JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. El0, PAGES 23,291-23,316, OCTOBER 25, 2001

Overview of the Mars Global Surveyor mission

Arden L. Albee Divisionof Geologicaland PlanetarySciences, California Institute of Technology,Pasadena, California

RaymondE. Arvidson McDonnellCenter for the SpaceSciences, Department of Earth and PlanetarySciences WashingtonUniversity, St. Louis, Missouri

Frank Palluconi and Thomas Thorpe Jet PropulsionLaboratory, Pasadena, California

Abstract. The Mars Global Surveyorspacecraft was placedinto Mars orbit on September 11, 1997, and by March 9, 1999,had slowlycircularized through aerobraking to a Sun- synchronous,near-polar orbit with an averagealtitude of 378 km. The sciencepayload includesthe Mars Orbiter Camera, Mars Orbiter Laser Altimeter, Thermal Emission Spectrometer,Ultrastable Oscillator (for Radio Scienceexperiments), and Magnetometer/ ElectronReflectometer package. In addition,the spacecraftaccelerometers and horizon sensorswere usedto studyatmospheric dynamics during aerobraking. Observations are processedto standardproducts by the instrumentteams and releasedas documented archivevolumes on 6-monthcenters by the PlanetaryData System.Significant results have been obtainedfrom observationsof the interior, surface,and atmosphere.For example, Mars doesnot now have an activemagnetic field, althoughstrong remanent magnetization featuresexist in the ancientcrust. These resultsimply that an internal dynamoceased operationearly in geologictime. Altimetry and gravity data indicate that the crustis thickestunder the southpole, thinningnorthward from the crateredterrain to the northernplains. Analysis of altimetrydata demonstratesthat Mars is "egg-shaped"with gravitationalequipotential contours that showthat channelsystems in the southern highlandsdrained to the north, largelyto the Chrysetrough. A closedcontour in the northernplains is consistentwith the existenceof a greatnorthern ocean. Emission spectraof low-albedoregions show that basalticrocks dominate spectral signatures on the southernhighlands, whereas basaltic andesites dominate the northernlowlands. The bright regionsshow nondiagnostic spectra, similar to that of dustin the atmosphere.Signatures of aqueousminerals (e.g., clays, carbonates, and sulfates)are noticeablyabsent from the emissionspectra. High spatialresolution images show that the surfacehas been extensivelymodified by wind and that layeringis nearlyubiquitous, implying that a complexhistory of eventsis recordedin surfaceand near-surfacematerials. Altimetry data implythat both permanentcaps are composedof water ice and dust,with seasonalcovers of carbondioxide frost. Finally, the altimetrydata, coupledwith thousandsof atmospheric profiles,are providingnew boundary conditions and dynamiccontrols for the generation and testingof more realisticdynamic models of the globalcirculation of the atmosphere.

1. Introduction to exploreMars shouldbe an orbiter to obtain a complemen- tary suiteof observationsof the atmosphere,surface, and in- Observations from the two orbiters and two landers that terior for a full Mars year from a near-circular, Sun- were part of the 1976Viking missiondemonstrated that Mars' synchronousorbit. The Mars Observer(MO) missionwas historyis complexboth geologicallyand climaticallyand that developedto conductthese global studies [Albee et al., 1992]. the currentenvironment and climateare both dynamic[Kieffer Unfortunately,that spacecraftwas lost in August1993, just as et al., 1992]. Perhapsas many questionswere raised about it approachedMars orbit insertion. Mars aswere answeredthrough analyses of Viking data. Issues Mars Global Surveyor(MGS) is the firstmission conducted includedthe origin and history of the putative fluvial and underthe auspices of the NASA Mars Surveyor Program. Thins lacustrinelandforms, the compositionand mineralogyof the program has as its foci understandingthe current and past majorcrustal units, the compositionand dynamicsof the polar climates of Mars, whether or not life started and evolved on caps,and a myriad of questionsassociated with atmospheric the Red Planet, and the geologicaland geophysicalevolution circulation.By the early1980s it wasclear that the nextmission of the planet. The historyand currentstate of water are inte- gratingthemes for the program.MGS utilized sparecompo- Copyright2001 by the AmericanGeophysical Union. nents,science instruments, and designsfrom Mars Observerin Paper number 2000JE001306. a configurationonly abouthalf the sizeof the Observerspace- 0148-0227/01/2000JE001306509.00 craft. Further, MGS waslaunched on a Delta asopposed to the

23,291 23,292 ALBEEET AL.: OVERVIEWOF THE MGS MISSION

largerTitan rocket.As a result,the two heaviestinstruments Table 1. MGS Mapping Orbit Characteristics fromMars Observer are slatedto fly on latermissions in the program. Characteristic Attribute Theformal science objectives forthe MGS mission are to (1) Nodal period 117.65 min characterizesurface morphology at highspatial resolution to Index altitude 378 km quantifysurface characteristics andgeological processes, (2) Altitude above surface 368-438 km determinethe composition and map the distribution of surface Semimajoraxis 3775,2 km Inclination 92.96 ø minerals,rocks, and ices and measure surface thermophysical Eccentricity 0.01 properties,(3) determineglobally the topography,geodetic Sun synchronous 1400/0200LT (relativeto meanSun) figure,and gravitational field, (4) establishthe natureof the Earth to Mars range 0.578AU (minimumon May2, 2000) magneticfield and map the crustal remnant field, (5) monitor 1.666AU (maximumon July21, 2000) globalweather and thermal structure of theatmosphere, and Solarconjunction July 1, 2000 (6) studysurface-atmosphere interaction by monitoringsur- facefeatures, polar caps, polar thermal balance, atmospheric dust,and condensate clouds over a seasonalcycle. variationdue to the J3 component ofthe gravity field, thereby Theseobjectives and the basic mission design are consistent providing more stable spacecraft operation over an extended with the Mars SurveyorProgram themes and the earlierrec- period[Uphoff, 1984]. The 88-revolution near-repeat pattern ommendationsof the SolarSystem Exploration Committee resultsin groundtracks spaced -242 kmapart at theequator [1983].Several other objectives have been adopted for the atthe end of 7 days(Figure 1). Each successive cycle fills in the MGS Missionas part of its inclusionin the MarsSurveyor spacing and under ideal conditions would result in 26 global Program:(1) providemultiple years of on-orbitrelay commu- cyclesand a spacingof-3 kmat theequator at theend of the nicationscapability for Marslanders and atmospheric vehicles 2-yearmapping mission. The Sun synchronicity was chosen as fromany nation interested in participatingin theInternational 1400/0200LT, relativeto themean Sun position, to balance the MarsSurveyor Program and (2) supportlanding site selection requirementsof the various instruments.The Mars Orbiter planningfor future Mars Surveyor Program missions through Camera(MOC) Teampreferred late afternoonfor distinctive acquisitionof high-resolutionimaging and other relevant data sets. shadowsin the images, whereas the Thermal Emission Spec- trometer(TES) Teampreferred an orbitcloser to 1300LT, In thispaper an overview of theMGS Mission and its major wherehigher ground temperature would result in a higher accomplishmentsto date are presented. Subsequent papers in signal-to-noiseratio. However, the true Sun position actually thisissue provide detailed results from analyses of keymea- variesduring the courseof themission from nearly an hour surementsfrom the instrumentsaboard the spacecraft. aheadto nearlyan hour behind the mean 1400 LT Sunposi- tion. 2. Mission Design Duringthe mapping orbit configuration at Mars the space- craftis continuously nadir-pointed, rotating at theorbital rate, The scienceobjectives for MGS presenteda numberof asthe high-gain antenna tracks Earth and the solar arrays track challengesfor the mission design. The spacecraft, instruments, the Sun. The instruments are mounted on the nadir platform, communications,ground data system, and required orbit had permittingthe use of simple,fixed, line-scan instruments, each to be designedto permita logicallyintegrated payload of withits own computer and without moving parts. This config- scienceinstruments to acquirehigh-quality, global observa- uration,along with adequate data storage, downlink rate, and tionsof Mars' atmosphere,surface, and interior over an entire powermargins, makes it possiblefor all instruments, including Mars year (687 days).Further, the data would need to be radioscience, tobe operated simultaneously andcontinuously processedand distributed in a timelyfashion for analysis by the witha single10-hour pass per dayon a 34-mDeep Space collectiveplanetary community. These overarching goals re- Station(DSN) groundstation. quiredfinding a configurationthat allowedall instrumentsto Theuse of chemicalpropulsion to reach the required map- beoperated simultaneously andcontinuously in the mapping ping orbit would require a largemass of fuel,far beyond the orbitwhile data were transmitted to Earth and the solar panels injectioncapability of the Delta II launchvehicle. Therefore maintainedSun-orientation to provide power. thespacecraft was designed tofacilitate the use of aerobraking Designinga specificmapping orbit to fulfill the scienceob- to attainthe mappingorbit (Figure2). After Marsorbital jectives,the extended radio relay period, and the Mars plane- insertion,aerobraking utilizes repeated dips into the upper tary protectionrequirements required meeting many con- atmosphereusing the atmosphericdrag on the spacecraftto straints.In turn, the designof the mappingorbit placed slowit soas to reachthe low-altitude circular mapping orbit. significantconstraints on the designof thespacecraft and the Sucha periodof aerobrakinghad not been incorporated into instruments.In general, these constraints were met by a low- thearchitecture ofany previous mission, and the challenge was altitude,near-circular, near-polar, Sun-synchronous orbit with mademore difficult by lack of knowledgeof thestructure and a shortrepeat cycle (Table 1). The chosencombination re- dynamicsof theMars atmosphere, especially at theaerobrak- sultedin a "frozen"orbit with a periodof 117.65min andan ing altitude. 88-revolutionnear-repeat cycle of -7 sols(Martian days). The indexaltitude of 378 km is a measureof the differencebetween the semimajoraxis length from the centerof massand a mean 3. SpacecraftDescription and Configurations equatorialradius of Mars (3394.2km)..The height of the TheMGS spacecraft was designed to carrythe science pay- spacecraftabove the "actual" surface of Marsranges between loadto Mars,place it intothe desired mapping orbit with the 368 and 438 km. Thisfrozen orbit essentially balances the useof aerobraking,maintain its proper pointing and orbit as a secularmotion of theargument of periapsisdue to J2by the three-axisstabilized platform for acquiring mapping data, and ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,293

SHIFT = 28.6 ø D

Nightside Ground Track

Equator

\ Dayside \ GroundTrack

40 ø 30 ø 20 ø 10 ø 0 o Areographic Longitude Figure 1. Mappingorbit groundtrack for 7-solrepeat cycle. The dashedlines are daysideground tracks, and the solid lines are nightsideground tracks. return the acquired data to Earth. The spacecraft'sdesign, As shownin Figure 4, the spacecraftassumes five different especiallythe electronicarchitecture, is basedon that of Mars major configurationsduring various phases of the mission.In Observer[Albee et al., 1992; Cunningham,1995]. Most of the the launch configurationall of the appendagesare in stowed major electronicassemblies are spareMars Observerunits that positionand fixed to the body of the spacecraft.During cruise havebeen retrofittedto eliminateany identifieddiscrepancies. the solarpanels were deployed,whereas the HGA antennawas The designis generallysingle-fault tolerant with redundancy not. The HGA was pointed toward Earth, and the spacecraft managedby central computers. rotates slowlyabout that axis, the array normal spin configu- LockheedMartin Astronautics(LMA), Denver, Colorado, ration. During aerobraking,as shown in Figure 4, the solar built the spacecraft(Figure 3). In order to meet the stringent panelswere sweptback towardthe payloadso that the engine massrequirements, the spacecraftstructure was constructedof faced into the flow. During propulsivemaneuvers the config- lightweightcomposite material. it was divided into four sub- uration was similar, but the solar panelswere sweptback to- assemblies:the equipmentmodule, the propulsionmodule, the ward the propulsionmodule. In the mappingconfiguration the solar array support structure, and the high-gain antenna spacecraftis three-axiscontrolled using input from the horizon (HGA) supportstructure. The equipmentmodule houses the sensorto keep the instrumentspointing toward nadir. Gimbal avionicspackages and scienceinstruments. Its dimensionsare drivesenable the solarpanels to track the Sun and enablethe 1.221 x 1.221 x 0.762 minX, Y, and Z, respectively.With the deployedHGA to track Earth. In safe and contingencymodes exceptionof the Magnetometer/ElectronReflectometer, all of the spacecraftlocates the Sun and then conesat one revolution the scienceinstruments are bolted to the nadir equipment per hundred minutesabout the Sun vector until contactwith deck,mounted above the equipmentmodule on the + Z panel. Earth is reestablished. The Mars Relay antenna is the tallest instrument,extending 1.115m abovethe nadir equipmentdeck (Figure 3). 3.1. Command and Data Handling The propulsionmodule serves as the adapterwith the launch The commandand data-handlingsystem is built aroundtwo vehicle,contains the propellanttanks, main engine,and pro- redundant flight computersthat run in parallel. The system pulsion feed systems,and has the attitude control thrusters consistsof spareMO hardwarewith the exceptionof two solid mountedon the four cornersof the module. Two solar arrays, state recorders(1.5 Gbits each) that replacedthe three MO each 3.531 x 1.854 m, provide power. Including the structure tape recorders for recording scienceand engineeringdata. that holds the array to the propulsionmodule, each panel Softwareresides in two redundantMarconi 1750A flight com- extends4.270 m from the sideof the spacecraftin a Y direction puters (128 Kwords RAM, 20 Kwords PROM): one in the and is further extendedby a 0.813-mspring-mounted flap that engineeringdata formatter and one in the Payload Data Sys- was added to increasethe ballisticcoefficient during aerobrak- tem (PDS). The softwarein the flight computerscontrols al- ing andmaintain the heatingrate below 0.38 W/m 2. The two most all the spacecraftactivities. While one computer is in magnetometersensors are mountedon the end of each array active control, identical software runs concurrently in the between the array and the flap (Figure 3). When fully de- backupunit. The flight softwarefunctions include attitude and ployed, the 1.5-m-diameterHGA sits on the end of a 2.0-m articulation control, command processing,some telemetry boom mounted to the +X panel of the propulsionmodule. functions, power management and battery charge control,

ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,295

High-Gain High-Gain Antenna • AntennaGimbals Magnetometer

Propellant Tank

Attitude Control Thruster Solar Panel (1 set on each corner of spacecraft)

Solar Panel

Magnetometer Mars Horizon Sensor

Solar Panel Electron Reflectometer

Mars Relay

Solar Panel MarsOrbiter CelestialSensor LaserAltimeter Assembly

MarsOrbiter ThermalEmission Camera Spectrometer Figure3. Viewof theMGS spacecraft showing major components and instruments. thermalmonitoring and heater control, and fault protection playback.Each unit can store up to 104hours of dataat the initiation and execution.Three levelsof basicfault protection 4-Kspsrecord rate and over 26 hoursat the 16-Kspsrate. modes are utilized. 3.2. Attitude Control Thereare three different types of datastreams and, because of therange in Earth-Marsdistance, each can be returnedat Three-axispointing control is providedthrough four reac- three different data rates. Instrument data and engineering tion wheels.Attitude information is providedby a Marshori- dataare collectedin discretepackets with identificationand zon sensorthat definesthe nadir direction, a star tracker for timinginformation in a regularschedule unique to eachdata inertialattitude, gyros and accelerometersfor measuringan- mode.The PDS collectsthe packets,formats the data into gularrates and linear accelerations, and multiple Sun sensors. transfer frames, and Reed-Solomonencodes the transfer Three reactionwheels are mountedparallel to the threespace- framesbefore storage in the recorder.The spacecraftthen craftaxes; the fourth wheel is skewedto the othersto provide appliesconvolutional encoding prior to transmissionby the backupredundancy. On January 18, 2001, the skew wheel was radio system.Reed-Solomon encoding requires 1.147 bits broughtinto useafter an electricalfailure in thex wheel (equalsone symbol per second (sps)) to encodeone bit of raw controls.The spacecraft attitude is controlled to highprecision data. by transferringmomentum to andfrom the rapidly spinning S&E1 is a combinedscience and engineeringdata stream reactionwheels. Sun sensorsare placedat severallocations that canbe recordedfor laterplayback or playedback in real aboutthe spacecraft. Their primary use is during attitude reini- timeat ratesof 4, 8, or 16Ksps. S&E1 data are played back at tializationafter a spacecraftanomaly. Data fromthese sensors a rate 5.333 times faster than the record rate, enabling the are sufficientto characterizenadir and HGA pointingto within spacecraftto return24 hoursof recordeddata during the _+3mrad (per axis,3 sigma). daysideportion of theorbit in a single10-hour DSN tracking 3.3. Telecommunications and DSN Utilization pass.S&E2 is a combinedscience and engineering data stream that returnsdata only in real time at ratesof 40 or 80 Ksps. TheX band(8.4 GHz) telecommunicationssystem consists ENG is a low-rateengineering-only data streamthat canbe of twoMO transponders,two Thompson 25-W traveling-wave recorded,played back in realtime, or both.The bit allocationstube poweramplifiers, the 1.5-m MO high-gainantenna for the instrumentsdiffer with the various data streamsand mountedon a 2-m boom,and four low-gainantennas (two for ratesin order to ensurethat all instrumentscan be operated receive,two for transmit). The frequency modulator (RF) feed simultaneouslyduring the various phases of the mission.Two of the HGA was modified to accommodatea Ka band engi- solid-statedata recorders provide redundancy. Each consists of neeringexperiment (REF) in whichX bandis converted to 32 two0.75-Gbit recorders that support simultaneous record and GHz andamplified to ---0.5W. TheX bandtransponders are 23,296 ALBEEET AL.' OVERVIEW OF THE MGS MISSION

Launch +z Configuration +Y

+x Array NormalSpin Configuration

Aerobrake +x Configuration +Y

VectorDuring •Direction Dragof PassVelocity

+z Toward Mars Nadir Mapping Configuration

Directionof Velocity Vector During Mapping +X

Figure4. MGSspacecraft configurations fordifferent phases ofthe mission (drag flaps are not shown; see Figure3). During mapping thespacecraft isbeing flown in the minus X direction toaccommodate the1-year delayin achievingthe mappingorbit. in themain spacecraft structure, but the power amplifiers and rate)to 500bits/s (750 commands per minute);the nominal the Ka band electronicsare locatedin an enclosureon the back rateis 125bits/s. Downlink data rates are as high as 85333 bits/s of the HGA. Whendeployed during mapping, two rotating when the Earth-Mars distance is small. joints(gimbals) allow the HGA to trackand point to Earth The 34-mhigh-efficiency (34mHEF) antennas of the DSN while the scienceinstruments observe Mars. The HGA was providemost of thetracking coverage for MGSbecause they usedin itsposition fixed to thespacecraft Y axis during cruise havethe capability to bothtransmit and receive X bandsignals. andaerobraking because it couldnot be deployeduntil all the Duringnormal operations in cruise,MGS operatedwith one burnsof the mainengine had beencompleted. Command 10-hourtrack per day, and an additional real-time 10-hour pass sequencessent by the flight operationsteam on Earth flow to aboutevery third day was added during the mapping period. MGSat ratesin multiplesof 2 from7.8125 bits/s (emergency During critical operations, including launch, maneuvers, and ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,297

Table 2. Characteristicsof Mars Global SurveyorInstruments

Mass, Power, W InstrumentName Description kg (AveragePeak) Data Rates,bits/s MarsOrbiter Camera imaging:narrow and wide fields of view,3.5- 21.4 13.5/18.3 700/2,856/9,120 (MOC) m focallength, 0.5-0.9 •m 11.4-mmfocal record 29,260/ lengthfor blueand 11.0mm for red 63,808 real time (0.04-0.45and 0.58-0.62 •m) ThermalEmission interferometerwith 6- to 50-•m spectral 14.6 12.3/18.2 1,104/2,485 record Spectrometer(TES) rangeand broad bands with 0.3-3.0 and 4,992 real time 6-100 •m band passes Mars Orbiter Laser Topography:50-cm-diameter telescope, 1.06- 25.9 34.2/35.9 618 record Altimeter (MOLA) •m laser,10 pulsesper second,160-m footprint,and 1.5-mrange resolution UltrastableOscillator oscillatorprovides precision frequency 1.3 3.0/4.5 N/A (USO/RS) referencefor Doppler measurements, especiallyduring exit occultations Magnetometer/Electron magneticfields in the rangefrom 0.004to 5.2 4.6/4.6 324/648/1,296 record Reflectometer 65536 nT; electronsfrom 1 to 10 keV (MAG/ER) Mars Relay (MR) Signalrelay receiver; interrogating at 437 7.9 9.0/9.05 8,000/128,000 MHz; receiveat 401 and 406 MHz relay rates

most of aerobraking,continuous tracking coverage was pro- variedas the solarrange increased and with the Sun angle, vided.In addition,continuous coverage has been provided for sincethe spinaxis was normally pointed toward Earth. special,weeklong science campaigns during mapping to take Duringmapping operations an autonomousgimbal drive advantageof specialorbit geometry conditions or to observe allowsthe solararray to trackthe Sun,acquiring power on the uniqueseasonal changes on Mars.In late1999, as Mars Cli- daysideof eachorbit. The orbit-averaged power produced by mate Orbiterand Mars Polar Orbiter approachedMars with the arraysranges from 980 W at Marsperihelion to 660W at theirtracking needs, the DSN developed the capability to track aphelion.Eclipses last from 36 to 41 minon eachorbit; depth severalspacecraft simultaneously with a singleantenna, signif- of batterydischarge is constrainedto be <27%, exceptduring icantlyenhancing the data return from MGS for thatperiod. emergencies.The powersupply electronics and battery charge assembliesare spareMO hardware. 3.4. Propulsion Propulsionis providedby a dualmode bipropellant system 4. Mars Global Surveyor ScienceInstruments usingnitrogen tetroxide and hydrazine. This dual mode differs fromconventional bipropellant systems in thatthe hydrazine is The scientificinstruments for MGS were designedwithin the usedby both the main engine and the attitudecontrol thrust- overall architecture of the mission.To meet the scientificob- ers,rather than having separate hydrazine tanks for each.The jectivesof this mappingmission, they had to both acquire mainengine is theonly one that used the bipropellant system. high-qualitydata and maximize the datareturned within the The mainengine maximum thrust is 659 N. It wasused for constraintsof the mission.Each was originallydesigned and majormaneuvers, including large trajectory corrections prior built for the Mars Observer Mission, from which the MGS to entryinto the mapping orbit. At launch,MGS carried --•385 missiondesign was derived. However, the massconstraints of Kgof propellant,and nearly 75 % of thatwas used during Mars the MGS missionsmade it necessaryto restrictthe payloadto orbit insertion. a subsetof the MO instruments,leaving two instrumentsfor The hydrazinesystem has four modules,each with three flighton latermissions. All theinstruments utilize the design 4.45-N thrusters.Each module containstwo aft-facingthrust- andspare components from the MO missionas well as the ersand one roll controlthruster. The hydrazinesystem is used same science teams. for orbit trim maneuversand unloadingthe reactionwheels MGS carriesfour onboardscience instrument packages and duringmapping. These thrusters were also used for keeping a RadioScience Experiment (Table 2). In additionto instru- the thrustvector in the velocityvector during main engine ment principalinvestigators and the Radio ScienceTeam burns,thereby saving significant propellant on the Marsorbit Leader(hereafter collectively termed PIs), there are a number insertion(MOI) burn. of InterdisciplinaryScientists associated with MGS (Table3). Visibleimaging and infrared spectral mapping provide infor- 3.5. Power mationabout the natureof the surfaceand processes operating Twosolar arrays, each 6.0 m 2, provide power for thespace- on the surface.Infrared and radio occultationprofiles of the craft.Each array consists of two panels,an inner and outer atmosphereprovide three-dimensional and time-varying tem- panel,composed of galliumarsenide and silicon solar cells, perature,ice and dustcontent, and pressureof the atmo- respectively.A combination with the more efficient and more sphere.The other instruments provide global determination of expensivegallium arsenide cells on one of thetwo panels was the topographic,gravitational, and magneticfields of Mars. utilizedbecause the panelswere sizedfor the aerobraking MOC, Mars Orbiter LaserAltimeter (MOLA), and TES are requirementsrather than the power requirements. When the coalignedon the nadirplatform. These instruments are de- spacecraftmoves into eclipseor turnsaway from the Sun, signedto acquiredata along the nadirground track without poweris providedby two nickel-hydrogen(NiH2) batteries, movingparts, although TES usesa forwardand aft scanning eachwith a capacityof 20 amp-hours.During cruise the power mirror to obtainatmospheric profiles. The instrumentshave 23,298 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

Table 3. MGS ScienceInvestigations, Lead Investigators,and InterdisciplinaryScientists

Instrument Lead Investigator ScienceInvestigation

Mars Orbiter Camera (MOC) M. Malin, Malin SpaceScience Systems, global synopticview, selectedmoderate- San Diego, California and high-resolutionimages of the surface Thermal EmissionSpectrometer (TES) P. R. Christensen, Arizona State surfacemineralogy; atmospheric dust University, Tempe, Arizona and clouds;radiation budget Mars Orbiter Laser Altimeter D. E. Smith, NASA Goddard Space globaltopography; surface reflectivity at (MOLA) Flight Center, Greenbelt, Maryland 1.06 •m Radio Science G. L. Tyler, StanfordUniversity, gravitationalfield; atmospheric Stanford, California refractivity(temperature and pressure) profiles Magnetometer/ElectronReflectometer M. H. Acuna, NASA Goddard Space global and local magneticfields and (MAG/ER) Flight Center, Greenbelt,Maryland interaction with solar wind Mars Relay J. Blamont, Centre National d'Etudes relay for plannedlander missions Spatiales,Toulouse, France

