Glossary 937
typically have remarkably little excess energy above their
solidus, viscosity can be the determining factor for the
magnitude and morphology of lava flow fields associated with
volcanoes and is often exponentially dependent on the core
temperature of the flow.
Viscous relaxation Process whereby topographic features
become subdued over time due to the flow of the surrounding
geologic material.
Volatile Any substance that outgasses or produces a
significant vapor pressure at a given temperature. Ice is a
volatile on Earth (T=270–300 K), but involatile in the outer
solar system (T<100 K). By contrast, the ices of CH 4 , CO, and
N 2 are volatile throughout the planetary region wherever
T>30 K. Also, chemical compounds or elements contained in
magmas that are generally released as gases to the atmosphere
during a volcanic eruption.
Vorticity A measure of the circulation of a region of the
atmosphere. Spots having cyclonic vorticity rotate
counterclockwise in the northern hemisphere and clockwise in
the southern hemisphere. Terrestrial hurricanes have cyclonic
vorticity. The large stable spots on the giant planets are
anticyclones.
Warm poles The alternating aphelion subsolar points on the
surface of Mercury at the 90◦and 270◦meridians.
Water ice The primary volatile constituent of comets. Water
ice comes in three forms: amorphous, cubic, and hexagonal.
Amorphous ice, believed to be the form in the deep interior of
cometary nuclei, is characteristic of ices formed at very low
temperatures. It has no crystalline structure. At higher
temperatures, typically 100–150 K, energy is available to convert
the ice to the lower energy cubic form; this transition releases
energy. A similar transition from cubic ice to hexagonal ice
also releases energy at∼180 K. Cubic ice and hexagonal
ice are collectively known as crystalline ice. The water ice
nearest to the surface of the cometary nucleus is thought to be
hexagonal.
Water-equivalent hydrogen (WEH) Gamma ray and
neutron spectrometers are sensitive only to the abundance of
hydrogen, which is sometimes expressed as the equivalent
weight fraction of water. If all of the hydrogen is in the form of
H 2 O, then the relationship between the weight fraction of
hydrogen (wH) and the weight fraction of water (wwater)is
wwater= 9 wH.
Wavenumber, wavelength Wavenumber is the inverse of
wavelength, having units of inverse length. In spectroscopy, the
wavenumberνof electromagnetic radiation is defined as
ν= 1 /λ, whereλis the wavelength in vacuum. This quantity is
commonly specified in cm−^1 , called a reciprocal centimeter, or
inverse centimeter.
Western boundary current Strong ocean current that runs
along the western edge of an ocean basin as a result of the much
slower eastward group velocity of Rossby waves (planetary
waves) relative to the westward group velocity. The Gulf Stream
is a well-known example.
Window A spectral region in a planetary atmosphere that is
relatively transparent between two regions that have higher
opacity. A window region can be important for remote sensing
of a planetary surface and for limiting the extent of a greenhouse
effect.
Yarkovsky effect A nongravitational force that arises from
the asymmetric thermal reradiation of incident sunlight on the
surface of a rotating body, that can lead to significant orbital
evolution of kilometer-sized and smaller objects.
Zodiacal cloud The cloud of interplanetary dust in the solar
system, lying close to the ecliptic plane. The dust in the zodiacal
cloud comes from both comets and asteroids.
Zodiacal light Diffuse glow seen on the Earth in the west
after twilight and in the east before dawn, that appears
wedge-shaped and lies along the ecliptic. It is widest near the
horizon and is caused by the reflection of sunlight from the
myriads of interplanetary dust particles concentrated in the
ecliptic plane.