Jovian Temperature and Cloud Variability during the 2009-2010 Fade of the South Equatorial Belt

2.8 MB·

by  Leigh N. Fletcher

Jovian Temperature and Cloud Variability during the 2009-2010 Fade of the South
Equatorial Belt
LeighN.Fletchera,b,G.S.Ortonc,J.H.Rogersd,A.A.Simon-Millere,I.dePaterf,M.H.Wongf,O.Mousisg,P.G.J.Irwina,M.
Jacquessonh,P.A.Yanamandra-Fisherc
aAtmospheric,Oceanic&PlanetaryPhysics,DepartmentofPhysics,UniversityofOxford,ClarendonLaboratory,ParksRoad,Oxford,OX13PU,UK
bDepartmentofPhysics&Astronomy,UniversityofLeicester,UniversityRoad,Leicester,LE17RH,UK
cJetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA,91109,USA
dBritishAstronomicalAssociation,BurlingtonHouse,Piccadilly,LondonW1JODU,UK
eNASA/GoddardSpaceflightCenter,Greenbelt,Maryland,20771,USA
fUniversityofCalifornia,Berkeley,AstronomyDept.,601CampbellHall,Berkeley,CA94720-3411,USA
gInstitutUTINAM,CNRS-UMR6213,ObservatoiredeBesanc¸on,Universite´deFranche-Comte´,Besanc¸on,France
hJUPOSTeam,C/OBritishAstronomicalAssociation,BurlingtonHouse,Piccadilly,LondonW1JODU,UK.
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Abstract
n
a Mid-infrared7-20µmimagingofJupiterfromESO’sVeryLargeTelescope(VLT/VISIR)demonstratethattheincreasedalbedo
J
of Jupiter’s South Equatorial Belt (SEB) during the ‘fade’ (whitening) event of 2009-2010 was correlated with changes to atmo-
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spherictemperatureandaerosolopacity. Theopacityofthetroposphericcondensationclouddeckatpressureslessthan800mbar

] increasedby80%betweenMay2008andJuly2010, makingtheSEB(7-17◦S)asopaqueinthethermalinfraredastheadjacent
Pequatorialzone. AfterthecessationofdiscreteconvectiveactivitywithintheSEBinMay2009,acoolbandofhighaerosolopacity
E(the SEB zone at 11-15◦S) was observed separating the cloud-free northern and southern SEB components. The cooling of the
h.SEBZ(withpeak-to-peakcontrastsof1.0±0.5K),aswellastheincreasedaerosolopacityat4.8and8.6µm,precededthevisible
pwhitening of the belt by several months. A chain of five warm, cloud-free ‘brown barges’ (subsiding airmasses) were observed
-regularlyintheSEBbetweenJune2009andJune2010,bywhichtimetheytoohadbeenobscuredbytheenhancedaerosolopacity
o
rof the SEB, although the underlying warm circulation was still present in July 2010. Upper tropospheric temperatures (150-300
stmbar) remained largely unchanged during the fade, but the cool SEBZ formation was detected at deeper levels (p > 300 mbar)
awithintheconvectivelyunstableregionofthetroposphere. TheSEBZformationcausedthemeridionaltemperaturegradientofthe
[
SEBtodecreasebetween2008and2010,reducingtheverticalthermalwindshearonthezonaljetsboundingtheSEB.Thesouthern
1 SEBhadfullyfadedbyJuly2010andwascharacterisedbyshort-waveundulationsat19-20◦S.ThenorthernSEBpersistedasa
vnarrow grey lane of cloud-free conditions throughout the fade process. The cool temperatures and enhanced aerosol opacity of
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the SEBZ after July 2009 are consistent with an upward flux of volatiles (e.g., ammonia-laden air) and enhanced condensation,
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obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. These changes occurred within cloud decks
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0in the convective troposphere, and not in the radiatively-controlled upper troposphere. NH3 ice coatings on aerosols at p < 800
0mbarareplausiblesourcesofthesuppressed4.8and8.6-µmemission,althoughdifferencesinthespatialdistributionofopacityat
.thesetwowavelengthssuggestthatenhancedattenuationbyadeepercloud(p>800mbar)alsooccurredduringthefade. Revival
1
0ofthedarkSEBcolorationinthecomingmonthswillultimatelyrequiresublimationoftheseicesbysubsidenceandwarmingof
7volatile-depletedair.
1
:Keywords: Jupiter,Atmospheres,composition,Atmospheres,structure
v
i
X
r1. Introduction GreatRedSpot(GRS)atitssouthernedge, istypicallyadark
a
brownstripeencirclingtheglobebetweentwoopposingzonal
Jupiter’s axisymmetric structure, consisting of bright zones flows (Fig. 1a): a prograde (eastward) jet at 7◦S (SEBn) and
and dark brown belts, can undergo dramatic visible changes theplanet’sfastestretrograde(westward)jetat17◦S(SEBs,all
over short time scales. The most impressive of these is the latitudes are given as planetocentric). The SEB is a site of in-
variability of the South Equatorial Belt (SEB, 7-17◦S), which tenseconvectiveactivityandlightningstorms(Ingersolletal.,
can change from the broadest and darkest belt on the planet 2004),andisoneofthefewlocationswherespectroscopically
to a white zone-like appearance over a matter of months. The identifiableammoniaclouds(SIACs, Bainesetal.,2002)have
SEB, which lies in Jupiter’s southern tropics and contains the been observed. However, this activity and the dark coloura-
tionoftheSEBwerecompletelyabsentwhenJupiteremerged
from behind the Sun during the 2010 apparition, replaced by
Emailaddress:[email protected](LeighN.
