Encyclopedia of the Solar System 2nd ed

144 Encyclopedia of the Solar System

strongly affected by the opacity of water vapor. The four
Pioneer Venusprobes carried infrared net flux radiometers
(points labeled “7” in Fig. 5), andVenera 11and 12 carried
an instrument working with weaker absorptions in the near
infrared (dashed line labeled “2”). These measurements re-
late to the atmosphere far from the probe and are not af-
fected by the difficulty, encountered by the gas chromato-
graph and mass spectrometers, of obtaining an undistorted
sample of the gas. It is likely that many of the divergences
are due to the extreme difficulty of measuring such small
quantities of a reactive molecule at the high temperatures
of the lower atmosphere, but some of the variations may re-
flect real effects of latitude or height. Particularly puzzling
has been the indication from the mass spectrometer on the
Pioneer Venus Large Probethat the mole fraction falls off
by nearly a factor of 10 between 10 km altitude and the sur-
face (Fig. 5, line “1”). It is likely that this result is incorrect;
it is not supported by remote sensing of this region in the
near-infrared windows.
The ratio of heavy to light hydrogen (D/H) (150 times
the value on Earth) was first measured on ions in the iono-
sphere and has been confirmed by the mass spectrometer
just mentioned and by analysis of spectra taken from Earth
in the near-infrared windows. In turn, the deuterium pro-
vides a valuable signature for distinguishing Venus water
vapor in the mass spectrometer from any contaminants car-
ried along from Earth. The likely enrichment process is
discussed in Section 6.

3. Middle and Upper Atmosphere

3.1 Temperatures

The middle atmosphere (stratosphere and mesosphere) ex-
tends from the tropopause at 65 km to the temperature min-
imum or mesopause at about 95 km (see Fig. 2). The upper
atmosphere lies above this level. Here, temperatures can
no longer be measured directly, but are inferred from the
scale heightsof various gases with use of thehydrostatic
equation. On Earth and most other bodies, this region is
called the thermosphere because temperatures in the outer
layer, or exosphere, are as high as 1000 K. The temperature
is much more modest on Venus; the exospheric temperature
is no more than 350 K on the day side. The corresponding
region on the night side is sometimes called a cryosphere
(cold sphere) because its temperature is not far above 100 K.
Measurements of these temperatures byPioneer Venus Or-
biterare shown in Fig. 6. The large temperature differ-
ence translates into a pressure difference that drives strong
winds from the day side to the night side, at all levels above
100 km.
On Earth, the exospheric temperature changes markedly
with solar activity, being perhaps 700 K at sunspot mini-
mum and 1400 K at maximum. The corresponding change
at Venus is much more modest, perhaps 50 K. Many of these

FIGURE 6 Diurnal variation of temperature in the upper

thermosphere. Scale heightsHwere measured by the mass

spectrometer onPioneer Venus Orbiterand converted to

temperatures by the formulaT=mgH/k. The measurements

sweep out the entire range of local solar time as Venus moves

around the Sun about 2^1 / 2 times. (From Hunten et al., 1984.)

differences are traceable to the fact that CO 2 , the princi-

pal radiator of heat, is just a trace constituent of Earth’s

atmosphere but is the major constituent for Venus (and

also Mars). Venus’s slow rotation is responsible for the very

cold temperatures on the night side, although the atmo-

sphere does rotate substantially faster than the solid planet.

[SeeMars:LandingSiteGeology,Mineralogy,and

Geochemistry.]

3.2 Ionosphere

The principal heat source for the thermosphere is the pro-

duction of ions and electrons by far-ultraviolet solar radia-

tion. The most abundant positive ions are O+ 2 ,O+, and CO+ 2.

As part of these processes, CO 2 is dissociated into CO and O,

and N 2 into N atoms. All of these ions, molecules, and atoms

have been observed or directly inferred (Fig. 7). Some of

the O+ions (with an equal number of electrons) flow around

FIGURE 7 Daytime (dashed) and nighttime (solid) number

densities of the major gases in the thermosphere obtained by

fitting a large number of measurements by the mass

spectrometer onPioneer Venus Orbiter.(From Hunten et al.,

1984.)