Glossary I

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

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. Perhaps the archetypal multi-ring impact basin, Mare 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.

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

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.

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.)

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

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.

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.

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.

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 ATOM - Atom occupying a region between lattice points.

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

INTRUSIVE - Refers to igneous rocks that crystallized underground.

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.

The bonding of MgO.

This is most common in atoms with large differences in electronegativity (ΔX>2). The electrons originally from the donor atom move to the vicinity of the other "acceptor" atom, binding the atoms together. 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 O^2- is used (~140 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.

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).

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.

• IAB group irons include the famous Toluca, Campo del Cielo, Odessa, and Canyon Diablo meteorites.Most IAB irons are coarse to medium octahedrites (although other structural classes do occur) and often contain abundant inclusions of troilite, graphite, and cohenite, and various silicates. Recent research suggests that both winonaites and IAB irons originated on the same parent body - a partially differentiated asteroid that was disrupted just as it began to form an Fe core and silicate-rich crust. The impact mixed silicates into molten Ni-Fe metal forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites.

• IIAB Group irons are classified as hexahedrites or coarsest octahedrites, making them some of the most Ni-poor iron meteorites known. Perhaps the most famous IIAB iron is Sikhote-Alin, a witnessed fall from Russia in 1947. Several thousand pieces with a total weight of over 70 tons have been recovered. Trace element abundances suggest that this group formed in the core of a differentiated C-type asteroid that was disrupted by several impact events.
• IIIAB Group irons consist of two closely related groups. The IIIA subgroup has mostly coarse octahedrite textures, whereas IIIB irons usually display medium textures. However, the two subgroups form a continuous sequence in structure and elemental compositions consistent with a common origin (the subgroups probably represent different parts of an asteroid's core). Group IIIAB includes some of the largest irons ever found (Cape York, Chupaderos, Morito, and Willamette). Some IIIAB members contain large nodules of troilite and graphite, but silicate inclusions are rare. Recent research suggests a close relationship between the IIIAB irons and the silicate-rich main group pallasites: both groups probably formed on the same parent body - a differentiated asteroid that was disrupted by a single impact event. IIIAB iron meteorites represent fragments of the core, whereas main-group pallasites are samples of the core-mantle boundary.
• IIICD Group irons mostly belong to the structural classes of fine and finest octahedrites, or ataxites. Several IIICD meteorites contain abundant silicate inclusions, similar to the inclusions in IAB irons. There are additional similarities in elemental compositions suggesting a close relationship between IIICD and IAB irons. However, group IIICD displays some unique features that clearly distinguish them from the IAB irons, e.g., presence of haxonite. This group includes the anomalous, troilite-rich Mundrabilla, one of the largest iron meteorites ever found.

• IVA Group irons are mostly fine octahedrites, and have extraordinarily low Ge and Ga abundances. Some IVA irons contain sparsely distributed small nodules of troilite and graphite, although silicate inclusions are rare to absent in most members. Recent research suggests that the IVA irons formed in the core of a small, differentiated asteroid disrupted by a major impact shortly after its formation. After re-accretion, the asteroid was again disrupted ~450 Ma ago. The famous meteorite Gibeon is a typical member of this group; over 30 tons have been recovered from its large, prehistoric strewn field in Namibia.

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 type from the Hubble classification scheme. These galaxies tend to smaller than other types, containing 10⁸-10¹⁰ stars, with an irregular overall shape.

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.

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.

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).

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.