JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. E6, PAGES 14,169-14,178,JUNE 25, 1999
Ganymede's ozone-like absorber: Observations by the Galileo
ultraviolet spectrometer
A. R. Hendrix, C. A. Barth, and C. W. Hord
Laboratoryfor Atmosphericand SpacePhysics,Universityof Colorado,Boulder
Abstract. We reporton UltravioletSpectrometer
observations
of Ganymedefrom four orbitsof
the Galileo primary mission. During 12 observationsequences,
regionson Ganymede'sleading,
trailing,andanti-Jovianhemispheres
were observedat severallatitudesandvaryingobservational
geometries.The measurements
showan ozone-likeabsorberconcentrated
in the polarregionsand
near sunriseand sunsetat low latitudes,with a clear correlationbetween ozone abundanceand so-
lar zenithangle.The absorptionmaximumis shiftedto the red of the gaseousozoneabsorption
maximum
(near2600A), confirming
disk-integrated
observations
by otherresearchers.
Ganymede'sabsorber
maybe ozonetrappedin theice lattice,whichwe proposeis dueto chargedparticle bombardlnentof the surface. At high Sun(small solarzenithangles),the ozone-likeabsorber
is destroyed.The ozonein the polarregionsis likely to be long-lived,on accountof the largesolar zenith angles.
1.
Introduction
ing the magnetometeron board Galileo [Kivelson e! al., 1996;
Gurnett et al., 1996]. Field lines at Ganymede'spolar regions
We discuss disk-resolved observations of Jupiter's icy
are open to Jupiter's magnetosphericplasma, while closed
moon Ganymede by the Galileo ultraviolet spectrometer
field lines, covering•_30ø-45 ø in latitude, shield the equato(UVS). The datareveal a complicateddistribution of an ultrarial regions. Polar caps on Ganymede,extendingdown to lativiolet absorberon Ganymede,which may be related to its
tudes of 40ø-45ø, were discoveredby the Voyager spacecraft
newly discoveredmagnetosphere[Kivelson e! al., 1996] and
[Smithet al., 1979]; the Galileo Near-InfraredMapping Specto the oxygen chemistrythat occurs in the icy matrix of the
trometer (NIMS) found that while water ice is present all over
surfacedue to photolysis. We first discusswhat is already
Ganymede'ssurface,there is less ice in the equatorial regions,
known aboutGanymede'ssurfacefromspacecraftand groundand the water ice in the polar regionsis finer grained [Carlson
based observations.
et al., 1996].
Early ground-basedmeasurements
showed that Ganymede
Featuresat 5773 A and 6275 A were detectedon Ganydisplays a hemisphericalbedo dichotomy,where its trailing
mede's trailing hemispherein ground-based
measurementsand
hemisphere(centeredon 270ø W, where 0ø W longitude alattributed to electronic transitions in pairs of oxygen moleways facesJupiter)is darker than its leading hemisphere(cencules [Spenceret al., 1995; Calvin et al., 1995]. The bomtered on 90ø W) [Millis and Thompson, 1975]. The albedo
bardmentby corotating magnetosphericplasma is likely the
dichotomyhas been attributed to enhancedbombardmentby
sourceof the molecularoxygen [Reimann et al., 1984], on the
Jupiter's corotating magnetosphereon the trailing hemibasis of the longitudinal variation in the absorption band
sphere. An additional leading/trailing differencewas noted
depth [Calvin et al., 1996]. Condensedpure oxygen is not
by the InternationalUltraviolet Explorer (IUE) [Nelsonet al.,
stable at Ganymede'stemperatures,so it was suggestedthat
1987]: Ganymede'strailing hemispherewas found to have an
the oxygenis trappedwithin the surfaceice (defect trapping or
absorber
centeredshortwardof 2800 A. This absorption
feaadsorption)[Calvin et al., 1996]. Additional spectrafromthe
ture was noted to be different from Europa's trailing hemiHubble Space Telescope (HST) Faint Object Spectrograph
sphereabsorber,
whichis centered
near2800A andis thought
(FOS) and images from the Wide Field Planetary Camera
to be due to implantationof sulfur fromthe magnetosphere
in(WFPC) showedthat the oxygen absorptionis concentratedat
teractingwith the surfacewater ice [Lane et al., 1981; Noll et
low latitudes[Calvin and Spencer,1997].