the pale ‘faded’ state (Fig. 2c). This missing jovian belt cap-
Fletcher)
PreprintsubmittedtoIcarus January5,2017
turedtheimaginationofamateurandprofessionalastronomers niques used to determine temperatures and aerosol opacities.
alike, and it prompted a program of thermal infrared imaging Section3presentsatimelineforthe2009-2010fade,whichis
of Jupiter’s faded SEB from the ESO Very Large Telescope usedtoprovideinsightsintotheunderlyingmechanismsforthe
(VLT) at Cerro Paranal in Chile. These data, along with sup- SEBfadeinSection4.
portingobservationsfromtheNASAInfraredTelescopeFacil-
ity(IRTF)andamateurobservers,willbeusedtodeterminethe
2. ObservationsandAnalysis
variations in temperature and aerosol opacity within the SEB
between 2008 and 2010 and provide insights into the under- 2.1. VLTImaging
lying physicochemical mechanisms responsible for these dra-
High spatial resolution imaging of Jupiter in the Q (17-20
maticmodificationstoJupiter’sappearance.
µm) and N (8-14 µm) bands from the ESO Very Large Tele-
The SEB fade and revival cycles appear to follow a repeat- scope (VLT) mid-infrared camera/spectrograph (VISIR, La-
able pattern, albeit at irregular and unpredictable intervals be-
gageetal.,2004)waspreviouslyusedtoprobetheatmospheric
cause the underlying physical causes are unknown. Excellent
structure of Jupiter’s Great Red Spot (Fletcher et al., 2010b)
historicalaccountsoftheSEBlifecycleatvisiblewavelengths
andtostudytheaftermathofthe2009asteroidalcollision(Or-
can be found in Peek (1958), Rogers (1995) and Sanchez-
tonetal.,2011;Fletcheretal.,2010a). Wewereawardedfour
Lavega and Gomez (1996). The 2009-2010 fade is the start
hoursofDirectorsDiscretionaryTime(program285.C-5024A)
of the fifth SEB life cycle since the first spacecraft encounter
tocharacterisethestructureandcompositionofJupiter’sfaded
withJupiter(Pioneer10inDecember1973),andthefirsttobe
SEBinJuly2010,whichwecomparetopreviousVISIRobser-
investigatedindetailinthethermalinfraredusingthehighspa-
vationsacquiredbetweenMay2008andNovember2009(Table
tialresolutionsandbroadwavelengthcoverageofmoderntele-
1)usingidenticalimagingtechniques.
scopes and instrumentation. Pioneer 10 and 11 visited Jupiter
Jupiter is too large to fit entirely within the VISIR field of
duringthe1972-1975fadedstatepriortotheJuly1975revival
view (32”×32”), so northern and southern hemispheres were
(Rogers,1995;Ortonetal.,1981). Aftera14-yearhiatus, the imagedseparately,oftenondifferentdates. Furthermore,ther-
SEBfadedandrevivedin1989-1990(Yanamandra-Fisheretal., mal imaging requires chopping between the target and an off-
1992;KuehnandBeebe,1993;SatohandKawabata,1994)and
source position to detect the jovian flux on top of the back-
1992-1993(Sanchez-Lavegaetal.,1996;Morenoetal.,1997).
ground telluric emission. However, the maximum chopping
The next ‘partial’ fade began in 2007: New Horizons obser- amplitudeofVLT/VISIRislimitedto25”,meaningthatsome
vationsinJanuary-February2007revealedtheabsenceofboth
ofthe‘sky’imageisobscuredbytheplanet,preventingtheuse
the chaotic turbulence and fresh NH3 ice clouds northwest of ofsomeregionsofthechopped-differencedimage(e.g., white
theGRS(Reuteretal.,2007;Bainesetal.,2007),butretrievals
regionsremovedfrom8.6-µmimagesinFig. 1). Furtherback-
of cloud opacity from VLT/VISIR still demonstrated a cloud- ground stability was achieved by offsetting the telescope to a
freeSEB(Fletcheretal.,2010b). Afadehadstarted,butavi-
skyposition60”fromJupiter(nodding).
olent revival began much earlier than expected, restoring dark
An imaging sequence typically featured eight filters (Table
coloration and turbulent activity by the end of 2007 (Rogers,
2) sensitive to (a) atmospheric temperatures via the collision-
2007a,b). This study concerns the fading event between May
induced hydrogen continuum and stratospheric hydrocarbon
2009andJuly2010.
emission;and(b)troposphericammoniaandaerosols(8.59and
ThegeneralpatternoftheSEBlifecyclecanbesummarised
10.77 µm). Each filter was observed twice, with a small 1-2”
as follows. After the cessation of turbulent rifting and con-
dither to fill in bad pixels on the detector for all eight filters.
vective events to the northwest of the GRS, the SEB (7-17◦S
A full imaging sequence required approximately 40 minutes.
