JOURNAL
VOL. 80, NO. 17
OF GEOPHYSICAL
RESEARCH
JUNE 10, 1975
The Mariner 10 Picturesof Mercury' An Overview
BRUCE C. MURRAY
CaliforniaInstituteof Technology,Pasadena,California 91125
The11papers
in thisissue,
comprising
theMariner10imaging
teamfinalreport,arebrought
together,
andthesalientresultsof the Mariner10imagingexperiment
at Mercuryaresummarized.
Thoseaspects
of
thedatasetacquiredwhichwereworkedby theteamareidentified,andotherareaswherefurtherworkis
needed are designated.
Because of its nearness to the sun as seen from the earth,
timeters
to metersof thesurface
materials
of Mercuryon the
averageare composedof polarizedsilicatesroughlysimilarto
the moonand that the planethasat presentverylittle or no atmosphere(lessthan around 1 mb).
On the basisof thesemeagerand disparatefactsabout Merhasplaced
Mercuryin a photographic
status
similar
to thatof cury, as well as on extensiverecentexperiencewith close-up
the moon in the early 1960's,beforespaceexplorationbegan, photographyof Mars and the moon,a setof experimentalobandsimilarto the photographic
knowledge
of Marssince jectivesfor a flyby imagingexperimentwereidentified[Murray
Mercuryhasbeenthe leastinvestigated
terrestrialplanetfrom
earth-basedobservatories.
The flybysof Mariner l0 on March
29 and September
21, 1974,haveproducedan enormousincreasein knowledge.As can be seenin Figure l, Mariner l0
Mariner
et al., 1971]:
9.
A total of 1673independently
usefulframeswere acquired
of Mercury in the first encounterand 540 in the second,most
of which are available from the National SpaceScienceData
Centerin Greenbelt,Maryland. Preliminarydescriptionsand
interpretations
of the photographic
datahavebeenpublished
[Murray et al., 1974a,b]. The presentset of ll papersis intendedto serveas a final report for the Mariner l0 imaging
team,althoughsomeadditionalrelatedpapersare expectedto
be completedin the comingmonths.In addition,production
of airbrushmaps of the photographedportion of the mercurian surface is under way in conjunctionwith the U.S.
GeologicalSurvey[seeDaviesand Batson,1975],and large
orthographically
rectifiedmosaics
areunderpreparation
at the
Jet PropulsionLaboratory(JPL). Thesewill alsobe available
in reducedform through the National SpaceScienceData
Center. An atlas of the Mariner l0 pi_ctures
is alsoin preparation, under the overall direction of Merton Davies.
It is a useful(and rare) circumstance
to be able to compare
what has actually been found in the exploration of a new
planet with what was anticipated.The two dominantfacts
about Mercury availablebeforethe flight of Mariner l0 were
that (1) the bulk densityof the planet is 5.45 g/cma, much
higherthan that of the moonor Mars andquitesimilarto that
of the earth and Venus,and (2) the surfacereflectionof visible
and radio waves, as well as the thermal emissionat infrared
and radio wavelengths,closelysimulatesthat to be expected
from the moon if it were likewise located at the orbit of Mer-
A firstobjectiveis to map andidentifythemajorphysiographic
provinceson the basisof topographicformsand albedovariations.All of the followingquestions
canbe answeredby an imagingexperiment.
What arethesimilarities
anddifferences
between
the majorsurfacefeaturesof Mercury,the Earth,the Moon, and
Mars?Are there impact cratersand other exogeriicstructures?
Are there endogenicstructures?
If mare-likefeaturesare present,
what is their distributionwith referenceto the dynamicaxis of
Mercury, and what is the nature of the mare boundaries?
Are
there correlations with Earth-based observations,both radar and
visual'?
At finer scalesthe topographicform and sizedistributionsof
craterscan be investigated.
Suchstudiesincludecomparison
with
slopesand shapesof lunarcraters,searchfor nonlunarcratering
processes,
variationsin crater distributionswith latitudeand
longitude,andestimationof the ageof the surface.Evidenceof a
possibleearly atmospherecan be sought.
Usingphotogrammetric
techniques,
planetaryradii at specific
pointscan becomputedandthusit maybepossibleto determine
whetherMercurydepartssignificantlyfrom a sphericalshape.By
followingfeaturesacrossthe diskas Mercuryrotates,theorientation of the spinaxismay beableto bespecified
with greaterprecisionthan can currentlybe determinedfrom Earth. Theseresults
can then be combinedwith imagesof surfacefeaturesto establish
a coordinate system.