al., 1995; Hendrix et al., 1998], creatingan SO2-1ikeabsorpLater measurements
by the HST FOS [Noll et al., 1996]
tion. Nelsone! al. [1987] suggestedthat an absorberin addiidentified ozone on Ganymede'strailing hemisphererelative
tion to SO2 is present on Ganymede's trailing hemisphere;
to the leading hemisphere. They indicate that the cross secthey suggestedozone,causedby magnetospheric
oxygen ions
tion of Ganymede'sozone is broadened and shifted somewhat
being implantedinto the icy surface.
fromgaseousroom-temperature
ozone, as it is present in the
The arrival of the Galileo spacecrafthas revealed that Gasurfaceice. The molecularoxygenandozoneare likely related.
nymede is even more complicated than Earth-based observaJohnson and Jesser [1997] describe a process by which 02
tions have shown. A magnetospherehas been discoveredusand 03 "microatmospheres"
are formedwhen magnetospheric
ions impact and destroythe surface,creatinga void or bubble,
where a small 02 atmosphereforms. Ozone may also form in
Copyright1999by theAmericanGeophysical
Union.
these bubbles through photolysis, and it was suggestedthat
photolysisis a source of the ozone on Ganymede[Nol! et al.,
Paper number 1999JE900001.
0148-0227/99/1999JE900001 $09.00
1995]. Possibly connectedto the charged-particle-induced
14,169
14,170
HENDRIX ET AL.' GANYMEDE'S OZONE-LIKE ABSORBER
oxygen chemistryis the discoveryof hydrogenatoms escaping
from Ganymede's surface, as detected by the Galileo UVS
[Barth et al., 1997]. Barth et al. [1997] also suggestedthat
UV irradiation of water ice may be the sourceof the escaping
hydrogenand the oxygen chemistry.
An alternative to having molecularoxygen in ice bubbles
was proposedby Fidal et al. [1997] and Baragiola and Bahr
[1998], who performedexperimentsto replicate the visible
wavelength range in 4.33 s, with 0.006 s integration time at
each channel.
Theseobservations
were performedduring the primarymission of Galileo, between June 1996 and November 1997; they
cover a variety of terrain types and latitudes and longitudes
on Ganymede,at severalspatialresolutions.Plate 1 shows an
imageof GanymedefromGalileo/Voyager and depicts the regions coveredduring each observationsequence.Table 1 dewavelength oxygen featuresseen by Calvin and Spencer scribeseach observationsequence,including the approximate
[1997]. They found that the condensedmolecular oxygen on
latitude and longitude covered during the observation. In
Ganymede's surface must exist in regions at temperatures this paperwe deal primarily with the higher resolution obsermuch lower than reported Ganymede surface temperatures, vations but use the lower resolution observations(GLOBAL)
suchas in permanentlyshadowedregions,cracksin the ice, or to supplementour higher resolution results. Plate 1 shows
the size of one field of view (FOV)during the GLOBAL sehigh-albedo locations.
A connection between the newly discovered magneto- quence,near 0ø N, 140ø W (this is the "standard" region dissphereand Ganymede'spolar capswas suggestedby ,Johnson cussedlater). The central locations of the other FOVs during
[1997], who thought they might be due to charged-particle the GLOBAL sequenceare shown in Table 1.
bombardment. In addition, Pappalardo et al. [1998] find that
the open field line regions correspondwith the polar caps of
3.
Photometric
Corrections
Ganymede. Thermal segregationhas also been suggestedas a
source of the caps [Purves and Pilcher, 1980], although this
The initial step in the analysis of the UVS Ganymedespecidea was challenged by Hilllet et al. [1996] through photra was to determine the measured reflectance. This was done
tometric analysis. Sieveka and ,Johnson[1982] suggestedthat
by averaging 14 spectra(60.67 s total). The averagebacka combinationof sputtering and thermalprocessingformsthe
groundradiation noise was subtractedfrom the resultant specpolar caps;they noted that on the trailing hemisphere,darker trum, which was then divided by the calibration curve and by
presumablyas a result of charged particle bombardment,the
a solarspectrum.The solarspectrumusedwasmeasuredby the
cap boundary is at a higher latitude than on the brighter leadSolar-StellarIrradiance ComparisonExperiment(SOLSTICE)
ing hemisphere.This may be becausefrost formationis inhib[Rottman et al., 1993] and was double boxcar smoothedto
ited in darker regions.
matchthe UVS spectralresolution.
