4.5 billion kilometers (3 billion miles) from the Sun, Neptune
orbits the Sun once in 165 years, and therefore has made
not quite a full circle around the Sun since it was discovered.
With an equatorial diameter of 49,528 kilometers (30,775
miles), Neptune is the smallest of our solar system's gas
giants. Even so, its volume could hold nearly 60 Earths.
Neptune is the densest of the four giant planets, about
64 percent heavier than if it were composed entirely of
most obvious feature of the planet in Voyager pictures is
its blue color, the result of methane in the atmosphere.
Methane preferentially absorbs the longer wavelengths of
sunlight (those near the red end of the spectrum). What
are left to be reflected are colors at the blue end of the
methane is not the only constituent in Neptune's atmosphere,
it is one of the most important. Methane cycles through
the atmosphere like this:
ultraviolet radiation destroys methane high in Neptune's
atmosphere by converting it to hydrocarbons such as ethane,
acetylene and haze particles of more complex polymers.
haze particles sink to the cold lower stratosphere, where
they freeze and become ice particles.
hydrocarbon ice particles gently fall into the warmer
troposphere, where they evaporate back into gases.
hydrocarbon gases mix deeper into the atmosphere where
the temperature and pressure are higher, mix with hydrogen
gas and regenerate methane.
convective methane clouds then rise great distances to
the base of the stratosphere or higher, returning methane
vapor to the stratosphere.
Throughout the process there is no net
loss of methane in Neptune's atmosphere.
is a dynamic planet, even though it receives only 3 percent
as much sunlight as Jupiter does. Several large, dark spots
are reminiscent of Jupiter's hurricane-like storms. The
largest spot is big enough for Earth to fit neatly inside
it. Designated the "Great Dark Spot" by its discoverers,
the feature appears to be an anticyclone similar to Jupiter's
Great Red Spot. Neptune's Great Dark Spot is comparable
in size, relative to the planet, and at the same latitude
(the Great Dark Spot is at 22 degrees south latitude) as
Jupiter's Great Red Spot. However, Neptune's Great Dark
Spot is far more variable in size and shape than its Jupiter
counterpart. Bright, wispy "cirrus-type" clouds overlaying
the Great Dark Spot at its southern and northeastern boundaries
may be analogous to lenticular clouds that form over mountains
42 degrees south, a bright, irregularly shaped, eastward-moving
cloud circles much faster than does the Great Dark Spot,
"scooting" around Neptune in about 16 hours. This "scooter"
may be a cloud plume rising between cloud decks.
spot, designated "D2" by Voyager's scientists, is
located far to the south of the Great Dark Spot, at 55 degrees
south. It is almond-shaped, with a bright central core,
and moves eastward around the planet in about 16 hours.
also measured heat radiated by Neptune's atmosphere. The
atmosphere above the clouds is hotter near the equator,
cooler in the mid-latitudes and warm again at the south
pole. Temperatures in the stratosphere were measured to
be 750 kelvins (900 degrees F), while at the 100 millibar
pressure level, they were measured to be 55 K (-360 degrees
F). Heat appears to be caused, at least in part, by convection
in theatmosphere that results in compressional heating:
Gases rise in the mid-latitudes where they cool, then drift
toward the equator and the pole, where they sink and are
bright clouds, reminiscent of cirrus clouds on Earth, can
be seen high in Neptune's atmosphere. They appear to form
above most of the methane, and consequently are not blue.
low latitudes (27 degrees north), Voyager captured images
of cloud streaks casting their shadows on cloud decks estimated
to be about 50 to 100 kilometers (30 to 60 miles) below.
The widths of these cloud streaks range from 50 to 200 kilometers
(30 to 125 miles), and the widths of the shadows range from
30 to 50 kilometers (20 to 30 miles). Cloud streaks were
also seen in the southern polar regions (71 degrees south),
where the cloud heights were about 50 kilometers (30 miles).
of the winds on Neptune blow in a westward direction, which
is retrograde, or opposite to the rotation of the planet.
Near the Great Dark Spot, there are retrograde winds blowing
up to 1500 miles an hour -- the strongest winds measured
on any planet, including windy Saturn.