DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Tethys Ocean
Tethys Ocean In geophysics, the continents
of Laurasia and Gondwanaland were separated
by the Tethys Ocean approximately 200 million
years ago. This ocean has closed, and the
Mediterranean Sea is a remnant. See continental
drift.
texture See cosmic texture.
Thalassa Moon of Neptune also designated
NVII. Discovered by Voyager 2 in 1989, it is
a small, irregularly shaped body approximately
40 km in radius. Its orbit has an eccentricity of
0.00016, an inclination of 0.21 ◦ , a precession
of 551 ◦ yr −1 , and a semimajor axis of 5.01 ×
10 4 km. Its mass has not been measured. It
orbits Neptune once every 0.3115 Earth days.
Tharsis Province A broad Martian topographic
rise centered on the equator at longitude
115 ◦ W. It stands as much as 10 km above the reference
datum, measures ≈ 8000 km across, and
it occupies ≈ 25% of the surface area of Mars.
It is the most pronounced region of central vent
volcanism on Mars. It has four large shield volcanoes
(Olympus Mons, Ascraeus Mons, Pavonis
Mons, and Arsia Mons, the summits of
which are concordant, standing 27 km above
the reference datum). Olympus Mons stands
25 km above the plains, and the other three
stand 17 km above the plains. All are considered
to have formed by successive eruptions of
low-viscosity lavas. The volcanoes account for
half the planet’s volcanic production from the
late Hesperian until the Amazonian.
The province is asymmetrical being twice
as steep at the northwestern limit as it is on
the southeastern limit. At the NW, it forms a
continuous slope (only broken by remnants of
old cratered terrain, e.g., Tempe Terra) with the
sparsely cratered northern lowland terrain, and
on the SE, it forms a continuous slope grading
into the high-standing, highly cratered southern
hemisphere terrain. It demonstrates a complex
and extended tectonic record, being at the center
of a vast radial fracture system that affects
half the planet, and a substantial free-air gravity
anomaly. Additionally, the huge equatorial
canyons of Noctis Labyrinthus start at the center
of the bulge and extend down the eastern flank
to form Valles Marineris.
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Thebe Moon of Jupiter, also designated JXV.
Discovered by S. Synnott in 1979, its orbit has
an eccentricity of 0.015, an inclination of 0.8 ◦ ,
and a semimajor axis of 2.22 × 10 5 km. Its size
is 55 × 45 km, its mass 7.60 × 10 17 kg, and its
density 1.6 g cm −3 . It has a geometric albedo of
0.05 and orbits Jupiter once every 0.674 Earth
days.
thermal anisotropy See kinetic temperature,
plasma stress tensor.
thermal bar Due to cabbeling, mixing of
two water masses with identical density but different
temperature and salinity generates denser
water. The thermal bar relates to the special
case of mixing two water bodies with temperatures
below and above 4 ◦ C. The mixed water
has close to maximum density and subsequently
sinks as thermal plumes. The term “bar” refers
to the phenomenon that the original waters cannot
cross the sinking plumes.
thermal boundary layer A portion on the
edge of a body with a high thermal gradient. In
convective systems (such as the Earth’s mantle),
this is commonly associated with the edges of
each convecting layer. The oceanic lithosphere
is one example of a thermal boundary layer
and illustrates clearly the importance of this
boundary layer for the convective process: the
layer thickness is very small at mid-ocean ridges
where new boundary layer material is erupted
from beneath, but the thickness increases moving
away from the ridge as heat is removed from
the plate by diffusion. This production of a
cool boundary layer causes negative buoyancy,
which allows the oceanic slab to plunge down
into the interior of the convective region, transporting
a heat deficit inwards (and by implication,
causing a net transport of heat towards the
surface). It is quite likely that there is another
thermal boundary layer at the base of the mantle,
where the seismically distinct D” layer is to
be found and where perhaps a reverse process is
taking place except that boundary layer material
appear to detach from the core-mantle boundary
in the form of blobs rather than slabs.
thermal bremsstrahlung See bremstrahlung
[thermal].