The changing colour of Saturn.

Cassini is go but Saturn has the blues. The launch of the Cassini mission to Saturn at 04.43 EDT on the morning of October 15 1997 was 'picture-perfect'.Both Hazel McGee and Nick James enjoyed the awesome spectacle of night turned to day as the Titan IV booster thundered into the Florida sky. As our Journal Editor and Papers Secretary trained their cameras on the floodlit launch site at the Cape, 800 miles to the north the Section Director trained a 350mm Cassegrain reflector on Saturn itself. The sleepy town of Coshocton nestles on the junction of the Muskingum and Tuscarawas rivers as they meander through the wooded valleys of Coshocton County in the state of Ohio. This is the heartland of the 'Ohio Amish' community and their black Amish buggies are a familiar site as they scuttle along the back roads and narrow trails that stitch together the neatly ordered patchwork of farms and fields. Coshocton is home to Thomas A. Dobbins, amateur astronomer, telescope maker, author and my host for a fortnight last October. We were privileged to be joined for part of this time by fellow enthusiasts Jeff Beish, Tom Cave, Charles Genovese, Diane Lucas, Alex Panzer and Donald Parker. 

Of 15 nights available for observing, 9 were clear, the seeing being good on most occasions and perfect on one particular night. Tom's telescopes are a 250mm Newtonian and a 350mm Cassegrain of his own construction, both of which are optimised for high resolution lunar and planetary visual work and video imaging. The main objects for study during my stay were Jupiter and Saturn. Unlike Jupiter, Saturn as seen through the telescope is not noted for showing much in the way of striking colour. The prerequisites for colour work are an adequate aperture telescope free from false colour with clean optics and a transparent sky. Even then, observers often fail to agree on the exact nature of the muted shades reported in Saturn's belts and zones. Yet colour was very apparent to Tom Dobbins and I on October 14 15. The brightest region on Saturn that night was the Equatorial Zone to the north of the narrow Equatorial Band, the brilliance of the zone enhanced by the passage of an elongated diffuse white area, the following end of which was reckoned to be on the CM at 04h 45m UT (system 1 = 215 deg.). This was our sole indication of any feature resembling a white spot and practically the entire planet was given scrutiny for such over the two weeks in question. Further south, the South Equatorial Belt appeared brown in colour, the South Temperate Zone cream with a tint of yellow. Contrasting with this, the hemisphere visible north of the rings had a marked blue cast. This hemispheric assymetry was noted by all the aforementioned observers and thought to be genuine, and not an artifact of atmospheric dispersion. 

Following his return to Florida, Don Parker secured CCD observations which confirmed the visual impression from Coshocton. Our most revealing view of Saturn was enjoyed on the night of October 20-21 when the ringed-planet was followed for 3 hours through both telescopes, the seeing being ranked at I on Antoniadi's scale for much of this time. On going to the telescope, Tom Cave remarked that during the course of his career as an observer (which spans six decades) he had never experienced better seeing and had enjoyed conditions of equal quality only on a number of nights that could be counted on the fingers of one hand. Due to mirror-currents occasionally bothering the larger Cassegrain (which have since been cured by the installation of electric fans) the best image was found to be that provided by the 250mm Newtonian at a power of X 552. What was seen that memorable night was eloquently summarised by Tom Dobbins as follows: 'Ring C exibited a steel gray colour and took on a remarkable granular appearance reminiscent of the texture of course sandpaper. The limb of the globe was distinctly visible through ring C , while in the ansae the space between the inner edge of the ring and the globe was sharply defined, with its outer edge separated from the inner edge of ring B by a division fully one-third as wide as Cassini's division. This was unmistakably a true gap rather than an intensity mininmum or a spurious effect of contrast. The inner third of ring B had a dusky tan or beige colour and gave the impression of containing exceedingly fine 'phonograph grooves', the only part of the entire ring system to have this appearance. A further abrupt graduation in brightness appeared at approximately two-thirds the distance from its inner edge to Cassini's division. No hint of radial spokes could be made out. Ring A exhibited a bright inner region adjacent to Cassini's division and Encke's division appeared as a broad but sharply bordered dusky region, while Keeler's gap appeared as a fine hairline located approximately three-quarters of the distance from the inner to the outer edge of ring A. The portion of ring A exterior to Keeler's gap was the darkest portion of the entire ring system except for the dark swath of Encke's division and ring C. The colour of ring A was reminiscent of the bluing on a rifle barrel and its outer edge was sharply defined, giving no indication of the presence of a faint exterior ring. I devoted my attention primarily to inspecting the rings rather than the globe but a few remarks concerning the latter are in order. In the exquisitely stable air, both the belts and zones of the planet took on a curious flocculent or minutely curdled aspect. The northern hemisphere of the planet had a striking bluish cast, contrasting with the warm hues of the southern hemiphere. The South Equatorial Belt had darkened edges (though I can notÛdescribe it as truly double) and a narrow but continuous equatorial Band was readily visible, paralleled by a broader diffuse streak to its south. The dusky South Polar Region had an azure tint. 

