viking 1 early results

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VIKING 1EARLY RESULTS


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Above: A sweeping 100" panorama of the Viking 1 landingsite on the Chryse Planitia basin, taken about 7:30 a.m. localMars time. Diagonal structure in middle is the meteorologyinstruments boom.

Cover photograph: The surface of Mars just before the ini-tial sample was taken on July 28, 1976. Sample was takenat -31" elevation (l ef t scale) and 215" azimuth (to p scale).Th e trench dug is shown in chapter 5.

Below: The historic first photograph sent back fr o m Marsminutes after the successful landing of Viking I on July 20,1976.


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NASA SP-408

Srreiitrfic arid Tecbmcal I?zformarion O f i r e 19761 5 1 . N A T I O N A L A ER O N A U T I C S A N D S PA CE A D M I N I S T R A T I O N

Washitzgton, D.C .


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FOREWORDEVEN fter fourteen years of planetary exploration by unmanned spacecraft, the conceptof dispatching automatic machines millions of miles from our home planet to penetratethe boundaries of the unknown still seems extraordinary. Man has never before undertakenanything like this. Where once sturdy seamen and valiant explorers risked years of theirlives-and their very lives-in a quest for glory, wealth, and the extension of their plitko-religious beliefs, we now rocket off sensitive electromechanical scouts to do our biddingand send back information about the new worlds they have encountered. This is truly anew process, characteristic of our times and skills, ideally adapted to the hostile characterand distances of the solar system.The Viking lander that began the on-site examination of Mars several weeks ago isincomparably the most versatile automated explorer ever built. (Imagine throwing a self-powered laboratory 460 million miles through space, soft-landing it delicately at a chosens p t on an alien world, and then commanding it to conduct and report on subtle biologi-cal, chemical, and physical measurements!) Seen whole, Viking is by far the most ambi-tious and venturesome automated exploration that man has ever attempted. It is a descend-ant of our earlier efforts on Earth to reach the poles or, before that, to cross the unknown

Why do we do this curious thing? W h y is man preeminently an exploring animal?Plainly our motives are many, and in te rtwkd. An unbounded curiosityseems to be part ofmans brain, an element in his genetic heritage. We have done this at least since a remotepredecessor felt a powerful need to see what was on the far side of a mountain. Earlyexplorations were driven by direct self-interest; heir goals were trade, land, power, andgold. Now we are i m p l l d Oj; motives chat are almost as coolly rational as those that gov-ern the design of our spacecraft: we explore other worlds that we may better understandour own. It is still self-interest, in a more intellectual form.

Mars is in some ways strangely earthlike. It is of a size with our planet, has days andnights, seasons of its years, and a thin atmosphere characterized by wind and clouds-i.e., weather. By some theories Mars may be a kind of proto-Earth, earlier in the sequenceof planetary evolution, a place where the geologic and atmospheric processes are far lesscomplicated than they are on our white-whorled blue planetary home. Thus knowledge ofMars may have the most immediate implications for bettering our knowledge of Earth,and every new insight in comparative planetology may be the greatest of treasures senthome by the Viking explorers.

oceans.

JAMESC. FLETCHER,dministratorNationalAeronatltics and Space Administration

August 16,1976

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PREFACESOMEmeasure of the rapidity of change in planetary exploration is reflected by the con-trast between the 1962 Mariner 2 mission to Venus and the 1976 Viking missions to Mars.In the first case, our first successful planetary flyby, a modestly instrumented probe man-aged a miss distance of 34700 km and returned several hours of data. In the presentcase, highly sophisticated orbiters and landers are each day returning rich harvests of newinformation about Mars. At this writing, the data stream began weeks ago and will verypossibly continue for months. Although the Viking spacecraft were designed for 60-daymissions, there is no evident technical reason why the exciting flow of new informationwill not continue a great deal longer than that.

During a mission such as Viking-an exhausting if exhilarating time of sleepdeficits, quick-looking the data, queuing up for future spacecraft commands, inter-relating results from many different instruments, and working around the anomaliesthat beset the best of spacecraft-it is almost impossible for an investigator to con-centrate on thorough and thoughtful analysis of the results received. So this little vol-ume, which includes no information later than August 13, 1976, is not put forward asinstant science but only as a preliminary and tentative account of early conclusions. Itshould be further noted that the findings reported here are those of the investigators con-cerned with each experiment. It can be confidently predicted that the views will be refinedand perhaps extended in some cases when this historic mission comes to its end.

JOHN E. NAUGLE,ssocitrtc ActniinistratorNational Aeronautics and Space Artministration

August 19, 1976

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CONTENTSForeword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 The Viking Mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32. The Viking Orbiter: Carrier. Relay. Observatory . . . . . . . . . . . . . . . . . 73. Entry and Landing: A Traverse of the Atmosphere . . . . . . . . . . . . . . . 254. On the Surface: A Look Around ........................... 315. Handling and Sampling the Surface ......................... 396. The Lander Environment 47...............................7. Composition of the Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 TheSearchforLife . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Appendix A-Viking Science Teams ........................ 65Appendix &Viking Key Personnel ........................ 67

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IntroductionThe Viking Project is a modern scientific adventure at the frontier of space. Like any

scientific exploration, it requires the imagination and labor of a multitude. In the fewshort weeks that the Viking 1 spacecraft have operated on and near Mars, discoveries havebeen made that have changed some theories, confirmed others, and, in the tradition ofscience, opened more questions than they have closed.

Mars has revealed much more variety than anticipated. In the search for the first landingsite, photographs were obtained showing some of the most remarkable features carved bywind and water ever seen by man. The forces of fluid in motion left an indelible historyon the surface of Mars that will be examined for decades. New features of enormousdimension, and always different,were found daily-wide gorges, scarps, faults, flat valleys,mottled erosion, ancient shorelines, deep basins, blocky terrain, knobby terrain, tablelands,sunburst craters, pedestaled craters, secondary craters, ejecta from cr at er st he descriptivegeology goes on and on.

Once cameras were on the surface the pictures revealed a very familiar scene. The Vik-ing 1Lander came softly to rest in a rocky desert with vast sand dunes reminiscent of theAmerican Southwest desert. Rocks range from pebble size to boulders several metersacross. Surprisingly or not, the true color of the landing site was red. Everything withinsight including the sky is some shade of pink or red. Many rocks reveal the weathering ofwind and time. The chemistry of the loose dirt collected by the soil sampler is an iron-richbasalt, familiar to the geochemist.

But the chemical surprises came h s t from the atmosphere and then from &e Sidagicalexperiments. Before Viking was launched, there was a flurry of interest in scientific circlesabout the amount of argon in the Mars atmosphere.Becauseof its inert nature this elementis important in tracing the history of the atmosphere. Guesses and theories as to argonabundance hovered around 20 percent. Another critically important atmosphere con-stituent, nitrogen, had never been detected. Viking has for the first time made directmeasurements of the atmosphere such that now a complete analysis, including manychemical isotopes, is available. Nitrogen was discovered, argon and its isotope measured,oxygen, its charged forms, and upper limits to the other noble gases were measured.

In the biological experiments that are still going on, a remarkable surface chemistry hasbeen discovered. The surface material is highly desiccated but made of minerals that arevery hydrated; water is chemically bound but very little absorbed in the surface. Thematerial is highly oxidized, appears to have oxygen adsorbed in its surface, and among itsconstituents are some very strong oxidizing components. All of this chemical activity isstill rather mysterious and poses a difficult milieu in which to study biology.

The weather on Mars during this summer period is benign by Martian standards: windsof only 10 to 15 m/sec and, so far, predictably from the east to southwest. Temperatureshave been somewhat surprising; the hottest part of the day occurs in late afternoon. Thesurface pressures at the landing site were within predicted ranges but have been steadily

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2 VIKING 1falling. This is consistent with an earlier discovery that the winter pole of Mars is nowcondensing out the carbon dioxide. There appears to be considerable cloudiness in thenorthern hemisphere during this time of the year.It appears that the explorations of Viking 1 have already made scientific history. TheViking Team is anxious to share our new knowledge with the world at large.JAMES S. MARTIN, R.Vik in g Project ManagerLcrngley Research Cen ter

GERALD. SOFFENVi ki ng Project ScientistLangley Research Center


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1TheViking MissionProgramGoals

The objective of the Viking mission is to advancesignificantly knowledge of the planet Mars by meansof observations from hk t i a n orbit and direct meas-urements in the atmosphere and on the surface. Par-ticular emphasis [is to] be placed on obtaining bio-logical, chemical, and environmental data relevant tothe existence of life on the p h e t at this time or atsome time in the past, or the possibility of life existingat a future date.By observing the physical and chemical compositionof the atmosphere, the daily and seasonal changes inwind, temperature, pressure, and water vapor contentnear the surface, the texture of surface materials, theirorganic and inorganic composition, and some of theirphysical properties, the Viking mission is enablingscientists to define the present conditions under whichany iviarrian bivivgid processes would have to takeplace. In addition, the Viking 1 Lander is attemptingto collect direct evidence as to whether biologicalprocesses are now occurring.

The Viking mission is providing information thatwill lead toward an eventual understanding of the his-tory of Mars. Visual imagery and infrared observationsof the surface from orbit are revealing the geologicprocesses that have shaped the planets surface features.They can also indicate past alterations in the composi-tion of the atmosphere and the surface materials. Suchinformation is, of course, relevant to the questions ofMars evolution as a planet, as well as to bio-organicevolution, and also to the development of our under-standing of Earths place in the history of the solarsystem.

