JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 104, NO. E6, PAGES 14,183-14,187, JUNE 25, 1999
Reply
R. A. Baragiola, C. L. Atteberry, and D. A. Bahr
Laboratoryfor Atomic and SurfacePhysics,Universityof Virginia, Charlottesville
M. Peters
Schoolof Medicine, Laboratory of Physics,Teikyo University,Tokyo, Japan
1.
Introduction
Although it was expectedthat oxygen, a product of the
dissociationof water, existsin thin atmospheresaround icy
satellites[YungandMcElroy,1977],the discoveryof condensed
oxygenon Ganymedehasbeen surprisingand perplexing.Several hypotheseshad been advancedto explain the finding,
whichwere then testedin simulationsat our laboratory[l/idal
et al., 1997;Baragiolaand Bahr, 1998] (to be referred to in the
followingas V97 and BB98, respectively).In the preceding
comment,Johnsonquestionsthe applicabilityof our laboratory experimentsto test conjectureson the radiolyticformation
of oxygenbubblesin the surfaceice by magnetospheric
ions.In
this reply we addressall the issuesposed,point to the qualitative nature of some of Johnson'shypotheses,and evaluate
them using published quantitative data, including some new
resultsfrom our laboratory.
We startwith a summaryof the evidence.In 1972,Carlsonet
al. [1973] observeda noninstantaneous
stellar occultationby
Ganymede,implyingthat the surfaceatmosphericpressureat
theterminators
wasat least10-3 mbar.A fewyearslater,Yung
and McElroy [1977] developeda model in which an oxygen
atmosphereof this magnitudewas producedby the photodissociationof desorbedwater followedby the escapeof hydrogen. The role of energeticion sputteringin producingan atmospherewas subsequently
pointed out by Lanzerottiet al.
[1978]. Based on a stellar occultationobservedby Voyager,
Broadfoot
et al. [1979]placedan upperlimitof 10-8 mbaron
the surfacepressureof 02, CO2, CH 4, and H20. Althoughthis
discrepancywith the earlier resultsof Carlsonet al. [1973]was
not addressed,later authors seem to have relied more on the
Voyagermeasurements,
andKumarandHunten [1982]showed
that the modelof YungandMcElroy[1977]admitteda solution
consistentwith Voyager's data. Recently,Hall et al. [1998]
confirmeda faint oxygenatmosphereon Ganymede,excitedat
the polesby energeticelectronsconstrainedby the satellite's
magneticfield. We have pointedout that the two occultation
experimentscould be reconciledif the atmosphericdensityis
variable,either as a functionof time or location(V97, BB98),
which is also suggestedby the time variability of volatileson
the satellite'ssurface[Domingueet al., 1998].
The first suggestionthat some form of molecular oxygen
might be presenton Ganymede'ssurfacecamefrom Nelsonet
al. [1987],who proposedthat the low ultravioletreflectanceof
the trailing hemispherewas due to ozone resultingfrom implantation of magnetosphericoxygenions into the surfaceice.
Ions would be preferentially implanted on the trailing side,
since the rotation of the magnetosphere is faster than
Copyright1999 by the American GeophysicalUnion.
Paper number 1999JE001023.
0148-0227/99/1999 JE001023509.00
Ganymede'sorbital motion.Observationsof Ganymedeusing
a ground-based
telescope[Spencer
et al., 1995]and the Hubble
SpaceTelescope(HST) [Calvin and Spencer,1997] showed
weak (<2%) absorptionbands in the visible, revealing the
presenceof solid 02. The bands are not detectableat high
latitudesand not in a largepart of the leadinghemisphere.The
band depth peaked near 265ø orbital longitude,which also
correspondsto that of maximumband depth in the ultraviolet
observedby Nelsonet al. [1987].No condensed-O
2 bandswere
visiblewithin errors for the disk centeredat a longitudenear
15ø, althougha large part of the trailing hemispherewas visible. The searchfor condensed02 on Europa, Callisto, and
Saturn'ssatellitesandringsproducednegativeresults[Spencer,
1998].The broad absorptionbandin the ultravioletwasfurther
studiedby Noll e! al. [1996]with the HST and by Hord et al.
