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Light and dark spots in the equatorial regions of Jupiter
Plonct.
Space Sci., Vol. 26, pp. 335 to 338.
LIGHT
Pergamon
AND
Press,
1978.
Printed
in Northern
Ireland
DARK SPOTS IN THE EQUATORIAL REGIONS OF JUPITER
G. C. BROWNE and A. J. MEADOWS Astronomy Department, University of Leicester, Leicester, England (Received
15 July 1977)
Abstrati-The lifetimes and motions of dark and light spots in the equatorial regions of Jupiter have been studied for the 1972 apparition. The difference in lifetimes of the two types of spot is demonstrated, as are their differing modes of formation and disappearance. The way in which the spots interact is examined in detail, and it is pointed out that this may have implications for the fluid dynamics of the Jovian atmosphere,
INTRODUcLlON The study of motions in the atmosphere of Jupiter from the observation of features often called ‘spots’ has been carried on since the nineteenth century.
However, until the past decade most measurements were made by direct visual observation (Peek, 1958). Photographic measurements have now become commoner, but the more accurate data thus made available have been limited by: (i) the restricted time coverage of the photographic material; (ii) the great effort involved in carrying out detailed reductions of Jovian photographs. The first of these limitations was effectively overcome by the establishment of the IPP (International Planetary Patrol). In 1969, a number of observatories situated at different longitudes round the Earth agreed to undertake regular photography of the brighter planets (Baum et al., 1970). The films produced were sent to the Lowell Observatory in Arizona for processing, and a complete set of images retained on file there for the use of investigators. The second problem-the significant investment of time required for reducing the photographic material-seems, at present, unavoidable, since only limited automation of the process is currently feasible. It follows that photographic studies should concentrate on questions that cannot readily be answered by visual observations. We have used IPP photographs of Jupiter for the apparition of 1972 to compare the properties of two main types of Jovian spot-the light and the dark. We have selected for especial study (i) observational indications of interaction between spots, (ii) the lifetimes of spots. The 1972 apparition was chosen because the equatorial regions of Jupiter in that year were both unusually active and unusually dark. This latter observation probably implies that an upper cloud layer of ammonia cirrus (which 335
might otherwise interfere with spot observations) was then generally absent. We wished to observe in the general area of the equatorial belt because its borders represent regions of very high shear, and this favours the formation of spots. (It has been asserted previously that the spots in these border regions are predominantly dark (Starr, 1973), but our observations have indicated as many light as dark spots.) OBSERVATIONS OF DARK SPOTS Three dark spots were observed in the NEBNTrZ region. We have designated these A, B and C, with spot A in the northern half of the NEB, spot B in about the same position, and spot C predominantly in the NTrZ.
Spot A was an elongated oval, with a long axis of lO-15”, dark in both red and blue light. It was already present on IPP images at the beginning of the apparition in March 1972, so no date of formation can be adduced. It was last measurable on 23 August, 1972, but there was no clear date of disappearance: the spot spread in longitude to cover some 40”, and decreased in intensity to merge with its surroundings. Hence, the conclusion to be drawn is that its lifetime was greater than 168 days. The longitude changed in a linear manner during this time by 81”. This implies a constant rotation period of 9 h 55 min 20 s, which is some 21 s faster than the System II rotation period. (The estimated standard deviation for this and all other periods quoted here is ?=l s.) Spot B was very similar to spot A in terms of shape and colour, though somewhat smaller. It is possible that it first formed on 14 March, 1972, though it was not then sufficiently distinguishable to be measured. It finally faded from sight on 22 October, but could not be measured accurately
336
G. C. BROWNE and A. J. MEADOWS
Spot A
Spot A
h
__
April
March
June
May
July
August
1972
FIG. 1. VARIATIONSIN THELATITUDEAND LONGITUDEOF SPOTA DURING1972.
