JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. El2, PAGES 33,223-33,240,DECEMBER 25, 2001
Landform degradationand slopeprocesses
on Io' The Galileo
view
ß
Jeffrey
M. Moore,
• Robert
J.Sullivan,Frank
C.Chuang,
• James
W.HeadIII,4
AlfredS.McEwen
sMoses
P Milazzo,
sBrianE Nixon,
4Robert
T Pappalardo,
4'6
PaulM. Schenk,
7andElizabeth
P.Turtles
Abstract.
The Galileo mission has revealed remarkable evidence of mass movement and
landformdegradation
on Io. We recognizefourmajorslopetypesobservedon a numberof
intermediate
resolution
t-250mpixel
-t)images
and
several
additional
textures
onvery
high
resolution(-10 m pixel- ) images. Slopesandscarpson Io oftenshowevidenceof erosion,
seenin thesimplestformasalcove-carving
slumpsandslidesat all scales.Manyof themass
movementdepositson Io areprobablymostlythe consequence
of blockreleaseandbrittle
slopefailure. Sputteringplaysno significantrole. Sappingasenvisioned
by McCauleyet al.
[1979] remainsviable. We speculatethat alcove-linedcanyonsseenin oneobservationand
lobeddepositsseenalongthe basesof scarpsin severallocationsmayreflectthe plasticdeformationand"glacial"flow of interstitialvolatiles(e.g.,SO2)heatedby locallyhighgeothermalenergyto mobilizethe volatile. The appearance
of someslopesandnear-slopesurfacetexturesseenin very highresolutionimagesis consistent
with erosionfrom sublimationdegradation.However,a suitablevolatile(e.g.,H2S)thatcansublimatefastenoughto alter
Io's youthfulsurfacehasnotbeenidentified.Disaggregation
fromchemicaldecomposition
of solidS20 andotherpolysulfuroxidesmayconceivably
operateonIo. Thismechanism
coulddegradelandformsin a mannerthatresembles
degradation
fromsublimation,
andat a
ratethat cancompetewith resurfacing.
1. Introduction
Such classification
Mass movementand landtbrmdegradationreducestopographicrelief by moving surfacematerialsto a lower gravitational potential. Landformdegradationcommonlyinvolvesa
reductionin material strength(e.g., lossor weakeningof an
interparticlebindingagentor decreasein mechanicalstrength
allowingdisintegrationor plasticdeformation)and transport
of this material underthe influenceof gravity. The identification of specific landform types associatedwith mass
movementand landform degradationpermitsinferencesabout
local sedimentparticle size and abundance,the natureof the
binding cement, and transportationprocesses. Generally,
schemes have been used to characterize
mass movements on Mercury, Venus, the Moon, Mars, and
the Galileansatellites[e.g., Sharp, 1973;Ma/in and Dzurisin,
1977; Malin, 1992; Schenkand Bulmer, 1998; Moore et al.,
1999; Chuangand Greeley, 2000]. The characterof extraterrestrialmovements
canonly be inferredfi'omthe resultingdeposits'morphologies
sinceresolutionis nearlyalwaysinsufficientto observeindividualparticleswithinthedeposits.
With few exceptions,detailsof slope modificationon Io
generallyare not obviousin Voyagerimages.Voyagerl images of Io revealed steep slopeson the flanks of isolated
mountains,along interior walls of calderas,and as scarps
within
plains units. McCauley et al. [1979] and Schaber
mass movements on Earth can be classified in terms of the
[1982]
reported
observations
of arcuateor digitatescarps,esparticlesizesand the speedof the materialmovedduring
pecially
those
in
the
south
polar
region,that suggested
that
transport[Sharpe,1939; k'arnes,1958, 1978;Coates,1977].
someslopeson Io weresubjectto recession
by masswasting.
Schaber[1982] reported lobate scarpsalong the flanks of
• NASAAmesResearch
Center,Mofl•ttField,California.
some mountainswhich he noted resembledlarge landslides.
2 Center
forRadiophysics
andSpace
Research,
Cornell
University,Schenkand Bulmer [1998] recognizedanddiscussed
in detail
Ithaca, New York.
a large debrisapronalong the southernmost
flank of Euboea
'• Department
of Geological
Sciences,
ArizonaStateUniversity,
Montes,evaluated,in part, fi'oma stereo-derived
topography
Tempe, Arizona.
4 Department
of Geological
Sciences,
BrownUniversity,
Providence,model.Schaber [1982] observedthe absenceof slumpsin
Rhode Island.
calderawalls and proposedthat this was strongevidencetbr
sLunarandPlanetary
Laboratory,
University
ofArizona,
Tucson,
Ari- substantialwall strength.
zona.
Galileoimages
acquired
at <200m pixel'•
6NowatAstrophysical
andPlanetary
Sciences
Department,
University In mostcases,
of Colorado,Boulder, Colorado.
7Lunar
andPlanetary
Institute,
Houston,
Texas.
Copyright2001 by theAmericanGeophysical
Union.
Papernumber2000JE001375.
0148-0227/01/2000J E001375509.00
resolutionand high incidenceangle and/orin stereowere requiredto reveal landtbrmson icy Galilean satellitescharac-
teristicof massmovement[e.g.,Mooreet al., 1999;Chuang
and Greeley,2000]. This is alsotruefbr Io. Herewe report
observations
madeduringthe first threecloseGalileofiybys
(I24, I25, and I27) of that satellite,which took place in late
1999 and early 2000. In this studywe identity and describe
33,223
33,224
MOORE ET AL.' MASS WASTING ON IO
;..:?
/...//
.
ß
2'5 km
ß
Figure1. A mosaic
of theTvashtar
caldera
complex
(centered
at •-60øN,120øW)shows
fourvolcanic
ventsin this
area,oneof whichwaseruptingduringthe I25 flyby, creatingthe pixelbleedseenon the image. Threeof the four
ventsaredistributed
acrossthefloorof thedepression
that,in turn,is partlyenclosed
withina plateaurising,•l km
abovethe surroundingplains. Along scarpsof this plateauseveraldifferentslopetypesare seen. Boxesindicate
the locationsof subsequent
figuresusedto illustrateindividualslopetypes(numberedby figure). The letter"A"
marksthe locationof alcove-rimmed
canyonsdiscussed
in the text. (Mosaicis composed
of images25I•040 and
2510041
withnominal
resolutions
of 183mpixel
'• superposed
ona portion
of a 1.3kmpixel
-•, global
mosaic
acquiredduringorbit C21. The view is oblique. Illuminationis fi'omthe left. North is up.)
slopemorphologies
seenin the new imagesof Io thatare in-
which was erupting during the I25 flyby [McEwen et al.,
dicativeof slopedegradation
andmasswastinganddevelop 2000]. Three of the four vents are distributedacrossthe floor
workinghypotheses
tbr the processes
and materialsinvolved of a 70 x 170 km depression
that, in turn, is partly enclosed
in the evolution of these landforms.
within a plateaurising abouta kilometerabovethe surrounding plains [McEwen eta!., 2000; Dtrtle eta!., this issue],
Four distinctslopemorphologic
typesare tbundalongthe
calderawalls and the extensiveinwardand outwardt•tcing
The I24, I25, and I27 Io observationsshow several distinct walls of the plateaumargins. Thesemorphologies,
described
degradationstylesof slopesassociated
with mountainsides, in detailbelow,reflectfour differentstylesor statesof slope
calderawalls, and plateaumargins,but degradational
style degradation and may also reflect variations in material
does not always correlatedirectly with tectonicor vol- strength and/or susceptibilityto modification processes.
canological
setting. We beginwith the rich varietyof slope Thesemorphologies
alsoareseenelsewhere
on theplanetin
morphologies
in the areaof TvashtarCatena(•63øN, 123øW) other Galileo observations.
2. Observationsand interpretations
seen in observations 25ISGIANTS01
and 27ISTVASHT01
(which also provide stereo information), then turn to other
Galileo observations
elsewhereon Io wheresimilarslope 2.1. Type U Slopes
morphologies
are f•)und. Finally,we discuss
two very high
Thesoutheastern
ventof theTvashtar
complexis a kidney"resolution
observations
that,whileveryselective
in coverage, shaped
pitwitha tlatfloorspanning
about
30x 60km(Figure
offeradditionalinsightsintoslopedegradation
on Io.
2). The steepwallsliningthispit generallylackdeepalcoves
The I25 mosaicof theTvashtarcalderacomplex(Figure1) or alternating
spursandgullies. Consistent
with interpretashowsfour recentlyactivevolcanicventsin this area,oneof tionsof similarpitsby manyprevious
workers,
thispit proba-
MOORE
ET AL.'
