JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 106, NO. El2, PAGES 33,285-33,290, DECEMBER
25, 2001
Radiolysis of water ice in the outer solar system:
Sputtering and trapping of radiation products
D. A. Bahr, M. Famfi, R. A. Vidal, 1 and R. A. Baragiola
Laboratoryfor SpaceResearch,EngineeringPhysics,Universityof Virginia, Charlottesville,Virginia, USA
Abstract. We performed quantitativelaboratoryradiolysisexperimentson cubicwater ice
between40 and 120 K, with 200 keV protons.We measuredsputteringof atomsand
moleculesand the trappingof radiolyticmolecularspecies.The experimentswere done at
fluencescorresponding
to exposureof the surfaceof the Jovianicy satellitesto their
radiation environmentup to thousandsof years.During irradiation, 02 moleculesare
ejectedfrom the ice at a rate that growsroughlyexponentiallywith temperature;this
behavioris the main reasonfor the temperaturedependenceof the total sputteringyield.
02 trappedin the ice is thermallyreleasedfrom the ice upon warming;the desorbedflux
startsat the irradiation temperatureand increasesstronglyabove 120 K. Severalpeaksin
the desorptionspectrum,which dependon irradiation temperature,point to a complex
distributionof trappingsitesin the ice matrix. The yield of 02 producedby the 200 keV
protonsand trapped in the ice is more than 2 ordersof magnitudesmallerthan usedin
recent modelsof Ganymede.We also found small amountsof trapped H20 2 that desorb
readily above160 K.
directlyfrom the ice by the incidentradiation.The ejectionof
H 2 and 02 from ice due to electronicprocesses
inducedby fast
Atmospheresaroundicy satellitesare generatedin part by ions (>10 keV/amu) dependson the squareof the electronic
thermal evaporationof the surface ices. In addition, if the stoppingpowerdE/dx (or linear energytransfer(LET)) and
atmosphere
is thin with respectto penetrationby fast (keV to dependsstronglyon temperature[Brownet al., 1978;Ciavolaet
MeV) chargedparticlesand Lyman a radiationfrom the Sun, al., 1982; Reimann et al., 1984; B•nit and Brown, 1990]. In
another sourceof desorptionoccursthrough the processof addition,and as expected,other specieslike H and OH have
sputtering or impact-desorption.That magnetosphericion been found to be ejected by ions of energiesbelow 10 keV,
sputteringof icesis a sourceof atmospheresaroundicy satel- where momentumtransfer (knock-on) collisionsdominate
lites was first pointed out by Lanzerottiet al. [1978], after sputtering[Haring et al., 1983, 1984; Bar-Nun et al., 1985].
laboratorystudiesdemonstratedsputteringyieldsof water ice Emission of those radicals has not been considered for the case
for ionizingparticleswere much higher than expected[Mc- of electronicsputteringby more energeticions, more relevant
Cracken,1974;Brownet al., 1978].Lanzerottiet al. [1978]pro- to the icy satellites.Sputteringyieldsof H 2 and 02 have been
posedthat sputteringwasmore importantthan sublimationin studied in the electronic sputtering regime but have been
that the desorbed
producinggaseousatmospheresaround Ganymedeand Eu- quantifiedin the pastindirectlyby assuming
ropa. Sincethen, surfaceerosionand atmospheregeneration flux consistsonly of these moleculesplus intact water moleby sputteringhave been modelednumeroustimes;represen- cules[Brownet al., 1984].
Evidencefor atmospherescame mainly from stellar occultativereferencesareJohnson[1990],Shiet al. [1995],Ip [1996],
Ip et al. [1997],Johnsonand Jesser[1997],Johnsonand Quick- tations[Carlsonet al., 1973;Broadfootet al., 1979] and from
enden[1997],Sauret al. [1998],and the paperscited therein. observationof optical emissionfrom the atmosphere[Hall et
Photosputtering
by solar radiationis an additionalminor at- al., 1995,1998;Barthet al., 1997;Feldmanet al., 2000].The first
mosphericsource.It wasfirst postulatedby Carlson[1980]for indication of atmospheric oxygen around Europa and
Saturn'sring particlesand observedin the laboratoryby West- Ganymedecame from the ultraviolet emissionmeasurements
of Hall et ai. [i995, i998], which supportedpredictionsby
ley et al. [1995a,b].