In this work, we use additional UVS measurementsto ex-
plore further the processesoccurring at Ganymede's surface,
especially those that may affect oxygen chemistry, to understandthe distributionof 02 and O3 on Ganymede'ssurface.
2.
Observations
The Galileo ultraviolet spectrometerwas built at the University of Colorado's Laboratory for Atmospheric and Space
Physicsand was describedby Hard et al. [1992]. The observations discussedin this paper were performedusing the F
channelof the UVS, which coversthe 1616- 3231 A wavelength range. The calibration is described in Hendrix [1996].
The observationswere performedin "full-scan" mode, where
the grating was stepped over the 528 channels covering the
The first step in correcting for photometricvariations in
brightness was to understandhow Ganymede's UV reflectancevaries with phaseangle. We divided all UVS observations of Ganymede (both disk-resolved and disk-integrated)
into leading(0ø - 180ø W) and trailing (180ø - 360ø W) hemispheredata. Including UVS data not discussedin this paper,
we used a phase angle range of 10.4ø - 81.8ø for the leading
hemisphere,and a phaseanglerangeof 1.3%86.7ø for the trailing hemisphere. We found that the single-lobed HenyeyGreensteinphasefunction [Hapke, 1993] with an asymmetry
parameterof g=-0.35 adequatelyfits the UVS Ganymededata at
all wavelengthsin both longitude bins. However, the phase
curve is not a perfect fit; this is becauseindividual terrain
types behavedifferentlyphotometrically. In spite of this, we
use this simple phase curve for all observedregions, aware
¸
0.6
o
]
[
!
!
!
0.6
0.4
o
0.4
¸
_o
0.2
0.2
0.0
0.0
200
220
240
260
280
300
wavelength(nm)
320
340
200
220
240
260
280
300
320
340
wavelength(nm)
Figure 1. Albedosfrom threedifferentlocations(seePlate 1). (a) Standardregionalbedo,(b) averagealbedos
from PTAH and DRKLIT regions;DRKLIT albedohasbeenoffsetby 0.05 for clarity. Statisticalerror bars are
shown.
HENDRIX ET AL.' GANYMEDE'S OZONE-LIKE ABSORBER
that the correction may not be perfect but assumingthat the
phase curve does not vary significantly with wavelength.
Thereforethe magnitudesbut not the shapesof the derived albedosare affected. In the analysisof the resultant albedos, we
focus not on variations in brightness but in variations in albedo with wavelength,which can indicate the presenceof absorptionfeatures. Ganymede's UV phase curve will be stud-
14,171
2.0
o
ied further in future work.
The next step in the photometriccorrectionwas to solve for
the single-scatter
albedo00(30of eachregionusinga simplified
version of the Hapke photometric function [Hapke, 1993, p.
233], where we ignored the opposition effect and multiple
scattering. Multiple scatteringmay be assumedto be negligible at these wavelengths. The opposition effectterm at ultraviolet wavelengthsis unknown for any terrain type; this will
be investigatedin a future study. To account for macroscopic
surface roughness, we use the terms from Hapke [1993, pp.
344-345] with 0=32 ø, which is consistent with the visible
results from Domingue and Ferbiscer [1997]' the surface
roughnessparameteris not expectedto vary with wavelength.
This photometriccorrectionis very simple and is not expected
to accuratelycorrectfor photometricvariations in all observations. However, in this studywe are concernedprimarily with
shapesof albedo ratios (discussed below), which should be
unaffectedover this short wavelength range by variations in
photometricparameters.
4.
Results
4.1
A!bedos
0.5
I
2OO
220
240
260
280
300
320
340
wavelength(nm)
Figure 2. Albedo ratio: PTAH averagealbedo/standardregion albedo.
which is not presentin the standardregion, and it absorbsbetween 2600 and 2800 A.
4.3 Comparisons With Ozone Cross Section
We follow the methodof Noll et al. [1996] and compareGanymede's ultraviolet absorption band with ozone, which ab-
sorbsstronglynear2600 .X,. The crosssectionof gaseous
ozone [DeMote e! al., 1994] is shown in Figure 3a. When we
shift this cross section 197 .X, to the red and broaden the full
width at half maximum
(FWHM) from- 400 .X,to - 950 .4,
The method
described
above was used to determine
the al-
bedo of each observedregion. Figure 1 displaysthree of these
albedos,binnedinto 15 .X,bins to reducescatter. The albedos
shownare the averagealbedo measuredduring three observation sequences,of three differentterrains and have distinctly
different shapes.The southern polar PTAH region is brighter
than the other regions, particularly at longer wavelengths.