Discipline InterdisciplinaryScientist Affiliation

Data Managementand Archiving R. Arvidson WashingtonUniversity, St. Louis, Missouri Geosciences M. Carr U.S. GeologicalSurvey, Menlo Park, California Polar AtmosphericScience A. Ingersoll California Institute of Technology,Pasadena, California Surface-AtmosphericScience B. Jakosky University of Colorado, Boulder, Colorado Climatology R. Haberle NASA Ames Research Center, Moffett Field, California Surface Processes and L. Soderblom U.S. GeologicalSurvey, Flagstaff, Arizona Geomorphology

significantlygreater computer and data storagecapability (Ta- cesses.Simultaneous wide-angle images provide positional and ble 2) than those on previousmissions, enabling operation meteorologicalcontext for the narrow-angleimages. continuouslyduring daysideand nightsideparts of the orbit, The MOC wide-anglecameras view Mars from horizon to with a single downlink each day. Most have editing and/or horizon at mapping altitude and are designed for low- compressionof their data and can accommodatethe differ- resolutionglobal and intermediate-resolutionregional studies. ences in downlink capability as the Earth-Mars distance The resolutionvaries from ---250m/pixel at nadir to ---2 km/ changes. pixel at the limb. In a single24-hour period a completeglobal mosiacof the planet is assembledfrom stripstaken eachorbit 4.1. Mars Orbiter Camera at a resolutionof at least7.5 km/pixel.Regional areas (cover- MOC is a three-componentimaging system (one narrow- ing hundredsof kilometerson a side) are imagedat a resolu- angleand two wide-anglecameras) designed to take high spa- tion of better than 250 m/pixel at the nadir. Theseimages are tial resolutionimages of the surfaceand lower spatialresolu- particularlyuseful in studyingtime-variable features such as tion, synopticcoverage of the surfaceand atmosphere[Malin et lee clouds,the polar cap edge, and wind streaksas well as al., 1992;Malin and Edgett,this issue].The camerasare based acquiringstereoscopic coverage of areasof geologicalinterest. on the "pushbroom" technique,acquiring one line of data at To avoid the need for a movable filter wheel, each camera has a time on a CCD line array as the spacecraftorbits the planet. a single red or blue filter. Images from the two wide-angle The camerasshare commonelectronics for storingand pro- camerasare combinedto provide color imagesof the surface cessing the data. They are "shuttered" by sending data- and atmosphere that distinguish between clouds and the processinginstructions to the instrument.The instructionsare ground and between cloudsof different composition.During keyed to accuratetimes, which representthe predictedposi- the circularizationphase of the mission,images were acquired tion of the spacecraft.The narrow-anglecamera was designed at a variety of Sun angles. In addition, data were acquired to samplethe surfacerather than to acquireimages of specific coveringspecial targets that were offset from the spacecraft targets. Therefore any targeting of MOC requires sending nadir track. This was done by rolling the spacecraftand "shut- commandswith predictedtimes (and equivalentpositions) well tering" at predictedtimes. in advanceof data acquisition.MOC can output data at a very More than 75,000 imageswere acquiredprior to the end of high rate and can take advantageof any extra real-time down- the primarymission, more than were acquiredby all previous link capability. Mars missions. Using the narrow-anglecamera at the mapping altitude, areasranging from 2.8 x 2.8 km to 2.8 x 25.2 km (depending 4.2. Mars Orbiter Laser Altimeter on availableinternal digitalbuffer memory)can be imagedat The primary MOLA objectiveis to determineglobally the ---1.4 m/pixel. Additionally, lower-resolutionimages can be topographyof Mars by generatinghigh-resolution topographic acquiredby pixel averaging;these images can be muchlonger, profilesat a precisionsuitable for addressingproblems in ge- rangingup to 2.8 x 500 km at 11 m/pixel.High-resolution data ologyand geophysics[Smith et al., this issue(a); Zuber et al., are beingused to studysediments and sedimentaryprocesses, 1992].The instrumentmeasures the round-triptime of flight of polar processesand deposits,volcanism, aeolian and fluvial a 1.064-/•mlaser pulsetransmitted from the spacecraftto the erosionand deposition,and other geologic/geomorphicpro- surface.Combining these data with accuratelocation and re- ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,299 locity of the spacecraftallows derivation of surfaceradii. On collimatedwith the main telescopeand usingthe samepoint- somepulses, reflections are alsoreceived from ice clouds.The ing mirror, is usedfor the bolometricthermal radiance(4.5 to measuredreflectivity and nature of the returned laser pulse -100 /xm) and solar reflectance(0.3-2.7 /xm) channels.A contribute to analysesof cloud dynamics,seasonal albedo pointing mirror capable of rotating 360ø providesviews to changes,and surfaceroughness and slope. space,both limbs, and to internal, full-aperturethermal and MOLA hasa diode-pumped,Nd:YAG lasertransmitter that visiblecalibration targets, as well as image motion compensa- emits 8.5-ns pulsesat 100-msintervals with an initial output tion. Each telescopehas six detectors,each with a -8.5-mrad power of 40 mJ per pulse.The receivingoptics, a Cassegrain field of view. These form a rectangulargrid of three framesin telescopewith a 0.5-m-diameterprimary mirror, focusesthe the crosstrack direction and two frames in the downtrack di- return signal on a silicon avalanchephotodiode detector. rection. The resultingspatial resolutionis about 3 x 2 km at MOLA has a singledata rate of 618 bits/s.At the mapping the mappingorbit altitude. The output from TES channelsis altitude each laser spot illuminatesabout a 160-m-diameter digitizedat 16 bits, processed,compressed, and formattedbe- circleon the surfacewith a spacingof --•300m (0.1 sec) per fore being sent to the spacecraftdata system.During opera- spot alongthe MGS nadir groundtrack. The range measure- tion, TES generates data at either 688, 1664, or 4992 bit/s ments are quantitized with 1.5-m vertical resolution before dependingupon the phase of the mission.More than 105 correctionfor orbit and pointingerrors. Relative error in alti- million spectrawere acquiredby the end of the primary mis- tude along profiles is 1-10 m, and the profiles are being as- sion. sembledinto a globalgrid referencedto Mars' centerof mass with high absoluteaccuracy. 4.4. Magnetometer/Electron Reflectometer The demonstrationthat a laser altimeter can operate for a The measurementobjectives of the Magnetometer/Electron longlifetime in spaceis a major accomplishmentof this instru- Reflectometer(M•G/ER) experiment were to determinethe ment.After firing --•350billion pulsesover a 2-yearperiod, the existenceand characteristicsof the global magneticfield, to laser energyhad decreasedfrom an initial 45 mJ to 25 mJ on characterizesurface magnetic features, and to determine the June 1, 2000. MOLA will be operated on alternate months nature of the solar wind interactionwith Mars [Acuhaet al., after solarconjunction to conservelaser power for later obser- 1992, this issue;Mitchell, this issue].The experimen•tincludes vations. However, almost 500 million shots were accumulated two redundanttriaxial fluxgatemagnetometers and an electron by the end of the primary missionon February 1, 2001. reflectometer.The vectormagnetometers provide in situsens- ing of the ambientmagnetic field in the vicinityof the MGS 4.3. Thermal Emission Spectrometer spacecraftover the range of -1 to 65,536 nT with a digital TES is designedto study the surface and atmosphereof resolution of 12 bits. The electron reflectometer measures the Mars using thermal infrared spectroscopy,together with local electron distributionfunction in the range of -1 to 20 broadbandthermal and solar reflectanceradiometry [Chris- KeV and remotely sensesthe strengthof the magnetic field tensenet al., 1992; Christensenet al., this issue].Objectives down into the top of the atmosphereusing directionalinfor- include (1) mapping the compositionof surfaceminerals, mation providedby the vectormagnetometer. This synergistic rocks,and ices;(2) determiningthe composition,particle size, combinationwas designedto increasesignificantly the sensi- and spatialand temporaldistribution of atmosphericdust; (3) tivity (factorof 10-100) and spatialresolution (about 3 times) determiningthe temperature,height, and abundanceof water achievablefrom the mappingorbit with the vector magnetom- ice and carbondioxide clouds; (4) studyingthe growth,retreat, eter alone. and total energybalance of the polar cap deposits;(5) mea- MGS lacksa boom to separatethe sensorsfrom the space- suring the thermophysicalproperties of the Martian surface craft bodyto reduceinterference by spacecraft-generatedmag- materials;and (6) characterizingthe thermal structureand netic fields. Instead, each magnetometersensor is located at dynamicsof the atmosphere. the outer edge of the articulatedsolar panels,--•5 m from the TES includestwo nadir-pointedtelescopes that sharea com- center of the spacecraftbus. This arrangementrequired a mon pointing-mirrorsystem. A 15.24-cmCassegrain telescope specialwiring designand implementationof extremelymag- feeds a Michelsoninterferometer that producesspectra from netically"clean" solar array panels.In addition,the spacecraft 1700to 200 cm-1 (--•6 to 50 /xm),at either5 or 10 cm-1 fields needed to be characterizedby calibration maneuvers, spectralresolution. The instrumentcycle time, includingcol- both in cruise and in orbit. The electron reflectometer sensor lection of the interferogram,mirror flyback, and electronic is mounted directlyon the spacecraftnadir panel. The instru- reset,is 2 s for 10-cm-1 operationand 4 s for 5-cm-1 opera- ment acquires2-16 vector samplesper secondof magnetic tion. The interferometer includes a visible interferometer that field data, dependingon the telemetry rate. Data acquisition, generatesfringes which are used to control the linear drive transmission,and commandfunctions are microprocessorcon- servo and to determine position in the interferogram. This trolled and canbe partiallyreprogrammed in flight. About 4.75 systemuses two redundantneon lampsthat producean emis- billionvector samples were acquiredby the end of the primary sion line at 703.2 nm for fringe generationand a continuum mission,--•200 million of them duringthe ellipticalorbit pe- that is usedfor a quasi-white-lightsource for determinationof riod. zero path difference.A digital signalprocessor is utilized to The significantadvantages of the elliptical aerobrakingor- perform fast Fourier transforms,providing high compression bits to the MAG/ER investigationwere recognizedearly in the over the raw data. The TES spectrometerhas a noiseequiva- planning of the observations.In these orbits the spacecraft lentspectral radiance near 1.2 x 10-8 W cm-2 sr-1 cm-1. This would dip below the bottom of the Martian ionosphere,allow- correspondsto a signal-to-noiseratio (SNR) of 490 at 1000 ing the MAG/ER experimentto achieveextraordinary sensi- cm-1 (10/xm)viewing a 270-Kscene. Calibration is achieved tivity and spatialresolution for the detectionof weak crustal by periodicviews of spaceand an internal referencesurface. fields.In addition,the high plasmadensities expected at these A separate 1.5-cm-diameter reflecting telescope, co- low altitudesrequired a differentmeasurement technique, so a 23,300 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

