utilize CO₂ as its sole source of carbon; derives energy from the oxidation of inorganic material (e.g. sulfides) or from photosynthesis; organism that does not feed on pre-formed organics.

Banded iron formation - BIF; alternating layers of iron-rich and iron-poor rocks. Most rocks of this type are older than about 2 billion years old and indicate the presence of oxygen.

Barophile - an organism that thrives at high hydrostatic pressure. A barotolerant organism can grow under high pressure but generally grows better at normal pressure.

Base - a chemical species which can bind protons (Bronsted base); a chemical species which has a 'lone pair' of electrons available for dative bonding (Lewis base).

Bentonite - rock comprised of altered volcanic ash and composed essentially of montmorillonite clay; capable of absorbing 7-10 times its own weight in water.

Biological marker - a chemical substance produced by an organism that serves as a signature for the presence or activity of that organism or group of organisms; indicates the existence of life contemporaneous with the age of the structure in which the biomarker is found.

Biomarker - see biological marker.

Bok globule - see molecular cloud.

Bolide - a large meteor.

Carbonaceous chondrite - a class of meteorites that contain chondrules imbedded in a material with a large percentage (~4%) of carbon.

Carbonyl group - the C=O 'fragment' in organic compounds.

Carbonylation - adding a CO group to a molecule.

Cataclysmic event - event hypothesized to have occurred on the Moon (and on Earth) at 3.9 Ga ago; Ryder coined this term to explain why no impact melts older than 3.9 Ga are found in lunar rocks. Compare with late heavy bombardment, terminal lunar cataclysm.

Cation - a positively charged ion (eg. Na⁺_(aq.)).

Cell - the smallest membrane-enclosed entity of an organism and capable of independent culture.

Chemoautotroph - see chemolithotroph.

Chemolithotroph - a microorganism that obtains its energy from oxidation of inorganics and utilizes CO₂ as its carbon source.

Chemoorganotroph - a microorganism that obtains its energy from oxidation of organic compounds and utilizes carbonaceous substrates (other than CO₂) for growth.

Chiral - an object (e.g. a molecule) having a non-superimposable mirror image; a common analogy uses hands: left hands are non-superimposable on (their mirror image) right hands.

Chirality - when the mirror image of a molecule is not able to superimpose on the original. This property becomes possible in an organic molecular system if the molecule contains one or more asymmetric carbon atoms (i.e. contains at least one C atom that is bonded to four different moieties). The opposite of a chiral molecule is an ACHIRAL molecule, which is superimposable on its mirror image.

Chondrite - an abundant class of stony meteorites characterized by chemical compositions similar to that of the Sun; they may or may not contain chondrules.

Chondrule - round silicate granules that lack volatile element; found in chondritic meteorites (chondrites), they are believed to be primitive Solar System materials.

Chromosome - the site of cellular DNA. The Archaea and Bacteria generally have one circular chromosome. One microorganism (Borellia burgdorferi) is known to have a linear chromosome. The Eukarya are multichromosomal.

Comet - a frozen celestial body, a few kilometers across. It consists of water ice and silicate and carbon dust particles and is very porous and fragile. There are probably billions of comets in the solar system, but most of them orbit the sun in a spherical shell at an average distance of more than a light-year and cannot be observed. Every now and then a gravitational disturbance causes one of these little worlds to be thrown into the inner solar system. When the sun heats its surface, a surrounding cloud of evaporated gas and dust develops; this is known as the coma. Later, solar radiation drives the coma into a plume, which is known as the tail. It always points away from the sun irrespective of the motion of the comet, and can be millions of kilometers long. The tail is sometimes split into two: one containing mostly dust particles and another containing mostly ionized gas. Some comets only pass the sun once, whereas others return to become periodic. The cometary coma and tail contain organic and inorganic molecular compounds like those we find in the interstellar medium; in fact the cometary body probably consists of frozen interstellar material, at least partly.

Cyanobacteria - the bacteria that can grow photosynthetically via oxygenic photophosphorylation. They are believed to have originated in the Archean time and are responsible for forming stromatolites. They are also thought to be largely responsible for the oxygen in Earth's atmosphere.

DNA - deoxyribo-nucleic acid; a nucleotide polymer composed of deoxyribonucleotides: adenine (A), guanine (G), cytosine (C) and thymine (T), joined by phosphodiester bonds. The genome of an organism is composed of DNA, and its genes are encoded by specific sequences in the DNA polymer.

