Fine structure

The mechanical stability of the ring system had been studied mathematically by P.S. Laplace, who had proposed that it is a multiplicity of extremely thin ringlets. During a lengthy study of the system under a range of illuminations, William Herschel found little supporting evidence. In fact, his only suspicion of fine structure was reported five years before Laplace advanced his theory: on four nights in 1780, when the rings were wide open and he could trace them all the way around he had noted in his log that he had perceived a division on the inner ('B') ring.

However, L.A. Quetelet, director of the Brussels Observatory, reported seeing a very fine line divide the 'A' ring when using a 10-inch refractor in Paris in December 1823. In England, Henry Kater, the Vice President of the Royal Society, had a 6.25-inch reflector of excellent quality, and on 17 December 1825 he ''fancied that I saw the outer ring separated by numerous dark divisions, extremely close, one stronger than the rest dividing the ring about equally'', from which he concluded that the 'A' ring ''consists of several rings''. The 'B' ring showed ''no such appearance''. Kater suspected the presence of such lines again in January 1826, but they were not in evidence the following month, and so he concluded that they were "not permanent" - a view that was reinforced when the rings were lacking in detail on an exceptionally clear night in 1828. John Herschel and F.G.W. Struve had independently attempted to verify Kater's sightings in 1826, using larger telescopes, but to no avail.

On 29 May 1838 Francesco de Vico of the Collegio Romano noted the presence of three dark lines, one in the middle of the 'A' ring and the others on the 'B' ring. On 7 September 1843 W.R. Dawes and William Lassell, utilising a 9-inch reflector at Lassell's observatory in Liverpool with very good seeing, saw a fine line dividing the 'A' ring. James Challis, director of the Cambridge Observatory, reported lines on the 'A' ring in 1842 and 1845 using the excellent 11.5-inch Northumberland refractor. On 21 November 1850 Lassell, now using his 24-inch reflector, noted a line near the outer edge of the 'A' ring, which Dawes, using his 6.5-inch refractor, independently saw two nights later, and again several times during the next week. Meanwhile, G.P. Bond at the Harvard College Observatory noted a line near the inner edge of the 'B' ring on 10 October, 11 November and 7 January 1851. Intriguingly, Bond did not see the marking on the 'A' ring reported by Lassell and Dawes, and they did not see his line on the 'B' ring. On 20 October 1851, in excellent seeing, C.W. Tuttle at Harvard noted the 'B' ring to be ''minutely subdivided into a great number of narrow rings'', starting close to its inner edge and extending over about one-third of its width, adding that ''the rings and the spaces between them were of equal breadth''. A week later, on 26 October 1851, Dawes saw the 'B' ring ''arranged in a series of [four] narrow concentric bands, each of which was somewhat darker than the next exterior one''. Warren De la Rue, who had an exceptional 13-inch reflector, observed the rings very clearly in 1856 and saw considerable structure, including in the transition from the 'B' to 'C' rings.

Rings 1856
In 1856 Warren de la Rue drew Saturn's rings with several fine subdivisions. Many of his contemporaries regarded such detail as illusory, however. Notice also his depiction of banding on the planet.

Reflecting upon the occasional fine structure in the rings, and the often irregular line of the planet's shadow on the rings, G.P. Bond suggested that the ring material might be a fluid. His colleague Benjamin Peirce agreed, and proposed that they were streams of a fluid whose density was somewhat greater than that of water - that is, exceeding 1.0 g/cm3.

Although Roche's conclusion applied to a body having the same bulk density as its primary, it was evident that companions with greater density could penetrate the critical radius until the gravitational tides overcame their structural integrity. It was therefore not straightforward to decide whether the rings were material that was prevented from coalescing to form a satellite when Saturn condensed out of the solar nebula, or were the debris of a moon that was recently disrupted after straying too close to the planet. The alternatives had a significant consequence: if the rings were a satellite that never formed, they would be composed of 'pristine' relics of the solar nebula, but if they were the debris of a shattered satellite the material would have been thermally 'processed' to some degree.

In 1855 J.C. Maxwell proved mathematically that the rings could be neither fluid nor gaseous bodies, because their rotation would stimulate the formation of waves, which would be disruptive. He also ruled out Laplace's proposal of a multiplicity of exceedingly fine ringlets. In 1857 Maxwell concluded that they could only comprise a myriad of small fragments, each of which was orbiting the planet in accordance with Kepler's law that the square of the orbital period is proportional to the cube of the mean distance from the centre of rotation. Therefore, the material at the inner edge of the system travels more rapidly than that farther out. In fact, the possibility that the rings were composed of small bodies had been suggested by J.J. Cassini in 1715

A drawing by E.L. Trouvelot of the Harvard College Observatory in 1872, at a time when the inner 'C' ring was so translucent that it was only partially visible crossing the planet's disk. Notice also the irregular profile of the planet's shadow line on the rings.

