Viking Press Handbook

8/8/2019 Viking Press Handbook

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VIKING PRESS HANDBOOK

Information Contacts:Nicholas PanagakosHeadquarters, Washington, D.C.(Phone: 213/354-6000 at JPL)

Maurice ParkerLangley Research Center, Hampton, Va.(Phone: 213/354-6000 at JPL)

W illiam Der BingHeadquarters, Washy ngton, D.C.(Phone: 202/755-3680

NOTE TO EDITORS: This press handbook contains detaileddescript ions of the various phases of planetary operationsrelated to th e Viking mission to Mars. It may be used asa source of information independently, or in conjunctionwith th e Viking Encounter Press Kit.

CONTENTSAPPROACH PHASE: June 9 Through June 19, 1976 ........ 1-6

Operations ........................................ 1-2Science............................................ 2-6MARS ORBIT 0ISERTION: June 19, 1976 ................ 6PRELANDING ORBITAL ACTIVITIES: ......................... 8-23

June 19 (MOI) Through July 4 (Separation) ........... 8Landing Sites...................................... 8-14Imaging From Orbit for Site Certification ........... 14-16Infrared Observations and Monitoring ................ 16-20Orbit Trim......................................... 20-22Pre-Separat ion Activities ............................ 22-23

SEPARATION: July 4, 3:38 p.m. PDT .................. 24

RELEASE NO: 76-116

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DESCENT AND LANDING: Jul- 4, 3:38 p.m. PDT(Separation) to 6:58 p.m. PDT (Touchdown) ......... 24-41Operation........................................... 24-33Science............................................ 3A-41

SURFACE OPERATIONS: ...................................... 42-56Sol 0 July 4, 6:58 p.m. PDT ThroughJuly 5 0:20 a.m. PDT ................................ 42-47

Sol 1 Through Sol 7 (July 5 Through July 12) ...... 47-48Sample Site Cerification......................... 49-54Science Experiments ............................... 55-56

SURFACE SAMPLING AND OPERATIONS STARTINGSOL 8 (JULY 12) .................................... 57-72

ORBITER SCIENCE AND RADIO SCIENCE................... .. 2-76VIKING 2 .............................................. 76-78DEEP SPACE NETWORK................................... 78-80DATA HANDLING ....................................... 81-86

Commanding and Controlling the Spacecraft...........83-86Prime Shift Schedule ...................... 86

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VIKING MARS ENCOUNTER

APPROACH PHASE: June 9 Through June 19, 1976Operations

A final course correction is made 10 days beforeViking 1 is scheduled to enter orbit around Mars. Thisapproach midcourse correction maneuver (AIC) consists ofa small change in velocity and direction to ensure thatthe Mars orbit insertion maneuver (110I), some 10 dayslater, results in an orbit that permits coverage of theprime landing site (A-1) in Chryse. The orbit will beadjusted as required after MOI.

Instructions fo r MOI were sent to th e spacecraft andloaded into its computer on May 22 and are enabled a dayafter the final AMC. These instructions are sufficient forthe spacecraft to enter an orbit around Mars even if no fur-ther commands can be sent to it before the date of the MOI.This safeguard is essential to the mission because Vikinghas only one opportunity to enter orbit around Mars. Shouldthis be missed the spacecraft would fly by the planet andcontinue on its orbit around th e Sun.The commands instruct the spacecraft on how to orientitself so that its rocket engine can thrust in the rightdirection, how long the rocket engine must thrust, and howthe spacecraft is to be reoriented after th e rocket firingis completed.During th e interplanetary cruise phase of the Vikingmission radio science investigations using a dual frequencydownlink from the spacecraft on Earth (X-band and S-band)measured properties of the interplanetary medium, particularlythe total electron content and its variations, are determinedby analyses of the differences in signal properties on thetwo downlink frequencies. From these measurements, the inten-sity, size and distribution of electron streams from the Sunand from solar storms are studied to increase understandingof the Earth-Mars region of the in:erplanetary space.Basic tracking data to navigate Viking towards Marsconsisted of very precise measurements of distance (range)and line of sight velocity (range-rate) between the space-craft and the Earth-based tracking stations. Range andrange-rate measurements are the primary data used to deter-mine orbi t and trajectory characteristics.

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NNow during the final approach to Mars, the radiosignals are used to determine the precise path of the spaci-craft after the final AMC so that it can he commanded intothe correct orbit around Mars by the MOI maneuver.The instructions for 4OI canbe updated five daysbefore it takes place. This allows for use of the additionalradio-derived navigational and orbital data obtained afterthe final AMC to refine the change in velocity and spacecraftorientation needed to enter the required orbit about Mars. Afinal correction, based on optical navigation through use ofthe TV cameras aboard the'Orbiter, as described later, may bemade, about 16 hours before MOI.

ScienceFive days before MOI the approaching spacecraft startsscience experiments directed to liars itself and to opticalnavigation.Now, TV images are obtainec1 cf the whole disc of Mars,of star fields, and of a satellite of Mars. Infrared scansof the planet give gross determinations of surface temtpera-tures and concentrations of water vapor in the Martian atmos-phere in preparation for more precise measurements to come.There are four reasons for the science experimentsduring final approach to Mars:* Instruments can now be calibrated for the firsttime with Mars as a target.a Some scientific measurements can only be made atthis stage of the mission, such as color picturesof the disc of Mars. Later Viking will be tooclose to the planet.* Observations of Mars' small moon Deimos againsta star background provide final, very precisenavigational data needed for an accurate 140I.* Look For changes to markings and present condi-tion of planet.Eight images (frames) of th e whole disc of liars aretaken on June 14 uising red and violet filters. The finalseries of optical navigation pictures begin on June 15.

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Orbtitdr Details antd Equuipmensmt LocationcP EPRO)DUCTBIII'Y 01? TRs

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The two TV cameras aboard the spacecraft alternatelyphotograph Mlars and a star background. Alternate camerasare used because of the great difference between the expo-sure needed for Mars and the stars. If one picture onlywere taken with Mars in the same frame as the starfield andgiven sufficient exposure to record the stars, the disc ofMars would be so overexposed as to make measurement of itsposition relative to the stars impossible. But since theaxes of the two cameras are known precisely, the positionof Mars in the image from one camera and the positions ofstars in the image from the other can be accurately related.Just before MOI 37, photographs of Deimos, one of the smallmoons of Mars, against the star background will provideadditional navigational information.

During the approach to Mars the science data are roughlyequivalent to data expected during a flyby of the planet. Thisscience information supplements observations made earlier bvMariners 6 and 7 (1969). The approach cf Mariner 9 to orbitMars in 1971 did not provide such science coverage becausethe planet was shrouded in a global dust storm at the time.Calibration of the imaging system during final approachis needed because th e cameras are susceptible to changes ciur-

ing the 11-month flight through interplanetary space.Approach pictures of Mars will show the rotation ofthe planet on its axis. Some are repeated in three colors-- so that colored pictures can later be reconstructed. Theseinitial color pictures are expected to be spectacular becauseMars should be free of major dust storms -- the planet isclose to aphelion, its greatest distance from the Sun, whilethe global dust storms are observed to take place around perihelion when Mars is closest to the Sun. The pictures willnot show a full disc because the spacecraft is approachingMars in such a way that its cameras see the planet as a half-moon phase. This is a view of Mars that can never be obtainefrom Earth.Extending from 50 to 24 hours before MOI, the sequenceof pictures gradually changes, from views of Mars in detailsimilar to that seen in the best Earth-based telescopes toviews showing craters ind a wealth of surface detail, includ-ing a first look by Viking at the landing sites.

