AppendixGlossary
Abiotic—Of or relating to nonliving things; independent of life or living organisms.
Accretion—The process by which an astronomical object increases in mass by the gravitational attraction of matter.
Albedo—The fraction of light that is reflected by a surface; commonly used in astronomy to describe the reflective properties of planets, satellites, and asteroids.
Amino acid—Any organic compound containing an amino (–NH2) and a carboxyl (–COOH) group. There are 20 α-amino acids from which proteins are synthesized during ribosomal translation of mRNA.
Aromatic compound—A major class of unsaturated cyclic hydrocarbons characterized by the presence of one or more rings of carbon atoms. The class is typified by benzene, which has a six-carbon ring containing three pairs of alternating single and double bonds.
Aromaticity—A property of the arrangement of chemical bonds in cyclic hydrocarbons which confers enhanced stability to aromatic compounds.
Asteroid—Small, rocky bodies in orbit around the Sun, found mainly between the orbits of Mars and Jupiter.
Astronomical unit (AU)—The mean distance of Earth from the Sun.
Biotic—Of or relating to living things; caused or produced by living organisms.
C-, P-, and D-type asteroids—Classes of asteroids sorted according to their spectral reflectance. Asteroid classes are correlated with position within the asteroid belt, with increasingly red objects farther from the Sun. C-type asteroids have a mid-belt location, and P- or D-types are in the outer belt. C-types are a very good spectral match to carbonaceous chondrites; beyond 2.7 AU (the so-called soot line), asteroids are very carbon-rich (P and D types).
Carbonaceous chondrite—A rare type of stony meteorite that is rich in carbon compounds and is thought to be relatively unaltered since the beginning of the solar system. Its spectrum (and probably also its composition) closely resembles that of the C-type asteroids.
Catalyst—A substance that enhances the rate of reaction by providing a lower-energy alternative pathway.
Centaurs—A family of small solar system bodies found between the orbits of Jupiter and Neptune. Their orbital characteristics indicate that they have not resided in their present locations for very long, leading to the suggestion that they are recently migrated Kuiper belt objects and that further evolution of their orbits might turn them into short-period comets.
Chiral—Describing a molecule configured such that it cannot be superimposed on its mirror image.
Chromatography—A family of techniques for separating components from a mixture. All take advantage of the fact that different substances diffuse through a given medium at different rates. Gas chromatography is a technique for separating gas mixtures, in which the gas is passed through a long column containing a fixed absorbent phase that partitions the gas mixture into its component parts.
Coma—The quasispherical envelope of gas and dust surrounding the nucleus of an active comet, created when the ambient heat causes the sublimation of cometary ices.
Cosmic rays—High-energy charged particles consisting of atomic nuclei, electrons, and protons, which originate from the Sun and from energetic astrophysical processes (e.g., those associated with supernovas).
Desorption—A physical or chemical process by which a substance that has been adsorbed or absorbed by a liquid or solid material is removed from the material.
Diastereoisomers—Stereoisomers that are not mirror images of each other.
Diffuse interstellar bands (DIBs)—Unidentified absorption bands detected mostly in the visible spectrum of reddened O- and B-type stars and observed ubiquitously in space. Recent results point strongly toward a gas-phase molecular origin, but the DIB carriers’ identification remains an extremely puzzling issue.
Enantiomer—Stereoisomers that are mirror images of each other. Enantiomers are optically active and rotate the plane of polarized light.
Exogenous delivery—Delivery of matter to a planetary environment via asteroidal or cometary impact.
Fischer-Tropsch (FT) catalytic process—A method for the synthesis of hydrocarbons and other aliphatic compounds. Typically, a mixture of hydrogen and carbon monoxide is reacted in the presence of an iron or cobalt catalyst to produce methane, synthetic gasoline, and other aliphatic compounds, with water and carbon dioxide as byproducts.
Free radical—A highly reactive chemical species carrying no charge and having a single unpaired electron in an orbital.
Fullerene—Any of various cagelike molecules that constitute the third form of pure carbon (along with the forms diamond and graphite), whose prototype C60 (buckyball) is the roundest molecule that exists. Fullerenes are a class of discrete molecules, soccerball-shaped forms of carbon with extraordinary stability (so named because their configuration suggests the shape of Buckminster Fuller’s famous geodesic dome).
Fumarole—A vent, usually in volcanic regions, from which vapors or gases are released.
Geminate recombination—The reaction with each other of two transient molecular entities produced from a common precursor in solution.
Hale-Bopp (comet)—Correctly known as C/1995 O1 (Hale Bopp), it is the brightest comet to appear in the night sky for many decades. Discovered by Alan Hale and Thomas Bopp on July 22, 1995, it reached perihelion on April 1, 1997, and was visible to the naked eye for many months. Its nucleus appears to be very large, about 40 km across.
Heat pulse—A rapid change in ambient temperature conditions over a wide field of view. The temperature change can be higher or lower than that of the normal, ambient, slowly changing temperature.
Infrared—The portion of the electromagnetic spectrum radiation with wavelengths in the range from 750 nm to 0.1 mm.
Interstellar medium (ISM)—The dust, molecular clouds, and neutral hydrogen that lie between the stars of this galaxy, generally in the plane of the Milky Way, but whose density is highly variable.
