Glossary Ii

ICE - Used by planetary scientists to refer to H2O, CH4, and NH3, which usually occur as solids in the outer solar system.

IDEAL GAS - Gas in which the mutual interactions of the gas particles are negligible, except for their momentary collisions. An ideal gas obeys the ideal gas law where P is pressure, n the number of gas particles per unit volume, T is temperature, and k is Boltzmann’s constant:

This may also be written:

where V = volume and R = ideal gas constant. Nearly all real gases obey the ideal gas law to very high temperatures and pressures, even those found in the interiors of stars.

IDEAL GAS CONSTANT - Constant that appears in the ideal gas equation; it is equal to 8.314 34 J/K/mol.

IGNEOUS ROCK - One of the three basic types of rock; formed by the solidification of magma.

ILMENITE- Ti-Fe oxide, TiFeO3, found in achondrites, lunar mare basalts, and shergottites.

IMPACT BASIN - Very large impact structure consisting of one or more concentric rings of peaks. Impact basins are proportionally shallower relative to their diameter compared with simple and complex craters. Basins are subdivided into central peak, peak ring and multi-ring basins. Mare Orientale on the Moon (photograph) is perhaps the archetypal multi-ring impact basin. Orientale contains four concentric rings with the inner ring bordering the mare basalt fill. The outer ring, known as the Cordillera Mountains, has a diameter of 920 km. The main two internal rings form the Rook and Inner Rook Mountains. The mountains in the rings rise up to 2–5 km above the surrounding plain.

Mare Orientale on the Moon. Cropped from image source:

IMPACT BRECCIA - Rock consisting of broken fragments of rock (clasts) cemented by a fine-grained matrix formed from regolith during subsequent impacts.

NWA-5207. Image source:

IMPACT CRATER - Crater formed by high-speed impact of a meteoroid, asteroid, or comet on a solid surface. Craters are a common feature on most moons (an exception is Io), asteroids, and rocky planets, and range in size from a few cm to over 1,000 km across. There is a general morphological progression from large to small craters: large craters often have several rings and smooth floors; intermediate complex craters tend to have a central peak (formed by melting and rebounding of the crust) and smooth floors; small simple craters have a rough bowl-shaped floor. Craters have been classified into three categories: simple craters, complex craters, and impact basins.

Because impact craters degrade at different rates depending on their environment, they are valuable indicators of the age of a surface and the extent to which resurfacing has taken place. On Earth, for example, craters are rapidly degraded and destroyed by weathering processes; ~120 are known, very few older than 1 Ga. Mercury, which lacks an atmosphere and is geologically inert, has a landscape peppered with craters dating to over 4 Ga.

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IMPACT MELT ROCK - Rock that has been made temporarily molten as a result of the energy released by the impact of a large colliding body. Impact melts include small particles, known as "impact melt spherules" that are splashed out of the impact crater, and larger pools and sheets of melt that collect in low areas within the crater. Melts are composed predominantly of the target rocks, but can contain a small but measurable amount of the impactor.

IMPACTITE - Slag-like glassy object found on surface of the Earth formed from rock melted by the impact of a meteor. This word is also applied to rocks that have been affected by impact (impact breccia, suevite, etc.) The backscattered electron photograph below shows the impactite’s complex internal structure.

IMPURITY - Substance incorporated into a semiconductor material that provides free electrons (n-type impurity) or holes (p-type impurity).

IN PHASE - Situation with waves when peaks and troughs of two waves directly match each other. When in phase waves of the same amplitude collide, the result is a new wave with twice the amplitude of the original two. This kind of behavior is often called constructive interference.

INCLINATION - Angle between the plane of an object's orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.

INCLUSION - Fragment of foreign material enclosed within the primary matrix of a rock or meteorite.

INDEX OF REFRACTION (n) - Ratio of the speed of light in a vacuum to the speed of light through a material:

where, c = speed of light in vacuum and v = speed of light through a material. The index of refraction varies depending upon the wavelength of interest.

INDUCED RADIOACTIVITY - Radioactivity produced in materials, especially metals, exposed to high-energy photons or neutrons.

INERT GASES - Noble gases.

INFLATIONARY SCENARIO - Modification of the Big Bang model in which a large cosmological constant exists temporarily early in the history of the Big Bang, leading to a rapid accelerating expansion of the Universe, which is then followed by the normal Big Bang model with a decelerating expansion.

INITIAL MASS FUNCTION - Distribution of masses created by the process of star formation. This empirical function specifies how many stars of a given mass there should be in a population of stars.

