GEOPHYSICALRESEARCHLETTERS,VOL. 24, NO. 17,PAGES2147-2150,SEPTEMBER1, 1997
Galileo ultraviolet spectrometerobservationsof atomic
hydrogen in the atmosphere of Ganymede
C. A. Barth, C. W. Hord, A. I. F. Stewart,W. R. Pryor, K. E. Simmons,W. E.
McClintock
Laboratoryfor Atmosphericand SpacePhysics,Universityof Colorado,Boulder
J. M. Ajello, K. L. Naviaux, and J. J. Aiello
Jet PropulsionLaboratory,California Institute of Technology,Pasadena
Abstract. Atomic hydrogenLyman alpharadiation(121.6
nm) has beenmeasuredin emissionfrom the atmosphere
of Ganymedewith the Galileo ultraviolet spectrometer.
An exosphericmodel with the following parametershas
beenfit to the observational
data:atomic hydrogendensity
directlyabovethe surface(radius2634 km) equalto 1.5 x
produced
andthatsomefractionof themshould
escape
from
the gravitational
field of Ganymede
[Yungand McElroy,
temperature 450 K.
A model calculation of the
photodissociation
of watervapor from surfaceice at 146 K
is usedto obtain the photodissociation
rate necessary
to
supply the hydrogenatoms that are escapingfrom the
exosphereof Ganymede. The calculatedescapeflux of
[Lanzerottiet al., 1978].
1977]. There has also beenthe suggestion
that charged
particlebombardment
of the surface
of Ganymede
should
produce
sputtering
of watervaporfromthe surface
ice and
produce
a watervaporatmosphere
with a partialpressure
104 atomscm'3 scaleheight2634 km, exospheric
thatmay be comparable
to that produced
by sublimation
Observations
The Galileoultravioletspectrometer
(UVS) consists
of a
atomic
hydrogen
is7 x 108atoms/cm
2 sec. TwoalternateCassegrain
telescope
anda Fastie-Ebert
spectrometer
[Hord
but speculativesourcesof the atomichydrogenescaping et al., 1992]. For the observations
describedin this paper,
from Ganymedeare photodesorptionof water ice by the field-of-viewis 0.1 x 1.0 degreesand the grating is
ultraviolet photonsin the wavelengthrange 120.5-186.0 stepped
through16 stepsup anddownthroughthe atomic
nm andsputteringof watericeby Jupiter'smagnetospherichydrogen
Lymanalphaline at 121.6 nm. Approximately
ion plasma.
equalamountsof time are spentmeasuringthe emission
line andthe background.In the analysisof the data,the
slit functionwas fit to the observationaldata using a least
squares
fit procedure.Thistechnique
successfully
separates
Introduction
the emissionline from the radiationnoise that is produced
tubesby the chargedparticlesin
Atomichydrogenis presentin theupperatmospheres
of in the photomultiplier
Jupiter's
magnetosphere.
the Earth,Venus,andMars. Its presence
hasbeendetected
During the Galileo flyby of Ganymedeon June 27,
by measuringLymanalpharadiationfromthe Sun that has
scannedthe atmosphere
been scattered by hydrogen atoms in the Earth's 1996, the ultravioletspectrometer
above
the
limb
for
seven
minutes
starting
whenthe field-ofatmosphere
[Kupperianeta!., 1959],in the atmosphere
of
view
was
2400
km
above
the
limb
and
ending a•er the
Venus[Barthet al., 1967],and in the atmosphere
of Mars
[Barthet al., 1971]. For thesethreeplanetarybodies,the field-of-viewhad crossedonto the disk of Ganymede. At
sourceof the atomichydrogenis, directlyor indirectly,the that time, the rangeto the limb was approximately5700
dissociation
of watervaporin theatmosphere.For all three km and the solar zenith anglewas 60ø. The sub-solar
planets,atomichydrogenis escapingfromthe top of the pointwasjust southof the equatorat a longitudeof 170ø.
atmosphere
by a varietyof mechanisms
includingthermal The limb crossingtook placeat a latitudeof 2 IøS and a
longitudeof 226øW. The atomichydrogenLyman alpha
or evaporative
escape.
