DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

I
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 × 10 6 km. Its radius is 730 km, its mass is
1.88 × 10 21 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
igneous
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
PV = 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
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