C

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C (alpha): Carbon atom linked to a chemical function we are interested in. More specifically, the carbon atom directly linked to the carboxylic function in amino acids. In alpha amino acids, the amino group is linked to the alpha carbon. The Greek letters alpha, beta, gamma . . . are used to describe carbon atoms separated from the function by one, two, three ... other carbon atoms.

CAI (Ca-Al Inclusions): Ca-and Al-rich inclusions abundant in some chon-drite meteorites.

Caldera: Circular km-sized structure due to the collapse of superficial formation induced by the emptying of an underlying magma chamber.

Carbonaceous chondrite: Chondrite with high carbon content. The famous Murchison meteorite is a carbonaceous chondrite.

Carbonate: (CO3)2~-bearing mineral e.g., Calcite = CaCO3; Dolomite = MgCa(CO3)2.

Carbonation: In Ca-, Mg-, K-, Na- and Fe-bearing minerals, chemical reaction of alteration resulting in the formation of carbonates.

Carbonyl: -(CO)-, this chemical function is found in carboxylic acids, ketones, aldehydes, amides in the peptide bond and many other organic molecules.

Carboxylic acid: Organic molecule containing a COOH group.

C-asteroid: Asteroid containing carbon. C-asteroids are the parent-bodies of carbonaceous chondrites.

Catalysis: Chemical process such that a substance (catalyst) increases the reaction rate by changing reaction pathway but without being chemically modified during reaction. Enzymes are very efficient catalysts able to increase reaction rates by several orders of magnitude and also able to limit the number of secondary products and therefore to increase reaction selectivity.

CCD (charge-coupled device): Silicon photoelectronic imaging device containing numerous photosensors (often at least 1000 x 1000). The most used astronomic detector in the visible wavelength domain.

Cell: Complex system surrounded by a semipermeable membrane that can be considered as the basis unit of all living organisms.

Cenozoic or Caenozoic( Era): Period of time (Era) ranging from 65 Ma to today, it is also called Tertiary Era but in addition it also includes the Quaternary Era.

Chemical sediment: Sediment formed by direct precipitation of ions dissolved in water.

Chemolithoautotroph: Chemotroph that uses CO2 as only source of carbon.

Chemolitotroph: Chemotroph that takes its energy from the oxidation of inorganic molecules. On Earth, the first living organisms could have been chemoli-totrophs.

Chemoorganotroph: Chemotroph that takes its energy from oxidation of organic molecules.

Chemotroph: Organism that takes its free energy from the oxidation of chemicals.

Chicxulub: Large (180km in diameter) impact crater, located in the Gulf of Mexico. It is assumed to be the result of the collision of a big (10km in diameter) meteorite, 65Ma ago. This impact is considered as the cause of the important biological crisis at the Cretaceous-Tertiary boundary that led to mass extinction of thousands of living species, such as ammonites, dinosaurs, etc.

Chirality: Property of an object (and therefore of a molecule) to be different from its mirror image in a plane mirror. A hand is an example of a chiral object (in Greek: cheir means hand). Any object is chiral or achiral and if it is chiral, it can exist as two enantiomorphous forms (called enantiomers for molecules).

Chlorophylls: Pigments of major importance in the oxygenic photosynthesis. The chemical structure of all chlorophylls is based on a porphyrin ring system chelating an Mg2+ cation. Chlorophylls are found in higher plants, algae and some micro-organisms.

Chloroplast: Subcellular structure that plays a fundamental role in photosynthesis in all photosynthetic eukaryotes. Chloroplasts have more probably an endosymbiotic origin.

Chondre: Small spherical aggregate of radiated silicate minerals (typically 1mm in diameter) that is frequent in stony meteorites and especially in chondrites. Olivine is the main component of chondres.

Chondrite: Undifferentiated stony meteorite unmelted and frequently considered as a very primitive object. Chondrites have the same composition as the Sun except for volatile elements.

Chromatography: Preparative or analytical chromatography: experimental method based on the properties of all molecules to be absorbed more or less selectively by a solid phase (the stationary phase) and therefore to migrate at different rates when they are "pushed away" by a mobile phase that can be a ga s (gas chromatography or GC) or a liquid (LC or HPLC for high-performance LC).