The following additional phenomenacan be searchedfor:
featureswhich showpeculiaralbedoand photometricfunctions
suchas rayed craters;indicationsof radiationdarkeningand its
relationshipto that of the Moon and the Galilean satellites;
evidencesof transitoryfrostsin the terminatorregions;regional
color differences,suchas are presentin the lunar maria.
Due to the extraordinary skill of the engineeringgroups
cury.The first fact impliesthat Mercuryasa planetmusthave
a verymuchhigherproportionof iron thanMars or the moon. from JPL and the Boeing Company (prime spacecraftconIndeed,takinginto accountthe self-compression
of materials tractor),it has beenpossibleto pursueall theseexperimental
at highpressure
in the interiorof theearth,theconclusion
had objectivesin satisfactorydetail, in somecasesfar exceeding
(exceptfor determinationof spinaxis
beendrawnearlierthat Mercurymusthavea somewhathigher our originalexpectations
percentage
of iron than eventhe earth[Kaula,1968;Reynolds orientation). Mercury has been found probably to be a
and Summers,1969]. Whether the planet was composedof a differentiatedplanet with a large earthlike iron core and,
homogeneous
mixtureof iron andsilicatephaseor insteadhad rather surprisingly,to exhibit a surfacehistorycloselysimilar
beendifferentiatedinto a largeiron coresimilarto that of the to that recordedon the moon. There are significantimplicationsto the historiesof the otherterrestrialplanetsfrom this
earth with a thin silicate mantle could not be determined from
pre-Mariner10 observations.
The secondfact, the telescopic lunarlikeexternalrecordand probablyterrestriallikeinterior
similarityto the moon, meansthat at leastthe uppermostcen- of Mercury.
In the sequenceof 10 papersthat follow are presentedthe
principalfindingsof the Mariner 10 televisionteam through
Copyright¸ 1975by the AmericanGeophysicalUnion.
2342
MURRAY: MARINER 10 MISSION
2343
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Fig. 1. Resolutionversuscoverage:All curvesin the four partsof Figure 1 are derivedby the sameguidelines.Theseinclude(l) cumulativefractionalcoverageas a functionof surfaceresolution,(2) eliminationof redundantcoverageof the
samelocation,and (3) adjustmentof resolutionfor foreshortening.
SeeMurray et al. [1971]and DaviesandMurray [1971]
for details.(a) Mars imagingcoverageversusresolutionplotsfor pre-1965earth-based
results(indicatedby stippling),1965
extensionby Mariner 4, 1969increaseby Mariners6 and 7, and 1972increaseby Mariner 9. (b) Lunar imagingcoverage
versus
resolution
plotsfor pre-1964earth-based
results
(indicated
by stippling),
1965extension
byRanger,and1967increasedueto lunar orbiter coverage.The immenselysuccessful
lunar orbiter seriesof spacecraftphotographedselectedsites
on the lunar near sideat resolutionsto 1 m and then proceededto producelow- and medium-resolution
photographyof
both near and far sideto extendlunar coverageto 100%;almostall of the resultsare at leastan order of magnitudebetter
thanground-based
results.(c) Mercuryphotographic
coverageresultingfrom thefirstandsecondencounters
of Mariner l0
ascomparedwith previousground-basedresults.(d) Coverageversusresolutionplotsshowingstatusof the moon, Mars,
and Mercury comparedto pre-spaceage coverageof the moon (indicatedby stippling).It can be seenthat presentcoverage and resolutionof Mars and Mercury are roughlycomparableto the coverageand resolutionof the moon obtainable
before the spaceage began.
the analysisof picturesfrom both the first (Mercury 1) and
second(Mercury 2) Mercury encounters.
The Mercury 2 resultsare presentedfor the first time in the
immediatelyfollowingpaper by Strom et al. [1975a],in which
it is shownthat the MerCUry2 picturestend to reinforcethe
conclusions
developedafter the firstencounterand, especially,
emphasizethe uniquenessof peculiar hilly and lineated ter-
rainsthat wereobserved
in the Mercury1 pictures.