Since Ganymedehas water ice on its surface,it is line was measured44 timesduringthis sevenminute limb
period. Sincethemeasuremeres
wereaveraged
expected
thattheremay be watervaporin the atmosphereobservation
in groupsof four,the emissionratewas determinedevery
above the water ice as a result of sublimation and that the
distance(distancefrom the
watervapor will be photodissociated
by solar ultraviolet 225 km from a planetocentric
radiationproducinghydrogenatoms. A model calculation centerof Ganymede)of 5000 km down to the limb (the
of the photochemistry
of a watervaporatmosphere
on radius of Ganymedeis 2634 km). Figure 1 shows the
Ganymedeshowsindeedthat hydrogenatomsshouldbe emission rate in kiloRayleighs as a function of
planetocentric
distance.TheLymanalphabackground
of a
nearbyregion of the sky (ecliptic latitude -31ø, ecliptic
longitude345ø) was measuredseveralminutes after the
Copyright
1997bytheAmerican
Geophysical
Union.
limb
observation
and determined
to be 0.61
kR
with
a
statisticaluncertaintyof 0.04 kR. Using this measuremere
and a model [Pryor et al., 1996], the emissionrate of the
Papernumber97GL01927.
0094-8534/97/97GL-01927505.00
2147
2148
BARTH
ET AL.: ATOMIC
HYDROGEN
Lyman alpha sky backgroundin the directionof the limb
observations
(eclipticlatitude-21o,eclipticlongitude305ø)
was determined to be 0.58 kR.
Exosphere Model
Sincethe densityof theatmosphere
of Ganymede
is low,
the analysisof datarequiresthe use of exospheric
theory.
An exosphere
is the regionof planetaryatmosphere
where
collisionsbetweenthe neutralconstituents
essentially
do
not occur. The theoryof the densitydistributionfor a
spherical
atmosphere
hasbeendeveloped
by Chamberlain
[1963;ChamberlainandHunten,1987].
Figure 1 showsthe resultof an exosphere
model fit to
ON GANYMEDE
least-squares
fit procedure
(correlation
coefficient0.46). As
is shownin Figure 1, the 1/r functionfits the observations
aswell asthe exosphericmodel.
Sincethe observations
of theLymanalphaemissionrate
aremadeby viewing the exosphere
transversely
fi'omthe
outside,it is necessary
to take into accountthe geometryof
thespherical
exosphere.
As partof the development
of the
exospheric
theory, Chamberlainhas evaluatedintegrateddensityfunctions
for ballisticandescapeorbits. Usingthe
functionsgiven in Tables IX.2 and !X.3 in Chamberlain
andHunten[ 1987],we calculatea volumedensityof 1.5 x
!04atoms
cm'3atr = 2634kmdirectly
above
thesurface.
Escape Flux
the Lyman alpha limb observations. This model was
Thosehydrogenatomsthat havea velocitygreaterthan
generatedusing an exospherictemperature
of 450 K and a the escapevelocity for Ganymede,2.74 km/sec, and are
scaleheightof 2634 km. The figurealsoshowsthe Lyman moving upwardhave the opportunityto escapefi'omthe
alphasky background
of 0.58 kR. This model exosphere gravitational
field of Ganymedeif they do not undergoany
was fit to the datausinga leastsquaresfit procedure
which further collisions. If the collision cross-section between the
gavea correlationcoefficientof 0.42. One result of this fit
hydrogen atoms and the major constituent of the
is that the slant column emission rate at r = 2634
km
(directlyabovethe limb) is 1.14kR with an uncertaintyof
0.20 kR.