Lanzerotti
et al. [1978]andJohnsonet al. [1981]that sputtering
The atmosphereabovewater ice surfaceswill contain not
only water moleculesbut also dissociationfragments(H, O, of ice will produce02 atmospheresaroundthosesatellites.In
OH) from collisions
of the incidentradiationwith atmospheric contrast, finding condensed oxygen, 02, and ozone on
water molecules,and productsof chemicalreactions(H2, 02, Ganymede[Spenceret al., 1995;Noll et al., 1996]was surprisetc.), both as neutralsand ions [Yungand McElroy,1977].In ing,sinceboththesespecies(but in particular02) are unstable
addition to being createdin atmosphericradiation processes, in condensedform at the relatively low pressuresand high
radicalsand stablemoleculescan alsobe producedand freed temperatures of this satellite. The existence of 03 on
Ganymedewaspostulated[Nelsonet al., 1987;Noll et al., 1996]
to explain a UV absorptionband seen in the ratio of reflec•Visitingscientist
fromINTEC-CONICET,SantaFe, Argentina.
tance spectraof the trailing and leadinghemispheresof this
Copyright2001 by the American GeophysicalUnion.
satellite.However,there are significantdifferencesin the positionsof Ganymede'sUV absorptionband measuredby Noll
Paper number 2000JE001324.
0148-0227/01/2000JE001324509.00
et al. [1996]and by Hendrixet al. [1999].Solidozoneis consis1.
Introduction
33,285
33,286
BAHR ET AL.: RADIOLYSIS
OF OUTER SOLAR SYSTEM WATER ICE
tent with the existenceof solid 02, detected in the visible
reflectanceof the trailinghemisphereof Ganymedeby Spencer
et al. [1995],Calvinet al. [1996],andCalvinand Spencer[1997].
These authorsproposedthat 02 is formed by radiolysisinducedby magnetospheric
ions and that 02 is trappedin bubbles insidethe surfaceice. This interpretationwas questioned
by Vidal et al. [1997]and by Baragiolaand Bahr [1998]on the
groundsof laboratoryresultsthat showedthat 02 couldnot be
retained in water ice in sufficientamounts to produce the
observedabsorptionbandsin Ganymede.Also, studiesof 02
recordedthe differencebetweenmassspectraobtainedwith
the ion beam on and off. The spectrometerwas calibrated
routinelyusingsublimationfluxesfrom pure and mixed ices,
measuredabsolutelywith the microbalance.To minimize interferencewith backgroundwater in the vacuumsystem,we
usedisotopicwatercontaining
70% H2180and30% H2•60.
The isotopiclabelingwasdoneusing•80 and not with the
more commondeuterium(D20), becausethe largemassdifferencebetweenH and D may affectradiolyticprocesses.
For
determination
of partial02 yieldsweused•802.In all cases
we
The uncertainties
in the parabsorptionband shapesand positionsin the red (around600 correctedfor isotopicabundance.
nm) showedthat 02 had to be in the form of a solid, not a tial sputteringyieldsamountedto ---30%.
liquid or gas [Baragiolaand Bahr, 1998].These spectroscopic To searchfor the trappingof radiationproductsin icewe did
desorption(TPD) of icesirradiated
laboratory data imply that solid 02 exists at temperatures temperature-programmed
much colder than the averagetemperaturesof Ganymede's to a fluence of 1.5 x 10•7 H+/cm 2 at 200 keV. The TPD
surfaceice. Suchlow temperaturescould occur at very bright method consistsof warming the ice at a rate of ---1 K/min and
of differentgasesevolvingfrom
surfacepatches,where atmospheric02 could be condensed recordingthe partial pressures
the
ice
with
the
mass
spectrometer.
