Imaging experiment: The Viking Lander

ICARUS 16,~92--110 (1972)

Imaging Experiment: The Viking Lander T. A. MUTCH Brown University, Providence, Rhode Island 02912

A. B. B I N D E R lit

Research Institute, Tucson, Arizona 55701

F. O. H U C K Langley Research Center, Hampton, Virginia 23365

E. C. L E V I N T H A L Stanford University, Stanford, California 94305

E. C. MORRIS U. S. Geological Survey, Flagstaff, Arizona 86001

CARL SAGAN Cornell University, lthaca, New York 14850 AND

A. T. YOUNG Jet Propulsion Laboratory, Pasadena, California 91103 R e c e i v e d M a y 5, 1971 T h e V i k i n g L a n d e r I m a g i n g S y s t e m will consist of t w o i d e n t i c a l facsimile c a m e r a s . E a c h c a m e r a h a s a h i g h - r e s o l u t i o n m o d e w i t h a n i n s t a n t a n e o u s field of v i e w of 0.04 o, a n d s u r v e y a n d color m o d e s w i t h i n s t a n t a n e o u s fields of view o f 0.12 ° . C a m e r a s are p o s i t i o n e d one m e t e r a p a r t t o p r o v i d e stereoscopic c o v e r a g e of t h e near-field. T h e I m a g i n g E x p e r i m e n t will p r o v i d e i m p o r t a n t i n f o r m a t i o n a b o u t t h e m o r p h o l o g y , c o m p o s i t i o n , a n d origin o f t h e M a r t i a n surface a n d a t m o s p h e r i c f e a t u r e s . I n a d d i t i o n , l a n d e r p i c t u r e s will p r o v i d e s u p p o r t i n g i n f o r m a t i o n for o t h e r e x p e r i m e n t s in biology, organic c h e m i s t r y , m e t e o r o l o g y , a n d physical properties.

the Martian surface, any discussion of anticipated results has a strong element of speculation. The scientific objectives of. the Imaging Experiment can be described under five headings: cartography, biology, geology, meteorology, and astronomy.

2. INTRODUCTION

The purpose of this article is to give the reader some familiarity with the Viking Lander Imaging Experiment, the equipment, and the probable scientific yield. The experiment is distinct from the Orbital Imaging Experiment which is a separate engineering effort, supervised by another team. Several cautionary notes are in order. Because the launch date is approximately four years in the future, many questions of instrumental design and operational strategy are still under discussion and negotiation. Because so little is known about the morphology and composition of © 1972by AcademicPress, Inc.

Cartography 1. Location of lander by observation of brighter planets, brighter stars, and satellites of Mars. 2. Mapping of landing site. 3. Correlation of features identifiable in both lander and orbiter pictures. 92

LANDER IMAGING

Biology

93

followed, in December 1969, b y NASA selection of a team responsible for the mission experiment. This group includes A. B. Binder, F. 0. Huck, E. C. Levinthal, E. C. Morris, T. A. Mutch (Team Leader), Carl Sagan, and A. T. Young. Because the science team does not have prime responsibility for instrument construction, the "chain of command" leading from scientist to instrument is complicated. Very briefly, the science team leader interacts formally with the Viking Project Office. This office directs the prime mission contractor, The Martin Marietta Corporation. Martin Marietta, in turn, directs the subcontractor for camera construction, the I T E K Corporation. was

1. Search for living or fossil forms. 2. Observation of any redistribution of the substrate b y living forms {e.g., burrows, tracks, and trails). 3. Determination of possible biological causes for temporal variations in dark and bright areas. 4. Search for technological artifacts.

Geology 1. Morphological characterization and interpretation of land forms. 2. Estimation of regolith thickness and particle size distribution. 3. Determination of rock morphology, texture, color, and, b y inference, mineralogical and chemical composition. 4. Definition of any rock structures indicative of igneous, sedimentary, structural, or metamorphic activity (e.g., stratification, folds, joints, faults). 5. Search for evidence of rock weathering or sediment transport.

Meteorology 1. Observation of dust storms and particle transport b y wind. 2. Observation of clouds. 3. Estimation of aerosol particle distribution b y observation of solar aureole, brightness of celestial objects, and distribution of sky brightness.

Astronomy 1. Observation of satellites of Mars. 2. Accurate location of Martian axis of rotation in space. I I . TEAM ORGANIZATION

The management of the Viking mission has proceeded in two stages. In February of 1969 teams of scientists were selected by NASA to assist in the planning of experiments, particularly to define the critical scientific measurements which should be made, and to evaluate candidate instruments for making those measurements. Team members selected for the imaging experiment included A. B. Binder, E. C. Levinthal, E. C. Morris, T. A. Mutch, and Carl Sagan. This first phase

II. CRITIQUE OF INSTRUMENTAL CAPABILITIES

The experiments contained in planetary spacecraft reflect a tug of war between competing goals. On the conservative side are constraints on money, weight, and power; also requirements for simplicity and dependability. These goals often clash with a desire for the most sophisticated "state-of-the-art" equipment, for precise and comprehensive measurements, and for inclusion of "exotic" capabilities designed to test extremely provocative b u t highly speculative scientific models. The Viking camera experiment reflects all of these constraints and goals. To appreciate the evolution of the experimental strategy, it is helpful to compare the 1969 report of the Viking Lander Science Instrument Teams (VPO M73112-0) with the 1970 Viking Mission Definition (VPO M75-123-1). The former document summarizes the conclusions of scientists who participated in the early Viking planning activities. Tabulated in this report are a variety of scientifically desirable capabilities. These include: 1. A camera with a low-resolution instantaneous field of view of 0.1 ° and a high-resolution field of view of 0.01 °. 2. Stereoscopic capability with a base of at least one meter. 3. Generation of visual color b y combining three narrow-band images.

