GEOLOGICAL SURVEY CIRCULAR 640 

A Strategy for the 

Geologic Exploration 

of the Planets 

PREPARED ON BEHALF OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
A Strategy for the 

Geologic Exploration 

of the Planets 

By M. H. Carr, Editor 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 6.40 

Prepared on behalf of the 

National Aeronautics and Space Administration 

Washington 19 70 
United States Department of the Interior 

WALTER J. HICKEL, Secretary 

Geological· Survey 

William T. Pecora, Director 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page 

Introduction, by M. H. Carr . . . . . . . . . . . . . . . . . . . 1 

· Scope and purpose . . . . . . . . . . . . . . . . . . . . . . . . 1 

Relevance of geologic studies . . . . . . . . . . . . . . . . 1 

Origin and evolution of the solar system . . . . . . 2 

Origin and evolution of life . . . . . . . . . . . . . . . . 2 

Man's terrestrial environment . . . . . . . . . . . . . . 3 

Stratigraphy and structure , by M. H. Carr 

and J. F. McCauley . . . . . . . . . . . . . . . . . . . . . 3 

Applicability to the planets . . . . . . . . . . . . . . . . . . 3 

Surface heterogeneities . . . . . . . . . . . . . . . . . . . 3 

Nature of processes . . . . . . . . . . . . . . . . . . . . . . 4 

Sequence of events .. .. .. •. . . . . . . . . . . . . . . . 5 

Geologic framework . . . . . . . . . . . . . . . . . . . . . 5 

Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Data requirements...... . ....... . . . . . . . . . . . 7 

Flyby photography . . . . . . . . . . . . . . . . . . . . . . 7 

Orbital photography . . . . . . . . . . . . . . . . . . . . . 7 

lllumination . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Resolution . . . . .. . . . . . . . . . . . . . . . . . . . . . 7 

Areal coverage . . . . . . . . . . . . . . . . . . . . . . . . 9 

Color............................... 9 

Polarimetry . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Advanced orbital missions ............... 10 

Lander photography ..................... 10 

Lithology, attitude, and texture 

of surface rocks . . . . . . . . . . . . . . . . . . . . . . 10 

Surface topography . . . . . . . . . . . . . . . . . . . . 11 

Analytical support . . . . . . . . . . . . . . . . . . . . 11 

Drillhole measurements . . . . . . . . . . . . . . . . . . . . . 11 

Summary .. .- ............. . ................ '11 

Geodesy and Cartography, by D. W. G. Arthur, 

R. M. Batson, and F. J. Doyle ... ! ......... 12 

Definition of objectives ................. . ... 12 

Data-acquisition systems . . . . . . . . . . . . . . . . . . . . 13 

Visual imaging systems ................... 13 

Other imaging systems . . . . . . . . . . . . . . . . . . . 15 

Support data ....... . · . . ................. 15 

Flyby and approach photography ............ : 15 

Orbital photography ....................... 16 

Elevation determinations . . . . . . . . . . . . . . . . . 17 

Vertical stereophotography . . . . . . . . . . . . . . 17 

Geodesy and Cartography--Continued 

Orbital photography--Continued 

Elevation determination--Continued 

Page 

Convergent stereophotography . . . . . . . . . . . 17 

Photoclinometry ...................... 18 

Radar altimetry . . . . . . . . . . . . . . . . . . . . . . . 18 

Soft-landing missions . . . . . . . . . . . . . . . . . . . . . . 18 

Photography . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Data reduction . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Summary· ................................. 20 

Geophysics, by M. H. Carr, G. G. Schaber, 

and R. L. Wildey ....................... 21 

Size, shape, mass, and rotational constants . . . . . . 21 

Seismology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

Heat flow ............................... 24 

Magnetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

Physical properties of near-surface material . . . . . 25 

Infrared radiometry ........ ' ............. 25 

Microwave emission . . . . . . . . . . . . . . . . . . . . . 26 

Bistatic radar .......................... . 26 

Radar topographic mapping . . . . . . . . . . . . . . . 27 

Terrestrial 'observations . . . . . . . . . . . . . . . . . 27 

Coherent side-looking radar . . . . . . . . . . . . . 27 

Summary .......... ·······················. 28 

Geochemistry, by M. H. Carr, Priestley 

Toulmin, 3d, and I. A. 'Breger . . . . . . . . . . . . . 28 

Elemental composition . . . . . . . . . . . . . . . . . . . . . 29 

Sampling ......... · . . . . . . . . . . . . . . . . . . . . . 29 

Accuracy and precision ... . ......... . ..... 30 

Analytical techniques . . . . . . . . . . . . . . . . . . . . 31 

Isotopic composition and age determinations . . . . 32 

Mineral composition . . . . . . . . . . . . . . . . . . . . . . . 32 

Organic radicals ............... . ........... 33 

Orbital measurements ...................... 33 

Multiband photometry ................... 33 

Far-infrared spectroscopy ( 5-50}1) . . . . . . . . . . 34 

Y -ray spectrometry . . . . . . . . . . . . . . . . . . . . . . 34 

X -ray spectrometry . . . . . . . . . . . . . . . . . . . . . . 34 

Supporting Earth-based research .............. 34 

Summary ................................. 35 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

ILLUSTRATIONS 

Page 

Figure 1. ·Graph showing dependence of feature recognition on illumination angle . . . . . . . . . . . . . . . . . . . . . . . 8 

2. Diagram showing conditions for photogrammetric reconstitution ...................... . ..... 14 

III 
A Strategy for the Geologic Exploration 

of the Planets 

By M. H. Carr, Editor 

INTRODUCTION 

By M. H. Carr 

Scope and Purpose 

The geology of the planets bears directly on 

three basic aims of lunar and planetary 

exploration: determination of the origin and 

evolution of the · sol or system; determination of 

the origin and evolution of life; and clarification 

of the nature of the processes shaping man's 

terrestrial environment (National Academy of 

Sciences, 1966 ). The purpose of this report is to 

suggest guides to the orderly geologic 

exploration of the planets and the assignment of 

priorities to specific experiments and to indicate 

where supporting Earth-based research can be 

most profitably pursued. The kinds of data that 

are most relevant to geologic interpretation are 

specified, and the sequence in which these data 

should be acquired is outlined. The report does 

not provide detailed consideration of the 

engineering problems involved in acquiring the 

data, but the exploration suggested appears 

practical in view of present flight plans and 

probable future flight opportunities. 

The strategy is concerned almost exclusively 

with geologic problems; other topics are 

discussed only where they relate to geology and 

only insofar as they illuminate a particular 

geologic problem. The term "geology" is used in 

its broadest sense and is considered to mean 

study of the solid parts of the planets. 

Geophysics, geochemistry, geodesy, and other 

disciplines concerned with the solid planets are 

1 

all included in the general term. Planetary 

atmospheres are almost completely disregarded, 

not because they lack relevance but because 

they are not normally studied by geologic 

techniques. This limitation restricts much of the 

discussion to the terrestrial planets and to the 

solid satellites of the outer planets. Jupiter and 

Saturn in particular are difficult to consider in 

geologic terms because they may have no solid 

surfaces. Attention is focused on the inner 

planets because they are closest to Earth and are 

easiest of access. Successful flyby missions have 

already been made to Mars and Venus, and 

missions to Mars, Venus, and Mercury are in 

advanced planning stages. 

The strategy proposed is intended as a 

supplement to earlier more general papers. It has 

been written largely within the framework of 

recommendations of the Woods Hole 

Conference (National Academy of Sciences, 

1966) and the MacDonald Committee on Space 

Research (National Academy of Sciences, 1968). 

We have relied heavily on several earlier reports, 

particularly those of the Space Science Division 

of the Jet Propulsion Laboratory (Adams and 

others, 1967a,b, and Mackin and others, 1969). 

This paper differs from the aforementioned 

largely in the emphasis on geologic problems. 

The work was supported by NASA (National 

Aeronautics and Space Administration) 

contracts W-12, 650 and W-12, 872. 

Relevance of Geologic Studies 

Before examining in detail what specific 

experiments best fulfill geologic requirements, 
the more general question of the importance of 

geology in planetary exploration will be 

examined. 

Origin and Evolution of the Solar System 

Adams and others (1967a,b) examined the 

problem of the origin and evolution of the solar 

system and reduced it to these questions: 

1. Are the individual terrestrial planets and 

satellites chemically uniform or 

nonuniform? 

2. Did final accretion result in the present array 

of planets or satellites, or in an array that 

was subsequently altered? 

3. Was the cloud chemically homogeneous at 

the time of final accretion? 

4. What was the state of the sun-cloud system 

when it first appeared as a recognizable unit. 

5. W e r e there I a r g e-s c a I e elemental and 

isotopic nonuniformities in the contracted 

nebula? 

Because a II these questions con cern 

determination of the stage (or stages) in 

solar-system evolution when chemical 

fractionation took place, answers must 

ultimately depend on chemical measurements. 

Many of the necessary measurements will be 

_ made on planets and their satellites, to interpret 

them we are confronted with the problem posed 

by the first question. Are the planets and 

satellites chemically nonuniform? We connot 

assume a priori that a planet is chemically 

homogeneous and that a given analysis is 

representative of the whole planet. This is not 

true of the Earth and is not true of the Moon 

(see section "Surface heterogeneities"). To 

determine whether a planet is homogneous will 

require observations that will permit inferences 

regarding the structure of the interior. Moreover 

if general conclusions are to be drawn from 

specific measurements, it is not sufficient merely 

to establish non uniformity; the nature of the 

nonuniformity must be defined. To document 

nonuniformity, we cannot simply assume 

random variability and sample a planet 

statistically, because the only accessible part of a 

planet, its near surface, may be totally 

nonrepresentative of the planet as a whole. We 

must have a general understanding of the planet, 

and this can be achieved effectively only by 

studying the processes that have caused, and 

may yet be causing redistribution of materials 

within the planet. Only if the processes are 

understood can valid general conclusions be 

made regarding the significance of specific 

measurements. The sampling or type of 

measurement made on a planet should therefore 

be guided by geologic priorities based on this 

need to understand geologic processes, rather 

than cosmogonical · considerations, because we 

cannot proceed from the specific to the general 

without the geologic knowledge. 

Geophysics is concerned largely with 

do cum en ting internal heterogeneities and 

conditions. Chemical determinations on 

materials from the interiors obviously cannot be 

made directly; nevertheless, inferences are 

possible from measurements of physical 

properties. The value of any particular physical 

parameter is not important in itself, since it 

depends on local conditions; its importance lies 

in the fact that it places limits on chemical 

composition and internal conditions and 

provides data on internal processes. Stratigraphy 

is concerned directly with documentation of 

crustal heterogeneity. Even though the surface 

rocks consititue a minute part of the total mass 

of the planet, it is the part on which virtually 

every measurement is made. For this reason an 

understanding of surface materials is crucial. We 

must know how variegated the surface rocks are 

and how they were formed. The mode of 

formation is particularly important, for different 

processes vary in the extent to which they cause 

chemical change. Consequently, to understand 

the broader implications of surface analyses, the 

distribution and mode of formation of the rocks 

analyzed should be known, as well as the degree 

to which they typify all the materials of the 

planet. 

Origin and Evolution of Life 

The geology of planetary surfaces bears 

directly on the origin of life. This paper has been 

·written largely on the assumption that, in our 

solar system, life will not be found on other 

planets. But if life is found elsewhere, then the 

significance of geology will be enhanced and this 

strategy will need drastic revision. With life on a 

planet, the mode of rock formation, the physical 

2 
conditions of deposition, and the relative age 

will take on new significance. Not only will they 

reflect past conditions and events, but they will 

reveal the evolutionary path of life on the planet 

and the conditions under which life thrived. 

Paleontology, which has been totally ignored in 

this strategy, would become of paramount 

importance, and different types of missions 

would be required to meet its needs. 

Man's Terrestrial Environment 

One important result of planetary 

exploration will be increased knowledge of the 

Earth. Many fundamental geologic problems 

could be solved by detailed comparison of the 

Earth with other planetary bodies. The relative 

effects of size, original composition, and the 

presence of an atmosphere and hydrosphere on 

the e v o l u t i o n of the Earth are of particular 

geologic importance, and comparison of the 

Earth with other bodies will allow these effects 

to be assessed. 

Probably the most important result of 

planetary exploration from the point of view of 

Man's terrestrial environment will be an 

inproved understanding of the early history of 

the Earth. Features on the surface of the Earth 

are subject to relatively rapid destruction and 

modification largely as a result of the erosive 

action of water, and also as the result of tectonic 

and volcanic activity. As evidence of events and 

processes that took place early in Earth's history 

rarely survives, very little is known of the first 2 

billion years of the Earth's history. Mars, 

Mercury, and the Moon on the other hand, have 

no oceans, and the erosive effects of water are 

therefore virtually absent. Furthermore, because 

of their smaller size, we can expect less tectonic 

and volcanic activity than on Earth (Anderson, 

1969). Primitive features are liklier to survive on 

these planets, allowing their early history to be 

defined. Analogy with these more primitive 

planets may be the only way to arrive at an 

understanding of the Earth's early history. 

Geology will play a vital role in achieving the 

three basic aims of planetary exploration. A 

detailed discussion on how best to pursue the 

geologic studies in concert with other types of 

investigations follows. 

3 

STRATIGRAPHY AND STRUCTURE 

By M. H. Carr and J. F. McCauley 

Stratigraphy is the study of rock sequences; 

structural geology is the study of rock 

deformation and the features it has produced. 

The ultimate aim of both is to determine the 

history of the planet and the mode of its 

formation. A planet is not static and 

unchanging; its constituents are constantly being 

rearranged to form new rocks with new 

distributions. Changes may be only surficial, 

resulting from external mechanism such as 

meteorite impact. Or the changes may result 

from internal processes such as volcanism and 

tectonism. Whatever the processes, a partial 

record of successive events is preserved in the 

surface rocks and structures. The role of 

stratigraphy and structural geology is to 

reconstruct from this fragmentary record the 

sequence and nature of the events that have 

affected the planet and to devise a model for the 

evolution of the planet that is consistent with 

the observations. 

Applicability to the Planets 

Correct interpretation of any surface 

measurement depends on knowledge of the 

degree of heterogeneity of the planet, of the 

processes and sequence of events causing the 

heterogeneities, and of the local geologic 

environment at the site of measurement. One 

purpose of the study of stratigraphy and 

structure is to provide this necessary background 

information, and its relevance to the planets will 

be examined in this light. 

