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9.2 Venus
Venus is named for the Roman goddess of beauty and love, perhaps because it is the brightest object in
the sky after the Sun and Moon, and it is almost always seen near dawn or dusk. Venus is so bright that if
the air is very clear, you can sometimes find Venus in broad daylight. Of all the planets, Venus is most
like the Earth in diameter and mass. We might therefore expect it to be like the Earth in other ways;
however, over the past several decades, observations from Earth and with spacecraft and landers have
revealed that Venus and the Earth have radically different surfaces and atmospheres.
The Venusian Atmosphere
Venus's atmosphere is so thick that it completely hides its surface (fig. 9.9). The atmosphere is 96%
carbon dioxide as determined from its spectrum and from measurements with space probes. The kinds of
gases in the atmosphere determine what wavelengths of sunlight are absorbed, so astronomers examine
absorption lines in the reflected sunlight to find the composition and density of the gas. Moreover,
spacecraft have descended through the atmosphere to the surface and have sampled its atmosphere. In
addition to carbon dioxide, Venus's atmosphere contains about 3.5% nitrogen and very small amounts of
water vapor and other gases.
Figure 9.9
Photograph through an ultraviolet filter of the clouds of Venus. The picture is artificially colored and
enhanced to show the clouds clearly.
Q. If this were the view of Venus from Earth, what would it imply about the position of Venus in its
orbit?
answer
Spectra also reveal the nature of the Venusian clouds: they are composed of sulfuric acid droplets that
formed when sulfur compounds—perhaps ejected from volcanoes—combined with the traces of water in
the atmosphere. These clouds permanently cover the planet and are very high and thick, beginning at
about 30 kilometers (19 miles) above the surface and extending upward to about 60 kilometers (37
miles). Below the clouds, the Venusian atmosphere is relatively clear, and some sunlight penetrates to the
surface. The light is tinged orange, however, because the blue wavelengths are absorbed in the deep
cloud layer.
In Venus's upper atmosphere, wind speeds can exceed 350 kilometers per hour (210 mph), but near the
surface the winds move much more slowly, just a few kilometers per hour. The motion of the atmosphere
is driven by the Sun's heating near the equator, which causes the gas to expand most there. Its upper
layers then flow toward the cooler polar regions, where they sink and flow back toward the equatorial
regions. This produces a huge vortex near each pole, like the water running down a drain. The rapidly
changing shape of these vortices has been studied by the European Space Agency's Venus Express
mission (fig. 9.10).
Figure 9.10
Series of images of Venus's southern polar vortex made by ESA's Venus Express spacecraft at 24­hour
intervals. The images are taken in infrared light, and darker parts of the image correspond to where the
cloud tops are deeper. Each image is about 4000 km (about 2500 miles) across.
Venus's atmosphere is extremely dense. It exerts a pressure roughly 100 times that of the Earth's,
equivalent to the pressure you would feel under 1000 meters (3000 feet) of water. We will discuss in
section 9.4 why Venus has such a massive atmosphere, but there are other features of the planet that we
need to understand first. One of the most important of those features is that its lower atmosphere is
extremely hot, more than 750 K (about 900°F), hot enough to melt lead!
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The Runaway Greenhouse Effect
Venus's surface is hotter than Mercury's even though it is almost twice as far from the Sun. It remains
hotter than the hottest parts of Mercury even at night, and in fact there is very little change in temperature
between day and night. What makes the surface of a planet so similar to the Earth, and only slightly
nearer the Sun, so very hot?
Venus's carbon dioxide atmosphere creates an extremely strong runaway greenhouse effect. We
discussed in chapter 6 how gases in Earth's atmosphere allow sunlight to enter and warm the surface but
retard the heat so generated from escaping to space. That is, certain gases trap a planet's heat by
hindering infrared radiation from escaping to space (see fig. 6.23). Carbon dioxide in the Earth's
atmosphere traps heat, creating a weak greenhouse effect that keeps the Earth warmer than it would be
otherwise. Venus, however, has about 300,000 times more carbon dioxide than the Earth, and so its
greenhouse effect is correspondingly much stronger.
Measurements of hydrogen escaping to space made by the Venus Express spacecraft suggest that Venus
used to have much more water, with oceans probably covering much of its surface billions of years ago.
This suggests a dire scenario with cautionary implications for Earth. Venus may once have had an
environment similar to Earth's, but greenhouse warming caused its oceans to begin to evaporate. This
added further to the greenhouse effect because water vapor also absorbs infrared light. As the heat
trapping grew stronger, the oceans completely boiled away. Over time sunlight broke down the water
molecules in the atmosphere, allowing the light hydrogen atoms to escape and leaving behind the baked
dry surface we see today.