94

T . A . MUTCH E T A L .

4. Multispectral narrow-band filtering, both for generation of images in preselected pass bands and for generation of spectrometric curves for particular spots 0.1 ° in size. 5. Polarimetric measurements to aid in determination of textural and lithologic properties of surficial materials. 6. Use of auxiliary lenses to obtain highresolution (on the order of 10/~) pictures of the samples used for biological and chemical analyses. 7. Motion detector. A device which would systematically scan large fields of view over long periods of time to identify either biological or nonbiological motion, and to photograph the moving objects. The Viking Mission Definition indicates the camera specifications finally approved by NASA. The more important specifications are: 1. Two cameras with instantaneous fields of view (scanning resolution) of 0.04 ° and 0.12 ° . 2. Each camera can obtain visual color images as well as broad-band images. 3. Cameras will be positioned 1.5m above the Martian surface, assuming a nominal landing. 4. Cameras will be able to view two footpads and at least 90% of the area accessible to the surface sample. 5. In order to obtain stereographic images, cameras will be separated by a horizontal base of one meter. The scanning resolution of the flight instrument is 0.04 °, compared with 0.01 ° recommended by the instrument team. This specification was a matter of extensive negotiation for, although it does not uniquely determine picture quality, it nonetheless indicates instrumental precision. Obviously, scientific requirements are for high resolution, but there are no quantitatively persuasive arguments t h a t indicate critical threshold values. Such values are suggested for orbital cameras where candidate object resolutions range from meters to kilometers, and where linear resolution is more-or-less constant throughout the field of view. A comprehensive discussion of recognition thresholds

in planetary exploration is given by Masursky et al. (1970). For lander cameras the achievable linear resolution, depends, of course, on the distance to the object (Fig. 6). At 1.Sm, 0.01 ° subtends 0.25mm. We felt t h a t this resolution was highly desirable for interpretation of rock and soil fabric in the near-field. But the value of 0.01 ° was understood to be an optimistic limit to t h a t which was technically feasible. After the Planning Team Report was submitted, a number of engineering studies indicated t h a t a reasonable contractual requirement (as opposed to a planning goal) was 0.04 °. The adopted imaging resolution was specified on this basis. IV. INSTRUMENT DESCRIPTION 1. C a m e r a

Each camera contains an optical system, an array of photodiodes with their associated amplifiers, servomechanisms for vertical and azimuthal scanning, and a thermal control system. The camera system also includes an electronics package which is common to both cameras and is mounted within the temperature-regulated area of the lander body. The package contains analog and digital signal processing electronics, camera mode control logic, servo electronics for the scan drives, and power conversion modules. The cameras can scan at two different rates to match the two rates of data transmission : 16,000 bits/sec to the orbiter and 250bits/sec directly to Earth. A tape recorder in the lander will allow separate imaging and data-transmission times, but, for reliability, the camera can operate independently of the recorder. Light captured by the camera lens passes through a pin-hole to a photodiode which transforms it into an electrical signal. A nodding mirror on the object side of the lens provides the vertical line scans, and an azimuthal rotation of the line-scan assembly generates proper spacing between successive lines. Figure 1 shows a simplified diagram of the facsimile camera, and Fig. 2 the spectral response of the photosensors and color filters. Table I

LANDER IMAGING

INDUCTOSYN, POSITION TRANSDUCER

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LIGHT

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LENS UE MOTOR SENSOR ARRAY

CHOPPER

POTENTIOMETER

INDUCTOSYN POSITION TRANSDUCER

PELTIER COOLER

iiiiii! :.:.:.:.:

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F;G. 1. Schematic diagram of facsimile camera. summarizes the most important design goals. The optical subsystem consists of an objective lens, light baffles, a mechanical chopper, and a photosensor array. The f/8 lens has a focal length of 7.6cm. Of the 12 silicon photodiodes, one is for survey imaging; three are for color; four for high resolution; and four are presently unassigned. The pinholes covering the four highresolution diodes define an instantaneous field of view of 0.04 °, and are placed at

distances from the lens so t h a t objects are in best focus at 1.9, 2.7, 4.5, and 13.3m. Any one of the diodes can be selected, by command, to generate a picture. The modulation transfer function is required to be at least 0.37 for a spatial frequency of 0.08deg/cycle at the in-focus distances, and not to fall below 0.27 for this frequency from 1.Tm to infinity, provided the best-focus diode is used at each distance. The image degradation at infinity using the 13.3 m focus diode is insignificant. The pinholes covering the survey and

96

T. A. M U T C H ET A L.

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FIG. 2. S p e c t r a l r e s p o n s e c u r v e s f o r t h e s i l i c o n d i o d e d e t e c t o r a n d t h e t h r e e f i l t e r s u s e d f o r c o l o r images. TABLE

I

F A C S I M I L E CAMEttA D E S I G N GOALS

Characteristics

Survey

Color

High resolution

I n s t a n t a n e o u s field o f v i e w (deg) P i c t u r e e l e m e n t r e g i s t r a t i o n e r r o r (deg) A b s o l u t e a n g l e e r r o r (deg) elevation azimuth F i e l d o f v i e w (deg) vertical elevation horizontal azimuth D e p t h o f field ( m )

0.12 ±0.036

0.12 ±0.018

0.04 ±0.006

±0.3 ~0.15

±0.2 ±0.1

±0.2 ±0.1

Spatial

100 360 1.7 t o ~ (fixed f o c u s )

100 360 1.7 t o ~ (fixed f o c u s )

100 360 1.7 t o (4focuspositions)

~ 2 x 10 -6 ~ 2 x 10 -7 1000 : I

~ 2 x 10 -5 ~ 2 x 10 -6 1000 : 1

~ 2 × 10 -5 ~ 2 x 10 -6 1000 : 1

10 25

2 10

2 10

512 6

512 6

512 6

Radiometric Noise equivalent radiance (Wcm relay link to orbiter direct link to earth Dynamic range Radiometric accuracy (%) r e l a t i v e (1 - o) a b s o l u t e (1 - a)

Data

2 st-l)

format

Picture elements per line Bits per picture element

97

LANDER IMAGING

color diodes define instantaneous fields of view of 0.12deg. All pinholes are placed in the focal plane for objects at 3.7m, so t h a t t h e y are equally defocused at 1.7 m and infinity. The design requires t h a t the modulation transfer function be at least 0.17 for a spatial frequency of 0.24deg per cycle at the in-focus distance, and not fall below 0.08 for this frequency from 1.7 m to infinity. The photodiode array and integrally mounted preamplifiers are cooled by a Peltier cooler to -40°C. One of the video signals at the preamplifier outputs is selected by an electronic switch and then further amplified, filtered, and digitized to 6 bits per sample for transmission. Various sensitivities and partial dynamic ranges can be selected by using 5 gains and 16 offsets (Fig. 3). Radiometric and colorimetric calibrations are obtained by viewing external calibration targets. Contrast detection is limited not only by the modulation transfer function but also by the system noise, principally from the detectors and preamplifiers. The design requires t h a t the total noise equivalent power not exceed 1.7 × 10-12W at the rapid scan rate and 2.1 × 10-1sW at the slow scan rate. The noise equivalent radiance of this instrument may therefore be estimated from ¢O

NER =

NEP/AB f F;~R~ d~ 0

where A is the area of the aperture, B the solid angle of the instantaneous field of view, Fa the transmittance of optics and filters, and Ra the relative response of the photodiode. The following contrast detection thresholds have been established for spatial detail which is several picture elements in size, so t h a t contrast reduction by the optical transfer function is negligible. Variations of the illumination scattering function as small as 0.3% should be detectable under average conditions when using the highest gain setting and the slow scan rate. This detection threshold may rise to 2.4% when using the rapid scan for high resolution imaging. In the color mode, albedo variations of 0.2% should be detectable when using the slow scan, providing t h a t the illumination scattering function is larger than 25%. Detectable albedo variations may rise to 1.6% when using the rapid scan. The vertical line scan and azimuth rotation mechanisms are controlled by separate servo systems, each using a de torque motor, an inductosyn for fine angular resolution over a narrow range, and a potentiometer for coarse angular resolution over the complete range of motion. In addition, the vertical line scan servo also uses a tachometer. The rotors of the vertical line scan components are integrally mounted on the mirror shaft, and the stators are mounted on the

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Radiant power, nW FIG. 3. T h e f a c s i m i l e c a m e r a c a n o p e r a t e a t 16 s e n s i t i v i t y command.

a n d 5 g a i n levels. S e l e c t i o n

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98

T. A. MUTCI-I E T A L .

u p p e r housing which also contains the optical s u b s y s t e m . T h e r o t o r s of t h e a z i m u t h r o t a t i o n c o m p o n e n t s are m o u n t e d on a s h a f t which is an extension of t h e u p p e r housing, a n d t h e s t a t o r s are m o u n t e d on t h e lower housing which is fixed to a mast. T h e vertical line scan servo nods t h e m i r r o r u p a n d d o w n a t a r a t e of 4.6 scans per second for t h e o r b i t e r relay, a n d 0.07 scan per second for direct t r a n s m i s s i o n to E a r t h . T h e scan covers 36 deg in t h e s u r v e y a n d color m o d e a n d 12deg a t high resolution. (The m i r r o r reflection doubles this angle.) Video i n f o r m a t i o n is acquired d u r i n g b o t h t h e u p a n d d o w n scans w i t h high efficiency; t h e design goal is 9 0 0 . T h e initial viewing m i r r o r position can be set in 10-deg intervals, to p e r m i t scanning f r o m 4 0 d e g a b o v e to 60deg below t h e p l a n e n o r m a l to t h e optical axis. T h e a z i m u t h servo r o t a t e s t h e u p p e r housing while the m i r r o r is reversing its scanning direction. I n high-resolution imaging, r o t a t i o n is 0.04deg per line; in t h e s u r v e y m o d e it is 0.12deg per line; a n d in t h e color mode, 0.12deg e v e r y t h i r d line. I n t h e last mode, successive color diodes are selected sequentially, line b y line. TABLE I I N U M B E R OF B I T S I N V A R I O U S P I C T U R E FORMATS, AND T I M E S N E C E S S A R Y TO O B T A I N AND T R A N S MIT T H A T D A T A AT 1 6 , 0 0 0 B I T S / S E C . a