and, with considerable conviction, by Thomas Wright in England in 1750, but neither had been able to argue the case mathematically. Nevertheless, the idea had gained sufficient support for Mr Filda to reflect in 1776: ''It is thought that the ring around Saturn consists of a great number of Saturnian satellites in close proximity.'' Maxwell proposed that the impermanent structure in the rings corresponded to 'waves of disturbance' traversing the disk as the gravitating particles influenced one another, and were therefore similar to surface ripples. In the long term, Cassini's Division, the only 'real' gap in the rings, might be impermanent. The irregular and changing line of the planet's shadow on the rings was interpreted as evidence that some particles were in slightly inclined orbits. The 'C' ring must be composed of the same material as the main rings, but be more sparsely populated so that sunlight is reflected less efficiently and the material appears darker. Nevertheless, the particle density was sufficient to cast a hazy shadow on the planet. Indeed, even when the 'C' ring had been too faint to be seen, its shadow had been seen crossing the globe and misinterpreted as a dark equatorial belt. In line with O.W. Struve's hypothesis, it was presumed that the 'C' ring was material in the process of spiralling down towards the planet, gradually eroding the ring system. In 1855 Cambridge University had offered its Adams Prize to the first person to resolve the nature of the rings, and it was promptly issued to Maxwell for his penetrating analysis.

Daniel Kirkwood in America conducted a mathematical study of the asteroid belt in 1866, by which time almost 100 objects had been catalogued. He discovered that there were statistically significant 'zones of avoidance' with mean distances from the Sun matching orbits whose periods were in resonance with Jupiter's. Evidently, the planet's immense gravitational field was perturbing the asteroids in their orbits. In 1867 he turned his attention to Saturn's rings and concluded that Cassini's Division is a 'zone of avoidance' due to perturbations by the moons beyond. He showed that material at this radius would have an orbital period one-half of Mimas's period, one-third of Enceladus's period, one-fourth of Tethys's period and one-sixth of Dione's period. The resonance with Mimas alone would be sufficient to 'sweep' this gap clean. In 1871 Kirkwood found that the orbital period of Encke's Division was three-fifths that of Mimas. Evidently, the simpler the fraction, the stronger the perturbation; thus while Cassini's Division is broad and stable, Encke's is a temporary thinning of the material. Kirkwood noted that material orbiting at the transition between the 'B' and 'C' rings would have a period one-third of that of Mimas. The ephemeral nature of the fine ring structure made it possible to reconcile the view of Bond and Lassell that the 'C' ring was a continuation of the 'B' ring, with occasional subsequent reports of a narrow gap. In January 1888, a few months before Alvan Clark's 36-inch refractor was commissioned at the Lick Observatory in California, J.E. Keeler, visiting, sneaked a preview to test the optics, and was delighted to find a narrow division near the outer edge of the 'A' ring. Although referred to in America as 'Keeler's Gap', this actually marked the return of Encke's Division, which had been absent for some time.

The asteroidal 'zones of avoidance' analogy reinforced Maxwell's assertion that Saturn's rings are composed of a myriad of particles. In 1888 Hugo von Seeliger in Munich made photometric measurements to test whether the rings were composed of

On 7 January 1888 J.E. Keeler tested the Lick Observatory's 36-inch refractor on Saturn, and resolved a very thin division near the outer edge of the 'A' ring.

discrete particles. He concluded that only such a configuration would display the observed surface brightness in varying angles of solar illumination. He ventured that the 'particles' ranged up to a metre or so across. The albedo and phase variations of the reflected sunlight indicated that they do not have smooth surfaces. In fact, measurements of ring brightness with phase angle implied that the material in the primary rings - the 'dense' part of the structure - occupies just 6 per cent of the available volume. In 1933 E. Schonberg in Breslau undertook a similar analysis, and concluded that while there may be a significant number of large objects, the mean size must be only a few microns. In the 1870s, H.C. Vogel in Germany had noted that the spectrum of the ring system was very different to that of the planet, being a simple reflection of the solar spectrum, which indicated that the rings are free of vapours. The high albedo implied that the material is water ice. It was therefore concluded that the rings comprise a few large snowballs orbiting in a multitude of tiny ice crystals.

