Saturn's atmosphere

Data from Pioneer 11, Voyagers 1 and 2, and the Cassini spacecraft, combined with ground-based observations, show Saturn’s atmosphere to be composed largely of hydrogen mixed with helium. Clouds of ammonia ice and other chemical components are sources for the various configurations on the planet’s visible surface. This hydrogen-helium envelope likely covers a layer of metallic hydrogen that surrounds Saturn’s rocky core.

Overview

Archaeologists have found recorded observations of Saturn dating from several hundred years BCE, inscribed in cuneiform on kiln-baked bricks. It was not until more than two millennia later that Galileo, using one of his early telescopes, was first able to see both the planet and its rings—though he did not recognize them as such. Original drawings made by Galileo indicate that he interpreted the rings as solid. At times he drew them as if they were open handles on a cup, and at other times as filled-in semicircles connected to the planet. In 1794, Sir William Herschel, carefully compared many weeks’ worth of observations of subtle markings to deduce a planetary rotation period of 10 hours, 16 minutes, and 0.4 seconds. In the twenty-first century, Saturn is theorized to have a mean density of 690 kilograms per cubic meter, considerably less than that of water. It is therefore accepted that Saturn must be composed mainly of the lightest element, hydrogen. The presence of hydrogen was difficult to detect spectroscopically, but methane and ammonia were both observed in 1932.

Ground-based observations of Saturn have always been fruitful, as well as exciting, and continue to be so. However, astronomers’ knowledge of the Saturn system was increased dramatically by four spacecraft encounters with the planet. The first was Pioneer 11, later called Pioneer Saturn, which made its closest approach to Saturn on September 1, 1979. It made many important observations of the entire Saturnian system but was concerned mainly with the nature of the surrounding radiation and particle environments. Images made by the photopolarimeter showed a butterscotch-colored planet with an indistinct pattern of subtly varying belts and zones. In other words, Saturn’s atmosphere appeared much more subdued than Jupiter’s.

More data were gathered during the first of the two Voyager encounters. Voyager 1 made its closest approach to Saturn on November 12, 1980. A few months later, its sister spacecraft, Voyager 2, made its closest approach on August 26, 1981. It was the outbound trajectory of the Voyager 2 spacecraft that was expected to provide good images of Saturn’s southern hemisphere. Unfortunately, soon after closest approach, a problem arose with Voyager 2’s scan platform, the movable platform upon which several of the instruments—including the two cameras—were mounted. Thus, most of the images obtained during the departure portion of the flyby show primarily the northern hemisphere.

Continuing analysis of all these observations has led to a reasonably consistent picture of Saturn’s atmosphere, although much remains uncertain. One of the most obvious features of Saturn is its shape. It is flattened at its poles more than any of the other planets. Measurements estimated by the International Astronomical Union (IAU) in 1985 for Saturn’s polar and equatorial radii were 53,543 and 60,000 kilometers, respectively.

The shape of the planet, in association with measurements of other quantities, such as its gravitational field, composition, and heat production, is an important constraint on models of the planet’s interior. The major chemical component of Saturn is hydrogen, but other elements are present as well. In particular, a heavy central core, composed of some sort of ice or rock, is proposed. To be consistent with gravity measurements, this core must contain between 10 and 20 percent of Saturn’s mass. This proportion is considerably larger than the 2 or 3 percent that would be expected if Saturn had the same composition as the sun. This central region may extend to about one-fifth of Saturn’s radius.

Outside the core lies the hydrogen-helium atmosphere. In the visible part of the atmosphere, hydrogen is in its normal gaseous form, but deeper inside Saturn it is compressed by the weight of the overlying atmosphere. About halfway between the center and the visible surface, the pressure is so great, around three million bars, that hydrogen must change form and behave like a metallic fluid. For comparison, the Atmospheric pressure at Earth’s surface is about one bar.