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A similar sequence is repeated by Viking 2 as itapproaches Mars six weeks later, providing valuable scienceinformation about large-scale changes on the planet in theperiod between arrival of the two spacecraft.Fifteen days before M1CI, the infrared thermal mappinginstrument (IRTM) which is mounted on the same scan platformas the cameras, goes through alignment and calibration test-ing. In the final day before MOI this instrument is usedfor low resolution thermal mapping of the Martian surface,i.e., 20 samples across the planet at a range of 193,000kilometers (115,000 miles) for longitude coverage of temper-

ature changes of the surface of Mars.F"ve davs befo::e i0I the infrared water vapor experi-rent -- ars A-mosphere Water Detector (14AWD) -- also on thescan platform makes a first scan of th e planet. A similarscan is then repeated daily. The best water vapor data areobtained within the period of one and a half days before MCIwhen several scans are made.

MARS ORBIT INSERTION: June 19, 1976The purpose of this maneuver is to place the spacecraft

in an orbit that requires only minor corrections to permitthe Viking Lander to touch down later in the Chryse landingsite, when this site is certified as safe by observations fromorbit.The spacecraft is first rolled, then yawed, then rolledagain to facilitate alignment of the high-gain antenna towardEarth so that good communications can be maintained with thespacecraft in the burn attitude at its great distance of 380million km (236 million mi.) from Earth. While no scienceexperiments take place during MC0I, engineering data continueto flow back to Earth except for the brief period between thetwo roll maneuvers when communications are temporarily inter-

rupted as the high gain antenna points away from Earth. Whenthe engineering telemetry data are again received at Earththis is a good indication that the spacecraft has orienteditself ready for the burn. First indication of a successfulburn is the abrupt change in the Doppler residual graph whichis displayed on screens to mission controllers and others,showing that the trajectory has changed from that followedbefore the burn :arted.After tle MOI burn the spacecraft reorients itselfready to make its science experiments in orbit.

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CL Composition One Science Retarding Potentialcc Atmospheric Topic Analyzer, Pressure andStructure Temperature Sensors,LuJ;tut Accelerometers andRadar AltimeterVisual ..................... 2 Facsimile Cameras,Color/Stereo CapabilityBiology ............ .......... 3 Analyses for Photo-synthesis, Metabolismand GrovwthMolecular Analysis (organic) . . Gas ChromatographMass Spectrometer(GCMS)Mineral Analysis (inorganic) . .. X-Ray FluorescenceSpectrometerMeteorology----------------- Prcssure, Temperature,Wind Velocity andWind Direction SensorsSeismology 3-Axis Seismometeruj Magnetic Properties of Soil - . - Magnet Array on Soila Sampler, Cameras Used

Z for Visual Study ofParticlesPhysical Properties of Soil . * * Analysis of Visual andEngineering Data fromApplicable Instrumentsand Experiments:

* Cameras-VisualStudy of SoilCharacteristics (e.g.,clumping, grain size,cohesion, adhesion,etc)* Soil Sampler-(withcameras) Trenches,Engineering ForceMeasurements,Porosity, BearingStrength

C Orbiter/Lander Location, Orbiter/Lander RadioAtmospheric and Planetary and Radar Systemsc Data, Interplanetary Medium

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Infrared thermal mapping consists of three types ofcoverage of the planet:* From high altitude -- low resolution scanning mapsover the whole of the planet.* From intermediate altitude -- better resolutionscanning maps but slowly filling in details ofthe whole planet over several revs.* From low altitude near periapsis -- highest reso-lution obtained at a fixed pointing of the instru-ment so-that the spacecraft's motion carries thetrack across the planet's surface.During the first 14 days following MOI, infraredmapping is directed toward siLe certification. On the firstfew revs the instrument obtains representative observationsin the general area of the Chryse landing site to providebasic information about the surface of the planet at severaldifferent wavelength bands. The instrument has a resolutionof about 8 km (5 mi.) at the landing site when observed fromperiapsis.On Rev 4 through 8 detailed observations are made ofthe landing site and the surrounding area. The aim is toascertain how the thermal properties of the surface varyaround the site. Over a Martian day boulders have a dit-ferent thermal profile from sand. San(d reaches higher day-time temperatures and lower nighttime temperatures than aboulder field. However, the sunrise and sunset temperaturesare almost the same for both. Unfortunately when the Orbiteris over th- landing site and obtains best infrared resolution,the time is sunset on Miars. Thus a direct temperature measure-ment does not readily determine whether the site has bouldersor is a sandy surface. The site is also observed from theOrbiter before dawn, when a difference could be found, butthen the Orbiter is far from the landing site and the resolu-tion is not good.The infrared instrument is also used to measure thetemperature of the Martian stratosphere and to monitor thistemperature every day. The temperature measured is typicallyat an altitude of 20 km (12 mi.), i.e., at the 0.3 mb pressurelevel when looking straight down through the atmosphere.

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The infrared instrument will make box scans of theatmosphere to determine the stratospheric temperature overareas of the planet. On Rev 3 one of these scans will begina series of measurements to determine the atmospheric sta-bility above the landing area, and this type of infraredatmospheric survey continues until one day before separationof the Lander from the Orbiter. If a large instability isdetected the landing may be delayed.Although water vapor information is not critical tothe certification of the landing site for Mission 1, the

EIAWD experiment obtains data prior to separation. Scans ofthe planet on Rev 1 provide instrument calibration. Thenthe instrument takes a first detailed look at the concentra-tion of water vapor, several times a day, in the atmosphereabove the Tharsis-Coprates area. These observations continueuntil separation with the objective of establishing the diurnalvariation of water vapor in the Martian atmosphere in thisarea which is similar to the Lander 1 landing site. The land-inc3 site can only be seen in the late afternoon.This mission will continue with a landing at PrimeSite A unless something negative is revealed by the sitecertification observations from orbit and from Earth. In

addition to the Earth-based radar data, Earth-based opticaltelescopes keep close watch on liars to check for major duststorms. This planetary patrol activity started years beforeViking's approach to Mars organized by the planetary patrolprogram centered at Lowell Observatory, Ariz.Orbit Trim

A series of three orbit trim maneuvers can take placebetween MOI and Lander separation. The first takes placeat periapsis on Rev 2 to remove an error in the period ofthe orbit and to synchronize the Orbiter period with therotation period of Nlars. The trim also adjusts the orienta-tion of the orbit, if this is required, so that the orbit'speriapsis point in space is d rectly over the landing-site,and the landing site is directly beneath this at the sahoetime. The Orbiter must arrive at its periapsis at preciselythe same time that the landing site, carried around the planetby planetary rotation, arrives beneath the periapsis of theelliptical orbit of Orbiter.

A second trim maneuver takes place on Rev 5, to furtheradjust the orbi t pC rnod and orientation.-more-


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On Rev 10, if needed, a small trim maneuver can takeplace at periapsis-to correct the altitude of the periapsisand to fine tune the period of the orbit to the exact tim-ing required for the landing.Pre-Separation Activities

With the landing site certified and the orbit adjusted,the landing can be attempted. First, at about 36 hours beforethe scheduled separation time -- i.e., on Rev 13 (July 3) --a set of commands is transmitted to the Lander for it to enterthe Martian atmosphere and reach the surface at th e landingsite.This same set of commands is transmitted to MartinMarietta Aerospace,-Denver, Colo., where it is used on acomputer to fly a test version of the Lander spacecraft dozn4 to a hypothetical Mars. This computer simulation takes placeto ensure that there are no mistakes in the command sequence.Thirty hours before separation the Lander switches on

N1 its power systems. A pre-separation checkout of th e Landerthen takes place. This is very similar to the checkout madebefore Viking was launched frDm Kennedy Space Center and toa fl ight check made during November 1975. This pre-separationcheckout takes about five hours to complete.Immediately afterwards the Orbiter recharges th ebatteries of the Lander so that they are fully charged priorto separation.On Earth, engineers analyze all the telemetered datareturned from the checkout to make sure that everything isfunctioning correctly on th e Lander. Navigation calculationsare also made to double check the elliptical orbit of theOrbiter and the landing trajectory for the Lander. At nineand a half hours before separation the command instructionswithin the spacecraft can be modified if found necessaryThere is a final option to update th e instructions and refinethe landing sequence at three and a half hours before separa-

tion. This acts as a backup in case th e modifications failedto get to the spacecraft at the nine-and-a-lhalf-lhourtransmission.Any modifications to the landing instructions are alsoverified by the hybrid computer-spacecraft simulation atMlartin-Denver before they are used in the real spacecraft.About one and a half hours before separation, th eLander is again powered up and all is now ready.

