DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Einstein Universe
where Rab is the Ricci tensor, gab is the metric
tensor, and R =Ra a is the Ricci scalar. The
divergence of the Einstein tensor vanishes identically.
See gravitational equations, Ricci tensor.
Einstein Universe The historically first cosmological
model derived by Einstein himself
from his relativity theory. In this model, the universe
is homogeneous, isotropic, and static, i.e.,
unchanging in time (see homogeneity, isotropy).
This last property is a consequence of the longrange
repulsion implied by the cosmological
constant which balances gravitational attraction.
This balance was later proved, by A.S. Eddington,
to be unstable: any small departure
from it would make the universe expand or
collapse away from the initial state, and the
evolution of the perturbed model would follow
the Friedmann–Lemaître cosmological models.
Because of its being static, the Einstein Universe
is not in fact an acceptable model of the actual
universe, which is now known to be expanding
(see expansion of the universe). However, the
Einstein Universe played an important role in
the early development of theoretical cosmology
— it provided evidence that relativity theory is
a useful device to investigate properties of the
universe as a whole.
ejecta In impact cratering, material that is
tossed out during the excavation of an impact
crater. The ejected material is derived from the
top 1/3 of the crater. Some of the ejected material
falls back onto the floor of the crater, but
much is tossed outside the crater rim to form
an ejecta blanket. Ejecta blankets on bodies
with dry surface materials and no atmosphere
(like the moon and Mars) tend to display a pattern
with strings of secondary craters (craters
produced by material ejected from the primary
crater) radiating outward from the main crater.
Glassy material incorporated into the ejecta can
appear as bright streaks, called rays. A radial
ejecta blanket is typically very rough within one
crater radii of the rim — few individual secondary
craters can be discerned. Beyond one
crater radii, the ejecta blanket fans out into radial
strings of secondary craters. On bodies
with a thick atmosphere (like Venus) or with
subsurface ice (like Mars and Jupiter’s moons
of Ganymede and Callisto), impact craters are
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typically surrounded by a more fluidized ejecta
pattern, apparently caused by the ejecta being
entrained in gas from either the atmosphere or
produced during vaporization of the ice by the
impact. The extent of the fluidized ejecta blankets
varies depending on the state/viscosity of
the volatiles and the environmental conditions.
In supernova physics, the ejecta are the material
blown from the stars as a result of the explosion.
Ekman convergence The stress on the
Earth’s surface varies from place to place and
hencesodoestheEkmantransport. Thisleadsto
convergence of mass in some places, and hence
to expulsion of fluid from the boundary layer,
called the Ekman convergence. In other places,
the Ekman transport is horizontally divergent,
i.e., mass is being lost across the sides of a given
area, so fluid must be “sucked” vertically into
the boundary layer to replace that which is lost
across the sides, called Ekman suction. This
effect is called the Ekman pumping.
Ekman layer (Ekman, 1905) The top or bottom
layer in which the (surface or bottom) stress
acts. The velocity that was driven by the stress is
called the Ekman velocity. Typically, the atmospheric
boundary Eckman layer is 1 km thick,
whereas the oceanic boundary Eckman layer is
10 to 100 m thick.
Ekman mass transport The mass transport
by the Ekman velocity within the boundary Ekman
layer is called the Ekman mass transport or
Ekman transport. In steady conditions, the Ekman
transport is directed at right angles to the
surface stress. In the atmosphere, the transport
is to the left in the northern hemisphere relative
to the surface stress. In the ocean, the transport
is to the right in the northern hemisphere relative
to the surface stress.
Ekman pumping See Ekman convergence.
Elara Moon of Jupiter, also designated JVII.
Discovered by C. Perrine in 1905, its orbit has an
eccentricity of 0.207, an inclination of 24.77 ◦ ,
and a semimajor axis of 1.174 × 10 7 km. Its
radius is approximately 38 km, its mass 7.77 ×
10 17 kg, and its density 3.4 g cm −3 . It has a