Subtractingthe 0.58 kR Lyman alpha
background,the column emission rate attributedto the
atmosphere
of Ganymedeis 0. 56 kR. Using • emission
atmosphere
is takento be3 x 10-•5cm2, thenthecritical
level occurswhere the column density of the major
constituentis 3 x l0 TMcm'2. This critical level is the
exobase and the collisionless region above is the
exosphere.The escapeflux of hydrogenatoms fi'omthe
ratefactor
of 6.06x 10'5photons/sec-atom,
a slantcolumn exosphereis a functionof thehydrogenatomdensityat the
densitythroughthe sphericalatmosphere
is determined
to
be 9.21 x 10TMatoms cm'2. The emissionrate factoris criticallevel, the mostprobablevelocityof the atoms,Vm,
and the energy parameter,)•c, which appearsin the
calculated
using a measurement
of the Lyman alpha solar exponentof the escapeflux equation [Chamberlainand
flux at 1 AU by the SOLSTICE instrumenton UARS Hunten, 1987]. Using the parameters
that were usedto fit
(Rottman
et al., 1993)of3.02x 10TMphotons/cm
2 sec. the atomichydrogendistributionon Ganymede,T = 450
The assumptionis madethat the flux in the centerof the K, )•c = 1, Vm = 2.74 km/sec, and assumingthat the
solarlineis 3.02x 10• photons/cm
2 secJr. Thisfluxis criticallevelis at r = 2792 km wherethe atomichydrogen
adjustedto themeandistanceof Jupiterat 5.205 AU. We density
is 1.2 x 104 cm'3 theescape
fluxof atomic
also fit a 1/r functionto the observational
data using a hydrogen
iscalculated
tobe7 x 108atoms/cm
2sec.
Photodissociation and Photodesorption
.....
0.8
•
•
'
_ , , ,
[]
I
'i
•
I
/
'
I
A likely sourceof the hydrogenatomsin the atmosphere
of Ganymedeis the photodissociation
of watervaporby
solar ultravioletphotonsof wavelengthshorterthan 242
nm whichproduceshydrogenatomsandhydroxylradicals.
A possiblesourceof watervaporin the atmosphere
is the
sublimation
of waterice on thesurface.Duringthe Galileo
flyby of Ganymede, the photopolarimeterradiometer
instrument (PPR) measured the 17-gm brightness
T
temperatureof the surface and found the maximum
temperatureto be 152 K [Orton et al., 1996]. The Galileo
near infiared mapping spectrometer(NIMS) which
0.4 ., •. i .... , .... • .... •, ,},, • ....
2500
3000
3S00
4000
4S00
5000
SS00
PlonetocentricDistonce(kin)
Figure 1. Observedemissionrate of atomic hydrogen
Lyman alphaemissionfromthe atmosphere
of Ganymede
as a functionof the distancefromthe centerof Ganymede
(squares).The errorbarsthatareplottedonthedatapoints
show the statisticalvariationdue to the signal and the
radiation
noise.
The
observations
are fit
with
an
exospheric
model that has a temperature
of 450 K (solid
line). Also plottedis a 1/r fit to the observations
(dotted
line) andthe Lymanalphasky background
(largedashes).
The scatterin the datapointsis most likely causedby the
radiationenvironment
in Jupiter'smagnetosphere.
measured
the depthof the waterice 2.0-gm band showed
thatwaterice is non-uniformlydistributedoverthe sin'face
of Ganymedewith more ice presentin the polar regions
(seeFig.7, Carlsonet al., 1996). Extensiveanalysisof
thethermalproperties
of theiceonGanymede
suggests
that
theicetemperature
maynot be 152 K, but may be as low
as 112 K [Spencer,1987]. Ice at thesetemperatures
would
not have a vapor pressurehigh enoughto provide a
photodissociation
rate large enough to support the
calculated escape rate.
A calculation using a
photodissociation
fi'equency
of 1.7x 10-7sec
4 [Yttt•g
and
McElroy, 1977] shows that 146 K is the lowest
temperature
wherethe photodissociation
rate exceedsthe
calculated
escape
rate. For a modelat 146 K, the vapor
BARTH ET AL.: ATOMIC HYDROGEN ON GANYMEDE
2149
pressure
ofwateris 2 x 104mb,thecolumn
density
of of atomic hydrogencalculatedfrom the Galileo UVS
of atomichydrogenwith the outwardflow of
watermolecules
is 5 x 10•smolecules/cm
2,theexobase
is observations
measured
by the Galileo
at an altitudeof 125km (r = 2792 km) andthe escapeflux hydrogenions from Ganymede
is 7 x l0 g atoms/cm
2 sec.
plasma instrumentation(PLS) [Frank et al., 1997].