Surface Heterogeneities 

The immediate aim of stratigraphy and 

structure is to reduce the enormous complexity 

of a planet's surface to comprehensible 

proportions by dividing the near-surface rocks 

into units and mapping their distribution and 

attitude. The complexity of the Earth's crust is 

well know, but the variegated nature of other 

bodies in the solar system is less widely 
appreciated. The Moon is the only other body in 

the solar system whose surface has been studied 

in any detail; its heterogeneity is now well 

established. The basic distinction between mare 

and terra has long been recogn~zed. Gilbert 

(1893) pointed out the existence of a blanket of 

material surrounding Mare Imbrium; several 

additional widespread terra units such as the 

Orientale blanket (McCauley, 1967) and the 

Cayley Formation (Morris and Wilhelms, 1967) 

have since been recognized and mapped from 

telescopic observations. Subtle differences in the 

color and reflectivity of the maria documented 

from telescopic observations (Whitaker, 1966, 

McCord and Johnson 1969) showed that the 

maria also are heterogeneous and susceptible to 

subdivision (Carr, 1966). It was with the 

acquisiti9n of Lunar Orbiter IV photography 

that different provinces of the terra could be 

fully recognized and differences in the maria 

adequately documented (Wilhelms and 

McCauley, 1969). Wilhelms and McCauley 

ascribe some of the regional differences to 

volcanism, but whatever the cause of the 

differences, they are real and exemplify the 

heterogeneity of the terra. Similarly detailed but 

larger scale mapping of the Apollo landing sites 

(Trask, 1970) shows that differences also occur 

within the maria and that the mare material 

cannot be treated as a single homogeneous unit. 

The Moon therefore shows considerable 

heterogeneity and stratigraphic studies are 

essential for its orderly scientific exploration. 

Much less is known about the surfaces of 

other planetary bodies, but telescopic 

observations do reveal markings on all bodies 

whose surfaces can be observed. The surface 

markings of Mars, which probably result from 

albedo variations, are especially well known 

from the extensive observations of Lowell 

( 1908), Slipher (1962), and deVaucouleurs 

(1965). Recent Mariner photography has shown 

that the Martian surface is, in addition, 

topographically variegated. The Mariner Mars 

'69 experimenters have pointed to cratered areas 

much like the lunar highlands, to crater-free 

areas such as Hellas, and to areas of "chaotic" 

terrain. Markings have also been observed on 

Mercury and on the Jovian satellites Europa, Io, 

Ganymede, and Callisto ( Katterfeld and others, 

1968). Color differences have been detected on 

4 

Mars and some Jovian satellites and the 

differences have been attributed to chemical 

heterogeneity (McCord and Adams, 1969). The 

surfaces of many planetary bodies appear to be 

heterogeneous. It will be necessary to establish 

the nature of the differences so that the events 

and processes leading to the present 

configuration can be un<:}erstood. 

Nature of Processes 

Data from the surface of a planet must be 

interpreted in the light of processes that resulted 

in its present configuration. Processes affecting a 

planet can be classified into two broad 

categories; those such as volcanism and 

tectonism, related to internal forces, and those 

such as impact and erosion, related to external 

forces. If a planet is internally inert, and has 

always been inert, and if the geology of the 

surface is controlled solely by external 

processes, then stratigraphic studies will be of 

limited value. If, however, the planet has 

experienced any internal activity, then chemical 

fractionation is likely and the significance of 

analytical data will depend directly upon the 

local geology. Stratigraphic studies are required 

therefore, first, to determine whether internal 

processes, particularly volcanism, have affected 

the surface, secondly, to document the geology 

in detail if internal effects are present. 

The first close look at Mars provided by the 

Mariner IV photography indicated that Mars is 

more Moon-like than Earth-like. Meteorite 

impact appears to have played a prominent role 

in sculpturing the surface, and it is expected that 

the same is true of Mercury. This does not, 

however, imply that internal effects are absent. 

One result of impact on the Moon is to mask 

volcanic effects; nevertheless, such effects have 

long been recognized from telescopic and orbital 

observations (Shoemaker and Hackman, 1962; 

McCauley, 196 7) and the volcanic nature of 

mare materials is now confirmed by direct 

examination of lunar samples. The fact that 

volcanism has occurred on the Moon stro~gly 

suggests that it has also occurred on planets of 

comparable or greater size, thereby greatly 

enhancing geologic interest in these bodies. 
Sequence of Events 

The significance of any event on the surface 

must be viewed in its historical context. If, for 

example, volcanism ceased 1 billion years after 

the planet formed, the implications regarding 

bulk composition are far different than if 

volcanic processes are still active. Stratigraphy is 

largely concerned with placing rock units in 

historical sequence and relating them to specific 

events in the past. This historical perspective 

distinguishes stratigraphy and the discipline of 

geology in general from most other natural 

sciences. In the absence of fossils and as a result 

of the difficulty of getting absolute ages, 

geometrical relations as observed by 

photography will be the principal source of the 

data on relative ages. On the Moon, geometrical 

methods have been highly successful in 

establishing a global stratigraphy, largely because 

there are extensive marker horizons in relation to 

which other units can be dated. Two examples 

are the Imbrian blanket (Fra Mauro Formation) 

and the Orientale blanket. Both are extensive 

deposits that appear to have formed during a 

very short period of time. Structures have a 

similar importance for dating purposes, and in 

some areas of the Moon the presence or absence 

of Imbrian sculpture is the dominant criterion 

for determining relative ages. The now 

well-established techniques used on the Moon 

(Shoemaker and Hackman, 1962; Wilhelms, 

19 6 6 ) are directly applicable to planetary 

imagery, and we can expect the same degree of 

success in unraveling crustal stratigraphy given 

photographic data of comparable quality. 

Definitive information on stratigraphic relations 

will however finally depend on surface-based 

observations; crucial relations inferred from the 

imag~ry must be checked at some stage by man, 

or by means of automatic roving vehicles or 

drilling techniques. 

Geologic Framework 

The stratigraphy and structure of a planet 

are fundamental in that they provide the 

framework within which all other measurements 

must be evaluated. A chemical analysis, a 

heat-flow measurement, or a seismic profile can 

5 

be understood and' correctly interpreted only if 

the geology near the place of measurement is 

known. A chemical analysis of a salt dome in 

Texas does not give the composition of the 

surface of the Earth in the south-central part of 

the United States, nor can a value for heat flow 

in Yellowstone National Park be extrapolated to 

the whole of the western United States. For any 

measurement to be understood, the geologic 

context must be known. This is so fundamental 

to geologic thinking that to state it appears trite 

and obvious, yet it is a concept that has found 

only limited acceptance with regard to the 

exploration of other bodies in the solar system. 

Without geologic analysis, the surface of the 

planet is in danger of being treated as a 

homogeneous unit; a chemical analysis is likely 

to be considered as indicative of the chemistry 

of the whole surface, and the mineralogy of one 

part might be considered as the mineralogy of 

the whole. Such interpretations · are 

demonstrably nonsensical for the Earth and the 

Moon, and there is little reason to think that 

they will prove any more valid on other planets. 

Measurements made on the surface of a planet 

must be interpreted in light of the local 

stratigraphy; and to do otherwise could lead to 

gross errors in interpretation. 

Methodology 

Because of dependence on remote-sensing 

data, the techniques of stratigraphic and 

structural analysis of the planets differ from 

those normally used on the Earth. Terrestrial 

rock units are defined on the basis of lithology, 

chemistry, and mineralogy as seen at an outcrop. 

The relative ages of different units are 

determined largely by relations observed in 

vertical sections or in outcrop patterns; 

correlations are made by comparison of 

successions of lithologies, by tracing specific 

horizons on the ground, by diagnostic fossils, 

and by absolute dating methods. In general the 

stratigrapher proceeds from particular 

observations seen at specific outcrops to a 

synthesis of general patterns. The reverse 

procedure must be used in studying the geology 

of a remote surface. General patterns are 

observed from remote-sensing data and the 
significance. of the patterns is later checked and 

supplemented by measurements and 

observations at a restricted number of ground 

sites. The procedure is similar to that used 

terrestrially in geologic exploration of remote 

regions, such as the Canadian shield, where 

initial reconnaissance is by aerial photography, 

aerial gravity, and magnetic surveys; areas of 

interest are located from the remote-sensing data 

and a restricted number are checked on the 

ground. 

The techniques used in analyzing the surface 

of the p I a n e t s will be s i m i I i a r to those 

already used on the Moon. For stratigraphic 

purposes, photography has been the most 

valuable source of remote-sensing information. 

Nearly all basic lunar stratigraphic units and 

relations have been determined from the visual 

imagery. On the · basis of differences in 

topography and reflectivity, the surface of the 

Moon has been divided into many units on the 

assumption that those differences result from 

differences in the materials exposed at the 

surface. The relative ages of adjacent units are 

determined by superposition and intersection 

relations, and a Moon-wide chronology has been 

established by dating units with respect to 

extensive marker horizons such as the Fra Mauro 

Formation. Variations in gravity, emissivity in 

the infrared spectrum, radar reflectivity, and 

color have had value largely as interpretative 

aids, but not in defining and mapping units 

because they have not been determined with the 

same linear resolution as the visual black and 

white image. 

Stratigraphic techniques are success~ul on 

the Moon because the lunar surface does not 

undergo regional variations in external 

conditions. The surface of the Earth is greatly 

affected by external conditions such as those 

produced by climate and vegetation; as a result, 

a specific rock unit may exhibit a variety of 

surface properties. On the Moon the princiapl 

external process modifying the surface is 

bombardment by meteorites, and because this is 

essentially isotropic, differences in properties of 

the lunar surface must reflect differences in the 

properties intrinsic to the -surface materials. 

Similar conditions probably hold for Mercury. 

On Mars and Venus regional climatic patterns 

6 

may affect surface forms, and geologic 

interpretation must be made in light of possible 

climatic effects. 

The techniques that have been applied to the 

Moon are directly applicable to Mars, Mercury, 

and many planetary satellites, but additional 

techniques may prove useful. In particular, 

color, being partially dependent on 

mineralogical composition, may prove to be a 

valuable aid in identifying and characterizing 

geologic units on planetary surfaces. 

Once the geologic units have been defined 

and their relations to one another determined, 

the results can be portrayed on geologic maps. 

Although the geology of the Moon has been 

interpreted largely in the light of surface 

landforms and albedo, the resulting maps are 

truly geological, for they incorporate the three 

basic ingredients of a geologic map: rock units, 

age relations, and geometric position. They are 

quite distinct from topographic maps, terrain 

maps, and physiographic maps, which are 

concerned with surface form. Unless t.he surface 

topography is controlled entirely by surficial 

effects, such as wind action in a thick debris 

layer, we can expect to make geologic maps of 

several of the planets. 

Complete characterization of the rock units 

recognized from the imagery will require 

ground-based measurements. A major purpose of 

a lander will therefore be to determine the 

lithology, texture, mineralogy, and petrology of 

the rocks at the landing site. Another objective 

should be to determine the vertical and lateral 

variations in these properties so that the 

stratigraphic relations at the site can be 

compared with those inferred from the vertical 

imagery. The choice of site for establishing 

ground control will clearly be critical. In the 

initial stages of exploration, sites should be 

located on geological units that have a wide 

distribution and in areas where there is the 

minimum of stratigraphic ambiguity. 

Subsequently, sites may be chosen to clarify a 

specific scientific problem, such as presence of 

volcanic activity, or they may be located in an 

area where a vertical section is accessible, as near 

a fresh crater or steep vertical wall. In late stages 

of exploration, surface units and their mutual 

relations will be most efficiently studied by 

some kind of traversing vehicle. 
Data Requirements 

The first stage in the geologic exploration of 

a planet is to identify and outline the 

distribution of the basic units of the crust from 

the visual imagery supplemented by other 

remote-sensing data. The second stage is to 

establish "ground truth" at critical locations by 

obtaining chemical, mineralogical, and 

petrological data from landers. It is important 

that sufficient data be obtained in the first stage 

so that landing sites can be effectively chosen 

and the data obtained related to a regional 

framework. No matter how many landers are 

deployed, determination of the geologic 

environment at each site and interpolation 

between the sites will depend on the quality and 

coverage of the available regional imagery. If 

adequate imagery is lacking, the subsequent 

exploration program could be jeopardized by 

lack of the basic information needed to interpret 

or extrapolate data from individual sites. It is 

therefore pertinent to examine in detail what 

photographic data are required to meet the 

needs of later exploration. 

Flyby Photography 

The initial photographs of any planet will 

surely be from flybys. These missions will be 

reconnaissance in nature, and no systematic 

coverage can be expected, the purpose being in 

large part to determine crustal style and identify 

general problems. For optimum utility, an early 

reconnaissance mission should strive for imagery 

at a wide range of scales and broad coverage at 

low resolution. The ideal plan would result in 

photographs of the whole disk during approach 

and in subsequent photographs each at a larger 

scale and nested within the previous frame. This 

has two advantages over a plan in which all 

photographs are taken at the highest resolution: 

the broad geography of the body is established, 

and the nature of the terrain at different scales is 

determined. The second advantage is important, 

as there is no way of knowing in advance the 

optimum scales for obtaining information on 

landforms. Systematic stratigraphic studies are 

not possible from flyby photography; for this 

purpose, orbital coverage is required. 

7 

Orbital Photography 

Orbital photography will be the single most 

important source of stratigraphic information; 

its usefulness for stratigraphic purposes depends 

on three main factors: resolution, areal coverage, 

and illumination. Color, polarization, 

photometric fidelity, and stereo overlap, though 

useful, are of secondary importance in the very 

early stages of exploration. 

Dlumination 

Imagery is a source of information on two 

basic properties of the surface of a planet, 

topography and reflectivity; detailed 

information on both properties is necessary for 

stratigraphic work. Albedo differences are best 

seen at or near vertical illumination; topography 

is best observed at low angles of illumination 

when contrast and resolution are enhanced and 

shadows bring the topography into relief. The 

optimum illumination angle depends on the 

roughness of the terrain. For lunar terrain, 

illumination in the 5° -15° range has proved 

best. In very smooth terrain, illumination at the 

low end of the range is desirable so that 

low-relief elements can be discerned, whereas in 

rough terrain higher angles are necessary to 

prevent too much of the image being in shadow. 

The optimum angle must be determined from 

preliminary terrain analysis. Ideally, vertical 

photography at both low and high illumination 

should be obtained. Oblique photography has 

very limited use for systematic work because of 

geometric distortions, scale and resolution 

changes within a single frame, and because 

distant features may be hidden by near features. 

Resolution 

The term "resolution" as it is applied to 

visual imaging experiments often leads to 

confusion with regard to system capability. 

Photographic resolution has traditionally been 

given in terms of separable white and black line 

pairs per millimeter of the image. In vidicon 

imaging experiments, on the other hand, the 

scale width of a single TV line, or in other cases 
the width of two TV lines, at a particular 

altitude, is often given as an index of system 

resolution. The size of the smallest objects about 

which something can be said either in a 

topographic or geologic sense is considerably 

larger than the width of two TV lines, and is a 

function of overall target contrast (fig. 1). In 

any given television picture, for example, small 

high-contrast objects such as young fresh craters 

will be more detectable than larger, subdued, 

older craters. In the ensuing discussion 

resolution is meant to imply the smallest relief 

element that can be recognized, acknowledging 

that resolution is not fixed by the imaging 

system alone, but depends also on illumination 

and the . character of the surface being 

photographed. 