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SEPARATION: July 4, 3:38 p.m. PDT(Times include 18 minutes radio transmission time fromMars to Earth)

The separation sequence loaded in the Orbiter andLander is fully automatic. But before it can be initiatedthe Orbiter has to receive a "go" signal from Earth, a sig-nal which is transmitted about 45 minutes prior to separa-tion time. Should this signal no- get through to the space-craft, the separation cannot be executed and the landing isaborted. Another 14--- days then have to elapse before thelanding can be tried again. This is the time needed forLander temperatures to stabilize, to repeat the pre-separationcheckout and to instruct the spacecraft on a new descenttrajectory.

When the separation command is received by the space-craft, the automatic sequence begins. At separation timepyrotechnic devices fire and release the Lander from theOrbiter. Springs then push th e two spacecraft apart. Com-mands to orient to a certain attitude fo r the de-orbit burnand the duration of the de-orbit rocket thrust have beenstored in the Lander. At th e pre-programmed time th e de-orbitengines on the aeroshell burn fo r about 20 minutes and decel-erate it from the elliptical orbit it pursued with the Orbiter.The Lander commences its three-hour coast toward atmosphericentry. The new path is an ellipse which carries the space-craft into the atmosphere of Mars on a path leading to th elanding site.DESCENT AND LANDIN!G:July 4, 3:38 p.m. PDT (Separation) to 6:58 p.m. PDT (Touchdown)

OperationEollowing the de-orbit burn, the spacecraft coastsalong its descent ellipse. Telemetered data are sent inshort bursts via the Orbiter which transmits them to Earthat the 4 kilobit per second rate. The Deep Space Network(DSN) station receiving th e data during descent is Goldstonein the Mojave Desert, Calif. The transmission in short burstsrequires quick lock-on to the signal each time it is receivedat the antenna.

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| Separation Orbtr4) Deorbit

i; OrientationI A,, ~Deorbit iBurn

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-LLanded

Separation, Deorbit, Entry andLandin7g Sequence


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Six minutes before entering the atmosphere, the storedcommands orient the spacecraft for its encounter with therarefied upper atmosphere of Mars. The x-axis is aligned sothat the spacecraft has a -20 degree angle of attack; i.e.,the x-axis is 20 degrees below the velocity vector.This angle is needed for science purposes. Just beforeentry into the upper atmosphere at about 243,000 meters(800,000 feet) above the surface a preprogrammed pitch placesthe spacecraft in a -1. degree orientation which is auto-matically maintained until aerodynamic forces cause thespacecraft to maintain this -11 degree angle of attack.In this attitude the spacecraft experiences a lift sothat it does not plunge in too steeply so as to overheat.As soon as the atmosphere decelerates the spacecraft by .05 g(as sensed by a. on-board accelerometer) the pitch/yaw attitudecontrol of the spacecraft is disabled. Now the spacecraft'scontrol concentrates on damping any pitch or yaw motions.Aerodynamic forces are in general sufficient to maintain thespacecraft in its correct attitude, but the damping action isrequired to prevent any aerodynamic instabilities fromdeveloping.The spacecraft continues to be slowed down by atmos-pheric drag, its pro'.ective aeroshell preventing the entryheat from penetrating to the Lander. Gradually the entryvelocity of 4:600 ir/s (15,000 ft/s) is reduced until at6,000 m (20,000 ft.) above the Martian surface the spacecrafthas slowed to about 250 m/s (820 ft/s). At this point asupersonic parachute is deployed and the aeroshell is jetti-soned. The parachute slows the lander until it is travelingat about 65 m/s (200 ft/s) at an altitude of 1,200 m (4,000ft.). Then the three throttleable rocket engines are startedand the parachute is jettisoned. Deployment of the parachuteand start of the rocket engines is controlled by an onboardnavigator which uses an altimeter and other measurementsaboard the spacecraft. The Lander continues toward the su::-face using the descent rockets to slow its speed further andto maintain its attitude. These three engines orient the

spacecraft so that their combined thrust vector opposes th evelocity vector of the spacecraft.

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Two limiting altitude/velocity profiles had beenstored in the computer memory of the spacecraft beforeseparation from orbit. These represent permissible limitsof velocity at each altitude based on the amount of pro-pellant stored in the spacecraft and the thrust capabilityof the rocket engine. Applying too great a decelerationearly in the descent would bring the spacecraft's velocityto its terminal value too high above the Martian surfacewith insufficient propellant left for descent. Applyingtoo small a thrust would let the spacecraft impact the sur-face with plenty of propellant still unused. Either wouldbe disastrous to the mission but, if the Lander's velocityat each altitude is kept within the limits, it can landsafely.If the Lander enters its descent phase under condi-tions of no wind, its onboard computer allows it to veritably"coast" to the upper contour of altitude versus velocity and4 then to follow this contour to the surface like a pilot fol-lows a glide path indicated by an airplane's instruments. Ifhowever there is wind, th e spacecraft follows a contour thatis an interpolation between the two limits, like a pilotadjusting for a cross wind. In either case, the Lander reachesa height of about 16.8 m (55 ft.) above the Martian surfacewith a remaining velocity of 2.4 m/s (8 ft/s) and continuesdown to the surface at this terminal velocity. As soon as a

sensor on any one of the three footpads touches the surface,the rocket engines are switched off.Viking has landed. On Earth we won't know of thisuntil almost 18 minutes later because of the time it takesradio signals traveling at the speed of l ight to bridge th e380 million km (236 million Aii.) gap between Mars and Earth.The Lander relays data to the Orbiter during its des-cent. This data is relayed to Earth. So by the tiime th edata about the Viking entering the atmosphere of Mars arereceived at Earth, the landing has been completed. Becauseof this time delay it is not possible to "fly" the Landerdown to the surface. Everything has to be automaticallycontrolled from the Lander's onboard computer.Earth-received time fo r touchdown on Mars is scheduledfo r 6:58 p.m. PDT July 4 including 18 minutes radio trans-mission time from Mars. This could vary by five minuteseither way. Should the landing be delayed to July 5 or 6,the landing time changes by about 36 minutes later fo r eachday of delay.

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Landing is scheduled for about 4:00 p.m. Mars time,on Sol 0 (each Sol, Mars day, is 24.6 Earth hours long).The first midnight at the landing site following the timeof landing is designated the start of Sol 1.A safe, roft landing will be quickly known from thedata telemetered to Earth via the Orbiter. Viking has beensending information all the way throughout its entry intothe Martian atmosphere and descent to the surface. As wellas science data, engineering information about the attitude

of the spacecraft and the sequential landing operation, para-chute deployment, aeroshell jettison, landing engine start,is given in the telemetered record. But, of course, thisinformation is received at Earth always just less than 20minutes after the actual event has taken place at the Lander.