after
2 October. The estimated +12 210 _34 days. There was a reduction
lifetime
is
in longitude
of 84” (relative to System II) in 190 days: if this occurred linearly, the corresponding rotation period is 9 h 55 min 24 s. However, the rotation period certainly increased around the beginning of July, as will be noted below, so it may be inappropriate to quote an average value. Spot C was lighter than spots A and B in both red and blue light, but very similar in shape and size. It was measurable from 16 March, 1972, until
-Spot
2 October, though it was possibly present as a faint feature after this latter date. Hence, its lifetime was greater than 201 days. The measurements imply a linear reduction in longitude of 105$ in 201 days, but there is some suggestion of a change in the rotation period around the middle of July. It should be noted that none of the three spots showed any significant change in latitude throughout their existence. It appears that some kind of interaction occurred between spots B and C in July, 1972. In the middle of June, the two spots were some 20” apart in
B
---spot
80
I March
I April
June
I
I
I
I May
c
July
August
1
September
FIG. 2. VARIATIONSIN THELATITUDEAND LONGITUDEOF SPOTSB AND C DURING1972.
Light and dark spots in Jovian equatorial regions longitude, but, by the middle of July, they were separated by only 3”. Spot B had increased its rotation period by some 17 s to equal the System II period. On 19 July, the rotation period of spot C increased by 8 seconds from the previous value of 9 h 55 min 15 s. Thus both features decreased their rotational velocity during this period; but, whereas the change for spot B was only temporary, that for spot C was permanent. Since we carried out the above measurements, Minton has discussed the motions of a number of spots on Jupiter (Minton, 1976). Three of these appear to be our spots A, B and C, for which Minton gives rotation periods of 9 h 55 min 20.07 s&O.27 s, 9 h 55 min 24.34 s+O.64 s and 9 h 55 min 16.03 s f 0.63 s, respectively. These values are in excellent agreement with ours (so long as the change in spot C after mid-JuIy is ignored). Hence, our methods of measurement appear to produce valid results. On the other hand, the colours that Minton assigns to these spots differ from ours. He claims that spots A and B were red, whilst spot C was blue. We believe that spot C was redder than spots A and B, and would classify the latter two simply as dark spots. The point is of some importance, since differently coloured spots may have different dynamical properties. In view of the extensive study of Jovian colours we have been carrying out, we feel our description may be more firmly based. OBSERVATIONS
337
half of the NEB. They began as small spots, but ten days later had become much larger than Dl-3. D4 ended by combining with D7 by 4 July, whereas D5 broke up into three smaller spots in the next few days. Their lifetimes are therefore estimated to +2 have been 34 f 1 days and 38 _4 days, respectively. It is worth noting that D4 and D.5, unlike the previous three spots, survived a passage through, or beneath, a veil. Their rotation periods differed from each other. The motion of 04 corresponded to a rotation period of 9 h 54 min 16 s; whereas D5 had a period of 9 h 54 min 57 s before 29 June, and a period of 9 h 54 min 02 s afterwards. D7 formed between D4 and D5 about 15 June, 1972, with the same period as D5. It rapidly caught up, and +2 merged with, D4 after a lifetime of 18 _4 days. D6 formed ahead of D4 at about the same time as D7. Its rotation period before 1 July, 1972 was 9 h 54 min 36 s, changing after 3 July to 9 h 53 min 11 s. It disappeared in mid-July, either by merging with D4, or by merging witb a veil, giving a lifetime of 32 f 1 days. Finally, D8 emerged from a vei1 on 18 July,1972, then merged with another veil in mid-July, giving it
OF LIGHT SPOTS
Eight light spots near to the edge of the NEB in the region 250-350” (Jovian longitude System II) were observed for the period May-July, 1972. We have designated them DI-8. Dland D2 formed some time after 28 April, 1972, and had similar motions, corresponding to a common rotation period of 9 h 54 min 02 s. However, this average period disguises a change around 1-2 June: prior to this date the periods were 9 h 54 min 14 s, and afterwards they became 9 h 53 min 13 s. In the latter part of June, both spots ended their careers by merging with a faint veil lying in the northern half of the equatorial region. The respective lifetimes of Dl and D2 were therefore 54*3 days. The spot D3, which was similar in appearance to Dl and D2, appeared after the passage of a faint veil on 27 May and merged with another veil on 24 June. Hence it had a lifetime of 29 days. Its rotation period was constant at 9 h 53 min 25 s. At the end of May, 1972, two light-coloured features (D4 and D5) appeared in the equatorward
June
July
1972 FIG. 3. VARIATIONSINTHELONGITUDESOF
DURINGJUNEANDEARLYJULY, CATE UNCERTAIN
MEASUREMENTS. ~ILA~A~OMAR~.
