VOL. 80, NO. 17
JOURNAL
OF GEOPHYSICAL
RESEARCH
JUNE 10, 1975
Surface History of Mercury' Implications for Terrestrial Planets
BRUCEC. MURRAY,1 ROBERTG. STROM,
•' NEWELL J. TRASK,a AND DONALD E. GAULT4
A workinghypothesis
of Mercury'shistoryis presented.
We inferthe surfaceof Mercuryto recorda
sequence
of eventsbroadlysimilarto thoserecordedon the moon, implyingsimilarhistoriesof impact
bombardment.The oldestterrainson Mercury seemto be better preservedfrom modificationby ejecta
from subsequentlyformed impact basinsbecauseof higher surfacegravity. The large lunarlike impact
craterson Mercury can be interpretedas part of a distinctepisodeof bombardmentwhich may have
affectedall the terrestrialplanetsabout 4 b.y. ago. The light crateringaccumulatedon the surfacesof the
Mercuriansmoothplainsis similarin diameter/frequency
relationship
to that of thelunarmariaandof
the oldestMartian plainsunits,consistent
with recentinterpretations
of lunarandMartianfluxhistories
by Wetherill(1974)and Soderblom
et al. (1974).A straightforward
interpretation
of the MerCurian
surfacerecord thus supportsrecent order of magnitudeincreasesin age estimatesof many Martian
featuresdiscoveredby Mariner 9 but is not conclusive.The largecoreinferredfor Mercurycombinedwith
the lack of recognizableevidenceof pastatmosphericactivity is more easilyunderstoodin termsof radially heterogeneous
accumulationthan in termsof differentiationof a homogeneous
planet.Early corecooling may be reflectedby widespread
evidence
of crustalshortening.
However,Mercury'ssurfaceseemslittle affectedby any tectonic,atmospheric,or volcanicprocesses
for the last 3 b.y. or so, raisingquestions
concerning
(1) the relationshipof the originof Mercury'smagneticfieldto that of earth'sand (2) the
primarycauseof volcanicflooding,whichmay havebegun,and ended,approximatelysynchronously
on
Mercury and the moon.
lunar
]NTRODUCTION
Our goal in this paper is to portray a plausiblehistory of
Mercury as suggested
by the Mariner 10data and to call attention to the implicationsfor the history of the other terrestrial
planets.We view this first 'working hypothesis'for the history
of Mercury not as a definitive statement but as an initial
flamework to stimulateand focussubsequentanalysesand interpretation.
A convenientway to discussthe historyof Mercury as suggestedby the Mariner 10televisionpicturesis to postulatefive
periodsdelineatedby successive
variationsin the modification
of the surfaceby externaland internal processes:
(1) accretion
and differentiation, (2) terminal heavy bombardment,(3) formation of the Caloris basin, (4) flooding of that basin and
other areas,and (5) light crateringaccumulatedon the smooth
plains.
The sequenceof events recorded on Mercury's surface
closelyresemblesthat of the moon, althoughthereare interesting differencesin detail. Whether the absolutetime periods
representedby thosesequencesalso are closelysimilar is one
principal question in the interpretation of the Mariner 10
photography.The other major issueis whether the smooth
plains of Mercury, like those of the moon, are primarily of
volcanicorigin. Our working hypothesisis that Mercury is indeedmoonlikein both respects,absolutehistoryand extensive
volcanism, and we will outline the basisfor our judgments.
However, very divergent interpretations, including major
differencesin the early historyof the two objectsand evennonvolcanicorigin of the plains, cannot be ruled out entirelyon
the basisof the analysescarried out so far.
maria
and of thousands of kilometers
in horizontal
dimension,emphasizingstratigraphic,geographic,volumetric,
and albedo relationships.Figure 1 presentsadditional data
supportinga volcanicrather than impact origin of the plains
east of the Caloris basin. By analogy with the earth [Bowen,
1928] and the moon [Ringwoodand Esserie,1970],large-scale
volcanismon Mercury would imply the existenceof a silicate
mantle probably of at least several hundred kilometers in
depth [Murray et al., 1974, note 20]. Yet the models of
ReynoldsandSummers[1969]indicatethat a silicateoutershell
of only 500- or 600-km thicknessrequires all the silicate
availablein a planetwith a meandensityof 5.45. Henceon the
basis of the television data alone it can be inferred that the
planetis mostlikely composedof a largeiron coreenclosedby
a relatively thin silicatelayer from which the smoothplains
originated.