The albedo of the standard region (describedbelow) is lower
than PTAH's and is flatter at longer wavelengths. The
DRKLIT albedo, corresponding to a leading hemisphereregion, is flatter than the albedos of the other regions. Generally, leading hemisphereregions have albedo that are rela-
tively flat longward of-2900 .X,;this is also seenin UVS
spectraof regions on Europa's and Callisto's leading hemispheres. This spectral characteristicwill be investigated and
discussedin a later report.
4.2
Albedo
Ratios
We display the variation in albedo fromplace to place on
the surfaceby ratioingthe albedoof eachregion to the albedo
of a "standard" region (part of the GLOBAL sequence). The
albedoof the standardregionis shownin Figure l a. We have
chosen this as the standard
albedo
as it was observed
with
relatively low spatialresolution,so it is not the albedo of any
particular, and possibly anomalous,region; the albedo was
measuredwhen the solar and emission angles were approximately equal. Furthermore,the standard region covers an
equatorial portion of the leading hemisphere,near 140ø W
longitude,so it is an appropriateregion to which to compare
trailing hemisphereand polar data. The albedoratio of one region (PTAH) to the standardis shown in Figure 2. The ratio
indicates that an absorber is present in the PTAH region
which is similar to the FWHM of solid O3 [Faida e! al., 1989],
we obtain the "ozone-like" cross section also shown in Figure 3a.
For this
cross section
we have
also normalized
the
magnitudeof the absorptionmaximumto correspondwith that
of gaseousozone. As was noted by Noll e! a/. [1996], the
shifting and broadeningof the O3 crosssection may be caused
by the presenceof O3 in water ice.
We note that in deriving the ozone-like cross section, the
shift to the red is greater than the shift required to fit the HST
data of Noll e! al. [1996]. This is simply due to the albedo ratio used: No// e! al. [1996] ratioed the disk-integrated trailing hemispherealbedo to the disk-integrated leading hemispherealbedo; we are using disk-resolved albedos and have
somewhat arbitrarily chosen a leading hemispherestandard
region. Using a differentleading hemispherestandardregion
(or a leading hemispheredisk-integratedalbedo) would likely
resultin a different shapefor the crosssection. This is why we
refer to the absorber as "ozone-like":
we are utilizing it
mainly to quantifythe relative amountof absorptionacrossthe
surface. Laboratory measurements
of ozone in ice under Ganymede's conditionsare neededto understandabsolute quantities of the absorber.
We use this derived
cross section
in Beer's
law to deter-
mine the column densityof the ozone-like absorberat each observed location. Figure 3b displays the ratio of the PTAH albedo to the standard albedo, along with a model fit to the albedo ratio: model=A exp(-oN), where A is a constant derived
to fit the ratio magnitude, csis the ozone-like cross section,
and N is the column density derived to fit the absorption
band;N includesthe slant angle to the location of the observation, 1/g+l/go. For the PTAH region we obtained a best fit
column
density
of 4.7x10•6cm-2.
14,172
HENDRIX
ET AL.' GANYMEDE'S
OZONE-LIKE
ABSORBER
b
i
1200
i
1000
8OO
/
/
6OO
i
i
/•
i
2.0
A
\\\\ \
/
_
\
,x ......
1.5-
A=1.92,
N=4.7e16
,x
-
o
\
_
o
iI
4OO
Q
iI
2OO
1.0-
_
-
_
0.5
o
200
220
240
260
280
300
320
340
......
200
220 240
wavelength(nm)
260
280
.500 .520 .540
wavelength(nm)
Figure3. (a) Laboratory-measured
crosssectionof gaseous
ozone(solidline)versusozone-likecrosssection
derived
usingGalileoUVS Ganymede
albedo
ratio(dashed
line),(b) PTAH/standard
albedoratiocompared
to modelfit withcolumndensity
N=4.7x10
•6cm-2.
4.4 Distribution
of Ozone-Like
Absorber
tude,with a possible dependenceon amountof water ice present as may be indicated by visible brightnessof the region.