MASS WASTING
ON IO
33,225
The gradient of the lower slope componentappearsrelatively consistent,but this has not been confirmedquantitatively to be within the range of likely anglesof repose
particulatematerials. Even if this couldbe verified,a Richter
slope [Bakker and Le tteux, 1952; see alsoSelby, 1993, Plate
16.2], in which only a thin veneerof debriscoversa sloping
rock core, could be present. Most debris weatheredfrom a
Richter slope would have to be transportedelsewhere,however, or possiblyburied and hiddenat the slopefoot by volcanicmaterials. While thesepossibilitiescannotbe ruled out,
there is, again, no evidencetbr substantialburial of the slope
since it tbrmed or tBr a transportmechanismto carry away
debris. Similar difficulties and uncertaintiesapply to another
alternative in which the cliff-tbrming unit could representa
resistantcaprocklayer protectinga lower, more friable unit
Figure2. TypeU (unmodified)
slopes
exhibitsteep,unmodi- revealed by its lower gradient. In this scenario,the lower
fled wallslininga caldera,generallylackingsignificantmodi- slope componentis not primarily debris but a sloping rock
t•.ce subject to degradation and debris removal, possibly
fication. Width is ---30 km. View is oblique.
causingminor undercuttingof the upper, more resistantunit
in the process.No evidencetbr undercuttingof the upperunit
bly is a collapsecaldera[Radebaugheta!., this issue]. A low can be resolvedalongthe breakin slopewherethesetwo units
internal ledge runs continuouslyaround the caldera floor. meet.
This ledgeformsa steep,shortclift; so it probablyrepresents
In summary,type D slopes,as seenalong the westernedge
material fi'om a tbrmer, higher level of the caldera floor, of the plateauat Tvashtar,have consistentslopeprofilesthat
rather than accumulations of disaggregateddebris mass- are characterizedby a steep,cliftZtbrmingmemberoverlying
wastedt¾omhigher exposureson the walls. We characterize a shallowergradientslope component. The lower, lesssteep
type U slopesas relatively steep,with poorly developedal- componentis probably dominated by debris derived fi'om
coves,spurs,and gullies. A basalledgemay be presentas in many individualsmall releasesof materialdisaggregated
from
the currentexamplefrom the southeastern
calderaat Tvashtar, the cliff-lbrming memberdirectlyabove. The lack of deepalor anynumberof initial morphological
variationspresent,but coving along the scarp brink suggeststhat disaggregation
the importantdistinction is that the collective characteristics rarely involveslarge releasesof materialthat would substanof type U slopesprovide no convincingevidenceof signifi- tially affectthe transverseprofile.
cant degradationbeyond minor spur and gully modification
and are largely unmodifiedby masswastingat the observed 2.3. Type A Slopes
scale.
Slopesin theeasternandsoutheastern
portions
of theplateauat Tvashtarhavesharpbrinksand are deeplyand con-
2.2. Type D Slopes
Outward facing slopes along the western margin of the
plateauat Tvashtarconsistof cliff-lbrming walls abovea less
steepslopecomponentthat descends
to the surrounding
plains
(Figure 3). The characteristictwo-componentslope profile,
including the proportion of cliff exposureto the subjacent,
lesssteepunit and respectivegradientsof each, appearsrelatively uniform along the slope, but resolutionlimitationsand
compression
and radiationartifactsin the imageslimit quantitative determinations[K!aasen eta!., 1999]. The transverse
profile along the scarpbrink is irregularbut is not dominated
eitherby convex-or concave-outsegments.
The overall slope configurationappearsconsistentwith debris accumulating as talus immediately below the clifftbrming material, presumably produced by numerousrelatively small mass-wastingevents each involving release of
minor amountsof material t¾omthe upper, cliff-forming exposures. Cliff t•.cesstill composeabouthalf the slopereliet;
and this, combinedwith the poorly developedalcoving revealed along the transversebrink profile, suggeststhat the
scarpmay have recededonly slightly from its original position. However, theseestimatesare complicatedby the possibility that not all disaggregatedmaterial (and not all of the
original relief of the slope) remainsin view. Additional debris may be buriedto unknowndepthby latervolcanicmaterials depositedon the surroundingplains,or someportionof
the debrisvolume might have beentransportedaway.
Figure 3. Type D (debris) slopesconsistof cliff-forming
walls abovea less-steepcomponentthat descendsto the sinroundingplains(left). Width is ---35 km. View is oblique.
33,226
MOORE ET AL ß MASS WASTING
ON IO
Figure 4. Type A (alcove)slopeshavesharpbrinksand are deeplyand consistentlyscalloped,tbrminga seriesof
adjacentalcoves. Gradientsbecomelesssteepat lower elevationsto tbrm concave-upslopeprofiles. The lower
portionsof theseslopesare more concave,longer,and rougherthan the (straighter,shorter,and steeper)lower
components
of type D slopes. In manyinstancesthe hummockymaterialsin the lower portionsof eachalcoveextendoutwardas irregularlobeswith very low telleft Width is- 90 kin. View is oblique.
sistentlyscalloped,tbrming a seriesof adjacentalcoves. Gradientsbecomelesssteepat lower elevationsto tbrm concaveup slopeprofiles(Figure4). Headwallsof the alcovesappear
featurelessat availableresolution,but slopesbecomeroughel'
at lower elevationswheregradientsdecreaseandmergeonto
the surrounding
plains. The lowerportionsof theseslopesare
more concave, longer, and rougher than the (straighter,
shorter,and steeper)lower componentsof type D slopes. In
many instances
the hummockymaterialsin the lowerportions
of each alcove extend outwardas irregularlobeswith very
low reliet} suggestingthesematerialsoriginatedfi'om the alcovesandmoveddown and laterallyaway. Thesehummocky
lobestypically extendof the orderof an alcovewidth out onto
the surroundingplains,but in many casesthe terminationsof
these lobesout on the plains cannotbe reliably recognized
among or distinguishedfi'om other scatteredlow-relief hummocksand small, very low mesasthat all have marginsand
relief similar to the low-relief lobesextendingfrom the al-
Despitemorphologicalhints that sappingcould be involved,
there are no direct indicationsof fluid flow on the canyon
floors or of reservoirsaway t¾omthe canyonswhere canyon
materialmighthavebeendeposited.Suchligatures
couldhave
been buried by youngel' volcanic materials on the canyon
floors and elsewhereor simply be too subtleto be recognized
in the images.
For instance,it is possiblethat the canyonfloors resemble
the surroundingplains as well as plateausurthcessimply becausethe canyonstbrmed tectonically fi'om these surthces
ratherthan erosionally. The origin of a large,-30-km alcove
systemopeningto the southti-omthe southeastern
cornerof
the plateauat Tvashtar (Figure 4, right) could be due to particularly active erosionand transportation(e.g., by sappingor
plastic ice detbrmationwith subtlerelief on the floor of this
alcoveSystemrepresentinga low-relief remnantdebrisfield).
However, in supportof a tectonichypothesisit is notablethat
the tbur largestcalderasof the Tvashtarsystemare distributed
coves.
along a southeastwardtrend, suggestingtectoniccontrol(FigType A slope morphologyis consistentwith scarpreces- ure 1). The large alcove systemopeningof the southeastern
sion involving ti•wer, larger individualmass!novementsthan Tvashtar plateau is located along this trend, suggestingthat
tbr type D slopes,as suggested
by the sizesof the alcovesand this alcove systemis in a likely placetbra structuraldepresthe volumesof materialin the lobesextendingfi'omthem. It sion of this size to occur. Furthermore, the subtle low relief
is unclearif a volume discrepancyexistsbetweenthe alcoves on the tloor o1'the alcove systemis morphologicallyidentical
and hummockymaterialswithin and extendingI¾omthem. to various expressionsof low-relief plains elsewherein the
For this reason, it is difficult to be certain how tb.r these area that have no demonstratedrelationship to slope procslopeshave receded. Crude estimationsthat restoreall hum- esses. A tectonichypothesisimpliesmuch lessbackwasting
mockymaterialto till the alcovesand reconstruct
the prior than the erosionalhypothesis. Uncertaintyon this issueimslope topographysuggestslopeshave recededdistances plies uncertaintyin estimationstbr recessionof type A slopes
similarto theiroverallheight(slightlythrthcr,relatively,than on Io. Without further evidence, it remains ambiguous
type D slopes).
whethertype A slopesindicateonly local recessionwith total
However,type A slopesalsooccuralongthe wallsof sev- volumesrelatedcloselyto currentalcovegeometryor indicate
eralcanyons
thatpenetrate
upto 40 km intotheeasternpartof much greateramountsof recessioninvolving the transportathe plateau,and it is possiblethat the mechanismtbr remov- tion of correspondingly
largervolumesof materialsignificant
ing thematerialfi'omthesecanyons
is the sameasor closely distancesaway ti'omthe slope.
relatedto slope processesproducingtheir alcovedwalls. In
Hummocky materials in the lower portionsof each alcove
this scenariosomemechanism(suchas sapping)erodesthe extendingoutwardas irregular,low-relief lobescouldsimply
baseof the walls,removingmaterialt?omthe slopeface(par- be landslidedeposits. An alternativeemplacementmechatially by undermining),
andevacuating
the overlyingdebris. nism is solid-statecreep. This speculativesolid-statemecha-
MOORE
ET AL.:
MASS WASTING
ON IO
33,227
'Figure
5. TypeT (terraced)
slopes
arecharacterized
bysubparallel
terracing
athigher
elevations
and(inmost
cases)
hummocky
material
ending
at distinct,
sometimes
convex-up,
lobate
margins
at lowerelevations.
