Giant Planets Atmospheres. PHYS Week 5, Part 2

Transcription

1 Giant Planets Atmospheres PHYS Week 5, Part 2

2 Jupiter This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in The colors have been enhanced to bring out detail. Zones of light-colored, ascending clouds alternate with bands of dark, descending clouds. The clouds travel around the planet in alternating eastward and westward belts at speeds of up to 540 kilometers per hour. Tremendous storms as big as Earthly continents surge around the planet. The Great Red Spot (oval shape toward the lower-left) is an enormous anticyclonic storm that drifts along its belt, eventually circling the entire planet.

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4 Jupiter with Io and Europa Voyager 1 took this photo of Jupiter and two of its satellites (Io, left, and Europa) on Feb. 13, Io is about 350,000 km above Jupiter's Great Red Spot; Europa is about 600,000 kilometers (375,000 miles) above Jupiter's clouds. Although both satellites have about the same brightness, Io's color is very different from Europa's. Io's equatorial region show two types of material -- dark orange, broken by several bright spots -- producing a mottled appearance. The poles are darker and reddish. Preliminary evidence suggests color variations within and between the polar regions. Europa is less strongly colored, although still relatively dark at short wavelengths. While the dominant large-scale motions are west-to-east, small-scale movement includes eddy-like circulation within and between the bands. Jupiter is about 20 million km from the spacecraft. (JPL)

5 Figure 10.1 Jupiter Jupiter as seen by the Cassini spacecraft on its way to Saturn. The storm system called the Great Red Spot is visible to the lower right. Unlike many spacecraft photos, the colors here are only slightly enhanced. (NASA/JPL) 5

6 PIA02863: Planetwide Color Movie The first color movie of Jupiter from NASA's Cassini spacecraft shows what it would look like to peel the entire globe of Jupiter, stretch it out on a wall into the form of a rectangular map, and watch its atmosphere evolve with time. The brief movie clip spans 24 Jupiter rotations between Oct. 31 and Nov. 9, Various patterns of motion are apparent all across Jupiter at the cloudtop level seen here. The Great Red Spot shows its counterclockwise rotation, and the uneven distribution of its high haze is obvious. To the east (right) of the Red Spot, oval storms, like ball bearings, roll over and pass each other. Horizontal bands adjacent to each other move at different rates. Strings of small storms rotate around northern-hemisphere ovals. The large grayish-blue "hot spots' at the northern edge of the white Equatorial Zone change over the course of time as they march eastward across the planet. Ovals in the north rotate counter to those in the south. Small, very bright features appear quickly and randomly in turbulent regions, candidates for lightning storms. The clip consists of 14 unevenly spaced timesteps, each a true color cylindrical projection of the complete circumference of Jupiter, from 60 degrees south to 60 degrees north. The maps are made by first assembling mosaics of six images taken by Cassini's narrow-angle camera in the same spectral filter over the course of one Jupiter rotation and, consequently, covering the whole planet. Three such global maps -- in red, green and blue filters -- are combined to make one color map showing Jupiter during one Jovian rotation. Fourteen such maps, spanning 24 Jovian rotations at uneven time intervals comprise the movie. Occasional appearances of Io, Europa, and their shadows have not been removed. The smallest visible features at the equator are about 600 km across. In a map of this nature, the most extreme northern and southern latitudes are unnaturally stretched out. (NASA/JPL/University of Arizona) 6

7 Figure Winds on the Giant Planets The speed and direction of the east west winds on Jupiter, Saturn, Uranus, and Neptune, as measured by Voyager, are shown on these photographic maps. Latitude is shown in the left column of each chart, with the equator being 0. The dark line shows the wind speed at each latitude (measured relative to the planet s core). Notice that Saturn has very strong winds at the equator. (NASA/JPL) 7

