Gases/Gaseous objects/Saturn

Saturn is studied using gaseous-object astronomy.

"Measurements taken by NASA's Cassini spacecraft have shown that the ringed planet might have a longer day than originally calculated from measurements taken by the Voyager 2 probe more than 20 years earlier."^[1]

"While an uncertainty of 15 minutes may appear small compared to the approximately 10.5-hour rotation of Saturn, it is actually important to know [the rotation] accurately. The rotation period has an important effect on understanding Saturn's atmosphere dynamics and internal structure."^[2]

"When Voyager 2 visited Saturn in 1981, the probe measured the planet's rotation period at about 10 hours, 39 minutes. But when Cassini first arrived at the planet in the early 2000s, it determined Saturn spun once on its axis every 10 hours and 47 minutes, and those numbers changed each time Cassini took a look."^[1]

"Voyager and Cassini relied on measurements of the planet's radio radiation, but because those measurements shifted with each observation, they proved unreliable."^[1]

"Radio radiation isn't the only method for studying the rotation of a gas giant. For planets where the magnetic field is tilted in relation to the axis the planet rotates around, measurements of the magnetic field can reveal how quickly the planet spins. However, Saturn's magnetic field lines up with its rotation axis, which prevents scientists from relying on that measure."^[1]

"A third option involves measuring how long it takes for a cloud in Saturn's atmosphere to travel around the planet. But the rotation of the atmosphere doesn't necessarily line up with the rotation of the planet, making this method challenging."^[1]

A "more mathematical approach [involves searching] for solutions for the rotation period by using coefficients to represent the interior, then [searching] for the rotation period that the most solutions calculated."^[1]

"We did not want the derived period to be associated with a specific internal structure, so we accounted for many possibilities within their physical range. There are many solutions, but it was found that they tend to give a similar rotation period."^[2]

"The theoretical estimate returned a rotation of almost 10 hours, 33 minutes."^[1]

This is "in very good agreement with previous estimates that used different methods."^[2]

"The newly returned calculation relied on studies of the planet's well-defined gravitational field. As Cassini traveled around the planet, it measured the tug of Saturn on the spacecraft, determining the strength or weakness of the gravitational pull."^[1]

"The advantage of our method is that it is not associated with a specific interior model for Saturn, does not rely on the cloud-tracking wind properties that have large variability, and allows for the large range of solutions constrained by the measured physical properties of the planet and their uncertainties."^[2]

"Saturn is rather thick around the middle, more than even Jupiter, which could indicate a fast spin. However, Helled pointed out that winds also affect oblateness, so strong winds around the equator could lead to a bigger bulge."^[1]

"Such a rotation period for Saturn implies that the latitudinal wind structure is more symmetric, containing both easterly and westerly jets, like we see on Jupiter."^[2]

A huge storm (great white spot) shown in the image on the right, churning through the atmosphere in Saturn's northern hemisphere overtakes its own trail in this true-color view from NASA’s Cassini spacecraft. Note that the trail of the disturbance in the atmosphere has apparently moved closer to the equator than the storm itself.

File:Satstorm.gif

In 1990, the Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that was not present during the Voyager encounters and in 1994, another, smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice.^[3] Previous Great White Spots were observed in 1876, 1903, 1933 and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.^[4]

Wind speeds on Saturn can reach 1,800 km/h (1,100 mph) ... Voyager data indicate peak easterly winds of 500 m/s (1800 km/h).^[5]

"This NASA Hubble Space Telescope image [fourth down on the right] of the ringed planet Saturn shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth (about 7,900 miles). Hubble provides new details about the effects of Saturn's prevailing winds on the storm. The new image shows that the storm's motion and size have changed little since its discovery in September, 1994."^[6]

"The storm was imaged with Hubble's Wide Field Planetary Camera 2 (WFPC2) in the wide field mode on December 1, 1994, when Saturn was 904 million miles from the Earth. The picture is a composite of images taken through different color filters within a 6 minute interval to create a "true-color" rendition of the planet. The blue fringe on the right limb of the planet is an artifact of image processing used to compensate for the rotation of the planet between exposures."^[6]

"The Hubble images are sharp enough to reveal that Saturn's prevailing winds shape a dark "wedge" that eats into the western (left) side of the bright central cloud. The planet's strongest eastward winds (clocked at 1,000 miles per hour from analysis of Voyager spacecraft images taken in 1980-81) are at the latitude of the wedge."^[6]

