[4] The Aftermath of the GRS-LRS Encounter, 2008 July.

Jupiter in 2008: 

A rare Little Red Spot squeezes ‘through the eye of a needle’.

John H. Rogers,  British Astronomical Association

Non-Technical Summary

This month, astronomers have watched a very rare collision between Jupiter’s Great Red Spot (GRS) and a smaller copy of it, a Little Red Spot (LRS). It happened just as a third red spot, called Oval BA, was also passing alongside the GRS.  These ‘spots’ are huge anticyclones, whose reddish cloud colour may indicate that they are particularly vigorous and deep-rooted. This was a unique opportunity to see how Jupiter’s atmosphere would behave when they collided.  This report describes how amateur astronomers around the world have produced a thorough record of the event.

As the LRS approached the GRS from the west, it was suddenly swept up by the powerful winds that circulate around the GRS, and within a few days it was being shredded as it squeezed into the very narrow jet-stream that separated the GRS from oval BA.  Several fragments of it emerged in the form of smaller white or orange spots.  Several of these were apparently caught up by the fastest winds around the periphery of the GRS and performed almost a full circuit of the GRS before disappearing.  A slower-moving remnant emerged east of the GRS and for a few days it again showed traces of the reddish, high-altitude cloud layer that distinguished the original LRS; a white remnant of this spot still persists a week later. 

Background

Red colour appears in Jupiter’s clouds in regions of very fast winds, or after intense weather activity, or in exceptional anticyclones.  The GRS is the largest of these anticyclones and the only permanent red oval. Smaller ones (LRSs) appear sometimes in other latitudes: in recent years there has often been one in the NNTZ, and much attention was directed to a large oval in the STZ (called Oval BA) when it turned reddish in 2006. We also reported a LRS in the NTZ in 1997-1998 [Ref.2], possibly similar in origin to the present S. Tropical LRS.

The present LRS appeared in the South Tropical Zone (STropZ), i.e. in the same latitude as the GRS, which is very rare.  The only previous examples were in 1986 and 1990-93, and there was probably a similar example in 1889-1890 [sic], very like the present one although colour was not noticed at that time. [For details see Ref.1.]

The present LRS was the latest result of an unusual tendency to eddying in the STropZ, which has been noticed since summer 2006.  Last year this eddying was concentrated in two sites (called South Tropical Disturbances: STrD’s), and one of these produced a dramatic spectacle as it caused dark vortices travelling on a jetstream to perform U-turns.  During solar conjunction, the STrD’s disappeared leaving just two ovals, which seem to be the outcome of all this eddying.  They were seen in 2008 Feb. at the start of the apparition.  One of these (which we called Oval 2) is the LRS – red, and bright in methane-band images – as we first identified in images taken by Tomio Akutsu (Philippines) and Anthony Wesley (Australia) on 2008 March 1-2.  Its length was about 3500 km.

Along with those of 1986 and 1889-1890, it belonged to an uncommon class of ovals in the STropZ, which arise from a STrD and share its prograding motion (negative DL2).  Its speed (DL2) varied between -5 and -12 deg/month, and it was at 24.1 deg.S (+/- 0.45 deg., SD: JUPOS data from Hans-Joerg Mettig and colleagues).  All these properties distinguish these ovals from the more common class of anticyclonic spots in the STropZ, which are slower-moving (typical DL2 ~ 0 to +8), further north (typical latitude 23.3 deg.S, as in 1994 [ref.3] and 1999/2000/2001/2002 [our BAA reports]), and white or brown or ring-shaped but never red. (There was one exception: a long-lived white spot which reddened in 1990 and 1993 along with the GRS as the SEB faded; it later collided with the GRS, in 1997, although it was then white.)  It is not known why there are two classes of S. Tropical anticyclonic ovals with different origins and properties.

The LRS was drifting slowly towards the GRS (Figure 1).  The GRS is a vast anticyclone with a circulation period of 4.5 days [ref.4].  Other anticyclonic circulations in the STropZ have been observed impinging on it in previous years:

1)  When the p. end of the great STrD (also an anticyclonic circulation) encountered the GRS from the W, it  used to be swept round the S side very rapidly then re-form on the E (p.) side [ref.5]. 

2)  Vortices on the SEBs jet-stream at 20 deg.S commonly encounter the GRS from the E and are swept around its N edge, and their remnants may be merged into it, as was beautifully shown in the Voyager movies.

3)  The larger ovals at 23 deg.S drift slowly up to the GRS from the E, and the collision of one of these in 1997 was observed in some detail [refs.2 & 3]. It was swept more slowly around the N edge and part was then expelled to the N, though part was retained in the GRS which continued disturbed for several months.