The atmosphere of Saturn seems to have been relatively quiet through the 1997-98 apparition with few reports concerning spot activity communicated to the Section. David Weldrake observed from the site of the Castle Eden Walkway Observatory near Stockton-on-Tees on the night of August 9-10 with his 300mm Newtonian and found a 'small localised brightening' in the EZ(S); it crossed the CM at 23h 50m UT (system I = 129 deg). Dr Richard McKim did much to uphold the reputation of the Association at a special Mars conference held in Tucson, Arizona last October, but still found time to observe Saturn from an altitude of 7000 feet on Mount Lemmon using a 250mm Newtonian belonging to Daniel Joyce. A small white spot was seen interupting the EB, in transit at 07h 30m UT (system 1 = 41 deg) and was confirmed by DanielTroiani and Don Parker who were also present. In his notes, Richard described the northern hemisphere as being 'grey or bluish-grey' and in ring A 'I caught an inner division and the other observers, an outer one. In reality I think we saw the shaded part in the centre of the ring which edges Keeler's gap and Encke's division'. As part of their three part paper 'Saturn 1943-1981:A Visual Photometric Study', Richard McKim and Keith Blaxall conducted a painstaking reduction of many thousands of visual intensity and colour estimates gathered over a 38 year period by the Saturn Section. In the part dealing with colour changes, the authors wrote 'The question of seasonal colour changes on Saturn was investigated with striking results...We concentrated upon times when both hemispheres of the planet were visible together. Rather than detailing the colours of specific features the general tone of one hemisphere was assessed by colour references to the equatorial belt, temperate and tropical zones of each hemisphere'. Apparitions from 1947 to 1981 were tabulated in such a way that respective north and south hemispheres were assigned a colour or tone, where a 'warm' tone was taken to mean a yellow-grey, yellow, yellow-brown etc. and a 'cold' tone meant a pure grey, a grey-green or grey-blue colour. 

They then went on to say 'Clearly a hemisphere that has been eclipsed by the rings for a number of years is found to have a different colour to that which has been exposed to solar radiation for some time. On exposure to the Sun, a hemisphere attains a warmer tone. After eclipse by the rings the colour is much colder...Furthermore, the data suggest that a hemisphere begins to lose its warm tone as it turns away from the Sun.' Thus from 1947 to 1950, a period which saw 'late summmer' in Saturn's southern hemisphere and 'late winter' in the northern, the respective hemispheres were assigned a warm and cold 'tone' respectively. The Sun passed south to north through the ring plane on September 21 1950 marking the start of 'spring' in the northern hemisphere and 'autumn' in the southern. Results were less than perfect for the southern hemisphere from 1954 to 1965 but the northern hemisphere was assigned a 'warm' tone from 1953 to 1966, the seasons for the latter being 'spring and summer'. The Sun passed north to south through the ring-plane on June 15 1966 marking the start of 'autumn' for the northern hemisphere and 'spring' for the southern. The Consequently a 'warm' tone was assigned the southern hemisphere from 1966 to 1981, the last year for which observations were analysed. The solstice occured on May 21 1973, 'winter' commencing in the northern hemisphere and 'summer' in the southern. Meanwhile a 'cold' tone was assigned the northern hemisphere from 1968 to 1980 with the exception of 1971-75 when the northern hemisphere was hidden from view. The findings of McKim and Blaxall may be compared with ontemporary apparitions of Saturn. Dr William Sheehan was able to observe the planet with the 1060mm Cassegrain at the Pic du Midi and the 1020mm refractor at Yerkes, in 1992 and 1993 respectively, noting a pronounced blue colour to that part of the southern hemisphere recently exposed to the Sun, ie. visible to the south of the closing ring. Sheehan drew attention to the blue southern hemisphere and the conclusions drawn by McKim and Blaxall concerning periodic colour change on Saturn. 