"To the north of this arrowhead-shaped feature, the winds decrease so that the storm center is moving eastward relative to the local flow. The clouds expanding north of the storm are swept westward by the winds at higher latitudes. The strong winds near the latitude of the dark wedge blow over the northern part of the storm, creating a secondary disturbance that generates the faint white clouds to the east (right) of the storm center."^[6]

"The storm's white clouds are ammonia ice crystals that form when an upward flow of warmer gases shoves its way through Saturn's frigid cloud tops. This current storm is larger than the white clouds associated with minor storms that have been reported more frequently as bright cloud features."^[6]

"Hubble observed a similar, though larger, storm in September 1990, [sixth image down on the right] which was one of three major Saturn storms seen over the past two centuries. Although these events were separated by about 57 years (approximately 2 Saturnian years) there is yet no explanation why they apparently follow a cycle -- occurring when it is summer in Saturn's northern hemisphere."^[6]

File:Storm Progress-opo9011b.jpg

"[E]rosion from particles making up the icy rings of Saturn are forming rain water that falls on certain parts of the planet. ... tiny ice particles that compose the planet's distinctive rings are sometimes eroded away and then deposited in the planet's upper atmosphere. The droplets then create a kind of rain on the planet. ... charged water molecules rain down only on certain parts of the planet, which show up darker in infrared images. ... The magnetic connection creates a pathway for small ice particles in the rings to slough off into the planet's atmosphere, causing the "ring rain.""^[7]

"The most surprising element to us was that these dark regions on the planet are found to be linked — via magnetic field lines — to the solid portions of water-ice within Saturn's ring-plane"^[8].

"Saturn is the first planet to show significant interaction between its atmosphere and ring system ... The main effect of ring rain is that it acts to 'quench' the ionosphere of Saturn. In other words, this rain severely reduces the electron densities in regions in which it falls."^[9]

"It turns out that a major driver of Saturn's ionospheric environment and climate across vast reaches of the planet are ring particles located some 36,000 miles [60,000 kilometers] overhead ... The ring particles affect both what species of particles are in this part of the atmosphere and where it is warm or cool."^[10]

"Where Jupiter is glowing evenly across its equatorial regions, Saturn has dark bands where the water is falling in, darkening the ionosphere".^[11]

File:PIA17652color 690x690.gif

In the image down on the right, the "Cassini spacecraft captures three magnificent sights at once: Saturn's north polar vortex and hexagon along with its expansive rings."^[12]

"The hexagon, which is wider than two Earths, owes its appearance to the jet stream that forms its perimeter. The jet stream forms a six-lobed, stationary wave which wraps around the north polar regions at a latitude of roughly 77 degrees North."^[12]

"This view looks toward the sunlit side of the rings from about 37 degrees above the ringplane. The image was taken with the Cassini spacecraft wide-angle camera on April 2, 2014 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 752 nanometers."^[12]

"The view was obtained at a distance of approximately 1.4 million miles (2.2 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 43 degrees. Image scale is 81 miles (131 kilometers) per pixel."^[12]

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images.^[13][14]

"The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures."^[15]

"Although the NPSV formed during late northern spring, by the end of Cassini’s reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly formed NPSV was bounded by a strengthening stratospheric thermal gradient near 78°N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn’s long-lived polar hexagon—which was previously expected to be trapped in the troposphere—can influence the stratospheric temperatures some 300 km above Saturn’s clouds."^[15]

"Composite Infrared Spectrometer (CIRS)²⁷ reveal a significant surprise: the NPSV exhibits a hexagonal boundary that mirrors the well-studied hexagonal wave in Saturn’s troposphere^28,29,30. The meandering of the jet that forms the hexagon is believed to be a Rossby wave31 resulting from an instability of the eastward zonal jet near 78°N^29,32,33,34 and trapped within a waveguide formed by the zonal jets and Saturn’s vertical static stability profile."^[15]

"Saturn’s famous hexagon is not always restricted to the troposphere but can persist high in the stratosphere in the spring/summer, creating a hexagonal structure that spans more than ~300 km in height from the clouds to the stratospheric polar vortex."^[15]

The "tropospheric hexagon vertices do not move with the ~120 ms⁻¹ eastward cloud-top velocity of the jet at 78°N".^[15]