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11 PIA01093: Turbulence near the Great Red Spot True and false color mosaics of the turbulent region west of Jupiter's Great Red Spot. The Great Red Spot is on the planetary limb on the right hand side of each mosaic. The region west (left) of the Great Red Spot is characterized by large, turbulent structures that rapidly change in appearance. The turbulence results from the collision of a westward jet that is deflected northward by the Great Red Spot into a higher latitude eastward jet. The large eddies nearest to the Great Red Spot are bright, suggesting that convection and cloud formation are active there. The top mosaic combines the violet (410 nm) and near infrared continuum (756 nm) filter images to create a mosaic similar to how Jupiter would appear to human eyes. Differences in coloration are due to the composition and abundance of trace chemicals in Jupiter's atmosphere. The lower mosaic uses the Galileo imaging camera's three near-infrared (invisible) wavelengths (756 nm, 727 nm, and 889 nm displayed in red, green, and blue) to show variations in cloud height and thickness. Light blue clouds are high and thin, reddish clouds are deep, and white clouds are high and thick. Purple most likely represents a high haze overlying a clear deep atmosphere. Galileo is the first spacecraft to distinguish cloud layers on Jupiter.

12 Jupiter s Great Red Spot This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on Feb. 25, 1979, when the spacecraft was 9.2 million km from Jupiter. Cloud details as small as 160 km across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex end variable wave motion.

13 Figure The Great Red Spot This is the largest storm system on Jupiter, as seen during the Voyager spacecraft flyby. Below and to the right of the Red Spot is one of the white ovals, which are similar but smaller high-pressure features. The white oval is roughly the size of planet Earth. The colors have been somewhat exaggerated. (JPL/NASA) 13

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15 Figure 10.3 Galileo Probe Fall into Jupiter This artist s impression shows the Galileo probe descending into the clouds on its parachute just after the protective heat shield separated. The probe made its measurements of Jupiter s atmosphere on December 7, (NASA/ARC) 15

16 PIA01259: Galileo probe s entry point on Jupiter [left] - The arrow points to the predicted site at which the Galileo Probe will enter Jupiter's atmosphere on December 7, At this latitude, the eastward winds have speeds of about 110 m/s. The white oval to the north of the probe site drifts westward at 6 m/s, rolling in the winds which increase sharply toward the equator. The Jupiter image was obtained with the high resolution mode of Hubble's Wide Field Planetary Camera 2 (WFPC2). [right] - These four enlarged Hubble images of Jupiter's equatorial region show clouds sweeping across the predicted Galileo probe entry site, which is at the exact center of each frame (a small white dot has been inserted at the centered at the predicted entry site). During the intervening time between the first and fourth maps, the winds have swept the clouds 24,000 km eastward.

17 PIA00582: Jupiter's Multi-level Clouds The top left and right images, at 1.61!m and 2.73!m respectively, show views of the deep atmosphere down to a pressure about three times that at the Earth's surface. The middle image in the top row, at 2.17!m, shows only the highest altitude clouds and hazes because of strong absorption by H 2, the main constituent of Jupiter's atmosphere. Only the Great Red Spot, the highest equatorial clouds, a small feature at midnorthern latitudes, and thin, high photochemical polar hazes can be seen. In the lower left image, at 3.01!m, deeper clouds can be seen dimly against gaseous ammonia and methane absorption. The lower middle image, at 4.99!m,shows the planet's own heat from the deep, warm atmosphere. The false color image (lower right) indicates the temperature. Red areas denote photons from the hot deep atmosphere leaking through minimal cloud cover; green denotes cool tropospheric clouds; blue denotes the cold upper troposphere and lower stratosphere. The poles appear purplish, because small-particle hazes allow leakage and reflectivity, while yellowish regions at temperate latitudes may indicate tropospheric clouds with small particles. A mix of high and low-altitude aerosols causes the aqua appearance of the Great Red Spot and equator. NASA/JPL 17

18 PIA00602: True and false color hotspot mosaic True and false color views of Jupiter from Galileo show an equatorial "hotspot" on Jupiter and cover 34,000 km by 11,000 km. The top mosaic combines violet and near infrared to show how Jupiter would appear to human eyes. Differences in coloration are due to trace chemicals. The bottom mosaic combines three near-infrared wavelengths to show variations in cloud height and thickness. Bluish clouds are high and thin, reddish clouds are low, and white clouds are high and thick. The dark blue hotspot in the center is a hole in the deep cloud with an overlying thin haze. The light blue region to the left is covered by a very high haze layer. The multicolored region to the right has overlapping cloud layers of different heights. 18