planetocentric latitude) fades to a pale colour over a mat-
TheESOdatapipelinewasusedforinitialreductionandbad-
ter of months, obscuring the southern component of the SEB
pixel removal via its front-end interface, GASGANO (version
(SEB(S), 15-17◦S) and leaving a narrow northern component
2.3.01). Imagesweregeometricallyregistered,cylindricallyre-
(SEB(N),7-10◦S)whichhasalsobeenobservedtofadeinsome
projectedandabsolutelycalibratedusingthetechniquesdevel-
years.DuringthisunusualphasetheGRSappearsasaconspic-
oped in Fletcher et al. (2009b). Radiometric calibration was
uous red oval surrounded by white aerosols. The faded state achieved by scaling the observations to match Cassini/CIRS
can persist for 1-3 years before a spectacular revival begins
measurements of Jupiter’s radiance acquired during the 2000
withasingle,localiseddisturbance(theSEBD).Vigorouserup-
flyby (Flasar et al., 2004). Consequently we cannot investi-
tionsgeneratecomplexpatternswithbrightanddarkcoloration
gate absolute changes in Jupiter’s infrared emission, but we
throughouttheSEB,encirclingtheplanetandultimatelyrestor- canstudyrelativevariabilitybetweendifferentlatitudinalbands
ingthetypicalbrowncolour.
(see Fletcher et al., 2009b, for a complete discussion). Mea-
Theaimofthisresearchistoreconcilephysicochemicalvari-
surementerrorsofapproximately4-6%foreachfilterwereesti-
ability (temperatures, composition, clouds) derived from 7-20
matedfromthevariabilityoftheskybackgroundineachimage.
µmVLTimagingwithvisiblechangesinthealbedooftheSEB.
Alllatitudesinthisstudyareplanetocentric,alllongitudesare
The vertical temperature structure and spatial distribution of
quoted for Jupiter’s System III West (using the standard IAU
aerosolsduringthefadeareusedtodifferentiatebetweendiffer-
entmechanismsforthe‘disappearance’oftheSEB.Section2
introducesthesourcesofinfraredandvisibledataandthetech- 1http://www.eso.org/sci/data-processing/software/gasgano/
2
SEB Fade Sequence I: May 2008 - July 2009
VLT/VISIR 8.6 μm Visible Light IRTF/NSFCAM2 4.8 μm

1100
e (a) 2008-05-17
atitud 00 2008-07-10
L
c --1100
ntri
ce--2200
o
et
an--3300
Pl
--4400
Oval BA
--5500
1 80 1 70 1 60 1 50 1 40 1 30 1 20 1 10 1 00 9 0 8 0 2008-05-13 M. Salway 1 40 1 30 1 20 1 10 1 00 9 0 8 0 7 0 6 0 5 0
System III West Longitude System III West L ongitu de
1100
e (b) 2008-09-03
SEBn EZ Latitud 00 2008-09-24
SEBs SEB centric ----21210000
STrZ o
et
an--3300
Pl
--4400
Oval BA
T (K) --55001 90 1 80 1 70 1 60 1 50 1 40 1 30 1 20 1 10 1 00 9 0 2008-09-01/02 A. Wesley 1 8 0 1 70 1 60 1 50 1 40 1 30 1 20 1 10 1 00 9 0 8 0
150 System III West Longitude Sy stem III W est Lo ngitu de
1100
148 e (c) 2009-07-24
d
146 atitu 00 2009-07-20
L
144 c --1100
142 centri--2200 SEBZ B1
140 eto
138 Plan--3300
136 --4400
--5500
31 0 3 00 2 90 2 80 2 70 2 60 2 50 2 40 2 30 2 20 2 10 2009-07-24 M. Salway/A. Wesley 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 -1 0 -2 0
System III West Longitude System III West Longitude
Figure1:TheSEBfadesequenceMay2008-July2009,observedbyVLT/VISIRat8.6µm(left,sensitivetoaerosolopacityatp<800mbar);visiblelightfrom
amateurobservers(centre);andIRTF/NSFCAM2observationsat4.8µm(right,sensitivetoaerosolsabovethe2-3barlevel). Visibleimagestakenascloseas
possibletotheVISIRobservationshavebeenprovidedbyM.SalwayandA.Wesley. Suppressedemissionatboth4.8and8.6µmiscausedbyexcessaerosol
opacity.NSFCAM2imagesweretakenondifferentnightstotheVISIRimages,anddonotshowthesamelongituderange.Turbulentactivityisevidentinrows(a)
and(b)(the‘wake’oftheGRS),andthechainofthreebarges(B1-B3)arelabelledintheSEB(S)inrow(c).The2008conjunctionofOvalBAandtheGRScanbe
seeninrows(a)and(b).Largewhitearcsseenequatorwardof5◦Sinthe8.6-µmimagesareduetotheremovalofnegative-beamartefactscausedbythesmall20”
choppingamplitude.