In the 12 seconds following touchdown a quick seriesof automatic actions takes place within the Lander, and theoccurrence of these events on the telemetry record are thefirst indications that all is well.

The closing of any one of th e three footpad switchesturns off th e rocket engines and then their heaters; this issignalled in the telemetry record. This switch closure alsostarts a landed initiation sequence aboard the Lander whichturns on certain items of equipment within the spacecraft.The radio relay system from Lander to Orbiter is switched toincrease the bit rate from 4 kilobits/sec to 16 kilobits/sec.The command detectors are activated so that the Lander canreceive commands directly from Earth. The computer isinstructed where to start its sequence for landed operations.

The Inertial Reference Unit (IRU) which has helpedguide the Lander safely to the surface is allowed to run for12 seconds after the landing. This provides reference dataabout the orientation of the Lander so that the computer canmove the high-gain antenna from its stowed position and,within one minute of landing, point it toward Earth from thesurface of Mars. The IRU information can also be used todetermine the attitude of th e spacecraft relative to th elocal vertical. Thus the plane of the Martian surface atthe landing site can be determined if the Lander footpadshave each penetrated the surface about the same amount. Themeteorology boom is opened up ready to start experiments.The cameras, which may have been shaken from a stowed posi-tion during the landing, are comaanded back to the stowedposition.

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r -31-Viking Lander Communications

Earth

Orbiter

Lander to Earth /two-way radio link /* Lander science data* Engineering telemetry* Doppler and range signals* Commands Lander to Orbiterone-way relay radio link

o Lander science data* Engineering telemetry

/RIPRODUCB1LITY OF P t lli,ORiGhNAL PAGE IS POORLander on Mars

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ScienceThe Lander begins its science experiments before itreaches the surface of Mars. It investigates the upperatmosphere and ionosphere of the planet and measures temper-ature, pressure and acceleration down to the surface. Detailsof atmospheric constituents are also obtained among which arethe quantity of argon in the Martian atmosphere, a measure-ment that is very important to later experiments on the surface.There are three categories of the science experimentsduring entry:* Properties of the ionized or electrically chargedupper atmosphere.* Constituents of the neutral (un-ionized) atmosphere.* Temperature, pressure and density profiles of theatmosphere during the parachute phase.Electrical properties of the Martian atmosphere areimportant to scientists because the planet does not have astrong magnetic field like the Earth yet it reacts with thesolar wind to produce a bow shock and a pseu2domagnetopause.An instrument called a Retarding Poteatial Analyzer (RPA) islocated in th e aeroshell facing forward in th e direction of

motion of the spacecraft. This motion rams charged particlesinto the instrument which measures their energy spectrum. Theexperiment measures the energy of the ions. Heavier ionshave greater energies for the same temperature.The data returned from the instrument during the briefflight through the ionosphere will take weeks of analysis onthe ground to determine th e temperature of the ions and theirmasses. From this analysis an estimate can be made of whatthe ions are, and this will be confirmed by data from a sep-arate mass spectrometer experiment.It is expected that the ionosphere at about 150 km(95 mi.) will consist predominantly of ionized oxygen mole-cules with some ionized oxygen atoms and ionized carbondioxide molecules. At even higher altitudes there may behydrogen and helium ions detected. Radio occultation experi-ments made with Mariner spacecraft determined that the ionizedparticles peak in number at an altitude of about 150 km (95 mi.).The highest ionized layer on Earth is the F-layer which pro-vides an ion peak at about 300 km (190 mi.) varying with timeof day and with season.

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There is not enough oxygen on Mars for such an F-layerto form in the Martian atmosphere, and the ionized layer at150 km (95 mi.) is thought to be a similar layer to the lowerterrestrial layer called the'E-layer (at 115 km or 69 mi. onEarth).The experiment also detects electrons as well as ions.Two classes of electrons are expected to be recorded by theinstrument; low energy ionospheric electrons and high energysolar wind electrons. However, since the instrument wasdesigned for the detection of ions its electron detectioncapabilities are still to be demonstrated in a practical

application at another planet.The Martian ionosphere is not shielded, as is that ofEarth, from the solar wind -- the flow of charged particles/ streaming into interplanetary space from the Sun. There is,however, a bow shock at Mars (unlike the Moon where the solarwind impinges directly onto the lunar surface). A question4 to be answered is whether Mars has its own low intensity mag-netic field that holds off the solar wind or whether the bowshock is produced by current flows within the ionospheregenerating a magnetic field. There have been many theoriesput forward to account for how the solar wind interacts withMars. It is expected that the electron data obtained duringthe descent of the Viking Lander may provide better informa-tion about this interaction znd precisely where the boundariesare between the solar wind and the pseudomagnetopause, i.e.,how close to the planet the solar wind electrons penetrate.The discontinuity of the bow shock may be identifiedquickly from the returned data, but full analysis of thecomplex data will take some time. Although a Russian orbiterof Mars produced data that implied that the planet has a mag-netic field, the spacecraft was not in the best region tomake this observation. The Viking Lander however, penetratesthe shock front almost at the sub-solar point so it is in anexcellent position to observe the change from the solar windto the pseudomagnetopause.Viking instruments record the whole region through tht-solar wind, the bow shock, the pseudomagnetopause and thepeak of ionospheric activity.

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Observations from Lander of the constituents of theneutral atmosphere are important to subsequent experimentson the surface of Mars. These are made with the UpperAtmosphere Mass Spectrometer (UAMS) which identifies atomsand molecules in the atmospheric gases. Earth-based andspacecraft experiments suggest that the neutral atmosphereof Mars is predominantly carbon dioxide with minor constituentsof carbon monoxide and oxygen. There could be as much as5 per cent nitrogen in the Martian atmosphere but this wouldnot have been detected by these earlier experiments. Theremay also be inert gases such as argon in the Martian atmos-phere. This, too would have been missed in the earlierexperiments.The mass spectrometer carried by Viking Lander recordsatmospheric particles, atoms arid molecules, with mass numbersbetween 1 (hydrogen) and 50 (to include carbon-dioxide at 44)and in doing so reveals not only basic elements and moleculesbut also the isotopic variations of common elements.The argon question is important. Its solution has toppriority during entry science. There is indirect evidencefrom a Soviet spacecraft of a large amount of argon in theatmosphere of Mars. Mars 6 carried a mass spectrometer whichwas operated during the parachute descent phase to sample theMartian atmosphere in regions of relative high pressure. The

mass spectrometer had to be evacuated of gas for it to operate.Sputter ion pumps incorporated in its design used the principleof gettering with titanium to attain a high vacuum in theinstruments. But this process is inhibited if there are raregases such as argon present, because they do not enter intochemical reactions and cannot be taken from the pump bytitanium gettering. Since the Soviets did not obtain theexpected pumping of their instrument they concluded that arare gas must be present in large quantities in the Martianatmosphere to reduce the operation of the pump. They con-cluded that argon is the most likely gas.The effect of argon on the Gas Chromatograph MassSpectrometer (GCMS) carried by the Lander to sample theMartian atmosphere and later to make organic analysis ofthe Martian soil could be disastrous. This instrument hasa pump that would be affected by the argon and would be madeineffective. The strategy for use of the GCMS was originallyplanned so that it would first complete its atmosphericanalyses before making the organic analyses. This strategyensured that the instrument would not be contaminated byorganic samples from the soil before it sampled the atmos-pheric gases.