19 PIA01638: Jovian Lightning and the Daytime Storm A convective storm (left) and associated lightning (right) in Jupiter's atmosphere. The left image shows the dayside view. The right images show the box in the dayside view as it appeared 110 minutes later during the night. Multiple lightning strikes are visible in the night side images, which were taken 3 minutes and 38 seconds apart. The bright, cloudy area in the dayside view is similar in appearance to a region of upwelling in Earth's atmosphere. The dark, clear region to the west (left) appears similar to a region of downwelling in Earth's atmosphere. The presence of lightning confirms that this is a site of moist convection. The lightning originates below the visible ammonia cloud, which acts as a translucent screen, diffusing the light over a wider area. This apparent width can be used to infer the depth of approximately 75 km. 19

20 PIA01639: Water Cloud Thunderstorm Northwest of Great Red Spot This false-color picture shows a convective thunderstorm 10,000 km northwest of Jupiter's Great Red Spot. The white cloud in the center is a tall, thick cloud 1,000 km across, standing 25 km higher than most of the surrounding clouds. Its base extends off to the left and appears red in this representation. This red color indicates that the cloud base is about 50 km below the surrounding clouds. Most of the wisps and features in Jupiter's clouds are ammonia clouds forming at just less than Earth's sea level pressure. On Jupiter, water is the only substance able to form a cloud at about five times the Earth's sea level pressure. The red base of this thunderstorm is so deep that it can only be a water cloud. It is thought that this storm is analogous to an Earth thunderstorm, with the cloud's high, bright, white portion comparable to the familiar anvil cloud on Earth. Light at different wavelengths penetrates to different depths in Jupiter's atmosphere before being reflected by clouds. In this image, red represents data taken with the 756 nm filter, where Jupiter's atmospheric gases are mostly transparent and the light penetrates deeply. Blue and green represent data taken with the 889 and 727 nm filters, respectively, where the gases in Jupiter's atmosphere absorb strongly, so only high clouds can reflect the light. Thus, the blue and green areas depict higher clouds, while the red areas show deep clouds as well as higher clouds. 20

21 PIA10224: Jupiter Eruptions Detailed analysis of two continent-sized storms that erupted in Jupiter's atmosphere in March 2007 shows that Jupiter's internal heat plays a significant role in generating atmospheric disturbances. Understanding these outbreaks could be the key to unlock the mysteries buried in the deep Jovian atmosphere. This visible-light image is from NASA's Hubble Space Telescope taken on May 11, It shows the turbulent pattern generated by the two plumes on the upper left part of Jupiter. Understanding these phenomena is important for Earth's meteorology where storms are present everywhere and jet streams dominate the atmospheric circulation. Jupiter is a natural laboratory where atmospheric scientists study the nature and interplay of the intense jets and severe atmospheric phenomena. According to the analysis, the bright plumes were storm systems triggered in Jupiter's deep water clouds that moved upward in the atmosphere vi gorously and injected a fresh mixture of ammonia ice and water about 30 km above the visible clouds. The storms moved in the peak of a jet stream in Jupiter's atmosphere at 600 km/h. Models of the disturbance indicate that the jet stream extends deep in the buried atmosphere of Jupiter, more than 100 km below the cloud tops where most sunlight is absorbed. NASA/ESA 21

22 PIA10225: Jupiter Eruptions Captured in Infrared This infrared image shows two bright plume eruptions obtained by the NASA Infrared Telescope Facility on April 5, NASA/JPL/IRTF 22

23 Figure 10.16a Storms on Jupiter We show two versions of the same Galileo view of the large white ovals that are the most long-lived storms after the Great Red Spot. The top image shows how the scene might appear to the human eye, while the bottom is in false color (and includes infrared information). The left white oval is about 3/4 the size of the Earth. Note the cyclone-shaped storm between the two top ovals. (JPL/NASA) 23

