GEOPHYSICAL
RESEARCH
LETTERS,
VOL.24,NO.20,PAGES
2471-2474,
OCTOBER
15,1997
The Pele plume (Io): Observationswith the Hubble
SpaceTelescope
JohnR. Spencer,
• PaolaSartore•i,
2 GildaE. Balles•er,
3 AlfredS. McEwen,
4
JohnT. Clarke,3 and MelissaA. McGra•h*
Abstract. In July 1996, with the Hubble SpaceTele- 1). Imageresolutionwasimprovedby Lucy-Richardson
scope(HST), we observedthe Pele plumesilhouetted deconvolution.
against Jupiter at a wavelengthof 0.27pm, the first
A single F255W image taken on July 8 1995 shows
definitiveobservationof an Io plume from Earth. The a bright patch againstdark sky off Io's limb in (Figheight, 420 q- 40 km, was greaterthan any plume ob- ure 1). The imageis noisy,but the perfectalignment
servedby Voyager.The plumehad significantlysmaller of the bright patch with the expectedlocationof Pele,
optical depth at 0.34 and 0.41pm, where it was not and the lack of similar features off Io's limb elsewhere
detected. The wavelengthdependenceof the optical in this and other F255W frames, gives us someconfidepth can be matchedby a plume either of fine dust, dence that this is the Pele plume. The plume is about
with minimummassof 1.2x 100g andmaximumparti- 20 DN abovebackgroundsky brightness,corresponding
cle sizeof 0.08pm, or of SO2 gaswith a columndensity to an I/F value(the brightness
ratioedto a normallyof 3.7x 1017cm-2 andtotalmassof 1.1x 1011g. Our illuminatedLambert surface)of 0.004. This is similar
modelssuggestthat early Voyagerimagingestimatesof to average0.35pm Pele plumeI/F valuesseenby Voythe minimummassof the Loki plume [Collins,1981] ager 1 [Stromet al., 1981,Figure2]. No plumewasseen
may have'been too large by a factor of • 100. We may in 0.373 pm images taken 20 minutes earlier.
have detectedthe Pele plume in reflectedsunlight,at
An F255W image taken on July 7, when Pele should
0.27pm, in July 1995, but did not seeit 21 hoursearlier, have appeared on the opposite limb, does not show the
so the plume may be capableof rapid changes.
plume (Figure 1). Either the July 8 detection,or the
July 7 non-detection, is spurious,or the plume switched
on in the 21 hours between the frames. During the
1. The Pele Plume
Voyager 1 encounter the Pele plume was active for at
Pele was unique among the nine volcaniceruption least 9 days, but these data suggestthat more rapid
plumeson Io discovered
by Voyager[Stromet al., 1981]. variations are possible.
The Pele plume was definitely seen in three F255W
It wasthe highestplume(300 km), the mostoptically
thin, and was the only one not active duringboth en- imagestaken in Jupitertransit in July 1996(Figure1).
counters. The Pele plume was not detected in Galileo The plume appears dark against Jupiter. Subsequent
imagesin June, September,or November1996,but was F336W and F410M images do not show the plume,
seenin December1996 [McEwenet al., 1997a].High- though it was further from the limb when theseimages
temperature thermal emission from Pele has been seen weretaken(TableI). Partialsaturationof Jupiterin the
by Galileo'sSSI [Belton et al., 1996; McEwen et al., F336W image reducesthe precisionof measurementsin
1997b],and NIMS [Lopes-Gautier
et al., 1997]instru- this filter.
ments whenever observations have been made.
2.
HST
3. 1996 Plume Size and Density
Observations
To modelthe transit images,we ignorebackscattered
sunlight
from the plume.This is justifiedif the plume
2 (WFPC2)onHST in July 1995andJuly 1996(Table
activity levelwassimilaron July 24 1996and July 8
1995' the DN deficit in the plumein the July 1996
•Lowell Observatory,Flagstaff,Arizona
images,
comparedto the Jupiterbackground,
is about
•Institut d'Astrophysiquede Paris, Paris
320
DN,
compared
to
the
20
DN
brightness
of
the July
adept. of AtmosphericOceanic,and SpaceScience,Uni1995
plume
images.