The importance of resolution is easily 

understood, as the finer surface details that can 

be seen at higher resolutions permit more 

refined stratigraphic analysis. The Zond III 

pictures of the far side of the Moon, with 

resolutions of 10 km (kilometers) allowed little 

more to be deduced than the location of mare 

areas and the largest craters. No stratigraphic 

work was possible because surface textures 

60 10 80 90 

Sun elevation,in degrees 

Figure !.-Dependence of feature recognition on illumination angle (after Keene, 1965 ). 

8 
could not be seen. Stratigraphic work was 

possible from early telescopic photography of 

the near side, with resolutions of approximately 

2 km, but the only recognizable units, were 

those such as the Imbrian blanket (Fra Mauro 

Formation) that have very coarse surface 

textures. The big breakthrough in lunar 

stratigraphy came with the acquisition of the 

Herbig photography (taken by the Lick 120-inch 

reflector), with resolutions of approximately 

20Q-400 m (meters). This photography revealed 

the highly variegated nature of the lunar uplands 

as well as differences in the mare. Lack of broad 

contiguous coverage however prevented analysis 

of the entire front side at these resolutions. This 

was later rectified by Lunar Orbiter IV, which 

provided nearly complete coverage of the front 

side at even better resolutions. This latter 

mission was ideal from a stratigraphic point of 

view, since complete coverage at fairly constant 

resolution and illumination was provided. For 

the Moon, improvement in resolution beyond 

20m does not significantly improve geologic 

interpretation because regolith variations rather 

than the underlying rock units control the 

fine-scale topography. The optimum resolution 

at which to systematically photograph each 

planet must be established from earlier flybys, 

but on the basis of Moon and Mars ~xperience, 

0.5- to 1-km detection resolution appears to be a 

reasonable goal for early orbital imagery. . 

Areal Coverage 

The study of stratigraphy and structure from 

aerial photography hinges on recognition of 

regional patterns. · Broad regional coverage at 

resolutions such that surface patterns can be 

recognized is therefore essential. Photography of 

restricted areas, even if of excellent quality, has 

limited usefulness unless the general geologic 

context of the areas is known. The Mariner IV 

photography of Mars, for example, cannot be 

used for stratigraphic purposes because the 

individual frames cannot be placed in regional 

con text. The importance of areal coverage 

results from the importance of distributional 

patterns in the study of stratigraphy. The 

distribution of a rock and its relation to adjacent 

units often tells more about the origin and age 

of many rock units than does the surface 

9 

. morphology. The fact that the Fra Mauro 

Formation on the Moon occurs all the way 

around the Imbrium basin is far more indicative 

of its origin than is its surface texture. Similarly, 

very detailed photography of a particular fault 

scarp in the Southern Highlands might reveal 

very little concerning the origin of the scarp, 

whereas wide photographic coverage could show 

it to be part of a system of fractures radial to 

Mare Imbrium and therefore related in origin to 

the formation of the Imbrium basin. 

Distribution patterns are an essential element in 

geologic analysis, without which image analysis 

is reduced to inefficient exercises in comparative 

geomorphology. 

The success of any landing mission will 

depend in part on the scientific merit of the site 

and the suitability of the terrain. In order to 

make judgments on both these factors and to 

ensure that choice is made within the broadest 

range of possibilities, wide photographic 

coverage at moderate resolutions is required. 

Despite the importance of wide areal coverage, it 

is useful only if acquired at adequate resolution. 

Lunar experience suggests that increased 

coverage cannot justify degradation in resolution 

beyond 2 km and that the early photographic 

effort should be directed toward getting the 

broadest possible coverage resolution of -

0.5-2 km (6-8 TV-line widths). 

Color 

Despite the reservations stated above, 

information on color should be obtained on 

early orbital missions if coverage and resolution 

are not prejudiced. Color as a means of 

distinguishing chemical differences in the surface 

materials has been applied with moderate 

success to the Moon. This topic is covered more 

fully in the section "Geochemistry." 

Polarimetry 

The usefulness of photography through 

polarizing filters has yet to be demonstrated for 

the Moon, the Earth, or experimental surfaces. 

Polarimetry therefore should not be attempted 

at the expense of other potentially more useful 

aspects of a mission. 
Advanced Orbital Missions 

A strategy for advanced orbital m1ss1ons 

cannot be formulated effectively until the early 

reconnaissance program has been completed. 

Nevertheless, certain needs can be anticipated. 

After most of the planet has been photographed 

at a resolution of approximately 1 km, limited 

areas will need to be photographed at higher 

resolutions. Specific sites would be chosen for 

features typical of a given class or the solution of 

some specific geologic problem. Such detailed 

photography will bridge the gap in scale between 

observations on the ground and the 

reconnaissance orbital photography and will 

provide a basis for extrapolation of the 

reconnaissance photography to finer scales. The 

mission would resemble in part the Lunar 

Orbiter V mission, but should have additional 

capabilities, in particular, the capabilities of 

convergent stereophotography and multiband 

photography. 

Following a Lunar Orbiter V type of 

mission, emphasis should shift away from 

imagery toward other remote-sensing 

techniques, with particular emphasis on 

techniques that provide information on the 

chemical and mineralogical properties of the 

surface rather than those more dependent on 

physical properties. Possible remote-sensing 

techniques are discussed in the sections on 

"Geophysics" and "Geochemistry." 

Lander Photography 

A landed photographic system has three main 

geologic purposes: 

1. To examine in detail the lithology, texture, 

and attitude of the surface rocks at a specific 

site. 

2. To determine the topography in the vicinity 

of the spacecraft. 

3. To provide supplementary information for 

other experiments on the spacecraft. 

Lithology, Attitude, and Texture of Surface Rocks 

Since Mars and Mercury are not protected 

from meteorite bombardment by thick 

atmospheres, their surfaces are probably 

covered, at least partly, by a debris layer or 

regolith similar to that on the Moon. The 

Martian regolith is of particular interest because 

processes other than impact are likely to have 

· played an important role in its formation. 

Aeolian erosion and deposition, permafrost, 

carbon-dioxide "glaciation," and atmospheric 

weathering may all have left their imprint on the 

regolith. Detailed examination of the lithology, 

texture, and structure of the regolith in order to 

determine what processes have been most active 

in its formation will be one of the main tasks of 

a landed imaging system. 

On Mars, in contrast to the Moon, coherent 

rocks may be accessible in places for analysis 

and observation. The yellow clouds, widely 

interpreted as dust storms, may remove the 

regolith in places and expose bedrock. Coherent 

rock material will almost certainly be available 

for inspection, if not in place, then as blocks in 

the regolith. Much can be learned from the 

fine-scale texture and lithology of rocks. 

Layering suggests a sedimentary process, either 

in a magma chamber or on the surface. A 

texture of interlocking crystal forms indicates a 

crystallization stage during rock formation, 

either as a result of derivation from a melt or as 

a result of post-depositional recrystallization. 

Euhedral phenocrysts point to a . magmatic 

origin, rounded grains to a sedimentary origin. 

Textures of this type are generally visible at 

1-mm resolution and cameras can achieve this 

resolution without special optics. The ability to 

discern these fine-scale textures will depend on 

lighting; repeated photography of the spacecraft 

environment at different lighting conditions will 

bring out all the observable features. 

Optical microscopy is the most widely used 

technique for examining rocks. By no other 

single technique can a rock be so completely 

characterized or its origin so reliably 

determined. Petrographic microscopes have been 

successfully combined with TV systems and 

compact lightweight microscopes have been 

designed specifically for spaceflight programs so 

that petrographic work can be done remotely. 

But the utility of remote- petrographic 

microscopy is likely to depend more on the 

effectiveness of sample preparation than on the 

ability to design flight-qualified instruments. 

Thin-section work is the most informative type 

of microscopy; examination of crushed rock or 

fragmental debris from the regolith are far less 

10 
essential geologically and alone would not merit 

can elaborate microscope experiment. Making 

thin sections must be considered an integral part 

of the design of a petrographic microscope 

experiment and needs to be actively supported. 

A less desirable alternative to remote 

microscopy is visual examination of a sawed 

surface, perhaps coated with a wetting agent to 

improve contrast. A device to cut a very small 

.flat surface and a spray coater should not 

present great technological difficulties. If the 

material is reasonably coarse grained, then in 

addition to textural relations modal analyses 

may be made by point counts on pictures of the 

surface. P:rovision for sampling selected areas on 

the surface for chemical and mineralogical tests 

would greatly enhance the value of the device. 

The gross features of rocks in place are also 

related to origin. Bedding, cross-cutting 

relations, faults and folds are all necessary for 

understanding sequential relations, yet they 

cannot be examined adequately from a 

stationary vehicle nor from an orbiter. Some 

mobility is required because of the scale of the 

features involved and because a sufficiently large 

number of relations must be observed to 

confidently interpret them. An automated 

mobile vehicle to test the heterogeneity inferred 

from the orbital imagery and to sample a variety 

of geologic units for chemical and physical study 

must be considered for advanced missions. 

Surface Topography 

The conditions necessary for topographic 

-reconstruction from landed imagery are given in 

the section ''Geodesy and Cartography." The 

topography of any planet's surface is likely to be 

constantly changing as a result of contending 

destructional and constructional processes; 

evidence of the character of the processes will be 

preserved in the landforms. All features, 

irrespective of origin, will be modified by 

impact, and their state of preservation will 

indicate something about the rate of landform 

development. Detailed photography of the 

surface therefore provides basic information on 

the rate and nature of surface modifying 

processes. 

!Analytical Support 

One of the main functions of a landed 

imaging system is to provide support for other 

analytical instruments. The environment around 

the spacecraft will almost certainly be 

geologically diverse. Rocks of various types may 

be present both in the regolith and in the 

bedrock. An imaging system provides an 

essential means of monitoring what is being 

analyzed and what is available. When used in 

conjunction with a surface sampler, the imaging 

system will greatly improve the versatility of 

many analytical instruments on board inasmuch 

as it changes the sampling from a random 

process to one of intelligent control and 

selection. This applies both to the geological 

instruments and to the biological and 

engineering instruments. 

Drillhole Measurements 

The capability of drilling a hole and making 

in-hole measurements is of prime importance for 

stratigraphic studies. In addition holes are also 

needed for heat flow measurements, for 

detection of subsurface water, and for 

penetrating the regolith. Vertical sections may 

be observable also on crater walls, or fault 

scarps, and some of these may be accessible to 

roving vehicles, but a section is always available 

to a vehicle with drilling capability. A large 

number of in-hole measurements are feasible 

once a hole is dug. Resistivity, conductivity, 

porosity, and natural and induced radioactivity 

all can be logged to reveal stratigraphic 

discontinuities. Existing logging techniques 

require only the minimum of adaptation for use 

on planets. The principal technological difficulty 

lies not in logging but in drilling, as no 

satisfactory technique for remote drilling is 

available. A drillhole is so important that 

maximum efforts should be made to develop 

drlling techniques and to insure that advance 

systems will have a drilling capability. 

11 

SUMMARY 

1. Stratigraphic studies are concerned with 

documenting the nature, extent, and mutual 

relations of surface rocks in order to 
determine the history of the planet's surface 

and the nature of the processes controlling 

its evolution and to provide a framework for 

interpreting surface measurements. 

2. The basic data requirement is imaging, visual 

or radar, at a wide variety of scales, with 

substantial contiguous areal coverage at each 

scale. Initial requirement is global coverage 

at ground resolutions no worse that 1-2 km, 

then contiguous coverage of selected areas, 

constituting approximately 1 percent of the 

planet, with ground resolutions no worse 

than 100m. Imaging should be at both 

low-angle illumination (10° -25° ) for 

maximum topographic discriminability and 

high -angle illumination for albedo 

characterization. 

3. Colorimetric, photometric, and polarization 

demands on the imaging system should be 

secondary to the requirement for wide areal 

coverage at useful resolutions, but 

colorimetric characterization of the surface 

should follow acquisition of the initial 

imagery or be done concurrently if 

resolution and areal coverage are not 

jeopardized. 

4. Once moderate-resolution global coverage 

and higher resoulution coverage of selected 

areas has been acquired, then emphasis 

should shift toward remote-:sensing 

techniques other than imaging, particularly 

those such as visual and infrared 

spectroscopy that are sensitive to chemical 

and mineralogical differences. 

5. Lander imagery and direct chemical and 

mineralogical determinations on the surface, 

while essential for interpreting the nature of 

units identified from orbital photography, 

are of limited stratigraphic value unless 

accompanied by mobility or drilling 

capability. Development of the hardware 

necessary for drilling should be supported. 

GEODESY AND CARTOGRAPHY 

By D. W. G. Arthur, R. M Batson, 

and F. J. Doyle 

Deimition of Objectives 

. The literal meaning of geodesy is to "divide 

. the Earth." In application it has come to mean 

the determination of the size and shape of the 

physical Earth and the location of a newtwork 

of control points on the Earth's surface in fixed 

coordinate systems. Because of the 

intstrumental techniques employed for 

measurements on the Earth's stirface, geodesists 

have had to concern themselves with the 

positions of celestial bodies and with the gravity 

field. Their activities and interests overlap with 

those of astronomers and geophysicists, and as a 

consequence, geodesy has been divided into 

three nearly-distinct sujects: geometric geodesy, 

geodetic astronomy, and gravimetric (physical) 

geodesy. 

For most of its history, geodesy has had to 

depend on observations made from the surface 

of the Earth. The restricted range of these 

observations made possible the preparation of 

highly detailed and precise maps of localized 

areas, but expansion of gimensions and 

reference systems to continental and global areas 

was accomplished only with great difficulty, and 

some unresolved ambiguities still exist. The 

capability of observing from and to artificial 

satellites, available only in the past 10 years, has 

completely revolutionized geodesy. When 

applied to the planets, the new techniques will 

make -feasible the scientifically preferable 

approach of proceeding from the general to the 

specific in an orderly fashion, with the 

possibility of review at each level before 

proceeding to the next. 

Geodesy and cartography are intensely 

practical disciplines. They provide an 

indispensable reference system for recording and 

correlating the observations and deductions of 

all other planetary sciences. Because of this 

general utility, they are often looked upon as an 

exercise in technology and the inherent 

scientific content of the data obtained is 

neglected. 

The techniques of geodesy, 

photogrammetry, orbit analysis, and 

cartography can provide information on the 

following: 

12 

The basic planetary dimensions. 

A mathematical figure of reference. 

The orientation of the body in the celestial 

coordinate system. 

The rotational constants. 

A.defined system of coordinates . 
The location of any number of surface 

points in the defined coordinate system. 

The gravity potential expressed in spherical 

harmonics. 

Topographic and thematic maps: 

At small scales for synoptic coverage. 

At intermediate scales for regional 

coverage. 

At large scales for landing areas and 

other sites of particular scientific 

value. 

Surface albedo in various wavelengths. 

The size, shape, and rotational constants 

contain geophysical information about moments 

of inertia, internal composition, and rigidity. 