(a)
Prominent
terracing
alongthescarp
t•ce.(b) Lesst•ceterracing
butsmallprominent
parallel
scarps
behind
the
mainscarp.Figure
5ais---55kmwide,andFigure
5bis--35kmwide.Theviewisoblique.
nism might involve contributionsfrom a ballistically emplacedvolatile componentin pyroclasticdeposits,which, alter accumulatingon slopesto a thresholdthickness,could enable movementof the entire deposit, including debris shed
fi'omthe slopeitself; downslopeunderits own weight,analogousin someways to water-ice-coredactive rock glacierson
slopewalls, producingterracesand minorscarpsnearthe
head of the failure surt•ce. Slope material is transported
downand laterallyaway to form hummockydepositswith
marginsthat are resolvedas convex-upin the largestcases.
Subparallel
terracingon typeT slopesis themostconsistent
morphological
distinctionfi'omtype A slopes(which are
andindicates
a
Earth. This processwill be considered
in moredetailin sec- markedby alcovesat their higherelevations)
tion 3.4.
differentmass-wasting
style. Multiple terracingof type T
slopessuggests
thatsloperecession
in theselocations
is a
progressive
process
involving
manyseparate
larget•ilures.
2.4. Type T Slopes
In contrast,thereare no indicationsof evenminorterracing
TypeT slopesare characterized
by subparallel
terracingat
behindthe alcovesof type A scarps,suggesting
that alcovehigherelevationsand (in mostcases)hummockymaterial
creatingslopet•ilures thereare singulareventsor at least
endingat distinct,sometimes
convex-up,lobatemarginsat much smaller and/or rarer. This could mean that destabilizing
lower elevations but there are wide variations within this
category. Type T slopesat Tvashtarare mostprominent
alongthe southern-to-western
inwardfacingwallsof theplateau,andalongthesouthwestern
outward-lhcing
walls(Figure
5a). Lessprominent
examples
arefoundalongeastern
inward
t•cing plateauwalls (Figure 5b). The relativesize of the
lower hummockycomponentvariesconsiderably
with location and seemscorrelatedwith the degreeof terracing. The
ellEcts such as removal of toe material or raising of relief
(e.g.,by loweringthebaselevel)occurmorefrequently
along
typeT slopes.If so,thiswouldbe consistent
withmosttype
T slopesat Tvashtaroccurringalonginwardthcingplateau
margins,
wherethebaselevelmightbesubject
to fluctuations
fi'omactivityin the magmachambersystembeneaththe calderas. The seriesof t•ilures creatingthe type T slope along
the southwesternoutward thcing margin is harderto under-
hummocky
unitextendsupto 20 km fi'omthewesterninward
stand. The hummockyrun-outdeposithere is especially
t•cing wallswhereterracingis mostdeveloped,
extends<10
prominent,
andsubparallel
terracing
occurs
asmuchas50 km
km fi'omthe southerninwardthcingwalls and is not t•)undat
backfi'omthe slopemargin. Portionsof the plateauappearto
all alongthe foot of the easterninwardthcingwall. In the
havebrokenup in response
to lateraltensional
stresses,
with
latter case,where terracingis also much lessprominent,the
uplitlof theplateaumarginasa possible
stress
source.
lowerhummockyunitmay havebeenmuchsmalleror lower
to beginwith,thenwasburiedby volcanicmaterials
fromthe 2.5. Slope Exposuresat Other Localities
adjacent
calderas.Burialmightalsotruncate
theexposures
of
Many slopeson Io have morphologiessimilar to the four
other hummockydepositselsewherearoundthe walls rimmingthe large70 x 170km depression.
Alongthesouthwest- slope types describedabove in the Tvashtar area. Type A
ern outwardthcing walls, the hummockycomponentwith slopes,variationsof type T slopes,and perhapstype U slopes
convex-up
marginsis so prominent
thatit composes
moreof are found along plateaumarginsand mountainsidesseenin
the totalslopereliefthanthe subparallel
terracingt•rtherup- the Zal Montes and its surroundings (25ISTERM01 and
slope.
27ISZALTRM01
Galileo observations). The Zal Montes
TypeT slopemorphology
is consistent
with brittlefhilure, (Figure 6) consistof a 60 x 150 km plateauand a higher-relief
coherentslumping,and in someinstances
partialrotationof complexridgetrendingSSE tbr 200 km 'Turtleet al., this is-
33,228
MOORE ET AL.: MASS WASTING ON IO
Figure6. A plateau
in theZal Monteslocated
at •42øN,77øWexhibiting
twotypesof slopes
(images
2510060
and2510061,
260m pixel-•).TypeA slopes
aretbundnear"A,"andtypeT slopes
arelocated
near"T."Theview
isoblique.Illumination
is fi'omtheletl. Northistowardtl•etopletl.
sue]. A 120-km-diameter caldera lies between these constructs.
The area is seen closer to the terminator
in the 260 m
pixel
'• 125observation,
whichalsoincludes
an isolated
unnamedmountain
to thesouthwest,
butthe335m pixel
-•I27
observation(Figure 7) has more complete coverageof the
higher-reliefSSE trendingmountainridge. Type A slopesare
prominentlydisplayedalong the northernmarginof the plateau; the viewing geometry shows characteristiclobate depositsextendingup to 30 km fi'om individual alcoves(Figure
6). Subtlealbedopatternson the plainsbeyondthe lobatedepositscould mark where fluid-like material has flowed fi'om
the type A slopes. However, these patternsalso are very
similarto thosecommonlytbundelsewhereon plainst•r t?om
slopesand so may simply be lava flows. Shallowtroughsand
swaleson the southeasternpart of the plateautransformnear
the southernmargin into terracesof type T slopes. Hummocky lobes (thicker and more extensivethan those associ-
ated with type A slope alcoveson the other side of the plateau)extend40 km to the southfrom theseterracedscarps.
Subtlenear-horizontallineationsmark west thcingslopesof
the northernend of the higher-reliefmountainridgein the Zal
Montes(Figure 7). Thesecouldrepresentremnantsof layered
plainsmaterialuplitled with the main mountainmass[Schenk
and Bulmer, 1998; Turtle et aL, this issue]. Alternatively,
theset•aturescouldmark the locationsof slumpterracestypical of type T slopedegradation.For this scenariowe speculate that the massifprogressivelyshedrelativelyweakernearsurt•ce materialsaside during uplift as it emergedat the surthee;the original surthcematerialsprogressively
shovedaside
by the steep, emerging massif peak are the terraced,lower
gradientmaterialscomposingthe lower portionof the mountainside. Extended hummocky run-out depositsare absent
t?omthe plainsat the tbot of this slope,but thereare two possible explanations:(1) slumping materials simply may not
MOORE ET AL.: MASS WASTING
ON IO
33,229
Figure7. A ridgein theZal Monteslocated
at-35øN,73øW. Notethesubtlenear-horizontal
lineations
mark
westfacingslopes
of thenorthern
endof thehigher-relief
mountain
ridge,whichcouldmarkthelocations
of slump
terraces
typical
oftypeT slope
degradation
(images
2710059
and2710060,
335mpixel-I).
Theviewisoblique.
Illuminationis fi'omthe bottomlett. North is towardthe top leit.
materials);anda lower,lesssteepcomponent
consisting
of a
seriesof partlycompressed,
tilted, and potentiallyunstable
slumpsof whatwereoriginallysurfacedeposits,
covering
the
lower flanks of the uplifted massifand extendingoutward
ontothe surrounding
plains. Few detailsare exposedon the
slab-like, almost tkmtureless
upperflanks of this mountain
dera).
complex,similarto the upperflanksof the smallerisolated
Two mountaincomplexesassociatedwith Gish Bar Patera mountainto the northeast.SkythiaMons is a moreextended,
have beenshovedlaterally very tkr as the massifemergedat
the surIkceand/or (2) longer run-out landslidedepositsmay
havebeengeneratedeitherduringmassifupliI• or laterdegradationof the slopebut they may havebeencoveredor eroded
away by subsequent
volcaniceruptions(e.g., by lava flowing
north along the tbot of the slopefrom a small adjacentcal-
arecaptured
in theeastern
halfof the480-570m pixel
-•
lower-relief
24ISAMSKGI01 observation(Figure 8). Variationsof type T
slopedegradationare found on all of thesefeatures. A plateau extendswestwardfrom the largermountaincomplexnear
Patera. The flanks of Skythia Mons display alternating
groovesandridgesorientedonlyapproximately
perpendicular
structure than the mountains around Gish Bar
to localgradient(Figure9). Minor examples
of thesestria-
GishBarPatera.Thisplateauhasa lobate,digitatewestern tionsalongthe easternlowerflanksmightowepartof their
character
to expressions
of internallayering,but elsewhere,
especiallyalongthe southeast
margin,this texturedoesnot
convincingly
suggestexposures
of horizontalstrata. Instead,
the striationsalongthe southeast
margindefinearcuate(conat lowerelevations
layerwherethe massifemergesfrom the surt•tce.The alter- caveoutward)patternsthatare expressed
nativeexplanationwas also exploredby Schenkand Bulmer as a series of shallow alcoved terraces with arcuate scarps.
structural
disturbances
arelargelyresponsible
tbr the
[1998] for a t•ature associatedwith the Euboea Montes. Perhaps
Continueduplitl resultsin the weakersurthcematerialsbeing striatedrelief around the margin of this prominentplateau,
partly raisedand partly compressed
laterally as they are and along the southeastmargin these disturbanceshave
of theflanks.
graduallyshedaside. This process
produces
relativelyhigh- causedbackwastingof theplateauby slumping
The marginsof ShamshuMons(Figure10), imagedat 342
relief mountain slopeswith two elements:a steeper,upper
-• duringI27,showevidence
of typeA slopedegradacomponent
con'esponding
to themassifitself(probablywith a m pixel
variationsperhapslinkedto obminimumof adhereddebrisfi'omoriginallyoverlyingsurt•ce tion but with morphological
marginthat may be a landslidedeposit. As discussedpreviously tbr the smaller isolatedmountainto the northeast,an
alternativehypothesistbr this type of morphologyinvolves
massif uplit• disturbinga much weaker overlying surface
33,230
MOORE ET AL.' MASS WASTING ON IO
shadow. The cliffs appearmassive,with tbw hintsof layering
or of gullies or other featuresconsistentwith masswasting.
