Glossary 927
from the solar system by close encounters with the forming
giant planets. Pluto and Triton are thought to be among the
largest remnants of this population.
Ice Mixture of water, ammonia, methane, and other volatile
compounds in the interiors of jovian planets, not literally in the
form of condensed “ice.”
IDP Interplanetary dust particle, collected by aircraft in the
stratosphere.
Impact melt Melt of target rocks resulting from the waste
heat generated in an impact event. When solidified, it can be
either glassy or crystalline and contain clasts of rock and mineral
debris from unmelted portions of the target.
Inclination The angle between the plane of the orbit of a
planet, comet, or asteroid and the ecliptic plane, or between a
satellite’s orbit plane and the equatorial plane of its primary.
Inclination takes on values between 0◦and 180◦.
Insolation The flux of sunlight at all wavelengths falling on a
body. For the Earth this amounts to a flux of 1.368× 106 ergs
cm−^2 s−^1.
Integral of the motion Any function of the position and
velocity coordinates of an object that remains constant with
time along all orbits. In the circular restricted three-body
problem, the Jacobi constant is an integral of the motion. The
Jacobi constant can be approximated by the Tisserand
parameter.
Intercrater plains The oldest plains on Mercury that occur
in the highlands and formed during the period of late heavy
bombardment.
Intrusion Geological structure of igneous material that
forces its way into an existing formation.
Invariable plane The plane passing through the center of
mass of the solar system, which is perpendicular to its total
angular momentum vector. The invariable plane is inclined 0.5◦
to the orbital plane of Jupiter and 1.6◦to the ecliptic.
Ionopause The surface separating ionospheric plasma and
the solar wind in the vicinity of an unmagnetized planet.
Ionosphere Outer portion of an atmosphere where charged
particles are abundant.
Isolation mass The mass of a planetary embryo if it sweeps
up all the accessible solid material in its vicinity.
Jacobi constant An integral of the motion in the circular
restricted three-body problem. It is proportional to the total
orbital energy of the small body in a reference frame rotating
with the two massive bodies.
Jeans escape The process by which fast (energetic or hot)
molecules of an atmosphere escape into space. The energy
distribution of a gas at a given temperature has a hot tail—a few
atoms moving faster than the rest. If, at an altitude where
collisions between molecules are rare, the molecules in the hot
tail move faster than the local escape velocity, they can escape to
space. This process is fastest for hot atmospheres of light gases
(hydrogen, helium) on bodies with low gravity.
Jets The observed, collimated emission of gas and dust that
occurs in restricted areas on the surface of a cometary nucleus.
Jets are usually active on the sunlit side of the nucleus.
Joule heating Heating that occurs when a current flows
through a resistive medium. In the high atmospheres of the
giant planets, it may be an important process in heating the
atmosphere to high temperature as currents of charged particles
driven by magnetospheric electric fields collide with the neutral
atmosphere atoms, which provide resistance.
Jovian planet A planet like Jupiter, which is composed
mostly of hydrogen, with helium and other gases, but possibly
with a silicate/iron core. Also called a gaseous or a giant planet.
The jovian planets are Jupiter, Saturn, Uranus, and Neptune.
Jupiter-family comet An ecliptic comet with a Tisserand
parameter between 2 and 3. It is typically on a low to moderate
inclination orbit, with a semimajor axis less than that of Jupiter’s
orbit. Most Jupiter-family comets are in orbits that cross or
closely approach Jupiter’s orbit.
K or kelvin Unit of absolute temperature. The freezing and
boiling points of water are 273.16 K and 373.16 K, respectively.
Keplerian orbit The path that a body would follow if it were
subject only to the gravitational attraction of its primary, e.g. a
planet orbiting the Sun, a satellite orbiting a planet.
Keplerian velocity The speed with which a solid body
moves on a circular orbit about a larger body.
Kepler’s laws Three rules that describe the unperturbed
motion of planets about the Sun (and of moons about planets):
(1) Planets move on elliptical paths with the Sun at one focus.
(2) An imaginary line from the Sun to a planet sweeps out area
at a constant rate. (3) The square of a planet’s orbital period
varies as the cube of the semimajor axis of its orbit.
Kirkwood gaps Zones in the asteroid belt that have been
depleted of objects due to mean-motion orbital resonances with
Jupiter.
Klystron Vacuum-tube amplifier used in planetary radar
transmitters.
Kozai resonance A resonance where an object’s nodal
precession rate is equal in magnitude and direction to its
periapse precession rate. Objects within a Kozai resonance
undergo oscillations in eccentricity and inclination that are out
of phase (i.e., when one increases, the other decreases). Kozai
resonances affect the motion of Pluto and some comets and
asteroids in the solar system.
Kuiper belt Generally used to refer to the population of
trans-Neptunian bodies, i.e., those with semimajor axes> 30
AU. In a more detailed classification, which partitions the
trans-Neptunian population into the Kuiper belt, the scattered
disk and the extended scattered disk, the name “Kuiper belt” is
associated with a collection of bodies on essentially stable, low
inclination, low eccentricity orbits. Almost all Kuiper belt
objects discovered so far have semimajor axes<50 AU, which
argues for the Kuiper belt having an outer edge at
approximately that location.