SPOTS
Dl-D8
1972. BROKENLINESINDI?-HE EXTREMITIES OF A
338
G. C.
BROWNE
and
a total lifetime of 30 i 1 days. Prior to 1 July, the rotation period was 9 h 54 min 37 s, decreasing to 9 h 52 min 57 s after 3 July. Unlike the dark spots, there is some indication of latitude change in the later development of light spots. LWETIMES AND MOTIONS OF SPOTS
One of the most striking differences between spots A, B and C, on the one hand, and Dl-8, on the other, is their relative lifetimes. The former ranged from 168 to 210 days: the latter varied from 18 to 54 days. Spots A and B were almost certainly holes in the cloud deck. C may have been similar, or it may have been a dark red cloud. The spots Dl-8 were probably ammonia clouds, somewhat higher in the atmosphere. The difference between dark and light spot lifetimes appeared to be typical. It seems that, in the Jovian equatorial regions, holes in the cloud deck are more stable features than are higher lying clouds. Some spots appear to show a permanent change in their rotation periods during their lifetimes. For six of the light-coloured spots this consisted of a marked decrease in the rotation period, which, for four out of the six, was preceded by a brief increase. This change, in some cases, appeared to be connected with the passage of veils in the equatorial region, as did the beginning and end of some light-coloured spots. Indeed, one of our main conclusions is that the passage of such veils strongly affects atmospheric activity on Jupiter. The effect of such veils may sometimes remain unobserved because of a general higher-level haze on Jupiter. As we noted in the introduction, the 1972 apparition was noticeable for the absence of such haze in the equatorial regions. The changes in rotation period are also indicative of interaction between the spots. In this case, there is again a difference between the dark and light spots. The changes in rotation suggest some in-
A. J. MEADOWS
teraction between spots B and C. These were at different latitudes and showed no indication of latitude change throughout their lifetime. Hence, the interaction, if real, implies some type of meridional flow. Interaction was certainly observed between light spots. In particular, the pairs of spots Dl and D2, D4 and D5, D6 and D8, clearly influenced each other, exhibiting similar changes in motion at the same time. All these spots were at approximately the same latitude, so we are here dealing with latitudinal changes in flow. On the other hand, there is some slight indication of a decrease in latitude for these spots towards the end of their lifetimes. If this shift is real (the change in latitude is near the limits of the measuring procedure), it suggests that the spots were disrupted when they moved into a region of different zonal flow. The evidence from the dark and light spots taken together suggests that spot interactions can only be explained by changes in flow covering a fairly large region of the Jovian atmosphere. For we have noted interactions both in latitude and longitude, and at different heights in the atmosphere. Acknowledgements-We would Baum for allowing us access to use of facilities at the Lowell [GCB] wishes to acknowfedge search Studentship and receipt SRC.
like to thank Dr. W. A. the IPP files and for the Observatory. One of us support by an SRC Reof a travel grant from
REFERENCES
Baum, W. A., Millis, R. L., Jones, S. E. and Martin, L. J. (1970). The International Planetarv Patrol ProRram. icasus. 12, 435. Minton, R. B. (1976). Measures of Jupiter photographs1972 aooarition. Icarus. 29, 201. Peek, B. ‘6. (1958). The I%& Jupiter. Faber & Faber, London. Starr, V. P. (1973). A preliminary dynamic view of the circulation of Jupiter’s atmosphere. Pure appl. Geopfiys.