Langmuir probe operational mode was added to the electron orbit the MR took on a role asthe primarycommunication link spectrometer,using the outer caseof the electrostaticanalyzer for the Mars SurveyorProgram Deep Space2 microprobesand as a sweptbias collector [Mitchell, this issue]. a backuplink for the Mars Polar Lander. After the lossof these After only a few orbitsit wasestablished that Mars doesnot spacecraft,considerable effort was spentwithin the MGS mis- now possessa global magneticfield. However, the extended sionteam in attemptingto establishcommunication with them, time spentin the elliptical orbit made it possibleto character- but without positiveresults. Currently, discussionsare being ize remnant crustalfields from pole to pole at a sensitivityand held with the Mars '03 missionteam relating to usingMR to spatialdetail that couldnot havebeen achievedin the mapping provide a relay link during the landing of those spacecraft. missionas originallyplanned. Further, the interactionof the 4.7. Accelerometer and Horizon Sensor remnantmagnetic fields with the plasmacould be investigated. The MGS accelerometer[Keating et al., 1998;Bougher and 4.5. Radio Science Subsystem Keating,1999] and the horizonsensor [Martin, 1997] were also Radio scienceinvestigations of the gravity field and atmo- used as scienceinstruments during the aerobrakingportion of sphericstructure have been conducted since the first spacecraft the mission,while assistingin the operationof the spacecraftin visited Mars, utilizing the telecommunicationequipment on the elliptical orbit. They provided information on the density the spacecraftin conjunctionwith the equipmentat the DSN and densitychanges in the atmospherein a region where al- stations.Measurements of small perturbationsin spacecraft most no data were available previously.Use of the horizon velocityfrom Doppler shiftson transmittedsignals are usedto sensorcontinues during the mapping mission. infer detailed gravitationalfields from solutionsof simulta- neousequations using the measurements.During occultation 5. Mission Operations and Ground System of the spacecraftradio signalby the atmosphere,small Dopp- ler shiftscan be interpreted as refractivitychanges in the at- 5.1. Overview of Distributed Operations mosphere;from these, detailed temperature-pressureprofiles Flight operationsfor MGS were managedand conductedby of the atmospherecan be derived. the Mars SurveyorFlight OperationsProject (MSOP) at the The X band capabilityof MGS reducedsolar wind plasma Jet PropulsionLaboratory (JPL), an entity that alsobecame effects on the radio signal by a factor of 10 comparedwith responsiblefor the simultaneousoperation of Mars Climate previousS band systems.In addition,a temperature-controlled Orbiter and Mars Polar Lander. Surveyorflight operationsare UltrastableOscillator (USO) wasplaced on MGS [Tyleret al., characterizedby remote (from JPL) control of the science 1992, this issue]and was used as a frequencystandard during instrumentsand spacecraft.The scienceinvestigation teams one-wayradio scienceobservations, particularly during egress are locatedat the home institutionsof the PrincipalInvestiga- from the eclipseon eachorbit. Theseimprovements, combined tors (PIs), Team Leader, and InterdisciplinaryScientists. The with the low-altitude polar orbit and dedicatedgravity cam- spacecraftoperations are conductedby Lockheed Martin As- paignsduring favorablygeometric configurations for Doppler tronauticsat its Denver, Colorado, facility, where the space- measurements,have yielded unprecedentedspatial resolution, craft was built and tested.The MGS project performsmission global coverage, and accuracy of the derived gravitational management, navigation, data administration, and mission model.Vertical resolutionfor the atmosphericoccultation pro- planningand sequencedevelopment at JPL. Workstationsand files is closeto 10 m, providingan accuratecheck for infrared electroniccommunication links connectthe missionplanning measurements,especially during diametric occultation.More and data analysisactivities of the scientistsand engineerswith than 5000 spectrawere acquiredduring the primary mission. JPL, where tracking data are received from the DSN and The Ka band experimentwas operatedduring the cruiseto stored and where command messagesfor the spacecraftare Mars. Measurements showed that the use of Ka band for data assembled. transmissioncould increasedata capabilityby at least a factor The MGS missionutilizes data standardsfor packet telem- of 3 (5 dB) comparedto the X band system[Burman et al., etry and telemetry channel coding and a standardformatted 1997]. data unit for data transferamong ground systems. Mission data are stored in a central Project Database at JPL. Raw science 4.6. Mars Relay data consistof scienceand engineeringpacketized telemetry The Mars Relay (MR) systemwas originally provided to MO data, together with spacecraftposition and pointing informa- and then to MGS by the FrenchSpace Agency (CNES) to relay tion data availableas a supplementaryexperiment data record. data from the RussianMars '96 small penetratorsand Marsk- Planning productsinclude the up-to-date missionsequence hod Rover to Earth (J. L. Callas et al., manuscriptin prepa- plan, schedulesand commanding opportunities, and orbit/ ration, 2001). Data was to be routed from MR to the MOC viewingforecasts. Investigators and analystsaccess the Project data buffer for storage and subsequentdownlink as part of Databaseto participatein the planningprocess and to acquire MOC's data return. MR consists of a 1-m helix antenna raw data. mounted on the nadir panel and associatedelectronics, oper- The DSN acquiresand handlesscience and engineeringte- ating at UHF frequenciesnear 400 MHz. The antennapattern lemetry from the spacecraft,as well as radiometric data to (-3 dB) forms a 65ø cone emanatingfrom the tip of the supportnavigation and radio scienceobjectives. Multimission antennaand providescoverage to the horizonwith a 5000-km capabilitiesat JPL were used for telemetry processing,flight range for a 8-kbit/s data rate. However, the range drops to systemcommanding, data systemoperational control, and tele- --•1300 km for the 128-kbit/s data rate. The systemuses a communicationslink performanceassessment. The telemetry 1.3-W, 437.1-MHz beacon to activate transmissionof relay processingincludes error detection and correctionusing the data but essentiallyprovides only a one-waycommunication Reed-Solomon encodinginformation suppliedwith the data link from the surfaceto the MGS spacecraft. from the spacecraft,depacketization of the transfer frames, After the lossof the Mars '96 spacecraftbefore it left Earth placementof the packetsinto the Project Databasefor access ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,301 by the investigators,transfer of the engineeringdata for display mandsto their instrumentat any time. DSN coverageis sched- at the analysis'sworkstation, and monitoringof spacecraften- uled in advance.Using the skeletonsequence for eachmultisol gineeringdata for assessingthe health of the spacecraft.The cycle,the scienceinvestigators develop their observationplans, project also providesspacecraft performance analysis, space- producea set of files usingsequence software provided with craft navigation,and flight systemcommand sequencing. their SOPC, and transmit them to the Project Database as sequencerequests via the SOPC. Most requestsare updatesto 5.2. Science Operations Planning Computer tables in the memory of the individual instrument.Interactive A ScienceOperations Planning Computer (SOPC) is lo- real-time commandsare, by definition, those that affect an- cated at the home institutionof each Team Leader, Principal other instrumentor spacecraftsubsystem. They are taggedas Investigator,and InterdisciplinaryScientist. Each SOPC has suchand subjectedto validationin the commandprocess. The restrictedaccess and is connectedto the Project Databasevia use of these interactive commands is not common. Mission NASA Communications(NASCOM) circuits.The SOPCsen- operationspersonnel at JPL completethe commandprocess able the instrument teams to exercise substantial direct control by integratingthe independentlydeveloped sequence requests, of their instrumentsand enablethe interdisciplinaryscientists verifyingthat no spacecraftor missionconstraints are violated, to be directly involved in mission operations. Command se- and translatingthe sequenceinto a commandfile for uplink to quencesoftware is residenton each SOPC, enablingthe inves- the spacecraftby the DSN. tigators to prepare nearly all of the commandsrequired by their instrumentsto conductthe desiredexperiments and to 5.5. Data Return-Downlink Process forward thosecommands directly to the commandrequest file The MGS downlinkprocess consists of a daily playbackof in the Project Database. the spacecraftsolid state recorders plus a real-time data return about every third day. The sciencedata are placed in the 5.3. SPICE System Project Databasein packetsas formattedby the instrument. SPICE is an acronymused to describefive basic kernels of Averaged over the mission,the raw data rate is more than 120 data neededto providecritical information for processingsci- Mb/d. The SOPCsprovide the instrumentteams with accessto ence data files: S standsfor spacecraftephemeris, spacecraft the sciencetelemetry packets within 24 hoursof their receipt locationas a functionof time; P standsfor planetaryephemer- by the DSN and the SFOC. The scienceteams are responsible idesand selectedphysical and cartographicconstants; I stands for generating quick-look science data and/or examining for instrumentdescriptions, including identification codes used housekeepingdata to monitor instrumentperformance on the in E kernels and alignment offset anglesused in C kernel; C basisof the data in the experimentdata packet. standsfor inertial orientationof the spacecraftprimary coor- dinate systemin right ascension,declination, and twist angles and rate changesfor the coordinatesystem; and E standsfor 6. Science Data Management and Archiving event information, including nominal sequences,real-time Any one instrumenton MGS producesa large volume of commands,unscheduled events, and experimenter'snotebook data that when analyzed,will significantlyaugment our under- comments [Acton, 1991]. Ancillary data are packaged as standingof Mars. In addition, cross-instrumentanalyses allow SPICE kernels and made availablefor processingdata. questionsto be addressedthat are difficult or impossibleto The kernelsare usedtogether with a softwaretool kit sup- answerwith information from a singleinstrument. Because of plied by the Navigation and Ancillary Information Facility the scientificimportance of the new data from MGS and the (NAIF) at JPL to calculatethe basicinformation required by widespreadpublic interest in new resultsfrom Mars, formal the investigatorsin analysisof the experimentdata records. data sharingand releasepolicies have been established.Fur- SPICE kernels are generatedby various missionoperations ther, each experimentteam has committedto the generation teamson the basisof orbital tracking,together with instrument and timely release of archivesof MGS-derived productsto information and sequence data obtained from instrument NASA's Planetary Data System for distribution to the re- teams. MGS usesboth "actual" and "predict" SPICE files. search,education, and public communities. Predict SPICE kernelsare generatedin advanceas part of the sequenceplanning cycle. This capabilityis especiallyuseful for 6.1. Data Use Policy predictingwhen an instrumentwill have an opportunityto The data rights policy lays out broad guidelineson the use observe a feature of interest. The actual SPICE kernels are and release of data to ensure that the science done with MGS generatedwithin severalweeks after data acquisitionfor use data is maximizedand that the expertiseof MGS scientistsis during the scienceanalysis by each of the experimentteams. utilized in the best possibleways. The PIs have primary re- The SPICE data sets and the associated software tool kit are sponsibilityfor the acquisition,reduction, analysis, and pro- releasedregularly and archivedfor use with the archivedsci- duction of archival data sets for the data obtained from their ence data. instruments.They are alsoresponsible for publishingscientific resultsand for timely depositionof reduced data and docu- 5.4. Commanding-Uplink Process mentationwith the project.All MGS scientistsare encouraged The primarycommand processing consists of generatingpe- to participateactively in collaborativeefforts. The authorship riodic (typically1 week) StoredSequence Command loads that of collaborativepublications has generallyincluded all partic- control spacecraftbus operationsand update onboardspace- ipants in the reductionand analysisof data. craft ephemeridesand star catalogs.Each new load is gener- ated by an adaptive process,modifying skeleton sequences 6.2. Data Release and Archiving built and testedpreviously. However, sinceinstrument opera- A cornerstoneof the MGS data releasepolicy is that the tion is nearly independentof spacecraftoperations, each PI releaseof data to the PlanetaryData Systemarchive depends may request transmissionof noninteractivereal-time com- only on the amount of time required to accumulatesufficient 23,302 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

Table 4. Mars GlobalSurveyor Standard Data Products

Instrument Archive Internet Access Description

Accelerometer http://atmos.nmsu.edu/mgsacc.html accelerometercounts, periapsis orbital elements, angularrates and quaternions,thruster-on times,density profiles, constant altitude profiles MAG/ER http://www.igpp.ucla.edu/ssc/pdsppi/mgsmag.htm magnetometermagnetic field vectors,Electron Reflectometer omnidirectional flux MOC http://ida.wr.usgs.gov/ narrow-angleand wide-angleimages MOLA http://wufs.wustl.edu/missions/mgs/mola/ raw and precisionorbit correctedaltimetry profiles,gridded global topography maps Radio Science http://pds-geophys.wustl.edu/pds/mgs/rs/ occultationsummary file; atmospheric temperature-pressureprofiles; surface reflectionimages, tables, and geometry; ionosphericelectron density profiles; intensity powerspectra; line-of-sight acceleration profiles;spherical harmonic models of gravity field; Radio Sciencedigital maps SPICE http://wufs.wustl.edu/missions/mgs/spice.html spacecraftand planet ephemeris;Mars size, shape,and orientation(nominal pre-flight values);instrument mounting alignments and field-of-viewspecifications; orientation of spacecraft,Solar arrays, and high-gain antenna;high-gain antenna reference frames specifications;spacecraft clock coefficients; leapsecondstabulation TES http://wufs.wustl.edu/missions/mgs/tes/ observationalparameters; raw and calibrated radiance observations;raw and calibrated visual and thermal bolometer measurements; Sun, spacecraft,and planet geometricdata; positionalinformation for spacecraft,Sun, and Mars; auxiliaryobservation parameters; raw interferogramdata; real and complex FFT data; derivedsurface properties; derived parametersfrom planetarylimb observations; global derivedsurface property maps

data and informationto generatestandard products, to con- atmosphericpressure on the panelwhile the problemwas duct the work neededto generatethe products,and to check under assessment.Excellent, althoughunanticipated, science the validityof the results.The defaultperiod for this activity datawere acquiredduring this 1-monthperiod as alternative spansa nominal6-month period from the receiptof raw data planswere considered.The revisedmission plan preservedthe at the home institutionresponsible for generatingthe relevant mission architecture but delayed entry into the circular- standarddata productto releaseof documentedarchives con- tainingthe productsto the PDS. During thisperiod the Instru- ment Teams produce and validate standarddata products, submit for publicationtheir preliminaryanalysis, and work I 'ooI TOTAL with the PDS to assemblearchive volumes and Web-basedftp sitesfor releaseto the community.Table 4 liststhe standard • 250 productsand the Web accessand Figure 5 is a summaryof the numberand volumeof data productsreleased to the Planetary ,,• Data Systemas a functionof time. '-' 200

7. Mission Phases and Events o 150 The MGS spacecraftwas launchedon November7, 1996 (Figure2). The spacecraftmaintained a slowcontrolled roll in D 100 MOC a partiallydeployed state during its l 1-monthflight to Mars. ,>. After orbital insertioninto a 45-hour elliptical orbit on Sep- RS TES tember 11, 1997, MGS utilized repeated dips into the upper : 50 atmosphereto aerobrakeand attain the low index altitude m MOLA •,1 MAG/ER (378-km) circular-mappingorbit. MGS shouldhave reached 0 SPICE this orbit early in springof 1998. However, failure of a solar 1/1/98 1/1/99 1/1/00 1/1/01 panel damper during deploymentimmediately after launch resulted in damageto a panel, the extent of which did not Date become clear until about a month into aerobraking.At that Figure 5. MGS data volumeover time by individualexperi- time, periapsisaltitude was raised to 174-kmaltitude to lower ments. ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,303 mappingorbit for an entire Earth year, requiringa long phas- 50.0 ing period in ellipticalorbit and manymore-modest aerobrak- Capture- 9/1 )./97 PhobosEncounters: [263 X 54,000 km] P-476 8/7/98 ing passesinto the atmosphere.The spacecraftreached the 378 45.0. .'•. P-5018/19/98 •. InitialAerc 3raking P-5268/31/98 km circular orbit on February 19, 1999, but in the 0200 LT + [P-119 km] P-5519/12/98 40.0 + (relative to the Sun) position rather than in the originally + Aerobrakng Hiatus ,•._[1_74 x 4.• ,100km] planned 1400 LT position. 35.4'