Unlike earlier ventures in planetary exploration,Viking offers investigators the rich new dimension ofsimultaneous observation. The added value is im-mense. Simultaneity gives a chance to relate observa-

tions on a global scale with iindings tied to a spot onthe surface. The viewpoint and scale are so differentas to stretch investigators imaginations. Patterns ofvast aeolian deposits seen from orbit can be comparedwith the shape of aeolian deposits around a pebble.Infrared temperature measurements from the Orbiterindicate a freezing of part of the atmosphere ontothe south polar cap; and at the same time, a sensor atChryse Planitia feels the reduction of atmosphericmass.

Sometimes the viewpoints are SO far apart they arehard to reconcile. The Orbiter sees places where low-lying fogbanks of water ice appear and dissipatedaily-while the Landers biology instrument sees sur-face particles that react actively to a whiff of watervapor. Plainly Mars is nonuniform; plainly it will taketime and thought to understand all that we see.Detailed accounts of several of the earliest scientihcfindings of the Viking mission are scheduled to appearin a series of reports in the August 27 issue (Vol. 193,No. 4255 ) of Science.Mission Plan

Two identical Viking spacecraft, each consisting ofan Orbiter and a Lander capsule, were launched onAugust 20 and September 9, 1975. With arrivals atMars 7 weeks apart, Viking 1 and Viking 2 will con-duct many of their operations concurrently. However,the mission plan allows the Viking 1 Lander to com-plete its period of high-level activity before having toshare the Martian surface with the Viking 2 Lander.Each spacecraft arrives in the vicinity of Mars on atrajectory that would take it past the planet. Duringthe approach, the cameras and infrared sensors obtainglobal views of the entire disk of the planet in differ-ent spectral bands. Then a prolonged operation of itsrocket engine reduces the spacecrafts velocity SO that

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4 VIKING 1it is captured into orbit. The orbit is highly elliptical(fig. 1-1). At periapsis, which is placed over thelanding site, the orbit is 1500 km above the surface.An apoapsis altitude of about 32 600 km produces anorbital period that is synchronous with Mars siderealperiod of 24.6 hours, so that the spacecraft passes overthe landing site daily, at the same local time. Raisingor lowering the apoapsis altitude produces an asyn-chronous period that brings a different part of theplanets surface under each revolutions orbital track.The Viking 1 orbit is inclined 33.4O to the equatorialplane.After the landing site certification process (de-scribed in ch. 2 ) has been completed, preparations aremade to separate the Lander from the Orbiter. Whenit separates, the Lander is still enclosed in its aeroshelland base cover. The whole assembly is called thedescent capsule. Four of the aeroshells small rocketengines fire to slow the capsule into a descent trajec-tory. After coasting for several hours, the descent cap-sule enters the atmosphere and begins to deceleratebecause of aerodynamic drag. The aeroshells ablativeheat shield burns away, carrying with it the intenseheat of entry.

At an altitude of about 6 km, a parachute is de-ployed to slow the Lander further, and the aeroshellseparates from the Lander. The parachute and thebase cover are discarded at about 1.4 km, and theLanders own set of three terminal descent enginesbrings it down to a soft landing.

The deorbit and landing sequence is controlled en-tirely by the Landers computer, according to instruc-tions that the ground controllers fed into it beforeseparation. There is no possibility of real-time con-trol when it takes a radio signal 18 min to travel themore than 300 million kilometers that separate thetwo planets. At landing, the Landers computer hasenough instructions to operate the Lander and its in-struments for 60 days on its own. Once communica-tion with Earth is established, these commands aremodified and updated, normally every few days.

The Lander receives all its instructions directly fromEarth (fig. 1-2 ) . The daily rotation of Mars permitsabout 9 hours for communications. The Lander sendsits data to Earth in two ways. The transmitter thatcommunicates directly with Earth can operate forabout 70 minutes per day. At a data rate of 500 bits(computer binary digits) of information per second,

Lander separation(minimum coast period, 2.25 hr)/

Orbiter atLander entry

Lander separation(maximum coast

period, 6 hr)Orbiter atLander

Entry, 250 km(BOO 000 f t 1

Touchdown may occur\ Periapsis: 1 to 6 hr from sunset km /

FIGURE1-1.-Spacecraft relationships during landing.


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THE VIKING MISSION 5

Orbiter to Earth two-way radio linkOrbiter science dataLander relay dataEngineering elemetryDoppler and range signalsEarth

Lander to Orbiter one-way relay radio link.0 Lander science data

Engineering telemetry

0 Lander science data0 Engineering telemetry0 Doppler and range signals0 Commands

~~

FIGURE1-2.-The basic radio linksused in Viking 1 .

a b u t 2 million bits can be delivered each day overthis direct link. Later in the mission, increasing dis-tance will cut the data rate in half. A far larger volumeof data reaches Earth through the Orbiter relay link.The Lander can transmit data to the Orbiter wheneverthe latter is more than 2 5 O above the local horizonand within a range of 5000 km. This daily communi-cation window varies from 10 to more than 40 min,and while it is open the Lander sends 16000 bits ofinformation per second. At the time of this writing,the daily relay communication link has averaged about42 minutes. This has permitted the entry of about 40million bits of Lander data daily into the Orbiters taperecorder for transmission to Earth.

Since the total amount of scientific information ac-quired from the Lander is largely determined by theavailability of the Orbiter relay link, the mission planrequires an Orbiter to be in synchronous orbit overeach Lander during the Landers period of high-levelactivity. After the Viking 1 Lander has completed the

investigations that require a high data rate, it will gointo a reduced mission mode. This relieves the Viking1 Orbiter of its data relay duties and permits it to startits global walk. With the orbital period reduced to23.1 hours, the track then walks around the planet,shifting about 22.5O westward on successive revolu-tions. A large number of orbital science observationswill be possible of areas that cannot be adequately ob-served from the present synchronous orbit.

At present, the Viking 2 spacecraft is in an asyn-chronous orbit at a higher inclination, so that consid-erable global exploration is already in progress. Onthe assumption that the Viking 2 landing is successful,much flexibility exists in providing relay support foreither Lander with either Orbiter. At a later stage ofthe mission, a plane change maneuver is planned forthe Viking 2 Orbiter, to increase its orbital inclinationto about 75. This will permit its instruments to ob-serve the north polar region when the polar cap is atit s minimum size.


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6 VIKING 1On November 25, 1976, Mars and Earth will be in

conjunction, lined up on opposite sides of the Sun.Beginning about the middle of November, as con-junction is approached, communications betweenEarth and the Vikings will be interrupted by theeffectof the solar corona. In anticipation of the black-out, the Landers will be powered down to a safe con-dition, and the computers aboard the Landers andOrbiters will be loaded with sufficient instructions tocarry on by themselves. The primary mission willhave been completed, but the spacecraft are expectedto survive the blackout period and carry out an ex-tended mission. The ability to continue observationsover a full Martian year would provide a very impor-tant bonus for several of the scientific investigations.Mission Operations Strategy

As one can deduce from the mission plan, the Vik-ing mission operations are preeminently characterizedby their complexity and their flexibility. There will beas many as four space vehicles operating simultane-ously, at a vast distance and with an intricate patternof communication links. The conventional way to dealwith this complexity would be to operate in a pre-planned, highly programmed mode. Yet, if the scien-tific exploration potential of the Viking mission is tobe fully exploited, the operations must be capable ofadapting to what is being learned. From the beginning,the planning has been for an adaptive mission. Theoperations organization, procedures, and computerprograms have all been designed for the exceedinglydifficult task of maintaining flexibility and full controlconcurrently.

At any stage of the mission, there is a current Mis-sion Profile Strategy in being that describes how therest of the mission is to be conducted. As both sciencedata and data concerning the health of the spacecraftare acquired, they are used to revise this plan weeklyin a long-range planning activity. Long-range planningis primarily concerned with the period 11 to 17 daysprior to execution. It concludes with a Science Re-quirements Strategy that defines the desired mission toan intermediate level of detail.

Medium range planning starts with the Science Re-quirements Strategy, and concentrates on the period 6to 10 days prior to execution. A daily meeting resultsin a Final Mission Profile that lists all mission profileevents in time sequence. This is the basis for the se-

quence and command generation process, which ingeneral takes up to 5 days.Some activities, particularly aboard the Lander, needa faster response. A Lander Science ExperimentsOperations Strategy has been designed to have the gen-eral capability to respond to data that arrived on thenext-to-last communication downlink for many of theLander experiments. (This is, approximately, a 2-dayturnaround time on selected operations. ) Making thisvery demanding capability work smoothly has been amajor requirement on the mission operations system,and it has been met so well that the operation almostlooks easy.Mission Events to Date

Viking 1 was inserted into orbit on June 19, 1976.The Lander was scheduled to separate from the Orbiteron July 4 , if a landing site could be certified in theintervening time. Because the Orbiters visual imageryraised doubts about the suitability of the prime sitethat had been selected before the mission, the separa-tion was postponed to permit the examination ofother portions of the Chryse basin. A site about 900 kmwest of the prime site was finally certified, and theLander touched down there on July 20. The Landerslocation is 22.27 N, 48.00 W.The orbit of the Viking 1 Orbiter, which was ini-tially synchronous over the prime site, was adjusted onJuly 8 to let the Orbiter walk westward in thesearch for a more suitable landing site. Another ad-justment on July 14 stopped the walk, so that theOrbiter is now in synchronous orbit over the Lander.This track also permitted the Viking 1 Orbiter to ex-amine a broad area of Cydonia as a preliminary phaseof the Viking 2 landing site search.At the present writing (three weeks after the land-ing), all of the Landers instruments are working withthe exception of the seismometer, whose three sensingmasses have failed to uncage from their flight con-figuration. On July 28, the surface sampler deliveredsamples of the Martian surface to the biology, themolecular analysis, and the inorganic chemistry instru-ments. All three instruments have completed the firstcycle of experiments with these samples.On August 7 the Viking 2 spacecraft was put intoorbit. Its inclination is 55, and its present asynchro-nous period of 27.4 hours permits it to explore newareas on each revolution.