Also,
modeling
discussed
below
versity of Michigan, Ann Arbor
We imagedIo with the Wide-Field PlanetaryCamera
4Lunarand PlanetaryLaboratory,Universityof Arizona, suggeststhat for dust or gasplume modelsthat fit the
Tucson
transitobservations,
reflected
sunlightisfaintcompared
5SpaceTelescopeScienceInstitute, Baltimore,Maryland to absorbedor scatteredJupiter-light.
We modeled the plume as a uniform half-disk cen-
Copyright1997by theAmericanGeophysical
Union.
Papernumber97GL02592.
0094-8534/97/97GL-02592505.00
teredon the Pelevent (18 S, 256 W), includingonly
that portion that wouldprotrudeaboveIo's limb. This
model plume wassmoothedby a 0.07 arcsecboxcarfil2471
2472
SPENCER
ET AL.- PELE
PLUME
FROM
HST
Table 1. Details of HST images
UT
Date
95/07/07
95/07/08
96/07/24
96/07/24
96/07/24
96/07/24
96/07/24
UT
Mid
HST
Filter
Filter
Wavelength
Effective
Wavelength,
Exposure
Time
Name
Range,•um
•um
(s)
14:01
10:53
19:12
19:24
20:45
20:53
20:59
F255W
F255W
F255W
F255W
F255W
F336W
F410M
0.257-0.291
0.257-0.291
0.257-0.291
0.257-0.291
0.257-0.291
0.315-0.358
0.400-0.417
0.272
0.272
0.272
0.272
0.272
0.340
0.409
600
600
600
500
500
23
10
Sub-Earth
Lat.
W.
Long.
Solar
Phase
Angle
A
(AU)
R
(AU)
H
(km)
-2.9
-2.9
-1.8
-1.8
-1.8
-1.8
-1.8
345.5
162.4
171.9
173.7
185.1
186.2
187.1
6.8
7.0
4.2
4.2
4.2
4.2
4.2
4.494
4.503
4.239
4.239
4.240
4.240
4.240
5.328
5.328
5.196
5.196
5.196
5.196
5.196
1.2
2.8
8.1
13.9
87.4
97.8
106.6
Comments' Filter wavelengthrange is between filter transmissionhalf power points, after weighting by the solar
spectrum. Effective wavelength is also weighted by the solar spectrum. H is the projected distance between the Pele
plume sourceand 1o'slimb: this much of the bottom of the plume is hidden.
95/07/O7,
..,. -:-•..
....•k'
•.•
1,4-01 UT
•
.....
95/07/08,
' d'
10:55 UT
96/07/24,
-- ..........
19:18 UT
..... ...........
.::.::,:",•'•
..........'•, ...
ter, and was then added to each HST image. Model
sizeand brightnesswasadjustedto exactlyremovethe
(negative)plumeimage. The modelplumebrightness
was then translated into an optical depth by comparison
with the brightness
of theJupiterbackground
(Table2).
Althoughthe plumewasfurther from the limb when
the F410M image was taken, it shouldhave been de96/07/24, 20:44 UT
96/07/24, 20:59 UT(F410M)
96/07/24, 19:18 UT
tected had it been as large and optically thick at this
--"
.........
':•:"•
•;•:':
.... -.x
'...............................................................
:: •;;;•";:•
........
••• ....................................
•:•::::
wavelength
asin the F255Wfilter (Table2). The plume
....
:':'
'"'";
......................
"< '":•-•
.'ad•':':':•'"•::•
':'•"•'•'•:•?)•.;•'::{•?'•:d?•:k
} height,
420•40kin,makes
thisthelargest
plume
ever
':.'k
;' ex.