3
SEB Fade Sequence II: August 2009-July 2010
VLT/VISIR 8.6 μm Visible Light IRTF/NSFCAM2 4.8 μm

1100
e
ud 0 (a) 2009-08-12 2009-08-06
atit
c L--1100
entri--2200 B1 B1
c
o
net--3300
a
Pl
--4400
--5500
2 90 2 80 2 70 2 60 2 50 2 40 2 30 2 20 2 10 2 00 1 90 2009-08-12 T. Barry 3 30 3 20 3 10 3 00 2 90 2 80 2 70 2 60 2 50 2 40 2 30
System III West Longitude S ystem III W est L ongit ude
1100
EZ ude 00 (b) 2009-11-12 2009-08-27
SEBn atit
SEBs SEB entric L----21210000 B3 B2 B1
STrZ oc
net--3300
a
Pl
--4400
--5500
T (K) 3 50 3 40 3 30 3 20 3 10 3 00 2 90 2 80 2 70 2 60 2009-11-14 C. Go 3 80 3 70 3 60 3 50 3 40 3 30 3 20 3 10 3 00 2 90
System III West Longitude Sy stem III West Lo ngitu de
150 1100
148 ude 00 (c) 2010-07-13 GRS White Spot 2010-07-01
146 atit
L--1100
144 c
142 centri--2200 SEBZ
o
140 net--3300
a
138 Pl
--4400 Oval BA
136
--5500
6 0 5 0 4 0 3 0 2 0 1 0 3 60 3 50 3 40 3 30 3 20 2010-07-14 J.P. Prost 3 50 3 40 3 30 3 20 3 10 3 00 2 90 2 80 2 70 2 60 2 50
System III West Longitude System III West Longitude
Figure2: TheSEBfadesequenceAugust2009-July2010,observedbyVLT/VISIRat8.6µm(left,sensitivetoaerosolopacityat p < 800mbar);visiblelight
fromamateurobservers(centre);andIRTF/NSFCAM2observationsat4.8µm(right,sensitivetoaerosolsabovethe2-3barlevel).Visibleimagestakenascloseas
possibletotheVISIRobservationshavebeenprovidedbyT.Barry,C.GoandJ.P.Prost. Suppressedemissionatboth4.8and8.6µmiscausedbyexcessaerosol
opacity.NSFCAM2imagesweretakenondifferentnightstotheVISIRimages,anddonotshowthesamelongituderange.Thechainofthreebarges(B1-B3)are
labelledintheSEB(S)inrow(b).The2010conjunctionofOvalBAandtheGRScanbeseeninrow(c).Largewhitearcsseenequatorwardof5◦Sinthe8.6-µm
imagesofrowaareduetotheremovalofnegative-beamartefactscausedbythesmall20”choppingamplitude,whichwasincreasedto25”inNovember2009.
4
definition of System III rotation (1965), recently reviewed by maps in eight filters (7.9, 8.6, 10.8, 12.3, 13.0, 17.7, 18.7 and
RussellandDougherty,2010). 19.5µm)werestackedtoformrudimentaryimagecubesusing
Examples of the VISIR imaging at 8.6 µm between May the procedure described by Fletcher et al. (2009b). The tem-
2008 and July 2010 (where low fluxes indicate either elevated perature structure, composition and aerosol distribution were
aerosolopacitiesorcoolatmospherictemperatures)areshown derived from the 8-point spectra using an optimal estimation
in Figs. 1 and 2. These are compared to examples of ama- retrieval algorithm, NEMESIS (Irwin et al., 2008). Sources
teur images at visible wavelengths and Jupiter’s 4.8-µm emis- of spectral line data; conversion of line data to k-distributions
sionondatesclosetothoseoftheVISIRobservationsobtained and Jupiter’s a priori atmospheric structure were described in
by NASA’s Infrared Telescope Facility NSFCAM2 instrument detail by Fletcher et al. (2011). Two different retrieval ap-
(Table 3). Jupiter’s 4.8-µm emission is attenuated by aerosols proachesweretested:(i)asingle-stageprocessretrievingT(p),
above the 2-3 bar level, so dark regions indicate locations of NH ,C H andaerosolopacitysimultaneously,and(ii)atwo-
3 2 6
higheropacity. Althoughthedatesandspatialcoverageofthe stageprocessderivingtemperaturesfirst(fromthe7.9µmand
mid-IRand4.8-µmimagesdiffer,theirzonalpropertiescanbe 13.0-19.5 µm filters) before the retrieval of composition. At-
compared to provide insights into the vertical distribution of mospheric temperatures were derived as a vertical profile de-
cloudopacityduringthefadesequence(Section3). fined on a grid of 80 layers between 10 bar and 0.1 mbar,
whereas Cassini-derived profiles of aerosol opacity, NH and
3
C H (Fletcher et al., 2009a; Nixon et al., 2007) were simply
Table1: VLT/VISIRObservationsofJupiter’sBelts. UTtimesareapproxi- 2 6
scaled to reproducethe VISIR spectra. The two-stage process
mate,aseachfiltersequencerequiredapproximately45minutestoexecute.
was found to produce unreliable results because the compet-
Date Hemisphere Time(UT) ProgrammeID ingeffectsoftemperatureandaerosolopacitywereinseparable
2010-07-13 S 08:10 285.C-5024(A) withoutusingall8filterssimultaneously. Thusthesingle-stage
2010-06-23 N 08:50 285.C-5024(A) retrievalwasusedforeachoftheobservationsinTable1.
2009-11-12 S 01:30 084.C-0206(B) Radiances from each observation in Table 1 were averaged
2009-08-10 N 03:00 383.C-0161(A) over a 10◦-longitude range surrounding the central meridian,
2009-08-11 S 02:30 383.C-0161(A) andinterpolatedontoa1◦latitudegrid. Carewastakentoavoid
2009-08-12 S 03:00 383.C-0161(A) largeovals(theGRS,OvalBA)topreventspuriousresults,such
2009-07-24 S 03:30 283.C-5043(A) thatcentralmeridianscansarerepresentativeofthezonalmean
2008-09-03 S 04:00 081.C-0137(C) radiances on each date. Atmospheric temperatures, aerosol
2008-09-01 N 02:00 081.C-0137(B) opacity and the distributions of tropospheric NH and strato-
3
2008-05-17 S 10:00 381.C-0134(A) sphericC H wereretrievedsimultaneouslyforeachlatitudeto
2 6
determinemeridionalvariationsineachquantity. Examplesof
thequalityofthe8-pointspectralfitsateachlatitudeareshown
in Fig. 3, which compares synthetic radiances with measure-
mentsatthecentreoftheSEB(12.5◦S)onthreedifferentdates.