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The Viking Orbiter: Carrier, Relay, ObservatoryThe Viking 1 Orbiter is the second American

satellite to explore Mars from orbit. Like its predeces-sor, Mariner 9, it put itself into orbit, is examiningthe planets surface with several instruments, and istransmitting the resulting data to Earth. It also hassome additional duties to perform, Having carried adormant Lander into orbit, it turned its instrumentsto the detailed examination of the prime landing site.When that site revealed itself to be more hazardousthan expected, the Orbiter was released from its repeti-tive track to explore a broad region to the west untila suitable landing site could be found. Its orbit wasthen resynchronized over the newly certified site, andthe Orbiter became the launch base for the landingoperation. With the Lander safely on the surface, theOrbiter has become a facility for relaying data. Its in-struments monitor the region around the Lander sothat any changes they can detect (for example, atmos-pheric temperature, water vapor content, clouds, anddust storms) can be correlated with the Landers con-temporaneous observations. Meanwhile, since theorbital track covers the Cydonia region, the Viking 1Orbiters instruments are examining the area surround-ing the Viking 2 prime site. Other areas of the planetunder the orbital track are also being examined. Later,when it can once more be released from synchronousorbit, it will carry its three instruments on an excur-sion of exploration around the planet.Although the Orbiter (fig. 2-1) bears a facnily re-semblance to Mariner 9, the additional functions haverequired some important changes in design. The con-cluding section of this chapter describes the main de-sign features of the Orbiter and its instruments.The three instruments that constitute the Orbitersscientific payload are the visual imaging subsystem(VIS), the infrared thermal mapper (IRTM), andthe Mars atmospheric water detector ( M A W ) . All

three are mounted on one planetary science scan plat-form (fig.2-2), whose orientation with respectto theOrbiter is motor-driven about two axes. The instru-ments are boresighted to point in a common direction.With this arrangement, the three instruments can beaimed at the surface during much of the desired por-tion of the orbital period, while the Orbiter maintainsits solar orientation to generate electrical power mostefficiently. The scan platform can be commanded tolook a t any target within view on the planets surface.If stereoscopic observations are desired, the VIS looksahead just before passing over the site, and then looksback a t the same area.

The VIS consists of a pair of identical cameras andtelescopes. Its first, and most exacting, requirement isto cover the necessarily large Viking landing sites withcontiguous photography in high resolution, so thattheir suitability for a safe landing can be assessed.vlsuai images obrainrd honi &e pi5zpis ( d i dlow point) altitude of 1500 km have a ground resolu-tion of about 100 m. Application of this VIS capabil-ity to just a few of the many scientifically interestingareas of the Martian surface has already added greatlyto knowledge of the geologic processes that haveshaped the planet.The IRm consists of a group of infrared radiom-eters designed to measure and map variations in the

ORBITER IMAGING -AMMichael H . CarrWilliam A. Baum Harold MasurskyKarl R. BlasiusGeoffrey Briggs Lawrence A. SoderblomJames A. Cum Joseph VeverkaThomas C. DuxburyRonald Greeley

John E. GuestBradford A. Smith

John B.Wellman


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8 VIKING 1

Lo w gain anten na Propulsion

FIGURE -1 .-The main elements in the Or bit er spacecraft.

FIG UR E -2.-The science scan platfor m aboard the Orb iter.

~ ~~

THERMALAPPING TEAMHugh H. Kieffer Guido MunchStillman C. Chase Gerry Neuge bauerEllis D. Miner Frank Palluconi

temperature of the planets surface. Variations fromplace to place under similar conditions of solar illumi-nation can indicate differences in the composition androughness of the surface materials, as well as the ex-istence of areas where internal heat may be flowingout. Varfations in the nighttime cooling rate reflectdifferences in th e average size of surface particles. T heinstrument also measures stratospheric temperaturesover broad areas, and their variations with time.

The M A W D is an infrared spectrometer that isdesigned to map the distribution of water vapor overthe planet. Water vapor is a minor constituent of theMartian atmosphere that is of the greatest importanceto understanding the meteorology, the geology, andthe biology of the planet. L ife on Earth is completelydependent on the availability of water, since manybiochemical reactions take place in aqueous solutions.

WATER APORMAPPINGTEAMC. Barney FarmerDonald W. Davies

Daniel D. LaPorte


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THE VIKING ORBITER 9Measurements by the MAWD constituted one inputto landing site consideration. Later in the mission,when the Orbiter is in a nonsynchronous orbit, theM A W will observe the global distribution of watervapor and its variation diurnally and seasonally to at-tempt to discover the sources and the movements ofthe vapor.How the Viking 1 Landing Site Was Certified

A good landing site is one that affords a high prob-ability of landing safely and obtaining a surface sampleand that is likely to provide significant scientific in-formation about Mars and its history. The prime sitewas in the Chryse plain, near the mouth of the bigchannel system discovered by Mariner 9. Since thechannels appeared to have been formed by runningwater, it was reasoned that the material at the mouthshould represent the material gouged from the high-lands south of the basin, as modified by both windand water transportation.With regard to landing safety, it was recognizedfrom the beginning that any landing without thebenefit of visual guidance entailed some risk. TheViking landing dispersion ellipse (within which itcould be expected to land with 9 ercent probabil-ity) is 100by 22 0 km.Clearly, it would be impossibleto guarantee the absence of landing hazards within sovast an area. The purpose of site certification, then, isto maximize the probability of landing success.

The site factors that affect landing safety are topo-graphic configuration, surface roughness. bearing ca-pacity, and the prevalence of hard surface protuber-ances such as boulders. The last factor is of particularconcern because the Viking Lander has only 22 cm ofground clearance.

The main tool for assessing the site hazards is theOrbiters visual imaging subsystem. Features thatmight be a hazard to the Lander are mapped from theimages; then an assessment is made of the probabilityof the Landers encountering such hazards, given theaiming errors. Stereoscopic photography by the VISmakes it possible to plot the topographic configurationdirectly. The stereoplotting instruments and the photo-

LANDINGSITE STAFFH. Masursky J. F. NewcombN. Crabill E.D. Vogt

grammetrists trained in their use are available at theU.S. Geological Surveys astrogeology laboratories inFlagstail, Ariz. The problem is the coarseness of thedetail at the VIS monoscopic ground resolution of100 m, compared to the dimensions of the lander. Itcan be assumed that a site that is rough on the 100-mscale will be rough at the scale of the Lander. Un-fortunately, the reverse is not necessarily true. Oncethe areas that are visibly hazardous have been elimi-nated, the real usefulness of the VIS pictures is in pro-viding scientists with an understanding of the geologi-ca l processes that have been in operation at the site andin the surrounding regions. A knowledge of the proc-esses permits an estimate of the probability of landinghazards far below the limit of image resolution.

There are two sources of information about a sitesfine-scale surface characteristics. These are the VikingOrbiters infrared thermal mapper and radar observa-tions from Earth. Their information tends to be cryp-tic without the knowledge of surface processes thatcomes from the interpretation of the VIS images.The Viking IRTM is capable of providing informa-tion about the thermal inertia of a sites surface ma-terials. A predominantly sandy surface, for example,gets warmer in the daytime and colder at night than aboulder field. In the case of the Viking 1 site, un-fortunately, the Orbiter did not pass over the area ata suitable time of day.Radar observations can tell something about surfaceroughness at a scale of about 100 to 200 times thewavelength of the radar signal, which is comparableto the Landers dimensions. Plains that show extremeradar scattering are often rough on a fine scale. Sanddune areas on Earth, for example, exhibit such scatter-ing of airborne radar signals. Since the returned signalpower is also dected by the nature of the surfacematerials, radar data should be interpreted in the lightof the available geological information.

Radar observations of Mars can only examine a smallregion close to the sub-Earth point (the point closestto Earth at the time of observation). The rotation ofMars moves the sub-Earth point rapidly in longitude,while its latitude travels slowly across the Martian trop-ical and sub-tropical zones with the orbital motions ofthe two planets. As it happened, various parts of theregion surrounding the Chryse prime site at 19.5 Nand 34 W began to become accessible to radar ob-servation just a few weeks before Viking 1 went intoMars orbit. Observations made a few degrees south ofthe prime site indicated that the region was somewhat


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10rougher than the Martian average, but not decisively so.