For the TPD experiments
[Baragiolaet al., 1999a].The hypothesisthat 02 formsradiowe
did
not
use
any
heat
shield
around
the sample,to avoid
lyticallyinsidethe ice and is trappedin bubblesproducedby
radiationdamageis alsoproblematicbecausethe 02 bandsare effectsdue to trappingand releaseof desorbedgasesby the
indicatethe speciestrappedin the
seenmainlyin Ganymede'sequatorialregion,where the ener- shield.The measurements
ice, but one must bear in mind the possibilityof chemical
getic particle flux is mostly excludedby its magneticfield.
reactionsbetween trapped speciesduring warming. When
Argumentson the controversyof this topic have been pubcomparingdesorptionof water and radiolyticproducts,we
lishedrecently[Johnson,
1999;Baragiolaet al., 1999a].
took into account that water evolves from the whole area of the
Understandingand modelingsurfaceprocesses
in icy satelfilm (---1 cm2),whilethe products
originateonlyfrom the
litesare madedifficultby the scarcityof laboratorysimulations
irradiated
area(0.33cm2).ThustheTPD spectra
of sublimated
made under pertinentconditions.It is thereforeof interestto
water representboth irradiatedand unirradiatedregions.
obtain quantitativeresultson absoluteyieldsfor productionof
molecular02 and other speciesby radiolysisthat can be applied to astronomicalproblems.This is one of the motivations 3. Results
of the research described below. Another
motivation
for these
measurementscomesfrom recent discussions
aroundthe possibilityof life in Europa by Chyba[2000],who has proposed
that radiolytic02 producedin Europa'ssurfaceice canmigrate
to an underlyingoceanand fuel microbiallife at depthswhere
photosynthesis
is not possible.
3.1. Sputtering
We measuredthe partial sputteringyields of H20 and 02
from ice at 120 K due to 200 keV protonsat 48ø incidence,a
situationtypical of the ion flux at the icy Galilean satellites.
The yieldswere 0.7 _ 0.2 H20/proton and0.9 _ 0.3 O2/proton.
Thesevalueswere obtainedfor fluencesabove10TMH+/cm2,
above the range where 02 emission varies with fluence
[Reimennet el., 1984].The QMS revealeda verystrongflux of
The experimentalarrangementat the Laboratoryfor Space H atoms, which has not yet been quantified.As stated in
Researchat the University of Virginia has been describedin section1, H 2 is alsodesorbedduringirradiation,but measureseveralpublications[Westley
et al., 1995a;Shi et al., 1995;Vidal ment of its flux was not possiblein our experimentsusing
et al., 1997;Baragiolaet al., 1999b].Ice filmswere grownon a proton beamsowingto the backgroundH 2 gasarrivingfrom
cooled, gold-coated,quartz crystalmicrobalancethat has a the ion accelerator.The yield of H 2 wasestimatedby Browne!
sensitivity
of ---10TM
H20/cm2,or abouta tenthof a monolayer. el. [1984]to be twicethat of 02. We foundthat sputteringof
Vapor from a glassampoulefilled with degassedpure water 02 increasesnearly exponentiallywith ice temperaturebewasdirectedperpendicularly
ontothe goldsubstratethrougha tween about 40 K and 120 K, in agreementwith Brown e! el.
12 mm diameterarrayof microcapillaries,
whichwere ---50/•m [1984],while the sputteringof H20 is independentof temperin diameter and 500 /•m long. The substratetemperatureof ature below 120 K. The separatestudy of the temperature
will be publishedelsewhere.The main conclusion
---150 K and the perpendicularincidenceof the condensing dependences
is that the sputteringat 120
moleculeswere conditionsthat led to cubiccrystallineice of from our absolutemeasurements
02 molecules.
low porosity[Westley
et al., 1998].The ice filmswere invisible K is dominatedby emissionof synthesized
to the eye,implyingthe existenceof veryfew imperfectionslike
defectsor grain boundarieswhich could provideshort-circuit 3.2. Thermal Desorption
diffusionpathsfor trapped gases.
Temperature-programmed desorption experiments were
A massanalyzedbeam of 200 keV protons from an ion doneafter everyirradiationexperimentat a heatingrate of ---1
accelerator was rastered and collimated to a diameter of 5 mm
K/min. In Figure 1 we showthermaldesorptionspectrafor a 7
before impactingthe sample.The currentwaslimited to ---200 /•m water ice sampleirradiated at 80 K and in Figure 2 for
nA to avoidsignificantsampleheating.The angleof incidence similarirradiationsdone at 120 K. The water ice film disapwas 48ø with respectto the surfacenormal. The partial sput- pears at ---180 K; the precise temperature dependson the
tering yieldsof H20 and 02 were determinedwith a quadru- thicknessof the film and the heating rate. After the film is
pole massspectrometer
(QMS) tunedto differentmasses.