to Ganymede,the PLS measured
a
A speculative
sourceof the hydrogenatoms in the During the approach
ofhydrogen
ionswitha density
of30ions/cm
3
exosphere
of Ganymedeis the photodesorption
of surface coldplasma
water ice. Since the atmosphereis thin, the solar flowingoutwardat a velocityof 80 km/sec. This outward
ultraviolet radiation strikes the surface unattenuated. In the
flowofhydrogen
ionsof 2 x 10gions/cm
2secis similarin
wavelengthrange 120.5-186.0 nm, the solar flux at
magnitude
to thecalculated
escape
flux of hydrogenatoms,
2 sec. Oneof the suggestions
forthe
Ganymede
is 1.4x 10• photons/cm
2sec.Photons
in this 7 x l0g atoms/cm
wavelengthrangehaveenergiesbetween6.7 and 10.3 eV. sourceof the coldhydrogenionsis ion impacton the water
If 0.5% of the incidentuv photonseject atomichydrogen iceson the surfaceof Ganymede[Frank et al., 1997]. If
of the surfaceice of Ganymede
from the ice lattice, there would be a sufficientflux to indeedthe ionbombardment
supply the calculatedescapeflux of atomic hydrogen. is the sourceof hydrogenions flowingoutward,then it is
Laboratoryexperimentson photodesorption
show that plausibleto suggestthat this sameion sputteringmay be
water molecules are photodesorbedfrom water ice by producinghydrogenatomswith a similaryield. Thereare
Lyman alphaphotons(energy10.2 eV) with a yield of no laboratoryexperimentsthat have measured hydrogen
by ion sputtering.
0.7% at a temperatureof 100 K [Westley, 1995]. ionsbeingejectedfrom icy surfaces
Laboratoryexperiments
on the bombardment
of water ice
by low-energy electronsshow that atomic hydrogen Oxygen
(actuallydeuteriumwasusedin the laboratoryexperiment)
If the escapinghydrogenatomsare producedby the
is desorbedfrom the ice startingat a thresholdenergyof photodissociation
of waterin theatmosphere,
thehydroxyl
6.5 eV and that the yield of hydrogenatomsis largerthan radicals
thatareproduced
maystickto thesurface
ice. The
the yield of watermolecules[Kimmeland Orlando, 1995]. sticking
coefficient
of OH on icehasbeenmeasured
in the
Whetheror not photonsin the 6.7 to 10.3 eV energyrange laboratoryand foundto be 0.03 [Cooperand Abbatt,
produceatomichydrogendesorptionis not known.
1996]. On Ganymede,
the OH radicalswouldhavemany
collisions
with the surfaceandgraduallyaccumulate
in the
ice. If thehydrogen
atomsaredirectlyejected
fromthe ice,
Ion Sputtering
thehydroxylradicals
wouldremainin the ice andhavethe
SinceGanymede'sorbit lies within the magnetosphere
opportunityto reactand form variouscompounds
of
of Jupiter,the surfaceis constantlybombardedby the
oxygenandhydrogen The amountof ice that is lost
magnetospheric
plasmathat is corotatingwith Jupiter's because
of theescape
of atomichydrogen
maybe calculated
magneticfield. Using datafrom Voyagermeasurements,using the escaperate determinedfrom the Galileo
the erosionrate of waterice fromthe surfaceof Ganymede ultravioletspectrometer
measurements
of atomichydrogen.
hasbeencalculated[dohnson
et al., 1982, dohnson,1990]. Takingintoaccount
theday-nightcycleon Ganymede,
the
Usingthe assumption
that oxygenionsarethe principal averagethickness
of watericethatis lostis 1 m peryear.
constituent
of themagnetospheric
plasma,the flux of water Sincehydrogenatomsareescaping,
it is oxygenin some
molecules
fromthe surface
of Ganymede
is 1 x 10•ø formthatis accumulating
in the ice. It may be notedthat
molecules/cm
2 sec. This is the sameflux that occursfrom
molecularoxygenand ozoneare observedin the ice of
sublimation
of anicy surfaceat 125 K (Fig. 5, dohnson
et Ganymede[Spencer
et al., 1995;Calvinet al., 1996;Noll
al., 1982). Thus,if ion erosionwerethe principalprocess et al., 1996]. Theremay be a small amountof molecular
producing
a watervaporatmosphere,
the partialpressure oxygenin theatmosphere
of Ganymede.A columndensity
wouldbe 2 x 10-1•mb. Photodissociation
of sucha water of(1-10)x 10•4oxygen
molecules
cm'2hasbeeninferred
vaporatmosphere
wouldbeseveral
orders
of magnitude
too from an analysisof HST observations
of the ultraviolet
low to accountfor the calculatedescapeflux of atomic airglowof Ganymede[Hall et al., 1997].
hydrogen.