Once the spacecraft was in orbit an d VIS coverageof the prime site began to come in, it became a ppar enttha t the channels, instead of ending just upstream fro mthe site, ran right across it. As a mosaic of VIS photo-graphs shows (fig. 2-3), the channels are incisedsharply in the surface of the plain, leaving islandsto stand between them. T he channels provide evidence.of a strong northerly flow of fluvial currents thatcarved grooves and eddy channels in the islands. Theupstream end s of many islands, such as the on e at thebottom of th e figure, are marked by old craters, whoseoute r ramparts resisted the fluvial erosion. Thei r dow n-stream ends have the tails that are characteristic offluvial deposition.The channels themselves are marked with irregular,blotchy depressions where the surface was strippedaway by erosive action. Figure 2 - 4 shows such a p atchof scabland. Evidently, the prim e site was an area ofboth erosion and deposition by strong currents. Whilethe site was extremely interesting scientifically, itscomplexity made it very difficult to estimate the pro-portion of the surface that might be studded withfields of dangerou s blocks.Project officials made the decision to delay the land-ing in order to allow time to study the region to thenorthwest where the streams were more likely to havedeposited the less blocky material they transported.T he area was photographed by po inting t he cameras offto the west of the orbital track, rather than by shiftingthe track. Th e fluvial channels diminished in that di-rection, and the terrain appeared smoother.A new landing zone was tentatively selected, at aplace that would be observable by radar over the July4 weekend. T he observations were made from the verylarge radio observatory at Arecibo, Puerto Rico. Theyshowed an area of markedly weak radar signal returnscentered a t 44 W, in the zone that was unde r consid-eration for landing. Although the exact cause of therad ar anomaly at tha t location was not determine d,the indication was quite clear that the Lander shouldavoid it. Accordingly, the project officials decided onJuly 7 to alter the orb it the next day so that the Orbitercould explore further westward in the Chryse plain.

Photographs taken by the VIS in the next few daysshowed the smooth-appearing plain extending a fewmo re degrees to the west before the channels that oncedrained the highlands west of the Chryse plain wereencountered. Figure 2-5 is a mosaic of the region,showing the channels in the south and west parts and

FIGURE2-3.-Meandering channels in the northeast Chryseregion.

the ellipse centered on the lan ding site th at w as finallyselected. (The outer ellipse represents the 99 percentlanding dispersion probability zone, and the innerellipse represents a 50 percent probability.) Th e lowridges that twist across the smooth plain very closelyresemble the ridges in the mare regions of the Moon,which a re basaltic lava plains. O n the Moon, th e mar eridges are not as hazardous to a landing vehicle as theymay appear-in fact, Surveyor 5 landed safely on o nesuch ridge in 1967.The Arecibo radar data indicated that the total re-turned power was up to normal Martian levels in theregion to the west of the 44O W anomalous area, andthat the apparent fine-scale slopes averaged about 5at 4 7 . 5 O W , diminishing perceptibly to the west. T hesite that was certified, at 22-4 N, 47.5 W , r ep re -sented a good balance between the visible and radarroughness, in an area where it appeared that the sur-face processes were well understood.S o m e M a r t i a n F e a tu r e s O b s er v e d b y theVisua l Imag ing Subsys tem

In the course of surveying possible landing sites andobserving additional areas that are accessible from theorbital track, the VIS has taken many hhotographs ofnotable fe atur es of the M artia n surface that graphically


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12 VIKING1

FIGURE -5.-Mosaic of lan din g site in Chryse Planitia.

demonstrate some of the processes that have shapedthe surface. It has also photographe d some distinctiveatmospheric phenomena. A few of the spectacularphotographs are included here.

Figure 2-6 is a mosaic looking across a section ofValles Marineris, the huge canyon system discoveredby Mariner 9. The far wall, which is about 2 kmhigh, has collapsed repeatedly, in a series of massivelandslides. Th e apro n from the slide in th e centerovercoats previous aprons. Th e near wall also has afresh-appearing landslide apron. Evidently, collapse isan important part of the process of widening canyonshere. Streaks seen on the canyon floor point to wind

erosion as an agent for removing the debris. Stratifica-tion is evident in the upper part of the far wall. Thelayers indicate a succession of deposits that might belava flows, volcanic ash, or wind-blown m aterial.

Figure 2-7 shows a valley whose head (r ig ht ) is ajumbled mass of debris from the collapse of the sur-face. The streamlined forms extending down thevalley suggest that here the removal of subsurface ma-terial by flowing water may have been the agent ofcollapse. A possible source of the water is the meltingof subsurface ice. Ne ar t he to p of the picture, a sinuousrille very much like those that are common on theMoon wanders across the plain. Although the Apollo


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THE V IK IN G ORBITER 13

FIGURE?-h.-Section of th e Valles Marineris canyon system.


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VIKING 1

FIGURE2-7.-Jurnbled debris at the head of a valley near one Viking 2 site.15 astronauts landed near a large lunar sinuous rilleand briefly explored its bank, the origin of thesefeatures is still in dispute.

Relatively fresh impact craters on Mars appear dif-ferent from those typical of the Moon and Mercury.The material ejected from the craters seems to haveflowed as a fluid.

Figure 2-8 shows the crater Arandas, about 25 kmin diameter, with its conical peak and distinct rim.Th e material outside the rim forms an ap ron that endsin lobate flow scarps. In the lower left corner, th e flowhas been deflected around a small crater.

The crater Yuty, in figure 2-9, is another impactcrater whose ejecta blanket was formed by fluid flow.Th e leading edges of the flow evidently carried thelargest blocks of debris, which left a prominent ridgewhen the flow ceased. The debris flows shown hereand in the preceding photograph may be lubricatedby gas or water derived fro m melting and vaporizationof subsurface ice.Figure 2-10 shows an entirely different kind ofcrater-the caldera of the giant volcano Arsia Mons.A caldera is formed by the collapse of a volcanos sum-mit. Arsia Mons, in the Tharsis region, is about 17 km


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FIGLXE 2-8.-Flow patterns around the craterArandas.

FIGURE 2-9.-Flow patterns around the craterY u y .


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1 6 VIKING 1

FIGURE -lO.-The caldera of th e volcanic crater Arsia Mon s.

hig h, and its caldera is 1 00 km across. In this earlymorning photogra ph wispy clouds obscure much of th ecaldera's floor. Many lava flows can be seen on theflanks of the volcano.

The largest volcano on Mars (if not in the solarsystem) is Olympus Mons, whose summit protrudesthrough a wreath of clouds in figure 2-11. OlympusMons is about 600 km across at the base and approxi-mately 25 km high. Its multiringed caldera formedthrough a series of subsidence episodes. T h e cloudsextend up the flanks of the volcano to an a ltitu de ofabout 19 krn, leaving the summit cloud-free. Thecloud cover is densest on the western side of the

mountain, and beyond it (upper left) is a well-definedtrain of wave clouds. In the northern spring and sum-me r seasons this cloud cover builds u p du rin g the day,and becomes large enough in the afternoon to be seenfrom Earth. The clouds are believed to be composedof water ice condensed from the atmosphere as it coolswhile moving upslope.Ear ly Results of t h e I n f r ar e d T h e r m a lM a p p i n g E x p e r i m e n t

On e of th e importan t objectives of th e experim entis to investigate the diurnal temperature variation atmany areas of the planet. Since that req uires the obser-vation of each area at different times of day, data ofthat type will be quite limited until the Orbiter canbe released from synchronous orbit. In the meantime,the experime nt is acquiring other interesting kinds ofinformation.A global view is obtained when the Orbiter is wellaway from the periapsis of its orbit. Considerablethermal structure is apparent in such a view. Figure2-12 illustrates the global surface temperatures ob-tained 4% hours before periapsis on one revolution.The planetary disk is half illuminated, and surfacetemperatures o n the daylight side rise to above 240 Kat noon on the equator. (T he Kelvin tem perature scalebegins at absolute zero, so that tem peratures are alwayspositive numbers. The gradations are the same asthose of t he C elsius scale, and 0 C is equivalent to27 3 K .) Th e contour (isoth erm) interval in thefigure is 10 K on the daylight side, and 2 K o n t h enight side. The most conspicuous thermal feature isArsia Mons, the southernmost of three large volcanoeson the Tharsis ridge. It is near the morning termina-tor, and the high elevation puts the east-facing slopesin the sunlight while nearby areas are still at their un-expectedly low predawn temperatures. The compara-tively strai ght isotherms just east of t he mo rnin g ter-minator indicate that the temperature rises quite uni-formly at first. Toward noon the temperature is moreaffected by the area's surface reflectivity.About three hours before periapsis, the VikingIRTM can view the southern hemisphere to the southpole. These are the first observations m ade of th e polarregions in midwinter, and the results are interesting.Figure 2-1 3 illustrates the observed temperatures. Theisotherms are drawn at 10' intervals down to 150 K ,an d 1' intervals below that. The lowest temperatureobserved is 134 K, just off the south pole. Tempera-tures below 148 K had not been expected, because


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THE VIKING ORBITER 17

FIGURE -1 1 . 4 l y m p u s Mons, wreathed in clouds

that is the equ ilibrium temperature for th e sublimationof frozen carbon dioxide at the mean atmosphericpressure of 6 millibars. At th is stage of the w interseason, the polar cap of frozen carbon dioxide isactively growing, and as much as a fourth of the en-tire atmosphere m ay eventually freeze onto th e ground.

