embarks on a global upheavel
This news item is in press in the Journal of the BAA.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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
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.
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?
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.
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.
1: The new face of Jupiter in 2007.
(South is up in all images except for
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.
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.
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.
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.
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
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.
(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: