Ordinary Meeting, 2009 January 28
Jupiter in 2007 and 2008
Dr Rogers explained that what he had to say would neatly follow on from the previous talk: he would be providing an overview of the visual appearance of Jupiter over the past couple of years, as observed by amateurs. He began with an overview of the circumstances of Jupiter's 2008 apparition, as seen by the casual observer. He commented that since Jupiter had lain in Sagittarius throughout the whole year, at a declination a little south of –20°, this apparition had not been at all well placed for observation from the UK. At opposition, Jupiter had reached a peak altitude of only 16° in the UK sky. However, naked-eye observers with a good southern horizon had nonetheless been able to enjoy two conjunctions of Jupiter with Venus which had neatly bracketed the apparition: the first had occurred close to its beginning and the second had occurred close to its end. On the occasion of the first, around 2008 February 1, the pair had passed within 35' of each other, forming a brilliant conjunction. On the occasion of the second, around 2008 December 1, the pair had made a more distant conjunction at a separation of around 2°, but the addition of a three-day-old waxing crescent Moon to the pair's west had led to the event's being dubbed the 'smiley face' conjunction. The speaker noted that later on the evening of December 1, the Moon had gone on to occult Venus.
Dr Rogers then turned to describe the fine detail which amateur planetary imagers had been able to resolve with the use of modern webcams, some good seeing, and the stacking of large numbers of short exposure images. Among these imagers, the speaker reserved especial mention for local expert Damian Peach: whilst it had to be conceded that his images from Jupiter's 2008 apparition were far from his best, and certainly could not compete with those taken by the southern-hemisphere's amateurs, they were nonetheless highly impressive given that he had been working exclusively from the UK, against the problems of the planet's southerly declination at this apparition. Dr Rogers explained that the remainder of his talk would be about what could be learnt from such amateur images, with a bias towards images taken by those southerly-based amateurs who had had the best observing conditions.
The speaker identified three key milestones which amateur imagers of Jupiter had passed for the first time in 2008, and which he believed summarised their achievements rather well. The first of these was that substantially more fine detail had been recorded in the alternating latitudinal wind patterns across the planet's disk than had been recorded at previous apparitions. He explained that for many years it had been possible to observe and measure the speeds of Jupiter's well-known currents and jet streams. It had long been known that each of these had its own individual speed which could be measured by observing the movements of conspicuous spots in them. The resulting speeds were typically found to be around 1° of longitude per month relative to System I. The speaker went on to explain that there were much faster currents called jet streams on the edges of the belts at the equator which moved at characteristic speeds of several degrees of longitude per day. These too had been observable by amateurs in some detail for many years, but spacecraft observations had revealed, by looking at the movements of subtle textures in the clouds, that there was a smooth transition in rotation speed with latitude between these jet streams and the neighbouring belts, which amateur observations had not had sufficient angular or time resolution to record prior to 2008.
Dr Rogers explained that it was interesting that the first amateur observations of this zonal wind profile, made by a wide range of imagers and then collated and analysed by the JUPOS project over the course of 2008, agreed well with older data taken in situ by Voyager and Cassini. He explained that this might not have been expected, given that the planet had undergone a global upheaval in 2007, and that substantial changes had been observed in the visual appearances of many of the belts at that time. Specifically, he remarked that during the global upheaval, the generation of many new vortices had been observed in several of the belts. The South Equatorial Band (SEB) had been observed to become very faint before undergoing a vigorous outburst of spots which appeared to have revived it. The Equitorial Zone (EZ) had darkened substantially from its previous pale colour, but had since cleared again. And the North Temperate Belt (NTB), which had been absent for several years prior to 2007, had undergone a vigorous outbreak of spots and revived. In view of these planet-wide changes, the speaker explained that the agreement of the jet speeds measured by amateurs in 2008 with previous data – with the single exception of the NTB whose speed did appear to have changed – was surprising, and appeared to reveal something significant about the nature of the global upheaval. It appeared to have been a purely meteorological phenomenon which had not affected the underlying pattern of the planet's jets and winds.