At lower resolution this would probably be identified as a
type U (largely unmodified)slope. In someplaces,cliflk descendonly aboutthree fourthsof the way to the calderafloor
betbre changingabruptlyto a componentwith a muchlower
gradientthat descendsthe remainderof the total relief of the
slope. The relativelyconsistentgradientof this lower component suggeststhat it could be debrisaccumulatedfrom many
small releasesfrom cliff-tbrming materialimmediatelyabove.
However, severaldetails suggestthat an explanationas talus
might be too simplistic. The interpretationof the lowercomponentmaterial as talus would be more secureif there
were correspondingcavities or gullies immediatelyaboveto
suggestwhere this volume of material originated, but the
scarpbrinks appear as continuousin these locationsas elsewhere alongthe walls where cliffs extendall the way downto
the calderafloor. In one area the putativetalus hasan albedo
patternthat cannotbe explainedeasily as a shadowst¾omthe
clifik abovebut might be consistentwith fingersof dark talus
slumpingdown older, brighter exposuresof the sameunit in
minor stabilityadjustments.It is not clearwhy at somepoint
in the past portions of this putative talus would have been
brighter, but complicationsassociatedwith calderaactivity
could be involved. There are bright patchesimmediatelyadjacent on the calderafloor, and whateverbrightenedthis portion of the calderafloor might also have brightenedthe putative talus unit at one time; subsequentreleasesfi'omthe clifftk)rmingunit would be darker and would gradually cover
bright depositson the talus beginningat the higherelevations
immediatelybelowthe clifik.
Figure 8. The mountaincomplexnearGish Bar Patera(im-
ages2410111
and2410117,-•500
m pixel'•).Theplateau
at
left hasa lobate,digitatewesternmargininterpretedas a landslidedeposit. The view is oblique. Illuminationis fi'omthe
lefL North is roughlyup.
scurationby lava flows. Unlike type A slopesin many other
places,alcovesalong the northwesttkcing 11ankof Shamshu
Mons
have flat floors with
no traces of low-relief
lobes ex-
tending fi'om them. Lava apparently has flowed northeast
fi'om a small caldera into the trough betweenthe alcovesand
an outlying, parallel ridge, burying the lower portionsof the
alcovesand tbl'mingtheir flat floors.
Veryhighresolution
images
withresolutions
<10m pixel'•
were also obtainedby Galileo, but theseimagescover an extremely small portionof the satelliteso their contentis almost
certainlynot truly representative
of the entiresurtkceor processesthat occur there. Nevertheless,these imagesbegin to
show more of the actualgeologicalcomplexityof the surtkce
than more widespread,lower-resolutionimagescan. Chaac
calderascarp(Figure 11) is capturedin two imagesof the---9
m pixel'• 271SCHAAC_01
observation.
Mostof thescarp
length capturedin the imagesconsistsof a very steepclifftbrming member extendingfi'om a sharpbrink at the plateau
edge right down to the caldera floor; shadowmeasurements
indicatethis cliff-tbrming memberhas a total relief of 2.8 km
and descendsat a gradient of around 70ø [Radebattgheta!.,
this issue]. The walls are dimly lit by scattered light in
Figure 9. Striationsalong the southeastmarginof Skythia
Mons (•26øN, 97øW) define arcuate(concaveoutward)patterns that are expressedat lower elevations as a series of
shallowalcovedterraceswith arcuatescarps,which may be
the resultof backwastingof the plateauby slumpingof the
flanks
(image
2410110,
---500
m pixel-•).Theviewisoblique.
Illuminationis fi'omthe letL North is roughlyup.
MOORE ET AL.' MASS WASTING ON IO
33,231
Figure 10. Marginsof ShamshuMons (-•12øS,72øW),showevidenceof type A slopedegradation
but with mor-
phological
variations
caused
by thelowerportions
of theseslopes
beingobscured
by lavaflows(images2710065
& 2710066,
342m pixel'•).Theviewisoblique.Illumination
isl¾om
theupperletl. Northistoward
theupper
right.
Ridge setsseenon somemountainflanks, suchas Hi'iaka
Montes(Figure 12), appearinterleavedor en echelon. Ridges
bitBrcatein some casesand have convex to convex-rampterminations. Ridgesin cross-trendprofile appearroundedand
may be the surfaceexpressionof folds. The morphologyof
individualridgesand the arrangementof ridge setsmight best
be interpretedas the manitOstation
of compressional
foldsin a
surt•ce layer overlying a shallowdecollement,or perhapsa
ductile subsurt•ce[Pappalardo and Greeley, 1995]. This
ridge texture is most pronouncedon slopes,which suggests
that they t•)rmed by gravity sliding of a detachedsurt•ce
layer. The orientationof ridgesis always perpendicularto
gradientwhere this texture is found. If Io's mountainstbrm
by the tilting of preexisting crustal blocks, the shallower
ridged facet may be the original (formerly flat) premountain
surthce. Cumulative successions
of plume fallout, SO2 condensates,landslide material, and lava flows probably t•)rm
stacksof thin, areally extensivelayerswithin the plains. If
such stacks of material have weak layer-to-layer contacts,
they would be susceptibleto del•)rmationand detachment
when tilted. Schenk and Bulmer [1998] proposeda similar
configuration
asa contributor
to mountainlandslides
on lo.
Figure 13 showsone imaget?oman 1l-image observation
of a very small portion of Ot Mons obtainedat very high
resolution
(9 m pixel
-•) andlowsunthatweinterpreted
asa
close-upview of ridge textureon mountainflanks suchas is
shown in Figure 12. Preencounterpointing predictedthat
Figure 13 is locatedon the north flank of Ot Mons, but untbrtunately,the bestavailablelower resolutioncontextis a •-2
kmpixel
-• image
acquired
during
theEl4 encounter
(Figure
14). In the high-resolution
imagesthe surfaceis coveredby
E-W trending ridges ---1-2 km long and 0.25-0.5 km wide.
Just as with the ridges seen elsewhere at lower resolution
(e.g., Figure 12), the Ot Mons ridges exhibit pinchesand
swells. Unlike the ridgesin Figure 12, someof the Ot ridges
appearedbreachedor disrupted. These breachesor disruptionstake the lbrm of steep-walledamphitheaters.Theseamphitheatersare often elongateand trend the samedirectionas
the ridges. The floors of the amphitheatersare often covered
with dark material that might have moved downslopeto accumulatethere. Ten'acingwithin the walls of the amphitheaters might be layering exposedby the amphitheater-k)rming
process. If the Ot Mons flank ridges are indeed anticlinal
ridges,suchridgescommonlyhaveextensionall•aturesalong
their crests[Pappalardo and Greeley, 1995]. We speculate
that the amphitheatershave t•)rmed initially as anticline crestal fi'acturesand subsequentlywere widened into amphitheatersby an erosionalprocessthat involvesthe disaggregation of the outcrop.
Duringthe127encounter,
several
5.5m pixel
'l images
were
acquiredto look t•)r evidenceof layering and to investigate
Io's erosional processes. These images show complex patternswith pronouncedvariationsin textureandalbedo(Figure
15). Further complicatingthe interpretationsis the fact that
33,232
MOORE
ET AL ß MASS WASTING
ON IO
Figure 11. A portionof the Chaaccalderascarpillustrating
a typeU slope(toptwo imagesare locatedat ---12øN,
1
158øW,images2710009and2710010,---9m pixel-). Mostof thescarplengthconsists
of a very steepclif•:•brming
member(mostlyin shadow)extendingt?oma sharpbrinkat the plateauedge(at right)downto the calderafloor (at
let•); shadowmeasurements
indicatethat this clit]:t•)rmingmemberhasa total relief of 2.8 km and descends
at a
gradientof around70ø. The walls are dimly lit by scatteredlight in shadow,but the clifi• appearmassive,with
onlyflew
hints
oflayering,
orofgullies
orother
fieatu!'es
consistent
withmass
wasting.
•Illumination
ishighand
fi'omthe right tbr the top two fi'ames. The lower image(image2710036,---180m pixel' ) providescontext. The
bold boxes locate the top two fi'ames. Illumination is high and fi'om the let• tbr the context •?ame. North is
roughlyup.
MOORE ET AL'
MASS WASTING
ON IO
33,233
Figure 12. Ridge setsseenon somemountainflanks,suchas hereon oneof the Hi'iaka Montes(---2øS,83øW, im-
age2510064,
265m pixel-I),
appear
interleaved
orenechelon.