"Using the sinusoidal fits, [there is] a westward shift in the tropospheric hexagon vertices of 8.5 ± 1.1° over 963 days (the uncertainty comes from the quality of the sinusoidal fit). Offsets between the tropospheric and stratospheric vertices were <4° in February 2017."^[15]

"Most surprisingly, the boundary of the NPSV exhibited a hexagonal shape mirroring that observed in the clouds ~300 km below."^[15]

Subsidence "is occurring within the hexagon and that it extends from the cloud-forming region into the mid-stratosphere. In addition, the polar region exhibits an increased optical thickness of stratospheric hazes47,54 that is potentially related to aerosol production associated with Saturn’s aurora. Saturn’s main auroral oval, associated with the boundary between open and closed magnetic field lines, occurs at an average latitude of 76.5°N⁵⁵ and therefore completely encompasses the area of the NPSV, sitting ~1° south of the hexagon latitude (78°N)."^[15]

In the image down on the right "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. Such a storm, with a well-developed eye ringed by towering clouds, is a phenomenon never before seen on another planet."^[16]

The image "shows a swirling cloud mass centered on the south pole, around which winds blow at 550 kilometers (350 miles) per hour. [...] The clouds inside the dark, inner circle are lower than the surrounding clouds, which cast a shadow that follows the sun."^[16]

"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 kilometers (20 to 45 miles) above those in the center. This is two to five times greater than the tallest terrestrial thunderstorms and two to five times the height of clouds surrounding the eye of a terrestrial hurricane. Such a height difference arises because Saturn's hydrogen-helium atmosphere is less dense at comparable pressures than Earth's atmosphere, and is therefore more distended in the vertical dimension."^[16]

"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 kilometers (5,000 miles) across, which is considerably larger than a terrestrial hurricane."^[16]

"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."^[16]

"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."^[16]

"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."^[16]

"The images were acquired over a period of three hours on Oct. 11, 2006, when Cassini was approximately 340,000 kilometers (210,000 miles) from Saturn. Image scale is about 17 kilometers (11 miles) per pixel. The images were taken with the wide-angle camera using a spectral filter sensitive to wavelengths of infrared light centered at 752 nanometers. All frames have been contrast enhanced using digital image processing techniques. The unprocessed images show an oblique view toward the pole, and have been reprojected to show the planet from a perspective directly over the south pole."^[16]

The upper clouds are composed of ammonia crystals.

Infrared imaging has shown that Saturn's south pole has a warm polar vortex, the only known example of such a phenomenon in the Solar System.^[17] Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, believed to be the warmest spot on Saturn.^[17]

File:Pn 433 Image saturn rngs xray opt-1.jpg

The X-ray astronomy image of Saturn is on the left in the composite at right. The Chandra X-ray Observatory "image of Saturn held some surprises for the observers. First, Saturn's 90 megawatts of X-radiation is concentrated near the equator. This is different from a similar gaseous giant planet, Jupiter, where the most intense X-rays are associated with the strong magnetic field near its poles. Saturn's X-ray spectrum, or the distribution of its X-rays according to energy, was found to be similar to that of X-rays from the Sun. This indicates that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere. The intensity of these reflected X-rays was unexpectedly strong. ... The optical image of Saturn is also due to the reflection of light from the Sun - visible wavelength light in this case - but the optical and X-ray images obviously have dramatic differences. The optical image is much brighter, and shows the beautiful ring structures, which were not detected in X-rays. This is because the Sun emits about a million times more power in visible light than in X-rays, and X-rays reflect much less efficiently from Saturn's atmosphere and rings."^[18]

"[T]he soft X-ray emissions of Jupiter (and Saturn) can largely be explained by scattering and fluorescence of solar X-rays."^[19]

The second image at the right, "taken by the Chandra x-ray telescope, reveals that the rings of Saturn sparkle; in this x-ray/optical composite, they are visible as blue dots. This radiation’s source is likely fluorescence caused by solar x-rays as they strike oxygen atoms in the water molecules of the planet’s icy rings. As the image shows, most of the ring’s x-rays originate in the B ring—the bright white inner ring visible in this optical image—which is approximately 25,000 kilometers wide and 40,000 kilometers above the planet’s surface. X-rays may also be concentrated on Saturn’s left side, possibly because of their association with shadows in the planet’s rings that are known as spokes, or possibly as a result of the additional solar fluorescence caused by the transient ice clouds that produce spokes. Other Chandra observations of Saturn show that the x-ray brightness of the rings varies significantly from one week to the next."^[20]