Table3:NSFCAM24.8-µmdatausedinthisstudy
Calibratedradiancesweresimilarforallthreeepochs,withev-
Date Time(UT) CentralMeridian idence of a decrease in emission at 13.0 and 8.6 µm between
2008-05-27 13:53:39 334.9 2008and2010. Conversely,variationsofzonalmeanradiances
2008-06-16 13:45:26 103.3 at 10.8 and 12.3 µm (sensitive to NH3 and C2H6 respectively)
2008-07-10 13:06:44 96.1 werenegligible,suggestingthattheVISIRphotometrydoesnot
2008-07-27 12:52:05 224.6 showevidenceforNH3 orC2H6 variabilityduringthefade. It
2008-08-07 07:22:50 145.9 islikelythatdetectionofgaseousvariabilityduringanSEBlife
2008-09-24 06:22:15 133.2 cyclewouldrequirespectroscopyratherthanphotometry. Nev-
2009-07-20 14:09:23 27.6 ertheless,atmospherictemperaturesandaerosolscouldbereli-
2009-08-06 10:30:41 296.9 ably separated using the 8-point spectra and will be discussed
2009-08-18 13:19:15 46.9 inSection3.
2009-08-27 13:45:42 338.8 The retrieved haze opacity is driven by the 8.6-µm bright-
2010-06-24 15:50:41 319.9 nesstemperaturesplottedontheleftofFigs. 1-2,wherelower
2010-06-30 14:15:45 85.9 brightness temperatures are the result of higher aerosol opaci-
2010-07-01 15:47:43 292.1 ties. However,the8-pointspectralacktheinformationcontent
2010-09-03 08:26:22 305.4 required to determine the vertical distribution of this aerosol
2010-09-05 10:04:33 306.1 opacity. Cassini/CIRS spectral analyses (Wong et al., 2004;
Matchevaetal.,2005;Fletcheretal.,2009a)demonstratedthat
the8-11µmwavelengthrangewasbestreproducedbyacom-
pact cloud layer of 10-µm radius NH ice particles near 800
3
2.2. Mid-IRRetrievals mbar,althoughneitherthecompositionnorthealtitudeofthese
To separate the effects of temperature, aerosol opacity and aerosols were particularly well constrained. Using the CIRS-
compositionontheVISIRfilteredimaging,cylindricalradiance derived aerosol distribution, we scale the cumulative optical
5
Table2:VLT/VISIRFiltersusedinthisstudy.ApproximatepeaksofthefiltercontributionfunctionsarebasedonFletcheretal.(2009b).
Name Wavelength(µm) Sensitivity Approx. Pressure(mbar)
J7.9 7.90 StratosphericT(p) 5
PAH1 8.59 AerosolsandT(p) 650
SIV2 10.77 NH ,aerosols,T(p) 400
3
NeII 1 12.27 StratosphericT(p)andC H 6
2 6
NeII 2 13.04 TroposphericT(p) 460
Q1 17.65 TroposphericT(p) 200
Q2 18.72 TroposphericT(p) 270
Q3 19.50 TroposphericT(p) 400
depth of the aerosols above the 800-mbar level to reproduce thisinhibitionofconvectiveactivitycannotbeidentifiedinthe
the8-pointspectraateachlatitude. timesequencepresentedhere,itsignalledthestartofaremark-
abletransformationintheSEBoverthefollowing12months.
3. Results: TheSEBFadeTimeline
3.2. FormationoftheSEBZ(July-August2009):
Acomparisonofthevisible, 8.6and4.8-µmimagesinJuly
This following sections compare temperature and aerosol
2009(Fig. 1c)showsasubstantialalterationtothedistribution
distributions derived from VLT 7-20 µm filtered imaging to
of aerosol opacity within the SEB at a time when the visible
both IRTF 4.8-µm images of deep cloud opacity and ama-
colours of the SEB were largely unaltered. At this time, the
teur imaging of the visible coloration to reveal the sequence
SEB had a pale interior separating a narrow brown northern
of changes occurring during the SEB fade. The timeline of
component (the SEB(N), 7-10◦S) and a broad brown southern
SEBchangesbetween2008and2010issummarisedinTable4,
component (the SEB(S), 14-18◦S). Because there is minimal
alongwithaselectionofkeycharacteristicsofthefadeinTable
gas opacity at 4.8 µm, strong thermal emission implies a rela-
5.
tive dearth of aerosol opacity above the 2-3 bar pressure level
(e.g., Roos-Serote et al., 1998); whereas 8.6-µm is sensitive
3.1. InitialStateoftheSEB(Pre-May2009)
only to clouds and hazes above the 800-mbar level. An en-
Filamentary turbulent convective activity usually dominates hancementincloudopacityabovethe2-3barlevelinJune-July
theSEBtothenorthwestoftheGreatRedSpot(GRS)inare- 2009 caused a substantial reduction in 4.8-µm emission from
gion known as the ‘GRS wake.’ The chaotic activity is driven the SEB, restricting cloud-free conditions to the SEB(N) and
bytheconvergenceofasystemofcomplexatmosphericflows, SEB(S). The transformation at 8.6 µm was equally dramatic -
whereretrogradeflow(fromtheeast)at17◦Sisdeflectednorth- thefinestructuresthatcharacterisedtheVISIR8.6-µmimaging
wardaroundtheperipheryoftheGRStomeettheprogradeflow in2008hadbeenreplacedbyamorediffuseappearance,witha
(fromthewest)at7◦S.Thistypicalstatewasobservedthrough- narrowdarklane(8-13◦S)ofelevatedopacityatthe800-mbar
out 2008 (Fig. 1a-b), where the SEB between 7-17◦S has a level.ThislaneofelevatedaerosolopacityisknownastheSEB
higher temperature than the equatorial zone (EZ) to the north zone(SEBZ).