Another speculativesourceof the hydrogenatoms Uncertainties
escaping
fromGanymede
istheejectionof hydrogen
atoms
The calculationof the escapeflux is model dependent.
directly from the surfaceice by the impinging The
valueof 7 x l0 g atoms/cm:sec was calculatedfor a
magnetospheric
plasmaions. Jupiter'smagnetospheric
model
in which the hydrogenatoms are producedby
plasmawas measured
by the energetic
particlesdetector
photodissociation
in an atmosphere
with a temperature
of
(EPD) onthe Galileospacecraft
[Williamset al., 1992]. A
450 K and the atmospheric
densityis sufficientlyhigh to
calculation
based
onthese
observations
•gives
an ion
sputtering
rateof 4.8 x 10• molecules/cm
sec[W. Ip,
form an exobaseat 125 km.
If the hydrogenatoms are
ejectedfromthesurface
with excess
kinetic
energy,
thenthe
8
2
escapeflux maybe greaterthan7 x 10 atoms/cmsec. For
eithermechanism,
the densityof the neutralatmosphere
is
theimportantparameter
sinceit determines
whetheror not
thehot atomsequilibratebeforeescaping.
privatecommunication].
Laboratory
experiments
on ion
sputtering
of watericehaveshownthatin additionto water
moleculesbeing eroded,molecularoxygen, molecular
hydrogen,
andatomichydrogenare also ejectedfromthe
icysurface
[Bar-Nunet al., 1985]. If the sputtering
yield
of atomichydrogenis high enough,then ion sputtering Summary
may be a viablesourceof the atomichydrogenthat is
Atomic hydrogenhasbeenmeasured
in the atmosphere
escaping
fromGanymede.
For furtherspeculation,
we may compare
the escape
flux of Ganymede.The densitydirectlyabovethesurfaceis 1.5
2150
BARTH
ET AL.' ATOMIC
HYDROGEN
X 104atomscm-3 The altitudedistribution
may be
ß
ON GANYMEDE
Hall, D. T. et al., The Far-ultravioletOxygenAirglow of Europa and
Ganymede,Ap. ,I. Letters,in press,1997.
describedby a scale height that correspondsto a
Hord, C. W. et al., Galileo UltravioletSpectrometerExperiment,Space
temperatureof 450 K. A possiblesourceof the atomic
ScienceReviews, 60, 503-530, 1992.
hydrogenis the photodissociation
of watervapor that has Johnson, R. E., Energetic Charged-Particle Interactions with
sublimed from the ice on the surface. Using the water
Atmospheres
andSurface,Springer,Berlin, 1990.
Johnson,R. E., L. J. Lanzerotti,andW. L. Brown,PlanetaryApplications
vaporatmosphere
model,the escape
flux of aztOmic
of Ion Induced Erosion of Condensed-Gas Frosts, Nucl. Instrum.
hydrogen
is calculated
to be 7 x l0s atoms/cm sec.
Methods, 198, 147-157, 1982.
However,to supplythis amountof atomichydrogenfrom Kimmel, G. A. and T. M. Orlando,Low-Energy (5-120 eV) Electronphotodissociation
of water vapor, there needs to be a
Stimulated
Dissociation
of Amorphous
D20 Ice: D(2S),O(3p),and
substantial reservoir of ice on the surface that is at a
O(1D)YieldsandVelocity
Distributions,
Phys.
Rev.Letters,
75,26062609, 1995.
temperature
of 146 K or higher. While thehydrogenatoms
Kupperian,Jr. et al., Far UltravioletRadiationin the Night Sky,Planet.
that are producedfrom the dissociationof water have the
SpaceSci., 1, 3-6, 1959.
opportunityto escape,the oxygen atoms tend to remain Lanzerotti, L. J. et al., On the contributionof water productsfrom
behind.
The
amount
of water ice that is consumed
in
producingthe escapinghydrogenatomsis I nm per year.
The amount of oxygen that accumulateson Ganymede
shouldbe nearlyequalto that sameequivalentcolumn.