Ridges
bitercate
insome
cases
andhaveconvex
to
convex-rampterminations. Ridges in cross-trendprofile appearroundedand may be the surf•.ceexpressionof
tblds. Illuminationis fi'omthe top. North is roughlyto the right.
the imagesare quite oblique (60o-70ø emissionangle). The
scene shows a mesa or plateau scarp, largely facing away
from the spacecraft,and the surfacebeyondthe scarp'sfoot.
Shadow measurementsindicate the scarp exhibits •400 m of
relief and a slopeof •28 ø. There are hintsof layeringin the
scarpf•.ce. A •-l-km-wide belt or zone exhibitingbright and
dark swirl patterns hugs the base of the scarp. The swirl
belt's conformityto the scarpbasecan be interpretedto indicate that this material is being exposedby the retreatof the
scarp. While only a small portionof the scarpis imaged,the
portion visible is sinuousin plan form, unlike what would be
expectedof a l•.ult scarp. The swirl belt might be a lava flow
along the baseof the scarp,but generally,surl•ces beyond
scarpbasesare usuallyhighestnearthe scarp;thus lava flows
generallywould tend to not be scarphugging. Fartherout on
the plain beyondthe swirl belt, there appearto be small gullies cut into a blander, probablysmoothersurface,10-20 m
across and 100-200 m long, and roughly oriented perpendicularto the scarp(Figure 16). Beyondthe gullled smoothen'
belt is a thinner (---0.5km), darker, rougher belt or zone also
apparentlyroughly paralleling the scarptrace (though image
coverageis very limited). While this scenemight be entirely
Figure 13. Very
high
resolutionview of breachedridgeson the northflank of Ot Mons (•4øN, 216øW,image
ß
-!
2410037,9 m pnxel ). The breachestakethe formof steep-walled
amphitheaters.
Terracingwithin the walls of the
amphitheaters
might be layeringexposedby the amphitheater-forming
process.The view is near vertical. Illumination is from the leti.
North is down.
33,234
MOORE ET AL.: MASS WASTING ON IO
volatile enriched debris, analogousto terrestrial water-icecored rock glaciers.Although observationsappearconsistent
with mass wasting processesinvolving dry materials, it
should be kept in mind that mass wasting involving fluids
(e.g., debris flows, landslide depositswith long run-out distances lubricated by a fluid) could potentially involve thinly
distributedvolumesof relatively low relief that couldbe difficult to detect with available resolution. "Dry" mass movementsinvolve slope materialsthat releasein many small volumesor in fewer large volumesbecauseof either a reduction
in strength or a change in slope geometry (e.g., removal of
slopetoe by loweringof a calderafloor). Here the term dry is
being applied using a well-known terrestrialmassmovement
classification scheme [e.g., see Coates, 1977; Malin, 1992,
Figure 2] that comparesmaterial cohesionand particle size
against speed of movement. This scheme groups mass
movementsrequiringfluidizing lubrication(whetherfrom air
or water) to one extremeand thosewhich requireno lubricant
(and henceare dry) at the other. Figure 17 showshow slope
morphologychangesas the size of the slopefiailureincreases.
Type D slopes result fi'om the accumulationof many very
small, block- or boulder-sizedreleasesfi'om cliff exposures.
Larger, coherent fiailuresof slope materials reaching more
deeply behindthe slope fiacecreatealcovesand type A morphology. Even larger slope failures result in the terracing
seenin type T slopes.
Figure 14. Preencounter
pointing(arrow) predictionof the
In placessuchas Tvashtar all of theseslope morphologies
locationof Figure 13 on the northflank of Ot Mons (•-4øN, are presentin relatively closeproximity. How can this be ex215øW,image1410013).Thisbestavailablecontextis a •-2.6 plained? The inward facing slopesdisplaythreemorphologikm pixel imageacquiredduringthe El4 encounter.Illumi- cal styles: types U, A, and T. The outward fiacingslopes
nationis fi'omthe right. North is up.
show two of the same morphologies,A and T, plus an additional style, type D, that is not seenon the interiorwalls. One
possibilityis that strongermaterialsare more abundantcloser
theproduct
of volcanism
andtectonism,
theseriesof seem- to the calderathan fiartheraway. In this scenariothe ratio of
inglycircumffential
beltsor zonesbeyondthe scarpandthe silicate lavas to more volatile-rich pyroclastic depositsdeplantraceof thesetEatums
thatwejudgeto implya possible creaseswith increasingdistancefi'om the calderas,and siliexplanation
involving
theerosional
retreatof thescarp,possi- cate lavas might potentially producesteeperand more coherbly involvingthe decayof relief-supporting
materialin the ent linear scarps, while volatile-rich pyroclastic deposits
scarpface.
might tbrm potentiallymore unstablewalls and encouragealcove tbrmation.
3.
Discussion
The contribution
of this fiactor is difficult
to
assess or rule out with current data, but circumstantial evi-
dencesuggeststhat it is not necessarilythe mostimportant. A
differentscenariois that the tbur slopemorphologiessummarized in Figure 17 also define an evolutionary/timesequence,
in which type U slopesare leastevolved,tbllowedby type D,
and finally by either type A or T. Sucha time sequenceimplies that steepslopes(1) are createdas cliff-like scarpsand
(2) then progressivelydeteriorateas materials fiartherback
behindthe slopeface are weakenedsomehowfi'omproximity
to the surfiaceenvironment. In this scenario,slopedegradational style is a t•nction of age. Consideringthat volcanism
at the calderasis certainlythe high-frequencygeologicactivdiscuss these mechanisms in some detail.
ity in the Tvashtararea, it is not surprisingthat type U slopes
are tbund closerto the calderaswhere renewal is most likely
3.1. Block Release and Brittle Slope Failure
and that slope morphologiesthat might reflect greater expoCharacteristicslopeprofilesof the four slopemorphologies sure age and degradationaldevelopment are further away.
described in section 2 are summarized in Figure 17, along Outwardfacing slopesare lessfrequentlydisturbedso are left
with the most likely (or at least the simplest,most readily to moret•11ydevelopdegradational
morphologies.
supported)interpretations. Although processessuch as sapping cannotbe ruled out, we find no unambiguousevidence
3.2. Sputtering
tbr liquid dischargingat slope fiacesor debristransportreThe potentialeffectof magnetospheric
sputteringincreases
quiringthe presenceof a liquid. Low-relief hummockylobes
with decreasing
distancefi'omJupiter.HenceIo
extending from some slope faces could either be avalanche substantially
depositsor materials depositedfrom solid-stateflow of a shouldbe most affectedby this process. Sputteringat Io
Io shows clear and unambiguousevidenceof downslope
movementof material at a variety of scalesand probablyemployinga varietyof mechanisms.We now considerwhat may
be producingthesedifik•rentlandtbrmsand surtkcetextures.
On lo, mass movementsinvolving block releaseand brittle
slopetkilure, as well as other processes
suchas sputtering,
sublimation,sappingerosionI?omliquid SO2,the downslope
movementof warm, plasticallydeIbrmingSO2ice, and disaggregation I¾om chemical decompositionof solid S20 and
otherpolysulfuroxidesare candidateprocesses.We will next
MOORE ET AL.' MASS WASTING
ON IO
33,235
Scarp
Figure15. Veryhighresolution
images
(5.5m pixel
'l, images
2710005
and2710006)
showing
a mesa
orplateau
scarp,largelyt•cing away t?omthe spacecrat•,
andthe surfacebeyondthe scarp'stbot (---32øN,193øW,seecontext
map at top left). Shadowmeasurements
indicatethe scarpexhibits-400 m of relief and a slopeof-•28 ø. There are
hintsof layeringin the scarpface. While this scenemight be entirelythe productof volcanismand tectonism,the
seriesof seeminglycircumfrentialbeltsor zonesbeyondthe scarpandthe plan traceof theset•aturessuggestthe
possibilitythat it could also be the consequence
of erosionalretreatof the scarp,possiblyinvolving the decay of
relief-supportingmaterial in the scarpface. Original perspectivehighly oblique(---70ø). Imagesreprojectedto a
commonpointperspectiveview similarto that fi'omthe spacecratLIlluminationis t?omthe right. North is up.
would have a negligibleerosionaleffect on silicates,suchas
basalt, and some ett•ct on sulfur allotropes[Johnson, 1990].
Sputtering-inducederosionon Io would be most pronounced
on exposuresof unprotectedSO2 [Johnson,1990]. As such,
the amountof erosionof SO2dueto sputteringover the age of
Io's surt•ce (generally acceptedas not greater than ---1Myr
[e.g., Johnsonand Soderblom, 1982]) determinesthe maximum erosionsputteringcan causeon Io. Johnson[1990], in
Io. Thusotherprocesses
mustdominatelandtbrmerosionobservedby Galileo.