Time

Bits

5.26sec

8.44 × l04

10~56min

1.01 × 107

15.8sec 31.68rain

2.53 × 105 3.04 × 107

High resolution scan 20 ° × 3° 15.8sec 20 ° × 360 ° 31.68rain

2.53 × 105 3.04 × 107

60 ° × 360 ° Color scan

60 ° × 3 ° 60 ° × 360 °

W h e n the lander is encased b e t w e e n t h e aeroshell a n d t h e bioshield of t h e spacec r a f t t h e r e is n o t sufficient space for a rigid m a s t e x t e n d i n g a b o v e t h e u p p e r d e c k of t h e lander. Therefore, to r e a c h 1.5m a b o v e t h e surface of Mars, t h e c a m e r a s m u s t be a t t a c h e d to e x t e n d a b l e masts. F i g u r e 4 shows one possible configuration for an ex~endable m a s t . T h e c a m e r a folds d o w n against the side of t h e s p a c e c r a f t a n d swings into an u p r i g h t position a f t e r landing. A l t e r n a t e designs include a swingu p m a s t which lies on t h e u p p e r deck, or a telescoping p o p - u p m a s t . I n choosing a final design t h e critical factors will be s a f e t y of in-flight position, reliability of d e p l o y m e n t m e c h a n i s m , s t a b i l i t y of the m a s t in high winds, a n d weight. T h e m a s t design influences t h e m e t h o d of p r o t e c t i n g t h e c a m e r a window f r o m d u s t adhesion a n d abrasion. T h e c a m e r a presently conceived has a recessed w i n d o w to minimize t h e abrasion effects. H o w e v e r , additional p r e v e n t i v e devices will a l m o s t certainly h a v e to be included. One s t r a t e g y would be to t u r n t h e windows of t h e d e p l o y e d c a m e r a s to face a p r o t e c t i v e cover w h e n n o t in use. A n o t h e r solution would be to p r o t e c t t h e windows b y e x p e n d a b l e t r a n s p a r e n t covers t h a t could be discarded or m o v e d o u t of position w h e n d e g r a d e d b y dust. I n addition, it m a y be possible to d e p l o y t h e c a m e r a s in sequence so t h a t the first c a m e r a could t e s t the e n v i r o n m e n t before t h e second one was exposed. 3. Picture Reconstruction

Survey scan 60 ° × 3 °

2. Mast

a The vertical dimensions of the survey and h i g h r e s o l u t i o n s c a n s a r e fixed a t 60 ° a n d 20 ° r e s p e c t i v e l y , b u t t h e a z i m u t h a l d i m e n s i o n is identified by command.

Digital i n f o r m a t i o n t r a n s m i t t e d to E a r t h will be r e c o r d e d on m a g n e t i c t a p e a n d subs e q u e n t l y d i s p l a y e d in figure f o r m a t (Fig. 5). A quick-look i m a g e can be o b t a i n e d b y displaying the video d a t a on a television m o n i t o r coupled to a Polaroid film camera. A r c h i v a l films will be g e n e r a t e d in a film r e p r o d u c e r which e m p l o y s a m o d u l a t e d light m e c h a n i c a l l y scanned across t h e film. Color images will be o b t a i n e d b y r e p e t i t i v e scanning of color film using c a m e r a d a t a in t h r e e spectral b a n d s a n d inserting m a t c h ing filters in t h e light p a t h of t h e reproducer. Digital differencing of pictures of the

99

LANDER IMACIINU

same area, taken under identical lighting conditions, will be performed in order to determine seasonal and other changes within the field of view.

V. PICTURE SEQUENCING At launch the lander will be programmed for at least 30 days’ operation, in case of a communication failure. Following touchdown all updating commands will be transmitted directly from Earth to the lander over an S-band link. Opportunity for updating, then, will occur for a brief period of time each Martian day. Experimenters will be able to react to information received in the previous orbiter pass over the lander or in the previous direct communication period. Obviously these are much longer response times than for lunar

missions, and preclude any near-real-time command of the camera such as was possible in the Surveyor Missions. During each orbital communication link at least 10’ bits will be transmitted from lander to orbiter. This number of bits corresponds to a 60” x 360” survey picture, a 60° x 120° color picture, or a 20’ x 120’ 1high resolution picture (Table II). High priority picture assignments during the early part of the mission include a aomplete panorama in color and highresolution stereo image pairs for the entire sample area. As the mission proceeds, selected areas will be repeatedly imaged to letect any variable features. Picture timing will frequently be coupled to other experiments. It is desirable to photograph the soil sample site before and after each sample is obtained. Samples for biological and chemical analysis will

Ae STOWED

IN-FLIGHT -Deployed

DEPLOYED FIG. 4. A schematic view of the Viking lander camera in its flight position and three schematic views of the cameras in their deployed position on the Martian surface.

100

T. A. MUTCH E T A L.

1 Lander Camera System

(LCS]

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LCS Test Support Equipment

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Viking Lander

FIG. 5. Flow diagram for Viking lander imaging data. probably be taken within the first four days a n d eight times in the course of the mission. Both the magnetic and physical properties experiments depend upon camera data. For example, sequences of pictures are planned to determine strength of materials around footpads and in specially dug trenches. Finally, pictures m a y be desired when the meteorology experiment is operating or while the orbiter is monitoring the landing site. VI. MAPPING C0~TSIDERATIONS 1. Linear Resolutions

The best surface resolution obtainable with the Viking cameras will be 2.Smm per picture element pair in the immediate vicinity of the spacecraft. (Resolution is defined in a number of different ways, ,often leading to confusion. The values igiven throughout this section are obtained

simply by doubling the picture-element size, a convention frequently followed for space-flight cameras. Object resolutions will have slightly larger values but will vary considerably according to object shape, brightness contrast, sun angle, etc.) Figure 6 gives the resolution (A) and horizontal distance from the camera for 5 angles from the nominal horizon. I t is apparent t h a t half the information (i.e., half of a 60 ° frame) will come from the area between 0.9 and 2.6m from the camera, at resolutions between 2.5 and 4.2 mm/line pair. The Viking cameras will not have an unobstructed view of the Martian surface. As t hey turn in azimuth to look across the Lander, the ground area accessible to viewing will be limited as shown in Figs. 7, 8, and 9. With the best resolution of 2.5mm, the cameras will resolve 4000 and 2000 times smaller details than images taken from