We removed,the partial pressures
fall fast,at a rate determinedby
2.
Experimental Approach
BAHR ET AL.: RADIOLYSIS
OF OUTER SOLAR SYSTEM WATER ICE
33,287
the pumpingspeedand adsorption/desorption
at the system
walls(thisisdifferentfor waterthanfor 02). Besideswater,the
H•O_
2.0X10'7 other speciesthat can be clearlyidentifiedare 02 and H202.
Thermal desorptionspectraof 02 from the irradiatedwater
ice havea structurethat may be relatedto irradiation-induced
1.0x10'7 defectsand phasetransformations
in ice. Amorphousice crystallizesinto the cubiccrystallineform at 120-140 K, depending
•o
0.0 ' I'','','','
ß ....
' I'
on heatingrate and nanostructure[Sackand Baragiola,1993].
Cubicice transformsinto hexagonalice at 160-190 K, depend•
02
ing on conditionsthat are not well understood[Hobbs,1974].
Althoughour films are originallycrystalline(cubic),they may
u35xl 0.,ou)
be renderedpartially amorphousby ion irradiation, particularly at the lower temperatures,and that is a possiblereason
whywe observestructurein the sublimationof waterbelow150
a.
o :', : ', : ', :', .
.
.
. ß.
K in films irradiated at 80 K. Another possibilityis that sublimation is altered by complex defectsor thermally activated
H202.
chemicalreactions.An importantfindingis that 02 desorbsup
to the temperatureswhere the water film is removedcomlx10 -• _
_
pletely. If the trapped 02 would not move duringwarming,
then the desorbed 02 signal would drop once the ionirradiated layer had evaporated(the films are --•2.5 times
0 . I . I . I . I . I_.1•. I
• I ,
thickerthan the ion range). In contrast,the persistenceof 02
90 100 110 120 130 140 150 160 170 180 190 200
desorptionuntil the whole film is removedimplies that 02
Temperature (K)
diffusesinsidethe film duringwarming.We alwaysobservea
peak in 02 desorptionjust before the film disappears.This Figure 2. Same as Figure 1 exceptthat the irradiation temsuggests
a higher bindingcloseto the ice-substrateinterface. peratureis 120 K and the heatingrate is 1.5 K/min.
The 02 thermal desorptionspectrado not changemuchwith
irradiationtemperaturebelow80 K, exceptfor the appearance
of a "surface"peak at around40 K whenirradiatingbelowthat
temperature.
Thispeak,whichamounts
to 2 x 10•s 02, cor- versustime curve during TPD. When this value is dividedby
responds
to 02 that hasprecipitatedto the surfaceof the film the total energyof the projectiles(fluence x 200 keV), we
and desorbslike solid02 [Vidalet al., 1997].The total amount obtainGt, the numberof trapped02 per 100 eV of deposited
We findGt • 2.6 x 10-4 O2/100eV, independent
of
of trapped02 was obtainedfrom the integralof the pressure energy.
the irradiationtemperaturebetween40 K and 120K (Table 1).
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
ß
Onecanobtaina totalradiation
yieldG by addingto Gt the
2xl 0'7
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
o
1xl 0'7
•
o
:',
:',
:,
.......
o
ß
02
tween 0.2 and 4 x 10-6 at the fluenceof 1.5 x 10]7 H/cm 2. The
concentrationof H202 amountsto 0.001-0.02%, much lower
u)lx10
'•ø
•-
0 :', :
contributionGs due to sputtered02 (GTotal= Gt q- Gs), but
G t is the relevantnumber to usewhen calculatingthe formation of radiolytic 02 bubbles.We list also in Table 1 other
publishedvaluesfor liquid and solidwater obtainedby indirect
chemicalmethods[Lefort, 1955; Ghormleyand Stewart,1956;
Baverstock
and Bums, 1976] and by massspectrometry[Baragiola et al., 1999a].