andtheSouthTropicalZone(STrZ)tothesouth. Cloudopac- Fig. 4 shows how the establishment of the SEBZ modified
itymeasuredatboth4.8and8.6µmistypicallylow,sothatthe central-meridian brightness temperatures across the SEB be-
SEBappearsrelativelybrightatbothofthesewavelengths.This tween 2008 and 2010. These brightness temperatures are av-
is consistent with the typical view of upper tropospheric belt- eraged within 10◦ longitude of the central meridian for each
zone circulation (e.g., Ingersoll et al., 2004) whereby air rises filter, so are not true zonal averages. Hence, some of the ap-
attheequatorandsubsidesovertheneighbouringbelts, creat- parentvariabilityresultsfromthepresenceofdiscretefeatures
ingwarm,cloud-freeandvolatiledepletedconditionswithinthe (waves,vortices)closetothecentralmeridian. Changesassoci-
SEBandNEB(NorthEquatorialBelt). Smallwhitespotsand atedwiththeSEBfadebetween7and20◦Saremostdramaticat
riftingobservedinthevisibleinFig. 1a-bcoincidewithhigher 8.6 µm, with the whole region becoming darker in 2009-2010
aerosol opacities at 8.6 µm and cooler tropospheric tempera- than the south temperate region poleward of 25◦S. Variations
tures, indicating localised upwelling within the generally sub- at other wavelengths are more subtle: filters with contribution
siding belt. Such upwelling transports spectroscopically iden- functions sensitive to upper tropospheric temperatures (150-
tifiable NH ice (SIACs, Baines et al., 2002) upwards into the 300 mbar, approximately, for 17.6 and 18.7 µm, Table 2) re-
3
GRSwakeregion. mainedlargelyunchangedthroughoutthefadesequencewhen
Table 4 shows that this SEB activity continued until May- compared to mid-IR variability at other latitudes. Conversely,
June2009,whenamateurimagingrevealedtheabsenceofany close inspection of images probing higher pressures (p > 300
convective white spots (Rogers, 2010a), just prior to the first mbar: 8.6, 10.8, 13.0 and 19.5 µm) demonstrate the develop-
mid-IR images in July 2009 (Fig. 1c). Although the cause of mentofamulti-peakedstructure,withradiancemaximaat9◦S
6
Table4:Timelineofeventsinthe2009-2010Fade
Date Event PossibleImplication
2008-Sep Typical state of SEB (Fig. 1a): visibly brown, warm troposphere and Subsidence over much of the SEB, lo-
cloud-free;turbulentactivityNWoftheGRS calisedconvectiveupwelling.
2009-May-26 TurbulentconvectionNWoftheGRShadlargelyceased(Fig. 1c) Inhibitionofdiscreteconvectiveupwelling
bysomemechanism.
2009-Jun-05 Final bright convective spots observed NW of the GRS, SEB still ap- CessationofSEBturbulencecomplete.
pearsvisiblybrown
2009-Jun-15 First dark brown cyclonic barge (B1) appeared west of the GRS BargesareanewfeatureofthefadedSEB;
(Rogers,2010a)inpalebrownSEB(S)(e.g.,Fig. 1c) four more formed at progressively more
westerlylongitudes.
2009-Jul-20 High-opacitySEBZobservedat4.8µm(deepclouds)separatingcloud- Zone-likeconditionsbeginningatdepth.
freeSEB(N)andSEB(S),visiblybrowncolourstillpresent(Fig. 1c).
2009-Jul-24 NarrowlaneofSEBZhigheropacityobservedat8.6-µm(upperclouds); AerosolopacityinnewSEBZappearsdif-
SEB(N)andSEB(S)appeardiffuseandcloud-free(Fig. 1c) ferentin4.8µm(deepclouds)and8.6µm
(uppercloud)images.
2009-Aug-04 Fifthbrownbarge(B5)formsfurthestwestoftheGRS Circulation forming/revealing brown
bargeshaspropagatedwestward.
2009-Aug-06 Opacity of SEB(S) and SEB(N) equal at 8.6-µm, but 4.8-µm opacity SEB(S)fadewestofGRShadbegun.
considerablyhigheroverSEB(S)thanSEB(N);SEB(S)8.6-µmopacity
is higher west of the GRS than to the east (Fig. 2a); brown colour of
SEB(S)fadingwestofGRS
2009-Oct BrowncolouroftheSEB(S)eastoftheGRSbeginstofade,bulkofthe Fade had progressed around the planet
SEBacquiredapaleorangetint. fromitsstartingpointwestoftheGRS.
2009-Oct-04 BrilliantwhitespotobservednorthoftheGRS(Rogers,2010a) Uncertainconnectiontofadeprocess
2009-Nov All barges persist in faint red-brown southern SEB, only SEB(N) re- Fadehasnotyetcompleted,butcoolSEBZ
mains relatively cloud-free; SEB(S) has high cloud opacity at 4.8 and isfullyestablished.