We investigatethe presenceand strengthof the ozone-like To demonstratethe trends in albedo shapewith latitude and
absorptionfeatureat all observedregions on Ganymede. longitude, we display a"typical" albedo ratio from each reWhenthe albedoof eachregionis ratioedto the albedoof the gion observed in Figure 4, and explain the featureshere.
standard
region,we findthatthe absorptionfeatureappears
to Whenwe apply a similar analysisto all observedregionson
be related,in a complicated
way, to both latitude and longi- Ganymede,we find that some regions are fit well by this
o) DRKLT(37N, 106W)
I
2.0
!
I
!
I
A
b) MEMPHIS(12N,140W)
I
,0
i
!
i
i
A-0.90,
N=0
1.5
i
i
N= 2.0e 1.5
1.5
o
-
o
10-•
'
.,'•½.. zx.z•,,.,wvv•____
_,•,•
__.,,•%_../•zx,
-
1.0
0.5
I
200
i
220
240
I
260
I
280
i
300
0.5
i
320
340
......
200
220
wavelength(nm)
1..5-
280
500
520
540
d) NIPPUR(25N, 184W}
......
i
2.0
i
A- 1.57, N=1.Se16
o
260
wavelength(nm)
c) SIPPAIR(-19N, 182W)
2.0
240
az•
i
i
A=0.93,
-
l
1.5
A
o
"'""---.-.4•.'-'•as
i
N=2.0e16
o
1.0-
-
1.00.5
200
220
24-0 260
2ao
500
wavelength (nm)
520
540
200 220 240 260 280 300 320 340
wavelength (nm)
Figure4. Albedoratiosfromseveraldifferentregionsto showhow the absorptionfeatureis distributed
across
thesurface.Alsoshown
is themodelfit to thealbedo
ratio,indicating
thebestfit parameters
(N is col-
umndensity
in cm
-2)in themodel,
where
model=A
exp(-oN).
(a)DRKLIT
region,
(b)MEMPHIS
region,
(c)
SIPPARregion,(d) NIPPURregion,(e)NORTHPOLE region,(f)NORTHPOLE region,(g) TAMMUZregion,(h) AMONregion,
(i) NORTHPOLEregion,
(j) NORTHPOLEregion,(k) BRITRLregion,(1)HILAT
region.
HENDRIX
ET AL.' GANYMEDE'S
OZONE-LIKE
2.0
a ......
A A
A
-•'•=-•
o
-
1.0-
-
220
AA --••
o
1.0-
-
0.5
......
200
240
260
280
300
320
.......
200
340
220
240
o
2.0
......
0.5
A=1.57,
-
220
••r••
1.5
260
280
300
320
a
A ......
200
340
220
240
260
280
300
320
340
,wavelength(nm)
j) NORTHPOLE(62N,
225W'
i) NORTHPOLE(65N,
201W)
2.5
......
A
N=2.Se16
A
wavelength(nm)
2.5
540
a
1.0
-
0.5
240
520
o
......
200
500
.......
N=2.6e16
1.5-• A A A
1-0- z•
280
h) AMON(55N, 225W)
g) TAMMUZ(12N, 230W)
A=1.28,
260
wavelength(nm)
wavelength(nm)
2.0
A=1.75,
N=2.5e16
...^
1.5- -- -•
0.5
......
A•t•,
A=1.63, N=1.6e16
•
14,173
f) NORTHPOLE
(64N, 178W)
e) NORTHPOLE
(55N, 174W)
2.0
ABSORBER
......
AA
A=2.05, N=5.1e16
A=2.50, N=5.7e16
A
A
A
AAA
o
o
1.5-
1.0
200
2.0 •x•AA
A
_
2.0- •xAAA
-
1.0
......
220
240
260
280
300
320
......
200
340
220
1.,5
A=1.57,
A=0.60,
1.0-
520
540
1.0-
N=l.0e16
A
o
A
A
A
200
500
......
N=1.Se16
o
0.,5
280
I) HILAT(41N, 550W)
k) BRITRL(26N, 301W)
A......
260
wavelength(nm)
wavelength(nm)
2.0
240
0.0
......
220
240
260
280
500
520
540
200
A
......
220
240
260
280
500
wavelength(nm)
wavelength(nm)
Figure 4. (continued)
520
540
14,174
HENDRIX
ET AL.' GANYMEDE'S
OZONE-LIKE
i
I
I
"
I •'
I'
I
ABSORBER
I
....
I
!
.... ß
:
,•..
•.,
ß
•
o
0
o
ß
0
0
.!