96/07/24, 20:44 U•
•.:• •-•::•::•.•:.•;:•{•"-•"•?•,•.•.•
}:•5;:•;:;;•;??•,::•:•:•:•%
a........
.................
.::.•:::'
seen
on
Io.
December
1996
Galileo
show
aPele
•========================
.• ?•t•?::..
plume
of
similar
height
[McEwen
etimages
al.,1997a].
k
•
/ ::"•
•:;:--:':•27
--;•5:•::'
....• •:•?:'"•
'•:•'
'::
:•;•
.............
%.......•.w•:•:::•:
'"-:':...•;•.
4. Models'Dust Scattering
96/07/24, •0:5• UT
................
-......................................................................
..e..:::.:.½•:•?•'•,,•:•;,•-':-d?•sg?-•:•?•;
......................
....:.;•;::?:.:':.'..•.•,•s:•:•?•5•-;%•;•;•:•.....:•:•
Voyager
1 observations
of the Peleplumeat wave-
........... .:•:.?•
•]]::•;•:•;;•:•:•;•:•':'"'•"•]•:•s
.::;•:•;?:'•";-?•
"'•":
"-?•?:•½•:•:..
' ...........
,?::.:.•:•;•;:
•--.,??•:%
..........
lengths
> 0.35•m,
show
aneutral
color.
However,
the
:,::".:::::•::•:•':,•
............
:•:-•::•:
.....
:....ß'""?
:--:.
:''.:???::L:•:(:•:?•
"':•?•;".:-.]•'•.
:..:.:......•?'"'•:•?•"•:•Yd?•;;;::•
Voyager•
outer
Loki
plume
was
blue
inthe
0.35-0.6•m
'z::?;•:•:::•::':•;"•
.....
•:•a"•:•
.........
;•'
..............
•?'":"?":'
;"q•
%•?
.......
"::'5•'::•
•;•?•::::";
[Collins,
1981].
Collins
attributed
theblue
color
of
....................
•.<..:-•%;•:.:.:.:-•
'a::j""'"'<.:•:::,-.•k'--,:
.........
........:•-•::•:
.::::
•?.:.:..m•
•:•::::.•..•...:?'-'
• theouter
Loki
plume
toscattering
byparticles
much
.....
.......
::•:•
.................................................
:.•:3smaller than the wavelengthof light, and this may also
Figure 1.
Detections,probabledetections,and non- explain the HST color of the Pele plume.
detectionsof the Pele plume in HST WFPC2 images.
All are in the F255W filter except where noted. Io's
limb, equator, and the expectedlocationof the Pele
source are shown in all frames. The plume is proba-
Table 2. Plume optical depths, July 1996
bly seenagainstdark skyon 95/07/08, at F255W:the
arrows note imperfectly removed cosmic rays. How-
ever,it is not seenin 95/07/07 F255W imagestaken21
hoursearlier. The 1996imageshaveJupiterin the background,and mostare shownin both "raw"formatand
with plume visibility enhancedby ratioing eachimage
to a transposedand registeredcopyof itself: the plume
is then visibleas both a positiveimage (dark spot off
thelimb)andnegative
image(brightspotoffthelimb).
Pele Plume
Sub-Earth
Filter
F255W
F255W
F336W
F410M
W.
Long.
H,
km
Optical
Depth
Radius
172.8 •'
185.1
186.2
187.1
11.0
87.4
97.8
106.6
0.19-1-0.05
0.17-1-0.07
<0.15
<0.06
420-1-40
440+80
__
The Pele plumeis seenin silhouetteagainstJupiter in
the F255W images(the imagelabeled"19:18UT" is an
averageof the i9:i2 and 19:24UT images),but not in
the F336W or F410M images.
Comments:
H is defined as in Table 1
•Averageof first two 97/07/24 frames.
km
SPENCER ET AL' PELE PLUME FROM HST
\
.......
