140 Encyclopedia of the Solar System
(a)(b)FIGURE 1 Ultraviolet images of Venus’ clouds as seen from
Pioneer Venus Orbiteron February 5, 1979 (a) and on
February 26, 1979 (b). (Courtesy of NASA.)
Venera 15and 16 , which were radar mappers, andVega
1 and 2 , which dropped both probes and balloon-borne
payloads on their way to Halley’s Comet. Early missions
were devoted to reconnaissance, in particular to confirma-
tion of the high surface pressure and temperature inferred
from the microwave radio measurements. [SeePlanetary
ExplorationMissions.]
The composition of the clouds was another important
question, but it was actually answered first from analysis
of ground-based observations of the polarization of light
reflected from the planet. Although such measurements
were first made in the 1930s, the computers and programs
to carry out the analysis did not exist until the middle 1970s.
This analysis pinned down the refractive index and showed
that the particles are spherical; these two properties even-
tually led to the identification of supercooled droplets of
concentrated sulfuric acid (H 2 SO 4 ). Measurements from
thePioneer Venusprobes confirmed this composition and
gave much greater detail on the sizes and layering of the
haze.1.2 Measuring Techniques
Three principal techniques can be applied from Earth:
spectroscopy, radiometry, and imaging. They can be used
over a wide variety of wavelengths, from the ultraviolet to
the shortest part of the radio spectrum. Spectroscopy, as
mentioned earlier, was first applied in 1932 and led to the
discovery of CO 2. Little more was done until the middle
1960s, when traces of water vapor were found and a tight
upper limit was set on the amount of O 2. The develop-
ment of Fourier spectroscopy permitted an extension fur-
ther into the infrared, where CO, HCl, and HF were ob-
served. Radiometry, and especially polarimetry, eventually
led to the identification of the substance of the cloud parti-
cles. After the near-infrared “windows” were identified (see
Section 1.4), starting in 1983, spectroscopy of deeper parts
of the atmosphere provided important further information.
Visual studies, followed more recently by photography and
infrared imaging, disclosed the 4-day rotation of the cloud
tops and the 6-day period of a deeper region. Similar re-
marks apply to radio astronomical studies. Radiometry gave
the data that finally led to the establishment of the high
surface temperature, and millimeter-wave spectroscopy has
led to the interesting results on CO discussed in Section 3.4.
Until the early 1990s, all ground-based radio work used ra-
diation from the whole disk, but modest spatial resolution is
beginning to be available by interferometry (the technique
of combining the signals from several antennas).
Many of the same techniques have been applied from
flyby and orbiting spacecraft, but an important addition is
the radio occultation experiment, which tracks the effect of
the atmosphere on the telemetry carrier as the spacecraft
disappears behind the atmosphere or reappears from be-
hind it. On Venus, the regions observed in this way are the
ionosphere and the neutral atmosphere from about 34 to
90 km. At greater depths, therefractionof the waves by the
atmosphere is so great that the beam strikes the surface and
never reappears. In addition to carrying several instruments
for remote sensing,Pioneer Venus Orbiter(1978–1992)