The existenceof an iron core on Mercury also has beeninferred from the magneticfield observations[Nesset al., 1974,
1975].Nesset al. arguethat either an activedynamoor a fossil
magnetic field must be involved. We would emphasizehere
that regardlessof the origin of the magneticfield, a substantial
iron core is indicatedby Mercury's interactionwith the solar
wind. Thus the conclusionthat Mercury is a differentiated
planet is suggestedby two independentobservations.
Granted that Mercury very probablyis composedof a large
iron core and a silicate outer shell, when did that chemical
segregationtake place?
Regardless of how Mercury accumulated as a planet,
chemicaldifferentiationmust have beencompletewell before
any of the existingimpactcratersformed,sincemeltingor any
subsequent crustal plasticity would have modified the
ACCRETION AND DIFFERENTIATION
lunarlike appearancethat they still retain. In addition,if any
atmospherewas generatedduring or after accretion,it too
Strom et al. [1975a] have argued the casefor at least some
must have disappearedentirely before any of the present
volcanicextrusionson Mercury of morphologysimilarto the
topographic features formed. A very small amount of atmosphere
will markedlymodify the appearanceof secondary
• Division of Geological Sciences,California Institute of Technolcraters and ejecta blankets surrounding a primary impact
ogy, Pasadena,California 91125.
crater. For example,Mars, with an averagesurfacepressureat
2 University of Arizona, Tucson, Arizona 85721.
presentof only about 5 mbar, very rarely exhibitsthe superfi3 U.S. Geological Survey, Reston, Virginia 22070.
• NASA/Ames ResearchCenter, Mountain View, California 94040. cial featurescharacteristicof freshimpact craterson the moon
Copyright¸ 1975by the AmericanGeophysicalUnion.
and Mercury. Thus we conclude that profound chemical
2508
MURRAY ET AL.: MARINER 10 MISSION
2509
Fig. 1. Volcanicplainseastof Caloris basin.In this photomosaicof the Calorisbasinand adjacentareas,north is at the
top. The area of Mercury shownis about 2000 km in dimension.On the basisof the distributionof large(20-50 km) fresh
impact cratersthe smoothplainseast of Caloris basin outlined in white are inferred by us to be youngerthan both the
mountainousrim and the surroundingejectadeposits,as well as the older plains material which fills Caloris basin. These
relationshipsappearinconsistentwith the possibilitythat the smoothplainsin questioncould haveoriginatedas fluidized
ejectafrom the Caloris impactevent.A later volcanicorigin is indicatedinstead.
segregation
intoa larg•ironcoreandrelatively
thinsilicatebe emplaced into
mantle took place sufficientlybefore the formation of the large
craters, so that any lingering atmosphere had largely dissipated and a rigid lithospherehad come into existence.
TERMINAL
HEAVY
BOMBARDMENT
A period of heavy bombardmentis representedby the large
cratershundredsof kilometers in diameter grading into large
basins, including the Caloris basin itself. As is discussedby
Trask and Guest [1975], large craters appear in many areas to
an older host material, although in a few
regions the surfaceappears more nearly saturated with large
craters(> 50 km). In most of the older terrains, macroscopically smooth intercrater areas are discernedwhich must in part
predatethe formationof thoselargecraters(seeFigure 2, upper left).
On the moon, by contrast, the architecture of the present
surface is dominated by a heavy bombardment that ended
about 4 X 109years ago and by subsequentlava filling Older
surfacessaturated with large craters as well as small areas
2510
MURRAY ET AL..' MARINER 10 MISSION
similar to the extensiveintercraterplains of Mercury alsocan
be recognized.Terminal bombardmentrelationships
may be
more evident on Mercury than on the moon becausethe
greatersurfacegravityon Mercuryrestrictsthearealcoverage
oi• the debrisfrom the impactformationof the basin[Gaultet
al., 1975]. Also there may have been fewer large impactsin
comparison
to thegreatersurfacearea,althoughthat pointis
not clear. In any case,the existenceof largeintercraterareas
on Mercury implies that some earlier process obliterated
nearly all topographicremnants of the saturation bombardment that necessarilyconstitutedthe final stagesof planetary
accumulation(accretion). Otherwise,a surfacesaturatedby
largecrater formswould still survive(seeFigure 2, lower left).