Chromosome: Subcellular structure containing most of the genetic material of the cell. Prokaryotic cells generally contain only one chromosome made of a circular DNA molecule, while eukaryotic cells generally have several chromosomes, each of them containing a linear DNA molecule.

CIP (for Cahn-Ingold-Prelog): General nomenclature used in chemistry to describe chiral molecules and more generally stereoisomers. Following the International Union of Pure and Applied Chemistry (IUPAC), CIP nomenclature must replace all other nomenclatures including D, L Fisher nomenclature, except for amino acids and sugars for which the D/L nomenclature is still accepted.

Circular dichroism: The absorption coefficients of right and left circularly polarized light are different if the absorbing medium is chiral. By plotting the difference between the absorption coefficients as a function of the light wavelength, the curve obtained corresponds to the circular dichroism curve of the medium.

Circumstellar disk: Disk of gas and dust particles around a star.

Class [0, I, II, III] (astronomy): Classification of young stellar objects based on their electromagnetic emission in the microwave and infrared domains. It consists of an evolution sequence from the protostars (0 and I) to the T-Tauri stars (III).

Clast: Fragment of mineral or rock included in another rock.

Clay: Mineral family. Phyllosilicate (water-bearing sheet silicate), e.g., kaolinite, illite, smectite, montmorillonite.

CM matter: Pristine material of the Solar System, analogue to constitutive matter of CM carbonaceous meteorites (M = Mighei) and very abundant in micrometeorites.

Coacervat (droplet): Protein and polysaccharides containing emulsion. According to Oparin, model of protocells.

Codon: Triplet of nucleotides in a mRNA molecule that corresponds to a specific amino acid of a protein synthesized in a ribosome or that corresponds to a punctuation signal in protein synthesis.

Coenzyme: Small molecule that binds with an enzyme and that is necessary to its activity. ATP, CoA, NADH, biotine are examples of coenzymes. Coenzymes are often derived from vitamins.

Collapse (astronomy): Process that describes the formation of stars from dense cores. The process seems to be fast: less than 105 years.

Coma: Broadly spherical cloud of dust particles and gaseous molecules, atoms and ions surrounding cometary nucleus. It appears when a nucleus becomes active, generally when approaching the Sun.

Combustion: Used to describe any exothermal chemical reaction involving dioxygen and organic reactants. Sometimes used in astrophysics to describe the thermonuclear fusion reactions taking place in the stars and that, obviously, are also exothermal.

Comet: Small body of the Solar System with an average size of 1 to 100km, travelling generally on a strongly elliptic orbit around the Sun. Comets are constituted of ice and dust and are considered as the most primitive objects of the Solar System.

Cometary nucleus: Solid part of a comet (1-100km diameter), made of ice (H2O, CH3OH, CO...) and of dust.

Cometary tail: Part of an active comet; three different cometary tails are known depending on their composition (dust particles and molecules, neutral sodium atoms, ions).

Complex molecule (astronomy): Molecule containing more than 3atoms.

Complexation: Chemical term used to describe the noncovalent interaction of a molecule or, more frequently, an inorganic ion with other molecules (called ligands) to give a supramolecular system described as a complex.

Condensation: Chemical reaction involving two molecules and leading to the formation of a new chemical bond between the two subunits but also to the elimination of a small molecule (generally a water molecule). The formation of bonds between the subunits of many biochemical polymers are condensation reactions (examples: polynucleotides, polypeptides, polysaccharides).

Configuration: Term used in stereochemistry. Stereoisomers (except if they are conformers) have different configurations. As an example, butene can be cis or trans and it corresponds to two different configurations. In the case of chi-ral molecules like the D- and L-valine, the stereoisomers are enantiomers and they have "opposite" configurations. It is important to make a clear distinction between the relative configurations of two enantiomers and the absolute configuration of each of them.