[1996]andHendrixet al. [thisissue]with the Galileo ultraviolet
spectrometer.These latter researchersfound that the ozonelike bandsare deepernear the polesand absentat the equator,
exceptnear the terminator,just the oppositeto the behaviorof
the 02 bands. Ozone-like bands have also been detected in
spectraof Dione and Rhea [Noll et al., 1997].
The detectionof condensed0 2 has raised two main questions:(1) what is the sourceof this oxygen?and (2) how can
oxygenexistin a solidform if the temperatureof Ganymedeis
so high that the vapor pressureof oxygenwould exceedby
orders of magnitudethe atmosphericpressureinferred from
occultationexperiments?
Thesequestionsgenerateda seriesof
hypothesesthat we intended to test with laboratory experiments.Spenceret al. [1995] proposedthat condensedoxygen
might be stable at high temperaturesif trapped inside the
surfacewater ice.This raisedthe questionof howis 02 trapped
insidethe ice, whichled to two additionalhypotheses:
(1) 02
accumulatesdue to implantationof oxygenions from the Jovian magnetosphere,
and (2) O2 is formedinsidethe ice after
water is decomposed
by high-energyions(radiolysis).The second hypothesis
wasamplifiedby Calvinet al. [1996],who stated
that the oxygenwastrappedin "bubbles"producedby radiolysis, but added that oxygencould also be physisorbedin a
porous regolith, or be embeddedin clathrates.The idea of
irradiation-inducedbubbleswas further exploredby Johnson
and Jesser[1997].
Our firstexperiments(V97) determinedthat implantationof
magnetospheric
oxygenions,proposedby Spenceret al. [1995],
cannotproducethe observedvisiblebands.The markedweakeningof the absorptionwhen oxygenis dilutedin ice confirmed
the very strongdependenceof absorbanceon oxygendensity
(V97, BB98), which resultswhen two adjacentoxygenmolecules,(02)2, participatein the absorptionof one photon.The
findingsimplied that the hypothesisof oxygenbeing in gas
bubblesor in clathrateswould require a large columndensity
of oxygenincompatiblewith radiolyticproductionby magne-
14,183
14,184
BARAGIOLA
ET AL.: COMMENTARY
tosphericions.The radiolysis
hypothesis
lostfurtherground
uponthe discovery
of a magnetic
field [Kivelson
et al., 1996]
that wouldpreventmostmagnetospheric
ionsfrom reaching
the surfaceat the latitudes
wherethe (02)2 bandsare prominent.An alternativesource,ionsof sufficient
energyto penetrate the magneticfield at low latitudes,washypothesized
by
Johnson
andJesser
[1997].Theystated,withoutgivingdetails,
that energetic
ion bombardment
wouldproducethe required
amountof trappedoxygenin about 1 year. Our laboratory
experiments
(V97, BB98)wereintendedto test(1) if ion irradiationcan produceenoughoxygenin ice to accountfor the
observed(02)2 bands,and (2) if oxygenin ice is stableat the
temperatures
assumed
in the models.The interpretationof the
experimental
resultsand their significance
in testingvarious
hypotheses
is the subjectof Johnson's
comments.
They are
focusedon (1) the stabilityof 02 in ice,(2) the production
of
oxygenbubblesby ion irradiation,and (3) comparison
of laboratoryandsatellitebandintensities
andthe relatedquestion
of the temperature
of condensed
oxygenon Ganymede.
2.
Stability of Oxygen in Ice
Johnson
notescorrectlythat the fastdiffusionof oxygenin
amorphous
icemaynotholdin crystalline
ice.Notingthatour
iceshavea porosityof 10%,whichcouldleadto percolation
pathways,
hementions
thecaseof a particularly
largemolecule
(CC14)whichis stable,in laboratorytimescales
at 140K, under
a thinlayerof vapor-deposited
ice [Smithet al., 1997].Thisis
not an argumentagainstthe suitabilityof our experiments,
sinceall vapor-deposited
films,amorphous
or crystalline,
are
porous[Wesley
et al., 1998],includingthoseby Smith et al.