We are now in a position to further confirm their findings and state that only two years after the passage of the Sun north to south through the ring-plane and hence the beginning of 'autumn' in the northern hemisphere, this hemisphere appearsÛto display a blue tint. There seems little doubt that the mechanism for colour change is a seasonal one, depending on the strength of solar radiation falling upon the upper atmosphere of Saturn. As advanced by Andrew Ingersoll, the blue-green colour of a gas-giant may be explained by the greater depth of clouds rich in sulphur and ammonia in the atmosphere of a colder planet such as Uranus. Consequently sunlight reflected from the cloud-deck must pass through a deeper photochemical haze which if depleted of ammonia and rich in methane would efficiently scatter blue wavelengths but absorb those towards the red end of the spectrum. In the case of Saturn, as the solar flux decreases in line with lessening solar altitude, the depletion of ammonia would be enhanced as the latter freezes out to leave a methane-rich haze. Prior to 1995, the previous north to south passage of the Sun through the ring-plane took place in 1966. Two years later, the northern hemisphere of the planet was assigned a 'cold' tone and now we see a similar change of 'tone' or 'colour' at the apparition of 1997, one Saturnian 'year' later. Looking back to the Section report for 1968 we read that several observers reported fine structure in the rings, more so than in the apparitions prior to 1966 when Saturn was further south in the sky and conditions were less favourable to produce good seeing. 

David L. Graham

Section Director

Rings within rings.

Arthur Francis O'Donel Alexander directed the Saturn Section from 1946 to 1951, achieving much to get the Section back on its feet following the hiatus caused by the Second World War but is probably best known for being the author of 'The Planet Saturn: A History of Observation, Theory and Discovery'. First published in 1962, this classic volume remains the definitive guide to the history of Saturn as seen through the telescope. Alexander explored in depth the question of 'subdivisions' in the classical rings of Saturn glimpsed by various observers in the 19th.century. Captain Henry Kater was perhaps the first to do so when in the December of 1825 he reported no less than three subdivisions in the 'outer ring' of Saturn which we now call ring A, with a reflector of only 6-inch aperture by Watson. About this period the southern face of the rings were near their maximum presentation towards the Earth and Saturn would have been very well placed for observers in England. However it should be noted that Kater also observed Saturn with a 6-inch reflector by Dolland and saw nothing unusual in both the outer and inner ring (ring B). 

Observing Saturn from Berlin on 28 May 1837, Johann Franz Encke obtained a micrometer measure of a division within ring A, placing it one-third of the distance from the inner to the outer edge of the ring, the northern face of the rings then being tilted towards Earth. With the steady improvement of the telescope it seemingly became the fashion to report subdivisions in both ring A and B, and as explained by Alexander, these sightings may in part have been inspired by 'Laplace's theory that the stability of the ring system required it to be subdivided into a large nember of narrow rings'. Contemporary theory concerning the nature of Saturn's ring system is largely based upon the Voyager flybys of 1980 and 1981, but it seems more pertinent to compare the Coshocton observations with the best telescopic summary of the classical ring system, that by Professor Audouin Dollfus. Dollfus based his conclusions on observations spanning the years 1953 to 1982, largely with the instruments at the Pic-du-Midi. Dealing first with ring A, a division is located approximately one-fifth of the distance from the outer to the inner edge, which was always visible when seeing was good. Moving inward from the latter, there are three faint 'ripples' before reaching Cassini's division. Ring B can be divided into four zones with a ripple separating the first two, with a somewhat more prominant ripple or 'minimum' separating the second from the third. The fourth or final section of ring B contains two minima, following which a division separates ring B from ring C. Comparing impressions of the ring structure as depicted by Tom Dobbins and the Director on October 21, the subjective nature of visual observing is readily apparent and with both observers having to struggle to grasp detail on the threshold of resolution, some difference of interpretation is inevitable. Both agree on a division in ring A, located one-quarter of the distance from the outer to the inner edge, the darkest section of the ring being the central region and the lightest, that part immediately adjacent Cassini's division. Graham placed an intensity minimum two-thirds from the inner to the outer edge of the ring. Both observers could follow Cassini's division where the rings crossed the globe, this being possible due to the shadow of the rings on the globe being 'behind' Cassini's division at this part of the apparition. 