"One of a series, this image [at right] of Saturn was taken when the planet's rings were at their maximum tilt of 27 degrees toward Earth. Saturn experiences seasonal tilts away from and toward the sun, much the same way Earth does. This happens over the course of its 29.5-year orbit. Every 30 years, Earth observers can catch their best glimpse of Saturn's south pole and the southern side of the planet's rings. ... NASA's Hubble Space Telescope [captured detailed images of Saturn's Southern Hemisphere and the southern face of its rings."^[21]

The movie at right records Saturn "when its rings were edge-on, resulting in a unique movie featuring the nearly symmetrical light show at both of the giant planet's poles. It takes Saturn almost thirty years to orbit the Sun, with the opportunity to image both of its poles occurring only twice during that time. The light shows, called aurorae, are produced when electrically charged particles race along the planet's magnetic field and into the upper atmosphere where they excite atmospheric gases, causing them to glow. Saturn's aurorae resemble the same phenomena that take place at the Earth's poles."^[22]

Powered by the Saturnian equivalent of (filamentary) Birkeland currents, streams of charged particles from the interplanetary medium interact with the planet's magnetic field and funnel down to the poles.^[23] Double layers are associated with (filamentary) currents,^[24][25] and their electric fields accelerate ions and electrons.^[26]

"Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. ... In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules."^[27]

"The best view of Saturn's rings in the ultraviolet indicates there is more ice toward the outer part of the rings, than in the inner part, hinting at the origins of the rings and their evolution."^[28]

"Images taken during the Cassini spacecraft's orbital insertion on June 30 show compositional variation in the A, B and C rings. From the inside out, the "Cassini Division" in faint red at left is followed by the A ring in its entirety. The Cassini Division at left contains thinner, dirtier rings than the turquoise A ring, indicating a more icy composition. The red band roughly three-fourths of the way outward in the A ring is known as the Encke gap."^[28]

"The ring system begins from the inside out with the D, C, B and A rings followed by the F, G and E rings. The red in the image indicates sparser ringlets likely made of "dirty," and possibly smaller, particles than in the icier turquoise ringlets."^[28]

The image at right "was taken with the Ultraviolet Imaging Spectrograph instrument, which is capable of resolving the rings to show features up to 97 kilometers (60 miles) across, roughly 100 times the resolution of ultraviolet data obtained by the Voyager 2 spacecraft."^[28]

The image at second left Saturn's northern UV auroras. These exhibit changes in shape over the course of the observing interval.

"Saturn’s magnetosphere -- the big magnetic bubble that surrounds the planet -- is compressed on the side facing the sun, and it streams out into a long “magnetotail” on the planet’s nightside. Just like with comets, the magnetotails of Earth and Saturn are made of electrified gas from the sun."^[29]

"Now it appears that when strong bursts of particles from the sun hit Saturn, the magnetotail collapses and then reconfigures itself -- a disturbance of the magnetic field that’s reflected in the dynamics of auroras."^[29]

“We have always suspected this was what also happens on Saturn. This evidence really strengthens the argument.”^[30]

“We can see that the magnetotail is undergoing huge turmoil and reconfiguration, caused by buffering from solar wind. It’s the smoking gun that shows us that the tail is collapsing.”^[30]

"The north polar region of Saturn is pictured in great detail in this Voyager 2 image obtained Aug. 25 from a range of 633,000 kilometers (393,000 miles)."^[31]

"Two oval cloud systems some 250 km (150 mi) across are visible at about 72 degrees north latitude. The bright spot in the center of the leftmost cloud is a convective cloud storm about 60 km. (37 mi.)across. The outer ring of material rotates in an anti-cyclonic sense(counterclockwise in the northern hemisphere). A similar cloud structure of comparable dimension appears at 55 degrees north (bottom center of this picture). These northern latitudes contain many bright, small-scale cloud spots--only a few tens of kilometers across--representative of convective cloud systems. Across the top of this image stretch several long, linear, wavelike features that may mark the northernmost east-flowing jet in Saturn's atmosphere."^[31]

"In this orange-and-violet-image composite, the smallest features visible are about 16 km. (10 mi.) across."^[31]

In the second image at right, "[t]he gas planet's subtle northward gradation from gold to azure is a striking visual effect that scientists don't fully understand. Current thinking says that it may be related to seasonal influences, tied to the cold temperatures in the northern (winter) hemisphere. Despite Cassini's revelations, Saturn remains a world of mystery."^[32]