The Surface of Venus
Despite the extremely hostile conditions on Venus's surface, several Russian Venera spacecraft landed
there in the 1970s and 1980s and transmitted pictures back to Earth. These robotic spacecraft made a
variety of measurements and sampled the rocks, showing them to be of volcanic origin. The landers
lasted at most about two hours before succumbing to the high temperatures and atmospheric pressure.
The pictures (fig. 9.11) show a barren surface covered with flat, broken rocks and lit by the pale orange
glow of sunlight diffusing through the deep clouds. Two cameras on each spacecraft scanned a narrow
strip from horizon to horizon. Figure 9.11 combines portions from each camera to show a partial view of
the landscapes from the lander in the foreground to the horizon and the yellow sky.
Figure 9.11
Images from Venera 13 (left) and Venera 14 (right). The two cameras on each spacecraft scanned narrow
strips that intersected at each end. Portions of each strip are combined to provide a clearer sense of the
landscape. Distant hills are visible, as is nearby volcanic rock.
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The surface of Venus is hidden beneath its thick clouds, but planetary scientists can map its features with
radar stations on Earth or with spacecraft orbiting Venus. Just as radar penetrates terrestrial clouds to
locate a runway through fog, so too radar penetrates the Venusian clouds, revealing the planet's surface.
Figure 9.12 shows a radar map of Venus made in the 1990s by Magellan, a U.S. spacecraft.
Figure 9.12
Global radar map of Venus made by the Magellan Venus­orbiting satellite. Colors indicate the relative
height of surface features. Lowlands are blue; high elevations are orange.
Venus
Magellan met a deliberately engineered fiery doom in 1994. Its orbit was altered so that it plunged into
Venus's atmosphere. Analysis of its final tumblings gave astronomers data on the density of Venus's
upper atmosphere.
The radar map shows that Venus is less mountainous than Earth, with most of its surface being low,
gently rolling lava fields. Only two major highland regions, Ishtar Terra and Aphrodite Terra, rise above
the lowlands to form land masses similar to terrestrial continents. Ishtar, named for the Babylonian
goddess of love, is about the size of Australia. Ishtar is studded with volcanic peaks, the highest of
which, Maxwell Montes, rises more than 11 kilometers (about 7 miles) above the average level of the
planet. (Notice that because no oceans exist on Venus, “sea level” has no meaning as a reference height.)
The other major highland region, Aphrodite, bears the ancient Greek name for Venus and is about the
size of South America. Together, Ishtar and Aphrodite compose only about 8% of Venus's surface, a far
smaller fraction than for Earth, where continents and their submerged margins cover about 45% of the
planet.
Recent measurements by the Venus Express spacecraft comparing the infrared radiation emitted by the
highlands and lowlands regions suggest another similarity to Earth. The highlands rock has properties
similar to the granite in Earth's continents. This is consistent with the hypothesis that Venus once had
oceans, with volcanic activity forging granite when the water chemically combined with molten rock.
ANIMATION
Venus
The many odd and unique structures seen in the radar maps have proved puzzling to planetary geologists.
Venus is so similar in diameter and mass to the Earth that they expected to see landforms there similar to
those on the Earth. However, the features seen bear little resemblance to the features that result from
plate tectonics, such as continental blocks, crustal rifts, and trenches at plate boundaries.
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The Magellan spacecraft was able to detect features as small as about 100 meters (about 300 feet) across.
Figure 9.13 shows close­ups of some of the more intriguing images that Magellan transmitted. Brighter
regions reflected radar more strongly, which relates to the angle and roughness of the surface.
Figure 9.13
Magellan radar images of a variety of features on Venus, all shown to the same scale. The views look
straight down at the surface, with the radar illumination from the left. In addition to the individually
noted features, most of these images show long cracks or faults in the bedrock.
Although Venus has some impact craters and crumpled mountains, volcanic landforms dominate. These
include peaks with immense lava flows, “pancake­shaped” domes of uplifted rock, long narrow faults
(cracks), and peculiar lumpy terrain. All these features indicate a young and active surface, a deduction
borne out by the scarcity of impact craters. From the small number of craters, scientists have concluded
that virtually all of Venus's original surface has been paved over by volcanic activity. The surface we see
is probably at most half a billion years old, much younger than Earth's continental surface, and some
regions may be less than 10 million years old. Such estimates of crustal age are difficult to make,
however, because the Venusian atmosphere is so dense that all but the largest infalling bodies (bigger
than a few hundred meters) are broken up in it.