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However, if argon is present in substantial quantitiesin the atmosphere of Mars, the GCMS strategy of experimenta-tion must .be changed. So a top priority during entry scienceis to find out how much argon is present.The argon question also has important scientific impli-cations in connection with the evolution of Mars and itsatmosphere. On Earth the origin of argon and carbon dioxideis believed to be from outgassing. The carbon dioxide hasbeen mostly removed from the terrestrial atmosphere by naturalprocesses while the argon has remained. This argon is ofthree isotopes (chemically identical atoms having different

atomic weights); argon 36, 38 and 40. Argon 40 is the endproduct of the radioactive decay of potassium 40. Argon 36and 38 are believed to be original isotopes from the conden-sation of the solar nebula; they are in cosmic abundance.The Earth's atmosphere has 300 times as much argon 40 asargon 36, and five times as much argon 36 as argon 38. It isthought that without the presence of potassium 40, a planetwould end up with its argon being all argon 36 and argon 38.The science questions are, therefore; how much argon 40is present in the atmosphere of Mars? Is there the same iso-topic abundances as found in Earth's atmosphere?If the atmosphere of Mars formed in recent geologicaltime there would not have been time for quantities of argon40 to build up in it. However, if Mars has developed itsatmosphere over a long period of time, as is believed forthe terrestrial atmosphere, a proportion of the argon 40isotope similar to that on Earth would be expected.Fortunately, the UAMS is extremely sensitive to argcnand the experiment should easily detect the several isotopesof this element. The instrument is also sensitive to nitro-gen which may be another important constitu% '. t of th e Martianatmosphere previously undetectable. On Earth nitrogen isbelieved to have originated from outgassing of the terrestrialrocks. The volcanic features of Mars suggest that Mars, too,

must have experienced periods when rocks were outgassed bythese volcanoes. The UAMS can detect expected quantities ofnitrogen (a few tenths of a per cent) quite easily.

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The strategy of examining the data about the neutralatmosphere is:* Determine i*f there is argon present, and howmuch.* Ascertain its isotopic composition.e Look for mass 14 to check if the Martian atmos-phere contains nitrogen or carbon monoxide, orboth.* Look at all the other peaks on the curves ofdata from the mass spectrometer to determinethe combinations of atmospheric constituentsthat produce such patterns.The amount of argon will be determined within a fewhours of the landing. The other science questions will nC.be answered until returned data have been analyzed in depth.The third important experiment during entry is todetermine the profiles of temperature, pressure and densityof the Martian atmosphere as a function of altitude abovethe Martian surface. Two techniques are used: direct mea-surements derived from pressure and temperature gauges car-

ried on the Lander, and indirect deductions from the decel-eration of the Lander as it plunges through the atmosphere.The scientific view of the Martian atmosphere haschanged radically over the last decade. Before spacecraftflew by Mars, Earth-based observations of gas moleculescattering suggested a surface pressure of 80 mb (8 per centthat of Earth). However, scientists quickly amended this toaccount for the effects of dust which would produce similarscattering in a less dense atmosphere.Absorption spectroscopists suggested that there was5.5 mb of carbon dioxide present in the Martian atmosphere,

but other constituents were unknown. The broadening of thespectral lines of carbon dioxide implied that the totalatmospheric pressure might be as high as 18 to 25 mb.Then spacecraft flew by th e planet and radio occulta-tion experiments showed that the maximum pressure might beonly 6 mb. The spectroscopists were forced to give ground,but they still held out for a pressure of 20 ml).

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The latest uncertainty is the argon question. If asmuch potassium were present in the crust of Mars as in Earth'scrust, then a 3 mb partial pressure of argon might be expectedin the Martian atmosphere. This leaves 5.5 mb for carbondioxide, and possibly some nitrogen as gases emitted frormvolcanoes. Nitrogen can be held thermally on Mars becausethe planet has a relatively cool exosphere; i.e., nitrogenatoms are not thermally accelerated to high enough speed forthem to reach escape velocity (5,000 m/s) from Mars.Data from occultations of spacecraft and from infraredmeasurements made from Mariner 9 are somewhat erratic anddisplay no readily recognisable pattern of variation of atmos-pheric temperature with altitude. This could imply eitherthat variations are not within the limitations of the measure-ments techniques or that there are rapid variations. Theobservations also showed temperatures reaching below 140 K,which is 10 to 15 K less than the condensation temperatureof carbon dioxide. The question then is: why does the carbondioxide not condense from the Martian atmosphere? The onlyway to prevent this would be some kind of atmospheric lag inthe temperature cycle.The aim of the new atmospheric science with Viking istwo-fold: to improve the precision of earlier measurements,and to extend the measurements over a wider range of altitudes.A key reason for knowing the temperature profile inthe Martian atmosphere is because temperature differencesdrive winds. It is known that there are strong winds onMars and this would imply that there are large differencesin atmospheric temperatures. Dust storms also engulf thewhole planet at times, and these require strong winds fortheir initiation. How strong is questionable. It is esti-mated that winds of velocity 100 m/sec are needed to generatethe dust storms, yet temperature profiles used to computecirculation models of the atmosphere have so far resulted incomputed wind speeds of only 25 m/sec maximum.Based on current knowledge of Mars it is difficult toaccount for the dust storms and IIow the necessary winds canbe generated. Although the Lander is designed for winds upto 100 m/sec, knowledge of the actual winds are important toachieving a safe landing.

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SURFACE OPERATIONS:Sol 0 July 4, 6:58 p.m. PDT through July 5 0:20 a.m. PDT

Immediately after the Lander touches down on th eMartian surface events take place rapidly. Following th efirst few seconds of operations on the surface, as describedearlier for initiating the Lander sequence, this sequencefollows, during which engineering data are telemetered toprovide a status review of the various pieces of engineeringequipment on the Lander. Immediately following this, Vikingstarts its first picture taking sequence. Since the condi-tions on the surface of Mars are not known exactly at thetime of the landing, only one camera is opened to the Martianenvironment. This is Camera 2. The other camera is left inthe stowed position, its window protected against the Martianenvironment until Camera 2 has operated successfully withoutdamage.

Twenty-five seconds after touchdown, Camera 2 startsits first high resolution scan. Following the preprogrammedinstruction in the Lander's computer, the camera is directedto start its picture at a camera azimuth of 200 degrees andto sweep toward the right through about 60 degrees so as toinclude the footpad #3 and its leg. The camera view isdirected downward from -40 degrees to -60 degrees elevationso as to provide a close-up view of the Martian soil and howthe footpad has affected this soil during the landing. Asit is taken this picture is sent in real time at the 16 Kbit/sec rate from the Lander to the Orbiter where' t is storedon tape memory aboard the Orbiter.This first picture is directed to show the Martiansurface in as much close detail as possible because in theabsence of prior knowledge about the surface structure ofMars it is important to know quickly how the footpad inter-acted with the surface. From This interaction it is possibleto calculate approximately how much energy was absorbed bythe soil . The touchdown parameters are of vital importancein determining the relationships of the horizontal plane ofthe spacecraft, the plane of th e surface on which it haslanded, and the Martian horizon plane.

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4.Protective PostOuter HousingWindowand DoorAssemblyWindow \

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-45-To help in establishing these planes quickly, the

next picture, also with the number 2 camera, is a wide-angle par.orama starting at 105 degrees and sweeping as farto the right as the data link allows before communicationfrom Lander to Orbiter is lost for Sol 0. This second pic-ture begins at 6 minutes, 8 seconds after touchdown, andthe nominal link between Lander and Orbiter ends 6 minutes,25 seconds later, but the camera is not turned off untilalmost three minutes later to take advantage of communica-tions lasting beyond the end of the nominal link.

The total width of the second picture is 300 degreesand its height is 60 degrees; +20 degrees (above) to -40degrees (below) the horizontal plane of the spacecraft. Ifthe spacecraft is level and on level ground the horizon willbe 20 degrees or one-third of the frame below the top ofthis second picture.