HENDRIX
ET AL.' GANYMEDE'S
OZONE-LIKE ABSORBER
14,175
quantities(not as muchas in the 180ø-240
ø W reozone-like cross section, with varying column densities, and moderate
gion). At higherlatitudesnear330øW(HILAT observation,
someare not. Model fits are shown comparedto albedo ratios
Figure41), the absorptionfeatureis not very strong.
fromeach observedregion in Figure 5. Plate 1 displays the
column densities mappedout on a Galileo/Voyager image of
Ganymede. Table 1 also displays the best fit columndensity 4.5 Variations in Ozone-Like Absorber with Solar Angle
of the ozone-likeabsorptionfor the averagealbedo of each obThe derived columndensitiesshownin Figure 4, while not
served region.
stronglyrelatedto latitudeor longitude,do havea distinct
On the leading hemisphere(east of-120 ø W), where the
correlationwith solar zenith angle. The ozone-like column
DRKLIT and BRILED observationsequenceswere performed, density increases
with solar zenith angle. In Figure 5 we
the albedo ratios (Figure 4a)are generally blue, both at high
show the derived ozone-like column density versus solar zeand middle latitudes; the highest latitude observed in this
nithangle(angleto the Sunmeasured
fromthenormal).The 2 9
longituderangeis 37ø N. The blue albedoratio indicatesthat
no ozone exists in these locations.
pointsin Figure5 correspond
with the observations
shownin
Plate 1 and Table 1. The ozone-solaranglerelationshipis par-
The MEMPHIS region (middle latitudesnear 120ø-150ø W)
showsno absorption,making the albedoratio flat (Figure 4b).
This is also where the standard region is, although the
MEMPHIS observationwas performedwith a higher spatial
resolution.
Near 180øW, the absorption feature is present in small
amounts at low latitudes (SIPPAR and NIPPUR observations,Figures4c and 4d); at higherlatitudesthe amountof the
absorbervaries (NORTH POLE observations,Figures 4e and
4I). Moving slightly closertoward the centraltrailing hemisphere,the PTAH region(190ø-200øW)at high southernlatitudesshowsthe highest quantities of the absorber,as shown
in Figure 3b.
In the 200-240øW longitude region, the lower latitudes
(TAMMUZ observation,Figure4g) show the absorptionmore
stronglythan at similar latitudesnear 180øW; the higher latitudes(AMON observation,Figure 4h) have an even stronger
absorption,while the high northernlatitudes(NORTH POLE
observations,Figure 4i and 4j) show a very strong absorp-
ticularly strongin the 150ø-240
ø W longituderange,where
the most UVS observationsexist. The two trailing hemisphere
(largeplusandsmallopencircle)observations
alsofollow the
trend. The leadinghemispherelocations(large square,large
opencircle, and large asterisk)show negligible amountsof
ozonedespitesolarzenithanglebeinglarge. We thusdetecta
strongrelationshipbetweensolar zenith angle and derived
ozone-like columndensity; this relationship is clear on the
anti-Jovianand trailing regions,in contrastto the leading
hemisphere,
whereno ozone-likeabsorber
is measured.
5.
Discussion
The UVS results clearly show a relationship between derived ozone-like columndensity and solar angle, suggesting
that the ozone is destroyedby photolysis:
03 + hv•>O2+ O(1D).
(1)
The creationof the ozone-likeabsorberat Ganymedeis less
tion.
At middle latitudes near the central trailing hemisphere
(BRITRL observation,Figure 4k), the absorberis present in
clear. It is likely that the ozone is related to the molecular
oxygendetectedby Calvin and Spencer[1997]; we speculate
hereon the possiblecreationmechanisms
of this oxygenchemistry.
?
5
4-
3c-
2-
1-
0
0
15
30
45
60
75
90
solor ongle
Figure 5. Ozone-like columndensity versussolar zenith angle. Smalldiamondsand solid circle,GLOBAL; small plus,
PTAH; large cross,small and large asterisks,and smalltriangle, NORTH POLE; small circle, HILAT; large diamond,
AMON; small square,NIPPUR; large plus,BRITRL; large triangle,TAMMUZ; small cross,SIPPAR; large circle,DRKLIT;
large square,BRILED; mediumasterisk,MEMPHIS.
A possiblesourceof boththe ozoneand the molecularoxygen is chargedparticle bombardment[Noll et al., 1996; Calvin and Slvencer,1997;dohnson,1998.]. The evidencefor this
is that the ozone-like absorber appearsmore on the trailing
hemispherethan on the leading hemisphere,as displayed
throughthe albedoratios,wherethe standardalbedois a leading hemisphereregion. Other leading hemisphereregionsdo
not display the absorption feature. This leading-trailing
asymmetryin ozonewas first detectedby Noll et al. [1996].
