Saturns Atmosphere

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In 1704, in Opticks, Isaac Newton pointed out that 'white' light is a combination of a rainbow of colours, and that a glass prism will refract it. However, for some reason he either failed to notice, or chose to ignore, the thin dark lines that populate the solar spectrum. They were first remarked upon by J. Fraunhofer in Munich in 1814, but their origin was not recognised until 1859 when G.R. Kirchhoff and R.W Bunsen of the University of Heidelberg noticed that the lines that appear dark in the solar spectrum corresponded to lines which are bright in the spectra of incandescent gases.

In 1863, P.A. Secchi, the Italian pioneer of astronomical spectroscopy, spotted dark bands towards the red end of the spectra of Jupiter and Saturn, and concluded that their atmospheres were ''not yet cleansed'' of primordial gases as the Earth's had evidently been in its early history. Several years later, in London, William Huggins, who compared the realisation that a spectrum could reveal the chemical composition of a celestial object to ''coming upon a spring of water in a dry and dirty land'', mounted a spectroscope on an 8-inch refractor and independently discovered the lines towards the red end of Saturn's spectrum.

When P.J.C. Janssen in France first inspected Saturn's spectrum, he noticed that while it resembled that of Jupiter it had a distinctive line redward that he referred to simply as ''the red line'' because its origin was a mystery. In 1867 he transported his telescope to the 9,800-foot summit of Mount Etna on Sicily, the tallest volcano in Europe, in order to observe from above most of the tropospheric water vapour, and reported aqueous vapour in Saturn's atmosphere. However, his method involved fitting a prism onto his telescope and making visual comparisons between the planet and the face of the Moon, which was presumed to be arid. Unfortunately, using a micrometer, a visual inspection and documentation of the absorption features along the dispersion could take an hour or more, during which time local atmospheric conditions could easily change, and to make any comparison it was often necessary to wait several hours for the other object to achieve the same elevation, during which time conditions could alter further. As a result, little real progress was made until the introduction of photography enabled the spectrum to be recorded within minutes and compared at leisure; but to record a high-dispersion spectrum required the light grasp of a large telescope, and even then the state of the atmosphere could change while waiting for an object to achieve the requisite elevation. This was overcome by recording spectra in the laboratory for direct comparison. In 1875 Janssen established the Meudon Observatory in Paris and installed an excellent 33-inch refractor supplied by the Henry brothers to study the planets. In 1905 V.M. Slipher at the Lowell Observatory - established in 1894 by Percival Lowell at Flagstaff, Arizona - made a photographic study of the red absorption bands in Saturn's spectrum. There was evidently an as-yet-unidentified chemical in the planet's atmosphere that was leaving its imprint on the reflected solar spectrum. In 1909 he secured spectra using an emulsion that was sensitive into the near-infrared, and discovered more absorption features.

Orbiting twice the distance of Jupiter's orbit from the Sun, Saturn receives only a quarter of the insolation. In 1860 G.P. Bond determined that Jupiter radiates twice as much energy into space as it receives from the Sun. He reasoned that the giant planet must be in the process of contracting, transforming gravitational potential into heat, and concluded that the interior must be a very hot gas. In 1865, in Saturn and its System, the prolific populariser of astronomy, R.A. Proctor, wrote: ''Jupiter is still a glowing mass, fluid probably throughout, still bubbling and seething with the intensity of the primaeval fires, sending up continuous enormous masses of cloud, to be gathered into bands under the influence of the swift rotation of the giant planet.'' Saturn, being less massive, was thought to be at an 'earlier' stage of development. In 1882, in the second edition of his book, Proctor wrote: ''Regarding the cloud phenomena of the giant planets as generated by internal forces whose real secret lies deep below the visible surface of the cloud belts, we see that these forces must be of tremendous energy, must produce enormous changes in the cloud-laden atmosphere, with effects extending widely both vertically and laterally, and imply enormous heat in the whole frame of each planet.'' In fact, the giant planets were regarded as 'failed' stars. ''Over a region of hundreds of thousands of square miles in extent, the flowing surface of the planet must be torn by sub-planetary forces. Vast masses of intensely hot vapour must be poured forth from beneath, and, rising to enormous heights, must either sweep away the enwrapping mantle of cloud which had concealed the disturbed surface, or must itself form into a mass of cloud.'' This idea was so popular that in 1885 A.M. Clerke wrote in History of Astronomy during the Nineteenth Century: ''the chief arguments in favour of the high temperature of Jupiter apply, with increased force, to Saturn, so that it may be concluded, without much risk of error, that a large proportion of the bulky globe . . . is . . . heated vapours, kept in active and agitated circulation by the process of cooling.''

In 1923 Harold Jeffreys published the first of a series of papers showing that the 'failed' star model was untenable because the outer atmospheres of the giant planets are very cold, not hot. In 1924 he suggested that deep within their gaseous envelopes there were solid cores mantled with thick layers of ice. The details were different in each case, but for Saturn, whose great volume was offset by a very low density, he calculated that the envelope comprised about 20 per cent of the radius. He presented his results to a meeting of the British Astronomical Association in 1926, prompting considerable debate. The rarefied character of Saturn's envelope was indicated by an occultation in which the starlight was seen to progressively fade for a considerable distance 'within' the planetary limb. In 1926, W.W. Coblentz, C.O. Lampland and D.H. Menzel undertook radiometry at the Lowell Observatory employing a vacuum thermocouple to directly measure temperatures, and found that the 'visible' surface of Saturn was -150°C, some 15 degrees cooler than Jupiter. Although diehards put forward ad hoc models to explain why only the outermost layers were cold, and the interior was 'heated vapours', this in reality marked the end of the 'failed' star model of the giant planets.

