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igneous

Iapetus Moon of Saturn, also designated SVIII. Discovered by Cassini in 1671, it is noted for a surface where the leading edge has an albedo of 0.02 to 0.05, and the trailing edge has an albedo of 0.5. It has been conjectured that the difference is due to the leading edge accreting dark material (possibly from Phoebe) as it orbits. Its orbit has an eccentricity of 0.028, an inclination of 14.72◦, and a semimajor axis of

3.56 × 106 km. Its radius is 730 km, its mass is 1.88 × 1021 kg, and its density is 1.15 g cm−3.

It orbits Saturn once every 79.33 Earth days.

Icarus An Earth-crossing Apollo asteroid with perihelion 0.187 AU, aphelion 1.969 AU, approaching within 0.04 AU of the Earth’s orbit. Its orbital inclination is 22.9◦ and its orbital period is about 1.12 years. Based on its brightness, its diameter is 1 to 2.5 km.

ice A generic term referring to those substances of intermediate volatility, which are sometimes solid and sometimes gaseous. Examples of ices that are commonly encountered in astrophysical contexts are H2O, NH3, CH4, and CO2. Organic materials with intermediate volatilities are sometimes called organic ices, CHON (i.e., substances composed mainly of the elements C, H, O, and N) or simply organics.

ice age A period of geologic history during which considerable portions of the Earth were covered with glacial ice. There have been many ice ages in the geological history of the Earth; during some of them, the ice sheets were situated in the polar regions and some of them were in equatorial regions. In the last ice age, the Pleistocene, about one-ﬁfth of the Earth’s surface was covered by glacial ice. The climatic feature of ice ages is a very large temperature decrease. Over mid-latitude region, temperature can decrease more than 10◦ C and ice and snow can cover 20 to 30% of the Earth’s surface. The reasons for the ice ages may include the

long-term variations of solar radiation, orbit parameters of the Earth, and variations of climatic systems and processes of the rock lithosphere (including nonlinear feedback processes).

ICM-ISM stripping An important consequence of the interactions between a member galaxy in a cluster and the intracluster medium (ICM), resulting in stripping the galaxy of its interstellar medium (ISM) due to the ram pressure of ICM (Gunn and Gott, 1972). Suspected sites of the ongoing ICM-ISM stripping include some of the ellipticals and spirals in the Virgo cluster, and a few spirals in the cluster A1367. Observed signatures for the ICM-ISM stripping rather than other internal processes such as starbursts, cooling ﬂows, or gravitational interactions between member galaxies, consist in a relatively undisturbed stellar disk, together with a selectively disturbed HII region containing abruptly truncated Hα disk and one-sided ﬁlamentary structures above the outer disk plane. The ISM-ICM stripping is consistent with observational facts about spiral galaxies in clusters: many of them are deﬁcient in gaseous disks, those with truncated gaseous disks have relatively undisturbed stellar components, they sustain vigorous star formation in the inner disks but very little in the outer disks. The degree of severity of HII disk truncation signiﬁes the decline in star formation rates and the time elapsed since the ISM-ICM stripping began in cluster spirals.

ideal gas A theoretical gas consisting of perfectly elastic particles in which the forces between them are zero or negligible.

ideal gas [equation of state] The thermodynamic relationship between pressure, volume, number of particles, and temperature for an ideal gas. It is mathematically expressed by the equation P V = NkT , where P is the pressure, V is the volume, N is the number of particles, k is Boltzmann’s constant, and T is the absolute temperature.

igneous Referring to rock that has been in a totally molten state. Igneous rocks are classiﬁed as volcanic, or extrusive, if they have been extruded from the Earth in volcanic ﬂow. Examples are rhyolite, andesite, and basalt. Igneous

igneous rocks

rocks are classiﬁed as plutonic (intrusive igneous) if they formed totally subsurface. Granite, diorite, gabbro, and peridotite are plutonic. Igneous rocks are also classiﬁed as felsic, intermediate, maﬁc, and ultramaﬁc, depending on the silica content. The types listed above are in order felsic to maﬁc (periodotite is classiﬁed ultramaﬁc, with silica content below 45%).

igneous rocks Rocks that cooled and solidiﬁed from a magma.

illuminance (lux) The luminous power (measured in lumens) incident per unit area of a surface. One lux = one lumen per square meter.

immersed weight The weight of an object (or sediment particle) when underwater. Particle weight minus buoyant force.

impact basin Large craters (≥ 300 km in diameter) that exhibit many of the characteristics found in craters greater than a few tens of kilometers in diameter.

Terrestrial circular impact basins often display concentric rings of mountains (rising to several kilometers) and lack a central peak. Concentric ring structures are most common on the moon. Morphological differences in impact basins include the degree to which they appear to have been eroded, or the amount they have been modiﬁed by later processes such as volcanism. Examples of lunar basins include Imbrium and Orientale (greater than 900 km in diameter), and martian basins include Hellas (2000 km in diameter) and Argyre (1200 km in diameter). Because impact basins and their associated ejecta deposits are so large, they dominate the surface of the moon and other planetary bodies. See crater.


impact crater	See crater.
impact crater ejecta		Debris ejected from

an impact crater that surrounds the crater rim. Cumulatively ejecta fragments form a curtain of material.