Albert Marth, who studied under F.W. Bessel and assisted William Lassell when he took his 48-inch reflector to Malta in the 1860s, was subsequently appointed as director of the Markree Castle Observatory in Ireland, where he routinely published the ephemerides of satellites to assist other observers on a worldwide basis. Iapetus is the only large satellite able to pass through the shadow of the ring system, because its orbit is both remote and inclined. At Lick Observatory, E.E. Barnard decided to observe this rare occurrence in order to profile the density of the material across the rings. Unfortunately, Saturn was inconveniently positioned for the 36-inch refractor, so he had to employ a 12-inch. The sequence would start shortly before sunrise on 2 November 1889. As Saturn rose, he saw Iapetus emerge from the planet's shadow. Between the planet and the inner edge of the 'C' ring's shadow, its brightness was essentially constant, implying that this zone is more or less clear. It then faded progressively for 75 minutes as it penetrated the 'C' ring's shadow, but it was not entirely lost until it entered the shadow of the 'B' ring. The rising Sun terminated the observations. There was evidently no gap between the 'C' and 'B' rings at this time, and the ramp in the light curve was striking. Why there should be such a dramatic change in the density at that radius was not evident. Although it was frustrating that the dawn had interrupted the light curve, M.A. Ainslie, who had a 9-inch refractor at Blackheath, and J. Knight, who had a 5-inch refractor at Rye in Sussex, monitored a 7th magnitude star passing behind the 'A' ring on 9 February 1917, thereby providing a measure of the transparency across a chord of that ring. On 28 February 1919, W.F.A. Ellison employed the 10-inch Grubb refractor of the Armagh Observatory in Ireland to monitor Iapetus in eclipse, catching it after it had already cleared the 'B' ring and was illuminated by light through Cassini's Division. Shortly after noting its passage into the 'A' ring's shadow, Ellison was called away from the telescope for 50 minutes, but on his return he was able to monitor the remainder of the moon's passage through the shadow as it never completely disappeared, thereby confirming that the 'A' ring was translucent. William Reid in South Africa observed a star occulted by the 'B' ring on 14th March 1920, and found it to be much less transparent than the 'A' ring. Between them, therefore, these observers had provided a more or less complete profile of the system. The question was: Why were there such sharp transitions in density?

In 1895, in a paper entitled A spectroscopic proof of the meteoric construction of Saturn's rings, J.E. Keeler of the Allegheny Observatory, Pittsburg, reported measuring the radial velocities across the ring system employing the Doppler effect.2 A particle at the inner edge of the 'B' ring should travel at 21 kilometres per second in a Keplerian orbit with a 7.5-hour period. A particle at the outer edge of the 'A' ring should travel at 17 kilometres per second in a 13.75-hour orbit. Keeler's measured velocities were 20 and 16 kilometres per second. Slightly different, but supportive velocities were obtained a few months later by W.W. Campbell at Lick Observatory and by H.A. Deslandres at the Meudon Observatory in Paris. Differential rotation proved that the rings are composed of a myriad of discrete particles pursuing individual orbits around the planet.

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Progressive fading «hite _ in "C nng shadow

Slart to efrarge from Saturn's sha (taking about 10 minutes)

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On 2 November 1889, E.E. Barnard made the first observation of Iapetus passing through the shadow of the 'C' ring. Using Tethys and Enceladus as references, he composed this light curve of Iapetus's brightness.

Despite Maxwell's dismissal of fine structure as inconsequential 'density waves' crossing the surface of the ring system, some observers continued to record their appearance. Sidney Coolidge at the Harvard College Observatory noted four or five narrow lines on the 'B' ring on 20 March 1856, and Asaph Hall suspected the presence of lines on the 'B' ring while using the 26-inch Alvan Clark refractor of the US Naval Observatory in Washington on several occasions in 1875 and 1876. On 18 March 1884 Henri Perrotin and Norman Lockyer, jointly using the 15-inch refractor of the Nice Observatory in France, reported fine dark lines. Far more remarkable, however, was the report by E.M. Antoniadi, a Greek astronomer working as an assistant at Camille Flammarion's observatory at Juvisy in France, who, on 18 April 1896, not only saw a trio of widely spaced lines on the 'B' ring, but also half a dozen radial markings beyond Cassini's Division on the inner part of the 'A' ring. He promptly wrote to Leo Brenner and Philip Fauth urging that they look. On 26 April they were able to confirm the structure on the 'B' ring, but other than Encke's Division the 'A' ring was featureless. Although Antoniadi noted similar radial markings the following year, because Maxwell had proved that radial structure was impossible the reports were dismissed.

In the early-to-mid-twentieth century, T. Cragg, A.P. Lenham, O.C. Ranck, T.R. Cave and Leo Brenner, all of whom were skilled observers with excellent telescopes, reported seeing fine divisions. Percival Lowell, the founder of the observatory in Flagstaff in Arizona which bears his name, reported a number of fine 'divisions' in 1909, 1913 and 1914, and in 1915 he announced that for a time Encke's Division had 'doubled'. Cave saw a similar effect in 1954, but a few years later it had completely closed and was only an abrupt change in the brightness across the 'A' ring.