Justwhat crater obliteration processis recordedby that old
smoothsurface?Conceivably,it could be a relic from melting
associated with accretion
itself or a record of erosion from an
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7:
Fig.2. Cratered
terrains
onMercury,
themoon,andMars.(Upperleft)Mariner10photograph
ofthebright-rayed
crater Kuiper and surroundingcrateredterrain on Mercury centeredat 13øS,25øW. The solarelevationanglevariesfrom
about5ø to 2?ø (Flight Data Subsystem
2?,256). (Lowerleft) Mariner 10 photographregionin the northernuplandsof the
moon centeredat 60øN, 130øE. The solar elevation angle varies from 0 ø (i.e., the terminator) to about 25ø (Flight Data
Subsystem2669). (Upper right) Mariner 9 photographof crateredterrain in the Deuteronilusregion on Mars centeredat
30øN, 340øW.The solarelevationanglevariesfrom about17ø to 33ø (DAS number?,290,?43).All threeimageshavebeen
projectedat the surfacescaleindicated.BothMars and Mercuryexhibitextensive
intercraterplains,deficientin 10-(o 50km cratersin comparisonwith the moon. The Mercurian craters,however,resemblein distribution of morphologythoseof
the moonratherthan the highlydegradedpopulationpresenton Mars. From theserelationships
we infera periodofextensive crater obliteration on Mercury generally precedingthe terminal heavy bombardment responsiblefor the presentpopulation of large craters on Mercury.
MURRAY
E• AL.:MARINER
10 MISSION
2511
associatedatmosphericevent.However,thoseprocesses
should periodsof crater obliteration separatingthe main accumulahave operatedglobally, not leaving even slight remnantsof {ion of theplanetfroma heavyterminalbombardment.
Obsaturated surfaces. For this reason, and on the basis of local viously, future studiescan be expectedto challengethis inmorphology,we feel that it is somewhatmore likely that the terpretationand to searchfor an alternateexplanationbased,
intercraterplains on Mercury constitutea very early volcanic perhaps, on an episodic crater removal process such as
surface.Petrologicdata from the moon summarizedby ellbee catastrophicimpact ejecta.
[1975] suggestto somethat therewereearly volcanicepisodes
Another aspectof the heavilycrateredterrainson Mercury
on the moon now representedby occasionalfragmentsin the suggestssignificantdifferencesfrom the moon (and also from
breccias created by later bombardment. Another possible Mars). The intercraterareas especiallyexhibit compressional
obliteration processis fluidized flow generatedby large im- features,probablythrust or high-anglereversefaults [Stromet
pacts,as wassuggested
recentlyfor the origin of somesmooth al., 1975a];crustal shorteningmust have been taking place
plains
onthemoonpreviously
believed
to bevolcanic
(D. E. during the terminal heavy bombardment but evidently not
Wilhelms, personal communication,1975). Ob!lteration of much after that. Strom et al. suggesta net crustalshortening
early crater forms over somefraction of Mercury's surfaceby during this period correspondingto a few tenthsof a percent
hypotheticalcatastrophicbasin formation cannot entirely be reductionin the radius of Mercury. In contrast,significant
ruled out at present. However, we considerthe distribution crustalshorteningon a global scalehasnot beenrecognizedon
and morphologyof the Mercurian intercraterplainsto be in- the moon or Mars. Since the core mantle boundary is apconsistentwith suchan origin.In addition,the surfacegravity parently quite shallowon Mercury and the proportionalcore
of Mercury requires this cataclysmicprocessto be more volume is so large in comparisonwith the core volumesof the
localized on Mercury than on the moon.
moon and Mars, it is plausiblethat these unique compresIt is useful at this point to considerFigure 2. The most sional featurescorrespondto an early period of minor core
heavily crateredareasof Mars also exhibit intercraterplains, shrinkage.Strom et al. note that a phasechangefrom liquid to
i.e., also record a period of crater obliteration,as is shownin solid iron involvinglessthan 6% of a large core, or evenjust
Figure 2, upper right. But in contrastto Mercury, mostlarge the consequence
of a highercoefficient
of thermalexpansionof
Martian cratersare characteristicallymodified,probably as a solid metal compared to solid silicate,may be sufficientto
resultof atmosphericerosionand depositionduring and sub- produce the necessaryradius decrease.
sequentto the terminal heavybombardment.The Mercurian
Thus the oldest terrains on Mercury probably represent
craters(Figure 2, upperleft), on the otherhand,reflectthe full remnantsof an early period of crater obliterationfollowedby
distribution of morphologiesseen on the moon (Figure 2, a distinctepisodeof terminal heavybombardment.Crustal
lowerleft);i.e., theyrecordmerelythe normaldegradationas- shorteningwas taking place at that time, perhapscorrespondsociatedwith the impactprocessitself.
ing to core shrinkage.