The accordance continues with ring B, the ring being divisible into three sections, that bordering Cassini's division brightest and that adjacent ring C darkest where Dobbins had the impression of exceedingly fine 'phonograph grooves'. Graham placed two minima within the ring, the more prominant being positioned on the border of the first and second sections inwards from Cassini's division and can also be seen on the drawing by Dobbins. A particular surprise was the division separating ring B from C, which was plainly evident on both ansa. The drawings by Dobbins and Graham may be compared with that made August 17 1997 at 0h 50m UT by David Gray from Spennymoor, County Durham, using a 415mm Dall-Kirkham Cassegrain X 348 in conjunction with an apodising screen. Note the similarity in the depiction of both globe and ring detail but with the addition of dark spokes on ring B at the following ansa. The observational history of Saturn is littered with reports of fine structure within the ring system. The researcher is often left with the task of sorting out those reports which can be given credence from those which hint at a fervent imagination. As we have seen, Kater reported no less than three minor 'divisions' in ring A, the broadest central in the ring flanked by a lesser division either side, one of which was close to the outer edge of the ring. Discovery of this division is often attributed to James Keeler who saw it well with the Lick 36-inch refractor on Mount Hamilton, California in the winter of 1888 but there is ample evidence to suggest it had been seen previously. 

William Rutter Dawes stated that a 'very narrow and short line was discernible on the outer ring near its extremities' on November 25 1850 which he found 'obvious' four nights later. This was the eventful period in which Dawes independently discovered the crepe ring and the division is shown on his drawing of November 29 1850. As recently reminded by Dr William Sheehan and Stephen O'Meara, Harvard College Observatory's Phillip Sidney Coolidge glimpsed the outer division in ring A in 1854, his depiction of the outer ring being essentially the same as that by Kater, 29 years earlier. Not all observers reported subdivisions within the ring; observing from Malta with a newly commisioned 24-inch reflector in the autumn of 1852, William Lassell reported no division in ring A but 'an evident shade in the middle of its breadth and occupying about one-third of it'. In contrast, Mentore Maggini observing from Arcetri, Italy on October 25 1913, sighted not a darker shading central in ring A but a bright region of lighter intensity than the remainder of the ring, confirming a report by Fournier four years earlier. Harvard astronomer Charles Tuttle saw fine detail in ring B on October 20 1851, comprising many fine subdivisions within the ring. W.R.Dawes described ring B as containing four narrow concentric bands, each of which was slightly darker than the adjacent one, an appearance not unfamiliar to students of modern spacecraft imagery. To Dawes we can also attribute an early sighting of yet another division or boundary separating ring B from the newly discovered crepe ring in the winter of 1851, though Lassell who collaborated closely with Dawes was unable to detect it later that year despite access to a larger aperture telescope. 

