GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 12, PAGES 1565-1568,JUNE 15, 1995
Direct observationsof the Comet Shoemaker-Levy9 fragment G
impact by Galileo UVS
C. W. Hord, W. R. Pryor, A. I. F. Stewart,K. E. Simmons,J. J. Gebben,C. A. Barth,
W. E. McClintock, andL. W. Esposito
Labo•:atory
forAtmospheric
andSpace
Physics,
University
of Colorado,
Boulder,
Colorado
W. K. Tobiska,R. A. West, S. J. Edberg,J. M. Ajello, K L. Naviaux
Jet PropulsionLaboratory,Pasadena,California
Abstract. The Galileo Ultraviolet Spectrometer(UVS) team has nm the sensitivityof thistubedropssharply.An F-channelfixed
detectedthe Shoemaker-Levy9 fragmentG impacton Jupiterin wavelength
of 292 nm (1.4 nm bandpass),nearthepeakresponse
data recentlyplayed back from the spacecrafttape recorder. A to Jupiter (Fig. 1), was chosento maximize the probability of
20% brighteningof the disc-integratedsignal of Jupiter was detectingthermalemissionfrom the impact. This wavelengthis at
detectedat 292 nm during a swathacrossJupiterthat lasted 1.6 the energyemissionpeak for blackbodythermalradiationat a
secand was centeredat 1994-July18 (day 199)/07:33:31UT (all temperatureof 10n K. There are no distinctiveabsorption
or
timesin thispaperarecorrectedto be the timeof theeventasseen emissionfeaturesin the reflectedsolarspectrumat 292 nm.
from Earth). The emissionbrightness,when combinedwith
During the fragmentG observationon July 18, 1994, Galileo
simultaneousPhotopolarimeterRadiometer(PPR) measurements was 240 million km from Jupiter,whose angulardiameterwas
at 945 nm, is consistent
with thermalradiationat a temperature
of 0.034 deg. The planetundeffilledthe UVS instrumentslit (0.10ø
7800 (+500, -600) K emiuedoveran areaof 40 (+60, -25) km2. x 0.33ø). The angularrate of the scanplatform sweepswas such
No excesssignal was seenduring swaths5 1/3 sec before and that Jupitertraversedthe 0.1o width of the UVS slit in about1.6 s.
after the detection swath.
The observing geometry is shown in Fig. 2. Brightness
measurements
werereportedevery 7.6 ms, eachrepresenting
6 ms
of
integration.
Introduction
The Galileo spacecraft(Johnsonet al., 1992) witnessedthe
uniqueComet Shoemaker-Levy9 (SL9) impacteventson Jupiter.
The sun-Jupiter-spacecraft
phaseangleof 51 deg provideddirect
viewing of the fragment G impact event, which occurredjust
beforedawnbeyondthe limb of Jupiteras seenfrom Earth. The
G impact site was at planetocentriclatitude 43.66+1.0 ø and
system121Ilongitude 26.8+2.0ø (Hammel et al., 1995). Three
Galileo scanplatform instruments:the Near-InfraredMapping
Spectrometer (NIMS), the PPR, and the UVS performed
simultaneous,
boresightedmeasurements
duringthe periodwhen
theimpactof fragmentG wasexpected.The scanplatformswept
the instrumentalfields of view acrossJupiterfrom the dark to
brightlimb and thenback in a patternthat repeatedevery 10 2/3
sec. This paper will briefly discussand interpret the UVS
detectionof the fragmentG impactevent.
Observations
Galileo UVS data at 292 nm from 73 full or partial 1.6 s
traversesacrossJupiter were returned to Earth. Of this set, 33
were sequentialswathsfrom the period 1994-199/07:33:12to
07:36:06. A totalof 53 swathswereused(includingthedetection
swath)in this analysis.The wholedataset spannedthe period
from 07:31:44to 07:41:51. The averagecountrate from 52 such
traverses
(withno evidence
of excesssignal)was6.6 countsper6
ms at 292 nm, corresponding
to an energyflux at Galileo of
2.1x1044
wattcm'2 nm4. Onlyonetraverse
across
Jupiterat
1994-199/07:33:31,detailedbelow,producedUVS countlevels
significantly
differentfromtheaverage.The averagecountlevels
for eachtraverseweredistributed
according
to Poissonstatistics,
as expected, except for the one bright swath. Jupiter
measurements
5 1/3 s earlier and5 1/3 s later thanthe bright
swathwerenot significantlydifferentfromtheotherbackground
Experimental Design
Jupitermeasurements,
indicatingthattheUV flashwasbrief.