3.3. Sapping
A sappingmechanisminvolvingliquid SO2was proposed
by McCauleyet al. [1979] to explainthe irregularerosional
scarpsand brightdepositsin layeredterrainsimagedby Voyhissummary
of sputtering
studies,
givesa valueof 5 x 10• ager 1. Similar morphologieson Earth and Mars have been
molecules
cm'2 s'• tk)rthemaximum
sputter
erosion
rateof interpretedas due to the releaseof water to the surt•cecomSO_,on Io. Over a million yearsthis would resultin a surtb.
ce bined with masswasting [e.g., Carr, 1995]. Liquid water is
loweringof---I m tbr uncontaminated
massiveSO2ice (-1.5 g highly unlikely on Io, but SO2 will becomeliquid at some
cm'3)or---3m tbruncontaminated
porous
SO2snow.Surtiacedepth,dependingon the iothermalgradient(probablyhighly
would drive
modification on this scale would probably be undetectable variable in spaceand time). Artesianpressures
eveninthehighest
resolution
(-5 mpixel
'•) Galileo
images
of liquid SO2 to escapethroughcracks,fissures,or permeable
33,236
MOORE ET AL.: MASS WASTING ON IO
km
Figure 16. An orthographically
projectedexcerptfrom a portionof the rightimage(image2710006)in Figure 15
showingwhat appearto be smallgullies(seearrows)cuttinginto a blander,probablysmoothersurface. The gullies are 10-20 m acrossand 100-200 m long and are roughlyorientedperpendicular
to the scarp. Illuminationis
from above.
North is to the left.
layersalongthe edgesof scarps.The escapeof SO• to the it mustrely on locallyhighgeothermal
energyto mobilizethe
Ionian
surt•ce
(-•10
-• barpressure)
wouldcause
vaporization,
liquidSO2. As the Tvashtarregioncurrentlyis undergoing
andanexitvelocityof morethan350m s'•;theSO•would widespreadvolcanicactivity, suchgeothermalenergyis
snow back onto the surfaceup to 70 km from the vent. This
is entirelyconsistent
with the diffusewhitedeposits
seenextendingfrom the baseof manyirregularscarpson Io (seeil-
probablyavailable. New high-resolutionimagesof these
canyonsare plannedlbr the I31 encounter
(August2001) to
get a closerlook at the morphologyandto searchfor evidence
lustrations
of McCatdeyet al. [ 1979]).
of changesin thecanyonwalls.
As discussed
previously,deepalcovesintothe plateaunear
TvashtarCatena(Figure1, seeA) mighthavetbrmedby sap- 3.4. Plastic Deformation
ping. Canyons
---1km deep[Turtleeta!., thisissue]extendup
One of the possibleemplacement
mechanisms
tbr the lowto 40 km into the plateau. The rectangularpatternof alcove- relief lobesextending
fi'omalcovesalongtypeA slopesis
rimmedtroughsin the Tvashtarplateauresembles"fi'etted" solid-stateflow. This couldinvolveplasticdeformation
of
erosionalongthehighlands-lowlands
boundre3,
on Mars. The material with interstitialvolatiles (e.g., SO=), somewhat
fi'ettedmorphology
of thispartof theTvashtar
plateaumight analogous
to glacialflow of watericeor ice-cored
rockglabe an exampleof liquidSO•exploiting
andenlarging
struc- cierson Earth.Like liquidSO2sapping,thismechanism
also
turalpatterns
of weakness
(e.g.,jointsandfractures),
causing wouldneeda localconcentration
of geothermal
energyto
disaggregation
and furtherbackwastinginto the plateau. sotlentheSO2ice. Plastic
deformation
and"glacial"
creepof
Sappingdueto liquidwaterhasbeenproposed
tbr the initial interstitialwaterice hasbeenproposed
for the formationof
channalization
of frettedchannelson Mars [e.g., Milton, frettedterrainon Mars [e.g., Sharp, 1973;Squyres,1978;
1973; Cart, 2001]. If the sappinginterpretationis correct, Lucchitta,1984;Cart, 1995]. It is claimedthanin the Marthissuggests
thatSO• becameliquidat a depthof I km or less tian case plastic det•)rmationof warm water ice would not
whenthesecanyons
t•)rmed(perhaps
continuing
to the pre- necessarilycut channelsor obviouslystreamlineobstacles,
sent). However,liquid SO• is modeledto usuallyoccurat andthat ice mightsimplyflow outwardontoplains.Analodepthsf•trbelow4 km [Leoneand •ilson, 1999],soliquid gous processeson Io involving a different volatile would
SO2at depths
_<1kmmaynotbecommon.If sapping
occurs, leave tEw lasting traces at the resolutionsof the Galileo
MOORE ET AL.' MASS WASTING ON IO
TypeU (unmodified)
:•
33,237
behavior at temperaturesnormalized by their melting tem-
......................
:•' peraturesintermediatebetweenH20 and CO2. In any case,
H20 is apparentlythe stifl•st cosmicallyabundantice [e.g.,
Durham et al., 1999]; hence SO2 ice will 11owmore readily
than H20 ice at a given fi'actionof its melting temperature.
As warm H20 ice flows readily over very geologicallyshort
timescales,it is reasonableto assumesufficiently warm SO2
ice will also.
3.5. Sublimation Degradation
Sublimationdegradationis the result oF disaggregationof
relief-lbrmingmaterialsthroughthe lossof their cohesivebut
ultimately volatile, rock-lbrmingmatrix or cement[Moore et
al., 1996]. Sublimationdegradationhas been proposedas an
agentof kilometer-scalelandlbrmmodificationon Io [Moore
eta!., 1996] and other Galilean satellites[Moore et al., 1999;
Chuang and Greeley, 2000]. Sublimationprocessesrelevant
to (noncometary)bodiescontainingvolatilesin their surlhce
layershave beenmodeledby a numberof researchers
[Lebofsky, 1975; Purves and Piicher, 1980; Squyres, 1979; Spencer
1987; Colwell et ai., 1990; Moore et al., 1996, 1999].
Moore et ai. [1996], in their pre-Galileo study, proposed
that in the case of Io's polar regions (the area of highestresolutioncoverageby the Voyager missions)only H2S, sublimating l?om slopesthat lhce the Sun and have thin lags, is
volatile enough to causethe observederosion. They concludedthat SO2 is not a viable candidateas an agentof erosion for high-latitudelandtbrms.Models usedby Moore et al.
[1996], however, did indicate that SO2 might sublimate
quickly enoughunder a very thin (<5 cm), very low albedo
(<0.05) lag to modit•yequatorial landtbrmson scalesthat
mightbeobserved
in high-resolution(of
orderof 10m pixel
'•)
Galileo images. Moore et ai. [1996] predictedthat H2S surl•ce ice would be unambiguously
detectedon Io, particularly
at the poles. This prediction,however, has not been borne
out.
It is worth notingthat a numberof observations
are consistent with the occasionalpresenceof volcanically-released
H2S
Figure17. Stylizedprofilesof differentslopetypesdiscussed
in the text alongwith interpretive
restorations
of theirevolution.
Tvashtar and Zal Montes observations,except flow lobes
where movementwas not topographicallyrestricted. Similar
morphologicargumentshave been made t•)r the hypothesis
that some Martian terrains, such as the fi'etted terrain and
large lobate aprons are substantiallythe result of warm de-
formingice [e.g.,Squyres,1978;Carr, 1995]. An analogous
processmight be occurringon Io, but with a different "ice"
volatile, that of SO2.
Could solid SO2 flow downslope,or simply outwardunder
its own weight, if sufficientlywarmedfi'ombelow? Although
experimentalstress-strain
data lbr solid SO2 is unknownto us,
resultsfi'om recentsolid CO2 delbrmationexperimentsimply
that SO2may behaveplasticallyat temperaturesapproaching
its meltingpoint (198 K). Figure 18 is a plot from Durham et
al. [1999] showingthe flow of severalices,includingtheir re-
10'"
1.0
T•,IT
Figure 18. Plot from Durham et al. [1999] showingthe flow
centresults
lbrCO2.BothSO2andCO2exhibit
vander ofseveral
ices.Stress
isnormalized
bytheshear
modulus
(/t.)
Waals
intermolecular
bonding.
However,
theconfiguration
of andthetemperature
isnormalized
bythemelting
tempera-
CO2molecules
is linear,andthatof SO2is not,andsoSO2 ture(T,,). SO2ice flow mayfall between
thatof H20 and
will be morepolarthan CO2. Thus SO2ice may exhibitflow
CO2. Seediscussion
in text.
33,238
MOORE ET AL.: MASS WASTING
ON IO
but not stable H2S f¾ostdeposits. Nash and Howell [1989] the emplacementand subsequentdecompositionof S20/PSOpresentedspectroscopic
evidencefbr the transientoccurrence rich layerson timescalescompatiblewith the age of Io's surof condensedH2S on lo, but microwaveobservationsf•.iled to f•.ce and at a size scale that would be within the resolution of
detectatmospheric
H2S,suggesting
abundances
below10© Galileo images.
bar [Le!loucheta!., 1990]. Observationsby the SpaceTeleDisaggregationfi'omthermallyinducedchemicaldecomposcope Imaging Spectrograph(STIS) revealedenhancedH1 sition of solid S20/PSO shoulddegradelandfbrmsin a manLyman-o½
emissionnearIo's poles,perhapsdueto Iogenichy- ner that resembleserosionf¾omsublimation. If the disaggredrogen[Roesslereta!., 1999], but detailedanalysisof the gation-causingheat is primarily solar, typical landfbrmsthat
data has shownthat it is reflectedsolarradiationattenuatedby might be produced are rimless, irregularly shapedpits, unan SO2atmosphereconcentrated
nearthe equator[œeldmane! evenly retreating scarps[e.g., Moore eta!., 1996, 1999] and
a!., 2000]. Russelland Kivelson[this issue]presentevidence surfacesreminiscentof glacial ablationsurt•tcessuchas fields
fbrthetemporally
variable
presence
of H2S
+in Io'sexosphere.of cones or mounds [e.g., Malin and Zimbelman, 1986].