Thiscrucialquestion
of thedistribution
of morphologies
of
the M ercurian and lunar craters warrants, and surely will
receive,a major quantitativeeffort (which is now made much
more tractableby recentsuccess
in creatingusefulstereopairs
from originally isolatedMariner l0 frames).In the meantime,
careful examination of many individual Mariner l0
photographshasfailed to revealto usany consistentdifference
in
the
distribution
of
lunar
and
Mercurian
crater
FORMATION OF CALORIS BASIN
Theimpact
whichcreated
theCaloris
basinwasa major
eventin the historyof Mercury. It modifiedthe landscapeover
much of the illuminated hemisphereobservedby Mariner 10
and certainlyaffectedthe otherhemisphereaswell. In association with basinexcavationa ring of mountainsresemblingthe
lunar Apennine mountainsand hummockyterrain similar to
morphologiesother than the differenceidentifiedby Gault et that seen in the Orientale basin on the moon formed. Orientale
al. [1975]as arisingfrom the differencein surfacegravity.As a is a particularly usefullunar ringed basinwith which to comconsequence,
we infer that the cratersformed duringthe final pare becauseit was not floodedby extensivelava filling afterstagesof accretionhad been nearly completelyerasedbefore ward, like most other lunar basins. Strom et al. show that
the presentpopulationof large lunarlikecraterswas formed. many characteristicfeaturesof a lunar ringedbasinalsocan be
Otherwise,there would still be presentan obvious,evendomi- recognized
around
Caloris.
•,ccordingly,
allthese
features
are
nant, populationof degradedcraterforms, asthereis on Mars, ascribedto a single time inferval associatedwith the Caloris
reflectingthe persistenceof the crater obliteration process. basin impact.
This meansthat if the bombardmentpresentlyrecordedon
It is alsopossibleto correlatespeculativelythe peculiarhilly
Mercury were simply the end of a continuousbombardment and lineatedterrain recognizedon the incominghemisphereof
processpersistingsinceaccretionitself, the processof crater the Mercury I photographywith the Calorisbasinevent.That
obliteration must have (l) operatedat a rate comparableto terrain is apparentlyrestrictedto a locationapproximatelyanthat of crater formation and (2) terminatedabruptly.It seems tipodal to Caloris:nonewasobservedin the southpolar region
difficultto requirehypotheticalmelting,earlyvolcanism,or at- in the Mercury 2 photography[Stromet al., 1975b]nor on the
mospheric erosion to cease so abruptly. Conversely, the outgoinghemisphereasviewedin the first encounter.Furtherimpactingflux recordedon Mercury may not have beencon- more, local sequencerelationshipsindicate that the terrain
tinuoussinceaccretion;insteadthere may havebeenan exten- must have developednear the end of heavy bombardment
siveperiodof craterobliterationby volcanicor otherprocesses [Strom et al., 1975a], i.e., approximatelyat the time of the
followedby a distinctepisodeof late bombardment.Most im- Caloris impact. Schultzand Gault [1975]havespeculatedthat
portant, an episodicearly historyof Mercury is requiredeither the lineatedand hilly terrain is the resultof the focusingof the
way. We find an episodicbombardmenthistorymoreplausible seismicenergyfrom the Caloris impact at the antipodalpoint
at this point than the speciallyconstrainedepisodicoblitera- and therefore the terrain formed at the time of the Caloris imtion historywhichwould be requiredif the bombardmenthad pact. Smallerareasof similar terrain havebeenrecognizedanbeencontinuouslydecreasingfrom accretion.Accordingly,we tipodal to the lmbrium and to the Orientale basins on the
interpret the intercrater plains as recording one or more moon
andattributed
to focusing
ofejecta
rathertha•'seismic
2512
MURRAY ET AL.: MARINER 10 MISSION
energy[Moore et al., 1974];this mechanismis inhibitedin the
cordingan accumulatedflux comparableto that recordedon
caseof Mercurybythehighersurfacegravity.Thelessobvious
the lunar maria.