DNA polymerases - enzymes that participate in DNA synthesis.

Dark cloud - see molecular cloud.

Desorption - evaporation of a species (i.e. molecule) from a grain surface.

Diastereomer - a molecule containing more than one chiral C atom (e.g. ribose).

Endonuclease - see nucleases.

Enantiomers - mirror image isomers which are non-superimposable; chiral molecules.

Enzyme - a protein which speeds up (catalyzes) a chemical reaction, accelerating the rate of conversion of a substrate molecule into the desired product. Most enzymes are proteins but a few are made from RNA.

Eukarya - one of the domains of life; composed of organisms that have a membrane-bound nucleus and division of DNA by mitosis. They also have membrane-bound cell organelles like mitochondria, chloroplasts, etc. Examples: fungi, protozoa, plants, animals (including humans). Compare with Prokarya.

Eukaryote - a cell with a true nucleus, mitochondria, and chloroplasts (if the cell is photosynthesizing); have cells with a nucleus that contains the all the DNA. Eukaryotes include plants, animals, fungi, amoeba, algae, and many other organisms. Eukaryotes can be multicellular (many-celled) or single celled. Compare with prokaryote.

Excise - to remove a segment of genetic material by cutting it at each end. The two ends left over after excision are then ligated.

Exonucleases - see nucleases.

Extremophile - a microogansim that lives in extreme conditions

thermophiles

psychrophiles

barophiles

halophiles

alkalophiles

acidophiles

xerophiles

Extrons - sequences in a split gene that code for messenger RNA.

Fischer-Tropsch - a way of making organic molecules (especially long-chain functionalized aliphatic hydrocarbons) from CO and H₂ in the presence of a catalyst; similar processing can occur on a grain surface.

Fusion crust - black layer produced on the surface of a meteorite due to the atmospheric heating of the thin surface layer.

Genetic code - triplet nucleotide sequences (codons) that specify a specific amino acid in a protein chain. For example GAG codes for the amino acid glutamine. Ribosomes read the codons from an RNA copy of a gene, joining amino acids together to make the protein encoded in that gene.

Genome - the complete genetic code in a cell or virus.

Giant molecular cloud/complex - see molecular cloud.

Gravitational collapse - the collapse of a molecular cloud core due to its own self-gravity overwhelming all sources of internal supporting forces (such as thermal pressure, magnetic fields, and turbulent pressure). Collapse of the gas results in the formation of a star.

Halophile - an organism that requires very high salt (NaCl) concentration for growth.

Heterotroph - any organism that derives energy from the oxidation of organic compounds and most or all of its carbon from the assimilation of pre-formed and ingested/absorbed organic compounds; see also chemoorganotroph.

Hot core - Around a newly born hot star there is sometimes a bubble of surrounding dense gas which is the remains of the molecular cloud from which the star was formed. When this gas is heated to temperatures > 100-200 K by the star, the ice layer that covers all interstellar dust particles start to evaporate. This causes unusual chemical reactions to occur in the gas, resulting in a plethora of relatively complex chemical compounds like ethers, alcohols, carboxylic acids, cyanides etc. Most of the chemical compounds known to exist in the interstellar medium today has been seen only in such 'hot cores'. So far there are a dozen or two of these objects known in the Galaxy, but only a handful of them have been well studied.

Hydrocarbons - any compound containing only hydrogen and carbon (eg. methane, CH₄; benzene, C₆H₆)

aromatic

aliphatic

Igneous rock - a rock formed by the cooling of molten material.

Impact sterilization - the idea that the surface (crust) and interior (to a certain depth) of the Earth was heated to such a high temperature by impacting objects that no organism could have survived.

Intercalation - the 'slotting' of a molecule inbetween layers; e.g. flat benzene can intercalate between Watson-Crick base pairs.

Interplanetary dust particle - small cometary or asteroidal dust particles found in Earth's upper atmosphere.

Interstellar dust/grains - small particles in the interstellar medium which can be observed by extinction or reddening (obscuration) of starlight or emission by long-wavelength (mm, sub-mm, infrared) radiation. The sizes of these particles range from 1.e-8cm to 0.1 microns, with the abundances of the smaller radii being much greater than the larger ones. The dust grains are believed to be comprised mostly of carbon and silicates, with cores of elements such as Fe, Mg. Accretion of particles on the surfaces of grains can result in icy mantles. Chemical reactions can take place on the grain surfaces. In fact, it is believed that most of the H2 molecules in interstellar molecules is formed on grain surfaces, which are then ejected from the grains (desorption). Other more complex chemical species are also likely formed on grains.

Interstellar medium - ISM; The gas and dust particles that exists between the stars everywhere in a galaxy. In most places it is extremely tenuous; the average density of atoms is only about 1 per cubic cm. The elemental composition of the gas is roughly 75% H and 25% He (this came out of the Big Bang), plus small amounts of O, C, N, and heavier atoms (all of which have been manufactured in stars). Astronomers like to refer to elements heavier than He as 'metals'.

Interstellar reddening - the preferential scattering or absorption of blue light by small particles, allowing more red light to pass directly through; also known as 'reddening'.

Introns - intervening sequences in a split gene; do not appear in the ultimate RNA product.

Isomers - compounds with the same molecular formula but having different chemical and physical properties (e.g. acetic acid, CH₃CO₂H vs. methyl formate, CH₃O₂CH).

structural isomers

stereoisomers

enantiomers

diastereomers

Isua Region - location of the banded iron formations (BIFs) in Southern Greenland that are at least 3.8 billion years old.

Kerogen - aged and degraded carbon that was once 'handled' by living organisms; e.g. molecules found in coal.

Kuiper belt - a reservoir of perhaps hundreds of thousands of Solar System objects orbiting outside of Neptune; source of short-period comets.

Late heavy bombardment - the sudden and final deposition on the Earth and Moon of the largest planetesimals remaining after formation of the Solar System; Wasserberg coined this term (and hypothesized this event) to account for the lack of old (>4.0 Ga) rocks returned from the Moon during the Apollo missions. Compare with cataclysmic event, terminal lunar cataclysm.

Lattice - a quantum-mechanical model used to describe dust grain surface chemistry; a system of neighboring barriers and walls; a 3-dimensional array.

Ligand - any atom/anion(-)/molecule which is currently (or capable of) bonding to a metal atom/cation to form a 'complex' with that metal atom/ion (eg. water molecule, CO molecule, chloride ion, hydroxide ion, ammonia molecule).

Ligase - an enzyme which is capable of catalyzing the ligation (tying together) of two free ends of a nucleic acid molecule.

Lithosphere - outermost, rigid part of the earth that constitutes the tectonic plates; encompasses the crust, both oceanic and continental, and the uppermost part of the mantle.

Lunar impact glass - droplets of melt produced by energetic cratering events that quenched during ballistic flight.

Mantle - on interstellar dust grains; refers to layers of ice or other molecules which have accumulated around the grain's core.

Mean motion - resonances which result from gravitational relationships among Jupiter, the Sun, and asteroid.

Membrane - the boundary layer that separates intracellular fluid from the surrounding environment.

Metamorphic rock - rock that has been altered by conditions of high temperature and/or pressure.

Meteor - a heated glowing extraterrestrial body that moves through the atmosphere, having not yet hit the ground; friction between it and the atmosphere heats its surface and ionizes atmospheric molecules; a visual phenomenon (fireball, light trail) produced by a meteoroid entering the atmosphere at high velocity; a.k.a. 'shooting star'.

Meteorite - a body from space, believed to be asteroidal in origin, that hits the ground; classified as stony, iron, and stony-iron; generally named after a city or geographical landmark near where they fell or were found.

Meteoroid - a small (i.e. less than 10 m) natural rock or metallic object in space; larger bodies would be designated as asteroids or comets.

Methanogen - Archaea that generate methane in anaerobic environments; oxidize H₂ by transferring electrons from H₂ to CO₂ to form CH₄.

Mid-ocean ridge - ridge on the ocean floor where oceanic crust forms and from which it moves laterally in opposite directions.

Miller - Urey experiment - simulated lightening discharge in gas mixtures to generate metastable organic molecules.

Mineral - a naturally occurring inorganic solid element or compound with a particular chemical composition, or range of compositions, and a characteristic crystal structure.

Moho - abbreviation for Mohorrovicic discontinuity; marks the boundary between the crust and the mantle, marked by a rapid increase in the velocity of seismic waves.