110.2108.
Space Sci., Vol. 26, pp. 335 to 338.
LIGHT
Pergamon
AND
Press,
1978.
Printed
in Northern
Ireland
DARK SPOTS IN THE EQUATORIAL REGIONS OF JUPITER
G. C. BROWNE and A. J. MEADOWS Astronomy Department, University of Leicester, Leicester, England (Received
15 July 1977)
Abstrati-The lifetimes and motions of dark and light spots in the equatorial regions of Jupiter have been studied for the 1972 apparition. The difference in lifetimes of the two types of spot is demonstrated, as are their differing modes of formation and disappearance. The way in which the spots interact is examined in detail, and it is pointed out that this may have implications for the fluid dynamics of the Jovian atmosphere,
INTRODUcLlON The study of motions in the atmosphere of Jupiter from the observation of features often called ‘spots’ has been carried on since the nineteenth century.
However, until the past decade most measurements were made by direct visual observation (Peek, 1958). Photographic measurements have now become commoner, but the more accurate data thus made available have been limited by: (i) the restricted time coverage of the photographic material; (ii) the great effort involved in carrying out detailed reductions of Jovian photographs. The first of these limitations was effectively overcome by the establishment of the IPP (International Planetary Patrol). In 1969, a number of observatories situated at different longitudes round the Earth agreed to undertake regular photography of the brighter planets (Baum et al., 1970). The films produced were sent to the Lowell Observatory in Arizona for processing, and a complete set of images retained on file there for the use of investigators. The second problem-the significant investment of time required for reducing the photographic material-seems, at present, unavoidable, since only limited automation of the process is currently feasible. It follows that photographic studies should concentrate on questions that cannot readily be answered by visual observations. We have used IPP photographs of Jupiter for the apparition of 1972 to compare the properties of two main types of Jovian spot-the light and the dark. We have selected for especial study (i) observational indications of interaction between spots, (ii) the lifetimes of spots. The 1972 apparition was chosen because the equatorial regions of Jupiter in that year were both unusually active and unusually dark. This latter observation probably implies that an upper cloud layer of ammonia cirrus (which 335
might otherwise interfere with spot observations) was then generally absent. We wished to observe in the general area of the equatorial belt because its borders represent regions of very high shear, and this favours the formation of spots. (It has been asserted previously that the spots in these border regions are predominantly dark (Starr, 1973), but our observations have indicated as many light as dark spots.) OBSERVATIONS OF DARK SPOTS Three dark spots were observed in the NEBNTrZ region. We have designated these A, B and C, with spot A in the northern half of the NEB, spot B in about the same position, and spot C predominantly in the NTrZ.
Spot A was an elongated oval, with a long axis of lO-15”, dark in both red and blue light. It was already present on IPP images at the beginning of the apparition in March 1972, so no date of formation can be adduced. It was last measurable on 23 August, 1972, but there was no clear date of disappearance: the spot spread in longitude to cover some 40”, and decreased in intensity to merge with its surroundings. Hence, the conclusion to be drawn is that its lifetime was greater than 168 days. The longitude changed in a linear manner during this time by 81”. This implies a constant rotation period of 9 h 55 min 20 s, which is some 21 s faster than the System II rotation period. (The estimated standard deviation for this and all other periods quoted here is ?=l s.) Spot B was very similar to spot A in terms of shape and colour, though somewhat smaller. It is possible that it first formed on 14 March, 1972, though it was not then sufficiently distinguishable to be measured. It finally faded from sight on 22 October, but could not be measured accurately
336
G. C. BROWNE and A. J. MEADOWS
Spot A
Spot A
h
__
April
March
June
May
July
August
1972
FIG. 1. VARIATIONSIN THELATITUDEAND LONGITUDEOF SPOTA DURING1972.
