924 Encyclopedia of the Solar System
expected to respond rapidly to changes in sunlight, but if there
is substantial vertical circulation, the changes will occur more
slowly as the mixing increases the effective mass of the layer
under consideration.
Eccentricity A measure of the departure of an orbit from
circular. For an elliptical orbit, the eccentricity,e, is equal to
(1−b^2 /a^2 )^1 /^2 , whereaandbare the semimajor and semiminor
axes of the ellipse, respectively. Circular orbits havee=0;
elliptical orbits have 0<e<1; radial and parabolic orbits have
e=1; and hyperbolic orbits havee> 1.
Echo bandwidth Dispersion in Doppler frequency of a
radar echo, i.e., the width of the echo power spectrum.
Ecliptic, ecliptic plane The plane of the Earth’s orbit
around the Sun. The planets, most asteroids, short-period
comets, Kuiper belt objects and Scattered-disk objects follow
orbits with small or moderate inclinations (or tilts) relative to the
ecliptic.
Ecliptic comet A comet with a Tisserand parameter with
respect to Jupiter greater than 2. They generally have a small or
moderate inclination to the ecliptic. The designation of such
objects as comets does not necessarily imply visible cometary
activity because comets typically do not become active until they
pass well within Jupiter’s orbit. The term “comet” assumes that
the body contains a substantial fraction of water ice. Ecliptic
comets include Centaurs and Jupiter-family comets.
Ekman layer Idealized model of the planetary boundary
layer in which the mean flow (as in an atmosphere or ocean) is
modified near the ground by friction (either laminar or
turbulent). The resulting variation of flow speed and direction
with height is described by the Ekman spiral.
Elongation The angular distance between the Sun and a
planet or other solar system body as viewed from the Earth.
Embays To form a protective barrier.
Emissivity The ratio of radiant energy flux from a material to
that from a blackbody at the same temperature. A blackbody is
an ideal material that absorbs all radiant energy incident upon
it and emits radiant energy at the maximum possible rate per
unit area at each wavelength for a given temperature. A
blackbody has an emissivity of 1 across the entire spectrum.
Real materials have an emissivity between 0 and 1 for a given
wavelength.
Encke-type comet An ecliptic comet whose entire orbit is
interior to the orbit of Jupiter and which has a Tisserand
parameter with respect to Jupiter greater than 3.
Endogenic Forming from within
ENSO El Ni ̃no, “the child,” and the Southern Oscillation. El
Ni ̃no is the episodic appearance of warm water off the coast of
South America, often at Christmas time, that devastates
Peruvian fishing (usually one fifth of the world’s catch), causes
drought conditions in Australia, and weakens the monsoon in
India. The Southern Oscillation is the historical name for the
global (not just southern) atmosphere-ocean oscillation for
which El Ni ̃no years are the extreme.
Entropy Broadly, the degree of disorder, or randomness in a
system; in thermodynamics, a measure of the amount of heat
energy in a closed system that is not available to do work. In a
condition of low entropy (high efficiency), the system will
convert to energy a large portion of the heat transferred to it
from an external source (no actual system can utilize 100% of
the heat it receives).
Enzymes Proteins that catalyze, or accelerate, chemical
reactions.
Equal angle map For mapping, the surface of a planet is
subdivided into spatial elements called pixels on which
quantities such as counting rates and elemental abundances are
specified. In an equal area map, all of the pixels span the same
angle in latitude and longitude. Consequently, a parallel near a
pole is divided into the same number of pixels as a parallel at the
equator and the area of the pixels varies with latitude.
Equal area map The longitude-range for pixels at different
latitudes is adjusted so that all of the pixels have approximately
the same area. Consequently, a parallel near the pole is divided
into fewer pixels than at the equator. The span of pixels in
latitude and longitude at the equator is used to specify the map.
Equation of state Equation relating the pressure of a given
material to its temperature and density, typically derived from
experimental and theoretical considerations.
Equilibrium vapor pressure The ambient pressure of the
gas phase over a condensed phase when the gas and condensed
phase are in thermodynamic equilibrium (i.e., when the rate of
condensation from gas to ice equals the rate of sublimation from
ice to gas). In effect, vapor pressure is a measure of the amount
of gas an ice or liquid layer at a specified temperature will
evolve in a closed container (or planetary atmosphere). Vapor
pressures are extremely sensitive functions of temperature and
are also related to the composition and structure of a given ice
or liquid.
Escape velocity Minimum velocity required to escape from
the surface of a body to infinity.
Europium anomaly and Eu∗ Because the Moon is highly
reduced, europium is divalent on the Moon and hence is mostly
separated from the other smaller trivalent rare earth elements
because it is concentrated in plagioclase feldspar. The degree of
enrichment or depletion is given by Eu/Eu∗, where Eu is the
measured abundance and Eu∗is the abundance expected if Eu
had the same relative concentration as the neighboring rare
earth elements, samarium and gadolinium.
Exosphere The outermost part of an atmosphere,
characterized by very low densities and very long mean-free
paths, and usually isothermal.
Extended scattered disk Collection of objects with orbits
with semimajor axis>50 AU, large eccentricity, and perihelion
distance large enough to avoid destabilizing encounters with
Neptune. The apparent similarity with the orbits of objects
in the scattered disk suggests that the latter extended
further in perihelion distance in the past, due to a different