CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES : TECHNIQUES AND A PRELIMINARY ANALYSIS OF MARS AND VENUS

J. B. GARVIN, P. J. MOUGINIS-MARK,andJ. W. HEAD Department of Geological Sciences, Brown University, Providence, Rhode Island, U.S.A.

(Received 4 November, 1980)

Abstract. Characteristics of rock populations on the surfaces of Mars and Venus can be derived from analyses of rock morphology and morphometry data. We present measurements of rock sizes and sphericities made from Viking lander images using an interactive digital image display system. The rocks considered are in the gravel size range (16- 256 mm in diameter). Mean sphericities, form ratios, and roundness factors are found to be very similar for both Viking lander sites. Size distributions, however, demonstrate differences between the sites; there are significantly more cobble size fragments at VL-2 than at VL-1. A model calling for aphanitic basalts emplaced as ejecta or lava flows at the Viking sites is supported by the rock shape, size, and roundness data. Morphologic features pertaining to the modification history of a rock are considered for Mars and Venus. A multi-parameter clustering algorithm is utilized to objectively categorize martian and venusian rocks in terms of various criteria. Erosional markings such as flutes are demonstrated to be most important in separating VL-1 rock morphologic groups, while rock form (i.e., shape) represents the primary separator of subpopulations at VL-2 and the Venera landing sites. Fillets are common around VL-1 and Venera 10 fragments. Obstacle scours occur frequently only at VL-1. Cavities in rocks are ubiquitous at all lander sites except Venera 9. Eolian processes, possibly assisted by local solution weathering, are a strong candidate for the origin of cavities and flutes in martian rocks.

0. Introduction

Rock populations on Earth typically contain morphological information pertinent to their compositions, modes of origin, emplacement styles, and subsequent weathering histories (Oilier, 1969; Folk, 1974). Terrestrial analogues for the Martian environment (Morris et al. , 1972, McCauley et al. , 1979), permit some inferences to be made about the evolution of the Martian surface. Previous studies of block fields and fine particles on Mars (Moore eta!., 1977, Evans and Adams, 1979; Strickland, 1979), and Venus (Florensky et al., 1977; Keldysh, 1979), have identified rock subpopulations within the fields of view of the Viking and Venera landers, but have lacked the large-scale data base required for multiple-parameter morphological analysis. In this report, we provide an overview of a data collection and analysis scheme that has been developed for the interpretation of rock morphology from lander images (Garvin et al. , 1980). Emphasis is placed here on our approach to solving the problem of how to best characterize rock populations on planetary surfaces. It involves the collection of quantitative data such as rock size and sphericity, as well as qualitative information regarding morphological features. Full descriptions of the morphological attributes chosen with specific rock examples are presented. Data analysis techniques are also

Th e Moon and the Planets 24 (1981) 355-387. 0165--{)807 /81/0243--{)355$04.95 . Copyright © 1981 by D. R eidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A. 356 J. B. GARVIN ET AL. discussed in the context of identifying key characteristics of a rock that place it in a single category with similar rocks. Actual rock characteristics observed from Viking and Venera lander imagery are summarized. Finally, we present some speculations regarding the block fields on Mars and Venus, in an attempt to answer key questions such as their overall rock type, mode of emplacement, and modification history. These speculations arise from the analysis of planetary rock morphology and morphometry data sets and their comparison to terrestrial data.

1. Technique

1.1. THE PROBLEM To characterize planetary surfaces and the processes acting on them from a small number of lander sites is especially difficult when digital images are all that are available. For Mars and Venus this is the case. The two Viking landers on the martian surface have returned thousands of images of their vicinities with resolutions of up to 1 mm per picture element or pixel (Tucker, 1978). The Venera 9 and 10 spacecraft each returned a 180° panorama of their landing sites with an optimal resolution of about 10 mm per pixel (Keldysh, 1979). These resolutions are not sufficient to distinguish most mineral grains in rock, nor are they a replacement for the kinds of detailed information field and laboratory studies provide for terrestrial rock populations. However, a systematic qualitative and quantitative analysis of planetary surface images such as those returned from the Viking and Venera spacecraft provide information suitable for comparison with terrestrial data sets created by far more detailed studies.

