[3]  Jupiter embarks on a global upheavel  (report, 2007 Apr.17th)  

This news item is in press in the Journal of the BAA.

Jupiter embarks on a global upheaval

Once again, Jupiter graces the cover of this Journal (Figure 1) – this time, with major changes, which seem to be a long-awaited example of the grand phenomenon called a 'global upheaval'.  The last of these occurred in 1990.  This has added to the excitement of a flyby of  Jupiter by a spacecraft: New Horizons, en route to the outer solar system.  Observers in the southern hemisphere have risen to the occasion by producing increasingly impressive colour webcam images.

The New Horizons flyby

The prime target of New Horizons is Pluto, but the only planet it is visiting (according to the IAU's new definition) is Jupiter.  The flyby was planned to give the spacecraft the extra speed it needed, and the team used this opportunity to test all their instruments.  New Horizons is a much cheaper mission than Cassini, and it has returned less data from Jupiter – much of it trickling in for several weeks after the flyby.  Nevertheless, the flyby was entirely successful, and the data sets were unique and very revealing.

 Imaging was performed, intermittently, from 2007 Jan.8 to 22 (Figure 2a), so as to track currents over the whole planet.  The images were taken with the powerful telescope called LORRI, but being optimised for the dim lighting at Pluto, it takes images in white light, and at close encounter it would target features near the terminator to reduce the glare. After the initial imaging, priority was given to ultraviolet spectrometry of Jupiter's aurorae and the Io plasma torus.  Imaging of selected targets resumed on Feb. 24 using several instruments, especially LEISA which returned infrared images at numerous wavelengths (Figures 1 & 3).  Also, numerous spectra were taken of the satellites and their atmospheres and the plasma torus, and plasma spectrometers studied the interaction of the magnetosphere with the solar wind.  Closest approach was on Feb.28 at a range of 2.3 million km, just outside the orbit of Callisto.  This was close enough for LORRI to take full-disk images of the galilean moons and to record the erupting volcanoes on Io (Figure 4).  Visible colour images could be taken by the MVIC camera, but not of a surface as bright as Jupiter's, However, MVIC did return a striking view of the volcanic eruptions on Io's dark side (Figure 1). 

In Jupiter's atmosphere, the LORRI team wanted to target two archetypal features: the turbulent cyclonic region of thunderstorms following the GRS, and the newly-red anticyclonic oval BA.  To aim the camera accurately, they asked for help from the BAA Jupiter Section and our colleagues in the JUPOS project. This was a challenging request, as the pointing had to be finalised in 2006 September (when the speed of oval BA was varying unpredictably as it had just passed the GRS – see Journal cover, 2006 October), and the encounter would happen 6 months later, just after solar conjunction. So all concerned were very pleased to see oval BA in the centre of the mosaics returned after the flyby.  A pair of images taken 9.5 hours apart on Feb.26-27 (Figure 5) clearly showed the rapid rotation of the oval.

However, some features seen in the New Horizons images were most unexpected.  During solar conjunction, the thunderstorms following the GRS – which had been present continuously since 1995 – had disappeared.  Meanwhile, alongside oval BA, a prominent dark feature called a South Tropical Disturbance had appeared (Figure 2)– not seen since 1993. These same changes were also noticed, just a few days earlier, in the first good amateur images of the new apparition.

The changes in 2007.

To see how the planet has changed over the last year, you can compare the images on the cover of this issue with those on the covers of 2003 June, 2005 August, and 2006 October.  More details are on our web site:  http://www.britastro.org/jupiter/.

First, major changes have occurred in the South Tropical region.  Everything is quiet, for the first time for many years!  Since summer, 2006, there are no dark spots retrograding on the South Equatorial Belt (SEB), and no dark rim round the GRS.  And since autumn, 2006, there are no white spots in the SEB following the GRS.

The last time that long-running SEB activity stopped like this was in 1988.  Then the SEB faded (whitened) in 1989 and a spectacular SEB Revival occurred in 1990.  So we may see the SEB fading some time in 2007, with the GRS becoming truly red. We already suspect, in the latest images in 2007 March-April (Figure 1), that the southern SEB is turning from dark red-brown to a lighter grey, so the fading may be evident by the time you read this article.