Are the Venusian volcanoes still active? Eruptions have not been seen directly, but some lava flows
appear very fresh. Differences in the infrared radiation from some volcanic peaks (fig. 9.14) seen by
Venus Express suggest that these are relatively recent lava flows. In addition, electrical discharges,
perhaps lightning, have been detected near some of the larger peaks. On Earth, volcanic eruptions
frequently generate lightning, and some astronomers think the electrical activity indicates that Venus's
volcanoes are still erupting. Such eruptions might also explain brief increases in sulfur content detected
in the Venusian atmosphere, changes similar to those produced on Earth by eruptions here.
Figure 9.14
Perspective image of Venus's volcano Idunn Mons based on Magellan radar data in the top image. The
volcano appears to have relatively recent lava flows surrounding it, based on infrared observations made
by the Venus Express satellite, shown by a false­color overlay of temperature differences in the bottom
image.
The numerous volcanic peaks, domes, and uplifted surface regions suggest to some scientists that heat
flows less uniformly within Venus than within the Earth. Although some locations on Earth (Yellowstone
Park and the Hawaiian Islands, for example) are heated anomalously by “plumes” of rising hot rock, such
plumes seem to dominate on Venus. As hot rock wells upward, it bulges the crust, stretching and
cracking it. We may be viewing on our sister planet what Earth looked like as its crust began to form and
before smooth heat flows were established. Alternatively, Venus and Earth may differ for deeper reasons.
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The Interior of Venus
The interior of Venus is probably similar to the Earth's, an iron core and rock mantle (fig. 9.15).
Planetary geologists have no seismic information, so, as with Mercury, they must rely on deductions
from its gravity and density, which are similar to the Earth's. Why then are the surface features so
different?
Figure 9.15
Artist's sketch of the interior of Venus.
One important difference is the water content of the rock in these two worlds. Rocks that contain water
trapped in their structure melt at a lower temperature than similar rocks that lack water. Moreover, when
they become molten, they are “runnier,” which makes it easier for the melted rock to flow. As a result,
convection can be more vigorous in a planet whose rocks are rich in water compared to a planet with
drier rocks. Furthermore, because the water­rich rock melts at a lower temperature, the solid crust of such
a planet will be thinner. Other things being equal, we therefore expect wet Earth to have a thinner crust
than dry Venus.
A thin crust, such as we have on Earth, breaks into plates more easily. This breaking allows crustal
motion and activity to occur more or less continuously and at many places on the surface. For Venus, the
thicker crust holds heat in, keeping the interior hot, but the convective motions that develop are unable to
break up the thick crust.
Ultimately, however, the trapped heat must escape, and astronomers have proposed different hypotheses
for what then occurs. According to one idea, at points where the hot, rising material reaches the crust, the
surface bulges upward and weakens, and volcanoes may form. Where cooler material sinks, the thick
crust crumples into a continent­like region. This creates a planet with few and small continents and
whose surface is active but only in isolated spots.
Another idea is that the trapped heat gradually melts the bottom of the thick crust, thinning it and
allowing it to break up. This may happen over widespread areas, flooding large portions of the planetary
surface with lava in a brief time. The heat then rapidly escapes to space, the interior cools, and the crust
again thickens. Surface activity therefore subsides, but heat is trapped once more, and the process may
repeat but only at intervals of hundreds of millions of years.
Rotation of Venus
Venus spins on its axis more slowly than any other planet in the Solar System, taking 243 days to
complete one rotation relative to the stars. Moreover, its spin is retrograde (“backward”) compared with
the direction of rotation of the other terrestrial planets. Because the planet's spin is retrograde, the Sun
rises in the west and sets in the east. The slow rotation also makes the solar day there very long,
approximately 117 Earth days (fig. 9.16).
Figure 9.16
Venus spins slowly backward, so that a day lasts a little more than half its orbit.
This slow and retrograde spin has led some astronomers to hypothesize that Venus was struck shortly
after its birth by a huge planetesimal; the impact set Venus spinning backward, and tidal forces exerted
by the Sun have slowed it since. A less dramatic explanation of the spin is that Venus has been affected
by a combination of tidal forces exerted by the Sun—and perhaps the Earth and Jupiter—so that the tilt
angle of its rotation axis may have shifted over time. Venus probably has an interior similar to the
Earth's, but it rotates so slowly that it cannot generate a strong magnetic field as the Earth does.