7.1. Launch, Cruise, and Orbit Insertion 30.0 The MGS spacecraftwas launched from Cape CanaveralAir 10/12/97• Force Station, Florida, on November 7, 1996, aboard a Mc- Donnell Douglas Delta II 7925 rocket, a configurationwith 20.0 nine strap-onrocket motors. Weather conditionsdelayed the SciencePhasi• •g ._• launchuntil the secondday of the launchwindow. The 1062-kg Orbit(SPO) A-573 • spacecraft,built by Lockheed Martin Astronautics,traveled 11.6...... ;175x 17,850k• •] 9/23/98 • 10.0 nearly 750 million km over the courseof a 300-day cruise to 3/27/98 Solar o ( onjunchon • reach Mars on September11, 1997. Deployment of the solar 4/:9- 5/27/98 •erobra • '• arraysand checkoutof the spacecraftand instrumentsystems Phase 2 • • 2.0. was carried out early in cruise using the low-gain antenna. 0.0 .... , , , , , ...... • • • , , , , , , ...... After 4 months,communications were shiftedto the high-gain 9/3 12/24 4/15 8/5 11/25 3/17 1 bc = 2 weeks antenna,still in its fixed positionon the spacecraft.After de- Date (1997 - 1999) ploymentof the solararray panelsit becameclear that one of them was not locked in its proper position.After a seriesof Figure 6. MGS aerobrakingphase: orbit period and sizever- sus date. The location of periapsismigrated from 45ø north flexingexperiments it wasdecided to reversethe orientationof over the north pole and then down and over the south pole. that panel for the entry into Mars atmosphere. Upon reachingMars, MGS was insertedinto orbit by firing the main bipropellantrocket engine for a 23-min Mars orbit braking as alternative plans were considered[Albee et al., insertion(MOI) burn. A "pitchover"maneuver, keeping the 1998]. thrust vector parallel to the velocityvector, saved---20 m/s in After considerableanalysis, aerobraking was resumed on propellant [Espositoet al., 1998]. The initial MGS orbit was November 7, 1997, with reducedpressure on the panels.The highly elliptical with a period of 44.993 hours, a periapsis originalplan wasto reachthe mappingorbit prior to the period altitude of 262.9 km, an inclination of 93.258ø, and an orien- tation of---1800 LT relative to the sunline. of solar (superior)conjunction. The revisedmission plan de- layed the entry into the circular-mappingorbit for an entire Earth year, requiring many more aerobrakingdips into the 7.2. Circularization atmosphere,but with only one third as much pressurebeing During the third periapsis at Mars the spacecraft was exertedon the solarpanels. The dynamicpressure was reduced pointed to the nadir to enable the instrumentsto operate and fromthe planned 0.68-0.58 N/m 2 corridorto 0.25-0.15N/m 2. collectdata in order to verify that they were working properly. The spacecraftfinally reached its final 378-km near-circular On the third orbit, MGS initiated the aerobraking period, orbit on February19, 1999,but in the 0200 LT (relativeto the utilizing repeated dips into the upper atmospherewith the Sun) positionrather than in the originallyplanned 1400 LT solar panelsin a V configurationto slow it so as to attain the position.This delay of half a Mars year was undertakensince low-altitude (378-km) circular-mappingorbit. Aerobraking it made it possibleto operatethe spacecraftand its instruments was a key feature of the MGS mission,required becausethe in the geometricrelationships for which they had been de- injection capabilityof the Delta II is not sufficientto launch signed.However, the spacecraftwould ascendfrom south to the massof fuel required to use chemicalpropulsion to estab- north on the daylightside of the orbit in this position(Figure lish the mappingorbit. Accordingto plan the spacecraftwould 1). have reachedthe mappingorbit in the springof 1998, before Scientific observationswere obtained during most of the the solarconjunction period (see Figure 6 and Table 5). year period of delay.Low-altitude (high-resolution) data, es- However,at the low point of orbit 15 on October8, 1997,the pecially important for the magnetometerand gravity investi- spacecraftexperienced excessive flexing of one of the solar gations,were obtainedwith coverageover most of the planet. panels.The damper failure during solar panel deploymentin Suchcoverage was possiblebecause during this period of time early cruiseresulted in damageto the yoke of the panel, the the periapsisposition of the continuouslydecreasing elliptical extent of which did not become evident until the spacecraft orbit migrated from 45ø north over the north pole and then dipped far into the atmosphere.The orbit was quicklyraised down and over the south pole. Correct phasingof the entry higherin the atmosphereto lower the pressureon the panel. In into a 1400 LT circular orbit necessitateda substantialpause orbits 19-36 (October 13, 1997, to November 7, 1997) the (SPO-1 and SPO-2) in the aerobrakingoperation, during periapsisaltitude was only 175 km, and the spacecraftpassed in which sciencedata was collected from 372 elliptical orbits, and out of the ionosphereon each orbit. The scienceinstru- 175 x 17,800km, from March 27, 1998,to September23, 1998. ments were operated in the nadir orientation in the near- During the periapsis-passageportion of each 11.6-hour orbit periapsisportion of each 35.4-hour elliptical orbit, collecting the spacecraftwas turned to the nadir-pointingposition so that data as the spacecraftmoved from north to south. The first the instrumentspointed to Mars for ---22 min. MOLA, TES, extensive sets of scientific observations were collected from and MOC obtaineddata in near-normalmode duringthe nadir this assessmentor hiatus period during the hiatus from aero- portion of these orbits and acquired lower-resolutionglobal 23,304 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

Table 5. MGS Events Subsequentto Mars Orbit Insertiona

Periapsis Apoapsis Orbit Period,c Altitude, c Altitude, delta V, Eventsa Orbitb PeriapsisDate hours km km m/s

Mars orbit insertionburn (MOI) P-1 Sept. 12, 1997 44.993 262.9 54,026 973 Orbit nadir-pointedat periapsis P-2 Apoapsisburn P-3 Sept. 15, 1997 45.3 263 54,026 4.4 Aerobrakingperiod: initial Apoapsisburn P-18 Oct. 12, 1997 -115 Hiatus/assessmentperiod P-19 Oct. 13, 1997 35.4 175 45,100 (nadir-pointedat periapsis) Apoapsisburn P-36 Nov. 7, 1997 35.4 175 45,100 Aerobrakingperiod (AB-1) Apoapsisburn P-201 March 27, 1998 -125 4.4 Sciencephasing orbits (SPO1) P-202 March 27, 1998 11.6 175 17,800 (nadir-pointedat periapsis) Solar conjunctionperiod P-269 April 29, 1998 Sciencephasing orbits (SPO2) P-328 May 27, 1998 Phobosencounter P-476 Aug. 7, 1998 Phobosencounter P-501 Aug. 19, 1998 Phobosencounter P-526 Aug. 31, 1998 Phobosencounter P-551 Sept. 12, 1998 Apoapsisburn P-573 Sept. 24, 1998 11.6 175 17,800 11.6 Aerobrakingperiod (AB-2) -127 End of walkout 143 450 Apoapsisburn (ABX) P-1284 Feb. 4, 1999 61 Gravity/MOC calibration P-1285 Feb. 4, 1999 118 min 377 454 Adjustmentburn (TMO) P-1473 Feb. 19, 1999 22 Final partial orbit P-1683 March 9, 1999 Mappingorbits (MO) DEX-1 March 9, 1999 118min 367.8 438.5 Deploy HGS/go to nadir point DEX-247 March 29, 1999 Mapping configuration DEX-1000 May 29, 1999 Beta supplementconfiguration DEX-4110 Feb. 7, 2000 DEX-5000 April 20, 2000 End of nominal mission DEX-8506 Feb. 1, 2000

aMissionphases may be referred to as AB-1, AB-2, SPO-1, SPO-2, and MO. bPremappingorbits are labeled P-1 to P-1683,beginning with the periapsis of theMOI orbit.Mapping orbits are labeled DEX forDescending Equatorial Crossing. CDuringeach of the three aerobrakingperiods the orbit period and the apoapsisaltitude decreasecontinuously from the uppervalues to the lower values. image and thermal data during the rollout from the nadir points, 465 radio-occultationprofiles, and 1000 MAG low- positionto the Earth-point position.MAG/ER obtaineddata altitudepasses during this unexpectedbonus period of obser- throughoutthe entire orbit duringmost of thisperiod. Because vation prior to the primary mission.Moreover, atmospheric the HGA was not deployed,radio trackingwas limited to the data were obtainedover a range of daily times,other than the higher portionsof the orbit, where the spacecraftcould point fixed 0200/1400LT positionof the mappingmission. In addi- the HGA toward Earth. tion, data were acquired during three close encounterswith Science data were also obtained during the aerobraking Phobos. phases(AB-1 andAB-2), bothbefore and after the two periods Periodsof data acquisitionmay be referred to as the hiatus of science-phasingorbits (Figure 6). Although sciencedata period and the aerobrakingperiods (AB-1 and AB-2), sepa- acquisitionduring the aerobrakingphase was not in the orig- ratedby the science-phasingperiods (SP-1 and SP-2, separated inal missionplan, Mac, TES, ER, and the spacecraft'saccel- by solarconjunction). erometer and horizon sensorall acquired data, which were It was known from earlier missionsthat the atmospheric usedto predict atmosphericdensity on subsequentorbits. Dur- densityof Mars at the aerobrakingaltitude would showgreat ing the aerobraking passagethrough the atmosphere the variation over time as well as large orbit-to-orbit differences spacecrafthad the solar panelsin a V configurationwith the and that major dust stormsare most likely to developat the instrumentpanel in the lee direction.The accelerometerdata time of year at which aerobrakingwas to be initiated. So that provideddensity profiles of this atmosphericpassage. At the the spacecraftcould adjust its orbit, the densityhad to be closeof aerobrakingas the spacecraftexited the atmosphereit predictedfor each orbit in order to adjustthe spacecrafttra- rolled to point the HGA back toward Earth. During these jectoryto an appropriateand safedepth within the atmosphere rollouts,Mac and TES obtainedvisual and thermal coverage for the next aerobrakingpassage. More important,it was nec- over much of Mars. MAG/ER data were acquiredthroughout essaryto avoidany high-pressure excursions that coulddamage each orbit. Gravity measurementsand atmosphereprofiles the weakenedsolar panel and causeloss of the spacecraft. were obtained from radio tracking of the orbit, and acceler- Doppler tracking of the spacecraftand accelerometermea- ometer measurementswere obtained during the aerobraking suresprovided the key measurementsof the atmosphericden- portion of the orbits. MGS returned 2140 Mac images,11 sity. In addition,the scienceinstruments on the spacecraftas million TES spectra, 206 MaLA profiles with 2.6 million well as images from Hubble Space Telescopeand ground- ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,305

Begin Microprobe Moratorium Mapping Landing 6/22-7/13 EOM r- 3/9/99 1•4•1 •s• _l•u. V ß n n ,n ? Cover.I•orm. • ! OT!•I20TM3 ! 3/12f99 4/1/99 ... OTMs monthly... Maneuvers yz ! HGA Deploy ! I 3/29D9 ! ! BHGA ! 3]9-3/29 Mapping ! aMapping

Science

Events 'I

I-• ', ,o,•9 I I K• S&E27o,I 40K,, I 3/9/00 11111100 i I

i 512/99

I

Mars Geometry

I , •7 •7 I I I I I I I I MAR '99 JUN '99 SEP '99 DEC '99 MAR '00 JUN '00 SEP '00 DEC '00

Figure 7. Timeline for MGS mappingperiod showingscience campaigns, changes in data rate, and planned supportfor the Mars microprobeand Mars Polar Lander. Note that the formal end of missionwill be January 31, 2001 (PacificStandard Time). basedwhole-disk microwave observations were utilized to pro- duringaerobraking orbits (AB-1 or AB-2), brokenby the sci- vide advancewarning of dust stormsthat could trigger density ence-phasingorbits (SPO-1 or SPO-2), and broken in turn by excursionsat the aerobrakingaltitude. Plottingof densitydata solar conjunctionorbits (Table 5 and Figure 6), duringwhich against longitude displayedan unexpectedperiodicity with no data were returned. peaksnear 90ø and 270ø east longitude,and this dependency After more than 17 monthsin orbit, MGS reachedits map- wasutilized to providemore accurateestimates of atmospheric ping orbit on February19, 1999.For the first time a spacecraft density[Esposito et al., 1998;Keating et al., 1998]. had successfullydeployed aerobraking techniques to reach its During AB-2 the orbit period became shorter, and correc- desiredmapping orbit at Mars. Figure 7 and Table 6 showthat tionswere not made on everyorbit. During the final period of the orbit period had been reduced from 45 hours to 2 hours aerobrakingas the apoapsisreached 450 km the spacecraftwas during 299 days of aerobraking,which saved --•1250 m/s of "walked out" of the atmosphereby graduallyraising its peri- propellant.The MGS spacecraft(and the operationsteams) apsisto 143 km. A 2-day orbit lifetime constraintwas main- performed891 drag passes,executed 92 trajectorycontrol ma- tainedby makingfour smallburns so that if a spacecraftanom- neuvers,survived the onsetof a Martian duststorm that tripled aly occurred,there would be sufficienttime to recover and dynamicpressure values at aerobrakingaltitudes, and avoided perform a raise maneuver to maintain planetary protection collisionwith Phobosduring four closeencounters [Esposito et provisions.Aerobraking ended with a terminationburn (ABX) al., 1999;Johnston et al., 1999]. on February4, 1999,when the descendingnode had reacheda 0204 LT orientation, placing MGS in a 377 x 454 km orbit 7.3. Mapping (Table 5). At this point the periapsislatitude was --•60ø south, The MGS mappingmission was initiated on March 9, 1999, while continuingto drift --•3ø per day southward.On February after a period of final orbit adjustment,spacecraft checkout, 19, 1999, a transferto mappingburn (TMO) was executedto and instrumentcalibration (Figure 7). A problem was antici- place the spacecraftin the mappingorbit with periapsispas- pated with the strength of the damper used to control the sagelocked over the southpole. Continuousradio trackingwas deploymentrate of the HGA boom, sinceit was similar to the carried out betweenthe two burnsin order to provide a new one that failed on the solarpanel. As a contingency,one global and much more accurategravity field for Mars for use during data set was acquiredover a 3-week period with the HGA in subsequentmapping operations. the stowedposition. The spacecraftalternated between two The geometry for data acquisition during this period orientations:the standard nadir-mappingconfiguration for changedcontinuously. To indicatethis, data may be described nine orbits per day and one with the fixed HGA pointing as havingbeen acquiredduring hiatus (or assessment)orbits, toward Earth for data transmissionfor three orbits per day. 23,306 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