The ubiquity of lobate scarpsin the terrains exhibiting large impact
cratersis also supported.The next two papers,'Acquisition
and Descriptionof the Mariner 10 TelevisionScienceData' by
Danielsonet al. [1975] and 'IPL Processingof Mariner l0
Images of Mercury' by Soha et al. [1975], provide essential
backgroundinformationfor all who wishto pursuethe study
of the Mercury data. Danielsonet al. alsodiscuss
the resultsof
a preliminary searchfor any satellitesof Mercury, Stating5km diameteras the current upper limit. ThesePapersconstitutea valuablerecordof the techniquesand prioritiesapplied in this missionand thus may be relevantto somefuture
spacemissionsas well.
The paperby Klaasen[1975]on 'Mercurian Rotation Period
DeterminedFrom Mariner 10 Photography'exploitsthe fact
that the heliocentricperiod of the Mariner l0 spacecraftis
commensuratewith every two revolutionsof Mercury about
the sun; in addition, Mercury exhibitsa %.commensurability
between its axial spin and orbital revolution. As a consequence,identicalilluminationis encounteredby Mariner l0 on
each passage.Klaasenshowsthat limits can be placedupon
2344
MURRAY:MARINER 10 MISSION
periencedin the study of the surfacesof the moon and Mars,
especially,interactdirectlywith the data themselves.Certainly
the broad, rather far-reachinginterpretationspresentedin the
last paper in this serieswill triggervigorousdiscussion
and expressionsof alternatepointsof view. In so doing,an important
liminary
results
ofa surface
control
netanddiscuss
itsapplica- principleof planetaryexplorationmay well be illustrated;that
tion to the productionof cartographicproducts.Thesetech- is, eachnew planetaryobjectstudiednot only addsinformation
niquesrepresenta substantialevolution of those developed significantto its own historyand naturebut interactswith and
previouslyfor the Mariner Mars missions.The currentstatus accentuatesearlier information obtained about other planets.
of nomenclature is also reviewed.
Truly, planetary exploration is a nonlinear process.The opPhotometricmeasurementsof Mercury are discussednext portunity to add a whole new planetto our baseof knowledge
by Hapkeet al. [1975],takingadvantageof whathasprovedto about the terrestrialplanetshas beenof extraordinaryimporbebetterthanexpectedphotometricstabilityof theMariner 10 tance, the intellectualimplicationsof which will continueto
vidiconcameras.The photometry,colorimetry,and polariza- develop over succeedingyears.
tion resultsshowa generalsimilarityto thoseof the moon,but
Acknowledgments.The cumulativecoveragecurves of Figure 1
the authorsare able to distinguishcertain departuresfrom the were prepared by James Anderson of the California Institute of
lunar color and albedo relationships.Hapke et al. speculate Technologywho has contributed significantlyto the Mariner l0
that thesemercurianopticalpropertiesmay arise from small imaging experiment from inception to conclusion. Contribution
but significantcompositionaldifferencesfrom the materials 2578, Division of Geological and Planetry Sciences,California
Institute of Technology, Pasadena,California 91125.
that make up the surfaceof the moon. Gault et al. [1975]not
only describethe morphologyof the mercurianimpactcraters
REFERENCES
but show significantvariationsin form from those of the
moon. They argue persuasivelythat thesedifferencesreflect Danielson,G. E., Jr., K. P. Klaasen,andJ. L. Anderson,Acquisition
and descriptionof Mariner 10televisionscience
data at Mercury,J.
primarily the highersurfacegravityon Mercury as compared
Geoœhys.
Res., 80, this issue, 1975.
any departurefrom commensurabilityof the rotation rate of
the planet basedon comparisonof shadowpositionsof features contained in overlappinghigh-resolutionpictures. In
'SurfaceCoordinatesand Cartographyof Mercury,' Davies
and Batson[1975] presentboth the techniquefor and the pre-
,
to the moon.
Davies, M. E., and R. M. Batson, Surface coordinates and car-
tographyof Mercury, J. Geoœhys.
Res.,80, this issue,1975.
A preliminarygeologicterrain map of Mercury has been
Davies,
M.
E.,
and
B.C.
Murray,
The
Viewfrom Space,Columbia
preparedby Trask and Guest[1975] and is describedin their
University Press,New York, 1971.
paper. This is an essentialstep for unravellingthe major Gault, D. E., J. E. Guest,J. B. Murray, D. Dzurisin, and M. C. Malin,
historicaleventsof the surfaceas well as providinga basisfor
Somecomparisions
of impactcraterson Mercuryand the moon,J.
Geoœhys.
