Encyclopedia of the Solar System 2nd ed

Venus: Atmosphere 141

actually penetrated the upper atmosphere once per orbit,
and took advantage of this by carrying a suite of instruments
to make measurements in situ. Two mass spectrometers
measured individual gases and positive ions; aLangmuir
probeand a retarding potential analyzer measured elec-
tron and ion densities, temperatures, and velocities; and
a fifth instrument measured plasma waves. Higher-energy
ions and electrons, both near the planet and in the solar
wind, were measured by a plasma analyzer, and important
auxiliary information was provided by a magnetometer. In
addition, the atmospheric drag on the spacecraft gave an
excellent measure of the density as a function of height.
A large number of probes have descended part or all the
way through the atmosphere, and the Vega balloons carried
out measurements in the middle of the cloud region. All
of them have carried an “atmospheric structure” package
measuring pressure, temperature, and acceleration; height
was obtained on the earlyVeneraprobes by radar and on
all probes by integration of the hydrostatic equation. Gas
analyzers have increased in sophistication from the simple
chemical cells onVenera 4to mass spectrometers and gas
chromatographs on later Soviet and U.S. missions. In some
cases, however, there are suspicions that the composition
was significantly altered in passage through the sampling in-
lets, especially below 40 km, where the temperature is high.
A variety of instruments have measured the clouds and their
optical properties. Radiometers observed the loss of solar
energy through the atmosphere, and others have observed
the thermal infrared fluxes. Winds were obtained by track-
ing the horizontal drifts of the probes as they descended,
and the balloons as they floated.Venera 11–14carried radio
receivers to seek evidence of lightning activity.

1.3 Composition

The fact that carbon dioxide is indeed the major gas was
established by a simple chemical analyzer on theVenera 4
entry probe. The mole fraction was found to be about 97%,
in reasonable agreement with the currently accepted value
shown in Table 1. The next most abundant gas is nitrogen;
though it is only 3.5% of the total, the absolute quantity
is about three times that in the Earth’s atmosphere. The
temperature profile is illustrated in Fig. 2, along with a
sketch of the cloud layers.
Many of the strange properties of the atmosphere can be
traced to an extreme scarcity of water and its vapor and the
total absence of liquid water. On Earth, carbon dioxide and
sulfuric, hydrochloric, and hydrofluoric acids are all carried
down by precipitation, a process that is absent in the hot,
dry lower atmosphere of Venus. They all then react and
are incorporated in geological deposits; the best estimates
of the total amount of carbonate rocks in the Earth give a
quantity of CO 2 almost equal to that seen in the atmosphere
of Venus. Free oxygen is undetectable at the Venus cloud
tops; one molecule in ten million could have been seen.

TABLE 1 Composition of the Venus Atmosphere

Mole fraction Mole fraction

Species at 70 km at 40 km

%CO 2 96.5 96.5

N 2 3.5 3.5

ppma He ∼ 12 ∼ 12

Ne 7 7

Ar 70 70

Kr ∼0.2 ∼0.2

CO 5170 45

H 2 O ≤ 145

SO 2 0.05 ∼ 100

H 2 S? 1

COS 0.25

HCl 0.4 0.5

HF 0.005 0.005

O 2 <0.1 0–20

% D/H 1.6 1.6

aParts per million.

FIGURE 2 Temperature profile from the surface to 200 km

altitude, obtained from different experiments in thePioneer

OrbiterandProbes. ONMS, orbiter neutral mass spectrometer;

OAD, orbiter atmospheric drag; BNMS, bus neutral mass

spectrometer. The cloud region with its three layers has been

sketched in. (From Hunten et al., 1984.)