1 .2. THE APPROACH In order to characterize the blocks visible in planetary surface images in a rigorous fashion, a scheme for data collection must be derived. This standardization permits more rapid collection of data and forms a basis for comparison between populations.

1.1 .1. Data Gathering A. The data. An interactive digital image processing system (Garvin et al., 1980) was employed to display mosaics of VL-1 and VL-2 images covering a camera elevation band between - 20° and - 40° (0° representing the horizon) around the spacecraft. Distortion of objects below - 40° (less than 2m from the camera) is extreme (Mutch, 1978), and rocks in the - 40° to - 60° range (1.5 to 2.0 m from camera) can be elongated by as much as a factor of two. This is because the images are cylindrical mercator projections with the least distortion at 0° elevation, and maximum distortion at the pole (- 60° elevation). The field of view above - 20° (> 3.8 m from the camera) was not imaged with sufficient resolution (< 3 mm/pixel) for the detailed evaluation of cavities and flutes on rock surfaces. Only high resolution images taken with BB2 and BB3 black and white diodes were considered. This choice of camera diodes means that rocks approxi- CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 357 mately 3m from the cameras(- 20° to - 40° camera inclination angles) are in the best focus, and that only rocks from ~ 2.0 to 4.0 m from the cameras were studied. Lander images were also chosen so that they gave the most uniform lighting conditions in terms of solar elevation for the azimuth ranges of each mosaic. In general, high sun images were used (> 50° solar elevation), while lower sun images (20° to 30° solar elevation) were also utilized for rock morphology determination. Table I lists the mosaic images for each lander and camera on Mars. It is important to note that these mosaics form the basis for the rock size and sphericity (i.e., shape) determinations, but that multiple images were considered when the overall morphology was evaluated, at which time both high and low sun images were essential. Stereo pairs were used when available, to aid in esti­ mations of parameters such as rock form.

TABLE I List of camera events included in the mosaics created as the basis for VL-1 and VL-2 rock morphology data collection. All elevations are - 20° top, - 40° bottom. Note that along with the images included here, others were used to provide multiple sun angle and stereo coverage of the areas. Lander/Camera Camera Event Label Start Azimuth Stop Azimuth (degrees) (degrees)

VL -1/Camera 1 11B058/036 180.0 220.0 11A114/019 220.0 320.0 VL-1/Camera 2 12A140/024 20.0 132.5 12A152/026 132.5 192.5

VL-2/Camera 1 21A115/016 152.5 227.5 21A057 /008 227.5 235.0 21G095/524 235 .0 240.0 21B022/033 240.0 310.0 VL-2/Camera 2 22H008/593 80.0 90.0 22A251/030 90.0 95.0 22H037 /597 95.0 102.5 22G237/590 102.5 115.0 22H025/595 115.0 172.5 22B012/032 172.5 195.0

The Venera 9 and 10 sites were measured from an 11" by 14" photograph using a digitizing table. Venera camera angular resolution is approximately an order of magnitude less than Viking (0.33° /pixel vs 0.04° /pixel; see Keldysh, 1979), so only the rocks within 2.5 m of the Venera spacecraft were measured for maximum compatibility of rock morphology data sets between planets.

B. Techniques for data gathering. The interactive digital image processing system was used in the collection of data from the Viking lander mosaics. Using this system, sections of the mosaics are displayed on a volatile display screen, contrast stretched, and optionally 358 J. B. GAR YIN ET AL. enlarged. An operator then selects a rock by moving a cursor ( crosshairs on the screen) about its perimeter. This process of outlining a rock triggers the computer to generate the major or length (L) and minor or width (W) axes for the rock based on the outline. By monoscopically ranging to the lowest elevation point on the rock, the absolute lengths of the axes are automatically computed as well as the least projection elongation or two­ dimensional sphericity (W/L; see Folk, 1974). The average rock diameter is then calcu­ lated by averaging the axes: (W + L)/2. After outlining a rock, the operator must qualitatively assess its morphologic features based on the 'menu' in Figure 1. A computer-driven digitizing table was used for this purpose in a manner similar to that employed for the interpretation of planetary impact craters (Arvidson et al. , 1974; Cintala et al., 1976a; Cintala et al., 1976b ). In this study, 8" by 10" and 11" by 14" photographic prints of areas included in the mosaics, but at high and low sun angles, were used for rock morphology determination. Because the Venera panoramas were only available in photographic form, they could not be analyzed for rock size and sphericity using the interactive image display system as was done for Viking images (digital computer tapes of the images are required).