The impactflash was seenduringa right-to-lefttraverseof
The UVS observingstrategyfor the fragmentG observation
was chosento searchfor thermalblackbodyemissionfrom the
impact event. The Galileo UVS instrumentusesa Cassegrain
telescope,an Ebert-Fastiespectrometer,
3 photomultipliertubes
anda scanninggratingdrive to recordUV spectrafrom 115430
Jupiter acrossthe UVS field of view. On such traverses,the
terminator
crossed
intothe slit first,followed0.45 s laterby the
brightlimb. The average
of 15 suchtraverses,
notincludingthe
impacttraverse,
is shownin Figure3 asa heavyline. Alsoplotted
nm (Hordet al., 1992). The UVS F-channel
hasa photomultiplier is the impact traverse;both curveshave been smoothedover 13
or 0.1 s. All thetraverses
werehand-aligned
beforethe
tube with a cesiumtelluridephotocathode
(160-320 nm) and is samples
suitablefor observingplanetsilluminatedby sunlight.Above300 average was taken, to counteract the effects of small errors in
pointingand/ortiming. The RMS correctionwas0.09 mrador 80
ms. Theimpactsiteentered
theslit0.42s beforethebrightlimb,
makingtheimpactswathwiderthanthebackground
swaths.In
Copyright1995 by the AmericanGeophysical
Union.
Figure3 theimpacttraverse
wasaligned
withtheaverage
at the
right-handedge,afterthe impactsitehad left the field of view.
Papernumber95GL01414
Thedifference
between
thesetwocurves
is shownasa histogram
0094-8534/95/95GL-01414503.00
representingthe averageexcesscountsseenin successive0.1 s
1565
1566
HORD ET AL.: FRAGMENT
Undisturbed Jupiter spectrum
8
'
'
'
I
'
'
'
I
'
'
'
I
G IMPACT
GALILEO SHOEMAKER-LEVY FRAGMENT-G GEOMETRY
I
.
Selected WavelengthRegion
for Fragment 0 Photometry
II
6.
2
Jupiter
0
i
Sun
i
220
a
I
t
J
240
•
I
260
a
•
•
I
280'
I
300
,
•
,
-- '
320
PPR
Wavelength(nm)
UVS
Figure 1. A Galileo UVS F-channeldisc-integrated
undisturbed
spectrumof Jupiterobtainedon 1994 day 195. An estimateof the
scattered light contribution has been subtracted. The fixed
wavelengthof 292 nm selectedfor the G fragmentobservationis
indicated.A SUSIM solarspectrum(Van Hoosieret al., 1988)is
alsoshown,degradedin wavelengthto matchtheGalileo UVS 1.4
nm resolution. The solar spectrumhas been convalvedwith the
UVS wavelengthresponsefunctionto simulatean observationof
NIMS
Figure 2. The observinggeometryfor the G fragmentexperiment
showingJupiteris much smallerthan the Galileo UVS slit which
is swept acrossit at right anglesto the slit length. During the
detectionswaththe slit encountered
first the darklimb of Jupiter
then the bright limb. The Galileo NIMS and PPR instrumental
thesunandthenarbitrarily
scaled.Jdpiter
isbluerthanthesun
from2400-3200]t, asfoundin earlierwork(Wageher
et al.,
fields of view are also indicated.
1985).
10-Is wattscm-2nm-1atthespacecraft
occurred
at thesametime
intervals.Thebeststraight-line
fit to the16intervals
containing
theimpactflashis alsoplotted.The average
excess
is 1.33ñ0.29 that PPR recordedan initial excesssignalof (1.1 + 0.2)x104s
wattscm'2nn-t
'l at 945nm(Martinet al., 1995).Thisleadsto an
counts/6
ms,or 20%of thesignalfromJupiteritself.
1-o error bars based on the counts in the 0.1-s intervals are energyflux ratio E292 nm/E945 nm of 3.9 + 1.1, which is
of thermalblackbody
radiationat7800(+500,-600)
shownon two representative
intervals.The standard
deviationof characteristic
K. Thereis someuncertainty
in the945nm excess(seeMartinet
theresiduals
aboutthebestfit is 1.2counts/6
ms,veryclosetothe
1.3counts/6
msexpected
onstatistical
grounds.
Whilevariations
in theflashbrightness
cannot
beexcluded,
onlytheresidual
near
1.4 s elapsedtime deviatessignificantlyfrom a normal
GLL
UVS
ß S-L9
G
_
distribution.