The harmonics of the gravity field contain 

information about anomalous mass distribution 

within the body. Regional and local topography 

imply internal and external mechanism of 

formation. Albedo measurements give 

information on regional and local differentiation 

and composition of materials. Coordinate 

systems and point locations are necessary for 

correlation of all other data, for specification of 

site locations, for navigation and guidance of 

eventual landers and surface mobile vehicles. 

Geodetic studies are therefore essential for the 

study of any planetary body with a solid 

surface. 

Data-Acquisition Systems 

It makes sense to consider the commonality 

of geodetic and cartographic sensors with those 

employed for other planetary sciences. The basic 

sensors-imaging systems, tracking systems, 

stablization systems-are essential to all 

disciplines, and minor concessions in 

optimization for one application can 

immeasurably increase the utility for another. 

An excellent example is the Lunar Orbiter 

Program, which was designed essentially to 

provide surface photographic coverage at 

medium and high resolution. That function is 

performed beautifully, but when landmark 

positions, elevations, terrain slopes, and control 

geometry were reql.1ired, the system was grossly 

inadequate. Imaging systems on future planetary 

missions must be designed and used in such a 

manner that accurate reconstruction of surface 

geometry is possible. 

Both geodesy and cartography depend in 

large part on scaled reconstruction of the surface 

by means of some form of imagery. It is 

appropriate therefore to review the conditions 

which are necessary for the photogrammetric 

reconstitution of models from photographs. In 

pure photogrammetry, only the photographs 

themselves are used; all external data are 

excluded. This is preferred, as camera position 

and attitude errors may be encountered. Also 

preferred for geodetic purposes are wide-angle 

cameras because their greater geometric strength 

results in much less restrictive reconstitution 

conditions. For wide-angle photography the 

necessary conditions for reconstitution are that 

five points must be identified on two 

photographs and that no three of these should 

lie in one plane containing the two camera 

positions. This is the well known Fourcade 

correspondence theorum. Base height ratios of 

0.2-1.5 are usable, 1.0 being about the 

optimum. The Fourcade correspondence 

theorum cannot be applied to narrow-angle 

photography, but pure photogrammetric 

reconstitution is possible if a minimum of three 

pictures with considerable convergence ( < 30° ) 

is acquired, with the same four points measured 

and identified on all three pictures (fig. 2). 

Reconstitution is possible from two strongly 

convergent photographs provided accurate 

orientation and positional information on the 

camera is available so that the correspondence 

settings can be performed and provided the 

cam era's internal geometry is appropriately 

calibrated. Despite their disadvantages for 

geodetic purposes, narrow-angle cameras have 

been used bn previous lunar and planetary 

missions and will be used on planetary missions 

presently planned because only with these 

cameras can acceptable ground resolutions be 

achieved from normal orbital altitudes. 

Given a choice, geodesists and cartographers 

will always opt for physical film return because 

of the inherently simpler and more accurate data 

reduction. Physical film return, while not 

impossible, is unlikely for unmanned missions 

other than to the Moon. High-resolution vidicon 

systems and combination film-scanning 

transmission systems such as on Lunar Orbiter 

can currently produce image-plane resolutions 

13 
I. Reoonstitution oenerally posslbfe 

NARROW 

3. Recon.,;tution generally PQIIJble . \ 

NARROW 

2. Reconstitution not possible 

without external data 

NARROW AND WIDE 

4. Reconstitution generally possible Figure 2.-conditions for photogrammetric reconstitution. 

14 
approaching 300 lines per millimeter. 

Development of such systems will certainly 

~o~tinue. The quality of the possible imagery is 

liklier to be\restricted by transmission capabilities 

than by camera design. A high-quality terrestrial 

aerail photograph contains approximately 1 oB 

to 1 Q9 bits; to obtain wide coverage at 

comparable quality, enormous amounts of data 

must be transmitted. 

Other Imaging Systems 

Infrared and microwave imaging systems 

may be useful for other planetary sciences but 

their inherent geometric problems and gross 

resolution make them undesirable for geodetic 

and cartographic applications. If, however, they 

are the only imaging sensors to be carried on a 

mission, their geometry should be analyzed and 

the system geometrically calibrated. Some useful 

geodetic information may be extracted under 

such circumstances. 

Planets with a visually opaque atomsphere 

(Venus) may be amenable to radar imagery. 

Again, geometric analysis and calibration 

beginning with sensor-system design are 

required. Radar imagery is susceptible to the 

same kind of reduction as optical imagery, but 

additional techniques are also available. In 

particular, the ranging capability of radar may 

be utilized for reconstruction of surface 

topography. 

Support Data 

Detailed tracking, timing, and attitude data 

are especially important if narrow-angle cameras 

are . used because reconstitution requires this 

external data. As a general rule, the precision of 

the time and tracking should permit vehicle 

location with an accuracy comparable to the 

surface resolution of the image. Usually sensor 

attitude can be determined to nearly an order of 

magnitude better than the sensor can be aimed. 

A particularly useful reference for attitude is the 

stellar field recorded either photographically or 

by star trackers. Properly time-coupled and 

calibrated with surface photography, this may 

body. Altimeter data (laser or radar, depending 

upon planetary conditions) provide a scale 

restraint for photogrammetric solutions, and in 

the absence of geometric imaging systems, can 

provide data on the size and shape of a planetary 

body. 

Flyby and Approach Photography 

The most useful photography for geodetic 

purposes is approach photography that 

encompasses all or most of the disk. In addition, 

the photographic period should be extended to 

provide continuity in scale between full disk and 

detailed photography of the surface. The main 

aims of approach photography are: 

Measurement of the profile of the disk to 

determine ellipticity. 

Determination of the relative 

three-dimensional positions of surface 

markings. 

Fitting of the surface markings to an oblate 

spheroid and fixing the axis of rotation 

with respect to surface markings. 

Determination of the position of the axis of 

rotation with respect to the celestial 

sphere. 

The aims are pertinent to all sizeable bodies in 

the solar system except those that have no solid 

surface. Two situations are encountered. Mars 

yields an example of the first, in which the 

planet rotates relatively rapidly; Mercury is an 

example of the second, where the planet rotates 

relatively slowly. For rapid rotation it is possible 

to make good use of the long-focus 

characteristics likely to be enforced by the 

requirements of other fields of study and to 

depend on the planets rotation to provide a 

baseline. In this case the inclination of the 

vehicle trajectory to the plane of the planet's 

equator is important. Clearly if the spacecraft is 

on the polar axis during approach, then as 

viewed from the surface of the planet it is not 

displaced by rotation. The spacecraft's position 

remains fixed and a three-dimensional 

triangulation is not possible. To utilize the 

planets rotation, the trajectory must be in a 

plane close to the equatorial plane of the planet. 

permit measurement of the celestial orientation To. establish a baseline for triangulation on a 

of the pole and the rotational constants of the slowly rotating planet, the planet's rotation is of 

15 
no assistance and image displacement must be 

caused entirely by spacecraft motion. Since this 

motion is almost directly toward the planet, the 

position in the sky, as seen from the planet, will 

change slowly so that poor geometry will result 

until near encounter. At encounter the direction 

of spacecraft motion is at a significant angle to 

the line joining the spacecraft and the planet's 

center, and so the spacecraft moves rapidly 

across the planet's sky and good geometry is 

possible. 

A similar geometric problem is encountered 

in establishing the position of a rotation axis 

with respect to the celestial sphere from 

approach photography. The approach trajectory 

is close to a straight line, and the position of the 

spacecraft on the line alone is insufficient to 

define the geometry since the planet and the 

cameras can be rotated as one around the 

trajectory without violating the tracking data or 

the photographic measures. To rigidly define the 

geometry, the orientation of the camera must be 

known. This can be determined from the 

spacecraft orientation provided the geometric 

relation between spacecraft and camera has been 

previously determined. The precision of the 

measurements will then depend on the precision 

that the spacecraft orientation can be 

determined. For successful geodesy the relation 

between camera orientation and spacecraft 

orientation must be precisely determined (in 

gravity-free environment to offset flexing of 

structural members) and the in-flight orientation 

of the spacecraft to the stars must be accurately 

known. 

Orbital Photography 

Systematic mapping of fixed surface features 

is necessary for guidance and navigation, for 

location of landing sites or sites for more 

detailed orbital examination, and to provide a 

base for portraying and correlating other 

analytical data. In order to achieve the 

appropriate resolutions, such systematic 

mapping must be done from orbital altitudes. 

For the Moon, a map series was recommended 

by the Geology Panel at the 1967 Conference on 

Lunar Science and Exploration at Santa Cruz, as 

follows: 

Orthographic, Mercator, and Polar 

Stereographic projections of the whole 

Moon at a scale of 1:5,000,000. 

Complete topographic coverage of the Moon 

at a scale of 1:1,000,000. 

Coverage of approximately 20 areas of 

scientific interest for landing sites and 

traverses at a scale of 1:250,000. 

Coverage of central parts of 20 areas of 

special interest at a scale of 1: 50,000. 

Coverage of landing locations in the central 

part of science sites at a scale of 1:5,000. 

This series may be considered representative of 

what might be desired for other planetary 

bodies, although the requirements may vary in 

detail according to size and surface 

configuration. 

Standards for the positional accuracy of 

maps at various scales, for the surface resolution 

required to produce the necessary content, and 

for the elevation accuracy r~quired for various 

contour intervals are as follows: 

Map Accuracy requirements, in meters 

Horizontal Optical Contour Vertical 

Map scale standard error resolution interval standard error 

5,000,000 1,520 250 1,000 300 

1,000,000 300 50 500 150 

250,000 76 12.5 100 30 

50,000 15 2. 5 25 8 

5,000 1.5 .25 3 1 

16 
For camera ·systems with strong geometry, the 

accuracy of positions and elevations established 

photogrammetrically will be one to three times 

the surface resolution. This means that a 

planimetric map having compatible resolution 

and horizontal positional accuracy as given in 

the above table can be produced from camera 

systems of relatively weak geometry. 

Elevation Determinations 

Although accurate planimetric maps are 

possible with weak geometries, the conditions 

for determining elevations are more restrictive 

and should be considered in more detail. 

Ph otogrammetry is the only rigorous 

photographic method. With the camera systems 

being flown, useful vertical resolutions can be 

achieved with convergent stereophotography, 

but those achievable by vertical 

stereophotography are marginal. Because of the 

difficulty of systematic coverage by convergent 

stereo photography, other methods of measuring 

elevations, such as photoclinometry or the 

traditional shadow-analysis technique, must be 

considered. 

Vertical Stereophotography 

In vertical stereo photography, the optical 

axis of the camera remains vertical and the 

baseline is fixed by the viewing angle of the 

camera and the overlap. Typical camera systems 

used so far on lunar and planetary missions 

incorporate a vidicon tube with a 10 mm square 

format and optimally 35 optical resolution 

elements per millimeter. The smallest elevation 

difference that can be measured is given by the 

following equation: 

HL10 

L1£!=----

sin o. 

ll=height of camera, 

L10=angle in radians subtended at 

the lens by the smallest 

resolution element in the 

image, 

o.=intersection angle at ground 

point formed by two homologous rays from two cameras. 

For a camera with a focal length 

of 50 mm 

For vertical cameras with 50 

percent overlap 

sin o.~ 0.1, 

therefore L1fi-=- H 

175 

The smallest differences in elevation that can be 

measured are 5. 7 km for an orbital altitude of 

1,000-km and 11.4 km for a 2,000-km orbit. 

These figures are independent of camera focal 

length if the overlap remains the same, since 

maintaining the overlap keeps the base/height 

ratio constant. The measurable differences are 

inadequate for most mapping purposes. Systems 

with wider angle lenses or higher resolutions 

than previously used must be flown if useful 

vertical stereophotography is to be obtained. 

Convergent Stereophotography 

Ph o togrammetrically useful 

stereophotography pairs can be obtained by 

convergent stereophotography even if the 

camera system is inadequate to provide useful 

vertical stereophotography. Instead of taking 

overlapping vertical photographs, the camera is 

tilted and multiple oblique photographs are 

taken of the region of interest. For vertical 

photographs the base/height ratio is fixed by the 

viewing angle of the camera and the overlap. 

With convergent stereophotography the 

base/height ratio depends on the tilt of the 

camera and some latitude is possible in choosing 

an appropriate ratio. 

Consider the case of two photographs of the 

same area, one vertical, the other taken with a 

tilt angle,~ 

17 
Hf10 

As before f1H=-=--. 

sin a. 

1 . 

f10 remains the same, being 1750 

for a 50-mm lens. a., the intersection angle at the ground 

formed by homologous rays from 

each camera position, is now the 

tilt angle t, 

H 

therefore f1H= 1750 . t s1n . 

For a 1,000-km orbit: 

H=2.2-km 15° tilt 

H=l.l-km 30° tilt. 

The vertical resolution improves proportionately 

with the focal length of the camera, 

since f10 ' becomes smaller without a 

corresponding decrease in a as in the vertical 

case. The increased resolution is of course 

accompanied by a decrease in the area covered. 

The disadvantages of convergent 

stereophotography with long-focal-length 

cameras are that a high pointing accuracy is 

needed and that systematic coverage requires a 

large number of frames to cover a relatively 

small area. The technique may be practical only 

for limited areas such as landing sites or sites of 

unusual scientific interest. 

Photoclinometry 

Photoclinometry is a method of extracting 

topography for nonstereoscopic photography 

obtained from orbiting vehicles (Watson, 1968). 

It is based upon the assumption that, if surface 

albedo is constant, variations in photographic 

density are functionally related to the surface 

slope. The relation is expressed by the 

photometric function required before the 

technique can be applied. Slope data is acquired 

on profiles across the photograph and integrated 

to provide elevations from which contours can 

be interpolated. While theoretically attractive 

because it does not impose severe restraints on 

angular field or stereoscopic overlap, the method 

is not truly practical because the normal albedo 

is not constant, the photometric function is not 

adequately known (particularly in the case of 

planets), and the data _ reduction effort is 

enormous. Although photoclinometry has been 

successful in characterizing different types of 

lunar terrain by statistical slope analysis 

(Rowan, McCauley, and Holm, 1969), the 

technique has had only limited success in terrain 

reconstruction to form topographic maps. The 

technique should be regarded as a stopgap 

measure to be used where other methods have 

failed. Since the data required for its 

implementation (zero phase and 

oblique-illumination photography) coincide with 

requirements for other purposes, no specialized 

photography is required. The only special 

requirement is that of photometric fidelity and 

this coincides with other scientific needs. 

Radar Altimetry 

0 n e possible way of making elevation 

measurements over wide areas is by means of 

radar altimetry. Profiles of the surface can be 

determined to accuracies of 1Q-100 m with 

existing systems which have only modest weight 

and telemetry requirements. A simple 

radar-altimeter experiment would also provide ~ 

valuable calibration data for photogrammetric 

purposes. 

Soft-Landing Missions 

Imaging systems on soft-landed planetary 

probes present a "man's eye" view of the 

landing site and provide a frame of reference for 

the various measurements made by other 

instruments in the probe. A landing site can be 

reconstructed from pictures provided the 

imaging system is specifically designed for 

making photogrammetric measurements and the 

optical geometry of the system has been 

precisely calibrated. However, the difference in 

ground resolution between pictures taken from 

orbit and pictures taken from a planetary 

surface is so great that the meaning of much of 

the information in pictures taken by a landed 

system is lost, unless the location of the lander is 

precisely known and photographed from orbit. 