File:PIA06177.jpg

"Saturn's northern hemisphere [as shown in the first image on the right] is presently a serene blue [...] as seen in this natural color image from Cassini."^[33]

"Images obtained using red, green and blue spectral filters were combined to create this color view. The images were taken with the Cassini spacecraft wide angle camera on Dec. 14, 2004, at a distance of 719,200 kilometers (446,900 miles) from Saturn. The image scale is about 39 kilometers (24 miles) per pixel."^[33]

In the image second down on the right, "Mimas drifts along in its orbit against the azure backdrop of Saturn's northern latitudes in this true color view from NASA's Cassini spacecraft. The long, dark lines on the atmosphere are shadows cast by the planet's rings."^[34]

The planet exhibits a pale yellow hue due to ammonia crystals in its upper atmosphere. Its exterior is predominantly composed of gas and it lacks a definite surface. The planet primarily consists of hydrogen. The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium.^[35] The proportion of helium is significantly deficient compared to the abundance of this element in the Sun.^[36] Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been detected in Saturn's atmosphere.^[37][38][39]

File:Early telescope subdued browns.jpg

File:Voyager 1 subdued browns.jpg

Early telescopic observations, i.e. pre-1979, as shown on the right revealed subdued browns of Saturn's upper atmosphere.

Later images from Pioneer 11 and Voyager 1 as the image on the left shows also exhibited subdued browns.

"A swing high above Saturn by NASA's Cassini spacecraft revealed this stately view of the golden-hued planet and its main rings. The view is in natural color, as human eyes would have seen it. This mosaic was made from 36 images in three color filters obtained by Cassini's imaging science subsystem on Oct. 10, 2013. The observation and resulting image mosaic were planned as one of three images for Cassini's 2013 Scientist for a Day essay contest."^[40]

"Saturn sports differently colored bands of weather in this image [second down on the right]. For instance, a bright, narrow wave of clouds around 42 degrees north latitude appears to be some of the turbulent aftermath of a giant storm that reached its violent peak in early 2011. The mysterious six-sided weather pattern known as the hexagon is visible around Saturn's north pole."^[40]

"When Cassini arrived in 2004, more of the northern hemisphere sported a bluish hue and it was northern winter. The golden tones dominated the southern hemisphere, where it was southern summer. But as the seasons have turned and northern spring is in full swing, the colors have begun to change in each hemisphere as well. Golden tones have started to dominate in the northern hemisphere and the bluish color in the north is now confined to a tighter circle around the north pole. The southern hemisphere has started getting bluer, too."^[40]

File:Saturn H2On.jpg

File:Eso1116a.jpg

On the left is Saturn imaged by the Stockholm Infrared Camera (SIRCA) in the H₂O infrared band to show the presence of water vapor. The image is cut off near the top due to the presence of Saturn's rings.

At right is an infrared astronomy image of Saturn. "This is the sharpest image of Saturn's temperature emissions taken from the ground; it is a mosaic of 35 individual exposures made at the W.M. Keck I Observatory, Mauna Kea, Hawaii on Feb. 4, 2004. The images to create this mosaic were taken with infrared radiation. The black square at 4 o'clock represents missing data."^[41]

"In the most precise reading of Saturn's temperatures ever taken from Earth, a new set of infrared images suggests a warm "polar vortex" at Saturn's south pole - the first warm polar cap ever to be discovered in the solar system. The vortex is punctuated by a compact spot that is the warmest place on the planet."^[41]

"The puzzle isn't that Saturn's south pole is warm; after all, it has been exposed to 15 years of continuous sunlight, having just reached its summer Solstice late in 2002. But both the distinct boundary of a warm polar vortex some 30 degrees latitude from the southern pole and a very hot "tip" right at the pole were completely unexpected. If the increased southern temperatures are the result of the seasonal variations of sunlight, then temperatures should increase gradually with increasing latitude. But they don't – the tropospheric temperature increases toward the pole abruptly near 70 degrees latitude from 88 to 89 Kelvin (- 301 to -299 degrees Fahrenheit) and then to 91 Kelvin (-296 degrees Fahrenheit) right at the pole. Near 70 degrees latitude, the stratospheric temperature increases even more abruptly from 146 to 150 Kelvin (-197 to -189 degrees Fahrenheit) and then again to 151 Kelvin (-188 degrees Fahrenheit) right at the pole."^[41]