Th e loss of atmosphere is even felt in the northernhemisphere, according to indications from rhe Viking1 Landers meteorology e xperime nt (discussed inch . 6 ) . One hypothesis to explain the low polartemperatures is that the active removal of CO:! fromthe atmosphere over the polar cap substantially in-

&


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18 VIKING 1

FIGURE -12.-G lobal surface temperatures.

creases the local concentration of noncondensable gasessuch as argon, nitrogen, and oxygen in the lowest layerof the atmosphere. As the partial pressure of CO2 isreduced, the equilibrium temperatures are also de-creased. Further observations are needed to rule outthe possibility of an alternative explanation, involvinga high-level cloud of frozen COz, thick enough to im-pede observation of the ground.Another interesting feature in the figure is thepresence of several temperature troughs, with a depthof 2 or 3 kelvins, that appear to emanate from thesouth pole. The arcuate shape of these troughs is

reminiscent of the shape of weather patterns aroundthe Earths poles. Further observations will establishwhether they represent a pattern of the global scaleatmospheric circulation.Early Results of the Atmospheric WaterVapor Mapping Experiment

Like the IRTM experiment, the water vapor map-ping experiment is temporarily limited in the accom-plishment of its objectives by the synchronous orbitin which it is operating. In order to i a p he variationsin the water vapor content with location and with


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THE VIKING ORBITER 19

FIGURE 2 -1 3 . 4 b s e r v e d remperatures of south polar re-gions.

time, the experiment needs to observe each area ofinterest at different times of day. Additionally, sincethe experiment is much concerned with seasonalvaria-tions, an extended period of observations is required.The measurements that have been made to date aremostly in the southern hemisphere, which is the dryone at this season. They show a strong latitude depend-ence, increasing gradually northward across the equa-tor.

Figure 2-14 is a map of the low-resolution observa-tions made about 30 min before periapsis on severalearly revolutions. A water vapor value (i n precipitablemicrometers, as explained in the instrument descrip-tion) is printed in the center of each en-degree squareobserved. The crosshatched line shows the position ofthe terminator. (The finer curves are not related tothe water vapor values; they are elevation contours onthe base map.) The local time of the observations isindicated across the top. In addition to the latitude

dependence, the plot shows another interesting point:the apparent independence of the values in a latitudeband from the time of day. This is somewhat surpris-ing, because at one location ( 10 N, 83O W ) that wasmonitored over a local time interval of about 6 hours,the water vapor content rose steadily from dawn untilnoon. Perhaps the monitored location has a differentmechanism (such as ground fog) for the release ofvapor from the solid phase, or perhaps the areas tothe west of that location simply have more water.Observations from nonsynchronous orbit shouldeventually clear up that question.The Orbiter

The design of the Viking Orbiter (fig. 2-1 ) differsfrom that of Mariner 9 in several important respects.To reduce its velocity enough to be captured into orbitwhile bearing its passenger, the Viking Orbiter carriedthree times as much propellant as Mariner 9. Thestructural augmentation required to support the largertanks resulted in a total Orbiter weight (with pro-pellant and without the Lander capsule) of 2325 kg.The requirement placed on the Viking Orbiter toprovide high-resolution images of the Viking landingsites has not only determined the characteristics of thevisual imaging subsystem; it has caused the use of anentirely new type of tape recorder for spacecraft datastorage. Whereas the Mariner 9 television camerasoperated on a 42-sec cycle, the Viking Orbiter camerasmust take a new picture every 4.5 sec to provide con-tiguous coverage of a landing site. This means thatvisual information must be recorded at a rate of morethan 2 million bits per second, and then played backat the very much lower rates set by the communica-tion link. The five available playback rates are 1, 2,4, 8, and 16 thousand bits per second. This uemen-dous disparity between the recording and playbackdata rates is dealt with by recording the visual infor-mation in parallel on Seven channels of an eight-chan-ne1 tape. The eighth channel is used to record datafrom the Orbiters other two scientific instruments,other Orbiter telemetry, and all the data transmittedby the Lander in the relay mode of communication.During playback, only one channel is read out at atime. Even with this approach, the recorders mechani-cal tape drive must operate through an extraordinarilywide range of tape speeds. The Orbiter has two taperecorders, each capable of storing 640 million bits(55 television frames plus the data stream recordedon the eighth channel).


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20 VIKING 1Local t ime

6:OOa.m. 7:OO 8:OO 9:OO 1O:OO 11:OO 12:oo i:OO 2:OO 3:OOp.m.

180' 170' 160" 150 140' 130" 120" 1IO" 100" 90" 80' 70" 60" 50" 40 30"West longitudeFIGURE -14.-Atmospheric water vapor observations.

The Viking Orbiter needs more electrical powerthan its Mariner predecessor. The four solar panels,with a combined area of 15 m2, generate 6 2 0 W atthe distance of Mars from the Sun. They differ in ap-pearance from those of the Mariner series becauseeach is divided into two subpanels. The division m adeit possible, by double folding, to stow these largerpanels within the launch-vehicle fairing. The Orbiternormally flies with the solar panels facing the Sun.When it must be in some other attitude, as well as atpeak-load periods, two 30 A-hr storage batteries sup-plement the solar power.

The Orbiter's communication system has a few du-ties to perform beyond those of the system used onMariner 9. It must receive signals from the Landerfor relay to Earth. It must also transmit simultane-ously in the S-band (at 2295 MHt) and in the

X-band (at 8415 MHz). The simultaneous transmis-sion capability is an important part of several radioscience investigations. Th e signals at both frequenciesare directed to Earth by the high-gain antenna, aparabolic dish 1.5 m across. Th e antenna, mounted o nthe side of the Orbiter, is motor-pointed about twoaxes. This enables the flight team to keep the anten-na's narrow beam directed at the Earth despite thechanging angular relationship between the Earth, theSun, and the Orbiter. T he high -gain antenna also re-ceives the S-band signals from th e stations of theDeep Space Network. During periods when the Or-biter is not oriented toward the Sun, and the Earth isnot in the beam of the high-ga in antenna, limited two-way communication in the S-band is maintainedthrough the low-gain antenna. The field of coverageof this antenna, which is moun ted ato p the Orbiter,


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THE VIKING ORBITER 21exceeds a hemisphere. The Viking Orbiter has a thirdantenna, mounted on the outer end of a solar panel,that receives the signals from the Lander that are tobe relayed to Earth. This communication between theLander and the Orbiter is one-way, at the UHF fre-quency of 380 M H t It was turned on shortly beforethe separation of the Viking 1 Lander, and operatedat a 4000-bit-per-second rate until touchdown, whenthe Lander immediately switched to the normal16OOO-bit-per-second rate. (In fact, the Orbiter te-lemetry event noting this change was one of the firstdefinite indications of the landing. )Visual Imaging Subsystem

The VIS, similar to that of Mariner 9,consists of twotelevision cameras and their associated electronics. A

camera comprises a telescope, a shutter, a filter wheel,and a vidicon tube (fig. 2-15). The two VIS camerasare the same in all respects, with Cassegrainian tele-scopes of 475-mm focal length, and identical six-sector filter wheels. The field of view of each camerais about 1.5' by 1.7, so that from the periapsisaltitude of 1500km each frame covers an area 41 by 46km on a side. The axes of the two cameras diverge byabout 1 . 4 O in the direction perpendicular to theground track. The cameras operate alternately, withthe shutter of one camera exposing a frame when theother camera is midway through scanning its frame.With each camera repeating its cycle every 4.5 sec, thecoverage of successive frames from the same camera iscontiguous along the ground track. Thus, the productof a photographic sequence near periapsis is a swath

Camera head "E" assembly

mera head "A"

Bus electronics"E" assembly

Bus electronics

Focus and deflection

Compartmentreference Lowtion Function

1 Bottom Shutter and filter wheel output electronics2 Left side Digital sequencing logic3 Top front Video amplifier chain4 Top rear Analog to digital converter5 Right side Vidicon analog controls6 Rear Vidicon power supply7A1 Front Shutter assembly7A2 Front Filter wheel assembly8 Bu s electronics Low voltage power supply

FIGURE2-1 5.-Visual imaging subsystem aboard the Orbiter.


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22 VIKING 1of contiguous coverage 80 km wide and about 1000km long.

The filter wheel in each camera makes it possible toselect the wavelength bands in which the image isproduced. Imaging of the same areas with differentfilters permits the differentiation of various surfacematerials and atmospheric conditions by spectral re-flectances. One filter in the wheel is clear, permittingpassage of the entire wavelength band, from 350 to700 nm, to which the vidicon target is sensitive.(Wavelengths between 350 and 400 nm are in thenear-ultraviolet, while the remainder of the band isvisible.) A red, a green, and a blue filter divide the fullband into three bands that overlap slightly. Of the tworemaining filters, the violet filter passes only the wave-lengths from 35 0 to 450 nm, and the minus-blue filterblocks those wavelengths while passing the remainderof the visible spectrum.A shutter allows the image formed by the telescopeto reach the faceplate of the vidicon during an inter-val that can be varied between 0.003 and 2.7 sec.When the vidicon target has been exposed to a lightimage, it holds the image in the form of a two-dimensional array of static charges. The target is thenscanned by an electron beam that neutralizes the im-age and converts it to a time-varying signal. The beamscans the target in 1056 lines, each composed of 1182picture elements (pixels).A pixel, a somewhat theoretical concept when thetelevision signal is in analog form, becomes a realentity if the signal is converted to digital form. Inthe Viking Orbiter, the flight data subsystem (FDS)digitizes analog information from all instruments.It accepts the signal from the VIS one scan lineat a time, dividing the signal waveform into briefsampling intervals that correspond to single pixels.Each pixel is assigned one of 128 discrete intensitylevels. Since 128 equals 2', it takes seven binary digits(bits) to distinguish that many levels. Each pixel be-comes a seven-bit word when it enters the tape re-corder. A frame of VIS imagery comprises 1% mil-lion pixels, or 8700000 bits of data. With picturestaken from periapsis, each pixel represents a squareon the surface about 40 meters on a side.