nature
of
the
lunar
occurrences
is
consistent
with
the
Other interpretationsof Mercury'shistoryare not ruledout
themlessplausirelationship
discussed
by Gaultet al. [1975]thatthematerial bythe Mariner10data,althoughweconsider
ble. For example,the crater obliteration processmight have
beencatastrophic,suchasmight accompanyearlylargebasin
formation. In that casethe terminal heavybombardmentneed
BASIN FLOODING
not havebeenepisodic.It is alsostillpossible,in theabsence
of
The smoothplainsof muchof the outgoinghemisphereand truly definitive morphological evidence, that none of the
partsof the incoming
hemisphere
of Mercuryrepresent
a smooth plains are of volcanic origin, although we think it
secondaryfilling of earlier basins,generallyat a time when the quite unlikely.
betweenancientbasinsis more visibleon Mercury becausethe
basin ejecta was restrictedto narrower rings.
impact flux had greatly decreased.There is plausibleevidence
[Trask andGuest,1975;Stromet al., 1975a]that fillingwasnot
simultaneous
and that much of the fill must be of volcanic
origin, rather than from ejectasheets,impact melt, or mass
wasting processes(also see Figure 1). Nevertheless,the total
time periodrepresentedby theseplainscomparedto the total
time interval recorded on the surface of Mercury may be
relativelybrief. No differenceaslargeas 1: 1.5in craterdensity
of the variousMercurian plainsin the equatorialregionshas
been recognizedso far [Murray et al., 1974].
In addition to theseplanet-widefilling episodes,probably
relatively close in time but neverthelessdiscrete,there was a
subsequentstructural modification of the Caloris basin itself.
Deformation continued after the end of filling, resultingin
open fracturesnow severalkilometersacrosswhich cut across
earlier ridgesto comprisethe presentsurface.This pattern of
tensional deformation
is localized to Caloris basin: whether it
was a subsequentuplift or subsidence,it can be ascribed
plausiblyto adjustmentto the principal Caloris event [Strom
et al., 1975a].
POST-FILLINC
PERIOD
The last and presumablylongestepisoderecordedon the
surfaceof Mercury is represented
by the light crateringpresent
over the entire planet and especiallyapparenton the smooth
plains. Bright-rayed craters are very common and tend to be
bluish in color in comparison to the underlying material
[Hapkeet al., 1975].Theyresemble
in morphology
and in
size-frequency
spectrumthe post-marecrateringof the moon;
the M ercuriancrater-frequencyrelationshipsmeasuredso far
all plot betweenthose of the Apollo 12 and Apollo 14 sites
[Murray et al., 1974].Thus sincethe formation of the plains,
Mercury has accumulateda light cratering similar to that of
the moon.
Equally significant is the absence of substantial surface
modificationarising from tectonic,volcanic, or atmospheric
processes.
No tangibleatmosphereaccumulatedsufficientlyto
transport fine particles,nor has there been any observable
deformationof the silicateouter shellof Mercury arisingfrom
motionsof any hypotheticalfluid core. The heat machineapparentlywasturnedoff after formationof the plains,and that
wasthe end of it, althoughfuture explorationof the 50%of the
Mercurian
surface not visible to Mariner
10 could reveal more
recent episodesof localized internal activity.
Thus to us the most plausiblesequenceof eventsrecorded
on the surfaceof Mercury is (1) major chemicaldifferentiation
either duringor very shortlyafter accretion,(2) after declineof
the original bombardment, a period of crater obliteration,
perhapsthroughextensivevolcanism,followedby an episode
of terminal heavy bombardment, global crustal shortening
taking place at this time, (3) formation of the Caloris basin
near the end of terminalbombardment,(4) volcanicflooding
of largeareasof the planetto form mostof the smoothplains,
and (5) light crateringsubsequent
to the filling episodesre-
COMPARISON
WITH SURFACEHISTORIESOF VENUS,
THE MOON, AND MARS
Studiesof returnedlunar sampleshavedemonstrated
that
heavybombardmentoccurredon the moon aslate as 500m.y.