18 
If an orbiter is present, a symbiotic relation 

between orbiter and lander can be developed 

from which highly detailed interpretations of 

planetary surfaces can be made. Even without 

support from an orbiter, considerable 

topographic information can be extracted from 

the lander imagery. Because the soft lander is 

stationary during data acquistion, the 

photographic problems are generally simpler 

than for orbiting spacecraft. 

Photography 

Imaging systems for planetary landers fall 

into two main categories: slow-scan television 

and optical-mechanical scanners. 

Slow-scan television pictures are 

geometrically similar to pictures taken by 

conventional film cameras. Panoramas can be 

taken with a slow-scan camera by rotating it 

about its perspective center some angle less than 

the field of view between each picture until the 

entire panorama has been covered. The resulting 

pictures can be pieced together as a mosaic. 

Optical-mechanical scanners are similar to 

the facsimile systems used by news services to 

transmit photographs from one facility to 

another. A camera of this kind suitable for 

surface planetary exploration incorporates a 

small mirror which rotates about vertical and 

horizontal axes and which reflects· images into 

the lens of the system. A tiny hole in the image 

plane, on the axis of the lens, admits light to a 

photosensitive device that converts luminous 

flux to an electrical signal for processing and 

transmission to receiving stations. As the mirror 

rotates about one of its axes, the focused image 

of the landing site moves across the pinhole 

above the photosensor, thereby transmitting the 

luminance along a narrow band of the scene. A 

picture is taken by making adjacent sweeps with 

the mirror until the complete panorama has 

been transmitted. Entire panoramas or segments 

of panoramas are recorded as single pictures 

without making mosaics. 

There are several advantages in using 

facsimile cameras on planetary landers. Their 

design is so much simpler, mechanically and 

electronically, . than slow-scan television cameras 

that they are inherently more reliable. Optical 

design is simplified because the pinhole is on the 

lens axis and optical aberrations outside the 

paraxial region need not be considered. They are 

capable of producing pictures of very high 

resolution from relatively small systems. 

Stere~scopic coverage, permitting topograhic 

mapping of the area photographed, requires 

photographs from two different points of 

perspective. This can be achieved byMoving the imaging system physically from 

one station to the other. 

Photographing the object space directly and 

then photographing its reflection in a 

mirror appropriately located. 

Providing two imaging systems separated by 

an appropriate baseline. 

Stereoscopic baselines may be either vertical or 

horizontal. Horizontal baselines present the 

most natural view of the scene, but vertical 

baselines are equally useable from a geometric 

point of view. 

In all cases the accuracy and extent of 

mapping depend on the length of baseline and 

the height of the perspective centers above the 

planetary surface. If the baseline is calibrated in 

advance, it will permit the establishment of a 

local coordinate system and map scale without 

reference to external control provided by orbital 

photography. Under certain circumstances the 

lander imaging system can photograph 

identifiable stars from which azimuth and 

perhaps position can be established. 

Spacecraft shadows provide data from which 

topographic measurements can be made. The 

location with respect to the imaging system of a 

shadow on a planetary surface can be computed 

as a function of the known size and orientation 

of the part casting the shadow and the angle the 

shadow subtends from the imaging system. A 

second method of spacecraft shadow 

measurement is the computation of shadow 

location as a function of the illumination angle, 

the angle subtended at the imaging system 

between the shadow point and the spacecraft 

part casting the shadow, and the distance 

between the imaging system and the spacecraft 

part. 

19 

Statistical slope information can be gathered 

by measuring shadows of natural features, and, 

theoretically, systematic slope measurements 

could be connected to provide map data. The 

geometric conditions for mapping by this 

method are so varied and complicated, and the 

number of observations required are so great, 
that this approach should be considered as an 

emergency · solution rather than a primary 

procedure. 

A mapping method called "Phototrig" may 

be used when the same surface features can be 

identified in pictures taken by both lander and 

orbiter. Feature heights relative to the imaging 

system are computed as a function of distance 

from the lander (measured on orbiter pictures) 

and vertical angle (measured on lander pictures). 

Contour lines are then interpolated between the 

spot elevations thus computed. As with all 

imaging systems from which map data are to be 

produced, geometric analysis and system 

calibration are indispensable. 

Data Reduction 

The camera focal lengths and picture 

formats useful for planetary exploration from 

spacecraft-whether flyby, orbiter, hard or soft 

lander-will generally be different from those 

employed in conventional aerial or terrestrial 

mapping. They may therefore be expected to be 

more or less incompatible with the 

photogrammetric ·procedures and equipment 

generally available. In addition, information 

about camera position and attitude-available 

through orbital tracking and attitude 

sensors-provides geometric constraints on 

triangulation not available to usual aircraft 

situations. This means that computer programs 

designed for conventional aerial control 

extension will be inadequate for planetary 

missions. 

It is therefore necessary that consideration 

be given from the initiation of a planetary 

exploration program to the complete system 

design, which includes not only the spacecraft, 

the sensors, and the mission, but also the 

hardware and software for extracting the 

maximum geodetic and cartographic 

information from the records obtained. 

Unfortunately, this has not been the case 

through the lunar program and the planned 

planetary flights. 

SUMMARY 

1. G e o de s y and cartography pro v i d e a 

reference system for navigation and guidance 

and for correlation and compilation of all 

data from the surface. 

2. The basic r e quire m en t , for geodesy is 

geometrically true, low to moderate 

resolution, wide-angle photography of the 

whole planet. For cartography the basic 

requirement is high-resolution wide-angle 

photography of areas of interest, the 

resolution depending on the intended map 

scale. 

3. Resolution demands and weight constraints 

have forced, and are likely to continue to · 

force, the use of undersirable narrow-angle 

camera systems for which reconstitution 

requires accurate knowledge of external data 

such as camera orientation and position. The 

narrow-angle cameras demand more 

stringent calibration procedures, a more 

restrictive imaging strategy, and more precise 

knowledge of camera orientation and 

position than wide-angle cameras; these 

factors must be considered in total system 

design at the inititation of any imaging 

experiment. 

4. The most necessary attribute of the imaging 

system, without which no metric analysis is · 

possible, is geometric fidelity. The widely 

used vidicon system are notoriously prone to 

distortion, ground-based work should 

therefore concentrate on developing 

geometrically more reliable systems, 

preferably film. 

5. Useful photogrammetric determinations of 

elevations with narrow-angle cameras from 

orbital altitudes is possible only with 

convergent stereophotography and then only 

with precisions of the order of . several 

hundred meters for most camera systems 

now being considered for planetary 

exploration. 

6. Nonvisual imagery can be used for geodetic 

and cartographic purposes, and with radar, 

the additional. ranging capability can be 

utilized for elevation measurements. 

. 7. 

20 

Metric analysis of lander imagery is 

complicated by rapid scale changes, 

uncertainties of orientation and masking by 

shadows, but facsimile cameras promise to 

provide geometrically stable imagery for 

quantitative work. 
GEOPHYSICS 

By M. H. Carr, G. G. Schaber, and 

R. L. Wildey 

Geophysics is concerned with the study of 

the planets by the methods of physics; it 

includes consideration of the structure, 

dynamics, and physical conditions of planetary 

interiors. Fundamental studies that can be 

related very directly to processes involved in the 

history and evolution of the planet are detection 

of radial subdivisions such as core, crust, and 

mantle; characterization of seismic activity; 

measurement of heat flow; and mapping the 

gravity and magnetic fields. Of lesser 

importance, but still of geologic significance, are 

measurements of the physical properties of 

near-surface materials. Abundant ground-based 

data cannot be expected from any planet as 

relatively few ground stations will be 

established. Even a very few surface geophysical 

measurements will enormously improve our 

knowledge of the planet, for little is known of 

the interiors of any planet other than the Earth. 

Many geophysical parameters such as size, shape, 

dynamical figure, surface reflectivity, and 

emissivity at various wavelengths can be 

determined remotely, either from spacecraft or 

from the Earth, and these are the measurements 

on which most data will be obtained early in the 

exploration program. 

Size, Shape, Mass, and Rotational Constants 

and others, 1967). Radius is important as a 

constraint on the internal mass distribution, and 

viscosity estimates are made from deviations of 

the geometrical figure from hydrostatic 

equilibrium. 

The dynamical figure of a planet is the shape 

of its gravitational field as determined from the 

orbital parameters of satellites. If the rate of 

rotation of a planet is great enough to cause 

significant polar flattening, then the moments of 

inertia can be determined from the dynamical 

figure, assuming hydrostatic equilibrium. Since 

the moment of inertia severely constrains values 

of internal densities, precise determination of 

the dynamical figure is fundamental. Very 

accurate determination of the dynamical figure 

requires tracking of several satellites, preferably 

in tight orbits with different inclination, but 

determination of the lower order harmonics 

results from normal tracking of an orbiter 

around a planet and will follow from any orbiter 

program. Higher order harmonics of the 

dynamical figure, similar to those encountered 

on the Moon (Muller and Sjogren, 1968), can 

result from near-surface anomalous mass 

distributions and may have important 

implications concerning the structure of the 

crust. These anomalies can be determined only 

by precise tracking of satellites unaffected by 

drag. For this purpose, a tracking transponder, 

preferably incorporating a nul-type 

accelerometer, should be placed in orbit around 

the planet. 

Moments of inertia can be derived from the 

dynamical figure only if the rate of rotation is 

great enough to cause significant polar 

Size, shape, mass and rotational constants flattening. If the rotation rate is too slow, then 

are of interest because they place constraints on the moment of inertia must be determined from 

two important properties of a planet, the the response of the planet's motion to torques. 

internal mass distribution and internal The principal response is a librational or 

viscosities. The internal distribution of mass may precessional motion; precise determination of 

reveal compositional differences, implying that these motions is needed for moments of inertia 

differentiation has taken place; internal calculations. Location of axes of rotation by 

viscosities are important in that they place limits approach photography and extended terrestrial 

on the temperature and pressure conditions in radar observations will aid in defining the 

the interior. Information on these parameters is planet's motion. 

derived largely from deviations from radial The moments of inertia of Mercury, 

symmetry and the effects of the asymmetry on although of great interest from a dynamical 

the planet's rotation. point of view because of its spin-orbit coupling 

The radius and geometry can be determined (Goldreich and Peale, 1967), are difficult to 

from radar measurements, approach determine owing to the slow rotation. However, 

photography, and occultation experiments (Ash refinement of figures for the mass and radius 

21 
will result from a flyby, and extended 

observation of an orbiter will give precise 

information on the geometrical and dynamical 

figures. Full-disk photography should be 

undertaken from all spacecraft so that the axis 

of rotation can be accurately located and any 

librational motion defined. 

The slow rotation of Venus prevents direct 

determination of its moment of inertia from the 

flattening. However, determination of the 

dynamical figure is still extremely useful for 

analyzing gravity anomalies and other 

indications of dynamic imbalance. Because the 

atmosphere of Venus will cause significant drag 

on a satellite, a tracking transponder 

incorporating a nul-type accelerometer should 

be orbited to precisely determine the dynamic 

figure. Determination of the libration of Venus 

is necessary for determination of the axial 

moment of inertia, and this appears possible 

from extended radar observations of the planet 

from Earth. The moment of inertia of Venus is 

of particular importance because of similarity to 

the Earth in mass and size. Analysis of the 

internal mass distribution will reveal to what 

extent the evolution of their interiors has been 

different. 

Mars rotates fast enough to cause significant 

polar flattening. At present the geometrical 

flattening is not well known, and the best figure 

conflicts with the dynamical flattening 

determined from the orbits of Deimos and 

Phobos. Precise values of the optical flattening 

and radius will be obtained fr0m the 

far-encounter photography and occultations of 

planned orbiters. Tracking of these orbiters will 

lead to more precise determinations of the 

dynamical figure and, as a result, of the moment 

of inertia and the internal-mass distribution. 

Later or biters containing a nul-type 

accelerometer and. tracking transponder should 

be flown to analyze local gravity anomalies and 

the general structure of the crust. 

Anomalous internal mass distributions may 

occur, especially for Venus and Mercury, which 

exhibit spin-orbit coupling. Nonradial symmetry 

would be revealed by noncoincidence of the 

centers of the geometrical and dynamical figures 

and by comparing their deviations from 

sphericity. This can be achieved by radar 

altimetry from an orbiting vehicle. Therefore 

inclusion of a radar altimeter, which is needed 

.for accurate analysis of topography also, should 

be considered in advanced missions. 

Seismology 

The seismicity of a planet provides indirect 

evidence concerning the thermal and dynamical 

state of its interior. Seismology provides the 

most effective way of measuring the dimensions 

of a core, crust, or any other subunit of the 

interior and is the most simple and direct way of 

locating internal discontinuities and internal 

movement. This information is of such 

importance in determing the origin and 

evolution of a planet that a seismic experiment 

should be given a very high priority and included 

early in any exploration program. 

The ideal seismic experiment requires a 

global network of seismic stations each with a 

wide variety of seismic detectors and the 

necessary power and facilities for data handling. 

The possibility of establishing such a network on 

any planet other than the Earth is exceedingly 

remote, but this does not rule out a meaningful 

scientific experiment. Much can be learned from 

a relatively simple seismic experiment which 

would not only provide basic scientific data but 

enable subsequent more complex and 

comprehensive systems to be better designed. 

The simplest experiment involves placing a 

single seismic detector on the surface of the 

planet, preferably. a short-period ( 1 second) 

single-axis seismometer, or, if telemetry and 

weight restraints permit, a short-period 

three-axis type. From such a detector, the 

seismicity or aseismicity of the planet could be 

established and estimates made of the magnitude 

and frequency of seismic events. The general 

level of seismic noise could be measured, and 

from the separation of the S and P waves 

approximate distances of events from the station 

could be inferred. In addition to its intrinsic 

scientific worth, the data would permit an 

accurate assessment of the advisability of more 

complex seismic stations for future missions and 

provide design constraints on such systems. A 

simple seismometer should be deployed at the 

earliest possible opportunity, preferably on the 

first lander to any planet. 