INNER-SHELL IONIZATION - Removal of an inner shell electron of an atom by high-energy particle bombardment. This is not a very efficient process: only about 0.1% of the incident electrons will produce K-shell vacancies in an atom. An atom remains ionized for a very short time (~10-14 second). Outer-shell electrons fall in to fill the resulting vacancy in a process of self-neutralization, yielding characteristicx-rays. The energy required to produce inner-shell ionization is termed the excitation potential or critical ionization potential (Ec).

INSTABILITY STRIP - Nearly vertical region on the Hertzsprung-Russell diagram occupied by pulsating variable stars (including RR Lyrae stars and Cepheids).

Instability strip

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INSULATOR - Material with very high resistivity (low conductivity) often used to prevent contact between conductors. The poor conductivity results from a completely full valance band. An insulator has no charge carriers at absolute zero and very few charge carriers at room temperature.

INTENSIVE VARIABLES - Parameters describing a system that do not depend on its size. Examples include temperature, pressure, and density.

INTERCLOUD GAS - Fraction of the gas in the interstellar medium that is not dense enough to form interstellar gas clouds. Intercloud gas is spread over most (~98%) of interstellar space and has 2 main components: the warm intercloud medium, which makes up most of the intercloud gas, and pockets of hot coronal gas.

The warm intercloud medium forms the bulk of the intercloud gas; it has temperatures of ~8,000 K and densities of 0.01-1 atoms/cm3. The H in the warm intercloud medium is both neutral and ionized. Ionization is produced by high-energy UV photons produced by stars that travel through interstellar space until they are absorbed. The ionized half of the warm intercloud medium can be observed as low surface brightness Ha emission. Approximately 90% of Galactic H+ occurs in the warm intercloud medium; the remaining 10 % is in HII regions. Ionising photons will eventually be used up in regions with a lot of warm intercloud gas, allowing the remaining H to remain neutral. These areas of neutral hydrogen are generally protected from UV photons by surrounding regions of ionized warm intercloud medium.

Hot coronal gas forms pockets with very low densities (<0.01 atoms/cm3) and temperatures comparable to the 106 K gas in the Sun's corona. This gas is probably heated by shock waves from supernova explosions, which heat the gas and carve out cavities in the surrounding interstellar medium. It appears that our own Solar System is located inside one of these bubbles of hot coronal gas. The LocalBubble is ~650 lightyears across and has a density of ~0.0065 atoms/cm3. It formed from the explosion of a supernova 100,000 years ago. Hot coronal gas can only be observed from space-based observatories since both the X-ray emission given off by the hot gas, and the signature interstellar absorption lines of O5+ and N4+ are blocked by the Earth's atmosphere.

INTERCRATER PLAINS - Regions on the surface of Mercury that do not show extensive cratering, but are relatively smooth.

INTERPLANETARY DUST PARTICLES (IDPs) - Extremely small (~10 μm in diameter) particles found on Earth (or collected during high altitude flights) that are probably from outer space. Their small size poses a problem for most common chemical and petrographic analytical techniques and thus research into IDPs is marked by the application of new analytical procedures and technologies. Their chemical compositions strongly suggest extraterrestrial origin. IPDs have high He abundances (a major component of the solar wind) and high nuclear tracks densities, produced as heavy atoms in solar flares penetrate mineral grains. Some IDPs match known meteorite classes, but most differ in crystal chemistry and petrography. This has led to the suggestion that IDPs may represent material even more primitive than meteorites, most of which appear to have been subject to alteration on asteroidal parent bodies. The most likely sources of IDPs are comets or asteroids. It has, however, been suggested that at least some IDPs come from other solar systems.

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INTERSTELLAR DUST - Grains of carbon and silicate ~0.1-1.0 mm in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction and scatters incident starlight, particularly blue light (which has a wavelength comparable to the dust grain's size), causing reddening. Cooling of interstellar gas and dust clouds occurs by infrared emission from the dust.

INTERSTELLAR MEDIUM (ISM) - Material between the stars, consisting of gas, dust and cosmic rays (high energy charged particles moving at nearly the speed of light). It comprises ~10% of visible matter in the disk of our Galaxy (Milky Way). Until recently it was generally assumed that silicates in the ISM were amorphous, but the Infrared Space Observatory (ISO) has discovered crystalline silicates. Crystalline silicates appear to be ubiquitous (except for the diffuse ISM) and sometimes even in very large quantities (>50 % of the small dust particles).

INTERSTITIAL - Term applied to ions or atoms occupying sites between lattice points.

INTERSTITIAL IMPURITY - Atom or ions of a species not normally found in a crystal, located between normal atomic sites.