Res., 80, this issue, 1975.
future geologicmapping. The crucial geologicissuesof the
nature of tectonismand the existenceof volcanismon Mercury
are then discussedby Strom et al. [1975b]. They argue that
Mercury exhibits a tectonicstyle distinct from the moon or
Mars; probablecompressional
featuresare manifestedin the
older terrains of Mercury, testifyingto an early period of
crustalshortening.They alsoaddressthe questionof theorigin
of the smoothplainsdepositsand concludethat at leastsome
of the mercurianplainsvery likely are of volcanicorigin.That
conclusioncarries significantimplications for the igneous
history of the planet. Finally, in the last paper, by Murray et
al. [1975], 'SurfaceHistory of Mercury: Implicationsfor Terrestrial Planets,' a working hypothesisis presentedfor the
historyof Mercury, drawingupon the previouspapersas well
as their preferencesamong many competing theories of
planetary history. Then the implicationsof that 'straw man'
history for other terrestrialplanetsare developed.
A substantialamountof analysisand processing
of the Mercurypicturesstill continuesunderthe auspicesof the Mariner
10 television team. Novel rectification procedures have
recentlybeen developedto permit stereoviewingof certain
portionsof the surface;this providesa powerfulnew tool for
photogeologicanalysisof Mercury. Other kindsof basicdata
compilationand analysisare also under way, includingpresentations of crater frequency curves developed from
orthographicallyprojectedpictures of the surface, detailed
color mappingof the plainsdeposits,and a refinedupperlimit
for the sizeof any hypotheticalsatellites.In addition,the Mer-
Hapk½, B., G. E. Danielson, Jr., K. Klausen, and L. Wilson,
Photometricobservations
of Mercuryfrom Mariner 10,J. Geoœhys.
Res., 80, this issue, 1975.
Kaula,W. M., AnIntroduction
toPlanetary
Physics:
TheTerrestrial
Planets,John Wiley, New York, 1968.
Klaasen,K. P., Mercury rotation period determinedfrom Mariner 10
photography,J. Geoœhys.
Res., 80, this issue,1975.
Murray, B.C., M. J. S. Belton, G. E. Danielson, M. E. Davies, G. P.
Kuiper, B. T. O'Leary, V. E. Suomi, and N.J. Trask, Imaging of
Mercury and Venusfrom a flyby, Icarus, 15, 153, 1971.
Murray, B.C., M. J. S. Belton, G. E. Danielson, M. E. Davies, B.
Hapke, B. T. O'Leary, R. G. Strom, V. E. Suomi, and N.J. Trask,
Mariner 10 picturesof Mercury: First results,Science,184, 459,
1974a.
Murray, B.C., M. J. S. Belton, G. E. Danielson, M. E. Davies, B.
Hapke, B. T. O'Leary, R. G. Strom, V. E. Suomi, and N.J. Trask,
Mercury'ssurface:Preliminarydescriptionand interpretationfrom
Mariner l0 pictures,Science,185, 169, 1974b.
Murray, B.C., R. G. Strom, N.J. Trask, and D. E. Gault, Surface
history of Mercury: Implications for terrestrial planets,J. Geophys. Res., 80, this issue, 1975.
Reynolds,R. T., and A. L. Summers,Calculationson the composition
of the terrestrialplanets,J. Geophys.Res., 74, 2494, 1969.
Soha,J. M., D. J. Lynn, J. J. Lorre, J. A. Mosher, N. N. Thayer, D. A.
Elliot, W. D. Benton, and R. E. Dewar, IPL processingof the
Mariner l0 imagesof Mercury,J. Geophys.Res.,80, this issue,1975.
Strom, R. G., B.C. Murray, M. J. S. Belton, G. E. Danielson, M. E.
Davies,D. E. Gault, B. Hapke, B. O'Leary, N. Trask, J. E. Guest,J.
Anderson, and K. Klaasen, Preliminary imaging resultsfrom the
secondMercury encounter,J. Geophys.Res., 80, this issue,1975a.
R. G. Strom, N. J. Trask, and J. E. Guest, Tectonism and volcanism
on Mercury, J. Geophys.Res., 80, this issue, 1975b.
Trask, N.J., and J. E. Guest, Preliminarygeologicterrain map of
cury3•results
canbeexpected
to contribute
to furtheranalysis Mercury, J. Geophys.Res., 80, this issue, 1975.
of the surfacehistoryand materials.More generally,detailed
photogeologicmappingand many topical analysescan be expected to develop as the large community of scientistsex-
(Received February 21, 1975;
revised March 3, 1975;
acceptedMarch 3, 1975.)