L A N D E R IMAGING NORMAL OF

HEIGHT

CAMERA

SURFACE

~

101 DISTANCE

TO

ABOVE

LANDING

AREA - 3 . 2 k i n

HORIZON

1.5 m

RESOLUTION

--

4.5

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HORIZONTAL

DISTANCE

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6

CAMERA

FIG. 6. Line pair resolution (A) of t h e Viking lander c a m e r a at different horizontal distances and different angles below a nominal horizontal plane.

the Mariner 1971 Orbiters (100m) and the Viking Orbiters (50 m). For comparison, the Surveyor images, with a resolution of 1 mm, represented an increase b y a factor of 300 over the best previous lunar surface resolution of 30cm achieved b y Ranger IX.

2. Stereoscopy 1

The methods used to reconstruct the physical features of a spacecraft landing 1 T h e a u t h o r s are i n d e b t e d t o 1=¢. M. B a t s o u for t h e i n f o r m a t i o n c o n t a i n e d in t h i s section. A fuller discussion is c o n t a i n e d in B a t s o n (1969).

J SOIL

SJ

F r o . 7. P o s i t i o n o f Viking c a m e r a s o n t h e l a n d e r a n d t h e i r r e l a t i o n s h i p t o t h e soil s a m p l e area. F i e l d s o f view for b o t h c a m e r a s are s h o w n in m o r e detail in F i g s 8 a n d 9.

102

T . A . MUTCH E T AL.

~:

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Oft. SAMPLE FIELD SHADED PLANE: GROUND TOPOF LANDER TOP OF FOOTPADS

FIG. 8. The field of view after a nominal landing and using both cameras. Solid black is part of ground obscured by spacecraft. Stippled pattern shows obscured areas on spacecraft. Lined pattern shows obscured parts of footpads. site differ from classical methods of photogrammetry. Stereoscopy is unsurpassed for topographic mensuration and interpretation, but when applied to surface-based, rather than high altitude images, its usefulness is limited by geometric requirements t h a t can be met in only a small percentage of the total image area, and by extreme variation in image quality caused by factors unique to surface-based pictures. The cartographic presentation of data from the Viking Lander imaging system presents unique problems. Maps at scales as large as l : l may be required, and contour lines on these large-scale maps may delineate small details such as fractures, vesicles, and individual grains and

fragments. Because of the difficulty of recognizing and interpreting such features from contour maps, the data will be presented as generalized contour maps with liberal use of symbols, as perspective grids on untransformed pictures, and as physical models. The difference in perspective between high altitude aerial photographs and pictures taken by surface-based systems is significant in several respects. High altitude pictures taken nearly normal to a planet's surface arc essentially maplike, in t h a t their horizontal scales are relatively constant across the field of view. Tilted high altitude pictures can be rectified to an apparently vertical view fairly easily. Rough planimetric measurements can be

103

LANDER IMAGING

15ft

~

~HADED PLAN:E GROUND TOP OF LANDER TOP OF FOOTPADS

FIG. 9. The stereoscopic field of view after a nominal landing. Solid black is part of ground obscured by spacecraft. Stippled pattern shows obscured areas on spacecraft. Lined pattern shows obscured parts of footpads. made on these pictures without elaborate photogrammetric computation because surface topographic relief is small with respect to its distance from the camera and has relatively little effect on picture scale. Pictures taken with surface-based systems, on the other hand, are extremely foreshortened, resulting in scale variations of more than three orders of magnitude in a single picture. Distances to objects, and object sizes, cannot be determined even approximately without some computation. Topographic relief which is large with respect to its distance from the camera has a significant effect on photo scale, and small topographic prominences obscure large surface areas from the camera. The clarity of stereoscopic fusion is controlled by two ratios : the ratio of local scale variation between one retinal image and the other (parallax differential), and

the ratio of binocular to monocular image area. Neither of these ratios is very significant in conventional high altitude photography. In surface-based pictures, however, the combination of large relief relative to object distance, extreme surface roughness, and large angles of convergence of rays from the perspective centers to the objects on the surface can result in such extreme values for either or both of the above ratios t h a t stereoscopic fusion is nearly impossible to achieve. E x t r e m e foreshortening in surface-based pictures makes stereoscopic fusion difficult because of rapid scale change from nearfield to far-field. Far-field stereoscopy is not so difficult to accommodate as nearfield stereoscopy, however, because the scale change ratios and monocular to binocular image ratios are usually more favorable than with highly converged

104

T. A. MUTCH E T A L .

b/d =0.1 FIG. 10. Stereoscopic field of view with obscured ground shown in black. Contours show the effectiveness of the stereoscopic effect for different regions. The base/distance ratio (b/d) is defined as the ratio of the effective baseline (b) to the distance (d) between a point on the surface and perspective center nearest to the point. Stereopsis is present in pictures taken with bid less than 0.1, b u t is not strong enough geometrically to make photogrammetric measurements sufficiently accurate for most mapping purposes. The optimum b/d ratio for near-field mapping with surface-based systems is approximately 0.25. n e a r - f i e l d p i c t u r e s . A l i t t l e p r a c t i c e is r e q u i r e d t o a c h i e v e s t e r e o s c o p i c f u s i o n in foreshortened pictures with small conver-

gence angles, but many highly convergent pictures cannot be accommodated under any circumstances.