But a LRS may be the most deep-rooted or vigorous of all anticyclones, and there had been no opportunity to observe one of these colliding with the GRS – at least, not since 1890.  A century later, we hoped that modern imaging would reveal the details of the interaction if and when this LRS encountered the GRS.  We watched to see if it would accelerate rapidly around the S edge of the GRS, and if so, whether it would merge with it or pass by or be torn apart. Methane-band  images would be of interest: for example, to see whether the  methane-bright cloud cap of the LRS would disconnect from the underlying vortex, as happened  during the merger of ovals BC and DE that formed oval BA.  And by coincidence, oval BA (which progrades relative to the GRS and passes it every 2 years) was precisely alongside the GRS during the interaction.

Observations

All the observations were from the worldwide network of amateur astronomers using CCDs or webcams; many made great efforts to record this phenomenon. With Jupiter at its furthest south, only observers in the tropics or southern hemisphere were getting really good seeing.  Just as with the Circulating Current observations exactly a year ago, the Australians had bad luck with winter weather, but Isao Miyazaki on Okinawa managed to get good images almost every night.  Chris Go in the Philippines was also especially involved in both making and interpreting observations.

We were keen to get images in the 0.89 micron methane band, to reveal the high-altitude cloud cap that is always a feature of a LRS.  Tragically, Tomio Akutsu, the only observer taking regular methane images, lost his telescope to a disastrous hurricane in the Philippines just a week before the collision.  But Chris Go was able to fill the gap as he obtained a methane filter just in time via Dr. Imke de Pater.  We were also very pleased that other observers came forward with methane images, including Antonio Cidadao (Portugal) and Bernd Gaehrken (Germany) – and even Damian Peach, who managed to get well-resolved methane images from England with the planet only 16 deg. up.  Caution should be applied in interpreting these images, as observers use a variety of methane filters with different bandwidths, and all except Go processed the images for clarity; nevertheless, all the features mentioned here are present in raw images.

Results

The best images on each date are shown in Figure 2.  In late June, we noticed that the LRS was slightly accelerating, and by June 28 it was very close to the W (f.) edge of the GRS (separated by no more than the diameter of the LRS).  On June 30, Miyazaki’s image showed it was clearly accelerating to the southeast, entrained by the rapid circulation around the edge of the GRS.  It was still a compact spot on July 1 (Peach’s methane image).  On July 2 it was stretched into a very narrow streak retaining some slight reddish colour, moving around the GRS, and the remains of this were probably still discernible on July 3 (in Cidadao’s methane image as a pale fringe to the GRS, and in Miyazaki’s colour image).  By then it was squeezing very tightly into the jet-stream passage between the GRS and oval BA, and we did not know whether we would see any trace of it again.

The first remnants to reappear were a string of 2 or 3 white spots lying obliquely across the north rim of the GRS, imaged on July 4, and retrograding around the N rim on July 5 and 6.  [We only identified them a few days later, re-examining the images after helpful correspondence with Dr. Glenn Orton, Dr. Agustin Sanchez-Lavega, and Sean Walker.]

They probably originated when the LRS was shredded on July 2, and travelled half-way around the GRS by July 4 – surprisingly fast for spots on the outer rim of the GRS, but possible if they moved with the fastest GRS rotation period (4.5 days) and its fastest wind speed (~ 180 m/s). From July 4 to 6, they travelled round the rim with a more modest speed, ~80 m/s (Figure 3b).  On July 7 the white clouds had apparently ended up in the formerly dark triangle at the W (f.) end of the GRS.

(One may also note that the f. half of the GRS was less methane-bright on July 3, then slightly greyish in the colour images of July 4 & 5.  However these subtle variations may not have been effects of the collision.)

Meanwhile, on July 5-6, as Far Eastern observers reported with excitement, reddish material began to re-emerge from the passage on the SE side of the GRS, and again comprised a methane-bright patch: some remnant of the elevated red cloud-deck of the LRS seemed to have survived the passage!  A longitude chart (Figure 3a) shows that the LRS accelerated smoothly around the GRS and the remnant decelerated symmetrically on the other side, having reached a speed of ~ -4.8 to 6.8 deg/day (64-94 m/s) (relative to System 2 and the GRS). (This is ~ 60-90 m/s relative to System 3, less than the rotation speed of the GRS, but more than the mean speed of the STBn jet which is 44 m/s.)

On July 7 and 8, this ‘LRS remnant’ was a distinct compact methane-bright spot, prograding away from the GRS, even though it was fading in visible light – just a pale orange smudge on July 7 and barely perceptible on July 8 and 9, although its f. edge was marked by a very dark grey streak being drawn out from the GRS rim.