Molecular cloud - a self-gravitating interstellar cloud of gas, comprised mainly of H₂(g) molecules with a 10% abundance by number of He, and other trace compounds (e.g., one of the next most abundant species is CO, with a relative abundance of 0.0001). They also have a dust component, normally about 1% by mass. Molecular clouds contain most of the mass of the galaxy, while occupying only a fraction of the volume. Star formation takes place in these clouds. Masses range from a few to 1.e6 solar masses, with radii from a pc to hundreds of parsecs. Mean densities are typically in the range of 100 to 1000 particles per cubic cm, and mean temperatures are usually low (10 to 50 K), although there can be denser and hotter regions inside these clouds, especially where star formation is taking place. Variants include 'dark clouds', 'Bok globules' (small, featureless clouds), 'giant molecular clouds or complexes (GMC's)' (larger, more massive clouds and active sites of star formation).

Molecular cloud core - a dense region of a molecular cloud, with spatial extent typically in the range 0.1 - 0.3 pc and mean densities > 1.e4 cubic cm. Star formation takes place in a dense molecular cloud core. Also referred to as 'ammonia core', since detection of cores is usually made by observations of NH₃ emission. 'Protostellar core' or 'prestellar core' refers to cores in which star formation or gravitational collapse is taking place. A 'hot core' is a core in which a young star is being born and is heating up the surrounding gas and dust.

Monomer - the single unit from which a polymer is made, usually a simple compound (eg. amino acid, nucleotide).

Monotonic Decline - the gradual deposition of planetesimals onto the Earth and Moon from 4.4 - 3.2 Ga; more objects were deposited early in the formation of the Solar System; deposition is virtually unnoticeable from 3.2 Ga to today.

Montmorillonite - a clay mineral used as a catalyst in the formation of RNA oligomers.

Nt - abbreviation for nucleotide; as in a 50 nt RNA molecule.

Nucleases - A variety of enzymes that hydrolyze nucleic acids (i.e. DNA, RNA). They usually show chemical specificity and are either deoxyribonucleases (DNase) or ribonucleases (RNase).

Exonucleases

Endonucleases

Nucleotide - mononucleotide; phosphate + sugar + base covalently bonded into one molecule.

Oligomer - long chain of monomers of length ~50. Compare to polymers.

Oort Cloud - trillions of incipient objects surrounding the Solar System in a 50,000 AU (radius) sphere; source of long-period comets.

Optical activity - the rotation of the plane of polarization by a chiral molecular species.

Organometallic - a chemical compound in which a metal is bonded to C.

Oxidase - an enzyme which is capable of catalyzing an oxidation reaction.

Oxidation - a common class of chemical reaction, in which one molecule loses electron(s) to another. Oxidation is electron loss; eg. methane (CH₄) can be oxidized to carbon dioxide (CO₂).

Oxidizing agent - a term describing a substance which usually accepts electrons from other compounds; an electron 'grabber'.

Oxidizing atmosphere - uses up free H molecules relatively quickly.

Oxidizing environment - uses up free metal relatively quickly.

Oxygen (O₂) sink - reservoir consisting of a chemical element or compound that combines readily with free oxygen and thus removes it from the atmosphere. During the early part of Precambrian time, sulfur (SO₂, SO₃), iron (Fe₂O₃, Fe₃O₄), and other elements and compounds served as important oxygen sinks, preventing (delaying) oxygen from accumulating in the atmosphere.

Panspermia - the idea that reproductive bodies of living organisms can exist throughout the Universe and develop wherever the environment is favorable; during the evolution of the Universe, conditions favorable to the development of life are postulated to have prevailed at different locations and at different times; 'seeding Earth with life'.

Parsec (pc) - an astronomical unit of measure. The distance required to be away from the solar system such that the orbit of the Earth subtends one second of arc (parsec = parallax second), numerically equal to 3.1e18 cm.

Peptide - linkage; the -CO-NH- grouping found in protein.

pH - a measure of the hydrogen ion concentration; the higher the H⁺ concentration, the lower the pH (more acidic).

Phosphate - PO₄^3-_(aq).

Photoautotroph - an organisms that can utilize light as an energy source and CO₂ as a carbon source.

Photoheterotroph - an organism that utilizes light energy while assimilating organic compounds as a carbon source.

Planetesimal - a small body that has formed in a protostellar/protoplanetary disk, through coagulation and sticking of dust grains in the disk. Accretion and coagulation of planetesimal can lead to the formation of larger, more massive bodies and, ultimately, it is believed, the formation of a protoplanet.