However, all theseH2S sightingsgive very little supportfbr Geethermally induced S20/PSO disaggregationmight also
substantial
H2Sdepositsin the nearsurf•.ce.
fbrm pits, voids, and other collapsefeatures. The erosionally
As was noted in section 2.5 the amphitheatersamong the enlargedamphitheatersof the Ot Mens ridge crests(Figure
ridgesseenon Ot Mens (Figure 13) are apparentlybeingen- 11) and the putativelyidentifieddifferentially erodedlayering
largedby someerosionprocess,in which casethe dark mate- and other surf•.cetexturesseenin the along the SE edgeof the
rial seen on their floors may be erosionaldetritus. The ap- Isum plateau(Figures15 and 16) are goodcandidates
fbr eropearanceof amphitheaterwalls is consistent
with sublimation- sional expressionsthat are consistentwith, thoughnot proof
degradation[Moore eta!., 1996, 1999], as are the putatively of, S20/PSO disaggregation. The S20/PSO disaggregation
identified differentially eroded layers seenat high resolution hypothesis,however, is an intriguing explanationfbr eroalong the SE edge of the Isum plateau(Figure 15). As these sional morphologieson Io, which could be attributableto
featuresdo not appearto be presentlyassociatedwith local, sublimation but are not so ascribed due to the absence of a
very low albedomaterial,the role of SO2sublimationin their suitable volatile.
erosionis probablysmall.
3.7. Implications for Volatile Abundances in Io's Upper
Crust
3.6. DisaggregationFrom Chemical Decomposition
An erosionalprocessthat so f•.r hashad little discussion
in
Both the eroded layered terrain and the long-lived PromeIo studiesis disaggregation
f¾omthermallyinducedchemical theus-typeplumesindicatethat the uppercrustof Io is rich in
decomposition.Hapke [1989] and Hapke and Graham[1989] volatiles. Hapke [1989] proposeda model in which surfaceof
discussedS20 and polysulfuroxide (PSO) and their potential Io is dominantlybasalticwith only thin (---1lum) coatingsof
contributionto spectraof Io surf•.cematerials. In the course PSO, S:O, and SO2,but McEwen and Lunine [1990] protested
of explainingwhy they thoughtS20 and PSO might be pre- that sucha model could not accounttbr the volcanicactivity
senton Io's surf•.ce,they reviewedthe temperature-dependentand that volatiles must be much more abundantin Io's upper
behaviorof thesematerials. Hapke [1989] statedthat S20 and crust. This view is strengthenedby evidence that PromePSO couldbe fbrmedwhen SO2gas is exposedto a dissocia- theus-typeplumesare producedby interactionsbetweensilltive environment such as the high temperaturesassociated cate lava and near-surfaceSO2 or sulfur in the surrounding
with hot eruptingmagma. Recentobservations[McEwen e! plains extending hundredsof kilometers fi'om the primary
al., 1998] of very hot magmason Io imply that the conditions vents [McEwen eta!., 1998; Kieffer eta!., 2000; Milazzo e!
fbr S20 andPSOfbrmation•.nddeposition
existandmaybe a!., this issue]. The eroded layered terrain imaged by Voycommon. Hapke [1989] notesthat S20 is stableif the tem- ager in Io's south polar region also suggestsa volatile-rich
peratureis lessthan about 115 K. As the temperaturerises compositionbecausethe plateaumaterialsappearto be comabovethis value, solid SO2doesnot sublimatedirectly but in- pletely removedin somemanner[Schaber, 1982]. The highstead disproportionatesinto S:O and PSO. Hapke [1989] resolution Galileo images support this impression,as disgoeson to note that the transitiont¾omS•_Oto PSO doesnot cussedabove. Both the Galileo and Voyager observations
occur at a fixed temperaturebut over a range fi'om -115 to suggestmore voluminousremovalof plateaumaterialsat high
---170K. Hapke and Graham [1989] reportthat PSO canfbrm latitudes,but the low Sun imagesnear the equatorare very
on surfaceswith temperaturesas high as 300 K, and it can limited in coverage. We shouldexpect greater volatiles at
exist in a lnetastablestateeven at this temperaturefbr months. high latitudes where the surt•ce is coolel', and this view is
Hapke [1989] commentsthat as the temperaturerises fi'om supportedby mappingof tbrward scatteringsurfaceunits in115 K, PSO repolymerizescontinuouslyinto chainsof in- terpretedas fi'eshSO2 fi'ost [Geisslereta!., this issue]. The
creasedS:O ratio, with attendantproductionof copiousquan- Prometheus-type
plumeshavean equatorialconcentration,
but
tities of SO2 and a considerabledecreasein volume. Thus as this is probably controlledby the lava eruptionstyles [McEPSO warmsit disaggregates.
•ven eta!., 2000]. However',note that there are many long-1
Hapke [1989] estimatesthat globally as muchas 50 •tm yr
lived eruptionsthat do not produceplumes,so presumablythe
of S20 from volcanoesmay be depositedon Io, which would uppercrusthas been depletedof volatiles in theseregions. In
result
in a layer5 cmthickin 103years.Muchthicker
depos-summary,the evidencesuggeststhat Io's uppercrustis rich in
its might be expectedin the vicinity of a long-eruptingvol- volatiles,at leastin places,at all latitudes.
cano. The S20/PSO depositswould probablybe interlayered
with dust-sized sulfur and silicate particles. S20/PSO-rich 4. Conclusions
depositscould be susceptibleto decompositionover a wide
1. We recognize foul' major slope types observed on a
range of temperaturesboth due to solar insolationand geoof intermediate
resolution
(-250 m pixel
'•) images
thermalheating. The ratesof this decomposition
would allow number
MOORE ET AL.: MASS WASTING
ON IO
33,239
ersen,and G.E. Ballester,Lyman-•: imagingof the SO2distribution on Io, Geophys.Res.Lett.,27, 1787-1790,2000.
pixel
-•)images.Slopes
andscarps
onIo ohenshowevidenceGeissler,
P., A. McEwen, C. Phillips,D. SimoneIll,R. Lopes-Gautier,
of erosion,seenin the simplesttbrm as alcove-arving slumps
and S. Doute, Galileo imaging of SO2 frostson Io, d. Geophys.
and severaladditionaltextureson very high resolution(-10 m
and slidesat all scales. Many steepslopeson Io probablyde-
grade
leaseand
through
brittle
dry
slope
mass-wasting
t•ilure.
processes
suchas blockre2. Sputteringprobablyplaysno role in the erosionof Io's
surface at the scale seen even in the highest-resolutionimages.
Res., this issue.
Hapke,B., The surt•tceof Io: A newmodel,Icarus,79, 56-74, 1989.
Hapke,B., and F. Graham,Spectralpropertiesof condensed
phases
of disulthrmonoxide,polysulfuroxide,and irradiatedsulfur,Icarus, 79, 47-55, 1989.
Johnson,R.E., EnergeticCharged-ParticleInteractionswith Atmospheres and Surfaces,Phys. Chem. Space, vol. 19, 232 pp.,
Springer-Verlag,
New York, 1990.
Johnson,T.V., and L.A. Soderblom,Volcanic eruptionson Io: Im-
3. A sappingmechanism
involvingliquidSO,,,proposed
by
McCauley et al. [1979] to explain the irregularerosional
plicationstbr surt•tceevolutionandmassloss,in Satellitesof Juscarpsandbrightdepositsin layeredterrainsimagedby Voypiter, edited by D. Morrison, pp. 634-646, Univ. of Ariz. Press,
ager 1, may have ibrmedthe rectangularpatternof alcoveTucson, 1982.
rimmedtroughsin the Tvashtarplateau(classifiedas a mani- Johnson,T.V., D.L. Matson, D.L. Blaney, G.J. Veeder, and A. Davies, Stealth plumes on Io, Geophys.Res. Lett., 22, 3293-3296,
festationof type A slopesin thispaper),whichresemblefi'et1995.
ted plateauerosionon Mars. If the sappingexplanationis Kieffer, S.W., R. Lopes-Gautier,A. McEwen, W. Smythe,L. Keszcorrect,this suggests
that SO2becameliquid at a depthof 1
thelyi, and R. Carlson, Prometheus:Io's wanderingplume, Science, 288, 1204-1208, 2000.
km or lesswhen thesecanyonsibrmed(perhapscontinuingto
the present). If sappingis, in thct, taking place, it must rely
on locallyhigh geothermalenergyto mobilizethe liquidSO2.
4. Fretted erosionalong portionsof the Tvashtar plateau
could also reflect plastic detbrmationand glacial flow of interstitial volatiles (e.g., SO2) that exploit and enlargestructural patternsof weakness(e.g..joints and fi'actures),causing
disaggregation
and t•rther backwastinginto the plateau. This
mechanismalso relies on locally high geothermalenergy to
mobilize the volatile. Alcoves with lobes emerging from
them seen elsewhere on the satellite may have tbrmed in a
similar manner.
5. The appearanceof some slopesand near-slopesurface
texturesseen in very high resolutionimagesis consistentwith
erosional styles that could be attributed to sublimationdegradation. However, it is difiScult to identi(y a suitable
volatile (e.g., H2S) that can sublimatelhst enoughto alter Io's
youthfulsurt:ace.