Conglomerate (chemistry): In the restricted case of crystallized chiral molecules, a crystalline state such that all molecules of the same chirality (homochi-ral) crystallize together giving a mixture of crystal that, themselves, are of opposite chirality.

Conglomerate (geology): Detrital sedimentary rock consisting of an accumulation of rounded fragments in a sedimentary matrix.

Continental margin: Submarine part of a continent making the boundary with the oceanic crust.

Continents: Emerged and associated shallow depth (< 300m) parts of the Earth's surface. Their average composition is that of a granitoid.

Continuum (astronomy): in emission (or absorption) spectroscopy, emission (or absorption) background of a spectrum extending over a large frequency domain. Frequently, lines are superimposed on the continuous spectrum. Blackbody radiation corresponds to a continuous spectrum.

Cool Early Earth: Period of time between Earth accretion (4.55Ga) and the late heavy bombardment (4.0-3.8Ga). This model considers that period of early Earth as quiescent with respect to meteoritic impacts, thus being potentially favourable for life development.

Core (geology): Central shell of a planet. On Earth the core mainly consists of iron with minor amounts of nickel and some traces of sulfur; it represents 16% of the volume but 33% of the mass of the planet. It is subdivided in a solid inner core from 5155 to 6378km depth and a liquid outer core from 2891 to 5155km depth.

COROT: French project for the search of extrasolar planets based on a 25-cm telescope and able to detect planets having a diameter equal to twice the Earth diameter and located at 0.5AU from its star.

Cosmic rays: Highly accelerated ions coming from the Sun (solar wind, essentially protons) or coming from other and extrasolar sources (galactic cosmic rays).

Covalent bonds: Interatomic bonds such that two atoms share one, two or three electron pairs leading to the formation of a single, a double or a triple bond. A covalent bond is described as a polarized bond if the two bonded atoms have different electronegativities.

CPT (theorem): general theorem of physics that assumes that physical laws are unchanged if, simultaneously, space is reversed (parity operation), time is reversed (the sense of motion is reversed) and matter is replaced by antimatter. In many cases, but not all, the CP theorem alone is valid.

Craton: Huge block of old (often Precambrian in age) and very stable continental crust (see also shield).

Crust (geology): The more superficial shell of the solid Earth. Its lower limit with the mantle is called Morohovicic (Moho) discontinuity. Together with the rigid part of the upper mantle it forms the lithosphere. Two main crusts exist 1) oceanic crust, basaltic in composition and about 7 km thick, it constitutes the ocean floor; 2) continental crust is granitic in composition, with a thickness ranging between 30 and 80km, it constitutes the continents.

Cryosphere: Part of the Earth surface made of ice.

Cumulate: Igneous rock generated by accumulation of crystals extracted from magma.

Cyanamid: NH2-CO-CN.

Cyanhydric acid: HCN, hydrogen cyanide. Triatomic molecule that during prebiotic period could have played the role of starting material for purine synthesis.

Cyanoacetylene or better cyanoethyne: H-CC-CN.

Cyanobacteria: Micro-organism belonging to the Bacteria domain and able to perform oxygenic photosynthesis. In the past, these micro-organisms were improperly called "blue algae". The cyanobacteria could be the ancestors of chloroplasts.

Cyanogen: C2N2.

Cysteine (Cys): Protinic amino acid containing three C atoms and a -SH group in its side chain. In a proteinic chain, two cysteine residues can be linked together by a -S-S- bond (disulfide bond) often used to stabilize protein conformation. Two cysteines linked together by a disulfide bond is a cystine molecule.

Cytidine: The ribonucleoside of cytosine. The corresponding deoxyribonucleo-side is called deoxycytidine.

Cytochrome c: One particular example of the large cytochrome family. Cy-tochrome c is a protein involved in the electron transfers associated to aerobic respiration. In the eukaryotic cells, cytochrome c is localized in the mitochondrias.

Cytoplasm: Whole content of a cell (protoplasm) except the nucleus whose content is called nucleoplasm.

Cytoplasmic membrane: also called plasmatic membrane or cell membrane. Cytosine (C): One of the nucleic bases of pyrimidine type.

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