CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 467
atmosphere of Venus in 1978, it discovered that deuterium is
about 150 times more abundant compared to normal hydro-
gen atoms than it is on Earth. Th is abundance of deuterium,
the heavy isotope of hydrogen, could have developed because
Venus has no ozone layer to absorb the ultraviolet radiation in
sunlight. Th ese UV photons broke water molecules into
hydrogen and oxygen. Th e oxygen would have formed oxides
in the soil, and the hydrogen would have leaked away into
space. Th e heavier deuterium atoms would leak away more
slowly than normal hydrogen atoms, which would increase
the ratio of deuterium to normal hydrogen.
Venus has essentially no water now, but the amount of
deuterium in the atmosphere suggests that it may have once
had enough water to make a planetwide ocean at least 25
meters deep. (For comparison, the water on Earth would
make a uniform ocean about 3000 meters deep.) Venus is now
a deadly dry world with only enough water vapor in its atmo-
sphere to make a planet-wide ocean 0.3 meter (1 foot) deep.
Venus’s present lack of water is one of the biggest diff erences
between that planet and Earth.
The Venusian Greenhouse
You saw in Chapter 20 how the greenhouse eff ect warms Earth.
Carbon dioxide (CO 2 ) is transparent to light but opaque to
infrared (heat) radiation. Th at means energy can enter the atmo-
sphere as light and warm the surface, but the surface cannot
radiate the energy back to space easily because the atmospheric
CO 2 is opaque to infrared radiation. Venus also has a greenhouse
eff ect, but on Venus the eff ect is fearsomely strong; that is the
explanation for why Venus, although it is farther from the sun
than Mercury, is actually hotter. Whereas Earth’s atmosphere
contains only about 0.04 percent CO 2 , the atmosphere of Venus
contains 96 percent CO 2 , and as a result temperatures on the
surface of Venus are more than hot enough to melt lead. Th e
thick atmosphere and its high winds carry heat around the planet
effi ciently enough to make surface temperatures on Venus nearly
the same everywhere. Th is evidently off sets the eff ect of the
planet’s slow rotation that should cause a large temperature dif-
ference between the day side and the night side.
Planetary astronomers think they know how Venus got
into such a jam. When Venus was young, it may have been
cooler than it is now; but, because it formed 30 percent closer
to the sun than did Earth, it was always warmer than Earth,
and that unleashed processes that made it even hotter.
Calculations indicate that Venus and Earth should have out-
gassed about the same amount of CO 2 , but Earth’s oceans have
dissolved most of Earth’s CO 2 and converted it to sediments
such as limestone. If all of Earth’s carbon were dug up and
converted back to CO 2 , our atmosphere would be about as
dense as the air on Venus and composed mostly of CO 2 , like
Venus’s atmosphere.
As you saw in the previous section, Venus may once have had
water on its surface, but that water would have begun to evapo-
rate at the temperatures of early Venus. Carbon dioxide is highly
soluble in water, but as surface water disappeared on Venus, the
ability to dissolve CO 2 and remove it from the atmosphere also
would have disappeared. Th e surface of Venus is now so hot that
even sulfur, chlorine, and fl uorine have baked out of the rock
and formed sulfuric, hydrochloric, and hydrofl uoric acid vapors.The Surface of Venus
Given that the surface of Venus is perpetually hidden by clouds,
is hot enough to melt lead, and suff ers under crushing atmo-
spheric pressure, it is surprising how much planetary scientists
know about the geology of Venus. Early radar maps made from
Earth penetrated the clouds and showed that it had mountains,
plains, and some craters. Th e Soviet Union launched a number
of spacecraft that landed on Venus, and although the surface
conditions caused the spacecraft to fail within an hour or so of
landing, they did analyze the rock and transmit a few images
back to Earth. Th e rock seems to be basalt, a typical product of
volcanism. Th e images revealed dark-gray rocky plains bathed in
a deep-orange glow caused by sunlight fi ltering down through
the thick atmosphere (■ Figure 22-2).
Th e U.S. Pioneer Venus main probe orbited Venus in 1978
and made radar maps showing features as small as 25 km in
diameter. Later, two Soviet Venera spacecraft mapped the north
polar region with a resolution of 2 km. From 1992 to 1994, the
U.S. Magellan spacecraft orbited Venus and created radar maps
showing details as small as 100 m. Th ese radar maps provide a
comprehensive look below the clouds.
Th e color of Venus radar maps is mostly arbitrary. Human
eyes can’t see radio waves, so the radar maps must be given some
false colors to distinguish height or roughness or composition.
Some maps use blues and greens for lowlands and yellows and
reds for highlands. Remember when you look at these maps that
there is no liquid water on Venus. Magellan scientists chose to
use yellows and oranges for their radar maps in an eff ort to
mimic the orange color of daylight caused by the thick atmo-
sphere (■ Figure 22-3). When you look at these orange images,
you need to remind yourself that the true color of the rock is
dark gray (How Do We Know? 22-1).
Radar maps of Venus reveal a number of things about the
surface. If you transmit a radio signal down through the clouds
and measure the time until you hear the echo coming back up,
you can measure the altitude of the surface. As a result, part of the
Magellan data is a detailed altitude map of the surface. You can
also measure the amount of the signal that is refl ected from each
spot on the surface. Much of the surface of Venus is made up of
old smooth lava fl ows that do not look bright in radar maps, but
faults and uneven terrain look brighter. Some young, rough lava
fl ows are very rough and contain billions of tiny crevices