Fundamental Astronomy

Fig. 7.38. A composed image of Jupiter taken by the
Cassini spacecraft in December 2000. The resolution is
is lower, the hydrogen is present as normal molecular
hydrogen, H2. At the top there is a 1000 km thick
atmosphere.
The atmospheric state and composition of Jupiter has
been accurately measured by the spacecraft. In situ observations
were obtained in 1995, when the probe of
the Galileo spacecraft was dropped into Jupiter’s atmosphere.
It survived nearly an hour before crushing under
the pressure, collecting the first direct measurements of
Jupiter’s atmosphere.
Belts and zones are stable cloud formations (Fig.
7.38). Their width and colour may vary with time, but
the semi-regular pattern can be seen up to the latitude
50 ◦ . The colour of the polar areas is close to that of the
belts. The belts are reddish or brownish, and the motion
of the gas inside a belt is downward. The gas flows
upward in the white zones. The clouds in the zones are
slightly higher and have a lower temperature than those
in the belts. Strong winds or jet streams blow along the
zones and belts. The speed of the wind reaches 150 m/s
at some places in the upper atmosphere. According to
the measurements of the Galileo probe, the wind speeds
7.14 Jupiter
114 km/pixel. The dark dot is the shadow of the moon Europa.
(NASA/JPL/University of Arizona)
in the lower cloud layers can reach up to 500 m/s. This
indicates that the winds in deeper atmospheric layers
are driven by the outflowing flux of the internal heat,
not the solar heating.
The colour of the Great Red Spot (GRS) resembles
the colour of the belts (Fig. 7.39). Sometimes it is almost
colourless, but shows no signs of decrepitude. The GRS
is 14,000 km wide and 30,000–40,000 km long. Some
smaller red and white spots can also be observed on
Jupiter, but their lifetime is generally much less than
a few years.
The ratio of helium to hydrogen in the deep atmosphere
is about the same as in the Sun. The results of
the Galileo spacecraft gave considerably higher abundance
than previous estimates. It means that there are no
significant differentiation of helium, i. e. helium is not
sinking to the interior of the planet as was expected according
to the earlier results. Other compounds found in
the atmosphere include methane, ethane and ammonia.
The temperature in the cloud tops is about 130 K.
Jupiter radiates twice the amount of heat that it receives
from the Sun. This heat is a remnant of the
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