24 Figure 10.8 Jupiter in Radio Waves This false-color image of Jupiter was made with the Very Large Array (of radio telescopes) in New Mexico. We see part of the magnetosphere, brightest in the middle because the largest number of charged particles are in the equatorial zone of Jupiter. The planet itself is slightly smaller than the circular green region in the center. Different colors are used to indicate different intensities of synchrotron radiation. (Imke depater/nrao) 24

25 Saturn's Auroras These images of Saturn's polar aurora were taken by NASA's Hubble Space Telescope on Jan. 24, 26, and 28. Each of the three images of Saturn combines ultraviolet images of the south polar region (to show the auroral emissions) with visible wavelength images of the planet and rings. The Hubble images were obtained during a joint campaign with NASA's Cassini spacecraft to measure the solar wind approaching Saturn and the Saturn kilometric radio emissions. The strong brightening of the aurora on January 26 corresponded with the recent arrival of a large disturbance in the solar wind. These results are presented in three papers, which appear in the Feb. 17 issue of the journal Nature. NASA/Hubble/Z. Levay and J. Clarke

26 Figure Saturn Over Four Years These beautiful images of Saturn were recorded by the Hubble Space Telescope between 1996 and Since Saturn is tilted by 27 degrees, we see the orientation of the rings around its equator change as the planet moves along its orbit. Note the horizontal bands in the atmosphere. (R. French, Wellesley College, et al./ Hubble Heritage Team, STScI/NASA) 26

27 Saturn and 4 moons This approximate natural-color image shows Saturn, its rings, and four of its icy satellites. Three satellites (Tethys, Dione, and Rhea) are visible against the darkness of space, and another smaller satellite (Mimas) is visible against Saturn's cloud tops very near the left horizon and just below the rings. The dark shadows of Mimas and Tethys are also visible on Saturn's cloud tops, and the shadow of Saturn is seen across part of the rings. Saturn, second in size only to Jupiter in our Solar System, is 120,660 km in diameter at its equator (the ring plane) but, because of its rapid spin, Saturn is 10% smaller measured through its poles. Saturn's rings are composed mostly of ice particles ranging from microscopic dust to boulders in size. These particles orbit Saturn in a vast disk that is a mere 100 meters or so thick. The rings' thinness contrasts with their huge diameter--for instance 272,400 km for the outer part of the bright A ring, the outermost ring visible here. The pronounced concentric gap in the rings, the Cassini Division (named after its discoverer), is a 3500-km wide region that is much less populated with ring particles than the brighter B and A rings to either side of the gap. The rings also show some enigmatic radial structure ('spokes'), particularly at left. This image was synthesized from images taken in Voyager's blue and violet filters and was processed to recreate an approximately natural color and contrast.

28 Saturn, Tethys and Dione Saturn and two of its moons, Tethys (above) and Dione, were photographed by Voyager 1 on November 3, 1980, from 13 million km. The shadows of Saturn's three bright rings and Tethys are cast onto the cloud tops. The limb of the planet can be seen easily through the 3,500-km-wide Cassini Division, which separates ring A from ring B. The view through the much narrower Encke Division, near the outer edge of ring A is less clear. Beyond the Encke Division (at left) is the faintest of Saturn's three bright rings, the C- ring or crepe ring, barely visible against the planet.