[Stevenson
et al., 1999].More important,all othersmallgas
molecules
testedby Bar-Nunandcoworkers
behavejustlike
oxygen(for a review,seeBar-Nunand Owen[1998]),slowly
desorbingfrom ice below 140 K. This groupof molecules
includesCO; thus Johnson'sassertionthat the behavior of CO
3. Is Radiolysisof Ice the Sourceof Oxygen?
Ganymede's(02)2 opticalcolumndensityr/can be calculatedfromd•o, the depthof the 1-0 absorption
band,andan
absorbance
of 11 cm-• (V97).Themaximum
disk-averaged
valuein Ganymede's
trailinghemisphere,
d•o = 0.018, gives
r/ • 2 x 1019O2/cm
2. Higherdensities
shouldresultif one
excludes
the polarregionsandthe part of the trailinghemisphereseenat orbitallongitudeof 15ø,where(O2): is not
detectable.As pointedout by V97, sincethe absorbance
of
oxygen
variesasa highpowerof density,
muchhighervaluesof
r/wouldresultif thecondensed
oxygen
werein lowerdensity
forms,suchas liquid,and evenhighervaluesif it were in a
densegas,the "microatmospheres"
postulated
byJohnson
and
Jesser[1997].
Cantheserelativelylargequantities
of oxygen
be produced
by fast-ion irradiation of ice? This is an essentialarea of controversy.We do not questionthat radiation bubblesor inclu-
sionsof trapped-gas
inducedby radiationare formedand sta-
blein manymaterials.
Whatwe question
is theamountof gas
trappedandJohnson's
assumption
that resultsfor metalsirradiatedin nuclearreactorscanbe extrapolated
to a materiallike
ice,whicheasilycracksunderstress,
amongotherdifferences.
Johnson
asserts
that O2 molecules
are produced
in ice by
penetratingradiationat a rate G(O2) -- 0.15 per 100 eV of
energydeposited
into electronicexcitations.
We havequestionedthisvaluefor severalreasons
(BB98).First,it appeared
in a second-hand
accountof a 1955experimentdoneunder
conditionsnot clearlyspecified,and it hasnot beenconfirmed
bymodernexperiments.
Second,
whiletestingtheoxygen-ion
implantation
hypothesis
of Spencer
etal. [1995],V97foundthat
G(O2) < 5 X 10-4 for 60 keV O•- ion irradiationof ice.
Johnson
claimsthattheseexperiments
are not appropriate
to
disprovethe radiation-bubble
hypothesis,
sincethe ionsused
wereverydamaging
andnotpenetrating
enough.However,the
assertion
wasnot accompanied
by an indicationof what is too
muchdamageor too little penetration,
or grounded
on any
is similarto the muchlargerCC14appearsincorrect.CO des- factualrelationship
betweenG and ion penetrationdepth.
orbsfromicemixturesalreadyabove27 K, asalsoreportedby Nevertheless,
we did newexperiments
with 100keV protons
Schmittet al. [1988],Kouchi[1990],andPalumbo[1997].We [Atteberry,
1998].Theseionsarerepresentative
of the proton
mustadd that, asmentionedby BB98,theseexperiments
do fluxat Ganymede
[Ipetal., 1997]and,compared
to 60keV O•-,
not rule out that condensed
oxygencouldbe trappedin inclu- they penetratemuch deeper:--•2 •m versus0.13 •m, and
sionsin a lessvolatile,transparent
materialor on ice capped sputtermuchless:2.9versus
87molecules/ion
[Shietal., 1995].
with such material.
To improveoxygen
retention,weusedcrystalline
icedeposited
We nowreturnto the questionof porosity.Johnsonargues at 150K, sinceJohnsonhasarguedthat crystalline
icewould
that the ice on Ganymedewouldbe "annealed,"an undefined retainmuchmoreoxygenthan amorphous
ice. We irradiated
termthatseemsto implycrystallinity
andthe absence
of po- thickicesamples
with3 x 1016H/cm2at70K andanalyzed
the
rosity.We notethat all published
analyses
of the photometric trappedgasesby evaporating
the film and analyzing
the desdataof Ganymede(for a review,seeDomingue
and l/erbiscer orbinggaswith a calibratedmassspectrometer.
Sincewe used
[1997])coincidewith the conclusion
that the icy regolithis isotopic
water(H2•sO),we couldavoidthe interference
of
porous.