after
2 October. The estimated +12 210 _34 days. There was a reduction
lifetime
is
in longitude
of 84” (relative to System II) in 190 days: if this occurred linearly, the corresponding rotation period is 9 h 55 min 24 s. However, the rotation period certainly increased around the beginning of July, as will be noted below, so it may be inappropriate to quote an average value. Spot C was lighter than spots A and B in both red and blue light, but very similar in shape and size. It was measurable from 16 March, 1972, until
-Spot
2 October, though it was possibly present as a faint feature after this latter date. Hence, its lifetime was greater than 201 days. The measurements imply a linear reduction in longitude of 105$ in 201 days, but there is some suggestion of a change in the rotation period around the middle of July. It should be noted that none of the three spots showed any significant change in latitude throughout their existence. It appears that some kind of interaction occurred between spots B and C in July, 1972. In the middle of June, the two spots were some 20” apart in
B
---spot
80
I March
I April
June
I
I
I
I May
c
July
August
1
September
FIG. 2. VARIATIONSIN THELATITUDEAND LONGITUDEOF SPOTSB AND C DURING1972.
Light and dark spots in Jovian equatorial regions longitude, but, by the middle of July, they were separated by only 3”. Spot B had increased its rotation period by some 17 s to equal the System II period. On 19 July, the rotation period of spot C increased by 8 seconds from the previous value of 9 h 55 min 15 s. Thus both features decreased their rotational velocity during this period; but, whereas the change for spot B was only temporary, that for spot C was permanent. Since we carried out the above measurements, Minton has discussed the motions of a number of spots on Jupiter (Minton, 1976). Three of these appear to be our spots A, B and C, for which Minton gives rotation periods of 9 h 55 min 20.07 s&O.27 s, 9 h 55 min 24.34 s+O.64 s and 9 h 55 min 16.03 s f 0.63 s, respectively. These values are in excellent agreement with ours (so long as the change in spot C after mid-JuIy is ignored). Hence, our methods of measurement appear to produce valid results. On the other hand, the colours that Minton assigns to these spots differ from ours. He claims that spots A and B were red, whilst spot C was blue. We believe that spot C was redder than spots A and B, and would classify the latter two simply as dark spots. The point is of some importance, since differently coloured spots may have different dynamical properties. In view of the extensive study of Jovian colours we have been carrying out, we feel our description may be more firmly based. OBSERVATIONS
337
half of the NEB. They began as small spots, but ten days later had become much larger than Dl-3. D4 ended by combining with D7 by 4 July, whereas D5 broke up into three smaller spots in the next few days. Their lifetimes are therefore estimated to +2 have been 34 f 1 days and 38 _4 days, respectively. It is worth noting that D4 and D.5, unlike the previous three spots, survived a passage through, or beneath, a veil. Their rotation periods differed from each other. The motion of 04 corresponded to a rotation period of 9 h 54 min 16 s; whereas D5 had a period of 9 h 54 min 57 s before 29 June, and a period of 9 h 54 min 02 s afterwards. D7 formed between D4 and D5 about 15 June, 1972, with the same period as D5. It rapidly caught up, and +2 merged with, D4 after a lifetime of 18 _4 days. D6 formed ahead of D4 at about the same time as D7. Its rotation period before 1 July, 1972 was 9 h 54 min 36 s, changing after 3 July to 9 h 53 min 11 s. It disappeared in mid-July, either by merging with D4, or by merging witb a veil, giving a lifetime of 32 f 1 days. Finally, D8 emerged from a vei1 on 18 July,1972, then merged with another veil in mid-July, giving it
OF LIGHT SPOTS
Eight light spots near to the edge of the NEB in the region 250-350” (Jovian longitude System II) were observed for the period May-July, 1972. We have designated them DI-8. Dland D2 formed some time after 28 April, 1972, and had similar motions, corresponding to a common rotation period of 9 h 54 min 02 s. However, this average period disguises a change around 1-2 June: prior to this date the periods were 9 h 54 min 14 s, and afterwards they became 9 h 53 min 13 s. In the latter part of June, both spots ended their careers by merging with a faint veil lying in the northern half of the equatorial region. The respective lifetimes of Dl and D2 were therefore 54*3 days. The spot D3, which was similar in appearance to Dl and D2, appeared after the passage of a faint veil on 27 May and merged with another veil on 24 June. Hence it had a lifetime of 29 days. Its rotation period was constant at 9 h 53 min 25 s. At the end of May, 1972, two light-coloured features (D4 and D5) appeared in the equatorward
June
July
1972 FIG. 3. VARIATIONSINTHELONGITUDESOF
DURINGJUNEANDEARLYJULY, CATE UNCERTAIN
MEASUREMENTS. ~ILA~A~OMAR~.