29 Figure 10.4 Cassini Mission Drops Its Probe In this artist s conception, the Huygens probe drops from the Cassini orbiter into the hazy clouds of Saturn s moon Titan. (NASA/JPL) 29

30 PIA09893: Beauty Marks Two dark spots drift across the northern skies of Saturn. The shadows are cast by the moons Tethys and Mimas. Tethys (1,071 km across) orbits farther from Saturn than Mimas (397 km across) and casts the larger of the two shadows here. This view looks toward the unilluminated side of the rings from about 46 degrees above the ringplane.the image was taken in visible light with the Cassini spacecraft wide-angle camera on March 30, The view was obtained at a distance of approximately 1.2 million km from Saturn. Image scale is 66 km per pixel. NASA/JPL/Space Science Institute

31 PIA09831: Probing the North The Cassini spacecraft probes Saturn's atmosphere, peering beneath the hazes that obscure the flowing cloud bands at visible wavelengths. Brighter areas in this view generally represent features higher in the atmosphere than darker areas. This view looks toward the unilluminated side of the rings and was acquired from about 38 degrees above the ringplane. The image was taken with the Cassini spacecraft wide-angle camera on Jan. 2, 2008 using a combination of spectral filters sensitive to wavelengths of polarized infrared light centered at 728 and 705 nm. The view was obtained at a distance of approximately 929,000 km from Saturn. Image scale is 52 km per pixel. NASA/JPL/Space Science Institute

32 PIA08410: Hissing Storm A bright, powerful, lightningproducing storm churns and coasts along the lane of Saturn's southern hemisphere nicknamed "Storm Alley" by scientists. Cassini detected this particular tempest after nearly two years during which Saturn did not appear to produce any large electrical storms of this kind. The storm appears as a bright, irregular splotch on the planet near lower right. This storm has now been continuously tracked by Cassini for several months, whereas previous storms observed by the spacecraft lasted for less than 30 days. The view looks toward the unilluminated side of the rings from about 5 degrees above the ringplane. Tethys (1,071 km across) is seen here in the foreground, and casts its shadow onto the high northern latitudes. NASA/JPL/Space Science Institute

33 PIA08411: Saturn's Long-lived Storm These two side-by-side views show the longest-lived electrical storm yet observed on Saturn by NASA's Cassini spacecraft. The views were acquired more than three months after the storm was first detected from its lightning-produced radio discharges on Nov. 27, See PIA08410 for an earlier color view of this storm. Cassini imaging scientists believe the storm to be a vertically extended disturbance that penetrates from Saturn's lower to upper troposphere. The view at left was created by combining images taken using red, green and blue spectral filters, and shows Saturn in colors that approximate what the human eye would see. The storm stands out with greater clarity in the sharpened, enhanced color view at right. This view combines images taken in infrared, green and violet light at 939, 567 and 420 nanometers respectively and represents an expansion of the wavelength region of the electromagnetic spectrum visible to human eyes. This view looks toward the un-illuminated side of the rings from about 3 degrees above the ringplane. Janus (181 km across) appears as a dark speck just beneath the rings in both images.

34 PIA09188: Saturn's Active North Pole This Cassini image shows Saturn's thermal glow at 5 microns. This allows the pole to be revealed during the nighttime conditions presently underway during north polar winter. Clouds at depths about 75 km lower than the clouds seen at visible wavelengths block this light, appearing dark in silhouette. To show clouds as features that are bright or white rather than dark, the original image has been contrast reversed to produce the image shown here. The nested set of alternating white and dark hexagons indicates that the hexagonal complex extends deep into the atmosphere, at least down to the 3-Earth-atmosphere pressure level, 75 km underneath the clouds seen by Voyager. The feature is nearly stationary, and likely is an unusually strong pole-encircling planetary wave that extends deep into the atmosphere. NASA/JPL/U Arizona