Ejecta deposited near the crater rim is coarser grained than ejecta far from the rim. In this way, deep-seated (coarse) materials that only just make it over the crater rim form the rim

deposits, while originally shallow materials are deposited at increasingly large radial distances. This produces an inverted stratigraphy in the ejecta deposits relative to the stratigraphy of the original target.

On the moon, ejecta patterns most often comprise continuous deposits, discontinuous deposits, and ﬁnally rays at the greatest distance from the crater rim. On Mars, ejecta is regularly lobate, implying it was emplaced by ﬂow across the surface, possibly having been ﬂuidized by near-surface ice. See impact excavation phase.

impact creep The method of moving material along a surface by the transfer of energy from saltating particles. A particle lying on a surface can gather sufﬁcient energy to move if it is struck by a bouncing (saltating) grain. The energy from a saltating particle is transferred to the stationary particle, causing it to move by traction.

impact excavation phase The second phase of impact crater formation following compression and preceding modiﬁcation. It is the phase during which the main mass of material is ejected from the crater as the impact shock waves expand via a hemispherical shell away from the point of impact. Material is thus thrown upwards and outwards at progressively lower levels and velocities, and forms a conical curtain of debris around the growing crater that leans outwards at angles that are approximately 50◦ from the horizontal. Toward the end of excavation, when the upward velocities are too low to launch excavated rock, the crater rim is formed by uplift and overturning of the crater edge. Close to the rim, the debris curtain is deposited at fairly low velocities and angles as continuous ejecta, whereas further out higher angles and velocities pursue such that debris from the curtain may strike the surface to produce discontinuous ejecta and secondary craters.

impact polarization The polarization of radiation from an emitted resonance line caused by excitation of the upper level of the relevant transition with a ﬂux of charged particles. Used as a diagnostic of energetic particle beams in solar ﬂares.

impulsive ﬂare

impact velocities A factor in crater formation that determines crater size and is controlled by the thickness of the atmosphere around the planet (assuming cosmic velocities are constant throughout the solar system for the same sized objects). Thus, the moon has a higher impact velocity than Mars. For example, a typical asteroid may hit Mars at 10 km/sec and the moon at 14 km/sec, causing the crater formed on Mars to be smaller by 0.66. By recognizing that smaller craters are produced on Mars, sizefrequency curves developed there can be displaced to make them directly comparable with those on the moon, etc. This is also applicable to the other solar system planets.

impermeable Does not allow penetration by a ﬂuid.

impulsive ﬂare Solar ﬂares with short duration electromagnetic radiation occurring low in the corona in compact volumina. In contrast to gradual ﬂares, impulsive ﬂares are not accompanied by coronal mass ejections. In more detail, the properties of an impulsive ﬂare are:

(a) the soft X-ray emission lasts less than one hour, (b) the decay constant of the soft X-ray emission is less than 10 minutes, (c) the duration of the hard X-ray emission is less than 10 minutes, (d) the duration of the microwave emission is less than 5 minutes, (e) impulsive ﬂares are always accompanied by metric type III bursts, about 75% of them are also accompanied by metric type II bursts, but metric type IV emission is observed only rarely, (f) the height in

the corona is less than 104 km, (g) the ﬂare occupies a volume between 1026 cm3 and 1027 cm3,

(h) the energy density is high, (i) the size in Hα is small, and (j) impulsive ﬂares are rarely accompanied by coronal mass ejections. If coronal mass ejections are observed in impulsive ﬂares, they tend to be small and slow with speeds well below the combined solar wind and Alfven speed which is required to drive a shock.

If an impulsive ﬂare gives rise to a particle event in interplanetary space, this event has properties different from the ones of particle events caused by gradual ﬂares. These properties include: (a) the event is electron-rich and the H/He ratio is about 10, (b) the 3He/4He ratio is of the order of 1, which is enhanced by a factor of

2000 compared to abundances in the corona and in the solar wind, (c) the Fe/O ratio is about 1.2, an enhancement by a factor of 8 compared to the corona or the solar wind, (d) the charge states of iron are about 20, indicative for the acceleration out of a very hot environment with temperatures of about 10 million K, (e) the particles event lasts (at the orbit of Earth) for some hours up to about 1 day, (f) particles can be observed only in a relatively narrow cone of ±30◦ around the source region, and (g) there is no interplanetary shock accompanying the event. Event rates are up to about 1000 per year during solar maximum.

