The Solar System

CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 475

The Rotation of Venus

Nearly all of the planets in our solar system rotate counterclock-
wise as seen from the north. Uranus is an exception, and so is
Venus.
In 1962, radio astronomers were able to transmit a radio
pulse of precise wavelength toward Venus and detect the echo
returning some minutes later. Th at is, they detected Venus by
radar. But the echo was not at one precise wavelength. Part of the
refl ected signal had a longer wavelength, and part had a shorter
wavelength. Evidently the planet was rotating; radio energy
refl ected from the receding edge was redshifted, and radio energy
refl ected from the approaching edge was blueshifted. From this
Doppler eff ect, the radio astronomers could tell that Venus was
rotating once every 243.0 Earth days. Furthermore, because the
western edge of Venus produced the blueshift, the planet had to
be rotating in the backward direction.
Why does Venus rotate backward, and very slowly? For
decades, textbooks have suggested that proto-Venus was set
spinning backward when it was struck off -center by a large plan-
etesimal. Th at is a reasonable possibility; you have seen that a
similar collision probably gave birth to Earth’s moon. But there
is an alternative. Mathematical models suggest that the rotation
of a Terrestrial planet with a molten core and a dense atmo-
sphere can be gradually reversed by solar tides in its atmosphere.
Notice the contrast between the catastrophic theory of a giant
impact and the evolutionary theory of atmospheric tides. It is
possible that both mechanisms played a role in causing Venus’s
peculiar rotation. Perhaps Venus’s very slow rotation is the expla-
nation for why there is no dynamo eff ect to produce a magnetic
fi eld.

A History of Venus

Earth passed through four major stages in its history (see
Chapter 20), and you have seen how the moon and Mercury
were aff ected by their own versions of the same stages. Venus,
however, has had a peculiar passage through planetary develop-
ment, and its history is diffi cult to understand. Planetary scien-
tists are not sure of all the details about how the planet formed
and diff erentiated, how it was cratered and fl ooded, or how its
surface has evolved.
Venus is only slightly closer to the sun than Earth, so you
might expect from the condensation sequence that it should
have a similar over-all composition with perhaps slightly higher
metal content than Earth. Instead, Venus’s uncompressed den-
sity is slightly less than Earth’s (look back to Table 19-2). Th e
density and size of Venus still require that it have a dense metal-
lic interior much like Earth’s. However, if the metal in Venus’s
core is molten, then you would expect the dynamo eff ect to
generate a magnetic fi eld. But, no spacecraft has detected a
magnetic fi eld around Venus; its magnetic fi eld must be at least
25,000 times weaker than Earth’s. Some theorists wonder if the

core of the planet is solid. If it is solid, scientists are puzzled by

how Venus got rid of its internal heat faster than Earth did.

Because the planet lacks a magnetic fi eld, it is not protected

from the solar wind. Th e solar wind slams into the uppermost

layers of Venus’ atmosphere, forming a bow shock where the

wind is slowed and defl ected (■ Figure 22-9). Planetary scientists

know little about the diff erentiation of the planet into core and

mantle, so the size of the core shown in Figure 22-9 is estimated

by analogy with Earth’s. Th e magnetic fi eld carried by the solar

wind drapes over Venus like seaweed over a fi shhook, forming a

long tail within which ions fl ow away from the planet. You will

see in Chapter 25 that comets, which also lack magnetic fi elds,

interact with the solar wind in the same way.

Studies of moon rocks show that the moon formed as a sea

of magma; and, presumably, Venus and Earth formed in the

same way and never had primeval atmospheres rich in hydrogen.

Instead, they outgassed carbon dioxide atmospheres as they

formed. Calculations show that Venus and Earth have outgassed

about the same amount of CO 2 , but Earth’s oceans have dis-

solved that CO 2 and converted it to sediments such as limestone.

Th e main cause of the diff erence between surface conditions on

Earth and Venus is the lack of water on Venus. Venus may have

had oceans when it was young; but, because Venus is closer to the

sun than Earth, it was initially warmer, and the CO 2 in the atmo-

sphere created a greenhouse eff ect that made the planet even

warmer. Th at process could have dried up any oceans that did

exist and reduced the ability of the planet to clear its atmosphere

of CO 2. As more CO 2 was outgassed, the greenhouse eff ect grew

even more severe. Venus became trapped in a runaway green-

house eff ect.

■ Figure 22-9

By analogy with Earth, the interior of Venus should contain a molten core

(size estimated here), but no spacecraft has detected a planetary magnetic

fi eld. Thus, Venus is unprotected from the solar wind, which strikes the

planet’s upper atmosphere and is defl ected into an ion tail.

Bo

ws

ho

ck

Ion tail

Solar wind

To sun