LANDER

E ach of the ratios discussed above varies with surface roughness, convergence angle, and illumination. The rougher the surface, the greater the local scale difference between pictures and the greater the number of surfaces imaged by one camera b u t not the other. Similarly, the greater the convergence (i.e., the larger the base/ distance ratio) the poorer the stereoscopic geometry. Under low angle illumination, however, shadows m a y obscure parts of the images which degrade stereopsis, and the stereoscopic effect m a y actually be enhanced. The horizontal base line employed b y Viking cameras presents the most natural view of the scene, but a stereoscopic effect exists only in part of the

IMAGING

panoramas (Fig. 10). Tests with stereoscopic surface-based pictures both of the lunar surface and of terrestrial test surfaces indicate t h a t optimum stereoscopic convergence for adequate photogrammctric measurement and stereoscopic fusion lies between 5 ° and 50 ° for average natural surfaces. Figure 11 is a plot of stereoscopic baseline-to-object distance ratios with respect to convergence. Convergence of 5 ° to 50 ° is obtained when object distances range from approximately 12 to 1.2 times the length of the stereoscopic baseline. Convergence angles less t han 5 ° but greater than the angular resolution limit of the imaging system produce stereoscopic models t h a t cannot be measured with

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]06

T. A. MUTCH E T A L .

useful precision, but which are suitable for qualitative interpretation because the ratios t h a t degrade stereopsis are not i m p o r tan t in this range. There are several methods of monoscopic p h o t o g r a m m e t r y with surface-based pictures. In general, t h e y are not as accurate as stereoscopic methods and are possible in only a few places in the panorama. For example, spacecraft shadows provide data from which topographic measurements can be made. Slope frequency distributions can be computed by measuring shadows of natural features, and theoretically, systematic slope measurements could be computed b y measuring shadow progression to provide map data. The geometric conditions for mapping by this method are varied and complicated, and the number of observations required is so great t h a t this method has not been a t t e m p t e d with surface-based pictures.

position within a few kilometers from astronomical observations alone, provided an accurate vertical reference is available. Observations of two or more celestial objects of known aerocentric position allow the determination of the point at which the Martian rotation axis intersects the Celestial Sphere (i.e., the coordinates of the Martian Celestial Pole). Fortunately, this does not require a local vertical to be observed. Again, we may expect to achieve an accuracy comparable to a picture element (a few minutes of arc). I f accurate orbit determinations for Phobos are made from Mariner and Viking Orbiter pictures, it may be possible to use lander-camera observations of Phobos to determine the lander position. The accuracy, however, will be several times poorer than can be achieved from star or planet observations referred to an accurate zenith. I f star transits can be timed to an VII. LOCATION OF L A N D E R : ASTRONOMICAL accuracy of about one second for a period OBSERVATIONS of about 100 days, the rotation period of the When en tr y data are analyzed following planet can he found with an accuracy comtouchdown, and after the lander has been parable to t h a t of existing determinations. tracked for several days from Earth, it A star near the Martian celestial equator should be possible to locate the lander with moves the diameter of one high-resolution an uncertainty of less than 0.5 ° in latitude picture element in about ten seconds, so and 0.2 ° in longitude. Astronomical obser- this measurement seems marginally posvations from the surface of Mars should sible, if a given line can be scanned repeateventually provide its position to better edly. I f the cameras continue to work for a than 0.1 o. The lander cameras will be able much longer period, one might hope to to detect the brighter planets, the satel- improve our knowledge of the rotation lites of Mars, and a few bright stars. rate or to look for variations in this rate. Observations of any fixed celestial object Much better angular resolution (seconds at different times of da y allow determina- of arc) or much longer periods of observation of the direction of the Martian tion (decades) would be required to observe Celestial Pole with respect to the lander, the expected Martian equinoctial preceswhich fixes the local n o r t h - s o u t h direc- sion, or any polar-wander phenoma, which tion. I f the local vertical can be determined provide important geophysical data on the with comparable accuracy, the lander's rigidity and density distribution of the aerographic latitude follows at once. The planetary interior. Early in the 90-day mission the orbiter interpolated time of meridian passage of an object of known celestial coordinates will take low altitude overlapping pictures then would give the longitude of the of an area at least 20km around the lander. The accuracy possible should be lander. Because orbiter pictures will have comparable to the size of a picture element a best resolution of 50m/line pair (unless (0.04 deg, or 2.4 min of arc). Since a minute it proves feasible to lower periapsis without of arc is about a kilometer on Mars, one violating the current quarantine protocols should be able to find the lander's for the unsterilized orbiters), the lander