after accretion [Tera et al., 1974]. However, there is not a
consensusas to whether such bombardmentwas episodic
[Stewart, 1975] or continuous and whether it was an event
restrictedto the moon or was solar systemwide [Hartmann,
1975; Wetherill, 1975].The Mercury re•ults seemrelevantti•
both questions.The terminalheavybombardmentrecordedon
Mercury probably could not have occurredappreciablymore
recentlythan the emplacementof the oldest lunar mare surfaces(,-•3.8 b.y. ago);it is difficultto imaginea populationof
large objects(i.e., capableof producingthe Calorisbasin)
impactingMercury long after accretionwhich would not also
have impactedthe moon. Most important, episodicterminal
bombardmentevidentlyoccurredon Mercury. Thus we find it
plausibleto correlatethe terminal bombardmentson both the
moon and Mercury as resultingfrom a distinct episodethat
affectedat leastthe inner solar systemabout 4 b.y. ago.
There still remainsthe seeminglycontrivedpossibilitythat
the Mercurian surface recordsan earlier episodicbombardment and the moon recordsa later heavybombardmentwhich
somehow did not affect Mercury. But Mars also exhibits a
heavily cratered surface (in equatorial and southern
hemisphericareas)which strikingly resemblesthe Mercurian
crateredterrain; i.e., it is only partially saturatedwith large
craters(seeFigure 2). Very large ringed basinsalsohavebeen
recognized[Wilhelms,1973].The simplestand, to us,the most
plausibleexplanationis that all threesurfacesrecordthe same
episodeof solar systembombardment,presumablyfrom objects originating in large aphelia orbits. It is not possible,
however,on the basisof the picture data alone (i.e., ignoring
dynamicalconsiderations),to excludeentirely the possibility
that the old cratered surfaces instead record late bombardment
of accretionaryobjects,objectsin heliocentricorbitssimilarto
those of the three planets.
If a terminal heavy bombardmentaffectedboth Mercury
and the moon, then Venus must have been bombarded as well.
Recent radar imagesof portions of the equatorial regionsof
Venus [Rumseyet al., 1974] show large circular craterlike
topographicformsin someareas.If thesefeaturesproveupon
higher-resolution
examinationto be of probableimpactorigin,
then they very probablyhave survivedat least4 X 109years,
sinceany more recentepisodeof bombardmentshouldhave
affectedMercury and the moon as well. Such a conclusionis
most significant,sinceit would mean that Venus, despiteits
similarity to earth in size and mass, has not manifesteda
global tectonic and volcanicstyle sufficientto obliterateall
suchancienttopography.Similarly, severeconstraintson the
erosionalcapacity of the extremely densehot Venus atmospherewould be implied.
MURRAYET AL.: MARINER10 MISSION
Extensiveareas on the moon and apparently on Mercury
were floodedby volcanismafter the end of late heavy bombardment, and internal activity on both planets evidently
ceasedafter that volcanicepisode.(Similar volcanicflooding
also occurredon Mars followingthe end of heavy bombardment [Soderblornet al., 1974; Wilhelms,1973] but continued
longer, along with subsequentshield volcanismand extensional deformation.) The timing and mechanismof volcanic
floodingon the moon has beenrelatedto specificmodelsof
lunar thermal historyand partial melting.(For a reviewof
these ideas, see Albee [1975].) In view of the profoundly
different structureof the Mercurian interior as comparedto
that of the moon, it seemsmostfortuitousfor a closelysimilar
historyof surfaceigneousactivity,and cessationof activity,to
have developedon both Mercury and the moon. It can be
speculatedthat somehowthe subsequentvolcanicfloodingon
both planets(and perhapsMars aswell) is a delayedbut direct
consequence
of the heavybombardmentitself.We canoffer no
plausible mechanism, however, to explain such a delayed
effect. Alternatively, in the unlikely (to us) event that the
smoothplains of Mercury are determinedconfidentlyto be
entirely of impact, rather than volcanic, origin, then the
difficultiesraised by the apparent similaritiesin igneous
history of the moon and Mercury would be avoided.
It is quite significantthat the light crateringon the flooded
2513
an episodewhichcharacterized
the entireinnersolarsystem.
The earth thereforevery probablyunderwenta comparable
bombardment
aswell.Sucha conjecture
is not new[Safronov,
1972]. However,now the bombardmentevent4 aeonsago
mustbe regardedasbeingmostprobableandincludedin any
reconstruction
of the earth'shistory.