There are two additional possible sources of the
hydrogenatomsthat are observedin the atmosphere
of
Ganymede. They arethe photodesorption
of waterice by
solarphotonsin the far ultravioletwavelengthrangeand
the sputtering
of waterice by Jupiter's magnetospheric
ion
plasma.
Galileansatellitesto the Jovianmagnetosphere,
Geophys.Res.Lett.,5,
155-158, 1978.
Noll, K. S. et al., Detectionof ozone on Ganymede,Science,273, 341343, 1996.
Orton,G. S. et al., GalileoPhotopolarimeter-Radiometer
Observations
of
Jupiterandthe GalileanSatellites,Science,274, 389-391, 1996.
Pryor, W. R. et al., Latitude variationsin interplanetaryLyman- data
from the Galileo EUVS modeledwith solar He 1083 nm images,
Geophys.
Res.Lett.,23, 1893-1896,1996.
Rottman,G. J., T. N. Woods,and T. P. Sparn,Solar StellarIrradiance
ComparisonExperiment 1:1 InstrumentDesign and Operation,ar.
Geophys.Res.,98, 10,667-10,677,1993.
Spencer,J. R., Thermal Segregationof Water Ice on the Galilean
Satellites,Icarus, 69, 297-313, 1987a.
References
Bar-Nun et al., Ejectionof H20, 02, H2, andH from Water Ice by 0.5-6
keV H andNe Ion Bombardment,
SurfaceSci., 150, 143-156,1985.
Barth, C. A. et al., Ultraviolet EmissionsObserved near Venus from
Spencer,J. R., W. M. Calvin, and M. J. Person,Charge-coupleddevice
spectraof the Galileansatellites:Molecularoxygenon Ganymede,ar.
Geophys.
Res.,100, 19,049-19,056,1995.
Westley,M. S. et al., Photodesorption
from Low-temperatureWater Ice
in Interstellar and CircumsolarGrains,Nature, 373, 405-407, 1997.
Mariner V, Science, 158, 1676-1679, 1967.
Williams, D. J. et al., Energeticparticle signaturesat Ganymede:
Barth, C. A. et al., Mariner 6 and 7 UltravioletSpectrometer
Experiment:
Implicationsfor Ganymede'smagneticfield, Geophys.Res.Lett.,this
UpperAtmosphere
Data,d. Geophys.
Res.,76, 2213-2227,1971.
issue,1997.
Calvin, W. M., R. E. Johnson,and J. R. Spencer,02 on Ganymede: Yung,Y. L. andM. B. McElroy,Stabilityof an OxygenAtmosphere
on
SpectralCharacteristics
andPlasmaFormationMechanisms,
Geophys.
Ganymede,Icarus, 30, 97-103, 1977.
Res. Lett., 23, 673-676, 1996.
Carlson,R. et al., Near-infrared spectroscopy
and spectralmappingof
Jupiterandthe Galilean satellites:Resultsfrom Galileo's initial orbit,
Science,274, 385-388, 1996.
Chamberlain,J. W., PlanetaryCoronaeand AtmosphericEvaporation,
Planet.SpaceSci., 11, 901-960, 1963.
Chamberlain,J. W. andD. M. Hunten,Theoryof PlanetaryAtmospheres,
2nd Ed., Academic, Orlando, Fla., 1987.
Cooper,P. L. andJ.P. D. Abbatt,Heterogeneous
Interactions
of OH and
HO2 Radicalswith Surfaces,ar.Phys.Chem.,100, 2249-2254, 1996.
Frank, L. A. et al., Outflow of Hydrogen Ions from Ganymede,
Geophys.Res.Lett.,thisissue,1997.
C. A. Barth, C. W. Hord, A. I. F. Stewart, W. R. Pryor, K. E.
Simmons,and W. E. McClintock, LASP, Box 590, University of
Colorado,
Boulder,
Colorado
80309-0590.
(e-mail:
barth@lasp.
colorado.
edu)
J.M. Ajello,K. L. Naviaux,andJ. J. Aiello, JPL,4800 Oak Grove
Dr., Pasadena,California 91109-8099
(ReceivedMarch 6, 1997;revised June20, 1997;
acceptedJune30, 1997.)