Venus: Atmosphere 147
and a third population (“Mode 3”), greater than 6μmin
diameter, is also found. The existence of distinct modes
is still not understood; the optical properties of all three
are generally consistent with sulfuric acid, although there
is some suspicion that the rare Mode 3 particles might be
solid crystals.
4.3 Cloud Chemistry
A cloud particle of diameter 1μm has a sedimentation veloc-
ity of 7.5 m/day at 60 km; this velocity varies as the square of
the size. Though small, these velocities eventually carry the
particles out of the cloud to lower altitudes and higher tem-
peratures, where they will evaporate. At still lower heights
the hydrated H 2 SO 4 must decompose into H 2 O, SO 2 , and
oxygen, all of which are (at least probably) much more abun-
dant beneath the clouds than above them (Table 1). Atmo-
spheric mixing carries these gases back upward. Nearly all
the water vapor is absorbed by the cloud particles. Above the
clouds, solar ultraviolet photons attack the SO 2 , starting the
process that converts it back to H 2 SO 4. An important inter-
mediate is the reactive free radical SO, and probably some
elemental sulfur is produced. Ultraviolet spectra (pertain-
ing to the region above the clouds) reveal the presence of
the small amounts of SO 2 shown in Table 1, but much less
than has been measured below the clouds.
Sulfuric acid is perfectly colorless in the blue and near
ultraviolet, and the yellow coloration that provides the con-
trasts of Fig. 1 must be caused by something else. Cer-
tainly the most likely thing is elemental sulfur, but yellow
compounds are abundant in nature, and the identification
remains tentative. The photochemical models do predict
production of some sulfur, but it is a minor by-product,
and the amount produced is uncertain. Probably the most
likely alternative is ferric chloride, particularly for the Mode
3 particles in the lower cloud.
4.4 Lightning
Electromagnetic pulses have been observed by the entry
probesVenera 11, 12 , 13 , and 14 ,byPioneer Venus Or-
biter, and byGalileo. For many years it seemed that the
most likely source was lightning, and many workers are
convinced of its reality. However, some searches for the
corresponding optical flashes have been negative, except
for one ambiguous interval fromVenera 9. A recent study
from the Earth does seem to have turned up a few optical
events. A close flyby by theCassinispacecraft saw no evi-
dence whatever of any impulses with a sensitive instrument
that, in a later Earth encounter, found them in abundance
(Gurnett et al., 2001). This is strong evidence against the
presence of lightning on Venus, at least at the time and
in the region that was observed. The negative results may
simply be because the flashes are too faint, but another con-
cern is that conditions on Venus do not seem propitious for
large-scale charge separation. On Earth, lightning is seen
during intense precipitation and in volcanic explosions. In
thunderstorms, large drops are efficient at carrying charge
of one sign away from the region where it is produced, and
the gravitational force is large enough to resist the strong
electric fields. This is not the case for small particles. There
does not seem to be enough cloud mass on Venus to gener-
ate large, precipitating particles, although they are difficult
to detect and may have been missed. As for volcanic explo-
sions, most of them are driven by steam; on Venus, water
is very scarce, and the 93-bar surface pressure means that,
other things being equal, any explosion is damped by a fac-
tor of 93 compared with Earth. In spite of these concerns,
lightning remains one of the more plausible explanations
for the radio bursts, but it is important to seek others.5. General Circulation
Careful tracking of entry probes, notably the four of Pioneer
Venus, has shown that the entire atmosphere is superrotat-
ing, with a speed decreasing smoothly from the 100 m/s
at the cloud top to near zero at 5–10 km. Winds in the
meridional direction are much slower. Because the density
increases by a large factor over this height range, the an-
gular momentum is a maximum at 20 km. Small amounts
of superrotation are observed in many atmospheres, espe-
cially thermospheres, but they are superposed on a rapid
planetary rotation. (A familiar example is the midlatitude
“prevailing westerlies” on the Earth.) In spite of a great deal
of theoretical effort and a number of specific suggestions,
there is still no accepted mechanism for the basic motion
of the Venus atmosphere, nor is it given convincingly in any
numerical general circulation model. What is needed is to
convert the slow apparent motion of the Sun (relative to a
fixed point on Venus) into a much more rapid motion of the
atmosphere. There must also be a slow meridional (north–
south) component, sometimes called a Hadley circulation,
to transport heat from the equatorial to the polar regions.
There are no direct measurements above the cloud tops,
but deductions from temperature measurements suggest
a slowing of the 100 m/s flow up to perhaps the 100-km
level. At still greater heights the dominant flow is a rapid
day–night one, first suggested on theoretical grounds and
confirmed by the large observed temperature difference.
But the flow is not quite symmetrical; maxima in the hy-
drogen and helium concentrations, and in several airglow
phenomena, are systematically displaced from the expected
midnight location toward morning. Possible explanations
are a wind of around 65 m/s or a wave-induced drag force
that is stronger at the morning side than the evening.6. Origin and Evolution
It is generally believed that the Sun, the planets, and their
atmospheres condensed, about 4.6 billion years ago, from