What would be the effectsof major basin-formingimpacts
on earth? Imbrium and Caloris were probably excavatedinitially to a depthof 100-200km; permanentmodificationof the
hostmaterialcertainlytranspiredto someappreciablygreater
depth. Such events on earth 4 b.y. ago must have created
physicaland chemicalheterogeneities
in the uppermantle.The
oldestconfidentlydatedrocksare 3.7-3.9 b.y. old [Goldichand
Hedge, 1974;Moorbath et al., 1973]. Whether the absenceof
earliersamplesreflectsa planet-widemetamorphism,asit does
on the moon, or is merely a matter of chancesurvivalis an
open question.In any case,the Mercury picture data, combined with lunar sample analyses,serve to emphasizethat
planetaryhistoryandgeologicalhistoryare almostconvergent
nearly4 b.y. ago. Continuingstudyof ancientterrestrialrocks,
aswell aseventualsamplereturn from otherterrestrialplanets,
may help unite the two subjectsfirmly.
The existenceof a large iron core, perhapsone evensustaining a currentlyactivedynamoprocess[Nesset al., 1975],also
carriesimplicationsfor the origin and historyof earth'score.
plainsof Mercuryis similartOthat on the mariaof the moon The formation of earth's core is sometimes linked to a concurin diameter-frequency
relationships;
similarfluxhistoriesover rent exhalationof volatilesto producemuchof earth'sprimary
the last 3-4 b.y. are implied if the Mercurian surfacesare of
comparable age. Relatively uniform impact flux histories
throughoutthe inner solarsystemfor the last 3-4 b.y. wereinferred recentlyby Wetherill[1974],who concludedthat the
impactingobjectsprobably originatedin large aphelia orbits
(like comets) rather than directly from the asteroid belt.
atmosphere during planet-wide differentiation following
homogeneousaccretion [Ringwood,1966]. The present terrestrial atmosphereis then ascribedto a long-term degassing.
M ons,for instance,
Wasinitiallyplacedat 40 m.y.Withan un-
ever evolved even minor
certainty of a factor of 2 or 3 [McCauley et al., 1972]. The
same diameter-frequencydata now would be inferred to in-
Conversely, if Mercury accumulated heterogeneously,i.e.,
with very early differentiation,then volatilesmay never have
accumulatedon the surfaceof Mercury in significantamounts.
(See, for example, Anderson [1973] and Kaula [1975].)
Generally, we find heterogeneousaccumulationto be more
probable for Mercury and, by analogy, for the earth and the
other terrestrial planets.
Regardlessof how Mercury formed, very probably it has
been a differentiatedplanet for well over 4 b.y., the inferred
age of the large impact cratersstill preservedon its surface.
Yet there is no evidenceafter the end of heavy bombardment.
of any modificationof the silicateouterlayersby 'hot spots'or
fluid convectionwithin that core. In contrast, hot spots and
Sucha process
is notexcluded
for MercurybytheMariner10
data but is burdenedby the needboth to developa thick cool
lithosphereand to removeentirely any secondaryatmosphere
Similar flux histories for Mars and the moon were inon Mercury before the end of heavy bombardment.
Atmosphericescapeis governedby solarirradiation and by
dependentlyhypothesizedby Soderblomet al. [1974] on the
basisof the similarityof diameter-frequency
versusareal surfacegravity; Mercury's nearnessto the sun, in comparison
coveragerelationshipsfor thosetwo objects.In contrast,many to Mars, enhancescertain atmosphericescapeprocesses,for
earlier analyseshad hypothesizedthat the apparent impact example. However, Mercury and Mars have similar surface
flux on Mars was 6-20 times greater than that Which had gravities,sothat differentialsolarirradiationwouldhaveto be
affected
themoon[Hartmann,
1973,1966;Anders
andArnold, afforded extraordinary effectivenessin governing the very
1965].Asa result,theageof thelargeshieldvolcano
Olympus differentatmospherichistoriesof the two planetsif Mercury
dicateby the Soderblom-Wetherill
hypothesis
an averageage
of thesurface
materials
Closer
to 500m.y.Similarly,theoldest
crateredplainson Mars,whichstrongly
resemble
the lunar
maria in morphology,albedo, and degreeof cratering,would
now be deemedto be of comparableage as well, i,e., 3-4 b.y.
