Venus: Surface and Interior 155
Generally speaking, the larger a planet, the longer it will
continue to lose energy and be geologically active. How-
ever, the details of the thermal evolution of a given body
can be quite variable. Venus and Earth provide perhaps
the quintessential example of variations in evolution. Most
explanations of how Venus and Earth ended up on differ-
ent geologic paths have to do with the history of volatiles.
Volatiles, mainly in the form of water, play a key role in en-
abling plate tectonics on Earth. The presence of even a small
amount of water in rock has a major effect on its strength
and on the temperature at which it will melt. The water in
thelithosphereis believed to be essential to making it weak
enough to break into plates in response to the motions of
convection in the interior. Theasthenosphereis the upper
part of the mantle, directly below the lithosphere, which has
a lower viscosity than the rest of the mantle and acts to lu-
bricate the motion of the plates at the surface of the Earth.
The low viscosity of the asthenosphere may be a result of
small amounts of melt. Melt would not be expected in the
asthenosphere unless at least a small percentage of water is
present. Thus, water appears to be an essential ingredient
in the development of plate tectonics.
Measurements made to date indicate that the surface
and atmosphere of Venus have very little water. In terms
of the strength of the crust, the extremely high surface
temperatures might be expected to offset the lack of water,
making the crust extremely weak. However, laboratory stud-
ies of rock strength at Venus temperatures have shown that
dry basalt (see Section 5) is stronger than wet basalt at Earth
temperatures. This extreme strength of the crust on Venus
likely contributes to the lack of lithosphere scale breaks that
are required to form plates. As we discuss later, there is also
evidence suggesting that Venus has no asthenosphere.
Recent studies have proposed that Venus exists in a “stag-
nant lid” convection mode rather than the “active lid” mode
predicted for Earth. When convective stresses exceed the
lithospheric strength, an active lid such as the terrestrial
system of plate tectonics is predicted. On Earth, conditions
such as weak, narrow fault zones, or the presence of a low-
viscosity asthenosphere, allow the convective stresses to ex-
ceed the lithospheric strength. On Venus, the present-day
lithospheric strength is apparently too high to allow plates
to develop. This model is consistent with the loss of volatiles
as key to differences on Venus and Earth.
Given the similarity in heat-producing elements and size
between Earth and Venus and the absence of plate tecton-
ics on Venus, how does Venus lose its heat? Venus must
be convecting in its interior. As we will describe, although
there is no evidence for plate tectonics, there is evidence
that mantle plumes contribute to heat loss. On Earth, hot
blobs of material form within the overall convecting pattern
in the interior. These plumes form hot spots, such as the
Hawaiian Island chain. The hot mantle material pushes up
on the lithosphere, creating a broad topographic swell. The
heat causes the lithosphere and crust to melt locally, thick-
ening the crust and forming surface volcanoes. On Earth,
the majority of the heat is lost where the upwelling mantle
creates new crust at midocean rises, and the cold litho-
sphere is pushed back into the mantle at subduction zones.
Hot spots account for<10% of Earth’s heat loss.
There are approximately 10 such hot spot features on
Venus. These rises are Atla, Bell, Beta, Dione, W. Eistla,
C. Eistla, E. Eistla, Imdr, Themis, and Laufey Regiones
(Fig. 6). Those features believed to be active today, such
as Atla, Beta, and Bell Regiones, have broad topographic
swells, abundant volcanism, and strong, positive gravity sig-
natures. Several rises also have rifts, such as Guor Linea at
W. Eistla (Fig. 7). These features are characteristic of hot
spots above a mantle plume. However, there are too few
hot spot features on Venus (∼10 on Venus versus 10–30 on
Earth) to account for a major portion of Venus’ heat budget.
In addition to the large-scale (1000–2000 km diameter) hot
spots on Venus, there are also smaller scale (mean diameter
of∼250 km) features called coronae (see Section 7). There
are∼515 of these features, which are unique to Venus.
There is considerable evidence that many, perhaps all, of
these features form above small-scale plumes. However,
even if all coronae represented small-scale plumes, they
would not be able to account for more than about one quar-
ter of the interior heat loss on Venus.
The relationship between the gravity and topography
provides evidence that Venus does not have a low-viscosity
asthenosphere. On Earth, a mantle plume must pass
through the asthenosphere before reaching the lithosphere.
(Note that there is some debate about the existence of
an asthenospheric layer beneath the very thick continental
lithosphere on Earth, but its existence below the oceanic
lithosphere, where the majority of plumes are observed, is
well accepted.) The plume tends to spread out in the rel-
atively weak asthenospheric layer, resulting in a reduced
amount of topographic uplift for a given plume size. Com-
paring the observed amount of uplift to the estimated size
and depth of the low-density plume provides evidence for
this behavior on Earth but not on Venus. On Venus, plumes
strike the lithosphere directly, thus causing more uplift for
a given plume size.
The relationship between the gravity and the topogra-
phy provide some insight into interior structure and con-
vection. The magnitude of variations in the gravity field as
compared to a given topographic feature is an indication
of the interior structure that supports a given topographic
feature. The strength of the lithosphere can support to-
pography. Variations in density in the interior can also sup-
port topography. A mountain can be supported by a thick
‘root’ of low-density crust, analogous to an iceberg floating
in denser water. Variations in the mantle temperature as-
sociated with convection can also support topography. The
gravity field of Venus has been carefully studied to estimate
the thickness of the strong, or elastic, part of the lithosphere,
the thickness of the crust, and the location of low–density,