Table 6. ScienceCampaigns and Important GeometricEvents Subsequentto Mars Orbit Insertion

Date L s Event Comments

April 24, 1999 opposition Sun-Earth-Marsangle at 178.62ø May 3-30, 1999 135ø sciencecampaign northern midsummer/southernspring May 2, 1999 Mars to Earth range at minimum 0.578 AU distance July 21, 1999 begin no surfaceoccultation Earth beta angle at 64.4ø Aug. 1, 1999 autumn equinox start of southernspring Aug. 2-8, 1999 181ø sciencecampaign start of southernspring/northern fall Aug. 13, 1999 begin no atmosphereoccultation Earth beta angle at 68.1ø Aug. 18, 1999 solar beta angle at maximum beta angle at 39.61ø, solar eclipseof 35.5 mins Oct. 6, 1999 Earth beta angle at maximum Earth beta angle at 75.49ø Oct. 16, 1999 southern declination at maximum Mars declination at -25.21 as seen from Earth Nov. 24, 1999 end of no atmosphereoccultations Earth beta angle at 66.7ø Nov. 25, 1999 perihelion solar distance at 1.381 AU Dec. 6, 1999 end of no surface occultations Earth beta angle at 62.9ø Dec. 13-20, 1999 261ø sciencecampaign start of southern summer/northern winter Dec. 25, 1999 winter solstice start of southern summer April 1, 2000 solar beta angle at minimum beta angle at 16.80ø, solar eclipseof 40.5 min May 29-June 5, 2000 359ø sciencecampaign start of northern spring/southernfall June 18, 2000 northern declination at maximum Mars declination at -24.22 ø, as seen from Earth June 22, 2000 begin commandmoratorium start solar conjunctionperiod, Sun-Earth-Marsangle <2 ø July 2, 2000 solar conjunction Sun-Earth-Mars angle reachesa minimum of 0.56ø July 13, 2000 end command moratorium end solar conjunctionperiod, Sun-Earth-Mars angle >2 ø July 21, 2000 Mars to Earth range at maximum distance of 2.621 AU Sept. 11-18, 2000 45ø sciencecampaign northernmidspring/southern fall Nov. 3, 2000 aphelion solar distance at 1.666 AU Nov. 27-Dec. 4, 2000 81ø sciencecampaign late northern spring/southernfall Dec. 15, 2000 diametric occultation Earth beta angle at 0.0ø, Earth occultationof 41.3 min Jan. 24, 2001 Earth beta angle at extended Earth beta angle at -1.5 ø minimum

The HGA was finally deployedon March 29, 1999, and con- hasbeen utilized duringthese periods of specialgeometrical or tinuousnadir pointingoperations were initiated,requiring the seasonalinterest. Campaign A was termed the GeodesyCam- HGA to track Earth and return data at the same time that the paign and had the objective of obtaining global stereo and instrumentswere acquiringdata. color coverageearly in the mission[Caplinger and Malin, this Unfortunately,the full motion of the HGA azimuth gimbal issue]. was and is still limited by an obstructionof some kind. The Additional real-time downlink became available in the fall of limitation on the motion did not restrict simultaneous Earth 1999 as the Mars Climate Orbiter and the Mars Polar Lander tracking and data acquisitionuntil February 2000. It proved spacecraftapproached Mars. This was a result of technical possibleto work around this problem by driving the gimbal developmentsthat enabled the DSN to simultaneouslytrack from the oppositedirection, the Beta Supplementmode. How- and receivedata from more than one spacecraftwith a single ever, radio occultationmeasurements in both hemispheresare antenna station. Much of this additional downlink was devoted not possiblein this mode over an extendedtime period (Feb- to acquiring additional characterization of the Mars Polar ruary 7, 2000, to April 22, 2001). During this period it remains Lander landingsite. MGS also had the responsibilityof using necessaryto periodicallypoint the HGA toward Earth to ac- its onboardradio relay antennato relay data from the micro- quire suchdata. probesand the landerand, after their loss,continued attempts The mapping orbit has a nodal period of 117.65 min, an to establishcommunications with the landed spacecraft. 88-revolutionnear-repeat cycle lasting -7 sols(Martian days), The nominal mappingmission will end on February 1, 2001 and an indexaltitude of 378 km (Table 1). Figure 7 providesa (January31 at 1643LT) at the start of orbit 8506 (Table 6). timeline for the mappingphase of the missionand Table 6 lists The planned3-year relay period followingthe mappingperiod the importantgeometric events during the sameperiod. In the was establishedto use the spacecraft'sMars Relay systemto nominal mappingmode the spacecraftis continuouslynadir- pointed, enablingthe instrumentsto acquire and record data provide a Mars to Earth relay functionfor future lander mis- on a continuousbasis. Daily, the recorded scienceand engi- sions of the United States or of other nations. During this neeringdata (S&E 1) are transmittedback to Earth duringa period a maneuverwould raisethe orbit to a near-circularorbit single 10-hour tracking pass using a 34-m DSN station. In of 400 km or more to assurethat the probabilityfor the un- addition,real-time data (S&E 2) may be collectedand trans- sterilized spacecraftimpacting the planet meets the interna- mitted at a high rate during additionaltracking passes sched- tional requirementsfor planetary protection. However, the uled about everythird day. Three S&E 1 rates and two S&E 2 additional knowledgeof the atmospherefrom MGS may re- rates are used at various times during the missionsince the move the need for a raise maneuver and allow an extension of useful telemetry data rate varies substantiallywith distance the mappingmission. Potentially, the mappingperiod could be from Mars to Earth (Figure 7). Telemetryis interruptedfor a extendedto about August 2004, more than doublingthe num- period aroundsolar occultation. The scientificreturn hasbeen ber of mappingorbits over that achievablein the prime map- significantlyenhanced by the use of the sciencecampaigns ping mission.In addition,MGS could serveas a relay orbiter shownon the timeline (Figure 7). Continuous24 h/d tracking for the 2003 lander missions. ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,307