Observers continued to dispute the existence of the division, with among others, Percival Lowell finding in its favour. Although the endeavours of most observers were devoted to other tasks, Saturn was not totally neglected during the Second World War, with Walter Haas directing a small bandÛof enthusiasts in the United States, finding two subdivisions in ring B and a gap between the latter and ring C. Meanwhile from France, Bernard Lyot made the most of the superb seeing to be had from the Pic du Midi, the south face of the rings being favourably tilted towards the Earth during this period. According to Lyot, ring A contained a narrow dark line near the outer extremity of the ring and a broad shaded area which occupied the middle and inner region of the ring, within which three 'minima' could be discerned. Ring B was split in two by a 'division' with two more adjacent the inner edge of the ring. A further division, about half the width of Cassini's division was seen to separate ring B from ring C. By the mid 20th century, the reality of subdivisions had its doubters, one sceptic being the pioneer of planetary science, Dr Gerard Kuiper, the opinion of whom was held in great respect. Kuiper examined Saturn on a 'nearly perfect night' in 1954 with no lesser telescope than the 200-inch Hale on Palomar Mountain after which he concluded that Cassini's division was the only true division, other 'divisions' being 'minor intensity ripples', notably one in ring A and three in ring B, there being no gap between ring B and C. By now, the use of the term 'ripple' or 'minima' was used in preference to 'division' or 'gap'. To quote Alexander writing from the viewpoint of the early 1960's: 'From all the observations of ring subdivisions cited...it seems evident that no true gaps in fixed positions are to be found in the ring system other than Cassini's division. The subdivisions seem more like surface ripples which come and go, not always in the same places. One near the middle of ring A usually called Encke's division though by no means always visible, seems to have been more often observed and more nearly constant in location than any of the others'. Pioneer and Voyager imagery have verified much of the structure reported in the rings by the classical observers, and of course revealed much more than could be seen from Earth. The gap close to the outer edge of ring A is real enough but as if to fuel the former controversy, the IAU official nomenclature committee bestowed the title 'Encke's division' upon it. Having continued the work commenced by Lyot, Audouin Dollfus found the visibility of what he referred to as 'gaps, concentric bright or dark sub-rings' was partly dependent on the phase angle of Saturn. Phase angle is defined as the angle measured at the centre of an illuminated body between the light source and observer, in this instance the illuminated body being Saturn. 

At or near opposition, when the phase angle is practically zero, ring B experiences a surge in luminance which reduces the contrast of delicate detail within the ring. At a phase angle of less than 0.5 degrees, the two minima on the innermost part of the ring merge and are not so dark as at a larger phase angle. The three minima central in ring A are well marked at phase angles greater than 2 degrees but appear merged at a phase angle smaller than 0.3 degrees. On October 21 1997, the night of excellent seeing enjoyed at Coshocton, the phase angle was 1.3 degrees. Saturn was sketched on five nights previous to this at which time a darker strip was placed central in ring A and only one prominant minima was seen in ring B. The division near the extremity of ring A, the second minima in ring B and the gap separating ring B from the crepe ring were seen only in the excellent seeing prevailing on October 21. Spoke-like features in ring B were discovered by Voyager and have been attributed to microscopic particles elevated above the ring by an electromagnetic phenomenon. They were 10 to 15% darker than the ring against which they were superimposed and had a lifetime of up to three hours or more. Observers have reported seeing spokes on ring B post Voyager, an example being an observation by by Sheehan and O'Meara from the Pic du Midi on August 2 1992 using powers of 800 and 1200 on the 1060mm reflector. Both observers agreed that the spokes were particularly striking on the following ansa and O'Meara also depicted them on ring C. Pre-Voyager reports of spokes are hard to come by but perhaps a series of observations by Etienne Trouvelot from 1873 to 1876 should be considered. Trouvelot had access to the 15-inch refractor at Harvard and the 26-inch refractor of the US Naval Observatory at Washington, in addition to his own 6-inch instrument. He reckoned to have seen both ring A and B display a 'mottled or cloudy appearance on the ansae' which may also have had a bearing on the form of the shadow of the globe on the rings. Trouvelot also saw what we would infer to be spoke-like markings on ring A adjacent Cassini's division. Observing from Juvisey on April 18 1896, Eugene Antoniadi recorded ring A fragmented by 'white spots separated by dusky intervals' following which C.Roberts saw the outer edge of Cassini's division as 'curiously indefinate and indented' on May 8. 

It should be noted that others observing near this time could see nothing out of the ordinary. To paraphrase Alexander, 'there has been much contradiction among the various observations of ring markings, and sceptics might be inclined to dismiss them all as due to optical and atmospheric causes, differences between depth of shading in adjoining areas, or even the bias of preconceived ideas: the difficulty is to decide which of these temporary appearances are truly objective and which are not'. One night last October the current writer was given an inkling as to the complex nature of the rings of Saturn. The Pioneer and Voyager flybys provided a unique snapshot of the ring system with a level of resolution impossible from Earth. With Cassini enroute to its destiny with Saturn early in the next millenium, one can but wonder which current questions will be answered and what new questions will be asked?

David L. Graham

Section Director