8.6µmandcannotbedistinguishedfromSEB(Fig. 2a)
2010-Jan End of 2009 apparition; SEB(N) narrow and dark; most of SEB pale Fadehadnotyetcompleted.
orange;5bargesdarkandconspicuous
2010-Apr Start of 2010 apparition, all of SEB is pale, brown barges still faintly SEBfadeproceededalmosttocompletion
visible whileJupiterwasobscuredbytheSun.
2010-Jul-01 SEB(S) is completely absent in 4.8-µm imaging, SEB(N) can still be Deep cloud opacity completely covers
seen (Fig. 2c). Visible images show brown barges have almost com- SEB
pletelyfaded,butstillpartiallyoutlinedbyfaintblue-greypatches.
2010-Jul-13 SEB(S)(narrowandundulating)andSEB(N)(broad)aredetectableat SEBfadehascompleted.
8.6 µm, barges cannot be distinctly observed (except at 10.8 µm, Fig.
5);centralSEBcoveredbyhighopacitycloud(Fig. 2c)
7
Table5:CharacteristicsofnoteduringtheSEBfade
Characteristic TypicalLatitude TemporalBehaviour
GRSWake 7-20◦S Filamentary turbulence normally dominates region NW of GRS (Fig. 1a-b; became
quiescentinMay-June2009(Fig. 1c);preludetotheformationofSEBZandthefade.
SEB(N) 7-12◦S Dark, narrow brown lane separating SEB and EZ (Fig. 1a); warm cloud-free condi-
tionspersistedthroughoutfade;opacityincreasedbutdidnotobscure4.8and8.6-µm
emissioncompletely(Fig. 2c).
SEB(S) 15-18◦S Broad dark band of variable width separating SEB and STrZ; began to fade west of
GRSinJune-July2009; eastoftheGRSbyOctober-November2009. ByJuly2010
deepcloudopacitycompletelyobscuredSEB(S)4.8-µmemission;whereasanarrow
laneoflowupper-cloudopacity(8.6µm)wasvisible(Fig. 2c).
SEBZ 11-15◦S Coolzone-liketemperaturesdevelopedJuly-August2009for p > 300mbarandper-
sisted to July 2010 (Fig. 5; Fig. 9); coincides with increased opacity of SEB centre
andreductionofzonalwindshearinuppertroposphere(Fig. 6).
SEB(S)Undulations 17-19◦S Unique characteristic of fully-faded state in July 2010 (Fig. 2c); zonal oscillations
ofcloudopacitywith5-6◦longitudewavelengthandretrogrademotion;furthersouth
thanthetypicalSEB(S)edge.
SEB(N)Projections 6-9◦S Persistent features of SEB(N), wave activity at the boundary between clouds of the
EZandSEBmixescloudopacity. Nochangeduringthefade.
BrownBarges 14-16◦S FivedarkbrownbargesintheSEB(S)(B1-B5)formedwestoftheGRSbetweenJune-
August2009(astheSEBZwasforming,Fig. 2). Allwerecloud-freewithwarmcore
temperatures. Not observed in previous fades (Sanchez-Lavega and Gomez, 1996).
BargesfadedbyJuly2010,butwarmcycloniccirculationwasstillpresent(Fig. 5).
GRSWhiteSpot 9-11◦S Whitecloud(vigorousconvectiveplume)nearNedgeofGRSinJuly2010;high8.6-
µmopacity(Fig. 10c),givesrisetoablue-greystreaknorthwestoftheGRS(Rogers,
2010a). Uncertain connection to the fade process, but may have also been present
during1989-1993fades(Rogers,2010a).
SEBD 7-20◦S ExpectedvigorousconvectivedisturbancethatwillsignalthestartoftheSEBrevival.
8
(SEB(N))and15-17◦S(SEB(S)),andazone-likeminimumat
12◦S(theSEBZ).ThetransitionfromtheturbulentSEBtothe
cool SEBZ by late 2009 is clearly shown in maps of 10.8-µm
brightnesstemperatures(Fig. 5),sensitivetotemperaturesand
(a) VISIR Radiances
ammonianearthe400-mbarlevel.
Wavelength (μm)
20.017.0 14.0 12.0 10.0 9.0 8.0 3.2.1. SEBZTemperatures
400 Thechangesinthemid-IRradianceinFig. 4manifestthem-
2008 Sep
selvesasvariationsintheretrievedtropospherictemperaturesin
2009 Nov
Fig. 6, particularlythoseathighpressures(480and630mbar
)
-1m 2010 Jul inthepanelsfandh), indicatingadifferenceinthelatitudinal
c
r/ temperature gradient between 2008 and 2009-10. Uncertain-
s
2m/ 100 tiesintheabsolutecalibrationoftheVISIRimages,combined
c with the broad pressure range covered by the weighting func-
W/ tion for each filter (Fletcher et al., 2009b) and the degenera-
n
( cies between temperature and composition, lead to consider-
e
c ableretrievaluncertaintiesfortemperaturesat p>500mbarin
n
a Fig. 6e-h, so that absolute cooling is difficult to detect at the
di
a SEB latitude. On the other hand, the presence of the distinct
R
cool zone (SEBZ) from 2009 onwards, relative to the warmer
SEB(N)andSEB(S),canbeseenat480and630mbarinFigs.