8. Overview of Scientific Results hemispheres;it further suggeststhat the hemisphericelevation differenceis primarily a long-wavelengtheffect due to large- Most of the scienceobjectives require mappingthat results scaledifferences in the crustalthickness. It hasbeen suggested in deriveddata setsthat havespatial and temporaldimensions. [e.g., Smith et al., 1999b] that the dichotomyboundary, as For the geoscienceobjectives, products are generatedthat manifestedin surfacegeology and regional topography,ap- depict surfaceand subsurfacecharacteristics as a functionof pears to contain three dominant contributions:(1) volcanic latitude, longitude,elevation, and season.For many climatol- constructionsassociated with Tharsis,(2) major excavatedde- ogy objectivesthe data sets are organizedby latitude, longi- positsapproximately circumferential to Hellas, with additional tude, altitude, and season.MGS has alreadyobtained a sys- contributionsfrom Isidisand probablyUtopia ejecta,and (3) tematicglobal characterization of Mars as it existstoday. This modificationof the interveningregion by fluvial processesas- characterization,supplemented by the data from the remain- sociatedwith the outflow channelsthat empty into Chryse der of the mission,will dramaticallyincrease our understand- Planitia. ing of the geologicand climatologichistory of Mars and the Knowledgeof both the gravityfield from RS and the surface evolutionof its interior and surfaceand will providedatabases topographyfrom MOLA at similarscales is key to understand- for comparisonwith Venus and Earth. ing the internal structureof the planet. Removal of the grav- itational signal due to the mass of the topographymakes it 8.1. Geoscience possibleto gain some understandingof the internal density Mars is a single-plateplanet, with a surface area that is anomaliesthat are associatedwith thermal or compositional slightlylarger than the combinedarea of all the plates that differences.MGS observationsof the gravity field of Mars make up Earth's continents,-144 million squarekilometers. [Tyleret al., this issue]show that the anomaliescorrelate well The systematicmeasurements made by MOC, MOLA, TES, with the principal featuresof the topography,including volca- MAG/ER, and RS have dramatically increased our under- nic constructs,impact basins, and Valles Marinei-is.However, standingof the geologyand geophysicsof the Red Planet. In the planet has respondeddifferently in its northern and south- the subsectionsbelow we summarizesome key resultsobtained ern hemispheresto major impactsand volcanicprocesses. The by the end of the first year of mapping. rough, elevated southernhemisphere has a relatively smooth 8.1.1. The Martian crust and interior. The most distinc- gravitationalsignature, indicating a stateof near-isostaticcom- tive and enigmaticfeature on the surfaceof Mars hasbeen the pensation,whereas the low, flat northernplains display a wider hemisphericdichotomy between the old, highlycratered south- range of uncompensatedgravity anomaliesthat indicatesa ern highlands(i.e., Noachian age crust) and the younger, thinner but strongercrustal layer than in the south. To first sparselycratered northern plains. MOLA data havebeen used order, the 6-km differencein elevationfrom pole to pole can to producea very high fidelity map of the shapeand topogra- be accommodatedby a 30-km thinning of the crust from the phyof Mars [Smithet al., thisissue (b)]. The globaltopography southpole to the north pole. In general,the dichotomybound- of Mars is now known to greater accuracythan Earth's conti- ary does not have a distinctgravitational signature. Mascons, nents in a root mean squaresense [Smith et al., 1999a].The gravityhighs associated with topographiclows, are presentin ellipsoidalshape is flattenedby -20 km owingto rotation,and the Argyre, Hellas, and Isidisbasins. The northernhemisphere the centerof figureis offsetfrom the centerof massby 2986 m showsevidence for buried impact basins, though none are alongthe polar axis,indicating that the north pole is -6 km large enoughto explainthe hemisphericelevation difference. lower than the southpole. As pointed out by Bills and Ferrari The gravitationalpotential pattern of Tharsisis approximately [1978], this offset requires that the densitydistribution for axisymmetricand contains the Tharsis volcanoes, but the Mars is not radially symmetricand is consistentwith a thicker OlympusMons and Alba Patera volcanoesseem to be adja- crust in the southernhighlands. In addition, the center of cent, separatefeatures. The gravity signatureof Valles Mari- figure is offset from the center of mass-1428 m toward the nerisextends into ChrysePlanitia and providesan estimateof Thatsis topographicrise. material removedby early fluvial activity. The topographyof Mars has a range of 30 km from the MAG/ER observationsduring the circularizationphase of lowestpoint in Hellas to the top of OlympusMons. This range the missionconclusively showed that Mars now lacksa global is the largestfor anyterrestrial planet and muchlarger than the internalmagnetic field [Acu•a et al., 1998].This resultresolved 20 km rangeof Earth. MOLA data alsoshow that the northern a long-standingcontroversy and placesan importantconstraint lowlandsare exceedingsmooth and level, and if Mars had on the present nature of the Martian core. More important, runningwater, water would collectin theseareas after draining however, MAG/ER observationsshowed crustal magnetic from the southernhighlands. The Hellas impact basin has a anomaliesof surprisinglyhigh strength in the older rocksof the total relief of more than 9 km. The main basin with a diameter southernhemisphere [Acu•a et al., 1999]. These anomalies of -2300 km is surroundedby a highlycratered annuluslying demonstratethat Mars had an early internal dynamo.The lack -2 km abovethe edge of the main basin.Material thrown out of suchhigh magneticstrength in the northern lowlandsand from Hellas accountsfor a significantcontribution to the the major volcanicconstructs associated with Tharsisimplies higher topographyof the southernhemisphere. that the internal dynamohad ceasedto operateby this time of The hemisphericdichotomy marks both a differencein ele- formation of these structures. vation and a differencein geologicsetting. Various hypotheses MAG/ER mappedparallel bands, -200 x 1000-2000 km, in to explainthese differences have included(1) thinningof the the southernhighlands that showreversals in the radial mag- northernhemisphere crust by mantle convection,(2) an early netic field directionfrom inwardpointing to outwardpointing period of tectonicplate recycling,and (3) large impactsin the in adjacentbands [Connerney et al., 1999]. Some showa mag- northernhemisphere. The MOLA data demonstratethat the netic moment an order of magnitudegreater than any known offset between the center of figure and the center of mass on Earth. The linear pattern, the reversalin polarity, and the accounts for much of the elevation difference between the two size of the magneticmoment each presentproblems in inter- 23,308 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION pretationthat are not yet resolved.On Earth, linear magnetic activity and possiblyat the present time (Figure 8b). Also, patternsof alternatingpolarity are associatedwith seafloor analysisof crater populationson high-resolutionMOC images spreadingand repeatedreversals of the globaldipole field of indicates that some lava flows within the Arsia Mons caldera Earth. Molten magma is injected from below, coolsbelow its may be as young as 40-100 million years [Hartmann et al., Curie temperature,and acquiresa permanent remanent mag- 1999]. netic moment aligned with the terrestrial dipole field. The TES spectral data indicate that there are two somewhat typicalspreading rate and the typicalreversal period lead to a different rock compositions,both basicallybasaltic, whose dis- characteristicwidth of ---10 km for suchfeatures. By compar- tribution can be spatiallymapped in the low-albedoareas over ison, Mars would require a faster spreadingrate or a longer the surface[Bandfield et al., 2000; Christensenet al., 2000b].In periodbetween magnetic reversals. The magnitudeof the mag- general, and½siticcompositions occur in the northern plains, netic moment introducesother problems for such a simple and basalticcompositions occur in the southernhighlands. In explanation.The moment,which is the productof the magni- fact, theseresults are consistentwith alpha proton X-ray spec- tude and effectivevolume of coherent crustalmagnetization, trometer (APXS) analysesof rocksat the Pathfinderlanding must be at least an order of magnitudegreater than for crustal site and rcanalysisof Viking Lander X-ray fluorescencespec- rocks on Earth. For strengthof magnetizationsimilar to ter- trometer (XRFS) data [Larsenet al., 2000]. The high-albedo restrialupper crustthe depth of coherentmagnetization would areas provide nondiagnosticspectra similar to that of atmo- have to be ---30 km. This would require an exceedinglyhigh spheric dust. Analysesof TES infrared spectra also demon- rate of heat loss since the crust must cool to a temperature stratefor the first time the abundantpresence of pyroxcneand lower than the Curie temperatureof the dominantmagnetic plagioclas½feldspar, the low abundanceor lack of quartz, and mineral throughoutthis entire depth interval on a timescale the absenceof a pcrvasiv½lyweathered surface, at least in the shorterthan the interval betweenreversals. Alternate explana- darker areas. The landers have shownthat the soilshad high tionsrequire an unusuallyhigh concentrationof the magnetic iron, sulfur,and chlorineand that the soil particlesare strongly mineralsor an exceedinglyvigorous convection leading to a magnetic,probably owing to an iron pigment. The soilsare very strongdynamo. Acura et al. [1999] suggestedan alterna- commonlyinterpreted as secondaryweathering products, such tive to reversalsof the dipole field. A thin, highlymagnetized as sm½ctit½,of mafic igneousrocks, possibly owing to palago- plate, broken into linear blocksand separatedby injectionof nization of basalt. However, a high abundanceof weathering material of lower magnetism,would showa similarpattern of mineralsis not confirmedby the remote-sensingobservations. reversalsin the radial magnetic field at the orbital altitude. Spectralmapping has establisheda unique area, ---350 x 350 However, at this point no fully satisfactorymodel is able to km alongwith severalsmaller nearby areas,that has abundant explainthese new observationsof the magneticfield. relatively coarse-grainedhematite [Christensenet al., 2000a, Magnetic anomaliesare not associatedwith the Argyre, Hel- las, or Isidis basins, suggestingthat the impact events that 2000c].The presenceof thismaterial in a thin fiat-lyinglayer suggestsan aqueousorigin. However, the searchfor carbonate, producedthese features postdate the cessationof dynamoac- sulfat½,or quartz-richrocks has been fruitlessdespite careful tion. Furthermore, the linear magneticpattern has been de- stroyedin the vicinityof Hellas, either throughreheating above analysesof many million spectra. the Curie point or by mechanicaldestruction of the coherent The MOC high-resolutionimages show an incredible array magnetization.The 9 km of relief associatedwith the Hellas of dcpositional and ½rosionalaeolian landforms, including basin impliesthat the lithosphereat the time of its formation dunes, sand sheets,and yardangsas well as a pervasivethin was of substantialthickness and possessedlong-term strength. layer of surfacedust. The nature and origin of neither the sand SinceHellas and its ejecta are extensivelymodified by crater- nor the dustis known.Ruff and Chtistensen[1999] haveshown ing, the magneticand topographicevidence both suggestthat that infrared spectrafor the bright and dark albedopatches on the crust in the southern hemisphere has cooled rapidly Mars are well matched by a fine and a coarseaggregate, re- enoughto achievea substantialthickness prior to the end of spectively,of pyroxeneand plagioclase.Thus the brightregions heavy bombardment. as well as the dark regions,and muchof the soil and dust,may 8.1.2. Geologic processes. Analyses of MOC data have be simplybasaltic material ground up by impact processesor shownthat layeredsequences of rock are muchmore common physicalweathering. The light coloreddunes may simplycon- than previouslythought in the uppercrust of Mars [Malin et al., sist of sand-sizedaggregate particles. Albedo mapping shows 1998;Malin and Edgett,2000b; Malin and Edgett,this issue].A that significantchanges in the pattern of light and dark mate- commonview was that the northernplains region was covered rial have occurredsince the Viking observations,indicative of with volcanicflows but that the heavilycratered highlands are currentaeolian activity [Jakosky and Mellon,this issue;Jakosky a megabrecciaof primordial crust. However, layered crust is et al., 2000]. Both albedo and thermal inertia mapping are exposedto a depth of almost 10 km on the walls of Valles being carried to higher resolution as the mapping mission Marineris, suggestingthat thick sequencesof volcanicrocks proceeds. may be presentwithin the southernhemisphere (Figure 8a). 8.2. Climate Further, the data showdifferent thicknessesand typesof layers acrossthe planet, somewith different albedo and some with Major MGS scienceobjectives are to addressthe volatile different resistanceto erosion [Malin and Edgett,this issue]. (carbondioxide, water) and dustcycles and the Martian atmo- These characteristicssuggest that layersare composedof vol- sphere over the course of a Mars year from the Sun- canic tuffs, aeolian deposits,or both. Complex sequencesof synchronouscircular orbit. The term climate is used because eventsand processescan be discernedin some images.For the emphasisis on the seasonalvariations rather than the daily example,partially denudedcraters reveal a period of impact variations of the atmosphere.However, the additional year into horizontallayers and subsequentcover by a thick friable spent in the elliptical orbit also allowed acquisitionof atmo- layer that hasnow been partially removed,probably by aeolian sphericdata at different times of day. ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,309

•...... •'•?"-*?•...... •:"'-'-:"•: .... ' ...... "..'•'-:' ...... '...... ':•..'•.::-:•:•!?•,.,,.:.'.•ø;.,'•:•t•:i•Z•':?•;;:...;; ...... :'•tT":'7 :•½•.:r•:.½•:½•.... •. • .... :. ••••••gt"•-*•:•' :'•t•':t•:½::•• ...... • ...... [11111[[11[I [I[ '1 [111111111111[[11111['[[[[111[ ]1 • ...... '•••••••""'"w'•'"• •":'•::•4•4•'"•'•'•'•. :':•t":"•'- " .... ½•:½•" • .... .'•'•.;•:•:•/Z•'<*•:";•....*•; ..... ;;;'" "•" '; '"'•;O•g'•"•;•'•:•:•2.•.'-• :•-:;•4.;•'-- ': ......

Figure 8. MOC imagesshowing layering: (a) a thicklayered sequence exposed in steepslopes surrounding a plateauwithin the VallesMarineris and (b) an e•umed craterwithin thinly layered deposits in KaseiVallis. 23,310 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

(b)

Figure 8. (continued) ALBEE ET AL.: OVERVIEW OF THE MGS MISSION 23,311

8.2.1. Structure and dynamics of the atmosphere. Al- provided seasonal and spatial global plots of temperature, though Earth and Mars have evolved atmospheresof very dusty content, cloud abundance,water vapor, and calculated different compositions,it was predicted and has been shown thermal winds [Smith et al., this issue(b)]. RS has provided that they have similar global circulationpatterns [Haberle, hundredsof precisevertical profilesof temperatureand pres- 1986]. Both have atmospheresthat are nearly transparentto sure in the lower atmosphere[Hinson et al., 1999]. sunlightand store almostno heat. Sunlightpasses through the Among other things, such data confirm the existenceof atmospherewithout heating it and heats the surface,which cross-equatorialHadley circulationduring part of the year, as then heats the atmospherefrom below. The thin atmosphere predictedby models.The numericalmodels typically utilized a and lack of oceansmean that temperature changesrapidly in fixed amountof dust uniformlydistributed in the atmosphere responseto diurnal and seasonalchanges in solar input. Both to meet thermalconstraints that requiredsome direct warming planetshave an obliquitythat leadsto significantlymore heat- of the atmosphere.More recentmodels have injected dust into ing of the surfacein the tropicsthan in the poles;the resulting the model at particulartimes and locationsor under appropri- temperaturegradient produces a pressuregradient that drives ate conditionsof calculatedsurface stress. New data providean a north-south(meridional) circulation.The planets have a intercomparisonof atmospherictemperatures derived from similar rotation rate, which determinesthe magnitude of the ground-basedmillimeter, TES, and Viking data, providing Coriolisforce deflectingthe movingair masses.Air riseswhere data on seasonaland interannual variability of temperature the solar heating is strongestand movestoward the poles and and dustloading [Clancy, 1999; Clancy et al., 2000;Smith et al., cools,sinking in the subtropicsto form circulatingcells (the this issue(b)]. MGS observationsshow that the atmosphere Hadley cells). On Mars, in the absenceof oceans,the rising can change rapidly from warm and dusty to cool and clear branchclosely follows the subsolarpoint, possiblyas far as 25ø [Conrathet al., 2000]. As was expected,the atmospherein the from the equator, and probablyleads to a singletransequato- southernsummer is relatively dusty and warm, but the north- rial Hadley cell during part of the year. The Coriolis force ern summer is cooler than expectedwith abundantwater ice deflectsthe northern moving upper branch to the east (the clouds.It is now clear that the radiative effects of the very westerlies)and deflectsthe equatorwardmoving lower branch common water ice clouds, and of the carbon dioxide ice clouds to the west (the easterlytrade winds). seen in the polar regions,will have to be included in future At middle and high latitudes,winds are predominatelyfrom modeling. the west, both at the surface and aloft. A band of intense winds Observationsduring the aerobrakingperiod of MGS have (the jet stream)blows in the upper atmosphereat an altitude providednew insightinto the physicalprocesses connecting the of ---30-40 km. Nearer the surface, high- and low-pressure upper and lower atmospheresof Mars [Bougherand Keating, storm systemsmigrate eastward,their strongwinds transport- 1999]. The period of MGS aerobrakingwitnessed the onset, ing heat toward the poles. These stormsare more regular in rise, and decayof a regional dust storm in the southernhemi- their timing on Mars, probably owing to the more rapid re- sphere.Rapid heatingand expansionof the lower hemisphere sponseof the atmosphereto temperatureperturbations. The expandedfaster than the actual increasein dust content. Ac- strengthof the jet stream dependson the latitudinal surface celerometerdensities at 130 km increasedby a factor of almost temperaturegradient, which on Earth tendsto be buffered by 3 over a 2- to 3-day period as the atmosphereexpanded by 8 the oceans.However, on Mars the winter seasonalpolar caps km. The decay of the effects of the storm took almost 2 extendto the midlatitudes,and the high temperaturecontrast months.Continued observations have led to the discoveryof a between carbon dioxide ice and adjacent soil leads to jet complexdensity wave structurein the upper atmosphere. streamsmany times strongerthan on Earth. However, in sum- Topographyis a major boundarycondition for global circu- mer, when the ice cap recedes,the surface soil has a more lation models and critical for mesoscale models, and the uniform temperature, and the jet stream and the associated MOLA altimetrydata are beingincorporated into thosemod- low-level stormsdissipate. els. The large relief near the dichotomyboundary and in the The highlyelliptical orbit of Mars meansthat its seasonsare Tharsisregion affectsthe lower branch of the Hadley circula- of unequal duration and intensity.Compared with the north- tion cell and produceswaves in the upper atmosphere.De- ern seasons,the southernspring and summerare shortand hot tailed topographyof both polar regionsand of the Hellas and and the fall and winter are long and cold. At perihelion, Mars Argyre basins is facilitating an understandingthrough me- receives---40% more solar input than at aphelion;the differ- soscalemodels of the role of downslopewinds in initiatingdust encefor Earth is only ---3%.This asymmetryhas influence on storms.We can also expect that the detailed thermal inertia the structureand dynamicsof the atmosphere,but it hasmajor maps [Jakoskyand Mellon, this issue],indicative of local dif- influenceon the seasonalcycles of carbondioxide, of dust,and ferencesin daily cooling and heating rates, will also be incor- of water. Mars is unique in that almost25% of its atmosphere porated into the mesoscalemodels. cyclesin and out of the carbondioxide ice polar capseach year, 8.2.2. Seasonal cycles of carbon dioxide, water, and dust. causinga like globalvariation in surfacepressure. Owing to the The general circulationof the presentMars atmospherecou- longerfall and winter, the southpolar ice cap growslarger than ples togetherthe seasonalcycles of carbondioxide, water, and the north polar cap, reaching---45 ø latitude. dust.Most obvious,of course,is the waxingand waningof the Models of the general circulation of the atmosphere of seasonalcarbon dioxide ice caps as nearly a quarter of the Mars, like the preceedingdescription, have been developedby atmospherealternately precipitatesand sublimatesfrom the a number of groups.These numericalmodels have reacheda surface.The abundanceof water in the atmosphereis closely highlevel of maturity,but theywere basedon surprisinglylittle coupled to the cycle of carbon dioxide. The maximum abun- data. For example, the single "classic,"but incomplete,tem- danceof water occursduring summer over the north pole. The perature profile from pole to pole is now replaced by thou- permanent(residual) water ice capbecomes a sourceof atmo- sandsof TES infrared profilesthat provide spatial and tempo- sphericwater during the summer,when the seasonalcarbon ral coverageover most of the Martian year. MGS has now dioxidefrost has sublimatedaway and exposedthe water ice. 23,312 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION

Viking observationssuggested that water ice was not exposed global storm every year, and what shutsoff the storm once it at the southernice cap in summerbut that it remainedcovered starts?What are the mechanismsand processesthat determine by carbon dioxidefrost. If so, it would be a water vapor sink whether a large local storm dies or whether it growsinto a growingat the expenseof the northern cap.Whether or not the regional or even a planet-encirclingstorm? carbondioxide ice coversurvives every summer is not yet clear, The layering in the polar depositsprovides evidence that but observationsover the period of the Mars SurveyorPro- settlingof dust and ice is and has been an importantprocess. gram shouldclarify this issue. It can be assumedthat dustparticles form nuclei for water and The permanentpolar ice capsare the largestknown reser- carbondioxide ice precipitationat times,but the importance voirs of water on Mars. Visually,the high-albedosouthern cap and timing of this processin the seasonalcycles of dust,water, appearssmaller, but new topographyindicates that ice, in part and carbon dioxideis not clear. The high-albedoresidual ice coveredby dust,forms a tabular disc,extending from eachpole cap overlieslayered deposits,thick stacksof alternatinglight to •--85ø latitude with a relief of about 800-1000 m at the edge and dark layerswith individualbands extending down to the [Smithet al., 1999b;Zuber et al., 1998].The similarityin topo- limit of resolution. High-resolution MOC images show an graphicprofiles and the comparativerheology of water ice and abundanceof pitted features, different in the north and the carbon dioxide ice suggestthat both permanent caps consist south,and presumablyformed by a sublimationprocess. Re- predominantlyof water ice. The permanentsouth polar cap is sidual ice dominatesnear the pole, and layered depositsbe- water ice rather than carbondioxide ice as previouslybelieved come more abundantaway from the pole, within troughsthat by many workers. However, TES thermal observations,like form the distinctiveswirl pattern and as a continuousfeature thoseof Viking, are currentlyconsistent with a surfacecarbon around the edge of the cap. The compositionof the layersis dioxide frost cover in all seasons.Depending on certain as- uncertain;wide proportionsof red dust, sand, ice, and void sumptions,the northerncap has a volumeof 1-2 x 106km 3 spacehave been suggested.Peripheral are circumpolardepos- [Zuberet al., 1992],and the southerncap has a volumeof its, commonlymantled by dunesand sandsheets. Fretted fea- 2-3 x 106km 3 [Smithet al., 1999b].A large,but unknown, turesand the shapesof cratersindicate that thesedeposits are volume of water is stored as ground ice, and most modelsof alsounderlain by or containabundant ice. MOLA topography the seasonal behavior of water include a substantial seasonal indicatesthe presenceof esker-like and kame-like features, interchangeof water betweenthe regolithand the atmosphere. suggestingthat the edge of the discof layered depositsrepre- Widespreadobservations of low-lyingwater ice cloudsseem to sentsthe edgeof a retreatingice cap, frozen in spaceas a relic substantiate that model. (J. Head, oral communication,2000). The dust cycleis very closelycoupled to the general circu- 8.2.3. Water on Mars. Water as vapor and solid, when lation becauseof the feedback between dust lifting and the not in the sunlight,can be presentin the atmosphereand polar intensityof circulation.Dust raised by the winds tendsto in- regionsof Mars, but liquid water is not stable under present tensifythe windsin a feedbackmanner, because the dust ab- atmosphericconditions. Calculations indicate that groundice sorbssolar radiation and heats the atmospheredirectly. MGS showed that the onset of a dust storm in the southern hemi- shouldbe stablethroughout the much of the year in the mid- latitudesand over the entire year in the polar regions.Obser- spherewas observed within daysas a markedpressure increase in the upper atmosphereof the northern hemisphere.Global vationsof patternedground, from Viking and MOC, indicate dust stormsoccur predominantly during the southernspring the currentor recentpresence of groundice in the soil.Abun- dant evidence has been presented to show that water also and summer,when Mars is near perihelion, becausethat is when the heatingof its atmosphereand the intensityof circu- existedin the past in the form of ground ice and groundwa- lation are greatest.Viking measurementsof the attenuationof ter-at times catastrophicallyreleased--and possiblyeven as sunlight suggestedthat the atmosphere never completely standingbodies of water[Carr, 1996]. The channelsprovide the cleared of dust in contradiction to reasonable settling rates. most dramatic evidencefor water on Mars; water emerged However, MGS has shown that water ice clouds are abundant, from the highlandssubsurface, carved channelsas it flowed contributingto the attenuation of sunlight, and in addition downstream,and emptied into the northern lowlands,where must play an important role in controllingthe vertical abun- the broad channelsend abruptly, possiblydebouching into a dance of dust. body of water. Less dramatic are the smaller valley networks, An important finding of MGS is the extent to which aeolian whosepattern suggestedthe possibilitythat they were formed processeshave modifiedthe surface[Malin et al., 1998]. Al- by surfacerunoff from rainfallduring a climaticperiod during most everywhereone looks, there is evidenceof atmosphere- which the atmospherewas denseenough and warm enoughto surface interaction in the form of dunes, sand sheets, wind stabilizeliquid water. erosion features, and even active dust devils and small local- MOLA topographyand MOC images are permitting the ized storms. In several places it has been shown that sand detailed mappingof the sinuousnetworks of outflowchannels transportis occurringcurrently. In general, dust is not lofted and ephemerallakes, their sources,and their final destination directly (it requiresvery strongwind), but dust is probably in a northern ocean [Head et al., 1998; Parker et al., 1989] injectedinto the air during saltationof sand-sizedmaterial. In (Figure9). Eventually,these studies will lead to understanding the past our thinking might have attributed most surfaceac- the heat source and the volume of ice that had to be melted to cumulationof atmosphericdust to the slowsettling of fine dust provide the necessarywater and the timing of theseevents in after great storms,but the role of small and regional dust the variouschannels. As yet, speciallytargeted MOC images storms(a continuumof scales)must now be considered.Mod- [Malin and Edgett, 1999] provide little evidencefor the pro- els of surfacestress using MOLA topographyare beingused to posed shorelineof an extensiveocean [Parkeret al., 1993]. predictlocations of evolvingdust storms, and thesepredictions However,MOLA topographysuggests the presenceof a bench will have to be confirmedby visualobservation. This approach at constantaltitude around the lowest parts of the northern shouldhelp to clarifysome major questions:why is there not a plains [Head et al., 1998]. Patternedground and crater char- ALBEE ET AL.' OVERVIEW OF THE MGS MISSION 23,313

From: Parker,Clifford, and Banerdt, Lunar and Planet• Sci, Conf.XXXI, March2000 Figure 9. Networksof valleysand ephemerallakes draining northward from Argyre Basininto the northern plains of Mars. 23,314 ALBEE ET AL.: OVERVIEW OF THE MGS MISSION acteristicssuggest the presenceof groundice within the north- eruptions,asteroid impacts, etc.). An improvedunderstanding ern plains,possibly residual from sucha bodyof water. of climatechange (with consequenteffects on biologicalpro- The high-resolutionMOC imageshave shown that the valley cesses)may be the most important contributionof MGS to networksseen in the Viking imagesare typicallynot dissected exobiologicscience. with the hierarchyof tributariescharacteristic of valley net- works formed from surface runoff [Malin and Cart, 1999]. Instead, they terminate in stubbybranches, typical of valley 9. Summary networksin regionsof groundwatersapping or of thermokarst As of June 1, 2000, the Mars Global Surveyorspacecraft had terrain. However,some MOC imagesdo showsinuous patterns completedover 14 monthsof operation(5500 orbits) of the suggestiveof sustainedfluvial flow and erosionover a periodof planned687-day Mars year of mapping.In addition,1 year of time, possiblyunder an ice cover.Moreover, high-resolution aerobrakingand science-phasingorbit observationsalso con- MOC images[Malin and Edgett,2000a] showgullies on slopes tributed to a unique and unexpecteddata set. Global maps of suggestiveof localizedmelting, seepage, and outflowof shal- the topography,the gravityfield, the magneticfield, the surface low ground ice. mineral compositionand thermal inertia, the albedo, the at- It shouldbe recognizedthat it took many years of studyof mosphere,and imaginghave been obtained.In addition,high- the Viking and Mariner 9 imagesto achieveour current un- resolutionimages have been obtainedfor -0.1% of the planet. derstandingof thesecomplicated processes. It is clear that it Whatever its future, Mars Global Surveyorhas demonstrated will also take a long time to studythe tremendousnumber of that a singlesuccessful mission can reshapeour understanding MOC imagesthat have been and will be returned to Earth. of a neighboringplanet. The differencethis time is that now all readersof this paper Note added at final submission:On February 1, 2001, at can quicklyretrieve some of the more than 70,000images for 1634 LT (PacificStandard Time) MGS completedits 8505th themselvesfrom the PlanetaryData Systemarchives and make orbit, marking the end of its primary mission,and embarked their own interpretations. upon an extendedmission. On June2, 2001, at 2138 LT (Pa- Despite the ambiguityin interpretingthe valley networks, cific Daylight Time) MGS completedorbit 10,000 with all there still seemsto havebeen an apparentchange in the mod- instrumentscontinuing to return data. ification rate of craters early in Mars history near the end of the heavybombardment. This observationretains the possibil- ity of the current thinking,i.e., that for most of the period of Acknowledgments. The research was carried out by the Mars heavybombardment Mars waswarm and wet and had a thick Global SurveyorProject at the Jet PropulsionLaboratory, California Instituteof Technology,under contract with the National Aeronautics atmosphereand that towardthe end of this period the atmo- and SpaceAdministration. A.A. wasProject Scientist, F.P. wasDeputy spherethinned greatly as carbon dioxide was removed and the Project Scientist,and T.T. was ScienceManager and subsequently surface cooled. Subsequently,perhaps for nearly 4 billion Mission Manager and Project Manager of the Project. R.E.A. was years,the Martian surfacehas been cold and dry, and aeolian supportedas a Mars SurveyorProgram Interdisciplinary Scientist by JPL contract1204044 to WashingtonUniversity. The authorsacknowl- action has been the dominant surfaceprocess. Clearly, the edge the contributionof the entire projectstaff, includingtheir col- ongoingsearch for carbonatedeposits on the surface,although leaguesat LockheedMartin Aerospaceand at all the homeinstitutions so far fruitless,remains very importantto the interpretationof of the instrumentPrincipal Investigators and the Radio ScienceTeam the history of water on Mars. Leader. Many of the key projectmembers and scientistswere part of It is generallyunderstood that the presenceof liquidwater is the early studiesin 1981-1982that led to Mars Observerand then to Mars Global Surveyor.Glenn Cunninghamwas projectmanager dur- critical to the evolution and retention of life. For this reason, ing the period from when Mars Observerwas lost until the beginning NASA's Mars Program has identified the increaseof our un- of the mappingperiod. His contributionto the amazingsuccess of derstandingof the historyof water on Mars as its principal MGS was outstanding;upon three occasionshe made difficult deci- crosscuttingand unifyingtheme. Many MGS observationsare sionsthat preservedthe scientificintegrity of the mission.As space- craft systemmanager throughout Mars Observerand MGS develop- directed toward trying to identify placeswhere liquid water ment, George Pace was always watchful for "creeping science might have existed in the past, e.g., shorelines,hot spring requirements,"but his selectionof a largerHGA nearlydoubled the deposits,carbonate deposits, etc. Suchareas would have a high capabilityfor sciencedownlink. Much of this work was drawnfrom a priority for future samplereturn missionsthat would seekfor varietyof projectdocuments and we thank their authors,particularly evidenceof past life on Mars. Evidencefor localizedground- Glenn Cunningham,Pat Esposito,Wayne Lee, and George Pace. Pat Espositocarefully checked the sectionson missionand missionoper- water seepsmay be of great importance.However, in general, ations.We thank Pat Esposito,Wayne Lee, Mike Malin, Tim Parker, liquid water, derivedfrom ground ice, has had an ephemeral and SusanSlavney for help in figure and table generation. presencein the presentclimate. Moreover, the new observa- tionsextend the period of cold and dry climateand do seemto pushany warmer and wetter climateperiod further back into References the early period of heavybombardment. At this point we are a Acton, C. H., Jr., The SPICE concept:An approach to providing long way from understandingthat criticalperiod in Mars his- geometricand other ancillaryinformation needed for interpretation tory.How did groundice becomewidely distributed? 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