The second image at right is "constructed from data collected in the near-infrared wavelengths of light, the auroral emission is shown in green. The data represents emissions from hydrogen ions in of light between 3 and 4 microns in wavelength. In general, scientists designated blue to indicate sunlight reflected at a wavelength of 2 microns, green to indicate sunlight reflected at 3 microns and red to indicate thermal emission at 5 microns. Saturn's rings reflect sunlight at 2 microns, but not at 3 and 5 microns, so they appear deep blue. Saturn's high altitude haze reflects sunlight at both 2 and 3 microns, but not at 5 microns, and so it appears green to blue-green. The heat emission from the interior of Saturn is only seen at 5 microns wavelength in the spectrometer data, and thus appears red. The dark spots and banded features in the image are clouds and small storms that outline the deeper weather systems and circulation patterns of the planet. They are illuminated from underneath by Saturn's thermal emission, and thus appear in silhouette. The composite image was made from 65 individual observations by Cassini's visual and infrared mapping spectrometer on 1 November 2008. The observations were each six minutes long."^[42]

The third image at right shows Saturn's northern polar region with "the aurora and underlying atmosphere, seen at two different wavelengths of infrared light as captured by NASA's Cassini spacecraft. Energetic particles, crashing into the upper atmosphere cause the aurora, shown in blue, to glow brightly at 4 microns (six times the wavelength visible to the human eye). The image shows both a bright ring, as seen from Earth, as well as an example of bright auroral emission within the polar cap that had been undetected until the advent of Cassini. This aurora, which defies past predictions of what was expected, has been observed to grow even brighter than is shown here. Silhouetted by the glow (cast here to the color red) of the hot interior of Saturn (clearly seen at a wavelength of 5 microns, or seven times the wavelength visible to the human eye) are the clouds and haze that underlie this auroral region."^[43]

Also on the right is a fourth image of Saturn's north polar region in infrared. "This striking false-color mosaic was created from 25 images taken by Cassini's visual and infrared mapping spectrometer over a period of 13 hours, and captures Saturn in nighttime and daytime conditions. The visual and infrared mapping spectrometer acquires data simultaneously at 352 different wavelengths, or spectral channels. Data at wavelengths of 2.3, 3.0 and 5.1 microns were combined in the blue, green and red channels of a standard color image, respectively, to make this false-color mosaic."^[44]

"This image was acquired on 24 February 2007, while the spacecraft was 1.58 million km (1 million miles) from the planet and 34.6 degrees above the ring plane. The solar phase angle was 69.5 degrees. In this view, Cassini was looking down on the northern, unlit side of the rings, which are rendered visible by sunlight filtering through from the sunlit, southern face."^[44]

"On the night side (right side of image), with no sunlight, Saturn's own thermal radiation lights things up. This light at 5.1 microns wavelength (some seven times the longest wavelength visible to the human eye) is generated deep within Saturn, and works its way upward, eventually escaping into space. Thick clouds deep in the atmosphere block that light. An amazing array of dark streaks, spots and globe-encircling bands is visible instead. Saturn's strong thermal glow at 5.1 microns even allows these deep clouds to be seen on portions of the dayside (left side), especially where overlying hazes are thin and the glint of the sun off of them is minimal. These deep clouds are likely made of ammonium hydrosulfide and cannot be seen in reflected light on the dayside, since the glint of the sun on overlying hazes and ammonia clouds blocks the view of this level."^[44]

"A pronounced difference in the brightness between the northern and southern hemispheres is apparent. The northern hemisphere is about twice as bright as the southern hemisphere. This is because high-level, fine particles are about half as prevalent in the northern hemisphere as in the south. These particles block Saturn's glow more strongly, making Saturn look brighter in the north."^[44]

"At 2.3 microns (shown in blue), the icy ring particles are highly reflecting, while methane gas in Saturn's atmosphere strongly absorbs sunlight and renders the planet very dark. At 3.0 microns (shown in green), the situation is reversed: water ice in the rings is strongly absorbing, while the planet's sunlit hemisphere is bright. Thus the rings appear blue in this representation, while the sunlit side of Saturn is greenish-yellow in color. Within the rings, the most opaque parts appear dark, while the more translucent regions are brighter. In particular, the opaque, normally-bright B ring appears here as a broad, dark band separating the brighter A (outer) and C (inner) rings."^[44]