In 1943 B.F. Lyot recorded numerous ring divisions that he had observed using the 24-inch refractor at the Pic du Midi Observatory. Unfortunately, Lyot died soon afterwards, but his assistant, Audouin Dollfus, published the observations in

On 18 April 1896, in addition to three subdivisions in the 'B' ring, E.M. Antoniadi was astonished to see radial markings on the 'A' ring.
In 1915, when the ring system was as 'open' as it ever gets, Percival Lowell and his assistant E.C. Slipher recorded at least 10 fine divisions in the 'B' ring, which Lowell described as being ''conspicuously striped''. (Courtesy Lowell Observatory; Memoir No.2, 1915.)

L'Astronomie in 1953. In fact, Lyot saw so much fine structure during moments of exceptional seeing while using a very high magnification and, indeed, a micrometer to measure it, that a generation of less fortunate observers dismissed it as illusory.

Harold Jeffreys pointed out in 1947 that Roche's derivation of the radius within which a satellite could not exist had been based on the assumption that the satellite had the same density as its primary, which in the case of Saturn is abnormally low. Jeffreys extended the theory, and found that a solid object could exist well within 2.44 radii as long as it had sufficient structural integrity to resist the tidal stress. By way of a test, he calculated that if an icy body some 200 kilometres in diameter was disrupted after straying inwards to the mean radius of the ring system, it would have broken into a fairly small number of substantial pieces but would not have produced a myriad of tiny fragments, and he therefore concluded that the rings are not the debris of a satellite that strayed too close. Nevertheless, he concluded that the 0.31 g/ cm3 surface density of the ring system suggested that it consists of lumps of ice so, if it is material that was inhibited from forming a satellite, it is likely to be 'cometary' in composition. After a spectroscopic study, G.P. Kuiper of the University of Chicago reported that the particles were either water ice or frost-covered rock.

In 1954 Kuiper made the first serious investigation of Saturn using the recently commissioned 200-inch Hale reflector on Mount Palomar. Upon inspecting the ring system using a very high magnification on a night of ideal seeing, he concluded that Cassini's Division was the only gap; Encke's Division was an abrupt change in the intensity on the 'A' ring; and the 'B' and 'C' rings were juxtaposed. Although he saw hints of three features in the 'B' ring, there was no evidence of structure.

In the mid-1960s, R.M. Goldstein of NASA's Jet Propulsion Laboratory used the large spacecraft communications antenna at Goldstone in California as a radar to characterise the surfaces of the Moon, Mars and Venus. In 1972, he was astonished

In 1943 B.F. Lyot made this drawing of Saturn's ring system using the Pic du Midi Observatory's 24-inch refractor during 'perfect' seeing using a high magnification and a micrometer to measure the fine detail. Although his contemporaries dismissed such fine structure as illusory, the Voyager missions spectacularly proved him correct.

In 1943 B.F. Lyot made this drawing of Saturn's ring system using the Pic du Midi Observatory's 24-inch refractor during 'perfect' seeing using a high magnification and a micrometer to measure the fine detail. Although his contemporaries dismissed such fine structure as illusory, the Voyager missions spectacularly proved him correct.

to find that Saturn's rings reflected microwave energy with an efficiency of 60 per cent. Mars, in comparison, reflected 8 per cent, the Moon 5 per cent and Venus 1.5 per cent. He inferred from this that, in addition to ice, there must be a substantial number of rocks containing a high proportion of metals in the ring system, many of them several metres across and with irregular surfaces. The presence of such 'evolved' materials implied that the rings could not be 'pristine' material left over from the formation of the planet. In a polarisation study published in 1929, B.F. Lyot had noted that the light reflected from the 'B' ring was polarised in a similar manner to that from silicate, whereas the 'A' ring was something for which he had not been able to find a suitable match. The polarisation measurements by Audouin Dollfus at Pic du Midi in 1958 implied that the 'B' ring particles had a mean radius of a few centimetres, were significantly non-spherical and were aligned non-randomly. Bit by bit, the nature of the ring material was being revealed, but it was a tortuous process and in some cases the facts appeared to be contradictory.

By the 1960s it was evident that Saturn has an equatorial diameter of about 120,000 kilometres and an oblateness of 10 per cent; that the 'C' ring starts some 10,000 kilometres above the cloud tops and extends outwards for another 17,600 kilometres, where it blends into the dense 'B' ring, although they are sometimes separated by a gap; the 'B' ring extends outwards for 25,600 kilometres to Cassini's

Division, which is 5,000 kilometres wide; and the 'A' ring is 16,000 kilometres wide. Although the rings are rendered invisible to small telescopes for several weeks during a ring-plane crossings, a large telescope will lose them only for a day or so. At such times, clumps can often be seen in the distribution of material. Estimates of the thickness of the rings had been progressively reduced over the years, decreasing from hundreds of kilometres to a mere 10 kilometres.

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