Thesizeof poresthataffectvisiblereflectance
islarger small
amounts
of 1602present
inourultrahigh-vacuum
system.
thanthemicropores
usuallyinvokedin gasadsorption
exper- We foundan upperlimit of 0.04 O2/H, whichcanbe translated
iments.However,microscopy
studies
of vapor-deposited
amor- into G(O2) < 4 x 10-5. It couldstill be arguedthat the
phousice[Lauferetal., 1987;Atteberry,
1998]revealroughness,penetrationis insufficient
so that 02 molecules
are sputtered
whichmay includepores,on a scaleof microns.Thesestruc- or releasedby radiationenhanced
diffusioninsteadof being
turesdo not disappearwhen the ice is warmedto 200 K. The retainedin the ice.This possibility
canbe takencareof by
questionof the crystalline
structureof Ganymede's
surfaceice assuming
that all the sputteredoxygenwouldbe retainedif it
is moredifficult.Ice grownin the laboratory
at lowtempera- wereproducedverydeepor if the surfacewascoveredby an
turesis amorphous
butwith somecontentof the cubicphase oxygen-impervious
layer.At 70 K, the temperatureof irradia[SackandBaragiola,
1993].Whetheramorphous
icecrystallizes tion, the yieldof sputtered02 moleculesis lessthana factorof
on a timescalerelevantto Ganymede's
regoiithcan onlybe 25 smallerthanthetotalsputtering
yield[Brown
etal., 1984].
answeredafter the temperatureof the ice is known, a non- Thus,includingsputteredoxygen,we arrive at the upper
trivial questionaddressedbelow.
bound,G(O2)< 2 x 10-4, morethan3 ordersof magnitude
BARAGIOLA
ET AL.: COMMENTARY
14,185
band positions of oxygen in different environments. This
broadeningwill not be the samefor the 1-0 and the 0-0 bands
becausethe 0-0 band positionvariesmore than that of the 1-0
band with temperature and among different states of condensedoxygen(V97, BB98). Therefore spectrafrom a source
like Ganymede,which likely containsa mixture of oxygenenvironments,will have broader and relatively less intense 0-0
bands.This may explain,at leastpartially,why there is a better
agreementbetween1-0 band shapesmeasuredin Ganymede's
spectrumand thosemeasuredin the laboratory,than with the
particularly
fastreactionin the solidstateis OlD + H20
0-0 band shapes.We note that, to compare laboratory and
OH + OH or H202 [Sedlacekand Wight,1989].Also, there are astronomicalobservations,
it is more sensitiveto use the posireactionswith atomic hydrogen,a major product of ice radi- tion of the 0-0 band and the shapeof the 1-0 band. It is the
olysis.H atom diffusing through ice will efficiently scavenge asymmetryof the 1-0 band what demonstratesthat the oxygen
atomic O by the reaction:H + O + H20 -• OH + H20. If O2 on Ganymedeis not in liquid or in gaseousform in "microatis formed, the moleculecan be readily destroyedby radiation mospheres"in the ice, sincethosephaseshave a symmetrical
1-0 band. Johnson also mentions that the recent HST obseror by the reactionsH + O2 -• OH + O or H + O2 + H20
HO 2 q- H20. These reactions,plus othersinvolvingions,have vationsof Calvin and Spencer[1997] suggestthat band posibeen discussedby Yung and McElroy [1977] and Spinksand tions shift with latitude. We feel that the noise level in the HST
Woods[1990], amongothers.
data, presenteven after substantialsmoothing,is too high to
lessthan the value usedby Johnsonand Jesser.Similar results
were obtainedat 100 K. We stressthat the upper limit for 02
productiontakesinto accountthe effect of limited penetration
and damagementioned by Johnson.
The inefficiencyof 02 productionfrom ice can be understood from extant knowledge about radiolysisof water ice.
Oxygenatoms (but not 02) are indeed a primary, although
unlikely,productof dissociationof water molecules.However,
O atomsare very reactive.Productionof 02 by O + O recombinationmustcompetewith other, more efficientprocesses.
A
warrant
4.
Are There Sufficient High-Energy Ions?
this conclusion.
The structure
that seems to exist at
around 562 nm, on the short-wavelengthside of the 1-0 band,
has about the same amplitude as oscillationsseen in other
regionsof the spectra.
