Fundamental Astronomy

Fig. 7.50. (Left) Neptune shows more features than Uranus.
In the picture of Voyager 2 the Great Dark Spot, accompanied
by bright, white clouds is well visible. Their
appearance is changing rapidly. To the south of the Great
Dark Spot is a bright feature and still farther south is another
dark spot. Each feature moves eastward at a different
Cloud structures are more complicated than on
Uranus, and some dark spots, like in Jupiter, were visible
during the Voyager fly-by (Fig. 7.50). The speed of
the winds are high, up to 400 m/s.
Like other giants, Neptune also has rings (Fig. 7.51).
The rings were discovered by Voyager 2, although their
existence was already expected prior the fly-by. Two
relatively bright but very narrow rings are at a distance
of 53,000 and 62,000 km from the centre of the planet.
Moreover, there are some faint areas of fine dust.
There are 13 known moons, six of which were discovered
by Voyager 2. The largest of the moons, Triton,
is 2700 km in diameter, and it has a thin atmosphere,
mainly composed of nitrogen. The albedo is high: Triton
reflects 60–80% of the incident light. The surface
is relatively young, without any considerable impact
craters (Fig. 7.52). There are some active “geysers” of
liquid nitrogen, which partly explains the high albedo
Fig. 7.51. The rings of Neptune. Ring particles are small and
best visible in the forward scattered light. There are several
brightenings in the outermost ring. One of the rings appears
to have a twisted structure. Neptune at left is overexposed.
(NASA/JPL)
7.16 Uranus and Neptune
velocity. (Right) Details of the Southern Dark Spot. The
V-shaped structure near the right edge of the bright area
indicates that the spot rotates clockwise. Unlike the Great
Red Spot on Jupiter, which rotates counterclockwise, the
material in the Neptune’s dark oval will be descending.
(NASA/JPL)
185