Polymer - chain of monomers of length > 50; longer than an oligomer.

Polymerization - bonding of 2 monomers together, with subsequent additions of more monomers, until a chain of n monomer residues is formed.

Prebiotic conditions - P (pressure), T (temperature), radiation flux, redox state, and composition at ~4 Ga ago; constraints on all of these variables are actively being sought.

Precambrian time - includes nearly 90% of geologic time ranging from 4.6 billion years ago (when the earth formed) to the start of the Cambrian Period about 4 billion years later (i.e. 600 Ma ago).

Prokaryote - a name previously applied to all microorganisms that lacked a nuclear membrane bu now replaced by Archaea and Bacteria; a plasma membrane surrounded by a cell envelope, enclosing an interior that includes DNA, ribosomes, proteins, and other cell components. Compare with eukaryote.

Protein - a chain of amino acids. The sequence of amino acids is specified by a gene. Each protein is made by a ribosome, and it folds up into a specific shape that is determined by the sequence of amino acids. The precise arrangement of amino acids determines the properties of the protein. Some proteins are enzymes, other are structural (e.g. keratin).

Protein chain - amino acids linked together.

Protoplanet - an object that is in the process of becoming a planet.

Protostellar disk/protoplanetary disk - the residual material being accreted onto a protostar/YSO. A disk that has formed during the later stages of protostellar collapse due to conservation of angular momentum in the gas and dust. Material from the disk is accreted onto the star by dissipation of angular momentum, presumably due either to viscous forces in the disk or magnetic braking. Early on the mass in a disk can be comparable to the mass of a star, easily obscuring the central star. As the star evolves, the disks dissipate or evaporate away. Emission from warm dust in a disk can be observed in the mm, sub-mm, and infrared. Planet formation takes place in disk. The solar nebula was the disk that accompanied our young sun.

Protostar - an object in the process of becoming a star. Usually referred to a core or portion of a core in the end stages of collapse or contraction, before the onset of nuclear reactions (or, `burning) takes place.

Psychrophile - a cold-loving organism that can grow optimally at temperatures as low as 12 - 15^oC; some can grow at temperatures below 0^oC.

RNA - ribo-nucleic acid; a polymer composed of ribinucleotides: uracil (U), adenine (A), guanine (G), and cytosine (C), joined by phosphodiester bonds. For a protein to be made from a gene a working copy of the gene is made from RNA. The RNA is read by the ribosome, which 'translates' it into protein. Some RNA molecules are capable of serving as enzymes, catalyzing chemical reactions.

RNA world - A proposed stage early in the evolution of life in which RNA acted as both genetic material and enzyme. RNA thus served both the informational and catalytic. requirements of life.

Racemization - the process occurring when an optically active reactant gives a product showing neither clean retention of configuration nor clean inversion of configuration. i.e. All D (or L) samples eventually become 50:50=D:L.

Reddening - see interstellar reddening.

Redox reaction - a chemical reaction involving changes in the formal oxidation numbers of elements in compounds (reactants vs. products); a chemical reaction involving electron transfer.

Reducing agent - a term describing a substance which normally gives electrons to other compounds.

Reducing atmosphere - containing significant free H₂(g).

Reducing environment - rich in reduced species (e.g. hydrocarbons, ammonia, hydrogen).

Reductase - an enzyme which is capable of catalyzing the chemical reduction of (i.e. 'sticking hydrogen atoms onto') a particular 'substrate' organic molecule.

Reduction - a common class of chemical reaction, in which one molecule gains electron(s) from another. Reduction is electron gain, equivalent to the addition of H atoms onto the C skeleton

Reflux - boiling a solution in such a manner as to allow vaporous materials to condense and fall back into solution.

Refractory - the more robust materials (e.g. silicate, metal, graphite) which make up dust grain cores. Compare with volatile.

Resonance - specific to the asteroid belt; resonances are associated with perturbed motion, involving 2 or more bodies (planetesimals), and are cumulative with time, altering elements of an orbit; originally recognized (Kirkwood, 1987) as regions in the asteroid belt that were devoid of bodies; see also mean motion, secular resonance.