A secondchallengeto the idea that a large concentrationof
oxygenbubblesis formed in ice by radiolysisby high-energy
ions arisesfrom noticing that the flux of high-energyions is
very low. «dal e! al. [1997] showedthat the strengthof the 6. What is the Temperature and Location
oxygenbandsat the temperatureassumedby the modelsre- of the Condensed Oxygen?
quired that bubblesbe formed over ion penetrationdepthsof
Our previouswork stronglysuggestedthat the sourceof the
the order of a centimeter or more. Indeed, in his comment, oxygensignaturesin Ganymede'sreflectancespectrummustbe
Johnsonstatesthat energeticions producingthe oxygenbub- colder than the equatorialdaylighttemperatures(90-140 K)
bles in the ice are protons with centimeter path lengths.He reportedby the Galileo photopolarimeterradiometer (PPR)
cautions,though,that it is essentialto model this energeticion [Orton et al., 1996]. After intensiveexperimentswith oxygen
bombardment and to confirm the hypotheseswith experi- and oxygen-waterices,we found that only experimentsat very
ments. We note that penetration to depths of centimeters low temperatureshave producedoxygenband positionsand
requiresproton energiesof the order of 30 MeV or larger, in shapessimilar to those of Ganymede.In addition, stabilityof
the favorablecase of normal incidence.The flux of protons the condensedoxygenduringprolongedexposureto the solar
with energy above 30 MeV can be calculated from Galileo flux requireslow temperatures.Two alternativecold sources
measurements
lip et al., 1997].The resultis 6 x 103protons/ were suggested
by V97: an atmospherichazeand a coldsurface
cm2/s;
theenergyfluxfor theseionsis 109keV/cm2/s.
Withthis source.Although the atmosphereof Ganymedeis very thin, a
fluxandtheuppervalueof G(O2) < 2 x 10-4 derivedabove, haze might still exist, stabilized by electrostaticforces, since
the02 production
rateis <2 x 10602/cm2/s.
Buildingup the both the haze and Ganymede'ssurfacemay acquirepositive
columndensity
observed
on Ganymede
requires>2.4 x 10•9/ surfacechargedue to irradiationby ions and ultravioletlight,
2 x 106S• 0.3millionyears,notthevalueof 1 yeargivenby and resultingelectronemission.Our currentknowledgeof the
JohnsonandJesser[1997].Thusit is unlikelythat the required electrostaticpotentials at Ganymede is insufficientto allow
oxygenconcentrationwould be built up, since the times for modeling.Nevertheless,we note that extremelylow temperaregolith gardening(which shouldliberate oxygentrapped in tures would apply to an oxygenhaze, since oxygen,being a
symmetricmolecule,would absorbonly minisculeamountsof
the ice) are of the order of 100-1000 years[Spencer,1987].
sunlightand thermal infrared from its environment.The temperature of high-albedopatcheson the surfacewas discussed
5, (02) 2 Band Shapesand Relative Band Depths
by us (V97, BB98). Although the argumentshave not been
Johnsonpointsout correctlythat the relative depthsof the challengedby Johnson,he claims that the low temperatures
1-0 and 0-0 bandsof (02)2 seenin different situationsshould needed for solid oxygenare "implausible,"sinceO2 must exbe addressed,but did not offer an analysis.The ratio of inten- perience the photon flux to be observed,and that any cold
sitiesof the 0-0 to 1-0 bandsis lowerin Ganymedethan for any trapswould be at the poles.
of the laboratory samples.The same is true about the total
First, we agree that cold traps can occur near the poles.
band area, a more meaningfulquantity,sincebandwidthsdif- Someof thesetrapswill be permanentlyshaded,and they will
fer. The discrepancyis likely real, but it can be affected by act as effectivesinksof oxygenand other condensablegases.
uncertaintiesin backgroundsubtractionand in the cancellation These sinkscan hold the large amount of oxygensuggestedby
of solarlineswhenprocessing
the telescopicdata.We note that the measurements
of hydrogenescapefrom Ganymede[Barth
band positionsand widthsdependon environmentfactorslike e! al., 1997;Frank et al., 1997]. Other regionswill be illumiO2 density,proximityto other molecules,and temperature.An nated by the Sun, but this doesnot mean that they will yield
apparent band broadeningwill result from a distributionof detectableO2 bands. Ice near the poles does not afford the
14,186
BARAGIOLA
ET AL.: COMMENTARY
large opticalband depthsneeded,due to the strongirradiation
by magnetospheric
ions,whichproducescatteringand absorption centers.Sinceoxygenfreezesat 55 K, temperatureslower
than thisvalue are neededbut only in the regionsthat contain
oxygen.Ganymede'ssurfaceis patchy,with very dark and very
bright regions,downto the scaleof tensof metersresolvedby
the Galileo solid-stateimager. The extraordinaryalbedo contrast in differentregionshasbeen mentionedin recentreports
[Beltonet al., 1996; Pappalardoet al., 1998; Prockteret al.,
1998]. BB98 pointed out that an infrared radiometerlike the
PPR, looking at a patchysurfacewith much lower resolution,
will yield a temperature close to that of the darkest (and
thereforewarmest)regions,due to the extremedependenceof
infrared emissionon temperature.Thus the PPR is not sensitive to the temperatureof bright surfacepatches.