_
_
_
Figure4 compares
thecountlevelson 52 traverses
(15 right-toleft, 37 left-to-right)with the impacttraverse.Many traverses
containdata dropouts,and the values in Figure 4 are for the
(a
central 1.25 s of each traverse,where the count rams are flat and
•;
dropoutsarethereforeeasilyallowedfor. Excludingthe impact
traverse,the averageof thesepointsis 6.61 counts/6ms with a •
standarddeviationof 0.20. The flashbrightness
of 1.33counts/6 z
ms thusrepresentsa 6-• detection. (Becausethe traverseof the
impactflashacross
theUVS fieldof viewdidnotfully overlapthe
1.25 s samplinginterval,the impacttraverseshowsan excessof
only 1.11 counts/6ms in Figure4.) The UVS observationat 292
nm coincides
with theinitialrisein signalobserved
by theGalileo
PPR at 945 nm on the sametraverseacrossJupiter(Martinet al.,
CT
TRAVERSE
6 OF
15TRAVERSES
4-
o
-2
I
0
I
I
ELAPSED
2
SECONDS
i
3
4
1995). The PPRdata,suitablyscaled,arealsoshownin Figure4.
The Galileo NIMS first detectedan enhancedsignalon the Figure3. TheUVS detection
swathacross
Jupiter
andtheaverage
subsequent
traverse,
5 1/3 s later(Carlsonet al., 1995).
of 15similarswathsareshown.Eachhasbeensmoothed
by 0.1 s.
A histogram of the difference between the two curves is also
Interpretation
shown.
The
straight
line
isaleast-squares
fittothe
excess
counts,
its lengthindicatesthe traverseof the impactsite acrossthe UVS
Comparison
of thedatafromtheGalileoNIMS, PPRandUVS field of view. Note the displacement
to the left of the excess
instruments
determines
the thermalhistoryand totalradiative countsdueto theflash,caused
by thedisplacement
of theimpact
outputof the G impact. The UVS excesssignalof (4.3 + 0.9)x sitefromJupiter's
centerof light.
HORD ET AL.' FRAGMENT G IMPACT
GLL
10
UVS'
S-L9
•
.
o
uJ
•
9-
G
•
to a more sphericalfireball. This areadeterminationis consistent
with the Carlsonet al., 1995, (NIMS) emittingareadetermination
,
[]
UVS SIGNAL AT 292 NM
-t-
PPR SIGNAL AT 945 NM (SCALED)
-
of 1250km2 for theswath21 s laterif thehotgashadcooledand
_
U.I
8"'
n
z
O
o
1567
expanded.NIMS actuallymonitoredtheexpansionandcoolingof
the fireball
for several minutes.
Summary
•_
The UVS experimentobservingat 292 nm showsthat a brief
initial UV flash was producedby the G fragmentwhen it entered
Jupiter's atmosphere. The initial flash was also detected by
'
7.-
13alileoPPR at 945 nm (Martin et al., 1995), allowing us to
estimatean initial temperatureof ~7800 K from the UV/IR flux
ratio. The UVS brightpoint falls on the rapidlyrisingpart of the
5
2
3
4
MINUTES
AFTER
5
6
94-199/07:30:00
Figure 4. Countsper 6 ms at 292 nm averaged
over1.25s are
shownasonepointpertraverse
of theUVS slit across
Jupiter
(i-I). The errorbarsindicatethestandard
deviationof 52 normal
traverses.
Notethebrighttraverse
associated
withthefragment
13
impact. Simultaneous
PPR data pointshave beenscaledfor
comparisonand aremarked(+).
PPR light curve, which may correspondto the fast passageof the
fragmentthroughthe visiblepart of the atmosphere(Martin et al.,
1995). If this is the case, the UVS data probably represents
thermal radiation emitted by the bolide as it descendedinto the
atmosphere.A bolide traveling60 km/s at an approachangleof
45 degreestraversesan altitude range of (60 km/s) x (1.6 s) x
cos(45ø) ~ 70 km during the 1.6 s period of UV signal. By the
subsequentswath 5 1/3 s later the affectedhot atmospherehad
expandedand cooled enoughso that its thermal emissionswere
below
the
UVS
detection
threshold
at-5500
K.