The speaker then turned to discuss in more detail the revival of the NTB which he had earlier alluded to; he explained that this was normally the fastest jet stream on the planet. He explained that, in its normal state, a series of conspicuous vortices ran along it, from whose motion the speed of the jet could easily be estimated. These had been monitored extensively between the mid-1990s and 2002, revealing that the jet had maintained a roughly constant speed of 120-130 m/s relative to System III over that time period. In 2002, however, the vortices had disappeared, leaving only a few flecks with which the jet speed could be measured. Observations since that time had suggested that the jet stream had been steadily increasing in speed. The speaker explained that in 1990, following a similar disappearance of the vortices from the NTB, there had been an outbreak of very fast-moving spots in the jet, moving at speeds greatly in excess of those normally observed. There had in the past few years been some anticipation that a similar outbreak might be seen again.
He could now confirm that such an outbreak had indeed been seen during the global upheaval of 2007: great white plumes had been seen at an altitude much above the planet's normal visible surface, becoming the brightest features on its disk at any wavelength accessible to amateur observers, and moving at speeds of up to 13°/day relative to System II. Dr Rogers identified the observation of these plumes as the second milestone which amateur imagers had passed in Jupiter's 2008 apparition, and reported that recent professional attempts to model them suggested that they were clouds of water vapour which had emerged from a water layer deep beneath the planet's cloud level. These models went on to argue that the high speed of these plumes could only be explained by the presence of a permanent fast jet at this depth1, and that in the coming months and years these plumes would disperse and the vortices which normally populated the NTB would reform. To date, amateur observations seemed to be in agreement with this analysis; they had recently revealed the formation of a deep orange belt, the formation of some subtle lozenges within it, and a slowing of the jet speed. If the belt did indeed revert to its pre-2002 behaviour as was now expected, this would represent the first continuous amateur observations of the increase and subsequent decrease in the jet's speed accompanying a complete cycle in its behaviour.
Finally, the speaker turned to discuss the merger of two red anticyclonic ovals in the southern hemisphere – the Great Red Spot (GRS) and a smaller red spot which had appeared at a similar latitude in 2008 and which had been dubbed the Baby Red Spot (BRS). The speaker added that the appearance of the BRS had been a highly exceptional event: the appearance of new red spots on Jupiter was very rare and that only a handful had ever been observed. Whilst it was well established that such spots were anticyclonic circulations, the exact reason for their red colour remained unclear. Empirically, though, it appeared that red spots were those which were unusually vigorous and deep-rooted as compared to other circulations. The speaker added that the precise origin of the BRS was somewhat unclear, since it had formed during Jupiter's solar conjunction of 2007-8 and its birth had been largely unobserved.
Returning to the story of the end of the BRS's life, the speaker explained that on 2008 June 30 the spot had approached the following edge of the GRS, and had subsequently been squeezed through a vanishingly narrow current between the southern edge of the GRS and the northern edge of Oval BA, which had happened to lie due south of the GRS at this time. At the time, there had been much speculation as to whether the BRS would be torn apart during this passage, or whether its deep rooting would be sufficient to ensure its survival. The speaker explained that in the event, it had reappeared a few days later on the leading edge of the GRS, appearing coherent but highly deformed. A white track, however, had also become apparent, connecting at one end to the emergent BRS, and which could be followed around a full 360° path around the rim of the GRS. The speaker favoured the interpretation that this track represented material which had been torn from the BRS by the GRS. Within a few days, the main part of the BRS's circulation had apparently been drawn back into the GRS and entirely swallowed, and so it appeared that it had been entirely torn apart over the course of two encounters with the GRS. The speaker noted that amateurs had never before seen the merger of two red spots on Jupiter, and so these observations were the third milestone which he identified in amateur observations of Jupiter's 2008 apparition.
The speaker closed his talk by showing video compilations of the amateur observations discussed earlier in his talk. Following the applause, the President invited Mr David Arditti to present Sky Notes.