The flight data subsystem feeds the digitized vis-ual data into seven data channels of the tape re-corder. Successive pixels enter successive channels.Channel 1, for example, will record the first, eighth,fifteenth, etc., pixel of each scan line. This system ofparallel recording produces on the tape an intricately

scrambled record of the image. The tape is playedback one channel at a time for transmission to Earth.It is only after all seven channels have been receivedthat the pixels can be sorted out to reconstruct theimage on the vidicon target.Infrared Thermal MapperThe IRTM (fig. 2-16) is a 28-channel infraredradiometer. Each of its four telescopes has an inter-ference-type filter that passes radiation in a selectedwavelength band. In the focal plane of each telescopeare seven antimony-bismuth thermopile detectors thatmeasure the intensity of radiation in the spectral band,or a portion of the spectral band, passed by that tele-scope's filter. The fields of view of the detectors aresplayed out in the cross-track direction. A detector'stypical 0.3' field of view covers an 8-km circle on thesurface from periapsis altitude. The forward motion ofthe Orbiter brings a ground point successively intoview of a detector in each spectral band.

Thermal emission from the Martian surface ismeasured in four bands: 6.1 to 8.3 pm, 8.3 to 9.8 pm,9.8 to 12.5 pm, and 17.7 to 24 pm. Seven detectorsrespond to reflected sunlight in the band from 0.3 to3.0 pm. One detector measures radiation between14.56 and 15.41 pm, in the C02 vibration band, tomeasure the stratospheric temperature.

6 in. Space view-.0cmElectronics

Planet view

1 Electronics DetectorsFIGURE -16.--Infrared thermal ma ppe r,


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THE VIKING ORBITER 23Mars A t m o s p h er i c W a t e r Detector

This instrument is designed to map the distributionof water vapor over the planet. Water vapor is a mi-nor constituent of the Martian atmosphere that is ofthe greatest importance in understanding the meteor-ology, the geology, and, above all, the biology of tlieplanet. Life on Earth is completely dependent on theavailability of water, since all biochemical reactionstake place in aqueous solutions. The prime landingsites for Viking were mainly places where watervapor was expected to be relatively abundant. Meas-urements by the M A W D to confirm this expectationconstitute one input to the certification of a landingsite. Later in the mission, when the Orbiter is in anonsynchronous orbit, the M A W D w ill observe theglobal distribution of water vapbr, locate areas wherehigher concentrations might indicate volcanic activityor subsurface ice conditions, and investigate the dailyand seasonal variations in the abundance of watervapor.

The M A W (fig. 2-17) is an infrared spectrom-eter operating at five selected wavelengths within thewater vapor absorption band at 1.38 pm. By measur-ing the prop ortion of the incide nt solar radiation th atis passed by the atmosphere at those wavelengths, itdetermines the amount of water vapor the radiation

has passed through. Comparison of the five channelsalso derives the atmospheric pressure at the levelwhere the absorption takes place, thus permitting anestimate of the altitude of the water-vapor-bearinglayer.

Radiation entering the instrument is focused by asmall telescope and reflected by a collimating mirroronto a diffraction grating of 12 000 lines per centi-meter. T he gra ting spreads out th e spectral band ontoan array of five lead sulfide detectors. The field ofview at any instant is a 0.12' by 0.92' rectangle, pro-riding a ground footprint from periapsis altitude thatis 3 km wide and 2 1 km long. A scanning mirror infront of the telescope sn'eeps the Martian surface in a15-position scan perpendicular to the ground track.Th e 15 cont iguo us rectangles covered in one scan havea combined width of 45 km. he scan is repeatedevery 4.5 sec.

The instrument is sensitive to variations in m-atervapor abundance of about one precipitable microm-eter of w ater. (T h e conventional way of expressingabundance is the thickness of the layer of rainwaterthat would be formed if all the water vapor in theatmospheric column above a given location could becondensed out. ) Abundances observed in the Martianatmosphere before the Viking mission have not ex-ceeded 50 pm .

Head electronicsIncoming radiation

Diffraction grating

Calibration assemblyOrder isolation f i bFore mtics housing

Neon reference 50Wavelength servo mo

Col I mator mirror

FIGURE-lT'.-Mars atmospheric water detector.


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3Entry and Lan-. A Traverse of the AtmosphereThe Viking Landers passage through the Martian

atmosphere during its landing provided an excitingopportunity for direct measurement of some of itsconstituents and properties. Since the Landers modeof descent changed several times during the passage,and the various insuuments operated through differ-ent altitude ranges, an account of the sequence ofevents (fig. 3-1) is an appropriate introduction tothe entry science investigations.

The descent capsule that separated from the Orbiterconsisted of the Lander, an aeroshell, and a base cover.The Lander was enclosed, the deorbit engines and theinstruments that first sensed the environment werepart of the aeroshell, and the parachute was in thecover.

The descent trajectory first took the capsule throughthe undisturbed interplanetary medium. This is amqpecized gzs of ions and electrons streaming awayfrom the Sun at hypersonic velocity, which is calledthe solar wind. Closer to the planet the Lander passedthrough a disturbed region where the solar wind isdiverted to flow around the planet. Beneath this inter-action region lies the Martian ionosphere, a zone ofcharged particles generated by photo-ionization ofthe Martian atmosphere. The retarding potential ana-lyzer that analyzed the charged particles (electronsand ions) was turned on shortly after deorbit. At thattime the aeroshell was facing away from the Sun sothat the instrument, which was sensitive to sunlight,could observe the interaction region without inter-ference.

The descent capsule encountered an appreciableatmosphere at about 250 km. In preparation for this,the upper atmosphere mass spectrometer was turnedon early to allow it to warm up, and the capsule wasoriented so that the aeroshell and its heat shield facedin the direction of travel. The capsde was traveling at

ENTRY SCIENCETEAMAlfred 0.C. NierWilliam B. HansonMichael B. McElroy

Alfred %ifNelson W. Spencer

about 16000 km/hr before atmospheric drag beganto slow it. The intense heat generated by the atmos-pheric drag was carried away from the aeroshell bythe ablation of the heat shields substance. Both insuu-ments ceased operating at about 100 km,where theatmospheric pressure exceeds0.003 millibar. The aero-shells other two nstruments, a pressure and a temper-ature sensor, continued to operate until the aeroshellitself was jettisoned.

The capsule experienced its peak deceleration some-where between 24 and 30 km above the surface. Fora while its path leveled off into horizontal flightbecause of the aerodynamic lift provided by the aero-shell. Continued deceleration caused the capsule toresume its descent. By the time its radar altimeterindicated an altitude of 6.4 km, it was traveling slowlyenough ( an estimated 1600 km/hr) to deploy a para-chute. Seven seconds after parachute deployment, theaeroshell separated from the Lander. The aeroshelIsremaining lift caused it to drift well away from thelanding site.

Temperature and pressure sensors on the bottomof the Lander were then exposed to the atmosphereand took over the measurement functions. The radaraltimeter switched to an antenna on the Lander tocontinue its measurements. The Landers three legswere extended to the landing position. About a min-ute later the parachute slowed the Landers fall toabout 60 m/sec. During this time the effects of windson the Landers horizontal travel could be observed.Then a signal from the radar altimeter at 1200 m

25


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26 V I K I N G 1

er

Entry to landing6400 m \ 6 to 13 min

FIGURE -l.-Diagram of sequence of landing events (not to scale).

ignited the Lander's three terminal descent enginesand caused the jettison of the parachute and the basecover.Retarding Potential Analyzer

This instrument measured the energy distributionof solar wind electrons and ionospheric photoelectrons,the temperatures of the electrons in the ionosphere,and the composition, concentrations, and temperaturesof positive ions. At the highest altitudes, the analyzerexamined the interaction of the solar wind with theupper atmosphere. This information is likely to beimportant to the understanding of the nature of theMartian atmosphere, because the planet's weak (ornonexistent) magnetic field should permit deepersolar wind penetration than occurs on Earth.

Data obtained by the retarding potential analyzer

during the descent of the Viking 1 Lander show thatthe major cqnstituent of the Martian ionosphere is0 ~ +singly ionized molecular oxygen). It is aboutnine times as abundant as COz' (singly ionizedcarbon dioxide), which is the primary ion producedby the interaction of sunlight with the Martian atmos-phere. This important new finding lends support totheoretical analyses by M. B. McElroy and J. C.McConnell that called attention to the reaction ofatomic oxygen with C02' that would produce carbonmonoxide and the more stable ion, Oz+.The tempera-ture of the observed ions at an altitude of 130 kmwas about 160 K.The retarding potential analyzer ( R P A ) consistedof a series of six wire grids located behind an openingat the front of the aeroshell and in front of an elec-trometer collector (fig. 3-2 ) .#The first, second, and


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ENTRY AND LANDING 27

Front face Electrometercircuit boards

r

Retarding gridsSuppressor gridShield grid I

cc0Cs

FIGURE 3-2.--Schematic diagram of the retarding potentialanalyzer.

sixth grids were grounded to the aeroshell The thirdand fourth grids were connected electrically and madeup the retarding grid. The electric potential onthe retarding grid was swept through a series of posi-tive and negative voltages. During the sweeps, a steadyvoltage of opposite sign was applied to the fifth, orsuppressor, grid. As the voltage was varied, differentportions of the population of ions and electrons couldpenetrate the grid structure and produce a current inthe electrometer. The cycle of voltage sweeps wasrepeated every four seconds. By knowing at what point

in a retarding voltage sweep a measurement of theelectrometer current took place, scientists can deter-mine the temperature and concentration of variousions and electrons.Upper Atmosphere Mass Spectrometer

This instrument analyzed the molecular compositionof the atmosphere during entry. It provided a qualita-tive and quantitative analysis of all electrically neutralgases whose molecular weight is 50 atomic mass unitsor less. It also measured their isotopic abundances. Aknowledge of the identities and concentrations of thevarious gases as a function of altitude is basic to un-derstanding the development of the atmosphere andthe processes that maintain the present balance.