Further
expansionof the hot gas led to the spectacularlimb plumes
al., 1995, footnote 8). Should the infrared excessat 945 nm be observedby the Hubble SpaceTelescope(Hammel et al., 1995).
twice thereportedvalue, the corresponding
temperature
wouldbe The Galileo NIMS team (Carlson et al., 1995) reported a
-6700 K. The assumptionthat thermal blackbodyemissionis subsequent"fall back" event at 1994-199/07:39:41 UT as the
presentis generally consistentwith the Carlson et al. 1995 plumes were recompressedand heated by gravity. The UVS
(NIMS) 0.7-5.0 I•m fireball spectraobtainedbeginningabout5 experimentdid not detect this secondaryevent, which involved
1/3 s after the UVS detection.TheseNIMS spectrashowa black- coolergasandredderemissionsthanthe initial impact.
body spectrumwith superimposed
absorptionfeatures. Because
of thebackandforthscanplatformmotion,we d• notknowif
highertemperatures
existed
between
measurements
whenJupiterAcknowledgements.
Thisresearch
was
partofacooperative
effort
onthe
wasnotin theUVSfieldof view. Forexample,
models
by Gfragment
impact
involving
theGalileo
UVS,Galileo
PPRandNIMS
Crawford
etal.,1995
f'md
temperatures
attheleading
edge
ofthe teams.
TheGalileo
Solid-State
Imaging
(SSI)
team
participated
inthe
observation
of otherimpactevents. We thankthe IPL Navigationand
cometaryprojectiles as large as 35,000 K. Ahrens et al. 1994
AncillaryInformationFacilityfor providingan up to dateleap-seconds
predictedbolide temperaturesof more than 10,000 K. A direct timingfile. We wouldlike to thanktheGalileoProjectfor makingthese
measurement
of thebolide
flashtemperature
byGalileo
PPR observations
possible.
This
research
was
supported
bythe
Galileo
Project.
(two-color
photometry
at 678and945nm)ontheQ1fragmentK. Tobiska
acknowledges
support
fromTelosandK. Naviaux
foundaninitialtemperature
of ~18,000K (Martinet al., 1995). acknowledges
support
fromNyma.
The totalthermalradiationemittedby impactG duringthe 1.6 s of
UVS detection,assuminga blackbodyat 7800 K, is 1.6x10]6 References
joules. For comparison,
a kilotonof TNT = 4.2x10]2joules,so
the thermalradiationemittedwas equivalentto 3800 kilotonsof Ahrens,T. J., Takata,T., O'Keefe,J. D., andOrton,G. S., Impactof
TNT.
CometShoemaker-Levy
9 on Jupiter,Geophysical
ResearchLetters,
By 5 1/3s aftertheUVS flash,thePPRsignalincreased
to its
1087-1090,
1994.
peaklevelof(2.2+ 0.2)x1045
wattscm'2nm
4, buttheUVSsignal Carlson,
R. W.,Weissman,
P.R.,Segura,
M.,Hui,J.,Smythe,
W. D.,
haddropped
toitspre-impact
background
level,where
it remained Johnson,
T.,Baines,
K.H.,Drossart,
P.,Encrenaz,
Th.,Leader,
F.E.,
for the rest of the time period covered in the data returned to
and the NIMS Science Team, Galileo Infrared Observationsof the
Shoemaker-Levy
9 G ImpactFireball:A PreliminaryReport,submitted
Earth.
This
suggests
that
theemissions
had
cooled
substantially,
toGeophysical
Research
Letters,
1995.
shifting
theemission
peakintheblackbody
spectrum
tolongerCrawford,
D.A.,Boslough,
M.B.,Trucano,
T.G.,and
Robinson,
A.C.,
wavelengths.
Fromthenon-detection
bytheUVSwecanseta2-
Theimpact
ofperiodic
comet
Shoemaker-Levy
9 onJupiter,
The
O upperlimit on the temperature
for thissubsequent
swathof
International
Journal
oflmpact
Engineering,
inpress,
1995.
~5500 K. Smytheet al. 1994(NIMS) founda temperature
greater Hammel,H. B., Beebe,R. F., Ingersoll,
A. P., Orton,G. S.,Mills,J. R.,
than 5000 K for this same swath.
Simon,A. A., Chodas,
P., Clarke,J. T., DeJong,
E., Dowling,T. E.,
Harrington,J., Huber,L. F., Karkoschka,
E., Santori,C. M., Toigo,A.,
The flux at the Galileo spacecraftat two wavelengthsuniquely
Yeomans,D., andWest,R. A., HubbleSpaceTelescope
imagingof
determinesboth the temperatureand the area of the emission,
Jupiter:Atmosphericphenomenacreatedby the impact of Comet
assuming
purelythermalemission.The hot emittingarearequired
Shoemaker-Levy9, Science,267, 1288-1296, 1995.
to producedthe flux at 292 nm of the UVS brightswathgiventhe
Hord,C. W., MeClintoek,W. E., Stewart,A. I. F., Barth,C. A., Esposito,
L. W., Thomas,G. E., Sandel,B. R., Hunten,D. M., Broadfoot,A. L.,
reflecting layer below the blast scattersthe emission into the
Shemansky,
D. E., Ajello, J. M., Lane,A. L., andWest,R. A., Galileo
upwardhemisphere.The actualgeometryis unclear:the derived
Ultraviolet SpectrometerExperiment,SpaceSci. Rev., 60, 503-530,
derivedtemperature
is foundto be40 (+60, -25) km2 assuming
a
areamay refer to a longnarrowtrail of hot gascreatedon entryor
1992.