If the horizon is not in this position, analysis canuse its position with the tilt of the spacecraft to determinethe slope of the ground on which the spacecraft is resting.These first pictures are black and white because theyare taken close to sunset on Mars when the low Sun will notilluminate the scene adequately for a color picture. Threetimes as much coverage of the Martian surface can be obtainedin black and white as could be obtained in a color photobecause a color photo requires three individual black andwhite photos taKen through three separate color filters.After these first pictures have been taken and relayedto Earth, me'-rology science starts on the Lander and con-tinues at planned intervals throughout each Sol thereafter.The meteorology instruments operate in short intervals oftwo minutes duration spaced two hours apart over each Mar-tian day, with some long observations lasting up to one hourtwice each Martian day. The sequence of observations of tem-perature, pressure, wind speed and wind direction is slowlymoved in Martian time so that data are ultimately gathered

to cover a whole day's changes.There are 20 observation periods for meteorology eachSol and by the end of Sol 20 the meteorology team expectsto have a very good idea of the diurnal variations of the

meteolorological measurements. These measurements are notonly of scientific interest but also of practical importanceto the other experiments. For example, diurnal variationsof wind speed may be such that the sampling boom cannot besafely extended at certain times of the day when it may befound that the wind speed is always too high.-more- R pj UU CJILITY OF ThE

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-4 -Very precise surface measurements of pressure areexpected since a twin of the pressure sensor has been oper-ating for many months on Earth and has not deviated fromits nominal reading by one count. It is rock steady in itsoperation.The meteorology of Mars poses an interesting questionabout the pictures to be returned from the Martian surfaces;will they show mirages? Mirages are expected on Mars becauseof great temperature differences between the ground and theatmosphere, as a result of the thin atmosphere of carbondioxide. The sunlight heats the surface of the planet butthis heat cannot readily be conducted or convected into theatmosphere. Radiation is absorbed by the carbon dioxide butnot as effectively as by water vapor in the terrestrial atmos-phere. The effect is that it is very difficult fo r the hotMartian surface to heat the atmosphere above it. Scientistsexpect that Martian surface conditions are such that "wetroad" type mirages may be seen on flat plains of Mars in theearly afternoon. Water sheet mirages may appear as lakes atdistances of one to one and a half kn from the Lander. Thisis further than the usual distance of slich mirages on Earth-- about 100 m (110 yd.) -- because th e atmosphere of Marsis so much thinner. The most likely time for the Martianmirages to occur is noon to 2:00 p.m., so they are not ex-pected on the first panorama of Sol 1 which is taken in lateafternoon. But they may show up on pictures taken on suLse-

quent Sols.Two other instruments are calibrated during the firstSol: the seismometer and the X-ray fluorescence instrument.This calibration is part of the first task of the seismologyteam to determine the seismic background of Mars, to ascer-tain the frequencies of vibrations of the Martian surface;e.g., the background noise produced by winds. Later, as thiscalibration progresses, the instrument is reconfigured --taking several Sols to do so -- so that its gain settingsare best suited to the natural environment of Mars and itcan search effectively for true seismic events. Since itis not possible to predict in advance what the backgroundor the seismic activity will be, the first few Sols ofseismology are very exploratory. The winds of Mars affecthe seismometer. The seismic team works closely with themeteorology team in subsequent interpretation of data sothat the background noise due to the wind can be understood.

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The first calibration of the X-ray fluorescencespectrometer instrument after landing operates the instru-ment without a sample and uses a calibration plaque withinthe instrument. Since the instrument is extremely sensi-tive to the presence of argon, this calibration test isexpected to reveal the presence of argon in the atmosphereof Mars and confirm the earlier UAMS determination, so impor-tant to effective operation of th e GCMS during subsequentsols.There is an advantage in checking for argon at th e

surface in tftis way. The UAMS experiment cuts off highin the atmosphere, and its detection of argon applies tothe upper atmosphere. While there is no reason to believethat the proportion of argon in the Martian atmospherevaries with altitude, the surface test gives added confidenceto making the decision on how the GCMS should be used.The calibration of the instrument is also importantto ensuring that there have been no changes to the X-rayspectrometer as a result of the landing stresses imposedon the Viking spacecraft. Any necessary changes to theinstrument will be made by Sol 7, ready for it to start itssampling of the surface materials.As the Sun sets at the Lander site on Mlars on thisfirst day, scientists have looked at the Martian surfaceand the panorama of the landing site; they know that theLander is operating.The Lander carries instructions within its computermemory to perform a 58-Sol mission in the event it cannotbe commanded from Earth. During the next few days the earlydata sent from Mars to Earth must be analyzed to determinehow much this program of activity is to be changed. Thisanalysis begins while the Viking Lander enters its firstMartian night.

Sol 1 Through Sol 7 (July 5 Through July 12)This period of Lander operations on the surface concen-trates first on certifying the site from which th e sampleris to scoop its samples for the biology, organic analysis,and X-ray fluorescence experiments. A sample site is includedamong the programmed instructions within the computer memoryof the Lander. But this site has to be evaluated before th eLander starts operations to pick up samples there. Additiunallytne Lander commences several of its scientific surface experi-ments during this sample site certification period.

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Sample Site CertificationSample site certification relies heavily upon theimaging capabilities of the two cameras carried by the

Viking Lander. When Viking reached the surface its computercarried instructions for it to operate independently of com-mands from Earth for 58 Sols and to perform all its scheduledexperiments. But this initial computer load for a prepro-grammed surface mission has two constraints:

* It is predicated on current knowledge of Mars.* It is limited by the size of the computer, e.g.,

it can only complete one cycle of the scienceexperiments.Even so, without any further commands from Earth, the

Lander would be expected to complete a competent mission onMars. The initial computer load provides a mission basedupon what the Lander should do in view of what is alreadyknown about Mars. The Viking mission, however, has thegreat potential of adaptability. Scientists want to usethis adaptability to convert the preprogrammed mission intoan adaptive mission; subsequent experiments reflecting whatearlier experiments reveal. Such an adaptive mission thenbecomes equivalent to a whole series of missions to Mars forthe price of one mission.

The first question to be answered is whether or notthe sampling sequence programmed into the computer memorywill aim the sampler toward a safe site. And time is ofessence, for the decision has to made by Sol 5 whether ornot to let the initial computer load continue to sample aspreplanned or change it to take a sample at another, safersite.

There are several factors that affect the decision;the sampler on the end of its extendable boom operates asa shovel to scoop up Martian soil. If the sample siteproves to be hard rock, the shovel cannot obtain a sample.If the sample site consists of large boulders, these cannotbe sampled. The ideal site has a mixture of particles liketerrestrial dirt.

The first two pictures of Sol 0 show th e nature ofthe soil, but they could very well be incorrectly exposed;too l ight or too dark. Moreover, tley (lo not give an accu-rate impression of th e distance of objects from the Lander.Experienced thotointerpreters can estimate the location ofthe samplirg site to within one foot from the pictures.

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In tests on Earth, it was found necessary to waituntil the first stereo pictures were obtained on Sol 3before safe measurement of distances of objects at thesample site could be derived.On Sols 1 and 2 photography continues with Camera 2only, until more information has been obtained alout theMartian environment.When the second picture for a stereo pair becomesavailable on Earth for evaluation, a special computer pro-gram allows an operator to run a track-ball over the stereoimage. He manipulates hand controls so that the spot ofthe track-ball appears to move over the surface where the

s-. le boom will stretch out to obtain its sample for theprep-rogrammed mission. The computer is then able to derivecoordinates along three dimensions for the surface of Marsalong this azimuth line. In tests on Earth with a fullscale Viking Lander on a mockup of a hypothetical Martiansurface model in the Atrium at the Jet Propulsion Laboratory,the process was demonstrated as providing an extremely accu-rate determination of the ridges and valleys and obstructionson that surface. In this way, the sampling site selected inthe preprogramming can be proved out for safety before thesampler begins its automatic operation. Hazards to thesampler include jabbing it into hard material or loweringthe boom onto a rock or a high ridge between the Lander andthe sample site. Since the cameras are at a high elevation,the danger from such a ridge or a rock could not be anti-cipated fully without evaluation of the stereo pairs.

