Rings around Neptune again evade sighting

Press, 1 July 1983, Page 14

Rings around Neptune again evade sighting

By

ERIC BEARDSLEY,

information

officer, University of Canterbury

Fifteen years ago, a research student at the Mount John University Observatory at Tekapo possibly detected a ring, or rings, round the giant planet Neptune; but a chapter of accidents prevented him from claiming an important astronomical discovery. As a cloudy sky began to clear just after 2 a.m. on Thursday of last week, observers at Mount John had every hope of confirming the finding. About 17 minutes later, Neptune, at present the most distant from the sun of the nine known planets, was due to pass in front of, or occult, a faint, distant star.

The planet’s shadow was predicted to sweep across the southern hemisphere of the Earth, including Australia and New Zealand. With a clear night sky observers would have had a splendid opportunity of probing Neptune’s upper atmosphere. The unexpected discovery of rings round Uranus during an occultation in 1977 sharpened interest in planetary rings. That interest was heightened three years later when a ring system round Jupiter was discovered by the Voyager 1 space probe. The rings round Saturn have long been known — they are readily visible through a binocular — though Voyager 1 photographs showed they are a multitude of narrow rings rather than the broad rings observed from Earth. Astronomers reasoned that if three of the giant planets possess rings, Neptune might also have them. However, Neptune is not much like its sister planets. The other three have regular satellite systems. Neptune has two moons. One of them, Triton, has an inclined retrograde orbit which will cause it eventually to break up, perhaps into a spectacular ring system. The other moon, Nereid, has a highly eccentric orbit. Voyager 2 is due to explore the Neptune system in 1989. Until its shutters open in six years, the best way of detecting planetary rings is during stellar occultations, with observers recording the intensity of starlight as the star passes behind the region round Neptune that could accommodate rings. Odhltation by Neptune of adptar

bright enough to be observed is comparatively rare. Neptune, in any case, is not easy to observe — it was not even discovered until 1846 — and only incomplete data have been recorded round the world during occultations. Two years ago, Dr James Elliot, of the Massachusetts Institute of Technology, who discovered the Uranian rings, arranged wide coverage of an occultation and found no evidence for rings round Nep tune.

In the same year, however, University of Arizona astronomers noted a significant gap in the signal they received ouring an occultation and interpreted it as being caused by the passage of a third invisible satellite of Neptune in front of the star. The occultation last week was therefore awaited with unusual interest, especially in view of the announcement last year by Dr Edward Guinan, of Villanova University, in the United States, that he had evidence for a ring system round Neptune. The evidence was the data he had obtained at Mount John when he was a student there in 1968.

When he had finished lecturing at Ham last Wednesday, Dr John Hearnshaw, a senior lecturer in astronomy at the University of Canterbury, drove to Mount John for an all-night vigil. With him were Dr Denis Sullivan, of Victoria University of Wellington, and a Ph.D. student, Mr Duncan Steel. Noriwest conditions prevailed in the Mackenzie Country and the only clouds in the sky lay low over the alps. Unfortunately, by 8 p.m. there was a southerly change and an hour later the sky was totally obscured. The astronomers thought the journey was in vain, but they checked predictions and calculations about the occultation and readied their recording equipment

By midnight the clouds were very thick — the mountains were covered in snow next morning'4-

and the technicians went to bed. Then at 2 a.m. the cloud cover broke and the technicians, hurriedly awoken, opened the domes housing the two 24-inch telescopes to be used and fixed them on Neptune. Photometers to measure the light were switched on. Those measurements would be recorded on a fast-moving strip chart. When the occultation began, promising results appeared on the strip chart almost immediately — a wide gap in light intensity for which a planetary ring was a possible explanation. The variations in the strip chart continued, even when the star was totally eclipsed and the observers could only conclude that thin cloud, invisible in the black sky, was giving them bogus results. Disappointments like that are familiar to astronomers, but occasionally they have good fortune too. The story of why it took Dr Guinan so long to come to the conclusion that he had probably observed a ring round Neptune at Mount John in 1968 was told in the journal "Science” last year. It was a cautionary tale about taking care of scientific observations, but it also revealed Dr Guinan had a lucky streak. He was not even looking for rings at the time. In April, 1968, Ed Guinan asked a Canterbury astronomer, Dr Noel Doughty, if he could interrupt the latter’s observations of eclipsing binary stars on the 16-inch telescope at Mount John so he could observe an occultation of Neptune. He wanted profiles of Neptune’s atmosphere from the photometric light curve of a star, first as it disappeared behind the planet, and later as it reappeared. It was a specially good occultation for that purpose because the star was as □right as the planet. Dr Doughty gave up the telescope and the high-resolution data were recorded on a strip chart while one second and 10-second averages went on to punched cards. However, there are sometimes slips twixt cup and lip. Ed Guinan

went home by ship to Europe and by plane through Russia. He pored over the strip chart during the flight and placed it in a "Time" magazine, which he left in the plane during a stopover. When he returned, the magazine and the chart had gone.

He still had the low-resolution punch card data, but they were useless for deriving any information about the atmosphere of Neptune. In any case the cards had been soaked during his sea voyage and were so badly warped they would have stuck in a card reader.

Ed Guinan thought the whole exercise was something of a washout, but he stored the cards away — and forgot about them until the N.A.S.A. probe discovered the Jovian rings and Dr Elliot found the Uranian rings, using a similar occultation observation. The cards were in bad shape when Dr Guinan, now a professor at Villanova, resurrected them. He was able to get a student, Craig Harris, to duplicate them one by one by hand, a task which was not completed until last year. The result, though, was quite spectacular: a light curve showing a sharp occultation by Neptune and then, three minutes later, a 30 per cent dip lasting about two minutes. Dr Guinan and a colleague, Frank Maloney, interpret the dip as evidence for a ring about 5000 km above the Neptune cloud tops. They announced their tentative findings to the annual meeting of the American Astronomical Society last year and claimed to have evidence from the Mount John data for at least two rings round Neptune. They were probably composed of ice and debris from a satellite shattered by Neptune’s gravitational forces. Others possibly missed observing them because they are so close to the planet and relatively translucent. Dr Elliot may be able to resolve the question. About a dozen southern hemisphere observatories planned to record the occultation last week. If they too suffered from cloud, Dr Elliot rose to the occasion. He observed the occultation from an airborne N.A.S.A. observatory flying out of Melbourne. His results will be awaited with interest.