As soon as these turbulent thunderstorms and vortices disappeared, two very different circulations appeared in the STropZ – South Tropical Disturbances. A STrD is a persistent dark structure spanning the STropZ.  The first images of 2007 Jan. revealed two STrD's, one of them just north-preceding red oval BA.  They were prograding as usual, and images from New Horizons and the Hubble Space Telescope showed clear evidence of the circulating currents at their p. and f. edges, which are probably the most essential feature but can rarely be observed. Unfortunately we could not observe how the STrD's originated, during solar conjunction, but they were probably the outcome of a strong tendency for eddying in the STropZ in summer 2006, observed on the JUPOS charts of the last few retrograding SEBs spots.

The situation resembles 1993 – the last time the SEB faded -- when there were two STrD's before the SEB Revival.  None have been seen since then.

Even more obvious to casual observers are the changes in the Equatorial Zone.  The EZ is mostly dark grey and brown.  This consitutes a coloration event, but of an unusual type where darkening is more obvious than coloration. This aspect has not seen since 1973-75, and no strong coloration at all since 1990-91. During the first half of 2006, the blue-grey projections and festoons from the NEB became much more prominent; and the northern EZ between them lost its long-term pale yellowish tint; meanwhile the centre of the zone became progressively darker brown and grey. This darkening of features all across the EZ has made a very striking picture, including great contrasts between the dark NEBs projections and bright patches in northern EZ. By 2006 August, there was also notable yellowish-brown tint in the southern EZ (hitherto very bright white).  Thus the coloration event spread southwards (as has been seen in some previous global upheavals), and the strong orange-brown tint of the SEB(S) in 2007 January may also have been part of it.

As the southern EZ darkened, the South Equatorial Disturbance became spectacular again.  This feature was described in our reports for 1999/2000 and 2000/01, and although it has been very inconspicuous since then, we have tracked it the whole time.  Now it has transformed into a pair of great white spots (Figure 1 & 2b), outlined by diffuse bluish and reddish shadings, with large-scale waves preceding them. 

Most recently, without warning, a spectacular new disturbance has broken out in the North Temperate region, on the extremely rapid jet stream that marks the NTB south edge.  Since the end of 2002, the NTB has been absent, replaced by white cloud – and during that time we have been able to trace the jet stream, gradually accelerating from DL1 =  -83 deg/month (135 m/s; 1995-2000; HST) to DL1 = -108 deg/month (146 m/s; 2005; Rogers et al., Icarus vol.184, p. 452).  The revival of the belt was expected, but what was not predicted was the hugely energetic outbreak which began on 2007 March 27 (images by Fabio Carvalho) and was discovered on March 29 (Zac Pujic – Figure 1).  It began with two brilliant white spots, erupting far higher than all other spots on the planet (according to methane-band and ultraviolet images by Zac Pujic), and travelling at DL1 = -156 deg/month (168 m/s). Smaller bright and dark spots are forming in their wake, with slightly slower speeds, and this turbulence appears to be breaking up the white cloud cover to restore the dark NTB.  Similar super-fast outbreaks occurred in 1975, 1980, and 1990, but have not occurred since then.  It poses a puzzle: Has the jet stream accelerated so much more in just two years? Or was the faster speed still present all the time below the visible cloud-tops?

A global upheaval

The conjunction of all these events fits beautifully into the definition of a global upheaval, as noted by Wynn Wacker in 1975 and seen again in 1990 (and partially in 1993).  Those were the dates of the last three SEB Revivals. (In the meantime, there was a global upheaval in 1978-1980 which had EZ coloration, a STrD, and vigorous activity on some jetstreams including a NTBs outbreak, but no SEB Revival.)  Indeed, in the Journal (1990 June) you can find a news item with the same title reporting many of the same phenomena as this one. 

The most spectacular event of a global upheaval is typically the SEB Revival. So if the SEB does indeed enter such a cycle, some time in the next two years observers should see a grand spectacle, reprising the great events of 1975 or 1990.