0.20 •
'•
Radius=O.O7/•,m,
Mass=1.25x109
g
Radius=O.O8/•,m
Mass=1.15x10
•g
'•
0.•
for the outerpart of the plume,for r _<0.01/•m.Our
Mie scatteringcode,approximating
the Loki plumeas
a uniformhemisphere
with a 0.34/•mI/F of 0.03 and
radiusof 200km,givesa plumemassof 9.3x 1010g for
Radius=O.O5/•,m.
Mass=2.80x109
g
0.25
,•
'•'•
I
\ /
kI
2473
r -- 0.01/•m, confirmingCollins' result. However,the
Mie codecanalsomatchthe colorandbrightness
of the
outer Loki plumewith r - 0.05/•mand m - 1.1 x 10ø
"-,I__'•.:.'•-'
g, comparable to our minimum dust mass for Pele. The
0.10
Loki plume may therefore be much lessmassivethan
ß .
deducedby Collins,if r is largerthan he assumed.
0.05
0.00
....
0.20
I
....
0.25
•
,
5. Models' SO• Gas Absorption
,
0.30
0.35
0.40
0.45
0.50
The extinctionof Jupiter light by the Peleplumein
the F255W filter imagemay alsobe explainedby the
W•velen9th, microns
op[•c•] •ep[•, comp•ed [o •us• •c•[[e•g
•cdbea •
[•e levi.
•e
o•c•]]•fio•s •
mo•e]• •e-
[•e •e
cu•es
presenceof SO2 gas, which would make Pele a "stealth
plume" [Johnsonet al., 1995]. SO2 gas absorption
[MannattandLane,1993]is muchmoreefficientin the
•e •ue [o ou• •ump[•o• o[ • •g]e pA•[•c]e•e • e•c•
mo•e]. •o•o•[•] bA• •ow [•e w•e]e•g[• •ge o• F255W filter than at longerwavelengths.The SO• abe•c• fi]•e• (•ble H), •a •e•fic•] e•o• b• •ow [•e sorptioncross-section
•r in the F255W filter bandpass,
u•ce•[•[]•
• [•e ob•e•ea optics] aep[•. •e [wo weightedby the spectrumof reflectedlightfromJupiter
(Mark Vincent,pets.comm.),is • 5 x 10-lø cm•. Assuming
SO• absorption
isin thelinearregime[Ballester
et al., 1994],the observed
opticaldepthof 0.19givesa
column abundance N • 3.7 x 1017cm-•. This value is
approximate,becausethe Manatt and Lane SO2 specof spherical particles with uniform projected optical trum is at 300 K, and hasrelativelylow spectralresoludepth, using Mie theory [Bobtenand Huffman, 1983, tion. This amountof gaswouldnot producedetectable
absorption in the F336W and F410M filters.
Appendix A], and assumingthe sulfur real refractive
Our inferred SO• abundanceis comparableto other
index of 1.957. For relevant particle sizes,the value of
We modeled the plume as a 420 km radius cloud
the complexrefractiveindexis unimportant. We de- estimates of local SO• abundance on Io. The darkness
termined the amount of direct Jupiter light scattered of Io's disk near Pele in 0.2-0.3•umHST imagessug-
out of the beam, the amount of Jupiter light scattered
into the beamfromotherpartsof the planet(whichwas
negligible),and the amountof light addedto the plume
image by backscatteredsunlight.
The HST plume color requires particles with radius
r _• 0.08/•m (Figure 2). The plumemassm neededto
match the F255W optical depth increaseswith decreas-
gested
N = 10ls cm-• [Sartoretti
et al., 1996],depen-
denton assumptions
aboutthe surfacealbedo.Voyager
infrared spectragave N - 5 x 10ls cm-• over Loki
[Pearlet al., 1979]. HST spectroscopy
detectedSO2
absorptionlocalizedabovePele in early August 1996,
shortlyafterourJupitertransitobservations
(McGrath
et al., pers.comm.),supportingthe SO• modelfor our
but not ruling out the presenceof dust in
ing particle size, becausesmallerparticlesare muchless observations
efficientscatterers.Forr - 0.07/•m,m - 1.25+0.3x10ø
addition.