The mystery of the redward absorption bands remained until 1931, when Rupert Wildt analysed Slipher's spectra. From the fact that the near-infrared absorption was stronger than that in the visible range, he inferred that the longer-wavelength absorption was induced by transitions in the vibrational states of molecular ammonia and methane. This was confirmed in the laboratory in 1933 by Theodore Dunham at the Mount Wilson Observatory. With this insight, and the recent measurements of the temperatures of the reflecting surfaces of Jupiter and Saturn, it was realised that Saturn's chilly outer envelope should show relatively less molecular ammonia and more methane, because the ammonia should have frozen out to form clouds of icy crystals. In 1929 B.F. Lyot had published polarisation measurements indicating the presence of clouds. It was the over-concentrated methane in the upper atmosphere that was producing the strong absorption features.

Saturn's banded structure is subdued in comparison to Jupiter's. In fact, Saturn at its most 'active' bears a striking resemblance to Jupiter at its most quiescent. The banded structure is also variable. At the turn of the century, E.E. Barnard observed a dark 'polar cap' and no less than five dark belts, four of which were in the northern hemisphere (this asymmetry was due to the ring system masking his view of the southern hemisphere). Several years later, when the rings were edge-on, he saw only two dark belts on each side of the equator. R.W. Wood of Johns Hopkins University noticed that in pictures taken in 1915 by the 60-inch reflector of the Mount Wilson Observatory the 'polar cap' and dark belts were conspicuous in violet light but virtually indistinguishable in infrared light - and limb darkening was also less pronounced. Evidently, the two emulsions were showing features at different depths in the planet's atmosphere.

Asaph Hall used the US Naval Observatory's 26-inch Alvan Clark refractor to observe Saturn intensively from 1875 to 1889. After finding the disk to exhibit little variability, on 7 December 1876, while checking Iapetus, he was surprised to find ''a white spot on the ball of the planet''. He was able to track the spot's motion around the equatorial zone until 2 January 1877, at which time it disappeared. A number of early observers had reported seeing small spots, but this was the first time that such a prominent spot had been seen. Having tracked it for 60 revolutions, Hall calculated a rotational period for the spot of 10 hours 14 minutes 24 seconds, which was in remarkable agreement with William Herschel's estimate of 10 hours 16 minutes, determined on the basis of irregular patterns in the latitudinal bands.

In 1891 A.S. Williams in England using a 6.5-inch reflector tracked another spot, and computed a similar period. In fact, over the next several years Williams tracked a number of small bright and dark spots in the equatorial zone and noted a continuously declining rotational period which he interpreted as evidence of long-term variability in the winds which were sweeping along the discrete features. Specifically, Williams concluded that ''the great equatorial atmospheric current of Saturn was blowing 66 miles an hour more quickly in 1894 than it was in 1891''. However, E.E. Barnard had often observed Saturn during this period using the 36-

A photograph of Saturn taken by the Mount Wilson Observatory's 60-inch reflector showing Cassini's Division separating the 'A' ring from the 'B' ring, the fact that the 'B' ring is the brightest part of the system, the 'C' ring silhouetted against Saturn's disk, and the shadow cast by the planet on the far side of the ring.

A photograph of Saturn taken by the Mount Wilson Observatory's 60-inch reflector showing Cassini's Division separating the 'A' ring from the 'B' ring, the fact that the 'B' ring is the brightest part of the system, the 'C' ring silhouetted against Saturn's disk, and the shadow cast by the planet on the far side of the ring.

inch Lick refractor, and had recorded none of these spots. On the morning of 16 June 1903, when Barnard aimed the 40-inch refractor of Yerkes Observatory at Saturn he was pleasantly surprised to find a remarkably bright spot at 36 degrees north latitude. He had been sceptical of reports of both white and dark spots by earlier astronomers with much smaller telescopes. Unfortunately, cloud prevented further observations until 24 June, but after following the spot he derived a period of 10 hours 39 minutes 21 seconds - somewhat longer than the period measured by Hall for his equatorial spot. Meanwhile, W.F. Denning, using his 10-inch refractor in Bristol, had independently discovered another white spot on 9 July 1903 with a period of 10 hours 37 minutes 56 seconds. This spot had also been seen by Catalan astronomer Jose Comas-Sola through his 6-inch refractor, who derived a period of 10 hours 38 minutes 24 seconds. T.E.R. Phillips in England saw a bright spot at latitude 36 degrees south with a similar period in 1910, and in America G.W. Hill derived a period for Saturn's equator of 10 hours 14 minutes 24 seconds. The variance in the derived rotational periods implied that, like Jupiter, Saturn's atmosphere rotates differentially, with the rate being fastest at the equator. In 1937, when the rings were edge-on and the axis was perpendicular to the line of sight, J.H. Moore at Lick used the Doppler effect and directly measured the rotational period as 10 hours 2 minutes - a figure that was significantly shorter than any determined by tracking spots in the equatorial zone. The period increased continuously with latitude, being approximately half an hour longer at 36 degrees, and almost an hour longer at 57 degrees.

On 3 August 1933, while observing Saturn through his 6-inch Cooke refractor in London, Will T. Hay, a comedian/actor with an enthusiasm for astronomy, noted a white spot in the equatorial zone. Its existence was promptly confirmed by W.H. Steavenson. It circled the planet in 10 hours 13 minutes. Over the next few weeks it became progressively more stretched out in longitude, the leading section became

A 'white' spot was discovered near Saturn's bright equatorial belt by W.T. Hay in 1933, who recorded its appearance in this drawing.

diffuse and the tail darkened. By mid-September it had merged into the equatorial zone. W.H. Wright believed it to be ''a mass of matter thrown up from an eruption below the visible surface, encountering a current travelling with greater speed than the erupted matter, which was carried forward by the current while still being fed from the following end''.

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