SPOTS
Dl-D8
1972. BROKENLINESINDI?-HE EXTREMITIES OF A
338
G. C.
BROWNE
and
a total lifetime of 30 i 1 days. Prior to 1 July, the rotation period was 9 h 54 min 37 s, decreasing to 9 h 52 min 57 s after 3 July. Unlike the dark spots, there is some indication of latitude change in the later development of light spots. LWETIMES AND MOTIONS OF SPOTS
One of the most striking differences between spots A, B and C, on the one hand, and Dl-8, on the other, is their relative lifetimes. The former ranged from 168 to 210 days: the latter varied from 18 to 54 days. Spots A and B were almost certainly holes in the cloud deck. C may have been similar, or it may have been a dark red cloud. The spots Dl-8 were probably ammonia clouds, somewhat higher in the atmosphere. The difference between dark and light spot lifetimes appeared to be typical. It seems that, in the Jovian equatorial regions, holes in the cloud deck are more stable features than are higher lying clouds. Some spots appear to show a permanent change in their rotation periods during their lifetimes. For six of the light-coloured spots this consisted of a marked decrease in the rotation period, which, for four out of the six, was preceded by a brief increase. This change, in some cases, appeared to be connected with the passage of veils in the equatorial region, as did the beginning and end of some light-coloured spots. Indeed, one of our main conclusions is that the passage of such veils strongly affects atmospheric activity on Jupiter. The effect of such veils may sometimes remain unobserved because of a general higher-level haze on Jupiter. As we noted in the introduction, the 1972 apparition was noticeable for the absence of such haze in the equatorial regions. The changes in rotation period are also indicative of interaction between the spots. In this case, there is again a difference between the dark and light spots. The changes in rotation suggest some in-
A. J. MEADOWS
teraction between spots B and C. These were at different latitudes and showed no indication of latitude change throughout their lifetime. Hence, the interaction, if real, implies some type of meridional flow. Interaction was certainly observed between light spots. In particular, the pairs of spots Dl and D2, D4 and D5, D6 and D8, clearly influenced each other, exhibiting similar changes in motion at the same time. All these spots were at approximately the same latitude, so we are here dealing with latitudinal changes in flow. On the other hand, there is some slight indication of a decrease in latitude for these spots towards the end of their lifetimes. If this shift is real (the change in latitude is near the limits of the measuring procedure), it suggests that the spots were disrupted when they moved into a region of different zonal flow. The evidence from the dark and light spots taken together suggests that spot interactions can only be explained by changes in flow covering a fairly large region of the Jovian atmosphere. For we have noted interactions both in latitude and longitude, and at different heights in the atmosphere. Acknowledgements-We would Baum for allowing us access to use of facilities at the Lowell [GCB] wishes to acknowfedge search Studentship and receipt SRC.
like to thank Dr. W. A. the IPP files and for the Observatory. One of us support by an SRC Reof a travel grant from
REFERENCES
Baum, W. A., Millis, R. L., Jones, S. E. and Martin, L. J. (1970). The International Planetarv Patrol ProRram. icasus. 12, 435. Minton, R. B. (1976). Measures of Jupiter photographs1972 aooarition. Icarus. 29, 201. Peek, B. ‘6. (1958). The I%& Jupiter. Faber & Faber, London. Starr, V. P. (1973). A preliminary dynamic view of the circulation of Jupiter’s atmosphere. Pure appl. Geopfiys.
110.2108.