LANDER IMAGING

107

organizational structures often associated with life, such as the capillary structure of leaves. The camera is also limited by its extremely restricted ability to explicitly resolve another general signature of life: namely, motion or rapid change of shape. The slow-scan design of the camera necessarily implies a degraded type of motion detection. Objects moving in the same direction as the azimuthal drive may be repeatedly imaged; objects moving in the opposite direction will be artificially condensed. The resultant images will include anomalies t h a t may be identified as motion-dependent, especially if there is opportunity to image the same field of view repeatedly. Repeated single-line scanning would provide enhanced opportunities for motion detection. The camera has the potential to recognize long-term changes in shape, numerical density, or color. Such changes might occur either on a daily or seasonal time-scale. Remote observations of seasonal variations VIII. ANTICIPATED SCIENCE RESULTS in the surface brightness and distinctions I t is hazardous to anticipate what between bright and dark areas have been specific features will be seen with the the basis of both biological and metereoViking cameras. Nothing is known about logical hypotheses about Mars. The imagthe character of the Martian surface at this ing experiment should contribute to an level of detail. Inevitably in making predic- understanding of these phenomena. tions one is biased strongly by what he One can imagine the observation of some knows to occur on the E a r t h and Moon. forms t h a t allow strong biological inferIn t h a t context the discussion which ences, such as dynamically unstable shapes follows should be considered exemplary either in the near-field or silhouetted on the rather t h a n definitive or all-inclusive. local horizon. The shape of a tree or a cow would be examples here on Earth. But even 1. Biology in this case a cautionary note is called for. Many of the scientific goals of the Wind-eroded pillars commonly taper toimaging experiment can be directly or wards the base and, at low resolution, indirectly related to the biological objec- would resemble the top-heavy shape of a tives of the Viking mission--the search for tree or bush. By definition, technological life on Mars. The results from all the experi- artifacts would indicate life. ments on this reconnaissance mission will The possibility of finding fossil life should serve to narrow the range of hypotheses not be discounted. Fossils include not only about the environment of Mars and its the direct remains of organisms but also present or past suitability for life. structures formed by organisms in connecThe most exciting outcome would be tion with their life activities : such things as pictures t h a t reveal forms whose mor- burrows, tracks, trails, and distinctively phology or motion is unequivocally as- laminated structures (e.g., algal mats and sociated with life. A positive result would bioherms). Fossil preservation on E a r t h is rest heavily on the recognition of resem- generally favored by sediment burial, but blances to terrestrial forms. Limitations of rapid crustal subsidence and sedimentary resolution prevent observation of fine deposition in many areas lead to a low

will not be visible. However, precise location of the lander on orbiter pictures by resection techniques will be possible if common landmarks are visible in both orbiter and lander pictures. Certainly, the orbiter pictures will be critically important in deciphering the large-scale character of the physiographic province in which the lander touches down and in monitoring large-scale diurnal or seasonal changes in brightness and color. It will be of particular interest to correlate albedo changes detected from orbit or from the E a r t h (e.g., the so-called wave of darkening) with local variations near the landing site. In this latter context the orbiter I R radiometer and spectrometer experiments will be useful in monitoring any changes in surface temperature and water vapor abundance respectively. Some of these changes may be related to transient features visible in lander pictures.

108

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MUTCH E T A L .

density of fossils encased in sedimentary rock. The depositional model which obtains on E a r t h may be exceptional. On the Moon, for example, a thin regolith of great antiquity has been suggested. The age and thickness ofsurficial sediments and sedimentary rocks on Mars will be a function of the tectonic activity of t h a t planet. I f Mars has not experienced the pervasive tectonic activity t h a t characterizes Earth, then fossils may be relatively common in a thin veneer of recycled surficial materials. 2. Geology The experience of Surveyor missions provides a background for anticipating some probable Viking results. Boulders in the near-field can be rudimentarily classified according to rock type, mode of origin, and extent of surficial modification. For example, basaltic and granitic rocks can be differentiated on the basis of color (assuming they are not covered by similar weathering crusts). Vesicles and flow fabrics provide evidence of extrusive igneous activity. Under favorable conditions sedimentary stratification or metamorphic cleavage can be distinguished. Admittedly it is easier to talk about these features in principle than to identify them in fact. For example, flow textures, sedimentary stratification, and metamorphic foliation can all produce the same general phenomenon--planar surfaces. The character of the Martian regolith may be quite different from t h a t on the Moon. Gault and Baldwin (1970) have pointed out t h a t the Martian atmosphere consumes all meteroids smaller than about 1000g. Primary craters smaller than about 60m will be greatly reduced in number. In addition, windblown fines will preferentially fill small craters. Accordingly the small craters so common in the vicinity of Surveyor spacecraft may be lacking in the Viking pictures. In addition, the pervasive comminution associated with small-scale lunar impacts may not be evident on the Martian surface. I f aeolian erosion and transport are effective modifying agents it is reasonable to expect evidence for this in the pictures. Possible diagnostic features include gravel

lag deposits, dunes, selectively sorted fine-grained silt or coarse-grained sand, ventifacts, and wind-swept rock surfaces (yardangs). Sagan and Pollack (1967) calculate minimum threshold drag velocities between 2 and 4m/sec (corresponding to wind velocities above the surface boundary layer of about 80 to 100m/sec for atmospheres of 5 to 15 millibars and particle sizes of about 0.3ram. For particles both larger and smaller the threshold values increase. This means that wind velocities of 80 to 100m/sec are necessary to get any particles into suspension. Such velocities are expected in a wide variety of times and places (Gierasch and Sagan, 1971). Once a particle is in suspension the turbulent velocities necessary to keep it aloft are significantly less. Even in the absence of turbulent support the Stokes-Cunningham fallout times are long. Mariner 6 and 7 pictures provide some empirical evidence for sediment transport. Cutts et al. (1971) have pointed out t h a t many craters in Meridiani Sinus have light marks consistently on the north side of crater floors. As they indicate, it is difficult to conceive an in situ process that would lead to this systematic distribution of light and dark floor areas within craters. The most obvious explanation is that sediment is being transported across crater interiors and collecting on the leeward sides, a common occurrence in terrestrial depressions. Sharp has been instrumental in pointing out the possible importance of freeze-thaw processes on Mars (e.g., Sharp, 1968; Sharp et al., 1971). One of the likely morphological structures resulting from freeze-thaw cycles would be polygonally patterned ground (Wade and deWys, 1968). On Earth these polygons commonly occur when contraction eraeks are formed in frozen ground during winter months and, during warmer months, are filled with surficial water which freezes as it drains downwards from the thawed soil into the permafrost zone. As this process is annually repeated substantial ice wedges with polygonal plan view are formed. Analogous features form where no liquid phase is present. I f the contraction cracks are