6. Disaggregationfrom chemical decompositionof solid
S20 and other polysult•r oxidesmay conceivablyoperateon
Io. This mechanismcould degradelandtbrmsin a manner
that resembles degradation from sublimation and at a rate
comparableto that of resurthcing.
Klaasen, K.P., et al., Calibration and performanceof the Galileo
Solid-Statehnagingsystemin Jupiterorbit, Opt. Eng., 38, 11781199, 1999.
Lebofsky, L.A., Stability of frosts in the solar system,Icarus, 25,
205-217, 1975.
Lellouch, E., M. Belton, I. de Pater, S. Gulkis, and T. Encrenaz,Io's
atmospherefrom microwavedetectionof SO2,Nature, 346, 639641, 1990.
Leone, G., and L. Wilson, The geothermalgradient of Io, Lunar
Planet. Sci. [CD-ROM], ,YX,¾,Abstract1358, 1999.
Lucchitta, B.K., Ice and debris in the fretted terrain, Mars, Proc. Lu-
narPlanet.Sci.Conf 14•', Part2, d. Geophys.
Res.,89,suppl.,
B409-B418, 1984.
Malin, M.C., Mass movementson Venus: Preliminary results from
Magellan cycle I observations,d. Geophys.Res., 97, 16,33716,352, 1992.
Malin, M.C., and D. Dzurisin, Landformdegradationon Mercury, the
Moon, and Mars: Evidencefrom craterdepth/diameter
relationships,d. Geophys.Res.,82, 376-388, 1977.
Malin, M.C., and J.R. Zimbelman, Surfacemorphologyof cometary
nuclei, Lunar Planet. Sci., XVII, 512-513, 1986.
McCauley, J.F., B.A. Smith, and L.A. Soderblom,Erosionalscarps
on Io, Nature, 280, 736-738, 1979.
McEwen, A.S., and J.I. Lunine, Comment on "The surt•tceof Io: A
new model"by BruceHapke,Icarus, 84, 268-274, 1990.
McEwen, A.S., et al., High-telnperaturesilicatevolcanismon Jupiter's moon Io, Science,281, 87-90, 1998.
McEwen, A.S., et al., Galileo at Io: Resultsfrom high-resolution
imaging,Science,288, 1193-1198,2000.
Acknowledgements. This investigationwas funded by
Milizzo, M.P., L.P. Keszthelyi,and A.S. McEwen, Observationsand
NASA's Galileo Project. We are grateful ibr the comments
initial modelingof lava-SO2interactionsat Prometheus,
Io, d.
of an anonymousreviewer. Thanksgoesto Pam Engebretson
Geophys.Res.,this issue.
for her help with figure preparation.
Milton, D.J., Water andprocesses
of degradationin the Martian landscape,d. Geophys.Res., 78, 4037-4047, 1973.
Moore, J.M., M.T. Mellon, and A.P. Zent, Mass wastingand ground
collapsein terrainsof volatile-richdepositsas a solarsystem-wide
References
geologicalprocess:The pre-Galileo view, Icarus, 122, 63-78,
Bakker,J.P.,andJ.W.N.Le Heux,A remarkable
newgeomophologi- 1996.
cal law, Proc. K. Ned. Akade. Wet., Ser. B Phys.Sci., 55, 399- Moore, J.M., et al., Mass lnovementand landtbrmdegradationon the
410, 554-571, 1952.
icy Galilean satellites:Resultsfrom the Galileo nominalmission,
Cart, M.H., The Martian drainagesystemandthe originof networks
Icarus, 140, 294-312, 1999.
andfi'ettedchannels,
d. Geophys.Res.,100, 7479-7507,1995.
Nash D.B., and R.R. Howell, Hydrogensulfideon Io: Evidencefrom
Cart, M.H., Mars Global Surveyorobservationsof Martian fretted
telescopicand laboratoryint?aredspectra,Science244, 454-457,
1989.
terrain,d. Geophys.Res.,in press,2001.
Chuang,F.C., andR. Greeley,Largemassmovementson Callisto,d. ?appalardo,R. T., and R. Greeley, A review of the originsof subGeophys.Res., 10.5,20,227-20,244,2000.
parallel ridges and troughs:GeneralizedmorphologicalpredicCoates,D.R., Landslideperspectives,
Rev.Eng.Geol.,3, 3-28, 1977.
tions from terrestrial models, d. Geophys.Res., 100, 18,985Colwell, J.E., B.M. Jakosky,B.J. Sandor,and S.A. Stern,Evolution
19,007, 1995.
of topographyon comets.II. Icy cratersand trenches,Icaurs,85, Purves,N.G., and C.B. Pilcher, Thermal migrationof water on the
205-215, 1990.
Durham,W.B., S.H. Kirby, andL.A. Stern,Steady-state
flow of solid
CO2: Preliminary results,Geophys.Res. Lett., 26, 3493-3496,
1999.
Feldman, P.D., D.F. Strobel, H.W. Moos, K.D. Retherford,B.C.
Wolven,M.A. McGrath, F.L. Roesler,R.C. Woodward,R.J. Oliv-
Galilean satellites,Icarus, 43, 51-55, 1980.
Radebaugh,J., L.P. Keszthelyi, A. S. McEwen, E.P. Turtle, W.L.
Jaeger,M. Milazzo, and the Galileo SSI Team, ?ateraeon Io: A
new type of volcaniccaldera?,d. Geophys.Res.,thisissue.
Roessler,F.L., H.W. Moos, R.J. Oliversen, R.C. Woodward Jr., K.D.
Retherford, F. Scherb, M.A. McGrath, W.H. Smyth, P.D. Feld-
33,240
MOORE ET AL.: MASS WASTING ON IO
man, and D.F. Strobel, Far-ultravioletimaging spectroscopyof
Io's atmospherewith HST/STIS, Science283, 353-357, 1999.
Russell, C.T., and M.G. Kivelson, Evidence for sulfer dioxide, suitor
monoxider,and hydrogensulfidein the Io exosphere,J. Geophys.
Analysis,and Control, edited by R.L. Shusterand R.J. Kriszek,
pp. 11-33, Nat. Acad. of Sci., Nat. Res. Counc.Transp.Res.
Board,Washington,D.C., 1978.
Res., this issue.
Schaber,G.G., The geologyof Io, in Satellitesof Jupiter,editedby
D. Morrison,pp. 556-597, Univ. of Ariz. Press,Tucson,1982.
Schenk,P.M., and M.H. Bulmer,Origin of mountainson Io by thrust
faulting and large-scalemass movements,Science, 279, 1514-
F.C. Chuang,Departmentof GeologicalSciences,ArizonaState
University,Tempe,AZ 85287.
J.W. Head Ill and B.E. Nixon, Departmentof Geological
Sciences,Brown University, Box 1846, Providence,RI 02912.
_head_I
I I• brown.edu)
Selby,M. J., HillslopeMaterialsandProcesses,
2nded., pp. 365- (james
369, Oxford Univ. Press,New York, 1993.
A.S. McEwen,M. Milazzo,andE.P.Turtle,LunarandPlanetary
Sharp,R.P., Mars:Frettedandchaoticterrains,d. Geophys.
Res.,78, Laboratory,Universityof Arizona, 1629 East UniversityBlvd.,
4073-4083, 1973.
Tucson, AZ
85721. (mcewen•pirl.lpl.arizona.edu;
arizona.edu;
turtle•pirlmail.lpl.arizona.
edu)
Sharpe,C.F.S.,Landslides
andRelatedPhenomena,137pp.,Cooper mmilazzo•pirlmail.lpl.
1517, 1998.
Square,New York, 1939.
Spencer,J.R., Thermal segregationof water ice on the Galileansatellites,Icarus, 69, 297-313, 1987.
Squyres,S.W., Martian frettedterrain:Flow of erosionaldebris,Icarus, 34, 600-613, 1978.
Squyres,S.W., The evolutionof dustdepositsin the Martian north
polarregion,Icarus, 40, 244-261, 1979.
Turtle,E. P., et al., Mountainson Io: High-resolution
Galileoobser-
J.M. Moore, Space SciencesDivision, NASA Ames Research
Center, MS 245-3, Moffett Field, CA 94035-1000.
([email protected].
gov)
R.T. Pappalardo, Astrophysical and Planetary Sciences
Department,CampusBox 392, Universityof Colorado,Boulder,CO
80309-0392.(pappalardo•lasp.colorado.edu)
P.M. Schenk,Lunar and PlanetaryInstitute,3600 Bay Area
Boulevard,
Houston,
TX 77058.(schenk•lpi.usra.edu)
R.J. Sullivan, Center tbr Radiophysics
and SpaceResearch,
vations,initialinterpretations,
andformation
models,J. Geophys.
CornellUniversity,308 SpaceSciences
Building,Ithaca,NY 14853Res., this issue.
Varnes,D.J., Landslidetypesandprocesses,
in Landslides
andEngi- 6801.(sullivan•cuspif.tn.cornell.edu)
neeringPractice, edited by E.B. Eckel, pp. 20-47, Nat. Acad. of
Sci., Nat. Res.Counc.HighwayRes.Board,Washington,
D.C.,
1958.
(Received
September
5, 2000;revisedFebruary15,2001;
Varnes, D.J., Slope movementtypes and processes,
in Landslide, acceptedMarch 12, 2001.)