The upper atmosphere mass spectrometer obtaineddata from 230 km to 100 km during the descent ofthe Viking 1 landing capsule. Figure 3-3 is a samplemass spectrum obtained at an altitude of about 135km. s expected, the main neutral constituent of theupper atmosphere is carbon dioxide, which producesthe peak at mass 44. he abundance of nitrogen, whichshows up at mass 2 8 (along with carbon monoxide)and a t mass 14, is 6 percent that of CO z at thataltitude. The peaks at 40 and 20 are due to argon,whose abundance relative to COZ s 1.5 percent. Molec-ular oxygen, at mass 32, constitutes about 0.3 percent.Atomic oxygen is detectable at mass 16. Smaller peaksin the spectrum are produced by gases containing theless common isotopes of these elements. The relativeabundances of the carbon and oxygen isotopes areclose to their terrestrial d u e s .

Nitrogen was detected in the Martian atmospherefor the first time during the Viking 1 entry. Theexistence of nitrogen is significant because at leasta small quantity of that etement has always beenregarded as necessary for the existence of life. Massspectra obtained at higher altitudes show a higherproportion of nitrogen, which is lighter than the otheratmospheric gases.

The measured abundance of argon indicates thatthe much higher values that had been inferred fromindirect data obtained by the Soviet Mars 6 probewere incorrect. This finding was of immediate prac-tical importance because it resolved a dilemma as tothe conduct of a Lander scientific investigation. Thelow argon abundance measured during entry permittedthe investigators to use the gas chromatograph massspectrometer (described in ch. 7) to d y ~ ehe un-contaminated gases at the bottom of the atmosphere


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28 VIKING 110-8

1 0 - 0Qiga; 0 - 2-

1 0 - 1 4 IDecreasing mass-

FIGURE-3.-Mass spectrum obtain ed at about 135 km .

before loading it with a sample of surface materialfor organic analysis. Argon in the concentrations pre-dicted by the earlier data would have endangered thefurther usefulness of the instrument.The upper atmosphere mass spectrometer (UAMS)is a double-focusing (electric and magnetic) massspectrometer in which the gases entering the port(fig. 3-4) are first ionized by bombardment with abeam of electrons. Most of the resulting ions have asingle positive charge (due to the loss of one electronby collision), and some are doubly charged. Some ofI iIon collectors(measuresnumber of

(measuresmass)

1 1FIGURE3-4.-Schematic of the upper atmosphere mass

spectrometer.

the ions are products of the dissociation of gas mole-cules by the electron beam. The ions are then acceler-ated toward the analyzers by a voltage that is variedwith time. The voltage between the plates of theelectric analyzer is varied concurrently with the accel-erator voltage sweep. After leaving the electric, ana-lyzer, the ions pass between the poles of a magnetfollowing curved paths. The path through the mag-netic analyzer of an ion that was accelerated by somegiven voltage depends on its mass-to-charge ratio.Mass spectra are obtained at the two fixed ion col-lector slits by the sweep of the accelerating voltage. Amass spectrum is shown as a plot of the quantity ofions collected at each mass-to-charge ratio.The intake port of the UAMS is at the surface ofthe aeroshell. It was covered by a protective seal beforethe Viking launch, and a vacuum was maintained inthe instrument until the seal was removed just afterthe deorbit burn. A mass spectrometer can only oper-ate under near-vacuum conditions; hence the UAMSbecame inoperable below about 100 km.Lower Atmosphere Structure Experiment

The primary objective of this experiment was toobtain vertical profiles of the density, pressure, andtemperature of the atmosphere from an altitude of90 km down to the surface. The first measurementsfrom which the density profile could be derived wereof the descent capsules retardation due to atmosphericdrag. The sensors were the accelerometers that were


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ENTRY AND L A N D I N G 29

10-4 0 20 40 60 80 100 120 140Altitude, km

FIG UR E -5.-Atmospheric pressure profile from 90 km tolanding (preliminary).100 I I 1 I I I 1

CO,condensationboundary

o 40 80 im 160 200 240 280Temperature, K

FIGURE -6.-Temperature of the atmosphere measured from90 km to landing (preliminary).

part of the Lander's inertial reference unit, whichprovided a continuous input for the guidance andcontrol of the descent.

Pressure and temperature measurements came atfirst from the two instruments in the aeroshell. Be-cause of the very high initial velocities, the pressuresensor actually measured the stagnation pressure (thepressure of the atmospheric molecules against theaeroshell surface), from which the ambient pressurecould be derived later. Similarly, the temperatureprobe, located near the aeroshell's outer rim, measuredthe recovery temperature of molecules flowing aroundthe aeroshell. During the parachute descent, after theaeroshell had been jettisoned and the Lander's ownsurface environment pressure and temperature sensorswere operating, the velocity was low enough to makethe dynamic correction of the readings negligible.

The altitude information needed for the consuuc-tion of profiles came from the radar altimeter. A by-product of the radar altimeter measurements was in-formation about the terrain elevation profile underthe Lander's path. The terminal descent and landingradar, which provided control information for thefinal phase of the landing, also measured the drift ofthe Lander with the wind during the parachutedescent.

Figure 3-5 shows the atmospheric pressure profilefrom 90 km to ground level derived from the h s tanalysis of the Viking 1 landing data. The pointsderived from the deceleration data and the stagnationpressures are indicated. They are consistent with eachother and with the directly sensed measurements 'Mow3.5 km. The pressure at the surface was 7.3 millibars.

The temperature of the atmosphere between 200and 140 km, btained from the RPA and UAMS data,averages about 180 K. The temperature profile from90 km to the ground is shown in figure 3-6. The Vik-ing 1curve is derived from deceleration data between90 and 30 km, and from direct sensing below 3.5 km.There are local temperature peaks at 64 and 30 km.The entire profile was above the condensation bound-ary of carbon dioxide (also shown) at the time ofentry.


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On the Surface: A Look houndThe function of the Viking Lander is to enable itsset of instruments to carry out experiments and ob-

servations on the Martian surface. After it has broughtthe instruments safely to the selected site, it mustmaintain a favorable environment for them, it mustexecute complicated instructions for their operation,and then it must transmit the resulting data in prop-erly arranged form back to the earthbound scientists.There is one important thing it must not do: bringlife to the planet in the form of terrestrial micro-organisms.

The Lander's body is basically a hollow six-sidedbox,about 0.5 m thick and 1.5 m wide, that providesa controlled environment for four of the instrumentsand for much of the equipment that supports andcontrols all of them. The box has three long and threeshort sides, so that i t looks like a triangle with bluntedvertices. The landing legs, which are attached outsidethe short sides, give the Lander a ground clearance of22 cm.

When the Lander is seen from above, its basic shapeis obscured by a cluttered superstructure that includessuch components as the terminal descent engines, thetanks that contained their propellant, and the twopower generators in their wind covers. The remainingprotuberances and appendages, which relate to thescience instruments and their data, are labeled in fig-ure 4 1 .

The electrical power for operating the Landercomes from a pair of 35-W radioisotope thermoelec-tric generators (RTG ) . Each generator contains abank of thermoelectric elements that convert the tem-perature difference between their ends into electricalpower. The source of heat for an RTG is the radioac-tive decay of plutonium-238. Unconverted heat is con-veyed by a thermal switch to the interior of the Landerbody as needed to maintain the internal tempera-ture during the night. The wind covers over the R T G s

conserve heat for this purpose during periods of highwinds. At times of high activity, the 70 availablewatts are supplemented by four nickel-cadmium bat-teries inside the Lander body. The batteries weredeveloped especially for the Viking Lander, as theyare the first of this type that can withstand the j i dsterilization treatment that the entire Lander was sub-jected to before launch: 40hours in an oven at a tem-perature of 113" C.

The operation of all the equipment aboard theLander is controlled by the guidance, control, andsequencing computer (GCSC).By the time the Land-er reaches the ground, the GCSC has accomplished itsmost time-critical task, but it still must run all thesurface activities. The GCSC has two identical general-purpose computer channels, each possessing a plated-wire 18000-word memory. One channel is opera-tional, and the other is in reserve. The operationalchannel executes its stored instructions in sequence,and it decodes and stores new commands from Earth.It also checks its own condition periodically. If itsstatus should become unsatisfactory during th e mis-sion, the GCSC would automatically switch over tothe reserve channel. It would then do just what ahuman controller should do under those circum-stances: shut down all nonessential activities and callfor help. Help would come in the form of new com-mands from Earth. In about a week the operationscrew could convey sdcient instructions to the newGCSC channel to enable it to carry on a sciencemission.

