Mass (kg)............................................1.02 x 10^26
Mean density (kg/m^3) ...............................1640
Escape velocity (m/sec)..............................23300
Average distance from Sun (AU).......................30.06
Rotation period (length of day) (in Earth hours).....19.1
Revolution period (length of year) (in Earth years)..164.8
Obliquity (tilt of axis) (degrees)...................29.6
Orbit inclination (degrees)..........................1.77
Mean temperature (K).................................48
Visual geometric albedo..............................0.51
Atmospheric components...............................74% hydrogen,
1% methane (at depth)
narrow, and contain concentrations of
particles called ring arcs.
Full Disk Neptune
On its approach to Neptune in August 1989, Voyager 2 captured this
image of the fourth and outermost of the giant gas planets. This image
shows two of the four oval cloud features tracked by the cameras. The
large dark oval near the left edge revolves around Neptune every 18
hours. The bright clouds immediately to the south and east of this oval
substantially change their appearance in periods as short as 4
hours. The second dark spot, at lower right edge, revolves around
Neptune every 16 hours.
The location of clouds in Neptune's atmosphere was used for testing the
accuracy of Neptunian weather forecasts to aid in the selection of
targets for the narrow-angle camera. Three of the four targeted
features are visible here, and all three are close to their predicted
locations. The Great Dark Spot with its bright white companion is
slightly to the left of center. The small vivid Scooter is below and to
the left, and Dark Spot 2 with its shiny core is below Scooter. Strong
eastward winds, up to 644 kph (400 mph), caused the smaller dark spot
to overtake and pass the larger one every five days.
Scooter with Spots
The three features visible here are among the most interesting on
Neptune. At the top is the Great Dark Spot, accompanied by bright,
white clouds that change rapidly with time. Below the dark spot is a
bright feature that scientists nicknamed "Scooter." Below Scooter is
the Small Dark Spot. All three features move eastward at different
velocities, so it is rare that they can all be photographed together.
The bright cirrus-like clouds of Neptune change rapidly, often forming
and dissipating over several hours. In this sequence spanning two
rotations of Neptune (~36 hours), Voyager 2 observed cloud evolution in
the region around the Great Dark Spot. The surprisingly rapid changes,
which occurred over the 18 hours separating each panel, show that in
this region Neptune's weather is perhaps as dynamic and variable as
that of the Earth's. However, the scale is immense by our standards
because the Earth and the Great Dark Spot are similar in size.
Bright Cloud Streaks
This Voyager 2 high-resolution color image, taken 2 hours before
closest approach, provides obvious evidence of vertical relief in
Neptune's bright cloud streaks. These clouds were observed at 29
degrees North latitude near Neptune's east terminator. The linear
clouds are stretched approximately along lines of constant latitude and
the sun is toward the upper left. The sides of the clouds facing the
sun are brighter than the surrounding cloud deck because they are more
directly exposed to the sun. Shadows are visible on the side opposite
This Voyager 2 image has been processed to obtain true color
balance. The processing allows both the clouds' structure in the dark
regions near the pole and the bright clouds east of the Great Dark Spot
to be reproduced. These and other features suggest waves are present in
the atmosphere and play a large role in the type of clouds that are
Small Dark Spot
This image was obtained when the infrared spectrograph was mapping
Neptune. It is the highest-resolution view of the small dark spot (D2)
taken during the flyby. Banding around D2 indicates unseen strong
winds, while structures within the dark spot suggest both active
upwelling of clouds and rotation about the center. The V-shaped
structure near the right edge of the bright area indicates that the
spot rotates clockwise.
This is the best image of the Small Dark Spot that was obtained by
Voyager. The small spot is thought to be a storm in Neptune's
atmosphere, perhaps similar to the Great Red Spot on Jupiter.
The Great Dark Spot
Feathery white clouds fill the boundary between the dark and light blue
regions on the Great Dark Spot. The spiral shape of both the dark
boundary and the white cirrus suggests a storm system rotating
counterclockwise. Periodic small-scale patterns in the white cloud,
possibly waves, are short-lived and do not persist from one Neptunian
rotation to the next. Depicted here is the last face-on view of the
Great Dark Spot that Voyager 2 made with its narrow-angle camera. The
image was shuttered 45 hours before closest approach.
The rings around Neptune are so faint that when Voyager's cameras
captured this image, the brightness of Neptune nearly made them
impossible to detect. The black box in the center of the image is where
Neptune was blocked out because no detail was visible given the long
exposure time required to capture the almost invisible rings. The
bright glow at the inside edges of the black box is some of the light
reflected from Neptune. Two sharp rings are visible, Leverrier and
Adams, and one faint ring, Galle, is barely visible inside them.
This portion of Neptune's Adams ring appears to be twisted. Scientists
believe it looks this way because the original material in the rings
was in clumps that formed streaks as the material orbited Neptune. The
motion of the spacecraft added to the twisted appearance by causing a
slight smearing in the image.
The irregular shape of Proteus suggests that it has been cold and rigid
throughout its history and subject to significant impact cratering. The
satellite has an average radius of about 200 km (120 mi) and is
uniformly dark with an albedo of about 6%. This image was taken from a
range of 870,000 km (540,000 mi).
Tiny, Dark Moon
With an average radius of only 200 km (120 mi), this tiny
half-illuminated satellite appears spectrally gray with an albedo of
6%. Hints of crater-like forms and groove-like lineations can be
seen. The graininess of the image is caused by the short exposure
necessary to avoid significant smear. This image was obtained August
25, 1989 from a range of 146,000 km (91,000 mi).
The pink hue of Neptune's largest moon, Triton, is thought to result
from a slowly evaporating layer of nitrogen ice. Triton is an oddity
among moons in that its orbit is highly tilted to the plane of
Neptune's equator, and it is in a retrograde orbit. These facts have
led scientists to believe that Triton formed independently of Neptune
and was later captured by Neptune's gravity.
This cantaloupe-textured terrain encompasses a region roughly 1000 km
(620 mi) across. Complex tectonic and volcanic forces involving icy
viscous fluids combined to develop the deformed pattern of this
landscape. This view was captured in late August 1989.
South Polar Cap
Triton's surface is covered with nitrogen and methane ice. Its surface
temperature is a mere 38 degrees Celsius above absolute zero. Dark
streaks across the south polar cap may be the result of recent
geyser-like eruptions of gas, dust, and ice venting from beneath the
cap into the satellite's near-vacuum atmosphere. The diameter of
Triton is 2700 km (1674 mi).
Lake-like features along the terminator record a time when these
regions of Triton's surface were fluid. This 200x200-km (124x124-mi)
view was acquired during Voyager 2's closest approach to Triton.
Voyager 2's parting look back at the Neptune system shows a beautiful
dual-crescent view of Neptune and its largest moon Triton.