"At 5.1 microns (shown in red), reflected sunlight is weak and thus light from the planet is dominated by thermal (i.e., heat) radiation that wells up from the planet's deep atmosphere. This thermal emission dominates Saturn's dark side as well as the north polar region (where the hexagon is again visible) and the shadow cast by the A and B rings. Variable amounts of clouds in the planet's upper atmosphere block the thermal radiation, leading to a speckled and banded appearance, which is ever-shifting due to the planet's strong winds."^[44]

The fifth infrared image of Saturn is a detailed false color image. "[T]aken in January 1998 by the Hubble Space Telescope [it] shows the ringed planet in reflected infrared light. Different colors [indicate] varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division."^[45]

"Two of Saturn's many moons have also put in an appearance (in the full resolution version), Tethys just beyond the planet's disk at the upper right, and Dione at the lower left."^[45]

The panoramic images at right "from NASA's Cassini spacecraft chronicle a day in the life of a huge storm that developed from a small spot that appeared 12 weeks earlier in Saturn's northern mid-latitudes."^[46]

"This storm is the largest and most intense observed on Saturn by NASA's Voyager or Cassini spacecraft. The storm is still active. As seen in these and other Cassini images, the storm encircles the planet - whose circumference at these latitudes is 300,000 kilometres. From north to south, it covers a distance of about 15,000 kilometres, which is one-third of the way around the Earth. It encompasses an area of 4 billion square kilometres, or eight times the surface area of Earth. This storm is about 500 times the area of the biggest of the southern hemisphere storms ... observed by Cassini."^[46]

"The highest clouds in the image are probably around 100 millibars pressure, 100 kilometres above the regular undisturbed clouds. These false colors show clouds at different altitudes. Clouds that appear blue here are the highest and are semitransparent, or optically thin. Those that are yellow and white are optically thick clouds at high altitudes. Those shown green are intermediate clouds. Red and brown colors are clouds at low altitude unobscured by high clouds, and the deep blue color is a thin haze with no clouds below. The base of the clouds, where lightning is generated, is probably in the water cloud layer of Saturn's atmosphere. The storm clouds are likely made out of water ice covered by crystallized ammonia."^[46]

"Taken about 11 hours -- or one Saturn day -- apart, the two mosaics in the lower half of this image product consist of 84 images each. The mosaic in the middle was taken earlier than the mosaic at the bottom. Both mosaics were captured on Feb. 26, 2011, and each of the two batches of images was taken over about 4.5 hours."^[46]

"Two enlargements from the earlier, middle mosaic are shown at the top of this product. The white lines below the middle mosaic identify those parts of the mosaic that were enlarged for these close-up views. The enlargement on the top left shows the head of the storm, and that on the top right shows the turbulent middle of the storm. Cassini observations have shown the head of the storm drifting west at a rate of about 2.8 degrees of longitude each Earth day (28 meters per second, or 63 miles per hour). The central latitude of the storm is the site of a westward jet, which means that the clouds to the north and south are drifting westward more slowly or even drifting eastward. In contrast, clouds at Saturn's equator drift eastward at speeds up to 450 meters per second (1,000 miles per hour). "^[46]

"Both of the long mosaics cover an area ranging from about 30 degrees north latitude to 51 degrees north latitude. The views stretch from about 138 degrees west longitude on the left to 347 degrees west longitude on the right, passing through 360/0 degrees west longitude near the far right of the mosaics."^[46]

"The images were taken with the Cassini spacecraft narrow-angle camera using a combination of spectral filters sensitive to wavelengths of near-infrared light. The images filtered at 889 nanometers are projected as blue. The images filtered at 727 nanometers are projected as green, and images filtered at 750 nanometers are projected as red."^[46]

"The views were acquired at a distance of approximately 2.4 million kilometres from Saturn and at a sun-Saturn-spacecraft angle (phase angle) of 62 degrees. Both the top and bottom images are simple cylindrical map projections, defined such that a square pixel subtends equal intervals of latitude and longitude. At higher latitudes, the pixel size in the north-south direction remains the same, but the pixel size in the east-west direction becomes smaller. The pixel size is set at the equator, where the distances along the sides are equal. The images of the long mosaics have a pixel size of 53 kilometres at the equator, and the two close-up views have a pixel size of 9 kilometres per pixel at the equator."^[46]