If the preselected site is hazardous to the sampler --it could be a hard rock or a miniature cliff-like surface --the initial program is then altered. The extension strokeof the boom can be shortened, or the boom can be commandedto a completely new site, which has also been evaluated fromthe stereo pictures. Finally, if necessary, the wholesequence can be cancelled. This would require a longer timeto resequence and would delay the sampling and many of thesubsequent experiments.On Sol 1 the camera produces high resolution hlack andwhite pictures and low resolution survey pictures in color.These pictures show the sample site and also the color-codedtest chart number two on the Lander which is used for cali-bration purposes.

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On Sol 2 the protective cannister cover on the samplercollector head is ejected onto the surface of Mars close tothe Lander footpad that was photographed on Sol 0. The areais now photographed again to provide further information aboutthe surface from the observed interaction of the cannistercover with the Martian soil. Other pictures obtained on Sol 2are high and low resolution black arid white views, coveringthe sampling site and a 25 x 25-cm (1 0 x 10-in.) grid patternon the top of the Lander's body. The latter is to study tihemotions of dust particles from the landing.If conditions are suitable for use of the cameras with-out danger to them, camera 1 is first operated on Sol 3. Izis moved from behind the protective dust cover, and photo-graphs are taken to repeat the survey of picture number 2(Sol 0) and the high resolution coverage within the samplingarea obtained during Sols 1 and 2. Two pictures are in color,two more in black and white. The Sol 3 camera operationsprovide a key picture event in producing the second of astereo pair of pictures tc'* be used in evaluating the samplingsite in detail.This same area photographed just after touchdown maynow show changes as a result of wind; however, the stereoeffect is still suitable for an analysis of the samplingsite. One of the mirrors mounted on the sampler housing isincluded in the field of view which provides a reflectedimage of the Martian surface beneath the Lander showing howone of the multi-nozzle rocket engines has disturbed thesurface during the landing.The decision whether to keep to the initial computerload fo r directing the sampling or to modify the commandshas preferably to be made by Sol 6 if the first sample isto be collected early on Sol 8. All commands that affecttemperature, power or mechanical motion at the Lander haveto be sent to the Lander in sufficient time for them to beverified bv a radioed response before they are executed.The two days provide opportunity to do this safely. A change

command could be sent on Sol 7 but there would be no oppor-tunity to repeat it if for some reason it did not reach thespacecraft.After the pictures have been obtained fo r samplingsite certification, the cameras continue obtaining otherimages associated with the general Lander imaging experiments.

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Science ExperimentsA capability of the cameras is their ability to obtainimages over six spectral bands extending into the near infra-red. This is comparable with Landsat capabilities and itallows "false color" pictures to be generated of Mars toreveal features that might otherwise be missed on a normalcolor rendering. The potential for spectacular color in such -pictures is immense.One aim of the spectral analysis of the surface withthe cameras is to try to obtain an objective answer to thequestion: is Mars really-red? This is important to anunderstanding of the processes that have molded the surface

and how the Martian crust has reacted with the Martianatmosphere; e.g., does the crust contain iron oxides?Pictures are transmitted from the Lander to the Orbiterat 16 kilobits/sec. Pictures are transmitted directly fromr'the Lander to Earth at 250 or 500 bits/sec only. A directlink is used every day for real time images to be sent directlyto Earth.Limit to the operation of the cameras is sandblastingfrom Martian dust. The cameras can be stored behind protec-tive shields if dust storras develop. Also, to prevent finedust from adhering to the camera windows and obscuring theview, carbon dioxide jets can be directed on command againstthe windows to blow it off.While the normal picture taking mode is a scan sequencewhich requires several minutes to build up a complete pictureand thus is not able to reveal motion of any object in thefield of view, the cameras can be operated in a single-line-scan mode. Scanning a single line continuously and layingthe scan lines side by side as in a normal picture, resultsin a picture in which differences along the line scan aresmeared across the whole picture. However, should there beany movement along that line scan during its repetition, thecomplete picture graphically draws attention to it by a cleardistortion of the smeared strips.The line-scan mode can also be uqed where it is notpossible because of data rate limitations, to take a wholeseries of pictures, say to show color changes over severalhours; for example, at sunrise or sunset. By using the singleline scan and a slow data rate the colors can be monitored foran hour or more to check how the sky changes over thoseinteresting periods of time at the beginning and ending ofa Martian day.

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-56-The qujstion of the quantity of argon in the atmosphereof Mars has earlier been discussed in connection with scienceexperiments during entry. If there are not more than a fewper cent of argon in the atmosphere, it is advantageous to

perform atmospheric analyses with GCMS on Sols 3, A and 5before using the instrument for organic analyses of surfacematerials. However, if there is a large amount of argon,the atmospheric analyses will not be performed because theargon will interfere with the operation of the instrument.Because the amount of argon cannot be determined untilthe Lander is actually on the surface, the atmospheric samp-ling is omitted from the preprogrammed mission. A gap in theprogram is left so that if the quantity of argon is smallenough a command sequence can be sent to the Lander to makethe atmospheric analyses without having to change the wholeof the preprogrammed mission by an insertion of new commands.The argon uncertainty is expected to be resolved within thefirst two Sols.The atmospheric analysis is directed toward establish-ing the relative abundances of carbon dioxide and other gasesin the Martian atmosphere with a view toward a better under-standing of how that atmosphere evolved. The results areexpected to confirm a bulk constituent of carbon dioxide withminor amounts of carbon monoxide and oxygen and probably someargon. Nitrogen, too, will be searched for since this gas isimportant to the evolution of life on Mars. A high argon con-centration would imply that the Martian atmosphere was oncemuch denser than it is now because argon is a chemically inertgas that has to accumulate in a planetary atmosphere becauseit cannot enter into chemical reactions with surface materials.It is also a heavy gas that cannot escape from the planet. Itscontinued production from the decay of radioactive potassiumleads to a gradual concentration in the planetary atmosphere.A high concentration means that a lot of atmosphere has beenproduced on Mars over its history and implies that atmosphericpressures on Mars may have been much greater than they are now.Such high pressures could have accounted for past periods ofplentiful liquic' water on the surface of Mars which, in turn,could have accounted for the sinuous channels and other liquiderosional features widespread on Mars today.Following the atmospheric analyses, if any are made,the GCMS is prepared for its organic analysis. On Sol 6,the analysis column is conditioned and the chromatographcleaned up for its organic analyses due to start as soonas a sample is delivered on Sol 8.On Sol 2, preparation begins for the biology experi-ments by setting up equipment and checking its readiness toreceive samples on Sol 8. On Sol 3, a 16-6iinuLa test analysistakes place with the Gas Exchange Biology Experiment.

