DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

trench excavation
trench excavation A method of investigation
for active faults to deduce when large earthquakes
occurred on the fault. Digging a trench
several meters in depth so as to cross the active
fault discloses disturbance of strata outcroppings
on the trench wall and their age of formation.
From such investigations, active periods
of large earthquakes over the past several thousand
years, recurrence time, and magnitude of
large earthquakes can be estimated. Such information
is very useful to forecast a forthcoming
large earthquake on the active fault. Recently,
Japanese archeological excavations have
discovered evidence for large earthquakes, such
as liquefaction of a ground and traces of ground
cracks.
triggered star formation The process where
star formation in one area can induce star formation
in a nearby region. This process can occur
by various mechanisms (e.g., stellar winds, supernovae)
in a star cluster which can compress
the nearby interstellar medium to densities sufficient
for new stars to condense.
triple-alpha process The set of nuclear reactions
whereby three helium atoms (alpha particles)
are converted to one carbon atom with
the liberation of about 10 18 ergs per gram of energy.
The triple-alpha process is a major energy
source in stars that have exhausted hydrogen in
their cores (see horizontal branch, clump stars,
asymptotic giant branch), and the main source of
carbon in the universe. It occurs at a temperature
near 10 8 K and only at relatively high density,
because the three helium atoms must approach
each other very closely, since the expected intermediate
nucleus, 8 B, is unstable to dissociation
back into two alpha particles. Another
reaction, 12 C(α,γ) 16 O, occurs at very slightly
higher temperature, so that helium fusion normally
results in a mix of carbon and oxygen
(more oxygen at higher temperatures). Some
of the nuclear reaction cross-sections involved
are still not adequately measured in the laboratory
or calculated, so that prediction of theC/O
ratio in the products remains somewhat uncertain.
See asymptotic giant branch star, clump
star, horizontal branch star.
© 2001 by CRC Press LLC
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triple junction In geophysics, a point on the
Earth where three plate boundaries meet.
triple point In physical chemistry, the temperature
and pressure under which gas, liquid,
and solid phases of a substance can coexist in
equilibrium. For water, at a standard pressure of
one atmosphere, this occurs at a temperature of
273.16 K (0.01 ◦ C). The (saturated) water vapor
pressure is 6.11 millibar under these conditions.
In meteorology, a junction of three distinct
air masses, leading to unstable conditions.
tripton Inanimate particulate matter in natural
waters.
tritium 3H; the isotope of hydrogen whose
nucleus consists of one proton and two neutrons.
Radioactive to beta decay to 3He with a half-life
of 12.26 years.
Tritium Unit A unit used to scale the amount
of tritium present in a particular sample of ocean
water. It is defined by convention as 10 18 times
the atom ratio of tritium to normal hydrogen.
See also tracers.
Triton Moon of Neptune, also designated NI.
It was discovered by Lassell in 1846. Its orbit
has an eccentricity of 0.00002, an inclination
of 157.4 ◦ , a precession of 0.5232 ◦ yr −1 , and a
semimajor axis of 3.55 × 10 5 km. Its radius
is 1350 km, its mass is 2.14 × 10 22 kg, and its
density is 2.1 g cm −3 . Its high albedo and large
heliocentric distance gives it a very low surface
temperature (34.5 K) and almost no atmosphere.
It orbits Neptune, retrograde, once every 5.877
Earth days. The peculiar orbit (retrograde and
highly inclined) makes it likely that Triton is a
captured body, probably from the Kuiper belt.
trochoidal wave theory A finite-amplitude
wave theory attributed to Gerstner (1802). The
water surface profile is a trochoid, which represents
a good match to many observed waves, but
the resulting predicted velocities do not match
observations well.
Trojans A class of asteroids that orbit at the
L4 and L5 Lagrangian points. These two points,
60 ◦ before and after the position of a planet in its