Ice shelves: Ice platforms generated by glaciers whose ice progresses from Antarctica land over ocean. Ross shelf is about 1000km wide and extents over 600km into the ocean. They are the source of very large tabular icebergs.

Ices (astronomy): Solid form (crystalline or amorphous) of volatile molecules like water, carbon dioxide or ammonia.

IDP: Interplanetary dust particle.

Igneous rocks: Magmatic rocks = due to magma crystallization.

Impactite: Heterogeneous breccia generated at depth by the impact of a large-size stellar body (> 10 000tons). It is made up of fragments of the rock substrate included in a vitreous matrix.

Inclination (1) (astronomy): Angle between the orbital plane of a solar object and the ecliptic plane in degrees (always lower than 90°).

Inclination (2) (astronomy): Angle between the orbital plane of an interstellar object and the "sky plane" (plane perpendicular to the "line of sight", i.e. the straight line joining the observer and the stellar object).

Indels: Acronym for insertions/deletions. Phylogenetic analysis on several DNA or protein sequences requires sequences with same length. During the process of alignment, alignment gaps (indels) must be introduced in sequences that have undergone deletions or insertions.

Interplanetary dust: Small grains left behind by asteroids and comets, and dispersed in a cloud including the whole Solar System.

Interstellar cloud: Cloud of gas (98%) and dust (2%). The gas is mainly H (diffuse cloud) or H2 (molecular cloud). Molecular clouds are called dense clouds (n(H2) > 103 molecules cm~3) or dark clouds if dense and cold (10-20K).

Intertidal environment: Environment between high and low tide. Also called tide range.

Intron: Noncoding sequence of nucleotides that separates exons. Introns are removed during the maturation processes of the three types of RNA by splicing.

Iodine (I): Halogen element. As iodine is incorporated in living organisms, including marine organisms, it provides information on interactions between living world and marine sediments.

Ion—molecule (reaction): Reaction between two gaseous reactants and initiated by ionizing cosmic rays, X-rays or UV radiation. They are important in interstellar clouds and in planetary ionospheres.

Iridium (Ir): Element belonging to platinum element family. Its concentration in Earth crust is extremely low. A local Ir enrichment as at the Cretaceous-Cainozoic (K/T) boundary is interpreted as a strong argument in favour of meteoritic impact or of deep originated volcanic activity.

Isochron (geology): Rectangular diagram plotting isotopic ratio of a disintegration system (abscissa = parental isotope; ordinate = daughter isotope) (e.g., 87Sr/86Sr versus 87Rb/86Sr). In this diagram, cogenetic rocks of the same age plot along a straight line whose slope is proportional to age. This method of age determination is widely used in geology.

Isocyanhydric acid: H-N=C.

Isocyanic acid: HN=C=O.

Isoleucine (Ile): Proteinic amino acid containing six carbon atoms and described as hydrophobic. Ile is considered as one of the prebiotic AA.

Isotopic ratio: Concentration ratio of two isotopes or concentration ratio of two isotopomers of a molecule (like H2O and D2O). Isotopic ratio can provide information on the age of a sample (when used in isochron calculation) as well as on its origin and source.

Isovaline: Hydrophobic nonproteinaceous amino acid. This constitutional isomer of valine contains five C atoms.

Isua: Region of Greenland where the oldest sediments so far recognized 3.865Ga (gneiss) are exposed. They contain carbon whose origin could be biogenic. (See also Akilia.)

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