35 PIA08332: Looking Saturn in the Eye Cassini stares deep into the swirling hurricane-like vortex at Saturn's south pole, where the vertical structure of the clouds is highlighted by shadows. The width of the shadow and the height of the sun above the local horizon yield a crude estimate of the height of the surrounding clouds relative to the clouds in the center. The shadow-casting clouds tower 30 to 75 km above those in the center, two to five times greater than the tallest terrestrial thunderstorms or the height of clouds surrounding the eye of a terrestrial hurricane. Saturn's hydrogen-helium atmosphere is less dense at comparable pressures than Earth's atmosphere, and is therefore more distended in the vertical dimension. The south polar storm, which displays two spiral arms of clouds extending from the central ring and spans the dark area inside a thick, brighter ring of clouds, is approximately 8,000 km across. Eye-wall clouds are a distinguishing feature of hurricanes on Earth. They form where moist air flows inward across the ocean's surface, rising vertically and releasing a load of precipitation around an interior circular region of descending air, which is the eye itself. Though it is uncertain whether moist convection is driving this storm, as is the case with Earthly hurricanes, the dark 'eye' at the pole, the eye-wall clouds and the spiral arms together indicate a hurricane-like system. The distinctive eye-wall clouds especially have not been seen on any planet beyond Earth. Even Jupiter's Great Red Spot, much larger than Saturn's polar storm, has no eye, no eye-wall, and is relatively calm at the center. This giant Saturnian storm is apparently different from hurricanes on Earth because it is locked to the pole, does not drift around like terrestrial hurricanes and because it does not form over liquid water oceans.

36 Figure Atmospheric Structure for the Jovian Planets In each diagram, the yellow line shows how the temperature (see the scale on the bottom) changes with altitude. The location of the main layers on each planet is also shown. (NASA/JPL) 36

37 Uranus in true and false colour These two pictures of Uranus -- one in true color (left) and the other in false color -- were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million km from the planet, several days from closest approach. The picture at left has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. The picture is a composite of images taken through blue, green and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. Beyond this boundary lies the hidden northern hemisphere of Uranus, which currently remains in total darkness as the planet rotates. The blue-green color results from the absorption of red light by methane gas in Uranus' deep, cold and remarkably clear atmosphere. The picture at right uses false color and extreme contrast enhancement to bring out subtle details in the polar region of Uranus. The very slight contrasts visible in true color are greatly exaggerated here. In this false-color picture, Uranus reveals a dark polar hood surrounded by a series of progressively lighter concentric bands. One possible explanation is that a brownish haze or smog, concentrated over the pole, is arranged into bands by zonal motions of the upper atmosphere. The bright orange and yellow strip at the lower edge of the planet's limb is an artifact of the image enhancement. In fact, the limb is dark and uniform in color around the planet.

38 Figure 10.6 The Strange Seasons on Uranus (a) The orbit of Uranus as seen from above. At the time Voyager 2 arrived (position 1), the south pole was facing the Sun. As we move counterclockwise in the diagram, we see the planet 21 years later at each step. (b) The amount of sunlight seen at the poles and the equator of Uranus over the course of its 84-year revolution around the Sun. 38

39 Uranus - clouds Time-lapse Voyager 2 images of Uranus show the movement of two small, bright, streaky clouds -- the first such features ever seen on the planet. The clouds were detected in this series of orange-filtered images taken Jan. 14, 1986, over a 4.6-hour interval (from top to bottom). At the time, the spacecraft was about 12.9 million kilometers (8.O million miles) from the planet, whose pole of rotation is near the center of each disk. Uranus, which is tipped on its side with respect to the other planets, is rotating in a counterclockwise direction, as are the two clouds seen here as bright streaks. (The occasional donut- shaped features that show up are shadows cast by dust in the camera optics. The processing necessary to bring out the faint features on the planet also brings out these camera blemishes.) The larger of the two clouds is at a latitude of 33 degrees; the smaller cloud, seen faintly in the three lower images, lies at 26 degrees (a lower latitude and hence closer to the limb). Their counterclockwise periods of rotation are 16.2 and 16.9 hours, respectively. This difference implies that the lower- latitude feature is lagging behind the higher-latitude feature at a speed of almost 1OO meters per second (22O mph). Latitudinal bands are also visible in these images. The faint bands, more numerous now than in previous Voyager images from longer range, are concentric with the pole of rotation -- that is, they circle the planet in lines of constant latitude.