On July 10 and 11, it looked as though the re-emerged LRS remnant was dissipating, as the orange patch had faded and the methane-brightness appeared diffuse over this whole region of STropZ.  But this was only temporary and the ‘LRS remnant’ apparently has survived through July 12-14, as a distinct spot which is white (July 12) or reddish (July 13-14) and methane-bright.  It is slightly further north than before, and nearly stationary at L2 = 108 (~10 deg. p. the edge of the GRS, but probably now drifting back towards it). So it may be a vortex that has settled down into the position of the slow-moving, non-red class of ovals. 

(Note that the very bright white spot just p. it is a pre-existing spot – one of a series of bright bays and dark streaks in this band, drifting towards the GRS at DL2 = +1.0 deg/day according to a JUPOS chart.)

Discussion

These observations show that the LRS was rapidly entrained by the GRS circulation and probably broke up on July 2, producing a group of white spots which circulated very rapidly most of the way around the GRS rim, and a slower-moving spot which emerged on the E(p.) side into the STropZ.  These may all have been fragments of a single disruption, those closest to the GRS moving around it fastest.  However it is not clear from the present observations whether the LRS was sheared horizontally (by radius from the GRS) or vertically (with ‘cap disconnection’) or both.  Methane-band images on July 1-3, before the conjunction, did  not fully resolve the LRS but suggested no cap disconnection.  We cannot say whether the white spots which moved around the rim on July 4-6 carried methane-bright caps, as they could not have been resolved from the methane-bright GRS. The spot which emerged p. the GRS on July 5-6 did retain or re-form some of the high-level, orange, methane-bright cloud cap, and although it was unstable at first, it seems to have evolved rapidly since July 11 into an anticyclonic oval with a methane-bright cap and slight reddish colour.  Therefore, this spot probably carried anticyclonic vorticity with it and represents the main part of the LRS.

Fortunately, professional observers were able to obtain v-hi-res images with HST and with IR observtories during the event, so they may have data to show more of what happened to the vorticity of the LRS, and to its clouds at different levels.

We will continue to look for effects on the GRS.  Figure 3a indicates that the GRS longitude (L2) increased from 125 to 127 just after the event.  This could be a reaction to the passage of the LRS, or it could just be the regular peak of the 90-day longitude oscillation, which is due at this time: this will be clearer when the next JUPOS chart is prepared.

We can also look for any effects on oval BA and the STBn jet-stream.  So far there are no obvious effects on either.  Oval BA passes the GRS every 2 years without lasting effect, though there are often albedo changes in the surrounding STB as it does so.  The STBn jet-stream was generating small dark vortices p. oval BA/GRS well before the LRS arrived, and has not produced any new spots nor any obvious deflection due to the LRS.  We will look out for any change in jet speed during the coming weeks.  This event is perhaps the best possible test of whether a prograde jet can be perturbed, as a substantial vortex was rammed into the jet between two very large stable anticyclones in close proximity.  Prograde jets on Jupiter are almost never perturbed by visible weather systems, and this event may put new limits on their imperturbability.

References:

1.  Rogers JH (1995)  ‘The Giant Planet Jupiter’ pp.197, 200, 206 (Camb. Univ. Press).

2.  Rogers JH (2001), ‘Jupiter in 1997’, JBAA 111, 186-198,

            &  (1997) JBAA 107, 333-335 [interim report]

3.  Sanchez-Lavega A et al.(1998)  Icarus 136, 14-26.

4.  Rogers JH (2008), JBAA 118, 14-20.

5.  Peek BM (1958)  ‘The Planet Jupiter’ (Faber & Faber)

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Figures:

Figure 1.  

Some images showing the new Little Red Spot, close to Oval BA and approaching the GRS.  The top row and the final image are in the methane absorption band, showing all three red ovals as methane-bright spots.

Figure 2.

(A)  A complete set of the best image(s) on each date during the event.  Monochrome images are in the methane absorption band.  Red arrow indicates the LRS and its remnants.

(B)  A selection of the best images, showing more of the planet.  Left, visible colour; right, methane band. Red arrows indicate the LRS and its remnants.

Figure 3.

Charts showing the motion of the LRS and its remnants.

(a) Longitudes in System II.

(b) Position angle for the small white spots on the rim of the GRS, July 4-6.  Position angle was measured from S to E after stretching the GRS image to be approx. circular. Uncertainty  +/- 10-15 deg.

[Notes: South is up in all figures.  If anyone needs a version with north up, or with less compression, ask and I may be able to provide them.  All images are copyright to the observers, and the compilations to JHR, but we will be happy to respond to reasonable requests to reproduce them.]