In impulsive ﬂares, particles are accelerated in a hot but conﬁned volume low in the corona. The enrichment in 3He and in heavier elements such as Fe can be explained by the process of selective heating: Particles are accelerated inside a close loop, giving rise to electromagnetic emission (the ﬂare). The loop is very stable, preventing particles from escaping into interplanetary space. As the particles bounce back and forth in the closed loop, they excite electromagnetic waves which can propagate into all directions, in particular also perpendicular to the magnetic ﬁeld lines. When these waves interact with the ambient plasma, they can accelerate particles. If these “secondary” particles are accelerated on open ﬁeld lines, they can escape into interplanetary space. Because the acceleration requires particles and waves to be in resonance, different particles are accelerated by different types of waves. If a particle species is common in the corona, such as H and 4He, the corresponding waves are absorbed more or less immediately; thus, these particles are predominately accelerated inside the closed loop and therefore do not escape into interplanetary space. Other waves, however, can travel larger distances before being absorbed by the minor constituents and therefore are more likely to accelerate these species on open ﬁeld lines. Since the escaping particle component is selectively enriched in these minor species, the acceleration process is called selective heating.

impulsive ﬂare Flares displaying impulsive spikes or bursts in their hard X-ray time proﬁles. These ﬂares are generally conﬁned in long (104 km) sheared loops and are considered as non-thermal X-ray sources.

inclination

inclination In geophysics, the angle at which the magnetic ﬁeld dips into the interior of the Earth; another term for geomagnetic dip angle; see also declination.

In astronomy, the angle the plane of the Earth’s equator makes with the ecliptic, i.e., the angle the pole of the Earth makes with the normal to the ecliptic; or in general the angle the pole of a planet makes with the normal to its orbit, or the angle a planetary orbit makes with the ecliptic; or the angle the normal to the plane of a binary star’s orbit makes with the line to the Earth. Most solar system planetary orbits have low inclinations (Pluto’s is the largest at 17◦) while asteroids and comets display larger inclination ranges.

incompatible element An element that enters the molten fraction when the degree of partial melting is low.

index, geomagnetic activity Disturbances of the geomagnetic ﬁeld are quantitatively characterized by numerous indices of magnitude. The International Association of Geomagnetism and Aeronomy (IAGA) has formally adopted 19 such indices. An oft-used index of general activity, the index K measures irregular variations of standard magnetograms recorded at a given observatory. After average regular daily variations are subtracted, the deviation of the most disturbed horizontal magnetic ﬁeld component in each 3-hour interval is derived. Auroral activity near the poles is the dominant source of these deviations, so an adjustment is made for observatory location in geomagnetic latitude, and the deviation is converted to a roughly logarithmic scale (ranging from 0 to 9 for deviations from 0 to 1500 nT). The most widely used of all geomagnetic activity indices is probably Kp (the“p” denoting planetary), intended as a worldwide average level of activity. The K indices from 12 stations between 48◦ and 63◦ geomagnetic latitude, at a wide range of longitudes, are combined to compute Kp. Detailed explanations of Kp and other indices may be found in Handbook of Geophysics and the Space Environment, A.S. Jursa, Ed., 1985. See geomagnetic ﬁeld.

index of refraction (n) The ratio of the speed of light in a material to the speed (c =

299792458ms−1 ) in a vacuum. The index of refraction enters essentially in computing the angel of refraction via Snell’s law. Typical indices of refraction are between 1 and 2. The index of refraction depends on the wavelength of the radiation being considered. See Snell’s law.

indices, geomagnetic Quantitative variables derived from observations, giving the state of the magnetosphere. They include the Dst index, which gauges the intensity of the ring current; the Kp index, which gives the variability of the ﬁeld; and the AE, AU, and AL indices, which provide information on the auroral electrojet and through that, indirectly, on the ﬂow of Birkeland currents. For details, see individual items.

induced Compton scattering Compton scattering induced by an extremely intense radiation ﬁeld, as found in compact radio sources and pulsars. Induced Compton scattering is a form of stimulated emission, in which a photon of a given frequency stimulates the emission of a second photon of identical frequency, phase, direction of motion, and polarization. Unlike stimulated emission due to the transition of an electron between two-bound states of an atom or ion, induced Compton effect transfers energy to electrons.

induced gravity (Sakharov, 1967) An alternative concept to the theories of quantum gravity in which the gravitational interaction is not fundamental, but is deﬁned by the quantum effects of the matter ﬁelds. The main mathematical problem arising in this approach is the calculability of the dimensional parameters: cosmological and gravitational constants. Only one of them may be deﬁned from the quantum ﬁeld theory of matter ﬁelds, which leaves the room to the ﬁne-tuning for the value of a cosmological constant. A form of induced gravity has been achieved in string theory. In this case, neither gravity nor other observable interactions are fundamental. All the interactions including gravity appear to be induced ones due to the quantum effects of (super)string. In particular, the Einstein equations arise as a consistency condition for the string effective action. See curved space-time, quantum ﬁeld theory in curved spacetime, quantum gravity.