Online Astronomy eText: The Planets
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Neptune as seen by Voyager 2 as it approached the planet on August 20, 1989
(Image credit, post-processing © NASA, Voyager 2, Calvin Hamilton)

Dark spots and high clouds on Neptune during the Voyager 2 flyby of 1989. The atmosphere of Neptune is almost entirely colorless hydrogen and helium, but appears blue because of trace amounts of methane gas, which primarily absorbs red light. The dark spots are high-pressure regions or anti-cyclones, while bright clouds are thought to be methane mists. (Image credit Voyager 2 Team, NASA, apod010821)

Above, a closeup of clouds in Neptune's upper atmosphere, two hours before Voyager 2's closest approach on August 25, 1989. The cloud "streaks" stretch for thousands of miles parallel to Neptune's equator. Individual streaks are 30 to 120 miles wide. Based on the position and slightly reddened color of the shadows cast by the clouds, they lie about 30 miles above the lower, more general haze. The overall color of Neptune is bluish due to absorption of red light by methane gas (most of the atmosphere is hydrogen and helium, but they have little effect on visible light save for random scattering). After Voyager passed completely featureless Uranus, it was a surprise to find that Neptune had obvious cloud and storm features. (Image credit Voyager 2, NASA, NSSDC Photo Gallery ID P-34709C)

Cloud detail as seen by Voyager 2, post-processed to obtain true color balance, with brightness and contrast adjusted to show details which would otherwise be washed out in brighter areas or obscured in darker ones. (Image credit, post-processing © Voyager 2, NASA, JPL, Calvin Hamilton)

The Great Dark Spot as seen by Voyager 2. The spot was a turbulent storm system similar in size (and latitude) to Jupiter's Great Red Spot, but unlike Jupiter's Red Spot, the Great Dark Spot proved to be only a temporary feature and was no longer visible by the time the Hubble Space Telescope was able to view the planet for the first time, in 1994. Wind velocities near the spot measured up to 1500 miles per hour, the fastest recorded on any planet. (Image credit Voyager Project, JPL, NASA, SolarViews)

A composite of five Voyager 2 images of Neptune, showing the planet's South Pole at center, the Great Dark Spot circling the Pole, and dramatically contrasting high clouds and deeper depressions centered on the Pole. It is interesting to compare this image to those of the polar regions of Saturn, which have been found to contain cloud formations similar to but far stranger than those observed in this image of Neptune. (The radial spokes in the image, which give an impression of non-circular structure in the polar clouds, are due to contrast problems with the imaging software. The image shown here was created more than a decade ago. Newer software would undoubtedly produce a smoother image.) (Image Credit Voyager 2, NASA, apod970529)

Neptune and Triton as seen by Voyager 2, looking back at the planet as it left the Solar System in late August, 1989. At the angle of this view the sunlight reflected by and transmitted through Neptune's atmosphere is considerably reddened, making it look less blue than usual. (Image Credit Voyager 2, NASA, JPL)

Both hemispheres of Neptune as seen by Hubble Space Telescope (images taken in 1994 and released in early 1995?). No sign of the Great Dark Spot remains, though bright clouds (shown in pink in this false-color composite) are still evident at various latitudes. (Image credit NASA, JPL, WFPC2 Science Team, NSSDC STScI PR95-09)

Cloud features as seen by HST (high contrast false color imaging of clouds in particular). Like the previous HST image (above), these images were taken in late (October and November) 1994, but released in early (April) 1995. (Image credit NASA/STScI, and Heidi Hammel (MIT), HubbleSite)

A false-color image of Neptune as seen by the HST in 2002. Since Neptune's year is 165 Earth years long, each season is more than 40 years long, and it has been Spring in the planet's southern hemisphere since the 1960's, well before Voyager 2 passed the planet in 1989. By the time this image was taken Spring was nearing its end, and in 2005 the southern hemisphere's Summer began. If the changes in the planet's appearance from earlier HST pictures are due to seasonal changes in solar heating, more dramatic chanages may lie ahead. (Image credit L. Sromovsky and P. Fry (Univ. Wisconsin - Madison) et al., NASA, apod030613)

Images taken in 1998, 2000 and 2002 show a dramatic increase in cloud activity presumably caused by seasonal changes in the southern hemisphere (as discussed in the caption for the previous picture). (Image credit NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison), Hubblesite)

A composite showing the relative sizes of Neptune and Triton as seen by the Hubble Space Telescope. The "almost true color" image of Neptune obtained with the Wide Field Planetary Camera (WFPC) shows the planet's South Pole at the bottom, while the Faint Object Camera (FOC) image of Triton shows its South Pole at lower left, so the alignment of the two objects is not the same. Note that the clouds on Neptune are white and the overall color is blue, as shown in the images at the top of this page. So the phrase "almost true color" should be interpreted as meaning "closer to actual color than completely false color images". (Image credit HST, Ted Stryk, Solarviews)