Figures1 and 2 alsoshowdesorptionof hydrogenperoxide,
a moleculethat hasalsobeenseenin our previousexperiments
with water ice photolyzedat 40 K [Westley
et al., 1995a,b]. The
amount of trapped H202 appearserratic, With G valuesbe-
than the 0.13% value reportedfor Europa [Carlsonet al.,
1999]. This result is consistentwith the infrared reflectance
....
', : I : ', : l
H•O•
2xl 0
1xl 0'•2
studiesof Moore and Hudson [2000],who could not find detectable amountsof peroxide after irradiatingpure ice with
protons(infraredspectroscopy
is lesssensitivethanmassspectrometry). In our experimentswe found no trapped ozone
within our sensitivityand can set the limit of its concentration
to an order of magnitudebelow that of H202.
0 , I . i . I . •1, I , I . • ,1 I , , I ,
90
100
110
120
130
140
150
160
170
180
190 200
Temperature (K)
Figure 1. Temperature-programmeddesorption from a 7
•m watericefilmirradiatedat 80 K by 1.5 x 10•7 H+/cm2 at
200 keV and 48ø incidence.The heatingrate is 1.1 K/min.
4.
Discussion
One cannoticein Table 1 very differentvaluesof G. Part of
the reasonis that differenttypesof irradiatingparticlesvaryin
thewaytheydeposittheir energy,producingdifferentdensities
of radicals.Ions like 200 keV protonshave sucha high stop-
33,288
BAHR ET AL.: RADIOLYSIS
OF OUTER SOLAR SYSTEM WATER
ICE
Table 1. RadiolyticProductionof 02 From CondensedWater
G (Total)
O2/100eV
G t (Trapped)
O2/100eV
Radiation
Energy,MeV
T, K
0.15
He
3 x 10-3
T
0
3.1 X 10 -2
8 x 10-3
•/
Ne
He
1.25
30
6
'-'300
---300
---300
He
H
H
H
H
29
0.08
0.2
0.2
0.2
---300
70
120
80
40
1.6 x 10 -3
2 x 10-4
7 x 10-4
4
2.7
2.4
2.6
x
X
x
x
10-s
10-4
10-4
10-4
Reference
a
Lefort [1955]
77
GS 1956
BB 1976
BB 1976
BB 1976
BB 1976
Baragiolaet al. [1999a]
this work
this work
this work
aGS 1956,Ghormleyand Stewart[1956];BB 1976,Bayerstock
and Bums [1976].
ping power (---60 eV/nm) that they producea "continuous" molecules,
makingthe absorptionstrengthstronglydependent
track of ionizationsand excitationsin the solid.The densityuœ •0IIpimseanddenbity.
Tiiu• ritei.8-%banddcptligivcb71• 2 X
ionizations(evaluatedfrom the stoppingpower and track ra- 10•9-2X 1020O2/cm
2 for solid02, depending
onthecrystaldius)is sohighthat there is efficientrecombinationof radicals line phaseof 02 [Landauet al., 1962;Vidal et al., 1997;Baralike OH, H, and O into moleculeslike HO 2 and H202, without giolaet al., 1999a].If the condensed
02 wasa liquid,• • 2 x
the need of intertrack diffusionalprocesses.
HO 2 and H202 102•O2/cm2;
evenhighervalues
wouldholdif the02 molecules
are precursorsto the formationof 02 by a subsequentelec- were presentas densegasinsidebubbles.Cooperet al. [2001]
tronic excitation[Bumset al., 1981;Siegeret al., 1998]. Direct calculatedenergyfluxesof ions and electronsand sputtering
productionof 02 in a singleion track,proposedbyBumset al. rates on the icy satellitesusingrecent Galileo data. They give
[1981],is unlikely,judgingfrom the very smallpartial sputter- totalenergy
fluxesof 5.4 x 109keV cm-2 s-• (Ganymede's
ing yield of 02 from water ice in the low fluence(singleion poles)and2.6 x 108keV cm-2 s-• (Ganymede's
equator),
impact)limit for denselyionizingMeV Ne ions[Reimannetal., whichproduceerosionratesof 0.004and0.0008/xm/yr,respec1984].
tively.Notice the decreaseof the energyflux on Ganymede's
It is important to note the differencebetween the total equatordue to the satellite'smagneticfield.
