DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Venusian Tessera
eccentricity of e = 0.0068, and an orbital inclination
of i = 3.395 ◦ . Its synodic period is
584 days. The average albedo of 0.75 refers
to the cloud tops. These clouds, predominantly
sulfuric acid drops, are embedded in an atmosphere
primarily composed of CO2. A strong
greenhouse effect maintains an average surface
temperature of close to 750 K. Venus’ interior
structure is probably similar to that of the Earth,
with a core composed of a mixture of Fe, Ni,
and S, surrounded by a silicate mantle. Radar
maps of the surface show it to be uncratered and
relatively young (< 10 9 years at the oldest) and
covered with lava flows that are still active in
some spots, but with no evidence of plate tectonics.
Venus has been the subject of many missions
including Mariner 2, Pioneer Venus, Venera 7,
Venera 9, and Magellan. Because its moment of
inertia is not known, the relative sizes of the core
and mantle can only be estimated from the mean
density of the planet. Venus has no satellites.
Venusian Tessera A distinct type of highland
region on the surface of Venus belonging to
the Plateau Highlands class (as opposed to Volcanic
Rises). Plateau Highlands are topographically
rugged, characterized by steep margins,
and lack volcanic landforms (lava flows are rare;
small volcanic constructs do not occur regularly
and there are no shield volcanoes). Tessera
form complex ridged terrain within the highlands.
They may have been created by shortening
the lithosphere so that the surface fractures
and buckles, and the crust thickens, similar to
the mountain-building process on Earth. In the
absence of plate tectonics on Venus, however, a
tessera-like landscape is formed.
vernal equinox The epoch at the end of the
Northern hemisphere winter on which the sun is
located at the intersection of the celestial equator
and the ecliptic; on this day the night and day are
ofequallengththroughouttheEarth. Thedateof
the vernal equinox is at the end of the Southern
hemisphere summer. The vernal equinox also
refers to the direction to the sun at this instant:
0 h RA, 0 ◦ declination. See autumnal equinox.
Very Long Baseline Interferometry (VLBI)
A technique for measurement of small scale
(milliarc second) features in radio sources by
© 2001 by CRC Press LLC
502
carrying out interferometry between widely separated
radio telescope sites, up to thousands of
kilometers apart.
Vesta The 3rd largest asteroid known; the
fourth asteroid to be discovered, in 1807. It is
located in the Asteroid Belt between Mars and
Jupiter. Vesta has a mean diameter of 576 km
andmassof2.76 ×10 20 kgandrotatesevery5.34
hours. As it rotates, Vesta’s brightness changes
which suggests that the surface is not uniform
either in terrain or composition. Images of Vesta
from the Hubble Space Telescope show dark and
light patches indicating a very diverse terrain,
consisting of exposed mantle, lava flows, and
a large impact basin. Spectra suggest that the
compound pyroxene is present, which is typically
found in lava flows. The composition of
Vesta is sufficiently different from that of other
known asteroids, so that Vesta has its own class.
Orbit: semimajor axis 2.3615 AU, eccentricity
0.09, inclination to the ecliptic 7 ◦ .13405, period
3.629 years.
virga Rain (or perhaps snow) falling from
a cloud but evaporating before it reaches the
ground.
Virgo A gravitational wave observatory under
construction near the Italian town Cascina.
The device is a laser interferometer, with two
arms at right angles. The length of each arm of
the laser interferometer device is 3 km. Operation
is expected to start in 2001. See LIGO.
Virgocentric flow The motion of the galaxies
in the local group toward the Virgo Cluster.
The gravitational force exerted by the mass of
the Virgo attracts all surrounding galaxies, including
the galaxies of the local group. The velocity
of approach toward Virgo lies in the range
vr ≈ 100 to 400 km s −1 . A correction for Virgocentric
flow should be applied to the radial velocity
measured for nearby galaxies, especially
if the recessional velocity is used as an indicator
of distance according to Hubble’s Law.
Virgo constellation (1.) A zodiac constellation
most visible in spring, covering the area
of sky approximately ranging from 12 to 15 h
in right ascension and from ≈−15 ◦ to ≈+15 ◦