It is this metallic region that could generate Saturn’s magnetic field. Researchers believe that irregularities in this region produce slight asymmetries in the magnetic field, which, in turn, regulates the radio emissions observed by the Voyager spacecraft. It is for this reason that the observed period of 10 hours, 39 minutes, and 24 seconds (with an estimated error of 7 seconds) of this radiation is thought to represent the rotational period of the deep interior of Saturn.

After hydrogen, the largest constituent of Saturn is helium, the second lightest element. The mass fraction of helium in Saturn has been estimated from infrared measurements to be 0.06 ± 0.05, significantly less than the value of 0.18 ± 0.04 found for Jupiter. This difference is thought to be caused by the fact that, at the lower temperatures found on Saturn, some of the helium becomes insoluble in the primarily hydrogen atmosphere. It is thought that it may form small droplets at some level and “rain out,” dissolving again at a deeper, and hence warmer, level. This process would warm the atmosphere somewhat.

The observed average temperature of Saturn, derived from infrared measurements, is about ninety-five kelvins, compared with an equilibrium temperature of eighty-two kelvins, the temperature that would be expected if the planet were entirely dependent on solar radiation for heat. The implication is that Saturn radiates 1.78 ± 0.09 times as much heat as it receives. Some of this energy is thought to be generated in the helium separation.

Because of its twenty-seven degree orbital inclination, larger than the twenty-three degree inclination of Earth, Saturn has seasons. The period of the Voyager encounters corresponded to early spring in the northern hemisphere. The large heat capacity of Saturn’s atmosphere, however, meant that the southern hemisphere, where it was early fall, would still be a few kelvins warmer than the northern hemisphere.

Although hydrogen and helium account for most of the mass of Saturn’s atmosphere, they are not responsible for the patterns seen in the Voyager images. Despite the fact that only gaseous ammonia has been observed spectroscopically, these patterns are thought to be the tops of ammonia clouds. Theoretical models predict that below the ammonia clouds there are clouds of ammonia hydrosulfide and water ice. There is, however, little observational confirmation of these lower cloud layers. The lack of observational data makes it difficult to model the behavior and possible interactions of these multiple cloud layers. Whether they produce precipitation—ammonia or water precipitation in the form of either rain or snow—remains a matter of speculation.

The presence of ammonia clouds still fails to explain the appearance of the planet, since a cloud of small ammonia droplets would not create Saturn’s observed butterscotch color. It should be remembered that the colors of many of the published spacecraft images are considerably enhanced (as “false-color” imagery) to make subtle variations distinguishable. Thus, an additional chemical component, termed a chromophore, has been postulated. Such a component must be capable both of existing at this level in the atmosphere and of providing the needed color. The complex radiation environment of Saturn’s upper atmosphere could produce many types of chemicals. Among those suggested as cloud chromophores are various compounds of sulfur, phosphorus, and hydrazine, as well as various mixtures of organic compounds containing both hydrogen and carbon.

A comparison of two images of Saturn’s atmosphere will show not only that there is an interesting variety of cloud shapes but also that they are all moving relative to one another. Most of the movement observed is longitudinal—that is, around the rotational axis. If the radio rotation period is used as the basic rotation rate, then the clouds show an alternating series of east-west jets. Close to the equator, these jets have a velocity of between four hundred and five hundred meters per second (the so-called equatorial super-rotation). The velocity decreases away from the equator until it reaches a retrograde speed of twenty-five meters per second at about forty degrees north and south latitudes. It then increases again to nearly 150 meters per second before decaying yet again. This pattern has been observed to eighty-four degrees north latitude, always with postgrade jets moving considerably faster than the retrograde ones. The mechanism responsible for generating these jets is still uncertain. The motions in Earth’s atmosphere are its response to an uneven solar heat input. The attempt to move warm air poleward produces, in the shallow (less than 1 percent of Earth’s radius) rotating atmosphere, streams of high- and low-pressure areas encircling the midlatitudes. A similar mechanism could operate on Saturn, with observed surface motions decaying rapidly with height. An alternative suggestion is that Saturn’s internal heat source produces a form of convection, and that the observed zonal flow around the rotational axis extends all the way through the planet, parallel to the rotational axis.

