Hadean (Aeon): Period of time (Aeon) ranging from 4.55Ga (Earth formation) to 4.0Ga (oldest known rock: Acasta gneisses). Hadean aeon belongs to Precambrian.

Harzburgite: Peridotite made up of olivine and orthopyroxene. It generally corresponds to residual mantle after lherzolite melting and extraction of basaltic magma.

HD 209458b: First exoplanet, whose previous detection by radial velocimetry method has been confirmed by the observation of a transit in front of its parent star (HD 209458).

Heavy element (astronomy): Any element other than hydrogen and helium.

Helium: Rare gas whose 3He isotope is used in geology as a marker of a recent degassing process from the deep mantle.

Hertzsprung—Russel diagram (HR diagram): In astronomy, a two-dimensional diagram with star temperature (or spectral type) as abscissa and star luminosity (or absolute magnitude) as coordinate. Temperatures decrease from left to right and the spectral types sequence is OBAFGKM.

Heterocycle: Cyclic organic molecule containing heteroatoms (i.e. atoms other than C) as constituents of the cyclic structure.

Heterotrophous: Organism that uses reduced organic molecules as principal carbon source for its biosynthesis. Nowadays, these reduced organic molecules are generally produced by other organisms. At the beginning of life, these reduced molecules were probably found in the environment.

Histidine (His): Proteinic amino acid containing an imidazole group in its side chain. Histidine residues (hystidyl) are frequently found in active sites of enzymes.

HMT: (C6H12N4); hexamethylenetetramine, could be a minor component of a comet nucleus.

Homeostasis: Property of a living organism to maintain unchanged some of its physicochemical characteristics even in the presence of a change in the environment. Homeostasis requires autoregulation.

Homoacetogens: Micro-organisms of the Bacteria domain producing acetate from H2 and CO2.

Homochirality: Of the same chirality. All proteinic amino acids are L, while ribose in RNA or ATP is always D. The origin of homochirality for the large majority of the chiral constituents of organisms remains an active research subject.

Homology (biology): Two structures in two different species are homologous (and therefore comparable) irrespective of their forms, if they are connected in the same way to identical structures.

Hornblende: Mineral. Inosilicate (double-chain silicate). [Nao-1Ca2 (FĂ©++,Mg)s_5(Al, Fe+++)o-2 Sig_eAlo-2O22 (OH,F)2]. It belongs to the amphibole group.

Hot Jupiter: Jupiter massive-like exoplanet, orbiting close to a star. Most of the extrasolar planets so far discovered belong to this type.

Hot spot: see mantle plume.

HPLC (high-performance liquid chromatography): Very efficient liquidphase chromatography performed under high pressure up to 100-400bars.

Hydrocarbons: Molecules containing only C and H atoms. If the hydrocarbon contains an aromatic system, it is called aromatic. If the hydrocarbon contains only tetracoordinated C atoms, the hydrocarbon is called aliphatic. Some hydrocarbons result from the polymerization of isoprene (2-methylbutene); they are called isoprenoid hydrocarbons. Latex contains isoprenoid hydrocarbons.

Hydrogen bond: Intermolecular but sometimes intramolecular low energy bond (about 20kJ/mol) involving generally an H atom linked to an electronegative atom like O, N, S and an atom bearing nonbonding electron pairs such as O or N. The H bond implies an H donor and an H acceptor.

Hydrogen cyanide: H-CN.

Hydrogen sulfide: H2S.

Hydrolysis: Cleavage of a molecule due to reaction with H2O. Hydrothermal vent: See black smoker.

Hydroxy acid: Carboxylic acid containing also an alcohol function. Glycolic acid is the simplest hydroxy acid.

Hydroxyl (group): -OH or alcohol group.

Hypoxanthine (6-hydroxypurine): Purine base and biological precursor of adenine and guanine.

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