L A N D E R IMAGING

109

filled with any solid material--fine sedi- efficient at several wavelengths. This may ment or hoar frost---thermal expansion of be done b y measuring the brightness of any the ground during the warm part of the constant celestial object while it rises or year will result in upward bulging. On sets; even the Sun can be used. The Mars similar opportunities exist for forma- distribution of brightness over the daytion of contraction cracks during cold time or twilight sky can be used to learn periods, filling of cracks with fine sedi- something about the particle size distribument, H 2 0 ice, or C02 ice, and subsequent tion and (during twilight) the vertical polygonal upbulging during warm periods. distribution of the scattering and absorbing The resulting patterned ground, if present, species. Halo and rainbow phenomena can should be identifiable in Viking pictures. be used, if visible, to measure the refractive Other features associated with freeze- index and crystalline form of aerosol thaw terrains such as slump structures and particles. Specifically, they may allow us particle sorting (stone rings) may also be to distinguish between ice cirrus and C02 preserved and recognizable. cirrus. It is just possible that scintillation Discussion of science goals to this point has emphasized features that would be (twinkling) of stars and planets may be visible in the near-field. This is in part a detectable very near the horizon. Such reflection of lunar experience where most observations would provide information spacecraft have landed in flat mare regions on winds and turbulence in the planetary with little local relief. Under these circum- boundary layer. Except near the Sun, the stances the surface available for detailed daytime sky on Mars should be dark examination is necessarily limited to enough to allow us to see the brighter stars distances a few meters from the space- and planets. craft. B u t it should be remembered that the Viking Lander cannot be so precisely 4. Local Phenomena and Global Evolution directed to a featureless plain, unless that Pictures of any planetary surface reveal plain is extremely large in size, for the a specific configurational response to many landing ellipse is some 840 × 400kin in processes interacting in a complex mansize. With this sort of positional uncer- ner. To that extent pictures are sometimes tainty the lander may well touch down in a discounted as scientific data since they region of great local relief, the best efforts of do not directly yield quantitative or the flight engineers to the contrary. I f so, "filtered" data. In the same vein it is there will be added opportunity for taking sometimes argued that localized geopictures of large-scale geologic features morphic or geologic features on a planetary exposed in local cliffs. Of particular surface are only distantly related to the significance would be such phenomena as origin of that planet, and are therefore of lava sequences, intrusive igneous rocks, secondary importance. sedimentary rock sequences, or structurAssignment of relative values to parally deformed rocks--all of which might ticular scientific questions is ultimately a bear witness to a "hot" tectonically active matter of personal taste, b u t we accept planet. the thesis that Viking experiments should address themselves to questions of large3. Atmospheric Observations scale planetary composition and evolution. The most obvious atmospheric pheno- The Lander Imaging Experiment has the mena visible from a planet's surface are potential to contribute to these goals in clouds. Several other types of atmos- the following ways : pheric phenomena should also be visible. 1. Test the thesis that macroscopic life is For example, the solar aureole provides or was present. information on the number and size of 2. Provide evidence of crustal melting. aerosol particles in a vertical path. Some 3. Provide first indications of the extent related information can be obtained b y measuring the atmospheric extinction co- of crustal differentiation, either b y igneous

110

T . A . MUTCH E T AL.

processes or by atmosphere-surface interaction. 4. C h a r a c t e r i z e t h e M a r t i a n r e g o l i t h a n d suggest the relative importance of meteoritic bombardment, aeolian action, and in situ " w e a t h e r i n g " i n d e v e l o p i n g t h e regolith. 5. I d e n t i f y w i n d - e r o d e d s t r u c t u r e s a n d , in so d o i n g , e m p i r i c a l l y e s t a b l i s h t h e existence of prevailing winds of high velocity. 6. P e r f o r m d i r e c t e x a m i n a t i o n o f a e o l i a n transport by suspension, saltation, or creep mechanisms. REFERENCES BATSON, R. M. (1969). P h o t o g r a m m e t r y with surface-based images. Appl. Opt. 8, 1315. CUTTS, J. A., SODERBLO1VI, L. A., SHARP, R. P., SMITH, B. A., AND MURRAY, B. C. (1971). The surface of Mars I I I - - L i g h t and dark markings. J. Geophys. ge8. 76, 343-356. GAULT, D. E., AND BALDWIN, B. S. (1970). I m p a c t cratering on M a r s - - S o m e effects of the atmosphere. Trans. Amer. Geophys. Union 51,343.

GIEaASeH, P., AND SAGAN,C. (1971). A prelimina r y assessment of Martian wind regimes. Icarus 14, 312. MASURSKY, H., et al. (1970). Television experiment for Mariner Mars 1971. Icarus 12, 10-45. SAGAN, C., AND POLLACK, J. B. (1967). A windblown dust model of Martian surface features and seasonal changes. Smithson. Astrophys. Obs. Spec. Rept. 255, 44 pp; also Nature 223, 791 (1969). SRARP, R. P. (1968). Surface processes modifying Martian craters. Icarus 8, 472-480. SHARP, R. P., SODERBLOI~I, L. A., MURRAY, B. C., AND CUTTS, J. A. (1971). The surface of M a r s - - I I . Uneratered terrains. J. Geophys. Res., 76, 331-342. VIKING PROJECT OFFICE (1969). Viking Lander Science Instrument Teams Report, Document No. M73-112-0, NASA, Langley Research Center, 81 pp. VIKING PROJECT OFFICE (1970). Viking Mission Definition, Document No. M75-123-1, NASA, Langley Research Center, 35 pp. WADE, F. A., AND DEWYs, J. NEGUS (1968). Permafrost features on the Martian surface. Icarus 9, 175-185.