BB98 pointed out that better averagesshould result at
longer wavelengths.De Pater et al. [1984] obtained a diskaverageddaylightbrightnesstemperatureof only 55 _+6 K at
6-cm wavelength.We noted that the kinetic temperature is
uncertain,due to the unknownemissivityof the surfacematerial. Nevertheless,this is a disk-integrated,averagedaily temperature.Lowervaluesfor surfacetemperaturesare feasiblein
regionsthat absorblittle sunlightbecauseof a high intrinsic
albedo,becausetheyreceivesunlightonlya few hoursa day,or
becausethey have high slopes,suchas bright crater rims.
Ganymede'strailing hemisphereneed not be entirely covered with cold oxygendeposits.For example,a high oxygen
concentrationin patchescomprisingonly a few percentof the
total area can give the observedband depthsof •2%. Small,
cold areason Ganymedewill continuecoolingduringthe night
and accumulatesolid depositsof oxygenand other gases.The
surfacedepositswill be irradiatedby ions,whichwill synthesize
ozone. In the morning, those areas exposedto sunlightwill
start to warm up. The bolometric albedo is expectedto be
higher after oxygencondensation,sincesolid oxygenis highly
scattering[Landauet al., 1962;Baragiolaand Bahr, 1998] and
since,as mentioned, oxygendoes not absorbsignificantsolar
radiation.Solid oxygenwill evaporatefrom the surfaceand at
the same time be replenishedby oxygenmigratingfrom the
very cold subsurfacesuggestedby microwave radiometry.
Evaporation will produce a local atmosphere but will also
resultin coolingof thesurface.
With a sublimation
rateof l0 is
exp(-1100 K/T) 02 cm-2 s-1 [Schlichting
andMenzel,1993],
evaporativecoolingof oxygenwill limit the surfacetemperature to below
40 K for a bolometric
albedo
of 0.9. These
low
temperatureswill be maintaineduntil the oxygenis depleted.
We note that, as pointed out by BB98, the disk-resolvedHST
observations
[Calvinand Spencer,1997]suggestthat condensed
oxygenis more prominent at morninglongitudesand may be
absentin the local afternoon.In the evenings,oxygenwill start
recondensing,and ozone will reform under sunlightand ion
irradiation,where it is detectable[Hendrixet al., this issue].
In this scenario,oxygenwill condenseon bright patchesat
other latitudes,but the faintnessof the (02) 2 absorptionbands
will limit their detectabilityin terrain that cannotprovidesufficient opticalpath lengths.This will be, in particular,the case
for the bright polar regionswhich, thoughthey likely contain
condensedoxygen,shouldhave a low opticalpath length due
to defectscausedby ion irradiation, The fact that not only 02
but also ozone are not detectablein the leading hemisphere
suggeststhe lack of sufficientoxygenand not a reducedpath
length in this hemisphere.This, in turn, may result from enhanced micrometeoritebombardment in the leading hemi-
sphere[Calvinand Spencer,1997],whichprecludesthe existenceof significantareasof high-albedoterrain. We stresshere
that condensedoxygenshouldexistin localizedpatches,which
are not representativeof an averagehemisphericterrain. In
particular, the trailing hemispheremay contain more very
bright patchesthan the leadinghemispherein spiteof a lower
overallbrightness.
Our scenario,althoughspeculative,
hasthe propertyof providing a consistentexplanationfor the wavelengthpositionof
the visible absorptionbandsof solid oxygen,for their shape,
andfor the variableatmospheresuggested
by the contradictory
occultation experimentsmentioned above. In contrast,the
other speculativescenario,involvingradiation-inducedoxygen
bubblestrapped in warmer ice, is inconsistentwith the observationsjust mentionedand is, further, not supportedby the
current knowledgeof ice radiolysis.
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