A second-generation seismic station should 

inc 1 u de both long- and short-period 

22 
three-component seismometers. From such a are already planned, and a simple seismic 

station, a far more complete picture of the experiment should be carried on the first 

seismic properties of the planet can be obtained landers, preferably three-axis short-period 

than from records of a single short-period seismometers capable of operating for 6 months, 

instrument. Comparison of the amplitude and to be followed later by six-component 

separation of surface and body waves enables seismometers. 

distances and focal depths to be estimated. Development of lightweight, compact, 

Distances and directions can be established with low-noise seismometers is well advanced. 

far greater accuracy than with a single Instruments have been designed that weigh less 

short-period instrument, and better than 5 pounds and can withstand loadings up to 

determinations. of attenuation rates are possible. 25 g. Such instruments developed for operation 

In addition, the seismic properties of the on the Moon can be adapted with little 

"crustal" materials can be deduced from the modification. The principal problem in 

dispersion of surface waves resulting from the establishing a seismic station is not in detecting 

wave-guide effect (Press and others, 1960). and recording the signal but in transmitting the 

The effectiveness of a seismic station is information. Most of the planned seismic 

vastly enhanced by the simultaneous operation experiments for the Moon are based on 

of a duplicate station. Many of the ambiguities technology of data analysis derived from our 

of interpretation are removed by having earth-based experience. To obtain relatively few 

duplicate records, and seismic events can be numbers, we analyze thousands of data bits 

more accurately located by using widely spaced from a seismogram. There are many possible 

stations. Far better estimates can be made of the ways to reduce the quantity of information 

variation of seismic velocity with depth than required for transmission and these ways should 

from a single station, and under certain be explored. One possibility is an event-triggered 

conditions shadow zones can be detected and mode of operation. Initial operation could be 

the presence of a core determined. continuous to determine the general seismic 

Consideration, therefore, should be given to the noise and appropriate trigger level; long-term 

simultaneous operation of two distantly spaced operation would be event triggered. 

stations early in the exploration program. A seismic station must of necessity operate 

Gravity-, tilt-, and strain-variometer experiments for a considerable length of time before a 

to examine free oscillations, tidal effects, and statistically significant number of events are 

secular deformation should be deferred until at recorded. No estimate can be made of this time 

least two six-component stations have been until the level of seismicity of each individual 

established on the planet. planet is obtained. A minumum 6-month 

The internal constitution of Mercury is of operation time should be aimed for, and the 

special interest because of the planet's high appropriate power sources for such lengthy 

density. Although no landers are planned at operation should be developed. 

present, a lander is feasible, and a simple seismic Active seismic experiments are normally 

experiment should have high priority on the first used to study near-surface discontinuities and 

lander. near-surface seismic velocities. These are of 

A thorough comparison of Venus and Earth considerably less importance than analysis of 

is necessary for an understanding of what factors natural seismic events, and this type of active 

control planetary evolution. A primary objective seismic experiment is given low priority on a 

is to determine whether Venus, like the Earth, is seismically active planet. However, active seismic 

divided into core, mantle, and crust. This is a experiments can be used to study the interiors 

problem that can be solved by seismic of planets where the number of natural seismic 

experiments, and the feasibility of designing a events is not adequate for analysis. Very little 

seismic experiment to survive in the 700° C and effort has been expended in attempting to 

100 atmospheres pressure that prevail on Venus define realistic active experiments for the Moon 

should be explored. and other planetary bodies, and it may be 

A successful seismic experiment appears to possible to carry out these experiments by using 

be particularly promising for Mars. Mars landers abandoned delivery vehicles. In anticipation of 

23 
some planets being aseismic, study of possible 

active experiments should be undertaken. 

Heat Flow 

The most significant geothermal 

measurement is that of the internal heat flow. 

From its value, much can be deduced concerning 

the thermal regime of the planet's interior and 

the thermal evolution of the planet (MacDonald, 

1964; Lee, 1967). A heat flow measurement is 

Determination of near-surface thermal 

properties at a landing site can be used to 

calibrate infrared radiometric measurements 

from an orbiting spacecraft. Because of the 

likelihood of lateral inhomogeneity, however, a 

number of measurements should be made, and 

they must be used with caution in the 

interpretation of radiometric data. 

Magnetics 

particularly difficult on a remote surface Detectipn of a planetary magnetic field 

because the measuring devices must be placed in significantly larger than that of the 

a hole drilled in the surface. Temperatures and interplanetary medium indicates the presenceof 

thermal conductivities must be measured in the an electrically conducting and convective core 

natural state at depths sufficient to avoid within the planet. The existence of such a core 

appreciable diurnal and seasonal temperature would show that the planet has undergone some 

variations. The depth required depends on differentiation in the past and that the present 

variations in surface temperature, the length of conditions of the core are such that the mobility 

days and seasons, and the local thermal of the core materials is possible. If the presence 

properties. In order to make the necessary of a core is demonstrated by seismic methods 

measurements, drilling of a hole at least 5 to yet no magnetic field is detected, then we can 

10m deep is required. Because strong lateral infer that either the core is nonconducting or 

gradients in both temperature and surface that conditions in the core are such that motion 

properties could cause strong lateral flux of the materials is not possible or that the forces 

components, several heat-flow measurements necessary to cause motion within the core are 

may be needed to evaluate the significance of absent. Should remanent magnetism be detected 

these shallow observations. The greatest in surfac~ rocks despite the present lack of a 

technical difficulty lies in drilling the hole. The field, then the conditions within the core must 

actual measuring devices are already developed; have been different in the .past. Magnetic 

they are lightweight and compact, draw very measurements therefore have important 

little power, and have very modest telemetry implications concerning both the history of the 

requirements. Such a hole would be useful for a planet and the present conditions deep in the 

wide variety of experiments other than heat interior. 

flow, and the feasibility of drilling on different Magnetic field measurements are relatively 

planetary surfaces should be vigorously simple and can be made remotely from flybys or 

explored. Until drilling is practical, attempts to orbiters with sensitivities limited only by the 

make in place heat-flow measurements should be magnetic properties of the supporting vehicle. 

deferred. The simplest magnetic experiment is 

Monitoring of the near-surface temperature measurement of total field strength with a 

regime by emplacement of temperature-sensing three-axis magnetometer from a flyby. The 

devices at shallow depths can give information experiment has very modest weight, power, and 

on diurnal and seasonal temperature variations. telemetry requjrements and should be included 

From the damping of these variations as a on all first looks at a planet. Subsequent strategy 

function of depth, and from the phase lag, local will depend on the results of the first look. If a 

values of thermal diffusivity can be estimated. field is detected, then followup experiments will 

Although these shallow thermal measurements be concerned with mapping the structure of the 

are not as fundamental as the internal heat flow, field since this will reflect the flow pattern 

they can yield very useful data on the thermal within the core. If no field is detected, then 

environment for other experiments and on the consideration should be given to methods of 

heat-exchange process at the planetary surface. improving the sensitivity of the measurements. 

24 
Jupiter is the only planet on which a 

magnetic field has been detected. An early 

spin-stabilized flyby or preferably a "tight" 

orbiter carrying a three-axis magnetometer to 

map the .magnetic field would provide sufficient 

information for harmonic analysis to reveal 

details of magnetohydrodynamics processes in 

the core and the possibility of other magnetic 

sources. As appropriate instruments are 

available, no new developments are needed. 

Should strong fields be detected on other 

planets, a similar program of magnetic mapping 

should follow for these also. 

Magnetic fields have not been detected on 

Mars and Venus from flybys (Fawcett and 

others, 1965; Brandt and Hodge, 1964), and if 

fields are present on ·either planet more 

sophisticated experiments are required than have 

been tried in the past. The possibility of using 

demagnetized spacecraft to improve sensitivity 

of magnetic measurements and to place more 

stringent limits on the magnetic fields should be 

explored. In the case of Mars, where a series of 

landers is planned, the feasibility of making 

magnetic field measurements at some distance 

from a landing vehicle by projecting the 

magnetic experiment away from the spacecraft 

should be studied. However, detection of 

magnetic fields of very low intensity is of 

secondary importance and should not preempt 

more fundamental and less risky experiments 

such as seismic and chemical measurements. 

The absence of a magnetic field at present 

does not preclude the existence of a field in the 

past. Evidence of the past field will be preserved 

in the surface rocks, and a simple experiment to 

determine to what extent surface rocks are 

magnetized should be made on an early lander 

to all planets. The simplest experiment would 

require a single-axis magnetometer and a sampler 

similar to the one carried on Surveyor. 

Magnetometer readings are taken as samples are 

moved toward it. Since the experiment is being 

conducted in the absence of a field, changes in 

magnetometer readings would indicate some 

rock magnetism and hence that a magnetic field 

existed in the past. Success of the experiment 

would imply the existence of a core and would 

further imply that present conditions within the 

core are static. 

Physical Properties of Near-Surface Materials 

Several remote-sensing techniques are 

available for measuring the properties of 

near-surface materials. These techniques have 

had only limited applicability to terrestrial 

geologic problems, largely because the 

appropriate measurements can commonly be 

made more accurately on the ground. But, 

because of the limited availability of 

ground-based information on planets, the 

importance of remote-sensing techniques as 

geologic tools is greatly enhanced. Some of the 

parameters susceptible to remote measurement, 

for example, porosity, density, and thermal 

conductivity, are difficult to relate to 

mineralogy and petrology of the underlying 

rocks; the absolute values of these properties 

have limited geologic significance. Nevertheless, 

the measurements are geologically useful, since 

their variation across the planet's surface can be 

used for mapping and for interpolation between 

ground measurements. Undue reliance on the 

optical response of the surface is thus avoided. 

Furthermore, although remote-sensing 

measurements rarely offer unique solutions, 

they do ~dd to the bank of d~ta with which any 

interpretation of the surface must be consistent. 

Infrared Radiometry 

Infrared radiometer measurements on the 

Moon (Shorthill and Saari, 1966) have proven 

extremely useful in interpreting the nature of 

the lunar surface and the relative age of craters. 

Systematic radiometric mapping will be similarly 

useful on Mars and Mercury. Of particular 

interest is the detection of thermal anomalies 

and correlation of these anomalies with optical 

features. Anomalies detectable on the nightside 

result principally from differences in the rates of 

decay of surface temperatures due to local 

differences in thermal inertia. Since thermal 

inertia is dependent on lithology, variations in 

nighttime temperatures are related to variations 

in rock lithologies. Thermal anomalies could 

result from internal heat sources, and although 

this is liklier, anomalies of this type would be 

most readily observed just to the darkside of the 

morning terminator, where surface temperatures 

25 
are lowest. Systematic radiometric mapping 

from an orbiter of nighttime surface 

temperatures, including the temperatures 

immediately before dawn, is therefore 

recommended for Mars and Mercury and should 

follow acquisition of broad photographic 

coverage at useful resolutions (z500 m) in the 

visible. 

Microwave Emission 

A planet undergoes both diurnal and 

seasonal variations in surface temperature. A 

thermal wave communicates these temperature 

fluctuations to subsurface layers, but with depth 

the wave becomes progressively damped in 

amplitude and retarded in phase. Because the 

overall microwave opacity of solids generally 

decreases toward longer wavelengths, microwave 

radiometers turned to lower frequencies can 

observe at progressively greater depth (Pollack 

and Sagan, 1965). With increasing wavelength 

we should therefore expect a decrease in the 

amplitude of the diurnal temperature 

oscillations and an increase in the phase lag. 

Furthermore, if the sole source of heat is the 

sun, and if the albedo is constant, then the mean 

surface temperature should be the same for all 

wavelengths. If the mean surface temperature is 

not the same, discontinuities in thermal 

properties with depth or an internal source of 

heat can be inferred. 

The geologic interest in passive microwave 

measurements results from the possibility of 

determing internal heat flow and from 

determination of near-surface properties (Drake, 

1966; Kaula, 1968). Theoretically the internal 

heat flow can be measured from the variation in 

mean surface temperature as measured at 

different wavelengths, provided the thermal 

. conductivities of the near-surface materials are 

known. In practice, surface properties may 

rarely be sufficiently well known that thermal 

gradients will be unambiguously measured; but 

in view of the importance of internal heat flow, 

development of microwave measuring 

techniques at several wavelengths and possible 

applications to planetary problems should be 

supported. 

Bistatic Radar 

Study of planetary surfaces by means of 

bistatic radar methods has been suggested by 

Fjelbo (1964) and Tyler (1966); such methods 

were applied to the Moon in a preliminary 

manner during the Lunar Orbiter I and 

Explorer 35 missions (Tyler, 1968). In the 

bistatic mode, radar signals transmitted from the 

Earth are received by an orbiting spacecraft after 

the signals are reflected from the planet's 

surface. The lia..,.l.fe and intensity of the signal 

depend on the configuration and properties of 

the near-surface materials. Surface roughness is 

particularly important in controlling the signal, 

as it affects the proportions and intensities of 

the specularly and diffusely reflected 

components. Surface roughness in the radar 

sense could be mapped bistatically with 

resolutions of several hundred wavelengths 

(Tyler, 1966); this is considerably better than 

what is possible by systems totally Earth based 

and is particularly useful for Venus, whose 

surface topography cannot be characterized 

visually. An early vehicle to Venus should test 

the feasibility of mapping in the bistatic mode 

by using the S-hand radio system. If mapping 

appears feasible in this mode then a spacecraft 

of the Pioneer/Imp class should be assigned to 

bistatic mapping of the Venusian surface. 

Radar measurements of the surface 

roughness of Mars and Mercury are less 

important than for Venus because the 

topography of these planets can better be 

described from visual observation. Radar 

reflectivity measurements are, however, of 

interest on Mars and Mercury because of the 

possibility of mapping dielectric constants and 

of detecting subsurface discontinuities. The 

dielectric constant is of particular interest as a 

means of detecting water. Dry materials rarely 

have dielectric constants in excess of 10, yet 

values can be much greater. than 10 if moisture is 

present. A radar receiver could therefore detect 

and map near-surface water, and, if the vehicle 

had an extended lifetime, the changes in the 

distribution of near-surface water with the 

seasons could be followed. The wavelength 

should be as long as compatible with instrument 

design (105-107 hertzes) since penetration is a 

26 
.function of wavelencth and surface roughness is 

oliesseririterest. -

Detection of subsurface discontinuities is of 

geologic interest especially if the surface is 

covered with a debris layer. The surface 

configuration of the debris layer on Mars, for 

example, may be in part controlled by 

atmospheric effects so that geologic mapping 

from the visible imagery alone may be difficult. 

Since radar signals penetrate to much greater 

depths than optical signals, radar measurements 

will be useful for detecting variations in the 

properties of rocks below the debris layer and 

variations in the thickness of the layer. The 

feasibility of bistatic mapping of Mars and 

Mercury at various wavelengths should therefore 

be examined. 

Radar Topograpbi~ Mapping 

From a geologic viewpoint the most 

important aspect of radar is that it provides a 

means of mapping the topography of a planet 

whose surface is obscured by the atmosphere. 

This is particularly important for Venus, which 

is very similar in radius to the Earth and so 

might reasonably be expected to have followed a 

similar evolutionary path. We know, however, 

that the atmosphere of Venus is drastically 

different from the Earth's and it is essential to 

know whether the solid parts of the planets 

exhibit comparable contrasts. Comparison of the 

topography of Venus with that of the Earth is 

an essential first step in evaluating the nature 

and causes of difference between the two 

planets. The presence on Venus of orogenic 

belts, rift zones, and sedimentary basins similar 

to those on Earth will have particular geologic 

significance. The necessary imagery could be 

obtained from either terrestrial measurements or 

orbital measurements. 

