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Neptune

Planet Profile
Mass (kg)	1.02 x 10^26
Diameter (km)	49528
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 in degrees)	29.6
Orbit inclination (degrees)	1.77
Orbit eccentricity (deviation from circular)	0.009
Mean temperature (K)	48
Visual geometric albedo (reflectivity	0.51
Atmospheric components	74% hydrogen, 25% helium, 1% methane (at depth)
Rings	Rings are 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.

Weather Forecast

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.

Cloud Evolution

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 the sun.

True-Color Image

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 visible.

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.

Huge Storm

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.

Faint Rings

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 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.

Twisted Rings

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.

Strange-Shaped Moon

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).

Captured Moon

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.

Cantaloupe Terrain

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).

Closest Passage

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.

Parting Look

Voyager 2's parting look back at the Neptune system shows a beautiful dual-crescent view of Neptune and its largest moon Triton.

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