Ribosome - the cellular organelle where protein synthesis occurs; represents a complex assemblage of proteins and RNA molecules. Ribosomes read the genetic code from the working RNA copies of genes, using these to synthesize the protein encoded by the gene. Ribosomes have an RNA core, which is largely responsible for protein production. The RNA core is supported and stabilized by a protein scaffold.

Ribozyme - catalytic RNA molecule; RNA, not protein, is the catalyst.

Rock - an aggregate of interlocking or attached grains, each of which is typically composed of a single crystalline mineral.

Secular resonance - resonance that results from the complex gravitational relationships involving the Sun, Jupiter, Saturn, and the asteroids.

Sedimentary rock - rock formed by the consolidation of loose sediment or by precipitation of crystals from an aqueous solution.

Shock wave - a front marked by an abrupt change in pressure caused by an object moving faster than the speed of sound in the medium through which the object is traveling; at the front, there is a jump in density, temperature, and pressure and chemical reaction rates may increase.

Stereoisomers - isomers that differ in the orientation of the atoms in space; e.g. enantiomers, diastereomers.

Sterilization - see impact sterilization.

Stromatolites - finely-layered fossilized microbial mats (in columns or mounds) that are found in rocks up to 3500 million years (My) old. Their origin is controversial, yet studies of modern stromatolites indicate that they are formed by the activities of cyanobacteria, in shallow marine environments. Stromatolites form the earliest-known reefs (2000 My old), some of which are up to 450 m thick. They are abundant in the Proterozoic (2500 - 900 Ma) but decline sharply in the Late PreCambriam (900 Ma).

Structural isomers - isomers that differ in the order in which atoms are bonded together; e.g. acetic acid and methyl formate.

Subduction - process that takes place in deep-sea trenches, where earth's lithosphere descends into the asthenosphere below; the forcing of denser crust material underneath lighter crust material, causing the crust to melt and mix with new rock from the mantle.

Sulfate-reducing bacteria - fermenting bacteria that obtain energy by converting sulfate compounds into sulfide compounds. These bacteria, which cannot tolerate oxygen, are common in the muds of swamps, ponds, and lagoons.

Terminal lunar cataclysm - a disastrous, suddenly commencing, short-lived cratering episode about 3.9 Ga ago that was supposed to have destroyed the lunar 'genesis rocks', primordial rocks older than ~4.0 Ga; the general phenomenon that the cratering rate on the Moon averaged much higher in the first 0.6 Ga than after 3.9 Ga; also known as 'late heavy bombardment', 'terminal bombardment', 'early intense bombardment', 'early heavy bombardment', 'terminal cataclysm'.

Thermal hopping - movement of atoms between lattice sites (to possibly react) because they possess a higher energy than the barriers of the lattice; enabled on dust grains during shock events.

Thermophile - an organism grows optimally at temperatures above 50^oC, with an upper limit of 75 - 80^oC.

Thiol group - SH (sulphur / hydrogen) grouping in sulfur-containing organic molecules.

Titration - addition of equal moles of acids and bases; acid-base neutralization.

Trapping - occlusion (inclusion) of other molecules (impurities) in layer of frozen amorphous H₂O.

Telomerase - makes the DNA ends of linear chromosomes; made up of an RNA portion and a protein portion, and probably dates back to the RNA world.

Tunneling - atoms (especially H) change lattice sites despite having too little kinetic energy to do so classically.

Ultramafic rock - very dense rock which is very poor in free quartz (SiO₂) and very rich in iron (Fe) and magnesium (Mg). These rocks characterize the earth's mantle.

Ultraviolet flux - ultraviolet (UV; 100 - 4000 Å) photon count per cm².

Volatile - icy materials on interstellar dust grains; these materials evaporate at a lower temperatures than the materials on the core. Compare with refractory.

Watson-Crick base pair - a hydrogen-bonded link between adenine (A) and thymine (T) (or uracil (U)) and between guanine (G) and cytosine (C)

Crick-Watson base pairing in DNA

Crick-Watson base pairing in RNA

xerophile - a microorganism that optimally grows under conditions with low levels of water availability (measured in units of water activity, a_w), usually in environments with high salt (NaCl) or sugar concentrations. Even if water is present in an environment, high concentrations of solutes such as sugar or salt render it unavailable.

young stellar object (YSO) - generic term for a young star which is contracting to the Main Sequence (MS). The energy emission from these objects may be from thermal compression / gravitational contraction as well as from nuclear burning in the. center of the star.

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