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-57-SURFACE SAMPLING AND OPERATIONS STARTING SOL 8 (July 12)

The first surface sampling is taken for the biologyexperiment. This is scheduled for 2:46 p.m. PDT on July 12(6:45 a.m. sol 8 Mars time)-. The collector head willcollect a sample of Martian soil and place it in the samplehopper on top of the body of the Lander. If a detector inthe hopper does not sense any delivered soil, the samplertries again twice automatically at the same site (it canbe preprogrammed for up to 15 tries). If there is stillno sample, it tries one more time before it shuts down toawait the next sampling try for the organic analysis aboutone our later. The biology analysis begins at 4:16 p.m.PDT, if a sample is delivered with the first cycles,taking 12 days to complete.The soil is distributed to the three biology experiments;gas exchange, pyrolitic release, and labeled release. Thefirst experiment is the pyrolytic release using a dry soilsample illuminated by simulated Martian sunlight. Carbon'I dioxide and carbon monoxide labeled with radioactive carbon 14are introduced into the test chamber and the experimentincubates the sample until sol 13 to check if any radioactivecarbon 14 is removed from the atmosphere.This experiment is based on a characteristic of manyterrestrial life forms that need carbon dioxide to synthesizecomplex carbon compounds for use in the living system. Thesmall sample of Martian soil placed in the incubation chamber

in its atmosphere labeled with radioactive carbon 14 isilluminated by a xenon lamp.No food or liquid water is provided. After the samplehas been incubated for five days (to sol 13) the atmosphereis flushed out of the test chamber and the sample heated tobreak the organic compounds into gases. These gases arecollected on an organic vapor trap. The organics are thenheated again to convert them into labeled carbon dioxide gas.If there are any organic molecules in the pyrolyzed samplehaving labeled carbon, they will be measured by a detector.The experiment thus shows whether or not Martian soil at thissite contains living systems that can extract carbon from the

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atmosphere and incorporate it into complex carbon-basedmolecules needed for their life processes.

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The first cycle of this experiment ends at about9:00 a.m. Mars time on sol 20. If a positive result isobtained, a control sample half of the original sample,is next sterilized by heat and put through the same test.-If he result of the first test was, indeed, due to livingorganisms in the Martian soil, the second control testshould,produce negative results. If it, too,,producespositive results, the effects cannot be attributed tocarbc;n-based Martian life forms. Some other explanationha-z, o be found. If the first test is however, negative,th'e next cycle of the experiment several sols later repeatsthe test,, but with the sample provided with some water vapor.A second biology experiment--theJabeled release experi-ment--begins with measurement of the background of radiationfrom the sample in its test chamber. This, experiment is

predicated oh the fact that given a food' labeled withcarbon 14, a Martian organism would be likely to ingest itand incorporate labeled carbon atoms into complex moleculesof its living system. Moreover, some of the labeled moleculeswould later be released by the living system as waste products.This transfer of carbon from a nutrient solution through theliving organism and back to the environment is searched forby this experiment. After the background test has been madefrom 14:50 on sol 8 to 04:50 on sol 10 (Martiantime) a small amount of extremely dilute aqueous solutionof aimple nutrients containing carbon 14 labels is injectedin.o the soil. The background radiation is then sampledover the next few sols; another injection is made and theexperiment continued,, Later, the sample is purged from thetest chamber. Again, should a positive result be obtained,e.g. radioactive carbon 14 be transferred from nutrientmolecules to the atmosphere, a second half of the originalsample, which was retained as a control, is sterilized andput through the same test sequence. If this controlproduces positive results, it is unlikely that the experi-ment has revealed the presence of organic life on Mars.However, if the control shows negative results, theexperiment may have detected a Martian life form.The nest sequence for a negative initial result ofthe labeled release experiment is to repeat the experiment,using a sample from a-different sampling site in the secondbiology sequence.This labeled release experiment complements the firstexperiment in that while the first looks for carbon dioxidepassing from the atmosphere to a living organism, the secondlooks for carbon dioxide passing from the living organism tothe atmosphere, Both are based on the assumption that Martianlife forms, like terrestrial life forms, would be expected tocycle carbon dioxide through the atmosphere with perhaps anintermediate food stage.

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A third biology experiment is the most complex. The gasexchange experiment provides several options. It assumesthat living systems must affect their environment as theylive, breathe, eat and reproduce. Part of the soil sampleis placed in the test chamber inside asmall cup. Theatmosphere is purged from the test chamber add replaced witha controlled atmosphere of helium krypton and. carbon dioxidewhich is injected at about 4:00 on sol 9 (Mars time).Immediately afterwards 0.5 cc of a rich nutrient are placedin the test chamber below the cup. This amount is insufficientto contact the sample directly but provides a humid atmosphere.The soil sample is incubated for one week (or more) each daychecking the atmosphere of the test chamber to seek evidenceof metabolism; i.e. looking for the presence-of hydrogen,nitrogen, oxygen and methane and for changes to the amountof carbon dioxide. These analyses are made at about 8:00 a.m.each Martian day.

In subsequent cycles of 'this experiment the nutrientliquid is increased so that it wets the sample. The atmos-phere is again sampled for presence of gases that might betransferred into it by living organisms or loss of gases tothe organism. Ti,.. nutrient liquid used differs from that ofthe labeled release experiment in that it contains just aboutevery, important amino acid, every known vitamin and many traceelements and is highly concentrated.Each of these biology experiments can be performed threetimes on each of the two Viking spacecraft. If positiveresults are not obtained until the third test sequence, afourth test is within the capability of the biology ,packageto provide for a control test to follow a positive third test.All biology samples. are essentially samples from the toplayer of soil.The first sample for the gas chromatograph mass spec-trometer's organic analysis is obtained on sol 8. Thisrequired only a, mall quantity of Mars' soil; less than 100milligrams. The gas chromatograph is purged before thesampling. The experiment ,seeks to establish whether organicmolecules are present in the soil of Mars and what kind. Theexperiment heats each soil sample and detects volatile com-ponents given off from the sample as a result of this heating.Higher temperatures break down even complex heavy organicmolecules into lighter volatiles that-canr be detected by theinstrument.

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Analysis of the components of organic matter takes sometime. A few abundant compounds can be quickly identified,but there are probably many components of low abundance whichhave-to be separated from the background of the instrumentitself, and it may take a&considerable time to identify them.Organic compounds that are thermally uistable decompose underheating so that the GCMS records their pieces. The experi-menters than have to try to derive what the original moleculeswere.

The next task\is to try to ascertain how such compounds'might have arisen 6niMars; i.e. from physical, chemical,photochemical or biological processes. Even as far as terres-trial organic compounds are concerned there is still somecontroversy about their origin. The GCMS is not a lifedetection experiment; but its results have important impli-cations to the search for'-life on Mars.

The second sample for this experiment is expected to begathered on Sol 22 and analyzed by Sol 27 or Sol 28. Thi.ssample is from below the surface; from the bottom of a trenchmade by the backhoe on the sampler head. Similar results areanticipated to those from the first ample; and the data fromthese first two samples is then used to plan the third sampling,-scheduled for about Dl 38.

But on Sol 8, the third sample is for inorganic'analysisof Martian soil with the X-ray fluorescence experiment. Thissample is obtained at 6:03 p.m. PDT on July 12 (about 10:30 a.m.Mars- time on Sol 8). Ideally it should consist of 30 cc (1.6,cu. in.) of particles all less than 12 mm (.48 in.) diameter tofit through the mesh of the screen: a sandy material is fine.If the Lander can only reach to areas of sheet rock a samplemight still be obtained for this experiment by waiting forwind-blown dust or pebbles to gather there. The sample armmight even be used as a deflector to cause wind-blown materialto fall into the sample funnel.

The experiment searches for elements 'in the sample andtries to determine their relative amounts. It detects elementswith greater,atomic weight than magnesium, but it cannotdifferentiate between isotopes of elements. Elements lighterthan magnesium can only bc detected as a group, but estimatescan be made of their relative abundances. It may also bepossible to estimate whether hydrates or carbonates arepresent on Mars.

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