A number of additional features have been noted on Saturn’s visible surface. In the southern hemisphere, there is a reddish spot that resembles a miniature version of Jupiter’s Great Red Spot. Near the equator, there are various wispy clouds, which appear to be stretched by the strong wind shears present in this region. Farther north, at around forty degrees north latitude, there are three brown spots, each one a few thousand kilometers across. Just to the south of these is an area in which white blobs of cloud, possibly of convective origin, appear, evolve rapidly, and dissipate. In the mid-1990s, the Hubble Space Telescope detected a large white storm near the equator that eventually grew to nearly twenty thousand kilometers in length before beginning to fade. This storm, referred to as Saturn’s Great White Spot, is hardly unique. Large white storms of this type have been noted in astronomical records on a nearly thirty-year period going back well into the nineteenth century.

Just to the north of the brown spots, centered at about forty-seven degrees north latitude, is the “ribbon feature,” a fairly light band, about five degrees wide, inside of which is a darker streak, around 1,000 kilometers wide, which threads an oscillatory north-south path along the center. The appearance of the individual peaks and troughs evolves rapidly over a few days, the average distance between adjacent peaks being 5,700 kilometers. This feature apparently represents some type of atmospheric wave. Measurements of the infrared emissions of this region show a large north-south temperature gradient, and it could be that this feature is the same type of atmospheric phenomenon as that which transfers equatorial heat northward on Earth.

Despite the nearly equatorial trajectory of the Voyager spacecraft, some images of the north polar region were obtained. These show it to be populated with many small, fluffy clouds, close to which, at about eighty degrees north latitude, was a regular hexagonal pattern, formed of long, thin, striated clouds that are moving at about one hundred meters per second. The hexagonal pattern appeared to be stationary relative to the radio rotation period, with the clouds passing around its corners. The driving mechanism of this feature was a mystery, although the close association between its rotation rate and the radio rotation period could be significant. The Cassini spacecraft returned images in 2007 of Saturn’s north polar region that again showed such a hexagonal pattern. This time the hexagon was nearly twenty-four thousand kilometers across. Data suggested that the nearly perfect hexagonal pattern extended down into Saturn’s clouds to a depth of nearly one hundred kilometers. According to University of Oxford planetary scientist Leigh Fletcher, the appearance of this vortex was surprising. Apparently gas moved toward the pole and was compressed and heated as it dropped into the depths of Saturn’s troposphere over the pole. The physical mechanism for this behavior remained unknown.

Cassini entered orbit around Saturn on July 1, 2004. During its primary four-year mission, the spacecraft returned impressive data about the ringed planet and its satellites, including the particles and field environment surrounding Saturn. The spacecraft remained in nearly perfect health, and therefore it was funded for a two-year extended mission to continue the scientific harvest.

Knowledge Gained

Spacecraft flybys of the Saturnian system and prolonged orbital observations by Cassini have dramatically increased astronomers’ knowledge of Saturn’s dynamic and complex atmosphere. Measurements of the hydrogen and helium abundance, the zonal circulation, the structure of the planet’s gravitational and magnetic fields, and the derivation of the radio rotation rate would have been very difficult, if not impossible, to obtain from Earth and even from the Hubble Space Telescope.

These Cassini observations have enabled scientists to refine considerably their models of Saturn’s internal composition and structure. This model envisages that the visible envelope, composed primarily of hydrogen and helium, extends about halfway to the planet’s center; below this covering is a region of metallic hydrogen and a small rocky core. The deep atmosphere is warmed by Saturn’s internal heat source and becomes cooler with distance from the core, reaching a minimum of about eighty-five kelvins at a pressure level of around one hundred millibars at the tropopause. Above this level, solar heating becomes significant, and the temperature starts to increase again. Visible clouds are thought to consist of ammonia ice crystals with, possibly, ammonium hydrosulfide and water-ice cloud layers below them. The existence of these lower cloud layers would depend on the amounts of the various elements in this part of the atmosphere.

