Venus: Surface and Interior 151
2. General Characteristics
2.1 Orbital Rotations and Motions
Venus orbits the Sun in a nearly circular path once every
224.7 Earth days. It is the second planet from the Sun,
located between Mercury and Earth. The plane of Venus’
orbit is inclined to that of the Earth by 3.4◦. Analysis of the
obliquity of Venus reveals that it has a liquid core, similar to
that of Earth. One day on Venus lasts 116.7 Earth days. The
rotation of Venus on its axis is not only extremely slow but
occurs in the opposite direction from all the other planets
(retrograde rotation), so that the Sun rises in the west.
When visible, Venus is the brightest planet in the night
sky due to its size and proximity to both the Sun and to
Earth. It’s easy visibility and the unusual pattern it makes in
the night sky have given Venus a special place inastrology
and made it an easy target for stargazers. Its proximity to
the Sun means that it never rises very high in the sky, but
it can often be seen as either the “evening star” in the west
or the “morning star” in the east.
2.2 Radius, Topography, and Physiography
The radius of Venus is 6052 km, only 5% less than the
equatorial radius of the Earth, 6378 km. The average den-
sity of Venus is 5230 kg/m^3 , somewhat higher than Earth’s
density. Thus, the acceleration of gravity at the surface is
8.87 m/s^2 , 90% of Earth’s. The radius of the Earth measured
at the poles is approximately 21 km less than the radius at
the equator. This difference is called the rotational bulge.
The Earth’s spin accelerates the equator more than the pole,
causing the pole to be flattened and the equator to bulge
out. The very slow rotation of Venus means that no such
flattening occurs, making it, on average, nearly spherical.
The topography on Venus is dominated by plains, which
cover at least 80% of the planet. There are also major high-
lands, including plateaus and topographic rises, as well as
rifts and ridge belts that stand out from the background
plains (see Fig. 1). Based on available data, the topography
of Venus is unique in our solar system. Most of the smaller
solid bodies, such as Mercury, Mars, and our Moon, as well
as many satellites, bear the mark of numerous small craters
and large impact basins, left from an earlier period in the
history of our solar system when large impactors were com-
mon. As we will discuss in more detail later, the absence of
impact basins and the small number of craters indicate that
the surface of Venus is relatively young. On average, it is
comparable to the age of the surface of the Earth.
Venus and Earth both have a large topographic range,
which results from the intense geologic activity that the two
planets have experienced. However, the distribution of el-
evations on the two planets is very different (see Fig. 2).
Earth’s topography is bimodal, while Venus’ topography is
FIGURE 1 Topography of the Earth and Venus in a sinusoidal
projection at a resolution of 1 pixel/degree. Note the long ridges
that dissect many of Earth’s ocean basins and the long mountain
belts that are the signature of plate tectonics. Venus has
numerous large highland regions, but the only long, quasi-linear
mountain belts occur in the northernmost highland region,
Isthar Terra.unimodal. The two peaks on Earth reflect the division be-
tween oceans and continents. Venus has no ocean and, as
we will discuss later, arguably no continents. The topogra-
phy on Venus differs from that on Earth in other significant
ways. Most importantly, Venus lacks the interconnected sys-
tem of narrow midocean ridges and long linear mountain
belts that are the hallmark of plate tectonics on Earth (see
Fig. 1). The absence of these features on Venus reflects fun-
damental differences in evolution between the two planets,
and will be discussed in greater detail later.2.3 Surface Conditions
The surface conditions on Venus can best be described as
hellish. The surface temperature at the mean planetary el-
evation is 437◦C (867◦F). The surface temperature at the
highest elevations is approximately 10◦C less. The surface
pressure is 95 bars, equivalent to the pressure under almost
1 km of water. The surface temperature varies by only about
1 ◦C over the course of a year due to the dense, insolating