INTERSTITIAL SUBSTITUTION - Substitution in which interstitial ions are added or removed. This is most common in cyclosilicate minerals, which have channel-like structures (e.g., beryl or zeolites) or in clay minerals, which can accommodate ions between their silicate sheets. The charge of interstitial ions is often balanced by substitutions in the crystal lattice. For example, for interstitial ions in the channels of beryl (R):

INTRINSIC SEMICONDUCTOR - Semiconductor comprised of a single type of molecule, with no impurities. Intrinsic, or pure, semiconductors are typically poor conductors of electricity. In an intrinsic semiconductor, each conduction electron in the conduction band leaves behind a hole in the valence band that also contributes to conduction. Si metal is an example of an intrinsic semiconductor.

INTRUSIVE - Refers to igneous rocks that crystallized underground.

INVERSION - Symmetry operation in which each point of an object is converted to an equivalent point by projecting through a common center (center of symmetry) and extending an equal distance beyond this center. If the center of symmetry is at the origin of the coordinates, every point (x, y, z) becomes (-x ,-y, -z). It converts an object or a structure into one of opposite "handedness," related to the first as is any object and its mirror image.

ION - Atom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more.

IONIC BONDING - Chemical bond formed by electrostatic attraction resulting when one atom “donates” valence electron(s) to another atom, resulting in filled energy shells for both atoms involved in the interaction. This is most common in atoms with differences in electronegativity >2. The electrons originally from the donor atom move to the vicinity of the other “acceptor” atom, binding the atoms together. The bonding of MgO is shown below.

Very few materials have bonds that at truly ionic; most ionic bonds are, at least, partially covalent. The fraction of ionic character of a bond, f, can be estimated from the electronegativities of the two atoms.

Physically one can understand the fraction of ionic character as the fraction of time spent as an ionic bond during resonance between ionic and covalent bonds.

IONIC RADII - Radius of an atom in its ionized state. Ionic radii depend on two factors: the degree of ionization and the type and number of neighboring atoms (coordination). The radius decreases as electrons are lost and increases as electrons are gained.
The length of an ionic bond reflects a balance between the electrostatic Coulomb attraction of the unlike charges and the mutual Born repulsion of the positively charged nuclei.

The effect of surrounding a cation with anions is to expand its radius. For example, Na+ with 4 neighboring oxygen atoms has a radius of 99 pm, but when surrounded by 12 oxygen atoms its radius is 139 pm.

Cation radii are derived by measuring the bond length (d) and subtracting the anion radius, R-. Since main ionic compounds are oxides, an experimentally determined value of the radius of O2- is used (~130 pm).

IONIZATION - Process whereby atoms lose one or more electrons to become cations. Ionization occurs by ionizing radiation or if an atom suffers a sufficiently violent collision. The “ionization potential” is the minimum amount of energy needed to remove an electron to infinity from the ground state. If the electron has already been excited to a higher level, less energy is needed to remove it. Where an atom has two or more electrons, the ionization potential for the second and subsequent electrons is greater than for the first electron.

Astronomers identify an atom that has lost a single electron by the Roman numeral II (the neutral atom, by I); whereas, cosmoschemists use the superscript +. For example, neutral hydrogen is denoted HI and ionized hydrogen by HII or H+. Doubly ionized He (helium that has lost both electrons) is denoted by He III or He2+. Very high degrees of ionization occur in some astrophysical settings (e.g., very high-temperature gases).

Ions that retain at least one bound electron can absorb or emit radiation, so producing spectral lines (emission and absorption lines) that differ in wavelength from those produced by neutral atoms. Photons are also emitted when an ion captures, or recaptures, an electron (this process is called “recombination”).

IONIZATION ENERGY - Amount of energy required to remove the highest-energy electron from an isolated neutral atom in the gaseous state. There is an obvious periodicity to the data, with maximum values for the noble gas elements and minimum values for the alkali metals. All values are positive, indicating that energy is always required to remove an electron from an atom.

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IONIZING RADIATION - Radiation with sufficient energy to eject electrons from electrically neutral atoms, leaving behind an ion. There are four basic types of ionizing radiation: α particles (4He nuclei); β particles (electrons); neutrons; and x-rays or γ-rays (high frequency electromagnetic waves). Neutrons are not themselves ionizing but their collisions with nuclei lead to the ejection of other charged particles that do cause ionization.

IONOSPHERE - Region of charged particles in a planet's upper atmosphere; the Earth's ionosphere is at an altitude of ~40-400 km.

