936 Encyclopedia of the Solar System
Tectonic framework The global or large-scale pattern of
fractures and folds formed by crustal deformation.
Tephra Generic term for all volcanic fragments that are
explosively ejected from a volcano.
Termination shock A discontinuity in the outer heliosphere
where the solar wind slows from supersonic to subsonic as it
interacts with the interstellar plasma.
Terminator The boundary between the illuminated and
nonilluminated parts of a planet, satellite, asteroid, or cometary
nuclei.
Terrane A particular type of terrain. Generally used to
denote the kind of terrain dominated or formed by a particular
geomorphic process regime, such as a volcanic terrane or an
aeolian terrane.
Terrestrial planet A planet like the Earth with an iron core
and a silicate mantle and crust. The terrestrial planets are
Mercury, Venus, Earth, and Mars.
Tesserae Intensely deformed terrain cut by at least two
directions of ridges and/or grooves.
Thermal desorption Process of heating to drive off volatile
gases.
Thermal diffusion Heat transport resulting from a
temperature gradient in a solid body.
Thermal emission Electromagnetic radiation emitted
by a body, typically at infrared wavelengths, due to its
temperature.
Thermal inertia A material property that is a measure of the
time it takes for the material to respond to temperature
changes. Thermal inertia is mathematically defined in terms of
the physical properties of the material as (kρC)^1 /^2 , wherekis
the thermal conductivity,ρis the bulk density, andCis the
specific heat capacity.
Thermal radio emission Continuous radio emission from
an object that results from the object’s temperature. Blackbody
radiation is a form of thermal radio emission.
Thermal wind A wind shear developed in one direction due
to a temperature gradient in an orthogonal direction.
Thermosphere, exosphere Outer parts of an atmosphere,
heated by ionizing radiation and cooled by conduction. The
exosphere is essentially isothermal and is also characterized by
very long mean free paths.
Tholeiitic Referring to basaltic rocks generally found
on the ocean floor, erupted from oceanic ridge zones or from
shield volcanoes. Such rocks are considered in the mafic
family.
Tholus (pl., tholi) Dome or shield. Small tholi are scattered
across Io.
Three-body problem Problem of the motion of three
bodies moving under their mutual gravitational attraction. In
the restricted three-body problem, the third body is considered
to have negligible mass such that it does not affect the motion of
the other two bodies.
Thrust fault A fault where the block on one side of the fault
plane has been thrust up and over the opposite block by
horizontal compressive forces.
Tidal heating Energy deposited in a satellite due to the
dissipation of energy from tidal deformation.
Time delay Time between transmission of a radar signal and
reception of the echo.
Tisserand parameter A nearly-conserved quantity in the
circular restricted three-body problem. The Tisserand
parameter for comets with respect to Jupiter is used to
recognize returning comets even if their orbits were changed by
a close approach to Jupiter, and to classify their orbits.
Transient crater The crater excavated in a hyper-velocity
impact, prior to the collapse of the surrounding crater walls.
Transition region The vertical zone in the Sun where the
temperature climbs from 20,000 K above the cool
chromosphere to 1 million K in the hot corona.
Triaxial ellipsoid A 3-dimensional surface defined by three
axes that are elliptical in cross section and used to describe the
shape of a body.
Troposphere Lowest level of an atmosphere dominated by
vertical mixing and often containing clouds, where temperature
falls off with height at close to the neutrally stable (adiabatic)
lapse rate. Earth’s troposphere contains 80% of the mass
of its atmosphere and most of the water vapor, and consequently
most of the weather. Terminated at the top by thetropopause.
On Earth, the troposphere extends to 14 km (equatorial)
and 9 km (polar); on Venus, to 65 km.Troposis Greek for
“turning.”
Turbulent concentration The concentration of large
numbers of similarly sized particles in stagnant regions in a
turbulent gas.
Type I migration Gradual inward spiraling of a planet
as it loses angular momentum via gravitational interactions
with nebular gas. This affects planetary embryos and rocky
planets.
Type II migration Change in the size of a planet’s orbit
when the planet is massive enough to clear a gap in the disk.
Migration is typically inward.
Van Allen belts Region in the Earth’s magnetic field, inside
of∼4 Earth’s radii, filled with energetic particles. Other
magnetized planets have similar radiation belts
Vernal equinox The direction of the Sun as viewed from the
Earth as it crosses the celestial equator moving northward. On
Earth, the vernal equinox denotes the beginning of spring in the
northern hemisphere.
Viscoelastic heating Heating (such as tidal heating)
produced by nonrecoverable (permanent) deformation in the
viscous portion of a viscoelastic body, i.e., a body that behaves
with both viscous and elastic components of deformation
Viscosity Property of a fluid that resists flow; fluid dynamic
stiffness or, in a sense, internal friction. For lava flows that