40 Neptune clouds with HST Three images of changing weather conditions were taken in 1994 on October 10 (upper left), October 18 (upper right), and November 2 (lower center). The temperature difference between Neptune's strong internal heat source and its frigid cloud tops (-260 degrees Fahrenheit) might trigger instabilities in the atmosphere that drive these large-scale weather changes. In addition to hydrogen and helium, the main constituents, Neptune's atmosphere is composed of methane and hydrocarbons, like ethane and acetylene. The picture was reconstructed from a series of Wide Field Planetary Camera 2 images taken through different filters. Absorption of red light by methane in Neptune's atmosphere contributes to the planet's distinctive aqua color; the clouds themselves are also somewhat blue. The pink features are high-altitude methane ice crystal clouds. Though the clouds appear white in visible light, they are tinted pink here because they were imaged at near infrared wavelengths.

41 Neptune During August 16 and 17, 1989, the Voyager 2 narrow- angle camera was used to photograph Neptune almost continuously, recording approximately two and one-half rotations of the planet. These images represent the most complete set of full disk Neptune images that the spacecraft will acquire. This picture from the sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. The large dark oval near the western limb (the left edge) is at a latitude of 22 degrees south and circuits Neptune every 18.3 hours. The bright clouds immediately to the south and east of this oval are seen to substantially change their appearances in periods as short as four hours. The second dark spot, at 54 degrees south latitude near the terminator (lower right edge), circuits Neptune every 16.1 hours. This image has been processed to enhance the visibility of small features, at some sacrifice of color fidelity.

42 Neptune s Great Dark Spot This bulls-eye view of Neptune's small dark spot (D2) was obtained by Voyager 2's narrow-angle camera on Aug. 24, The smallest structures that can be seen are 20 km across. Banding surrounding the feature indicates unseen strong winds, while structures within the bright spot suggest both active upwelling of clouds and rotation about the center. A rotation rate has not yet been measured, but the V-shaped structure near the right edge of the bright area indicates that the spot rotates clockwise. Unlike the Great Red Spot on Jupiter, which rotates counterclockwise, if the D2 spot on Neptune rotates clockwise, the material will be descending in the dark oval region. The fact that infrared data will yield temperature information about the region above the clouds makes this observation especially valuable.

43 Neptune - cloud changes The bright cirrus-like clouds of Neptune change rapidly, often forming and dissipating over periods of several to tens of hours. In this sequence spanning two rotations of Neptune (about 36 hours) Voyager 2 observed cloud evolution in the region around the Great Dark Spot (GDS) at an effective resolution of about 100 kilometers (62 miles) per pixel. The surprisingly rapid changes which occur over the 18 hours separating each panel shows that in this region Neptune's weather is perhaps as dynamic and variable as that of the Earth. However, the scale is immense by our standards the Earth and the GDS are of similar size and in Neptune's frigid atmosphere, where temperatures are as low as 55 degrees Kelvin (360 F), the cirrus clouds are composed of frozen methane rather than Earth's crystals of water ice.

44 Neptune bands This image of Neptune was taken by Voyager 2's wide- angle camera when the spacecraft was 590,000 km (370,000 miles) from the planet. The image has been processed to obtain true color balance. Additional processing was used to suppress surface brightness of the white clouds. 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 in this color photograph. Small trails of similar clouds trending east to west and large scale structure east of the Great Dark Spot all suggest that waves are present in the atmosphere and play a large role in the type of clouds that are visible.

45 Neptune cloud close up This Voyager 2 high resolution color image provides obvious evidence of vertical relief in Neptune's bright cloud streaks. These clouds were observed at a latitude of 29 degrees north near Neptune's east terminator. The linear cloud forms are stretched approximately along lines of constant latitude and the sun is toward the lower left. The bright sides of the clouds which face the sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the sun. These shadows are less distinct at short wavelengths (violet filter) and more distinct at long wavelengths (orange filter). This can be understood if the underlying cloud deck on which the shadow is cast is at a relatively great depth, in which case scattering by molecules in the overlying atmosphere will diffuse light into the shadow. Because molecules scatter blue light much more efficiently than red light, the shadows will be darkest at the longest (reddest) wavelengths, and will appear blue under white light illumination. The width of the cloud streaks range from 50 to 200 km, and their shadow widths range from 30 to 50 km. Cloud heights appear to be of the order of 50 km. This corresponds to 2 scale heights.