The total massof gasoverPele derivedfrom our 1996
g. Particles with r - 0.05- 0.08/•m are poor backscatif the exinctionis due to SO•, is 1.1 x 1011
terers, and the backscattered sunlight added to the observations,
plume is only 10-20% of the scatteredJupiter light g, about 100 timesthe minimummassof dust required
lost from it in the F255W filter, consistent with the
to explainthe sameobservations.
Rayleighscattering
for thesegasabundances
[Collins,1981],
plume's dark appearanceand justifying our neglectof is negiligible
as is refraction.
backscatteredsunlightin deriving optical depths. Particles smaller than 0.02/•m scatter too isotropicallyto
be consistent
with the observed
darkness
of the plume.
Application of the Mie scattering model to the ten-
6. Resurfacing Rates
Usinga ballisticapproximation,a 420 km plumeal-
tativeJuly8 1995observation
ofPelein backscattered
tituderequiresa launchspeedof 1.1km/s (43%of Io's
sunlight, assumingthe same plume radius as the 1996 escape
velocity,
but physically
plausible
[Keiffer,1982]),
observations,
givesm - 6 x 108g for r - 0.07/•m,about and a flight time of 1630sec,includingthe reductionin
half the July 1996 mass.
gravity with altitude. With a plume dust massof 10o
Collins [1981] used a Rayleigh-likeparticle scatter- g, a 420 km radiusfallout area, and a particledensity
ing law, valid only for r _<0.01/•m, to modelthe Voy- of 1 g/cm3, thisgivesa minimum
resurfacing
rateof
agerimages
of theLokiplume.He derivedm _>i0il g 3.5x 10-3 cm/yr,equivalent
to a globalrateof4.6x 10-5
2474
SPENCER ET AL.- PELE PLUME FROM HST
A. P., Io meteorology:
How atmospheric
pressure
cm/yr.Johnson
etal. [1979]
estimated
a global
plumeIngersoll,
is controlledlocallyby volcanoes
and surfacefrost, Icarus
resurfacing
rateof3.5x 10-4 cm/yr).Themuch
larger 81, 298-313, 1989.
global
plumeresurfacing
ratesof,-•10-2 - 1cm/yresti- JohnSon,T. V., A. F. Cook,C. Sagan,andL. A. Soderblom,
matedby [Johnson
andSoderblom,
1982]resulted
from
thehighLokiplumemassdetermined
by[Collins,
1981],
and may thus be too large.
Our minimum July 1996 Pele dust kesurfacingrate
Volcanicresurfacing
ratesandimplications
for volatileson
Io. Nature 280, 746-750, 1979.
JohnsonT. V., and L. A. Soderblom,Volcaniceruptionson
Io: Implicationsfor surfaceevolutionand massloss. in
Satellitesof Jupite?,edited
by D. Morrison,pp. 634-646,
givesonly10 micronsof deposition
in thefourmonths U. Arizona Press, Tucson, 1982.
betweenVoyageri andVoyager2, probablyinsufficient JohnsonT. V., D. L. Matson, D. L. Blaney,G. J. Veeder,
and A. G. Davies, Stealth plumeson Io, Geophys.Res.
to producethe major albedochanges
seenat Pele in
Left. 22, 23, 1995.
that interval. Either the plume is much more massive
Keiffer
S. W., Dynamicsand thermodynamics
of volcanic
than our lowerlimit, due to a largepopulationof pareruptions:implicationsfor the plumesof Io. In Satellites
ticleslargeror smallerthan 0.07/•m,or to a largegas
of Jupiter,editedby D. Morrison,pp. 647-723,U. Arizona
content,or it wasmuchmoremassiveduringthe Voy-
Press,,Tucson, 1982,
Lellouch,E., M. J. S. Belton, I. DePater, G. Paubert, S.
