Geology Reference
In-Depth Information
CHAPTER 6
Venus
6.1 Introduction
twentieth century demonstrated that the atmosphere is
predominantly carbon dioxide with small amounts of
water, but that the clouds were unlikely to be composed
of water droplets or ice crystals.
Earth-based radar observations enabled estimates of the
extremely high surface temperatures that were later con-
firmed by spacecraft. This led to the formulation of the
runaway greenhouse model by Carl Sagan and Jim
Pollack. In this model, solar energy penetrates the clouds
and is re
ected from the surface but cannot escape back to
space, which is similar to the processes that keep green-
houses on Earth warm. Radar data also enabled re
nement
of knowledge of the orbital characteristics of Venus,
showing that the planet rotates extremely slowly on its
axis; one day on Venus is equal to more than 116 Earth
days. Moreover, the direction of rotation is the opposite of
its orbit around the Sun, meaning that Venus is in retro-
grade rotation.
Because of its close proximity to Earth, a great many
missions have
flown either to Venus or past the planet on
the way to other destinations
(Table 6.1)
. Because visible
imaging of the surface is precluded by clouds, most of
these missions focused on atmospheric sciences rather
than geoscience. Some of the earliest missions of direct
geologic interest were the successful Soviet Venera land-
ers. Although the landers operated for less than two hours
each, these were remarkable engineering successes, given
the extremely hostile surface environment. For example,
in the 1970s Veneras 8 and 9 were soft landers and
returned the
first pictures directly from the surface
(Fig. 6.2)
, along with data on rock compositions. So little
was known about the surface conditions that lights were
carried on the landers to illuminate the terrain for taking
pictures because of the possibility that little sunlight
would reach the surface. As subsequent results showed,
the lights were unnecessary, but it was a wise precaution
nonetheless.
After the Sun and the Moon, Venus is the brightest object
in the sky, a consequence of sunlight being re
ected from
its dense clouds. The planet
s diameter, mass, and gravity
are nearly the same as those of Earth
(Table 1.1)
. Along
with the presence of an atmosphere, these characteristics
led some observers to refer to Venus as Earth
'
s sister
planet. Even as late as the 1960s, some serious researchers
thought that the surface of Venus was a wet, tropical
environment, possibly teaming with life.
With the dawn of the Space Age, Venus was revealed to
be substantially different from Earth. The surface temper-
ature is a hellish 480 °C and exceeds the melting point of
lead, while the dense carbon dioxide atmosphere is laced
with droplets of sulfuric acid that form dense clouds and
exerts a surface pressure of 95 bars, comparable to being
underwater on the sea
floor of Earth at a depth of 900 m.
This leads some wags to refer to Venus as Earth
'
s evil
sister. On the other hand, the geomorphology of Venus
displays features indicative of extensive tectonic and vol-
canic processes
(Fig. 6.1)
, some of which are similar to
those on Earth and could be active today, and a surface age
of no older than about 750 Ma, much like most of the
surface of Earth.
'
6.2 Venus exploration
In the early 1600s, Galileo trained his primitive telescope
on Venus and noted that the planet has lunar-like phases,
lending further credence to the Copernican Sun-centered
model of the Solar System. In the 1880s, the Russian
astronomer Lomonosov noted that Venus exhibits a gray
halo when viewed against the Sun, and he correctly
inferred that Venus has an atmosphere. Improvements in
telescopes and the use of spectroscopy in the middle of the
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