1568
HORD ET AL.: FRAGMENT
Johnson,T. V., Yeates, C. M., and Young, R., Space ScienceReviews
Volume on Galileo MissionOverview, SpaceSci.Rev., 60, 3-21, 1992.
Martin, T. Z., Orton, G. S., Travis, L., Tamppad, L., and Claypool, I.,
Observations of Shoemaker-Levy Impacts by the Galileo
Photopolarimeter
Radiometer,submittedto Science,1995.
Smythe,W. D., Carson, R. W., Weissman,P. R., Hui, J., Segura,M. E.,
Baines,K. H., Matson, D. L., Johnson,T. V., Leader,F. E., Taylor, F.
W., Encrenaz, T., and Drossart,P., Galileo Infrared Observationsof the
Shoemaker/Levy-9Impactsof Jupiter,BAAS,26, 1061, 1994.
Van Hoosier, M. E., Bartoe, J. F., Bmeckner, G. E., and Pdnz, D. K.,
Absolute solar irradiance 120 nm-400 nm, Astrophys.Lett., 27, 163168, 1988.
Wagener, R., Caldwell, J., Owen, T., Kim, S. J., Encrenaz, T., and
Combes,M., The Jovianstratosphere
in the ultraviolet,Icarus, 63,222236, 1985.
G IMPACT
Boulder,CO 80303. (e-mail: esposito•zodiac.colorado.edu)
J. J. Gebben,LASP, University of Colorado, 1234 InnovationDrive,
Boulder,CO 80303. (e-mail: gebben•pisces.colorado.edu)
C. W. Hord, LASP, University of Colorado, 1234 Innovation Drive,
Boulder,CO 80303. (e-mail: :hord@pisces.colorado.edu)
W. E. McClintock, LASP, University of Colorado, Boulder, CO
80303. (e-mail: mcclint@pisces.colorado.edu)
K. L. Naviaux, Jet PropulsionLaboratory/Nyma, MS 264-273, 4800
Oak
Grove
Drive,
Pasadena,
CA
91 109.
(e-mail:
knaviaux@gllsve.jpl.nasa.gov)
W. R. Pryor, LASP, University of Colorado, 1234 Innovation Drive,
Boulder,CO 80303. (e-mail:pryo•pisees.colorado.edu)
K. E. Simmons,LASP, Universityof Colorado,1234InnovationDrive,
Boulder,CO 80303. (e-mail:simmons@pisees.eolorado.edu)
A. I.. F. Stewart, LASP, University of Colorado, 1234 Innovation
Drive,Boulder,CO 80303. (e-mail:stewart•iac.colorado.edu)
J. M. Ajello, Jet Propulsion Laboratory, California Institute of
Technology,MS 264-744, 4800 Oak Grove Drive, Pasadena,CA 91109.
(e-mail:jajello•gllsvc.jpl.nasa.gov)
C. A. Barth, LASP, University of Colorado, 1234 Innovation Drive,
Boulder,CO 80303. (e-mail: barth@pisces.colorado.edu)
W. K. Tobiska,Telos/JetPropulsionLaboratory,MS 264-723, 4800
Oak Grove Drive Pasadena,CA 91109.
(e-mail:
ktobiska@
gllsve.jpl.nasa.
gov)
R. A. West, Jet Propulsion Laboratory, California Institute of
Technology,MS 169-237, 4800 Oak Grove Drive, Pasadena,CA 91109.
S. J. Edberg,Jet Propulsion
Laboratory,
CaliforniaInstituteof (e-marl:
raw@west.jpl.nasa.gov)
Technology,MS 264-744, 4800 Oak Grove Drive, Pasadena,CA 91109.
(e-marl:sedberg@gllsvc.jpl.nasa.gov)
(Received:February13, 1995, revised:April 7, 1995, accepted:April
L W. Esposito,
LASP,Universityof Colorado,1234Innovation
Drive, 13, 1995)