Gulkis, and T. Encrenaz,The structure,stability,and
If the observedplumeis dueto SO2gas,simplescalglobal
distributionof Io'satmosphere,
Icarus98, 271-95,
ing from the dustcalculation
givesa largelocalSO2
ager era.
1992.
deposition
rate of 0.39cm/yr. The Peleejectadeposits, R. Lopes-Gautier,
A. G. Davies,R. Carlson,W. Sfnythe,L.
however,
appearto bepoorin SO2frost[McEwenet al.,
Camp,L. Soderblom,F.E. Leader,R. Mehlman,and the
i988]. If the plumeweseeis dueto SO2,the ultimate Galileo NIMS team, Hot spotson Io: Initial resultsfrom
fate of that SO2 is an interestingquestion:perhapsit
flowshorizontallyto high-albedocold traps elsewhere
[Ingersoll,1989].
Galileo'sNear I.nfrared
MappingSpectrometer,
Geophys.
Res. L•ett.,this issue,1997.
Manatt, S. L. andA. L. Lane,A compilation
of SO2absorption cross-sections
from 106-403nm, J. Quant. $pectrosc.
Radiat. Trans. 50, 267-276, 1993.
McEwenA. S., T. V. Johnson,D. L. Matson,and L. A.
SoderblOm,
The globaldistribution,abundance,and staTheseobservations
showthat the largestof Io's plumes bility of SO2 on Io, Icarus 75, 450-4781988.
can be detected from Earth, opening up a new avenue McEwen, A. S., J. Spencer,D. Simonelli,T. Johnson,L.
Keszthelyi,and the GalileoSSI Team, Io plumeobserva7.
Conclusions
for studyof theseremarkable
phenomena.
The 1996
tions from Galileo and HST, Lunar Planet. Sci. XXVIII,
transit observations
may be due to dustor gas,but the
911,199")a.
tentative 1995 observationsof the plume in backscat- McEwen,A. S., D. P. Simonelli,D. Senske,K. P. Klaasen,
L. P. Keszthelyi,T. V. Johnson,P. Geissler,M. H. Carr,
teredsunlightwouldrequirea dustcomponent.
If the
plumeis due to gas,the F255W Jupitertransitimages and M. J. S. Belton, High-temperaturehot spotson Io as
seenby the Galileo:SolidStateImaging(SSI)experiment.
in Figurei providedramaticvisualconfirmation
of the
Geophys.
Res. LetL, this issue,1997b.
patcry-atmosphere
modelderivedfromtheoretical
and Pearl,J. (•., R. Hanel,V. Kunde,W. Maguire,K. Fox,S.
observational
considerations
[Ingersoll,1989;Lellouch Gupta,C. Ponnamperuma,
andF. Raulin,Identification
SO2andnewupperlimitsfor othergases,
Naet al, 1992;Ballesteret al., 1994;Sartoretti
et al., 1996], of gaseous
ture280,757-758,i979.
by directlyimaginga large,volcanically-derived
atmoSartoretti,P., M. J. S. Belton, and M. A. McGrath, SO2
spheric"patch"overPele. If the 1995observations
in
distributionson io, Icarus 122, 273-287, 1996.
backscattered
sunlightare real, they provideevidence StromRG, SchneiderNM, TerrileRJ, CookAF, andHansen
for very rapid time variationsin the Peleplume.
C., Volcaniceruptionson Io, J. Geophys.Res. 86, 85938620, 1981.
Acknowledgments. This work wasfundedby Space
Telescope
ScienceInstitutegrantsGO-6028and GO-6774.
J. R. Spencer,Lowell Observatory,1400W. Mars Hill Rd.,
Flagstaff
AZ 86001.(e-mail:[email protected])
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