IRON-WUSTITE BUFFER - Reference buffer to which the oxygen fugacity of a system may be compared:

IRON METEORITE - Meteorite composed mainly of iron (Fe) and nickel (Ni) in the form of two alloys, kamacite and taenite. Due to their metallic makeup and extraordinary weight, iron meteorites are easily distinguished from ordinary rocks. Also, because they rarely break up in the air and suffer much less from the effects of ablation during their passage through the atmosphere, they are usually much larger than stony or stony-iron meteorites. All known iron meteorites together have a mass of more than 500 tons, which is ~89% of the entire mass of all known meteorites. Yet they are comparatively rare, accounting for just 5.7% of witnessed falls.

There are two ways of classifying iron meteorites. The older, structural method is based on characteristic crystalline features that show up when the meteorites are sectioned, etched, and polished. This results in three subdivisions: hexahedrites (4–6 wt. % Ni), octahedrites (the commonest type: 6-12 wt. % Ni), and ataxites (> 12 wt. % Ni).

Structural ClassSymbolKamacite (mm)Ni (%)Related Groups
HexahedritesH>504.5 - 6.5IIAB, IIG
Coarsest octahedritesOgg3.3 - 506.5 - 7.2IIAB, IIG
Coarse octahedritesOg1.3 - 3.36.5 - 8.5IAB, IC, IIE, IIIAB, IIIE
Medium octahedritesOm0.5 - 1.37.4 - 10IAB, IID, IIE, IIIAB, IIIF
Fine octahedritesOf0.2 - 0.57.8 - 13IID, IIICD, IIIF, IVA
Finest octahedritesOff< 0.27.8 - 13 IIC, IIICD
Plessitic octahedritesOpl< 0.2, spindles9.2 - 18IIC, IIF
AtaxitesD->16IIF, IVB

The newer chemical method is far more precise but depends on sophisticated instruments to determine abundances of trace elements such as Ge, Ga, and Ir. The concentrations of the trace elements are plotted against the overall Ni content on logarithmic scales to resolve well-defined chemical clusters, each representing a distinct chemical group. Iron meteorites of each chemical group formed on a common parent body. Iron meteorites come mostly from the cores of small differentiated asteroids that were disrupted by devastating impacts shortly after their formation. Short descriptions of the major groups follow.

Canyon Diablo 1AB iron. Image source:

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A large number of ungrouped irons don't fit into any of the existing 14 chemical groups, and display unique structural and elemental compositions. Some have compositions similar to other ungrouped irons, and have been provisionally placed into several grouplets comprising less than five members each. The remaining ungrouped irons are unique, and probably represent single samples of their parent bodies.

IRREGULAR GALAXY - Galaxy with a very irregular shape and no obvious elliptical or spiral structure. These galaxies tend to smaller than other types, containing 108-1010 stars, with an irregular overall shape. They are classified as type Irr in the Hubble classification scheme. Irregular galaxies with a crude spiral-like structure are often classified as type Sm (or type SBm if they also have a central bar). Irregular galaxy NGC1427A is shown below.

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ISOBAR - Isotopes with equal atomic mass. For example, 10Be, 10B, and 10C are isobars. Only one isobar will be stable, the others are radioactive.

ISOBARIC - Process that takes place without any change in pressure.

ISOCHORIC - Process that takes place without any change in volume.

ISOSTASY - Mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which “floats” on the denser mantle. Isostatic gravity anomalies are derived from the assumption that the gravitational effect of the mass associated with the surface topography is approximately compensated by a deficit of either the density or the subsurface thickness of the crustal material.

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ISOTHERMAL - Process that takes place without any change in temperature. Isothermal processes are often described as "slow."

ISOTONE - Isotopes with equal numbers of neutrons. For example, 3H, 4He, 5Li, and 6Be are isotones, each having 2 neutrons. More than one isotone may be stable.

ISOTOPE- One of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H,2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus.

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ISOTOPOLOGUE - Molecular entity that differs only in isotopic compositions, e.g., C16O vs. C18O.

ISOTROPIC - Same in all directions (anisotropic = not isotropic; anisotropy - difference between different directions).

IUPAC NOTATION - Nomenclature for characteristic x-rays proposed by the International Union of Pure and Applied Chemistry (IUPAC). X-rays are described using the level symbol (K, L, M …) and an Arabic numeral for the sublevel for the final and initial states. Thus, K-L3 indicates a transition from the LIII subshell into the K shell, and is equivalent to Kα1 in Siegbhan notation. The IUPAC notation is consistent with the notation used for Auger electrons.

IVUNA METEORITE - Carbonaceous chondrite weighing 0.7 kg that landed near Ivuna, Tanzania, on Dec. 16, 1938. In 2001, a team from the Scripps Institution of Oceanography, the Leideon Observatory in the Netherlands, and the NASA Ames Research Center, demonstrated the presence of two simple amino acids, glycine and β-alanine, and linked Ivuna with a likely origin in the nucleus of a comet.

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