It is stillpossible,
of COurse,
thatthe Mercurianplainsare
older than the lunar maria, just enough older to have ac-
cumulated
virtuallyall of theirpresentcraterpopulations
sufficiently closeto the peak of the terminal heavy bombardment
episodeto mimicthe flux accumulated
on the lunar maria
over 3•/•b.y. Only under suchcoincidentalconditionscan the
lunar and Martian plains crater populationsbe ascribedto a
sourceof objectsexhibitingstrongdepletionbetweenthe orbits of Mars and Mercury, and the Martian surfacesconsequently be assignedages much younger than comparably
convection
amounts
in the terrestrial
of volatiles to its surface.
mantle
have
been attributed
speculativelyto an origin associatedwith the core-mantle
boundary. Of course,Mercury's core may have cooled to a
stablesolid configurationby the end of heavy bombardment;
cratered lunar surfaces.
indeedthe lobate scarpsmay recordthe end of that process.If
so, M ercury'spresentmagneticfield could not easily be atIMPLICATIONS FOR EARTH
tributedto an activedynamofield,theexplanationpreferredby
The Mercury picturessupportthe notion that a late heavy Ness et al. [1975]. Likewise, there may be somedifficultiesin
bombardment,dated from lunar samplesat •bout 4 b.y., was findinga plausiblethermal and electricalmodel of Mercury to
MURRAY ET AL:: MARINER
2514
10 MISSION
luStratedby Orientaleand associated
features),Proceedings
of the
permit a 'fossil'magneticfieldto survive4 b.y. Suchdifficulties
5th LunarScience
Conference,
Geochim.
Cosmochim.
Acta,Suppl.
in finding any plausible interpretation of the Mercury
5, 1, 71-100, 1974.
magneticfield perhapsmay be interpretedas providingnew MUrray, B, C., M. J. S. Belton,G. E. Danielson,M. E. Davies,D. E.
cluesto otherwisead hoc circumstances
hypothecatedwithin
Gault, B. Hapke, B. O'Leary, R. G. Strom,V. Suomi,andN. Trask,
the earth's core to account for earth's magneticfield.
Mercury's
surface:
Preliminary
description
andinterpretation
from
Mariner l0 pictures,Science, 185, 169, 1974.
Mariner 10'svoyageof exploration to the unknown planet
Mercuryhasenlargedour knowledge
notjustof that planet
but of the family of terrestrial planets, includingour own.
Ness,N. F., K. W. Behannon,R. P. Lopping,Y. C. Whang,and K, H.
Schatten,Magnetic field observationsnear Mercury: Preliminary
results from Mariner 10, Science, 185, 151, 1974.
Ness, N. F., K. W. Behannon, R. P. Lepping, and Y. C. Whang,
Acknowledgments.An earlier version of this manuscript was
Magnetic
fieldof Mercury
confirmed,
submitted
to Nature,1975.
thoroughlyand constructivetY
reviewedby Clark Chapmanand Wil- Reynolds,
R. T., andA. L. Summers,
CalculatiOns
onthecomposition
liam Hartmann of the PlanetaryScienceInstitute,Tucson,Arizona;
of the terrestrialplanets,J. Geophys.Res., 74, 2494, 1969.
by WilliamKaulaof the Universityof Californiaat LosAngeles;
and Ringwood,A. E., Chemicalevolutionof the terrestrialplanets,
by Don Wilhelms of the U.S. Geological Survey, Menlo Park,
GeOchim. Cosmochim. Acta, 30, 41, 1966.
California. As a consequence
the paper has beensubstantiallyrewrit- Ringwood,A. E., and E. Essone,
PetrOgenesis
of Apollo 11 basalts:
ten to presentour interpretationsmore clearly and to call attention
!nternalconstitutionandoriginof the moon,Proceedings
Apollo 11
more conspicuouslyto alternative possibilities.We feel that the
Lunar ScienceConference,Geochim.Cosmochim.
Acta, Suppl.1, 1,
resulting
paperhasbeensubstantially
improved
throughresponse
to
769,1970.
thereviewers'
effortsandwishto express
ourappreciation
fortheir Rumsoy,
H. C., G. A. Morris,R. R. Green,andR. M. Goldstein,
A
efforts.
Contribution
2579oftheDivision
ofGeological
andPlanetary radarbrightness
andaltitudeimageof a portionof Venus,
icarus,
Sciences,California Institute of Technology, Pasadena,California
23, 1, 1974.
91125.
Safronov,V. S., The initial stateof the earth and certainfeaturesof its
evolution,Izo. ,4cad.Sci. USSR Phys.SolidEarth, no. 7,444, 1972.
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