amountof 02 produced(measuredby GTotal)andthe amount
WithGt • 2.6 x 10-4 thebuildupof trapped02 is2.1 x
of 02 that maybe trappedin the ice.There hasbeenconfusion 10•3cm-2 yr-• in Ganymede's
equatorial
region.Theamount
in the literature betweenthesetwo quantities.What needsto
of trapped02 wouldbe lessthan2 x 10•6/cm
2 for a regolith
be used to calculatetrapped 02 is the restrictedvalue G t,
turnovertime of 100-1000 years causedby micrometeorite
whichis lower than G Tota
I. It is alsoimportantto realize that
impact [Spencer,1987]. This trapped amount is insignificant
G is not a radiolyticconstantbut that it dependson the typeof
compared
with• = 2 x 10•9-2x 1020O2/cm
2 column
density
radiation[Spinksand Woods,1990],fluence,the phaseof ice
of solid02 that canbe inferredfrom banddepths.Thusenergy
[Plonkaet al., 2000], temperature,and the presenceof impualone(usingour measuredvaluesof G t andthe
rities in the ice, which may "scavenge"radicals.All thesefac- considerations
total
energy
fluxes
expectedin the equatorialregion)rule out
tors are probablybehind the large variation in the valuesreradiolytic
02
bubbles
in ice as the sourceof the absorption
ported in the literature.
bands
of
condensed
02
on Ganymede.
The inefficiencyof 02 productionfrom ice can be understoodfrom the existingknowledgeabout radiolysisof water
ice. Oxygenatoms(but not 02) are a primary,althoughun- 5. Conclusions
likely, productof dissociationof water molecules.Especiallyat
Our experimentsshowedthat the sputteringof water ice at
low temperatures,the dissociatedO will likely recombineim120 K is dominatedby the emissionof 02. Warmingof radiomediatelywith OH, losingkinetic energyto the surrounding
molecules(cageeffect). A fractionof O can escapethe cage lyzedice releasedtrappedradiationproducts.Thermaldesorpwith a probabilitythat increaseswith the temperatureof the tion of 02 wasalreadyseenat the irradiationtemperatureand
ice. However, sinceoxygenatomsare very reactive,they will very stronglyabove 120 K. We also found small amountsof
more than an order of magrapidly form HO2, H202, and perhapsmore complexmole- trappedH202, at concentrations
with
cules,especiallyin the presenceof trappedcharges.Thus the nitude smallerthan reportedfor Europa but consistent
infrared
reflectance
experiments
of
proton
irradiation
of
oxyejectedatomicoxygenatomscomemostlikelyfrom the surface
gen-free
ice.
The
amount
of
02
trapped
in
ice
was
higher
than
layer.
We now discussthe productionof 02 trappedin the ice in in our previousexperiments,likely owing to higher impact
andmorecompacticesamples.
The radiationyieldG
comparisonwith observations
of condensed02 in Ganymede. energies
The maximumdisk-averagedstrengthof the 02 red bandsin of 02 producedand trappedin the ice per 100 eV of energy
Ganymede'strailing hemisphere,1.8%, givesdifferent values depositedby the 200 keV protonsis more than two ordersof
of the 02 columndensitydependingon the phaseassumedfor magnitude smaller than yields used in recent models of
thesolid02.Thisisbecause
thedouble
transitions
X 35;•---> Ganymede,which assumethe productionof 02 bubblesor
ions [Johnson
a•Aa (v = 1, 2) forming
theabsorption
bands
at577nmand inclusionsin ice by impingingmagnetospheric
628 nm [Landau et al., 1962] require a pair of adjacent02 andJesser,
1997].Thereforeit is morelikelythat the 02 bands
BAHR ET AL.' RADIOLYSIS OF OUTER SOLAR SYSTEM WATER ICE
33,289
seen on Ganymede result from some other mechanism,like
condensationof atmosphericoxygenon the surface.
Haring, R. A., R. Pedrys, D. J. Oostra, A. Haring, and A. E. de
Vries, Reactive sputtering of simple condensedgasesby keV
ions: Mass spectra,Nucl. Instrum. MethodsPhys.Res., Ser. B, 5,
Acknowledgments.We acknowledgesupport from NSF-Astronomy,NASA Officeof SpaceResearchandthe NASA Cassiniprogram.
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