10 6f-h. Temperaturecontrastsof1.0±0.5KbetweentheSEBZ
600 800 1000 1200 and the northern and southern components were measured at
Wavenumbers (cm-1) 630mbarinJuly2010(Fig. 6h). OfthethreeQ-bandfiltersin
(b) VISIR Brightness Temperatures Fig.4,onlythedeep-sensing19.5-µmfilter(whichhasthelow-
Wavelength (μm) est diffraction-limited resolution of all the images used in this
20.017.0 14.0 12.0 10.0 9.0 8.0 study)detectedtheSEBZ,confirmingthattheSEBZformation
occurred at depth. The cool SEBZ was not observed at lower
150
2008 Sep
pressures (150-300 mbar) sensed by 17.6- and 18.7-µm filters
2009 Nov
) wheretheSEBretaineditsusualwarmbelt-likeconditions.
K 2010 Jul
( Indeed, the temperature fluctuations in the stable upper tro-
e
r posphere between 15-300 mbar were small compared to those
u 140
at at other latitudes, particularly those associated with the NEB
r
e
p (see Appendix A). The SEBZ formation at depth did have a
m subtle effect at these lower pressures - both the retrieved tem-
e
T
s 130 peraturesnear240mbar(Fig. 6d)andtherawradiances(Fig.
s 4)demonstratea‘flattening’ofthemeridionaltemperaturegra-
e
htn dient (dT/dy) between 7-20◦S as the fade progressed. Instead
g of having a belt with a peak temperature in the centre (12◦S),
ri
B
the240-mbartemperaturesduringthefadedstatebecamemore
120
homogenised with latitude (dT/dy tends to zero between 10-
15◦S at 240 mbar, Fig. 6d). As vertical shears on the zonal
600 800 1000 1200 wind (du/dz) are related to dT/dy by the geostrophic thermal
Wavenumbers (cm-1) windshear equation, this reduction of dT/dy in 2009-10 com-
pared to the normal state of the SEB suggests a reduction in
Figure3: Comparisonofsynthetic8-pointspectra(lines)tomeasuredzonal the windshear on both the prograde SEBn jet at 7◦S and the
mean radiances (points) at the centre of the SEB (14.5◦S) at three different retrogradeSEBsjetat17◦Sinthetroposphere. Zonalflowas-
epochs-September2008(normalstateoftheSEB);November2009(midway
sociatedwiththesetwoopposingjetscouldthereforepersistto
throughthefade)andJuly2010(oncethefadehadcompleted). TheNEME-
SISretrievalmodeliscapableofreproducingthemeasuredradiancesinall8 higheraltitudes(i.e., nearthetropopause)duringafadedstate
filters.Therewerefewdifferencesinthecalibratedradiancesbetweenthethree thanduringthe‘normal’state.
epochs,exceptacoolingtrendat8.6and13.1µmasthefadeprogressedand Finally,stratospherictemperaturesabovetheSEBshowcon-
7.9-µmvariabilityduetoJupiter’squasi-quadrennialoscillation(QQO).Radi-
siderable variability between 2008 and 2010 (Fig. 6a) as part
ancesinthetoppanelwereconvertedtobrightnesstemperaturesinthebottom
panel.ThesensitivityofeachoftheeightfiltersisdescribedinTable2. ofJupiter’sQuasiQuadrennialOscillation(QQO,Leovyetal.,
1991), which affects the stratospheric temperatures between
20◦Nand20◦Swithaperiodof4-5years. However,nocausal
connection between the fades and the stratospheric oscillation
is found: (i) the regularity of the QQO means that it cannot
9
(a) 8.6 μm (b) 10.8μm
K) K)
e ( e ( 150
atur 170 atur 145 2008 Sep
per per 140 2009 Jul
m 160 m
e e 2009 Aug
ess T 150 ess T 113305 22001009 JNuonv/Jul
n n
ght 140 ght 125
Bri 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 Bri 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60
Planetocentric Latitude Planetocentric Latitude
(c) 13.0μm (d) 17.6μm
K) K)
ure ( 140 ure (
perat 135 perat 125
m m
e e
s T 130 s T 120
s s
e e
n n
ght 125 ght 115
Bri 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 Bri 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60
Planetocentric Latitude Planetocentric Latitude
(e) 18.7μm (f) 19.5μm
K) K)
e ( 130 e (
atur atur 130 2008 Sep
er er
p 125 p 2009 Jul
m m 125
e e 2009 Aug
T T
ss 120 ss 2009 Nov
e e 120 2010 Jun/Jul
n n
ht ht
g g
Bri 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 Bri 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60
Planetocentric Latitude Planetocentric Latitude
Figure4:Centralmeridianbrightnesstemperaturescansfrom2008to2010insixoftheeightfiltersobservedbyVISIR.Northernandsouthernhemisphereimages
werenotacquiredsimultaneously,soagapispresentattheequatorfortheearliestdates(Table1). ThelocationoftheSEBisdenotedbyverticaldottedlines.
Thebrightnesstemperatureshavebeenoffsetfromthe2010measurements(blackcurve)byarbitraryamountsforclarity.Themostdramaticchangeoccuredat8.6
µm(decreasedemissionduetotheincreasedaerosolopacityovertheSEB).TheformationofthecoolSEBZisalsoapparentinfilterssensitivetotemperaturesat
p>300mbar(10.8,13.0and19.5µm),butonlyasubtle‘flattening’ofthemeridionaltemperaturescanbeobservedat17.6and18.7µm(sensitiveto150-300
mbar).Brightnesstemperatureshavenotbeencorrectedforemissionangle,sothegeneraldecreasefromequatortopoleineachfilteristheeffectoflimbdarkening.
10