Terrestrial Observations 

Radar imagery, with spatial resolution 

comparable to telescopic optical photography 

(;::=2 km), has already been obtained of the 

Moon. With improved receivers and 

data-handling techniques, comparable 

resolutions are achievable for Venus from 

27 

terrestrial observations. The hemisphere 

ambiguity problem has already been solved, and 

rudimentary imagery has been obtained with 

existing techniques. Resolution is presently 

limited only by the ratio of signal to noise, and 

it is expected that this ratio can be significantly 

improved in the near future. Support of 

terrestrial radar observation of Venus with 

particular emphasis on improving spatial 

resolution is recommended. This has higher 

priority than any other measurement of Venus 

and is comparable in priority to obtaining 

orbital photography of Mars. A necessary 

supporting effort is to improve knowledge of the 

rotation and orbital constants of the planets and 

particularly of Venus. 

Coherent Side-Looking Radar 

Side-looking radar is the only means of 

obtaining high-resolution (100m) imagery of 

the surface of Venus or any other planetary 

body obscured by clouds. A side-looking radar 

system measures the reflectivity of the scattering 

medium as a function of position, and it uses the 

information to construct an image of the 

surface. The transmitter and receiver are both 

within the spacecraft and a signal is transmitted 

to the target surface at a fixed angle (generally 

45° -60° ). The reflected signal varies in 

in tensity, largely according to the relation 

between surface slopes and the spacecraft. The 

signal is measured as a function of track 

position, azimuth, and range. The positional and 

intensity information are combined to construct 

a radar image, which strongly resembles an 

optical image of the surface under oblique 

illumination. Use of synthetic aperture 

techniques permits high-resolution images to be 

formed from orbital altitudes without need of 

physically impossible antennas (Brown, 1969). 

The imagery can be interpreted by standard 

optical techniques, including stereophotography, 

provided the appropriate imagery is obtained, 

but several additional techniques unique to radar 

can also be applied. Since the side-looking radar 

is a ranging device rather than an 

angle-measuring device, such as a standard 

camera, topographic information can be 

extracted from the range data. It is not yet 
possible . to determine elevations from range data 

on a single radar image alone, but ranging 

techniques are being developed for construction 

of topographic maps (LaPrade and Leonardo, 

1969). Depth effects can be discerned with the 

use of multifrequency systems and imaging at 

different radar wavelengths. 

Utilization of side-looking radar will depend 

on rate of development of terrestrial radar 

techniques. Moderate-resolution ( 2 km) radar 

imagery of Venus niust be obtained during the 

next · decade. If this appears unlikely from 

terrestrial measurements, then side-looking radar 

ilnagery must be obtained, even if this requires 

all the payload of a Mariner-class vehicle. 

Side-looking radar of Mars and Mercury is not 

recommended for the foreseeable future unless 

obscuration of surface features from 

atmospheric effects proves to be a greater 

problem than anticipated. 

SUMMARY 

1. Precise determination of rotational constants 

and the optical and dynamical figures are 

required for analysis of internal mass 

distributions and internal viscosities. Radar 

observations of all the planets should 

therefore continue for better definition of 

motions and diameters in addition, Qrbiting 

tracking transponders, possibly with 

nul-type accelerometers should be flown to 

determine precisely the dynamic and optical 

figures and perhaps to identify near-surface 

mascons. 

2. The first landers to any planet should carry 

seismometers, preferably three-axis 

short-period instruments, and plans should 

be made for simultaneous operation of at 

least two widely separated stations. 

Depending on the earlier results, 

second-generation stations may include 

long-period instruments, more elaborate 

short-period instruments, or be capable of 

operating in an active mode. 

3. Plans for heat flow measurements should be 

deferred until a remote drilling capability is 

established, but .high priority should be 

attached to developing the necessary 

technology. 

28 

4. A first look at any planet should include 

measurement of the magnetic field. If there 

is no present field, then the first lander 

should carry a simple magnetometer 

experiment to measure the magnetization of 

surface rocks for evidence of past fields. If a 

significant· field is detected,· it should be 

mapped with a three-axis magnetometer on a 

"tight" orbiter. 

5. Measurement of. the physical properties of 

near-surface materials by remote-sensing 

techniques h as infrared radiometry or 

bistatic radar, while less important than 

determinations on interiors, is useful for 

understanding the nature and origin of 

near-surface materials, especially for Venus, 

where cloud cover prevents direct visual 

observations. 

6. Improved radar imagery of Venus is of 

highest priority; resolutions of 2 km are 

highly desirable. If these resolutions do not 

appear feasible from terrestrial observations 

within the next decade, then side-looking 

radar imagery must be obtained from an 

orbiting vehicle. 

GEOCHEMISTRY 

By M. H. Carr, Priestle~ Toulmin, 3d,and 

I. A. Breger 

An important concern of geochemistry is the 

bulk composition of the planets. Because 

chemical differences between the planets may 

reveal the nature of chemical heterogeneities in 

the original solar nebula and the mechanism of 

planetary accretion, it is important to acquire 

the data necessary for evaluation of their bulk 

composition. Obviously there is no way to 

chemically analyze a body as large and 

heterogeneous as a planet at a single gulp. Nor 

can chemical homogeneity be · assumed. The 

earth is certainly not homogeneous and most 

evidence points to a heterogeneous Moon. The 

overall bulk composition can be determined 

only be devising a sam piing and analytical 

program designed to evaluate and allow for 

compositional variability. To do this statistically 

on the assumption of random variability is both 

inefficient and, because only the very near 
surface part can be sampled, subject to great 

error. A plan must be devised that is based on an 

understanding of the geochemical regularities 

and geologic relations among the various rock 

types of the body. We should begin by 

determining the mineralogic and petrologic 

relations in order that the processes by which 

the major . ~ock units have been formed can be 

inferred and the relative proportions of different 

rock types can be deduced. Only when we have 

a fairly detailed understanding of the 

geochemical and petrogenetic processes that 

have operated on the planet and the history of 

their action, together with a satisfactory model 

for the internal structure of the planet, will we 

be in a position to arrive at a reasonable estimate 

of the bulk chemical composition. 

The most effective strategy for the 

geochemical study of a planet is to proceed 

systematically with the mineralogical and 

chemical characterization of its materials in 

accordance with geologically determined 

priorities. It is insufficient merely to chemically 

analyze the surface rocks. Determination of 

mineralogy, texture, lithology, and other 

properties of the rock that might be relevant to 

origin is necessary. Mineralogic composition is 

particularly important, as the mineral 

assemblage reflects both the chemical 

composition and conditions of formation and is 

far more diagnostic of rock type and formative 

processes than chemical composition alone. 

Mineralogy has · been greatly neglected in the 

exploration of the Moon; it must be included as 

a major aspect of the exploration and scientific 

study of the planets. Indeed if a choice must be 

made between a mineralogic and a chemical 

analysis, the geologist will generally opt for the 

mineralogic analysis because it contains much 

chemical information and a great deal more. 

Such · a choice should not be necessary, for 

mineralogical and chemical experiments can be 

readily integrated to provide a complete 

· analysis. 

The exploration strategy for the planets 

must be more comprehensive than that for the 

Moon, whose exploration has been conducted 

with the expectation of early return of lunar 

samples to the Earth. Difficult experiments 

could therefore be conveniently deferred until a 

sample was available on Earth. Since return of 

samples from· any of the planets presents far 

greater technological difficulties than from the 

Moon, we cannot anticipate early return. 

Difficult experiments such as age 

determinations, petrologic characterization, and 

isotopic and trace-element analyses cannot be 

delayed until samples are returned to Earth; the 

instruments necessary for performing these 

experiments remotely must be developed and 

deployed. 

The success of any geochemical exploration 

program will depend largely on how intelligently 

sites are chosen for analysis. A chemical analysis 

on the surface of a sand dune on Mars, for 

example, will not reveal much about petrologic 

processes. Information from orbiters will be 

crucial in selecting sites. Photography is likely to 

be most valuable but other techniques more 

sensitive to chemical composition, such as 

multizonal photometry, should also be used. 

The usefulness of these techniques depends not 

so much on the precision with which they can 

determine surface chemistry, but on their ability 

to detect differences. Because orbital 

measurements are important in giving areal 

dimension to specific surface analyses the orbital 

and landing programs should be planned in 

concert and strategies employed such that each 

mission plan can be modified by information 

gained from the other program. 

In summary, data from a lander should 

include both chemical and mineralogical 

information supplemented by information of 

petrologic importance such as texture, lithology, 

and structural position. The general significance 

of such information gathered at a restricted 

number of landing sites should be evaluated in 

light of all available geologic · information, 

including geophysical measurements on 

properties of the interior and orbital 

measurements of the variation in surface 

,properties. The following sections contain more 

specific discussions evaluating different methods 

of acquiring the necessary fundamental data. 

Elemental Composition 

Sampling 

Some of the broader aspects of the sampling 

problems were discussed in previous section, but 

29 
additional problems are encountered in the 

immediate vicinity of the spacecraft. Because of 

possible local variations, an analytical 

instrument must have the capability of making 

measurements on any nearby object of interest 

and of visually monitoring such measurements. 

This requires that a sampling tool be available 

and that the analyses be <;onducted in close 

conjunction with the imaging system, as was 

done successfully with Surveyor. 

Accuracy and Precision 

It serves very little purpose at this time to 

. specify accuracies and precisions for possible 

chemical determinations. This · will depend on 

what is technologically feasible and also on the 

variability in surface chemistry of the planet 

tmder consideration. Efforts to achieve extreme 

·First pr{ori ty e Lemen ts 

precision with relatively few analyses would be 

justifiable only in the unlikely case of a 

homogeneous planetary surface, whereas more 

analyses with lesser precision might be desirable 

on a planet with variable surface composition. In 

the table given below, the precisions listed are 

those adequate to characterize most terrestrial 

rock types. For even the most homogeneous of 

terrestrial rocks, the natural chemical variability 

generally exceeds the listed precisions (Landy, 

1961) so that to more accurately define the 

chemistry of the rock, multiple analyses are 

required as well as increased analytical precision. . 

This is somewhat academic for the planets 

because the presently achievable precisions fall 

far short of those listed in the table, and we are 

not likely to be presented with the choice 

between extreme precision on few analyses or 

moderate precision on many analyses early in 

the exploration program. 

ELement Expected range 

(percent) 

Precision needed to define 

most terrestriaL rocks 

(percent) 

Na----------------- Mg----------------- A1----------------- Si----------------- K------------------ Ca----------------- Fe----------------- H2oL---------------

~econd1 priority eLements 

0.5-4.5 

.5-20 

2.5-20 

15 -35 

.5-5 

.5-6.5 

.5-20 

S, ~i~ Mn, Ni, Cr, U, and Th 

Third priority elements 

Ba, Sr, Cu, Pb, Li, P, B, Cl, 

F, and Zn 

Priorities have been assigned to elements in 

the table. This could be done m a more 

sophisticated way if available chemical data were 

more carefully evaluated. A wealth of chemical 

data is available in ~rrestrial rocks, yet little 

· systematic work has been done in organizing the 

data · to separate natural variations from 

analytical e-rrors and to determine what 

30 

10 

10 

10 

3 

5 

10 

5 

abundances or what elemental ratios are most 

diagnostic of specific rock types or meteorites. 

The Geochemistry Group at the Santa Cruz 

conference sponsored by NASA suggested 

intensive study of existing data with the intent 

of evaluating the diagnostic value of specific 

abundances· and ratios. This group stated filar 

"The K/Ca ratio a much better index by which 

to classify chemical variations than analysis for 

AI, Mg, Si, and Fe. The K/Ca ratio varies by a 

factor of 500 in common rock types, where 

Mg/Si or Mg/Fe ratios vary by less than a factor 

of 10. Similarly, the Al/Si or Ca/Si ratios are a 

much better index for distinguishing chondritic 

material and ultramafic rocks from other rocks 

than the Mg/Fe or (Mg or Fe)/Si ratios. Probably 
the most distinctive chemical characteristic of 

chondrites is the Ni content or Ni/(Mg or Fe) 

ratio." (National Aeronautics and Space 

Administration, 1967) The study recommended 

should be undertaken to avoid making chemical 

determinations of limited diagnostic value. 

The importance of specific elemental ratios 

as indices of bulk differentiation of the planet or 

heterogeneities in the solar nebulae should also 

be explored. Adams and others (1967a) have 

approached this problem and have pointed to 

the importance of the Fe/Si ratio in determining 

homogeneity of the original nebulae, internal 

uniformity of the planets, and formation of 

preplanets. Other ratios may have comparable 

significance. 

Analytical Techniques 

A detailed evaluation of different analytical 

techniques -is beyond the scope of this report, 

and the following discussion is included more to 

illustrate problems than to suggest solutions. No 

one technique can be used to determine all the 

necessary major elements satisfactorily; different 

techniques may need to be combined if we are 

to have an adequate analysis. And since a 

mineralogic analysis is absolutely essential for 

understanding any chemical analysis, 

compatibility with phase-detecting devices is a 

principal criterior in evaluating the relative 

merits of the different techniques. 

The alpha-backscatter, the only fully 

flight-qualified instrument now available (fall 

1969), has been used successfully on the 

Surveyor program. The technique gives good 

results for the light elements, especially 

0, Mg, Al, and Si, which constitute most of any 

likely rock, but it has poor precision for heavier 

elements. Lack of precision for the heavy 

elements is particularly limiting for the highly 

diagnostic elements Ca, K, and Fe. The 

technique will encounter problems on Venus 

because of atmospheric absorption, but it might 

be appropriate for Mercury and Mars. 

X-ray fluorescence is a versatile technique 

with a high degree of compatibility with various 

methods of phase analysis and one by which 

both light and heavy elements can be examined, 

depending on the excitation source. The 

standard technique of using a high-energy 

primary X-ray beam to excite the sample is 

sensitive for all elements heavier than AI. To . 

determine th,e lighter elements, advantage can be 

taken of the rising alpha-particle excitation 

function with decreasing atomic number. An 

alpha-emitter such as Cm242 or Po210 can be 

used to excite the sample. The excited X-rays 

may be detected dispersively by means of 

goniometers or nondispersively by solid-state 

detectors. Continuing development of small 

multichannel analyzers and higher resolution 

detectors makes the nondispersive methods 

potentially very attractive because the 

instruments are mechanically simple and the 

resolution of present detectors ( < 200 electron 

volts) already permits unambiguous 

determination of adjacent elements heavier than 

Mg. 

Neutron activation may be of value in 

determining elements whose determination is 

not possible by one of the more general 

techniques or in low-level determinations of 

specific elements. The technique is particularly 

sensitive for N a, Al, Si, Ca, K, Mn, and Be and 

could be conveniently combined with an X-ray 

fluorescence unit using the same analyzer and 

the same detector (appropriately biased). The 

only additional requirement would be some 

form of neutron source. The sensitivity of 

neutron activation depends on the particular 

element and on the flux available to excite the 

sample, but potentially the technique can be 

used for trace-element analysis not possible by 

either <l· -scatter or X-ray fluorescence. 