IP/Halley (comet)—The most famous periodic comet. Its aphelion is beyond the orbit of Neptune, but it returns to the inner solar system every 76 years. Named after the 17th-century British scientist, Edmond Halley, who first recognized its regular pattern of reappearances. Studied by a fleet of spacecraft during its 1986 apparition, including the European Space Agency’s Giotto.
Isomer—One of two or more substances that have the same chemical composition but differ in structural form.
Kerogens—A family of chemical compounds that make up a portion of the organic matter found in sedimentary rocks. They are insoluble in organic solvents, non-oxidizing acids (HCI and HF), and bases because of their very high molecular weight. Each kerogen molecule is formed by the random combination of numerous monomers. When heated, hydrogen-rich kerogens yield crude oil and hydrogen-poor kerogens yield mainly gas.
Kinetic isotope effect—The effect of a difference in mass between two isotopes of the same element, such as a difference in reaction rate, vapor pressure, or equilibrium constant. The term includes effects on molecular or atomic properties; specific nuclear effects such as radioactivity are excluded.
Kuiper belt—A region of the solar system containing icy planetesimals distributed in a roughly circular disk some 40 to 100 AU from the Sun. Pluto’s orbit is believed to circumscribe the innermost region of the Kuiper belt.
Kuiper belt objects (KBOs)—A general name for the bodies found in the Kuiper belt.
Lacustrine—Of or pertaining to lakes.
Murchison (meteorite)—A carbonaceous chondrite, type II (CM2), suspected to be of cometary origin due to its high water content (12 percent).
Neutrino—One of a family of electrically neutral subatomic particles with little or no mass generated during some radioactive decay processes. Because they interact only weakly with matter, neutrinos are extremely difficult to detect.
Organic—Of or relating to any covalently bonded compound containing carbon atoms.
Outgassing—The ejection of gaseous material from the interior of a planet.
Oxidation/reduction—The change in the oxidation state of atoms or ions due to the “loss” or “gain” of electrons.
Photic zone—The richest and most diverse region in Earth’s ocean, extending downward to the maximum depth of sunlight penetration, approximately 200 m in the open sea. More generally, the region of a planet’s subsurface influenced by solar radiation.
Photolysis—The decomposition of a substance into simpler units as a result of the action of light.
Polar chemical bonds (in organic molecules)—Polarity is induced in chemical bonds when one atom attracts electrons more strongly than the other. A partial separation of charge results. Neither atom bears the full charge of an electron or proton, but one is slightly negative, and the other slightly positive. Such atoms are susceptible to attack in the course of chemical reactions. An atom bearing a partial positive charge is, for example, likely to attract electrons and to participate in the formation of a new chemical bond.
Polycyclic aromatic hydrocarbons (PAHs)—A class of very stable organic molecules made up of only carbon and hydrogen. These molecules are flat, with each carbon having three neighboring atoms, much like graphite. They are a standard product of combustion.
Protein—Any of a group of complex organic compounds, consisting essentially of combinations of amino acids in peptide linkages, that contain carbon, hydrogen, oxygen, nitrogen, and usually sulfur.
Protoplanetary disk—The disk of dust and gas surrounding a star out of which planets form.
Racemic compounds, racemic mixture, racemate—An equimolar mixture of the two enantiomeric isomers of a compound. As a consequence of the equal numbers of levo- and dextro-rotatory molecules present in a racemate, there is no net rotation of the plane of polarized light.
Radiolysis—The breakdown of molecules as a result of exposure to ionizing radiation.
Refractory material—A heat-resistant material that retains its strength at high temperatures, e.g., above the melting point of most metals.
Regolith—The layer of fragmented, incoherent rocky debris on the surface of a planetary body.
Saturated hydrocarbons—Organic molecules containing only single carbon-carbon bonds. As such they cannot incorporate additional atoms into their structure.
SNC meteorites—The family of shergottite, nakhlite, and chassignite stony meteorites believed to have originated on Mars.
Sputtering—A phenomenon occurring when energetic ionized particles impinge on the surface of a solid or liquid target, causing the emission of particles and erosion of the surface of a solid. The sputtered particles from the target can appear as charged or neutral atoms or molecules, atom clusters, or macroscopic chunks of material.
Stereochemistry—The study of how the spatial arrangement of atoms in a compound influences its structural properties.
Stereoisomers—Isomers that differ only in the arrangement of their atoms in space.
Strecker synthesis—The synthesis of α-amino acids by the reaction of an aldehyde or ketone with a mixture of ammonium chloride and sodium cyanide followed by acid hydrolysis of the amino nitriles formed.
Tagish Lake (meteorite)—A unique carbonaceous chondrite collected very soon after falling to Earth in a remote part of northwestern Canada in January 2001.
Tholin—A term used in planetary science to refer generally to organic heteropolymers.
T-Tauri phase—The early phase in the life of a star occurring soon after it has established hydrogen fusion reactions in its core. Such stars are characterized by vigorous surface activity (e.g., flares, eruptions), strong stellar winds, and irregular brightness variations.
Unsaturated hydrocarbons—Organic molecules containing double or triple bonds between adjacent carbon atoms, creating a possibility for further reactions to introduce additional atoms.