The seventh image down on the right shows thermal "infrared images of Saturn from the VISIR instrument on ESO’s VLT (centre and right) and an amateur visible-light image (left) from Trevor Barry (Broken Hill, Australia) obtained on 19 January 2011 during the mature phase of the northern storm. The second image is taken at a wavelength that reveals the structures in Saturn’s lower atmosphere, showing the churning storm clouds and the central cooler vortex. The third image is sensitive to much higher altitudes in Saturn’s normally peaceful stratosphere, where we see the unexpected beacons of infrared emission flanking the central cool region over the storm."^[47] The two wavelengths are 18 µm and 8.6 µm.^[47]

The eighth image down on the right shows a "Gemini North infrared image of Saturn and Titan (at about 6 o'clock position). Image obtained on May 7, 2009 (5:31 UTC) using the Altair adaptive optics system with the Near-infrared imager (NIRI). At the perimeter of Saturn's ring the F-ring is faintly visible. The F-ring was discovered in images from the Pioneer 11 spacecraft in 1979 and is normally not apparent in images taken with ground-based telescopes. Also apparent are several of Saturn's smaller moons. This color composite image made using data from three infrared filters (K' [2.0-2.1 microns], h210 [2.12 micron narrowband], and bracket gamma[2.17 micron narrowband]), the field of view is about 40 arcseconds across."^[48]

"[T]he PH₃ 1-0 rotational line (266.9 GHz) line [has been detected] in [the atmosphere of] Saturn".^[49]

"Three simultaneous radio signals at wavelengths of 0.94, 3.6, and 13 centimeters (Ka-, X-, and S-bands) were sent from Cassini through the rings to Earth. The observed change of each signal as Cassini moved behind the rings provided a profile of the distribution of ring material and an optical depth profile."^[50]

"This simulated image was constructed from the measured optical depth profiles of the Cassini Division and ring A. It depicts the observed structure at about 10 kilometers (6 miles) in resolution. The image shows the same ring A region depicted in a similar image (Multiple Eyes of Cassini), using a different color scheme to enhance the view of a remarkable array of over 40 wavy features called 'density waves' uncovered in the May 3 radio occultation throughout ring A."^[50]

"Color is used to represent information about ring particle sizes based on the measured effects of the three radio signals. Shades of red [purple] indicate regions where there is a lack of particles less than 5 centimeters (about 2 inches) in diameter. Green and blue shades indicate regions where there are particles of sizes smaller than 5 centimeters (2 inches) and 1 centimeter (less than one third of an inch), respectively."^[50]

"Note the gradual increase in shades of green towards the outer edge of ring A. It indicates gradual increase in the abundance of 5-centimeter (2-inch) and smaller particles. Note also the blue shades in the vicinity of the Keeler gap (the narrow dark band near the edge of ring A). They indicate increased abundance of even smaller particles of diameter less than a centimeter. Frequent collisions between large ring particles in this dynamically active region likely fragment the larger particles into more numerous smaller ones."^[50]

The image at left is Saturn's atmosphere and its rings shown "in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera."^[51]

"A large, bright and complex convective storm that appeared in Saturn's southern hemisphere in mid-September 2004 was the key in solving a long-standing mystery about the ringed planet."^[51]

"The complex feature with arms and secondary extensions just above and to the right of center is called the Dragon Storm. It lies in a region of the southern hemisphere referred to as "storm alley" by imaging scientists because of the high level of storm activity observed there by Cassini in the last year."^[51]

"Light rays here travel a much longer path through the relatively cloud-free upper atmosphere. Along this path, shorter wavelength blue light rays are scattered effectively by gases in the atmosphere, and it is this scattered light that gives the region its blue appearance. Why the upper atmosphere in the northern hemisphere is so cloud-free is not known, but may be related to colder temperatures brought on by the ring shadows cast there."^[33]

"Shadows cast by the rings surround the pole, looking almost like dark atmospheric bands. The ring shadows at higher latitudes correspond to locations on the ringplane that are farther from the planet--in other words, the northernmost ring shadow in this view is made by the outer edge of the A ring."^[33]

"Spots of bright clouds also are visible throughout the region. This view is similar to an infrared image obtained by Cassini at nearly the same time (see PIA06567). The infrared view shows a great deal more detail in the planet's atmosphere, however."^[33]

"Saturn's northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths."^[34]

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