[5] The collision of the Little Red Spot and Great Red Spot: Part 2

Jupiter in 2008: 

The collision of the Little Red Spot and Great Red Spot: Part 2

John H. Rogers,  British Astronomical Association

2008 August 8 th.


In early July, the South Tropical Little Red Spot (LRS) (or Baby Red Spot, as NASA have nick-named it) collided with the Great Red Spot (GRS) and was shredded by its vigorous winds.  Our earlier report, produced on July 15, described the initial events.  This report fills in further details of that initial collision, and continues the story up to early August.  Starting on July 2, the LRS seems to have been pulled into two main parts, the leading part marked by several bright spots orbiting rapidly on the rim of  the GRS, and the trailing part -- the main remnant of the LRS – re-emerging preceding (p.) the GRS.  HST images on July 8.9 showed these parts were still connected, forming a spiral completely encircling the GRS.  The remnant E of the GRS drifted N and then W, colliding with the GRS again on July 16.  Images from July 17 to 25 showed probable remnants of it orbiting slowly around the north side of the GRS.  Only one distinct product of these events survives: a dark streak which emerged from the GRS rim following the LRS remnant, and continued prograding to the E. However, there may be persistent smaller-scale effects both within and preceding the GRS.

Observations & Results:

Some extra images have been received showing important stages in the earlier events (Figure 4).  They include images by Paul Maxson on July 2 – the first to show stretching and disruption of the LRS as it was swept around the GRS – and July 9 – the last to show definite reddish colour in the pale ‘LRS remnant’ that had emerged p. the GRS.  They also include the publicly released Hubble Space Telescope (HST) image on July 8.9 (credit NASA, ESA, and Dr. Amy Simon-Miller of GSFC).

The HST image dramatically confirms the link between the two products of the LRS disruption that we reported earlier: first, a string of 2 or 3 white spots orbiting in the N rim of the GRS on July 4-6, and then, the more conspicuous LRS remnant emerging p. the GRS on July 5-6 (Figure 5).  The HST image on July 8.9 shows that the latter remnant was indeed a coherent, but highly distorted oval with streaks of orange colour; but this was connected to a bright lane forming a 360-degree spiral all the way round the GRS to a white spot inside its rim.  This spiral probably represented the complete LRS, disrupted on July 2 and pulled into a loop all around the GRS!  This is shown by labelled images (Figure 5) and position angle measurements (Figure 6).  The LRS portion closest to the GRS was dragged into and around it with a period of ~5 days, and the white spots imaged by amateurs on July 4-6 represented either this portion or further disturbance arising around it. (The PA measurements indicate a range of speeds, so some spots may have been transient, and the HST image also shows a disturbed sector within the GRS ahead of the tip of the spiral.) 

Meanwhile the portion of the LRS further from the GRS squeezed round its S edge and re-formed into the ‘LRS remnant’ on its p. side.  We can now describe the further fate of this feature (Figures 7 & 9) *[footnote].  On July 7-8 it was a pale orange, methane-bright spot, prograding away from the GRS.  On July 10-12, the orange tint had faded away but the remnant was still perceptible as a bright spot (at the p. end of a very dark streak extending out from the GRS rim). It halted at L2 = 108 (possibly merging with a pre-existing retrograding white spot). On July 13-14 it moved northwards, becoming slightly reddish again, and then headed back towards the GRS. Meanwhile on July 10-14 the methane-bright area was more extended, and covered both the bright spot, and the p. end of the dark streak which continued to prograde.  On July 15 the methane-bright spot seemed to lie on the boundary between the two.  Thereafter, amateur images showed no remaining methane-bright material (July 19 onwards). 

The LRS remnant came back to collide with the GRS again on July 16.  It was probably recorded as a tiny white spot drifting around the Red Spot Hollow on July 17-18 (with another similar spot ahead of it) (Figure 8). The drift rate was remarkably slow, equivalent to a period of ~15 days (Fig.6, spot 4). By July 19, it ended up in the larger, semi-permanent white spot complex at the northernmost point of the Red Spot Hollow, and that seemed to be the end of the story.  However, 3 days later, a white spot with dark rim broke out from this complex and continued retrograding around the Red Spot Hollow (nominal period ~17.5 days, or possibly a pair of spots travelling faster: Figs.6 & 8).  This unusually well-defined spot, at the head of a notable dark blue-grey sector of GRS rim, may well have been the last distinct appearance of the LRS remnant.  It was last seen on July 25 as it approached the f. (W) end of the GRS, where retrograding spots usually disappear. 

Meanwhile, the region p. the GRS was a rather chaotic scene of dark streaks and a few tiny bright spots (Figure 8), but I suspect that little of this was due to the LRS collision, although small-scale clouds or vortices from the LRS might well have spread into this region below our level of resolution. Features indicated on Figures 8 & 9 include:

--(Blue arrow): The one definite persisting outcome was the prograding dark spot that originally marked the p. end of the dark streak that emerged with the LRS remnant: DL2 = -3 deg/day. 

-- (Purple arrowhead): A conspicuous bright spot at L2 ~ 102 (July 12-21) existed beforehand, but its  retrograding drift was halted as it encountered the prograding dark streak (as well as a faster-retrograding SEBs white spot); thus it may have been influenced indirectly by the LRS disruption.  At this time it became a distinct reddish oval (July 17-20). It may be the same reddish oval, reappearing from the surrounding murk, that was recorded closer to the GRS (L2 = 107) on July 29-30.

--(Green arrowheads): Tiny white spots retrograding among the dark streaks (DL2 ~ +3 deg/day) and entering the Red Spot Hollow.  The LRS Remnant had joined this column, but the others were probably unrelated features.

The GRS may still be showing effects of the LRS collision. Its increase in longitude from L2 = 125 to 128.5 is more than expected from the usual 90-day oscillation, and may well be a reaction to the collision.  Its appearance on July 29-30 was still quite unusual: note a very dark blue-grey spot retrograding around the Hollow, another dark blue-grey rim section trailing behind it, and an orange annulus within the northern rim of the GRS, separated from the dark GRS core by a white strip. Thus, small-scale disturbance may continue for a little while.

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* Footnote:

There were pre-existing spots in northern STropZ: a band of dark streaks, with white bays becoming evident as they neared the GRS (including the one marked purple on the figures), all moving with DL2 = +1 deg/day.  Just north of this band, there were some tiny white spots in SEBs with DL2 = +3 deg/day (including the ones marked green on the figures).

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Figures (numbering continued from previous report):

(South is up in all images. Click for full size versions.)


Figure 4:  Some more images from earlier stages of the events.


Figure 5:  Annotated images showing how the multiple LRS remnants seen in amateur images relate to the 360-deg. spiral seen in the HST image.  The LRS is stretched continuously from July 2 onwards, until July 8.9 when it forms a noose completely encircling the GRS.


Figure 6:  Measurements of position angle (PA) of white spots orbiting around the GRS (continued from Fig.3b). For these measurements, the images were stretched to make the GRS approximately circular (as in Fig.5), and the PA of white spots was measured from south towards east. 


Figure 7:  Images from July 11-15, showing the extended methane-bright region, while the LRS remnant light spot (red arrow) drifted north and headed back towards the GRS.  The p. end of a dark streak from the GRS rim (blue arrow) continued prograding. 


Figure 8:  Set of the best images, July 14 to August 3, with spots colour-coded.  Methane images are not included as no methane-bright remnant was detectable after July 15.


Figure 9:  Longitudes of the GRS, LRS remnant, and adjacent spots (measured by JHR).


John H. Rogers, Ph.D.
Jupiter Section Director,
British Astronomical Association.