Ordinary Meeting and Exhibition Meeting, 2004 June 26


Observing Saturn in the Age of the Cassini Mission

The title of the talk, Mr Graham began by explaining, referred to the spacecraft bearing the name Cassini, now returning superb images of Saturn as it neared the date of an orbital insertion manoeuvre. It was anticipated that over the forthcoming four-year mission, extensive observation of the planet and its moons would be undertaken, and some very fine images were sure to be seen in that time. Cassini, the probe, was of course named after the man, Giovanni Domenico Cassini (1625-1712), onetime head of the Paris Observatory, and whose pioneering observations of the planet were commemorated by the naming of the prominent 'Cassini Division' in the ring system. The subject of this latter individual would, however, be left for another day.

Visual observations by Dr William Sheehan from 2003 September, during the previous apparition, opened the talk. Dr Sheehan had been working at the 36" refractor at the Lick Observatory on Mount Hamilton, primarily to observe the perihelic opposition of Mars, but had also found time to make some superb sketches of Saturn. In the same month, high-resolution CCD images from Christophe Pellier had revealed a vague feature in the South Tropical Zone (STrZ) which appeared to be a faint white spot. This was one of several similar features which had been noted by various observers during the apparition.

In the speaker's experience, astronomy books were frequently found to say that white spots erupted in the North Equatorial Zone around every 30 years, lasting only for a few months. However, it would now seem that high-resolution images such as those by Pellier, Peach, and others, were revealing that similar, but much smaller, features frequently erupted in the South Equatorial Zone between the large events which had been noted previously. Both phenomena appeared to be season dependent, occurring only in the summer of the respective hemisphere. It was perhaps counterintuitive that the smaller eruptions should be found in the southern hemisphere, the summer of which lay closer to the perihelion of the planet, and thus where the solar flux was stronger. However, the speaker pointed out that whilst the solar flux was indeed stronger at perihelion, the rate of orbital rotation was slower at aphelion, and so northern summer was a little longer than southern summer. In the case of spot formation, this effect appeared to dominate the closer proximity of the planet to the Sun.

On 2003 November 15, Saturn had occulted mag 8.4 star SAO 78867. The C-ring ingress had been at around 4h30 UT, and ingress and egress with the ball of the planet at 6h10 and 8h30 respectively, the occultation finishing with final A-ring egress at 11h20. In a brief window of around 5 minutes at 5h00, the star had been predicted to appear through the Cassini division, and the speaker displayed a successful observation of this brief appearance by Alan Heath. The star had also been visible as it passed through the Crepe Ring, a near-invisible ring of material beside the B-ring, as well as making an appearance in the gap between the inner edge of the ring system and the ball of the planet. Mr Graham showed a series of superb CCD observations by Jim Phillips, and an animated sequence by Ed Grafton in Houston. Particularly notable among Grafton's observations, made at around midnight local time, was the translucent appearance of the Crepe Ring as the star passed behind it, and the speaker complemented him on the quality of his work.

Later in the same month, around November 30, several observers had noted an outbreak of white spots in the STrZ. This activity seemed to mirror what had been seen during the 2002 apparition. A type of observation that the speaker wished particularly to recommend was the construction of photometric profiles of the rings. Presently, Damian Peach was the only observer apparently making such measurements. Once measurements had been amassed over several apparitions, it would be interesting to monitor the time-variation of the ring-components. Showing Peach's results, the speaker pointed out the clear dip in the profile which was the Cassini division, and a smaller dip which was the Encke division. It was hoped that these observations might prove scientifically interesting in due course.

The speaker noted that on one occasion in the previous apparition, a dark spot had been detected in Hubble Space Telescope (HST) images from the International Outer Planets Watch, and confirmation had come ten days later from Damian Peach. It was amazing that amateur planetary imagers could now see features which were previously exclusive to the HST. It seemed that these dark spots were most prominent when viewed in red light, whilst bright spots were most conspicuous in blue light because they were known to exhibit methane emission.

In the second part of his talk, Mr Graham moved onto the progress of the Cassini Mission itself. He was pleased to see another probe observing the Saturn system again, as Mars seemed to have got more than its fair share of attention in recent years. The probe had already started to return superb images, although it was still heading through the solar system on its way to Saturn, and would not enter a bound gravitational orbit of it until an orbital insertion manoeuvre on July 1. All the images were available on the Cassini website, and the speaker recommended all to have a look through them. Over the next four years – the estimated lifetime of the mission – it seemed likely that a rich array of images of the planet and its moons would be on offer.

The speaker first showed one of Cassini's images from February 9, when it had still been many millions of miles distant from the planet, but which was one of the first to show it in an orientation distinctly different from the view seen from Earth. Whereas the Earth's relative proximity to the Sun meant that terrestrial observers saw no visible phase to Saturn's disk, and any shadow cast by it onto the ring system lay hidden behind it, Cassini's approach trajectory now gave it a superb view of the day/night terminator, and the shadow of the ball cast onto the rings.

Displaying an image from a few weeks later, the speaker remarked that more detail had now come into view as Cassini drew closer, though the orientation was similar. Taken in the different passband, closer to the methane absorption wavelength, cold high-altitude clouds were visible as dark markings. A small south polar cap (SPC) was also visible.

In a series of images at 700nm, taken over a four-week period starting in late February, a number of white spots were apparent in the STrZ. These were moving at different speeds, and appeared to merge into a single spot between images. Turning its camera to Saturn's moon Titan, hazy images began to reveal the first signs of features being resolved on the disk. Much better images would come during a close fly-by shortly after orbital insertion, and it was into the atmosphere of this moon that the Huygens probe would descend on 2005 January 14.

Recent amateur images by Richard McKim and Christophe Pellier had been suggestive of the presence of indentation-like features on the northern edge of the South Equatorial Belt. Close inspection of some of Cassini's first images appeared to hint at a similar conclusion. The speaker was confident that the features seen by Cassini would turn out to correspond to those which amateur observers had seen previously.

Zooming in on the shadow cast by the ball onto the rings revealed a view reminiscent of that seen during the Voyager I flyby, and which had not been seen since. However, the Cassini images had so far revealed no hint of the spoke-like features which had been apparent in the images returned by this earlier mission. It would be interesting to see whether they would make an appearance at some point during the four-year mission.

Perhaps the most striking image of all those returned thus far was from the probe's close encounter with Saturn's distant moon Phoebe on June 11. Orbiting at a distance of eight million miles, and a mere 100 miles across, this moon was unusual in a number of respects. Its orbit was retrograde as compared to that of the other moons, and the plane much closer to the ecliptic than to that of the rest of the Saturn system. Furthermore, its albedo was very low – suggestive of an unusual composition. It was interesting that six years after its discovery in 1898 by William H. Pickering, the then BAA President had proposed that Phoebe might have had its origins outside the system, and have become gravitationally captured. Though this was now widely believed to be the case, it remained without decisive confirmation, and would be investigated by Cassini. Finally, the speaker once again urged members to download Cassini's images for home viewing – at half a megabyte the download time would be reasonable on even a dialup connection – yet the amount of detail was phenomenal, and could not be done proper justice on a projection screen. He noted in particular that the image of Phoebe was 2,000 pixels square, but despite being a mosaic of several smaller images, the stacking was so good that it was not possible to make out the edges of the individual frames.

Following the applause for Mr Graham's talk, the President invited Mr Martin Mobberley to once again present his regular Sky Notes.






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