540 PART 4^ |^ THE SOLAR SYSTEM
that, as in the case of Uranus, its blue-green color was caused by
methane in its hydrogen-rich atmosphere absorbing red light.
Neptune’s density showed that it is a Jovian planet rich in hydro-
gen, but almost no detail was visible from Earth, so even its
period of rotation was uncertain.
Voyager 2 passed only 4900 km (3050 mi) above Neptune’s
cloud tops, closer than any spacecraft had ever come to one of
the Jovian planets. Th e images it captured revealed that Neptune
is marked by dramatic belt–zone circulation parallel to the
planet’s equator. Voyager 2 also saw at least four cyclonic distur-
bances. Th e largest, dubbed the Great Dark Spot, looked similar
to the Great Red Spot on Jupiter (■ Figure 24-13). Neptune’s
Great Dark Spot was located in the southern hemisphere and
rotated around its center counterclockwise, with a period of
about 16 days. Like the Great Red Spot, it appeared to be caused
by gas rising from its planet’s interior. Unexpectedly, when the
Hubble Space Telescope began imaging Neptune in 1994, the
Great Dark Spot was gone, and new cloud features were seen
appearing and disappearing in Neptune’s atmosphere (Figure
24-13). Evidently, the cyclonic disturbances on Neptune are not
nearly as long lived as Jupiter’s Great Red Spot, which has per-
sisted for at least 400 years.
Th e Voyager 2 images reveal other cloud features standing
out against the deep blue of the methane-rich atmosphere. Th e
white clouds are made of crystals of frozen methane and range up
to 50 km above the deeper layers, just where the temperature in
Neptune’s atmosphere is low enough for rising methane to freeze
into crystals. (See Figure 24-5.) Presumably these features are
related to rising convection currents that produce clouds high in
Neptune’s atmosphere where they catch sunlight and appear
bright. Special fi lters can reveal these cloud belts in visual-wave-
length images and at infrared wavelengths (■ Figure 24-14).
Some observations made by the Hubble Space Telescope suggest
that atmospheric activity on Neptune may be related to fl ares
and other eruptions on the sun, but more data are needed to
explore this connection.
As on the other Jovian worlds, winds circle Neptune parallel
to its equator, but Neptune’s winds blow at very high speeds and
tend to blow backward—against the rotation of the planet. Why
Neptune should have such high-speed retrograde winds is not
understood, and it is part of the larger problem of understanding
belt–zone circulation.
Now that you have seen belts and zones on all four of the
Jovian planets (assuming that the very faint clouds observed on
Uranus are in fact traces of belt–zone circulation), you can ask
what drives this circulation. Because belts and zones remain
parallel to a planet’s equator even when the planet rotates at a
high inclination, as in the case of Uranus, it seems reasonable
to believe that the atmospheric circulation is dominated by the
rotation of the planet, and perhaps also by heat fl ow and
circulation currents in the liquid interior, but not by solar
heating.
Neptune was tipped slightly away from the sun when the Hubble Space Telescope
recorded this image. The interior is much like Uranus’s, but there is more heat.
(NASA)Celestial Profi le 10: Neptune
Motion:
Average distance from the sun 30.1 AU (4.50 109 km)
Eccentricity of orbit 0.010
Inclination of orbit to ecliptic 1.8°
Average orbital velocity 5.4 km/s
Orbital period 164.8 y
Period of rotation 16.05 h
Inclination of equator to orbit 28.8°Characteristics:
Equatorial diameter 4.95 105 km (3.93 D⊕)
Mass 1.03 1026 kg (17.2 M⊕)
Average density 1.66 g/cm^3
Gravity 1.2 Earth gravities
Escape velocity 25 km/s (2.2 V⊕)
Temperature at cloud tops 215°C (355°F)
Albedo 0.35
Oblateness 0.017Personality Point:
Because the planet Neptune looked so blue, astronomers named it after
the Roman god of the sea, Neptune (Poseidon to the Greeks). His wife
was Amphitrite, granddaughter of Ocean, one of the Titans. Neptune
controlled the storms and waves and was a powerful god, not to be
trifl ed with. His three-pronged trident became the symbol for the planet
Neptune.