Three units within the Lander's body handle theflow of scientific data from the various instruments.These are the data acquisition and processing unit(DAPU), data storage memory, and a tape recorder.The DAPU collects the engineering and scientificdata, converts any analog information to a digital for-mat, and feeds it as required to the data storage


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32 VIKING 1

S-band high gainantenna (direct)

Magnifying mirrorCamera test tar Meteorology sensors

Radioscience/\HF antenna (relay)

boom assembly

rnally mounted:BiologyGas chrornatograph-mass spectrometerX-ray fluorescence spectrometPressure sensor

iology processorFurlable boom

Collector head

~~

FIGURE -1 .-External features of the Viking Lander.

memory for short-term storage, to the tape recorderfor longer-term storage, or to one of the transmittersfor either direct or relayed transmission to Earth. Thedata storage memory can store 8200 words, of 24 bitseach. Its data are normally transferred periodically tothe tape recorder for bulk storage. The tape recorderuses a tape made of phosphor bronze coated withnickel cobalt as a recording medium. Its four trackscan store a total of 40 million bits.Direct transmission to Earth makes use of a high-gain antenna- parabolic dish 76 cm in diameter thatcan be pointed to Earth by a computer-controlled mo-tor drive. Transmission to the Orbiter uses a fixedUHF antenna.Lander Imaging Investigation

The broad objectives of the imaging investigationare to characterize the Martian landscape and its

variations, to perform celestial observations fromMars, and to provide support for the other investiga-tions.Just seconds after the Viking 1Lander touched downon the Martian surface, camera 2 began to photographa portion of the surface in the vicinity of Lander foot-pad 3. It took 5 min of camera scanning to producethat historic high-resolution photograph (fig. 4-2 ) ,and the imaging investigation was off to a runningstart. On that first day, the camera also acquired a low-

LANDER MAGING EA MThomas A . MutchAlan B. BinderFriedrich 0. HuckElliott C. Levintha l Carl Sagan

Sidney Liebes, Jr.Elliott C. MorrisJames A . Pollack

I I


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ON THE SURFACE 33

FIGURE -2.-The historic first photo from the Viking 1 Lander.

resolution panoram ic view covering some 300' of theterrain surrounding the Lander. It was apparent thatthe Martian surface at the site is strewn with bothblocky and angular rocks in a granular matrix of finematerial, and that some of the granular material hasbeen transported by winds.

Th e first photograph shows that footpad 3 barely pen-etrated the surface, and that some fine particles weredeposited inside the footpad's concave upper surfaceas a result of th e landin g. It is most likely that th edark band near the left edge of the picture is due tothe temporary shadorving of the site (f or a few tensof seconds) by a cloud of dust raised b:, the landing.The camera builds up its picture as a left-to-rightsequence of rapid vertical scans (see the cameradescription in a later section). so rhe time duringwhich the scene was darkened is readily estimated.

The area to the left of the dark band includes rockswith wind-deposited tails of fine granular material.Sh ado w i n the photograph show visible detail, mainlydue to the scattering of light in the atmosphere.A color photogr aph taken the day after the land ingshowed that the fine, granular m aterial is colored rust-red, and most of the rocks are coated with a stain ofth e same color. Some of th e rock exposures are darke rand appear less red. The Martian sky is bright (par-ticularly in the direction of the Sun), and is coloreda creamy pink. Apparently, the atmosphere carriesmany very fine particles (o n the order of 1p n ) in sus-pension: and chis dust is predominantly red: Le., i tabsorbs blue light. A later color photograph is on thecover of this book.

Camera 1 went into operation on the third M artianday and produced a new panorama (fig. 4-3 ) to sup-

FIGURE4-3.-Panorama produced by Lander camera 1


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34 VIKING 1plement the first-day panorama taken by the othercamera. The middle third of this panorama covers aportion of the landscape not visible to camera 2, whilethe left and right thirds provide stereoscopic coverageof the duplicated terrain. Looking from left to right,the Lander parts include the windscreen covering oneradioisotope thermoelectric generator, with paintedAmerican flag and Bicentennial emblem; the flat-topped housing of the seismometer; a grid painted onthe Landers deck to monitor dust accumulation; thestruts that support the high gain antenna; the secondRTG windscreen; the stroke gage of a landing leg;and the meteorology boom. The sky brightens in theSuns direction, at the left and far right. A horizontalcloud layer is visible halfway between the horizon andthe top of the picture. The landscape is gently undulat-ing, with several apparently shallow craters in the

middle distance and near the horizon, which is about3 km away. Angular rocks with a variety of textures-some pitted, some striated, some fine-grained and ap-parently dense-litter the surface. The large boulderto the left of the meteorology boom is about 8 m fromthe camera, and measures about 1 by 3 m. The surfaceto the right of i t is covered with dunes of wind-blown material. (This dune field is displayed strik-ingly on the title page.) The rock-free area just to theright of the meteorology booms deployment hingewas selected as the site for collecting the first surfacesample, and it was studied stereoscopically in order toprovide detailed instructions for the surface sampler.Figure 4-4 shows the site as photographed by thetwo cameras on the first- and third-day panoramas.The white lines are a grid of profiles generated by acomputer in response to the control motions of a

FIGURE -4.-Landing site as photographed by both cameras. White lines are part of a photogram-metric study.


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photogrammetrist who observed the pair stereoscopically. The instructions for the surface sampler opera-tion w ere based on the fourth profile from the left.

Th e removal and deposition of the fine material bywind action in the neighborhood of variously shapedrocks is exhibited in figure 4-5. The surface of finematerial is hollowed out where the upwind face of arock presents a sharp obstacle to the wind. Windstagnation on the leeward side of rocks causes thedeposition of taillike ridges. Evide ntly, the w inds tha tare strong enough to move these particles have a pre-vailing direction, from northeast to southwest.

The winds that the meteorology experiment hasmeasured since the landing have not moved particlesin the size range visible to the cameras. From time totime, the cameras are operated in the single-line-scanmode in order to detect the motion of wind-blownparticles, or possibly moving organisms. Th e right sideof figure 4-6 illustrates this mode. The camera main-tains a fixed azimuth while one line is repetitivelyscanned. If anything moved across the line of thecameras scan, the variation from left to right wouldshow up prom inently. To date, nothing has been seento move, If strong winds should occur a t the Landersite during the mission, a combination of the single-line-scan camera operation and the meteorology datacould determine the wind velocity necessary to trans-port these panicles.Astronomical observations also employ the single-line-scan mode to determine the elevations and timesof transit of the Sun and the two satellites Phobosand Deimos.

The imaging investigation has been providing sup-port for other Lander investigations, both withplanned sequences of photograp hy and with photo-graphs made specially to help solve the problems thathave arisen. Examples of such support photographyare in the chapters covering those investigations.Lander Camera System

Two identical cameras are positioned on shortmasts atop the Lander (fig. 4-1), about 80 cm apart.From their viewpoints 1.3 m above the surface, theyhave a clear view of the area that the surface samplercan reach.

They are facsimile cameras, operating on the prin-ciple used for many years to scan news photographsfor transmission by radio or telephone lines. Theprinciple is fundamentally differen t from tha t of thetelevision cameras in the Viking Orbiter. In a tele-

FIGURE 4-5.-Effeas of wind on fine surface material.

FIGURE -6.-Single-line-scan mode is shown at right.


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36 VIKING 1vision camera, a complete two-dimensional imagethat has been produced on a photosensitive target isread off, one picture element (pixel) a t a time, bythe scan of an electron beam. In a facsimile camera,the sensor can only see one pixel at a time, and theimage is presented to the sensor piecemeal by me-chanical or optical-mechanical scanning.Figure 4-7 illustrates the basic operation. The sceneoutside the camera is scanned in elevation by the nod-ding of a mirror. Each time the mirror nods, the pixelsalong one vertical scan line are presented successivelyto the sensor. The entire camera then rotates througha very small angle in azimuth, so that the line scannedby the next nod of the mirror adjoins the line previ-ously scanned. Only when the pixels are assembledback on Earth by successive exposure along scan lineson a film is an image formed.The actual configuration of a Lander camera is indi-cated in figure 4-8. The design reflects concern withprotection from the environment-temperature ex-tremes and wind-blown sand. The camera and themast on which it is mounted are covered with severallayers of thermal insulation. The only gap in the in-sulation is the slit over the entrance windows thatadmits light to the scanning mirror. To survive duststorms, the camera can be rotated so that the slit isunder a narrow post that serves as a dust cover. Be-cause of the post, the camera's field is reduced to342.5' in azimuth. Since there may be suspended dust

c

telemetryline/ -rum

FIGURE-7.-Basic principles of the Lander camera.

Scanningmirror andelevation -

driveOptics-

Photo -sensorarray

FIGURE -8 .4o nfigu rat io n of the Lander camera.

during much of the time the camera is operating,there are two entrance windows. If the outer windowis degraded by dust erosion, it can be swung out ofthe way to expose a second window. Dust that settleson the window can be blown off by brief jets of pres-surized carbon dioxide.

Instead of a single light sensor, the Lander camerahas an array of twelve tiny sensors in the focal planeof the optical system. Each sensor is a solid-statephotosensitive diode. All the diodes see the samescene, but during a scan the system is

viking 1 early results

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