The Voyagers provided an extensive album of images of Saturn, documenting cloud motions and their morphologies. Such images, however, have increased scientists’ understanding of the atmospheric dynamics only to a limited extent. Even the basic driving mechanism remains uncertain; it could be the planet’s internal heat source or the sun’s external heat supply. The extent to which Voyager observations have increased scientists’ knowledge can also be considered in terms of their understanding of atmospheres in general. The planetary atmosphere most comparable to Saturn’s is that of Jupiter, which is slightly larger, rotates slightly faster and has a similar hydrogen and helium composition. Jupiter is, however, much closer to the sun than Saturn, and it therefore receives several times as much solar energy.

Compared with Jupiter’s highly turbulent atmosphere, Saturn’s atmosphere, with its subtle belt-zone variations and widely separated spots, might appear to be relatively quiescent. Voyager observations, however, have shown that this is far from the case. Saturn’s atmosphere moves at velocities up to five hundred meters per second, compared with about two hundred meters per second on Jupiter. Jupiter's observations also suggested that Saturn’s alternating belt-zone pattern was associated with alternating eastward and westward jets. Saturn observations, which had no such obvious correlation, showed that this model was overly simplistic. Another interesting difference between the two planets is that on Jupiter westward jets reach much the same speed, relative to the radio rotation period, as eastward jets. The situation on Saturn is far less symmetrical, with eastward jets dominating.

Circulation patterns in Earth’s atmosphere are still not fully understood. Thus it is not surprising that scientists’ knowledge of the atmospheres of the outer planets remains fairly basic. The measurements of the Voyager and Cassini spacecraft contribute to that knowledge mainly by increasing the number of observations upon which models can be based.

One such Cassiniobservation was referred to by online National Aeronautics and Space Administration (NASA) reports as Saturn “riding the wave.” The wave pattern in the atmosphere is visible from Earth only every fifteen years. Earth has a similar oscillation, but its period is only two years. Jupiter has a similar oscillation, but with a four-year period. Saturn’s oscillation in question was under observation with ground-based telescopes and Cassini’s Composite Infrared Spectrometer. This oscillation involves temperatures in Saturn’s upper atmosphere switching with altitude in a hot-cold pattern that, when graphed in three dimensions, assumes a shape much like the stripes that wrap around a candy cane. These temperature oscillations result in winds changing direction from east to west and back again, and it is this pattern to which scientists were referring as the wave pattern discovered on Saturn.

Beginning in December, 2007, Cassini imaged a storm in Saturn’s southern hemisphere that produced lightning discharges with energies more than ten thousand times that of typical lightning on Earth. Cassini’s radio and plasma wave instruments actually picked up the lightning about a week before the storm itself could be identified by the spacecraft’s imaging cameras. Scientists then used the periodic appearance of the storm to confirm Saturn’s rotation rate.

In May 2013, Cassini captured close-up images of a hurricane-like vortex at Saturn's north pole. Although scientists intended to study the vortex to gain insight into similar Earth-based storms, Saturn's hurricane differed from those on Earth in several ways. The eye of the Saturn-based storm was 1,250 miles wide, more than twenty times larger than the eye of an average hurricane on Earth. It also appeared to be spinning four times faster than Earth's hurricane-force winds and staying at Saturn's north pole instead of moving the way Earth-based storms do.

Context

If it had not been for the Pioneer, Voyager, andCassini missions, knowledge of Jupiter and Saturn would be limited to that obtainable from Earth or from Earth-orbiting observatories such as the Hubble Space Telescope. Although probes have not entered Saturn’s atmosphere, they have still provided many observations that contribute to astronomers’ understanding of it. For the first time, researchers could see how the planet changes in appearance when it is viewed from different directions. Monitoring of each spacecraft’s radio signal as it disappeared behind Saturn provided information about the local atmospheric temperature profile (the variation of temperature with height). The proximity of these spacecraft to the planet also allowed the study of the structure of individual features.