Other evidence for bombardmentby the corotating plasma is
the molecularoxygenfound on the trailing hemisphere[Calvin and Spencer,1997], as well as Ganymede'salbedodichotomy,whereits trailing hemisphereis darkerthan its leading hemisphere[Millis and Thompson,1975].
Chargedparticle bombardmentof water ice produces H,
OH, O, H2, 02, and H [Johnson,1998]; this was discussedas a
possible sourceof the escapingatomic hydrogen detectedat
Ganymede[Barthet al., 1997]. It has been suggestedthat the
02 exists in the ice lattice, where bombardmentproduces defects in the ice, and movementof the defects(annealing) forms
larger bubbles in which 02 can accumulate(from oxygen atoms) [Johnsonand Quickenden,1997]. At low temperatures,
molecularoxygennot only is formedlessefficiently,but small
voids (bubbles) are created,producing a scatteringsurface.
Thereforethe lack of annealing at high latitudes results in
shorterphotonpath lengths,so the weak 02 absorptionbands
14,176
Table
HENDRIX ET AL.' GANYMEDE'S OZONE-LIKE ABSORBER
1.
Disk-Resolved
Sequence
Observations
Longitude*
Latitude *
Phase
Solar Zenith
Angle,deg.
GLOBAL ++
Number of Obser-
Derived Column
Angle,deg.
vations
+
Density,
cm'2
140ø W
0øN
29
13
1
none
152 ø W
0ø N
29
1
1
none
162 ø W
5ø S
29
9
1
6.2e 15
171 ø W
8øS
29
19
1
9.8e15
180 ø W
11ø S
29
28
1
1.0el 6
189 ø W
13 ø S
29
37
1
1.9e16
199 ø W
16ø S
29
47
1
2.3e16
210 ø W
18ø S
29
57
1
3.0e16
220 ø W
20 ø S
29
68
1
4.0e 16
151øW
48 ø N
29
49
1
2.0e16
169 ø W
41 ø N
29
45
1
1.6e16
180 ø W
37 ø N
29
46
1
2.0e 16
191 ø W
33 ø N
29
50
1
2.0e16
202 ø W
29 ø N
29
56
l
2.1 e 16
213 ø W
27 ø N
29
65
1
2.4e16
225 ø W
24 ø N
29
74
1
3.2e 16
ISIS-PTAH
190 ø - 201 ø W
67øS
24
68
4
4.7e16
NIPPUR
175ø-185 ø W
250-29 ø N
25
32
10
2.0e16
MEMPHIS
116 ø -162 ø W
90-22 ø N
29
12ø-52
8
2.0e15
AMON
220 ø 227 ø W
320-34 ø N
24
64
5
2.8e16
NRPOLE
157ø-244 ø W
540-66 ø N
29ø-32
57ø-84
19
1.6-3.7e 16
TAMMUZ
2270-238 ø W
13 ø N
26
66ø-77
9
2.6el 6
176ø-191 ø W
8ø-19 ø S
31
25ø-37
11
1.5e16
DRKLIT
51ø-136 ø W
260-38 ø N
10ø-19
26ø-71
25
none
BRILED
680-85 ø W
0ø-13 ø S
22ø-25
34ø-51
11
none
BRITRL
296ø-305 ø W
26 ø N
73
28
5
1.5el 6
HILAT
327ø-333 ø W
41 ø N
64
54
3
1.0el6
SIPPAR
*Latitudeand longituderangesare for entireobservationsequence
for all observations
exceptGLOBAL; for GLOBAL, geometriesshown
are for individualobservations
within the sequence,
andlatitudeandlongitudeshownarethe FOV boresightlatitudeandlongitude.
+Eachobservation
is anaverage
of 14 spectra.
++TheGLOBALobservation
centered
at 0øN, 140øW,wasusedasthe"standard"
regionin thisstudy,to whichotherregions
werecompared.
are not seen easily. An alternative to 02 accumulating in bubbles in the ice was suggestedby Baragiola and Bahr [1998],
who find that the 02 must exist only in very cold regions of
Ganymede'ssurface,suchas in cracksor other regions not exposed to sunlight.
Like the molecular oxygen at Ganymede's surface, the
ozone is probably created by chargedparticle bombardment.