Optical emission spectrometry, the 

traditional method of trace-element analysis, 

holds considerable promise for remote 

applications. The development of linear arrays 

of closely spaced photodetectors may have 

provided the key to utilization of this technique 

in space, but photography and subsequent 

photometric scanning is still a possibility. The 

method is sensitive to a large number of 

elements, can deal with very small samples, and 

has a wide range of response. Calibration to 

provide quantitative data may be the main 

problem with the technique. 

It is evident from this brief discussion that 

no single technique will fulfill all geochemical 

needs. Emphasis should be placed on integrating 

various methods and exploring the commonality 

of different components so that a complete 

geochemical package can be assembled, similar 

31 
in concept to the ALSEP (Apollo Lunar Surface 

Experiment Package) for the Moon. 

Isotopic Composition and Age Determinations 

The principal interest of isotopic 

compositions of the condensed part of a planet 

is that they provide a means of determing ages. 

Sequence and age are of fundamental geologic 

con cern. Only by placing rocks in proper 

sequence can we achieve some order out of the 

complexity. of a planet's surface and arrive at 

some understanding of the processes that 

resulted in its formation. It is of obvious 

geologic importance to know whether the 

surface of a planet that formed 4.5 billion years 

ago has been dead ever since or is still 

undergoing endogenetic change. Each alternative 

has vastly different implications concerning the 

chemistry and conditions of the interior. 

The isotopes of most interest are those of 

Rb, Sr, A, U, and Pb, and the only satisfactory 

technique of determining isotopic compositions 

of these elements is . mass spectrometry. Flight 

instruments are being developed that utilize a 

sputtering ion source and a double-focusing 

system, but percisions now achievable permit 

only the crudest of geochronological 

measurements. Mass spectrometry is potentially 

a very versatile analytical instrument, as it can 

be used for measuring elemental abundance as 

well as isotopic abundances. A major effort 

should be to obtain flight-qualified instruments. 

Mineral Composition 

X-ray diffraction is the only satisfactory 

technique for determining mineralogy. No other 

technique can provide the same combination of 

generality, specificity, and quantitative data. 

X-ray diffraction patterns of polycrystalline 

material can be interpreted (1) to identify most 

phases present with little or no ambiguity, ( 2) to 

determine quantitatively the composition of 

most solid-solution series, and (3) to determine 

quantitatively or semiquantitatively the modal 

proportions of phases in aggregate. The phase 

compositional data can be recalculated into an 

elemental analysis which, though somewhat less 

precise than that obtained by direct methods, is 

nevertheless useful for most interpretative 

purposes. (Interpretation of an elemental 

analysis involves, at least implicitly, calculation 

of a hypothetical mineralogical composition.) 

Several different diffraction techniques are 

possible. A procedure commonly used is to 

change the angular position of a detector with 

respect to the sample and X-ray source and 

measure the intensity of X-rays at different 

angular positions. Although this technique has 

been adapted for space-flight applications, the 

instruments are delicate and mech~ically 

intricate and several simpler alternatives are 

feasible. One promising approach is to use a 

curved . position-sensitive proportional counter 

(now being . developed at Illinois lnst. Tech. 

Research Inst., Chicago). The counter is in a 

fixed position, subtending an angle with respect 

to the sample such that X-rays from the sample 

hit different parts of the counter according to 

the angle through which they are diffracted. The 

resulting pulses are attenuated by the central 

wire of the detector with th~ result that pulse 

height depends on where the X-ray entered the 

detector and hence on the diffraction angle 

(monochromatic X-radiation is assumed). 

D-spacings in the sample can therefore be 

derived from the pulse-height distribution in the 

detector output. Photographic methods are also 

possible; film could be wound through an 

appropriate camera, then developed and 

scanned. Another possible diffraction technique 

is to have a nondispersive detector in a fixed 

position with respect to the sample and source. 

The diffracted X-rays are · then dispersed 

electronically with a multichannel analyzer 

rather than mechanically by means of a 

goniometer. All the above alternatives should be 

explored to avoid placing disproportionate 

reliance on goniometer systems. 

Another technique of mineralogic 

determination that deserves mention is 

differential thermal analysis (DTA). This 

technique, although of limited use for 

systematic mineralogy, can provide very useful 

mineralogical data. It is particularly important 

for identification of clay minerals, carbonates, 

and other minerals that undergo 

low-temperature transformations. The devices 

are compact and lightweight and have low power 

32 
and transmission requirements; they may 

therefore still be feasible when a more 

comprehensive mineralogical experiment is not 

possible. 

Organic Radicals 

A major objective of planetary exploration 

will be the search for extraterrestrial.life. Efforts 

of the organic geochemists will be primarily 

devoted to recognition, identification, and 

analysis of organic matter on the planetary 

surface or in its atmosphere. Should living 

substances not be detected, then such organic 

material might provide clues as to extinct life 

processes, or it might reflect compounds formed 

abiotically. Such substances may in turn provide 

evidence, by analogy, of substances similarly 

formed on Earth which ultimately were the 

progenitors from which life arose on our own 

planet. Abiotically-formed compounds could 

provide information on the chemical radicals 

that exist within the space of our solar system. 

The organic geochemist cannot ignore the 

environment in which the organic matter he has 

under study has been formed or found. 

Problems involving mineralogy, catalysis, and 

radiation chemistry, as well as the condensation 

or polymerization of organic compounds, are all 

interrelated. Organic matter may be formed 

biogenetically or may represent the abiotic 

products derived from condensed fragments 

such as C, CH, CH2, CH3, CO, N, NH, NH2, and 

other radicals interacting in planetary 

atmospheres or on planetary surfaces. The 

occurrence of high fluxes of cosmic radiation 

(Mercury and Venus in particular) or of 

ultraviolet radiation could readily provide the 

energy necessary to create the radicals in the 

vicinity of the planet or to convert 

low-molecular-weight con1pounds to high 

polymers on the planetary surfaces. Search for 

such compounds must be made. 

Gases expelled from a sample can be 

analyzed, quantitatively if desired, by infrared 

absorption or mass spectrometry with or 

without gas-liquid chromatography. Infrared 

techniques are also available for determining 

c13;c12 ratios in carbon dioxide that may be 

given off. Establishment of the c13;cl2 ratio 

would be significant in determining whether the 

33 

organic matter had the same ratio as the 

atmospheric carbon of the planet. A significant 

variation would lead one to suspect that the 

organic matter· was produced by a chemical or 

Hbiochemical" reaction accompanied by isotopic 

fractionation. Microscopic examination will also 

be necessary to determine the association 

between minerals and organic matter and to 

provide information as to whether the organic 

matter was deposited in place or was fluid at 

some time and flowed into its present location, 

where it then condensed or polymerized. 

Most of these techniques have already been 

developed for adaptation to landers, or are now 

under development. A package capable of 

performing gas-liquid chromatography, 

differential thermal analysis, mass spectrometry, 

microscopy, and possibly infrared analysis 

would provide a powerful set of tools for 

evaluation of the nature of the organic matter, 

its source, and its relation to other constituents 

of the planet being examined. It could also 

provide complementary information on isotope 

ratios and mineral species. 

Orbital Measurements 

Precise chemical and mineralogical 

determinations are not possible from orbiting 

vehicles. Nevertheless, extremely useful 

geochemical information can be gained from 

orbital programs. To evaluate the general 

implications of a few precise analyses from 

landers, we must have some knowledge of the 

geochemical variability of the planet's crust, and 

this is practically obtained only from orbital 

vehicles. By utilizing a variety of remote-sensing 

devices, geochemical variations can be mapped, 

geochemical anomalies identified, and limits 

placed on chemical and mineralogical 

compositions over wide areas. Unique chemical 

and mineralogical determinations cannot be 

expected from such measurements; in precision 

and generality, the results will be more like 

those of terrestrial gravity and magnetic maps 

and subject to a limited set of interpretations. 

Multiband Photometry 

Multiband photometry in the visible and 

near infrared is promising as a technique for 
mapping surface chemical and mineralogical 

differences. The technique has already been 

applied with some success to the Moon (McCord 

and Johnson, 1969). The method involves 

measuring the reflectivity of the surface at 

different wavelengths and utilizes the fact that 

the color of light reflected from crystalline 

material depends on the elemental and mineral 

species in the material. The method has several 

limitations, some practical and some theoretical. 

One practical limitation is that its utility is 

dependent on areal resolution and the number 

of wavelength channels in which measurements 

are made. Each spot measurement requires a 

large number of bits-because of the large 

number of channels-so that to build a mosaic of 

a planet's surface with useful resolution, say 

5 km, would require enormous transmission 

capabilities. This limitation applies to other 

spectral measurements discussed below. A 

theoretical limitation of the method is that the 

characteristic peaks are very broad and therefore 

not very specific, and any given spectrum is 

susceptible to a wide number of possible 

interpretations. Despite these reservations, 

development of the method should be 

supported, certainly for telescopic work, but 

also in anticipation that orbital measurements 

will be practical. 

Far-Infrared Spectroscopy (5-50ll) 

The far-infrared spectrum, being partly 

dependent on Si-0 polymerization, is in general 

more sensitive to different rock types than the 

visible and near infrared (Salisbury and Hunt, 

1969). Acid, intermediate, and basic rocks are 

readily distinguishable and measurements in the 

far infrared could be used to map the 

distribution of these rock types. In application it 

suffers from essentially the same limitations as 

measurements in the visible. 

Y-Ray Spectrometry 

Y -ray spectrometry from orbiting vehicles is 

an attractive possibility for a planet where gross 

differentiation of the crust is suspected. Because 

of atmospheric absorption, however, the 

technique is relevant only to Mars and Mercury. 

Measurements are, made of surface variations in 

natural Y -ray activity, which are largely the 

result of variations in the U, Th, and K contents. 

These elements are particularly diagnostic of 

"oceanic" and "continental" rocks on Earth and 

will have similar diagnostic . importance on other 

planets. Other major elements may be detected 

as a result of Y -ray activity induced by 

radiation from the sun. This is particularly likely 

for Mercury. 

X-Ray Spectrometry 

X-ray spectrometry may be possible on 

Mercury from an orbiter because of the high 

solar flux and thin atmosphere. The purpose of 

such an orbiter would be to map composition~ 

differences as revealed by the differences in 

X-ray spectra as a result of excitation of the 

surface by the sun's radiation. 

Supporting Earth-based Research 

The first and most obvious earth-based 

activity is in the field of instrument 

development, discussed above. As stated, the 

main . thrust should be toward exploring the 

commonality of components between different 

techniques and toward producing an integrated 

package for elemental and mineralogical 

analysis. 

The second, and in a sense more 

fundamental, kind of ground-based study should 

be devoted to acquiring basic scientific data 

needed to interpret the experiments performed 

at the planets. In part, this consists of the 

obvious direct calibration of instruments and 

simulation of experiments under conditions 

similar to those to be encountered on the 

planet's surface. The DTA response of a 

dehydration or decarbonation reaction, for 

example, may be quite different in the Martian 

environment than in a terrestrial laboratory. 

Effects of this sort may be evaluated in much 

the same way that flight hardware must be 

tested for operation under expected conditions. 

This work is properly part of the development 

procedure for specific instruments and 

experiments and can be done only in 

conjunction with experimental design. 

34 
A thin line separates these from other 

research programs that are designed to gain a 

better understanding of the conditions and 

processes on the planetary surface so as to 

predict and define the most significant 

measurements as well as to aid in their 

interpretation. For example, we need much 

more extensive data on the stabilities of phases 

that are curiosities on Earth but may be 

important constituents of the surface materials 

of other planets. We need especially to study 

minerals whose stabilities may ·be greatly 

enhanced or restricted by atmospheres very 

different from ours in composition, pressure, 

oxidation potential, content of H20, C02, CO, 

and NH3, and other parameters. 

To take a specific example 9 we know that 

the Martian atmosphere, with a surface pressure 

of 7 millibars, is largely C02. Such an 

atmosphere is so different from our terrestrial 

atmosphere that we cannot predict with 

certainty the nature of the weathering products 

that would result from its interaction with even 

common terrestrial rocks. We can predict that 

the mineral phases in equilibrium with the 

Martian atmosphere are likely to be more 

reduced and perhaps more carbonate-rich than 

their terrestrial counterparts, but we are not in a 

position to make ~ny quantitative predictions. A 

program of direct experimentation in 

. low-temperature reactions between common 

terrestrial rock types and simulated Martian 

atmospheres is one desirable route of attack, but 

it should be supplemented by systematic 

measurement, compilation, and evaluation of 

equilibrium data on the low-temperature regions 

of pertinent simple systems under similar 

reducing conditions. Carbona! compounds and 

oxalates, quite rare in terrestrial occurrences, 

may be much more common on Mars. The 

properties and stabilities of those containing the 

major rock-forming cations should be 

investigated. Similar stability studies are needed 

for conditions on other planets, especially Venus 

and Mercury. 

The extent to which the atmosphere has 

influenced more deep-seated rock-forming 

processes depends largely on the history of the 

planet. If a mobile interior and high rates of 

erosion and sedimentation have allowed 

accumulations of weathered debris to sink to 

sufficient depth to be metamorphosed, or 

35 

partially or completely melted, the character of 

the weathering products will obviously affect 

the resulting metamorphic or igneous rocks. 

Even if this has not occurred, volcanic rocks 

interact directly with the atmosphere in the 

process of their formation, and the resulting 

mineral assemblages may show the effects of the 

volatile constituents, especially those affecting 

the oxidation states of the magma. Again, both 

direct simulation and carefully controlled 

studies of selected key systems are desirable. 

Fortunately, the effect of fugacity on the 

crystallization of basaltic melts is of great 

terrestrial importance, and studies recently 

completed or currently underway may suffice, 

but their status should be evaluated from a 

planetologic point of view and supplementary 

work supported as needed. 

SUMMARY 

1. Geochemical work should begin with 

determining mineralogical and petrological 

relations so that the processes by which 

major rock units formed can be inferred and 

the relative proportions of different rock 

types deduced. 

2. Mineral assemblage is as important as bulk 

chemical composition for the mineralogy 

reflects both chemical composition and 

conditions of formation and is far more 

diagnostic of rock type and formative 

processes than composition alone. 

3. Difficult measurements such as isotopic and 

trace-element composition cannot be 

deferred with the expectation of early 

sample return to Earth but must be 

performed remotely. The necessary 

hardware should be developed. 

4. Since no single analytical technique can 

make all the necessary chemical and 

mineralogical determinations, emphasis 

should be placed on exploring the 

commonality of different systems so that an 

integrated chemical package can be 

assembled. 

5. 0 r b it a l chemical measurements such as 

multiband photometry and infrared 

spectroscopy, although imprecise and 

subject to a variety of interpretations, are 

essential for interpolation between ground 
stations and for choosing the best places for 

ground measurements. Detailed chemical 

mapping from an orbiter should follow 

acquisition of surface imagery. 

6. Concurrently with instrument development, 

theoretical studies on stability relations 

under planetary conditions should continue 

both to aid in instrument design and to 

ensure correct interpretation of the resulting 

data. 

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