Prior to these dedicated spacecraft missions, Saturn was a greatly appreciated but little understood object. Only its most global properties had been investigated. It was believed to be a typical “gas giant”—a smaller, colder, and less colorful version of Jupiter. Voyager's observations revealed this concept to be only partly true. The elemental abundances derived for Saturn are different from those of Jupiter, the most striking difference being the hydrogen-to-helium ratio, which, combined with the planet’s unexpectedly high temperature, led to the idea that its helium becomes depleted by becoming insoluble and precipitates into the lower atmosphere.

Very little was known about Saturn’s atmospheric dynamics before Voyager images were obtained. Tracking motions of the occasional atmospheric spots visible from Earth had suggested a predominantly zonal flow, strongest near the equator. It was only with the Voyager flybys that periodic radio emissions from the planet were observed. The association of these with the rotation of Saturn’s interior provided a plausible base velocity against which the motion of other features could be measured. Prior to this finding, velocities could only be given relative to an arbitrary feature.

The Pioneer 11 encounter provided some intriguing information about Saturn’s atmosphere; the two Voyager encounters supplied far more. All these flyby missions, however, could provide only a brief look at the planet. Ground-based observations suggest that the large-scale zonal flow is probably fairly stable. The appearance of the individual belts and zones can change, however, while smaller individual features can evolve quite rapidly. Some of these changes may be seasonal effects, while others could be caused by various types of instabilities in the same way that the fairly constant solar heating of Earth produces a perpetual sequence of low- and high-pressure regions.

To investigate these effects further, it was necessary to observe the planet over a longer period than is possible with a flyby mission. The Hubble Space Telescope was deployed from the space shuttleDiscovery in April, 1990. After a troubled start requiring repair missions, Hubble was able to begin taking unprecedented images, including on occasion some of Saturn. Although Hubble could capture images only at a much lower resolution than the Voyager spacecraft had achieved, Hubble did have the capacity to observe in spectral bands that could not be seen from Earth because of absorption by the atmosphere.

The obvious next step in Saturn investigation was to place an orbiter about the ringed planet, one that could use encounters with its many satellites in addition to propulsion system firings to alter its course so that the spacecraft could swing close to interesting objects; a principal focus on studying the large satellite Titan and its atmosphere was given to this follow-on mission. The National Aeronautics and Space Administration and the European Space Agency named the follow-up probe to the Voyager flyby mission Cassini after the astronomer who discovered several of Saturn’s satellites. Cassini was launched on October 15, 1997, and after several gravity assists in the inner solar system, it flew past Jupiter and then was redirected to enter the Saturnian system and conduct its primary science mission. That primary mission ended in 2008, but as the spacecraft was still in fully functional condition, a two-year extended mission was conceived and funded.Cassini arrived on station in Saturn orbit beginning July 1, 2004.

In its time observing Saturn Cassini found confounding aspects in Saturn’s dynamic and complex atmospheric structure. White spots, which were also seen with the Hubble Space Telescope, were seen to develop and diminish. However, perhaps the most perplexing atmospheric features were a hexagonal pattern about the north pole twenty-five thousand kilometers across and another similar feature found later to exist around the south pole. The nature and stability of features such as these polar ones remain under investigation.

Cassini'sprimary mission ended in 2008, but as the spacecraft was still in fully functional condition, a two-year extended mission was conceived and funded. The return from Cassini continued to be of immense scientific value such that extensions to its mission continued until September 2017. At this time NASA disposed of the spacecraft by deliberately flying it into Saturn's atmosphere.

Beginning in 2023, NASA’s James Webb telescope, a much more powerful instrument than the Hubble Telescope began to take imagery of Saturn. The Webb telescope immediately revealed several unknown aspects of the planet. One of these was the discovery of a plume of water vapor that transited from a sub-surface ocean on the moon Enceladus and into its E ring. The volume of water was believed to be equal to 300 liters (79 gallons) per second. As the water plume emerges from Enceladus, it emits water which forms a halo-like structure which extends into the outer ring.

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