The galilean moons resolved

In that news item in 1990, I also reported some then-rare observations resolving features on Io in transit, which suggested that one or both limbs might have darkened since the Voyager encounters.  Since then, images from Galileo, Cassini, and New Horizons have shown that large-scale, long-term changes are not occurring on Io's surface. Even the largest volcanic deposits fade away over a few years.  But now it is not uncommon for amateur images to resolve the dark polar caps of Io, as well as the largest dark areas on Ganymede: see the Journal (2003 June) and Figure 6.  These images also show the phase effect on these moons: as shown in Figure 6, they appear distinctly gibbous due to the gradient of illumination, in spite of the small phase angle.  This was undoubtedly the origin of the 'dark crescent Io' observation in 1990. 

John Rogers

Figure 1:  The new face of Jupiter in 2007. (South is up in all images except for Io.)

Top left:  The Great Red Spot, 2007 April 11, 16:03 UT (Stefan Buda, Australia).  In contrast to previous years, the GRS is an isolated orange oval and the SEB following it is entirely quiet.  The equatorial region is very dark but dramatically disturbed by the South Equatorial Disturbance, passing the GRS.

Bottom left:  The GRS on Feb.27 (New Horizons, LEISA).  This is a false-colour image made from three infrared wavelengths: red = 1.28 mm, green = 1.30 mm blue = 1.36 mm. This choice of wavelengths highlights hazes at different altitudes in the atmosphere.  The oval GRS looks white because it has thick cloud at all levels probed.  Credit for New Horizons images:  NASA / Johns Hopkins University Applied Physics Lab / Southwest Research Institute.

Centre:  Io on Feb.28 (New Horizons: a lo-res colour image from MVIS combined with a hi-res white-light image from LORRI). On the dark side, two volcanic plumes shine blue: at top, the 330-km high eruption from Tvashtar, lit by the sun; at bottom right, the smaller plume from Masubi, lit by Jupiter.  Below the Tvashtar plume is the bright red spot of the volcano itself, possibly a lava fountain.

Figure 2:  Left:  Jupiter on 2007 Jan.21, 05:52 UT (New Horizons, LORRI). Credit for all New Horizons images:  NASA / Johns Hopkins University Applied Physics Lab / Southwest Research Institute. Right: 2007 March 14, 18:47 UT (Anthony Wesley, Australia).  Arrow marks red oval BA; STrD-1 is north of it. 

Figure 3:  Jupiter in the infrared (1.53 microns): one of numerous images from the New Horizons LEISA instrument. This wavelength is sensitive to gas absorption and thus to the altitudes of clouds.  Note that oval BA is very bright, while STrD-1 is almost a negative of the visible image.

  Figure 4:  A close-up of Io from New Horizons (LORRI). Three volcanic plumes are indicated.  This is a combination of two images to bring out detail on the sunlit side (left) as well as the Jupiter-lit side (right). North is up.

  Figure 5:  Close-ups of oval BA from New Horizons (LORRI). Left, Feb.26, 17:41 UT.  Right, Feb.27, 03:12 UT.  South is up to match Fig.2.  Much of STrD-1 is also in the field of view.

  Figure 6: Hi-res amateur images resolving the moons in transit over Jupiter.  Top left:  Io and its shadow, 2006 April 14 (Dave Tyler and Damian Peach, Barbados).  Io (arrowed) moves over a white oval in the NEB. The dark polar caps and white equatorial band are well resolved. 

Top right:  Io, 2007 March 14 (Mike Salway and Anthony Wesley, Australia).  The left-hand edge is shaded due to Io's phase, 3 months before opposition.  Also see Fig.2.

Bottom:  Europa (upper) and Ganymede (lower), 2007 March 22  (Mike Salway, Australia).  Both show shading of the left-hand edge due to phase, and Ganymede shows an oblique dark band which includes the largest dark area. Galileo Regio. For Salway's movie, also including Io, see: 

http://www.iceinspace.com.au/downloads/20070323-jupiter_anim.gif