926 Encyclopedia of the Solar System
Gravitational instability Spontaneous collapse of a portion
of a protoplanetary disk due to mutual gravitational attraction.
This can refer to either the solid or gaseous component of the
disk.
Greenhouse effect Heating of a planetary surface above the
temperature that it would have been in the absence of an
atmosphere. The atmosphere transmits solar radiation in the
visible, but impedes the escape of thermal infrared energy
(usually due to absorbing clouds), thus creating the increased
temperature.
Gyro radius Radius of the orbit of a charged particle
gyrating in a magnetic field.
Gyrofrequency The frequency of the circular motion of a
charged particle perpendicular to a magnetic field.
Habitable zone The region of space around a star in which a
geologically active, rocky planet can maintain liquid water on its
surface.
Hadley circulation A major component of atmospheric
circulation driven directly by latitude-averaged heat sources and
sinks. Warm air rises in regions near the equator, flows poleward
at higher altitudes, and loses heat in the colder, higher latitude
regions. The cooler, denser air then descends and has a flow
component near the surface back toward the low-latitude heat
source, which completes a circulation cell. The near-surface and
high-altitude branches of the flow have eastward (“trade wind”)
and westward components, respectively, arising from Coriolis
forces. When the heat source is located on the equator, the
Hadley circulation tends to be symmetric about the equator, but
the Hadley circulation is asymmetric about the equator if the
heat source is located off the equator, as occurs during solstice
seasons on Earth and Mars.
Halley-type comet A returning comet with a semimajor axis
less than∼34.2 AU. Halley-type comets have Tisserand
parameters with respect to Jupiter less than 2.
Heat flow Heat emitted (or received) at the surface of a body
that is ultimately radiated to (or absorbed from) space.
Heavy elements In astrophysics,all elements other than
hydrogen and helium.
Heliocentric A Sun-centered coordinate system.
Heliopause Interface between the heliosphere and the
interstellar plasma; the outer boundary of the heliosphere.
Heliosphere The cavity carved in the interstellar plasma by
the solar wind, containing the solar system and plasma and
magnetic field of solar origin;
Heliospheric current sheet The surface in interplanetary
space separating solar wind flows of opposite magnetic polarity;
the interplanetary extension of the solar magnetic equator.
Heliospheric magnetic field Remnant of the solar magnetic
field dragged into interplanetary space by the solar wind.
Heterotrophy Literally, other-feeding; the condition of an
organism that is not able to obtain nutrients by synthesizing
nonorganic materials from the environment, and that therefore
must consume other life forms to obtain the organic products
necessary for life; e.g., animals, fungi, most bacteria.
Hill sphere Region around a secondary in which the
secondary’s gravity is more influential for the motion of a particle
about the secondary than is the tidal influence of the primary.
Hilly and lineated terrain The broken-up surface of
Mercury at the antipode of the Caloris impact basin.
Homopause Level in an atmosphere, above the stratosphere,
at which gases cease being uniformly mixed and separate by
diffusion, with the lighter elements diffusing upward.
Horseshoe orbits Librating orbits encircling theL 3 ,L 4 , and
L 5 Lagrangian points in the circular restricted three-body
problem. These orbits appear to be shaped like horseshoes in
the frame rotating with the mean motion of the system.
Hot Jupiter An extrasolar gas giant planet at a very small
orbital separation of 0.03–0.05 AU from its host star and with an
orbital period of a few days. The proximity of the discovered hot
Jupiters to their host stars is probably a result of inward orbital
migration.
Hot poles The alternating perihelion subsolar points on
Mercury at the 0◦and 180◦meridians.
Hot spots Regions of enhanced thermal emission on Io, a
marker of volcanic activity. The term does not imply a particular
eruption mechanism.
Hugoniot elastic limit Stress at which a rock or mineral’s
response to shock changes from elastic to plastic. Stresses above
the Hugoniot elastic limit cause the rock or mineral to deform
plastically.
Hydrated A mineral in which water molecules or hydroxyl
radicals are attached to the crystalline structure.
Hydrodynamic escape A limiting case of atmospheric
escape that occurs when the escape rate is so rapid that the
atmosphere at high altitudes reaches an outward velocity
comparable to the speed of sound. This occurs if the thermal
energy of the gas molecules becomes comparable to the
gravitational binding energy. Hydrodynamic escape allows the
upper atmosphere of a planet to escape wholesale, as opposed to
the usually slower processes of Jeans-type thermal leakage or
solar wind ion pickup.
Hydrogen cloud The huge cloud of atomic hydrogen
surrounding most comets. The hydrogen cloud is produced by
the dissociation of water and the hydroxyl molecule (OH).
Hydrostatic equation Relationship that says pressure is
equal to the weight of gas or liquid above the level of interest.
Hyperbolic orbit Unbound orbit in which the object
escapes the gravitational attraction of the central body: examples
are orbits of beta meteoroids and interstellar grains.
Hypsometry Geodetic observations of terrain elevations with
respect to sea level.
Ice dwarf The term given to the planetesimals believed to
have been created in large numbers during the formation of the
giant planets and later scattered to the Oort cloud or ejected