However, chargedparticles are unlikely to produceozone directly. Rather, 02 accumulatesin the ice, and subsequentradiolysis produces ozone [Johnson and Quickenden, 1997].
Laboratory experimentshave shown that the ozone-like absorptionseenat Ganymedeby the UVS can be reproduced. R.
Baragiola(personalcommunication,1998) bombardedan 02 +
I-I20 ice mixturewith 100 keV H+ ions and foundthe ozonelike absorptionfeaturein the laboratoryreflectancespectrum.
This suggeststhat charged particle bombardmentof Ganymede's trailing hemisphereproduces02 in the ice; further
bombardmentproducesO3 in the ice.
Complicating mattersof chargedparticle bombardmentis
the fact that Ganymede does have its own magnetosphere,
which shields the low-latitude surface from some particles
[Kivelsonet al., 1996]. Particles that are energeticenough to
crossthe closedfield linesincludeS+ (of at least400 keV) and
O+ (of at least800 keV)(J. Cooper,personalcommunication,
HENDRIX
ET AL.: GANYMEDE'S
1998) and makeup a significantportion of the trappedparticlesat Ganymede'sorbit [Ip et al., 1997]. Electronsand protons are not energeticenough to crossGanymede'sclosed
field lines and are deflected around Ganymedeor follow the
openfield linesintothe polarregions. It is difficultto understandGanymede'shemisphericasymmetries
when accounting
for this magnetosphere;
as was shownby Pospieszalskaand
dohnson[1989] and discussed
by Calvin and Spencer[1997],
the low-energy(cold) ions are thought to be responsiblefor
leading-trailing asymmetries,
whereasthe high-energy(hot)
ionsaffect both the leading and trailing hemispheres.We do
not yet know enough about Ganymede'smagnetosphere
to
fully understandwhat causesthesehemisphericasymmetries.
Furthermore,the amountof ice lattice damagedue to particles
of varyingenergiesis notclearandmayaffectthe amountof 02
andO3 formedat high and low latitudesby the particlestraveling alongthe openfield lines and the particlescrossingthe
closedfield lines. Calvin and Spencer[1997] suggestedthat
Ganymede'smagnetosphere
maynot be constantin strength,
allowingfor hemisphericasymmetries
to build up.
OZONE-LIKE
ABSORBER
14,1'77
Observationscoveringprimarilythe 150ø-240
ø W region
indicate variations in the amount of the ozone-like absorber;
this distributionis displayedin Plate 1 as well as in Table 1.
Figure5 showsa strongcorrelationbetweenthe derived
ozonecolumndensityandthe solarzenithangle,wherebyless
ozoneis presentat smallsolarzenithangles.
We haveproposeda combination
of chargedparticlebom-
bardment
andphotolysisfor the creationand destruction
of
the ozonedetectedby the UVS. Chargedparticlebombardment,constrainedprimarilyto Ganymede'strailing hemisphereandpolarregions,creates
ozonein the ice. At lowlatituderegionsthe ozoneis convertedby photolysisinto
molecularand atomicoxygenduring the day (particularlyat
smallsolarzenith angles);polar ozoneis likely long-lived.
The ozonecolumndensitythusdependson longitude(moreis
createdon the trailinghemisphere
wherethereis morecharged
particlebombardment),
andis inverselycorrelated
with solar
angle,wherethesolarangleis measured
fromthe localnormal
(moreozoneexistsat large solar zenith angles: at the poles,
andat lowerlatitudesat night,andearlyandlate in the day).
Other mechanisms to consider are the roles of solar radia-
tion and sputteringby chargedparticles. Ultraviolet photons
may break up the water ice molecules,producingmolecular
oxygenand ozone,althoughthe penetrationdepth is not as
great as with ions. A longitudinal variation in molecular
oxygenand ozonesignatures
is not expectedif solarradiation
were the primarysource,unlesssomeother processdestroys
the oxygenandozoneonly on the leadinghemisphere.Possible methodsof destructionof leading hemisphereoxygen and
ozone include micrometeorite bombardment [Calvin and
Spencer, 1997] and sputtering.
Acknowledgments.
Theauthors
thankR. Baragiola,
T. Orlando,B.
Pappalardo,
andJ. Cooper
for helpfulconversations,
T. Rosanova
for
theSSIimage,andB. Knappfor theSOLSTICEspectrum.R.E. Johnsonandan anonymous
reviewerprovided
constructive
comments.
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