fLUTE CR OSS ROU ND- OCCUL- CAVITY CAVITY CAVITY FLUTE FLUT E FLUTE FLUTE REL DUST BASAL LOCAL LI NEAR FORM MOAT SHAPES CU TTIN G FACETS NESS TAT ION SHAPE PARTICLES FE ATURES SIZE ORIENT. MOOII.LIT Y DEPTH GROOVE FLUTES DENSITY ALBEDO EFFECT ENVIR ON

N O NO N O N O NO N O N O NO N O NONE NO N O LOW NONE NO TROUGH NO CAV.S CAV.S CAV.S FLUTES fLUTES FLUTES FLUTES FLUTES FLUTES

OUST COMPACT/ ANGULAR N O YES UN IFOR ~ SHALLO NO NO N O LOW YES MEO ON YES DRIFT YES P L AT Y ROC K

OUST IN COMPACT/ sue- ELLIP- OU RI- ROCK ME.O. YES INOET MOO. YES YES YES MEO. INOET HIGH CAVITIES INOET INOE T £LON G ANGULAR TICAL •F LU TES CRUST

HETEAO- FACET OUST ON -FRAGMEN -NO"TCH LARG E INDET DEEP INOET INOET INDET HIGH VARIE S PLATY STYLE o• (FILLET) GENEOUS I N CAV.S ROC K "

RIIRALLEL ROU NO • TAIL RIOGE ELONG ROUND OTHER INOET NO INOET INOET FLUTE ' 0 ELLtPT. STREAK) INOET FA CETS I N OE T IN OET NO NO 'OWS FLUTE" R0ISE'

FLUTE UNOER · CAVITY ROUNO • INOET !NOET NO YES SHAPE: GROU ND YES FLATS YES OEPTH IRREG SIZE SHAPE

ROC K VERY SUAfACE CAVITY ELUPT. NO TWO· NO' ROC K on ' 0 YES IN OET NO N O N O ' 0' INOET INOET ELONG TEXTURE DISTRIB. CAV. S •IRREG. FLUT ES SIOED SIGN IF. BURl EO ROC K HON OG . (CC:W

NO ROVNO, INCREASE IN OET SM ALL CAV.S ELLIPT. N O YE S YES Y E S WITH I NOET '""""" DEPTH

IN TAI L MULTI• BIMODAL UNifOR M M OO . INDET MEO YES INDET IN OET INOET PLANAR WITH ~ACED DEPlH

HETERO · BIMO DAL DEEP INOE"T LARGE INOET INOET IN OET GENEOUS

HETERO - IN OET INOET IN OET INOET GENEOVS

IN OET

Fig. 1. Rock morphology attributes used in categorizing martian and venusian rocks. This table can be read by identifying an individual attribute at the top of a column, and reading down the list of alternatives until another attribute heading is reached (indicated by solid boxes). No attribute list is more than one column wide. 'Indet.' stands for indeterminate. See text for full descriptions. CHARACTE RIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 359

The digitizing table was used to collect morphological data for the rocks as described above. Accurate rock sizes are difficult to measure on the digitizing table, especially when camera inclination angles are not well known (see Keldysh, 1979). For this reason, we have not yet made rock size and sphericity determinations from the Venera images.

C. Rock morphology attributes. In order to quantify diverse rock features in a general manner, we have selected a set of parameters that describe the qualitative morphological form of each rock (Garvin et al., 1980). Traditional sedimentological criteria, as well as features diagnostic of certain rock weathering regimes, are stressed in the description of these rock populations. Certain features associated with fluid flow processes such as obstacle scours and flutes have been included, since they are mentioned in previous studies of the geology of the sites (Binder et al., 1977; Mutch et al., 1977) and provide infor­ mation on rock modification processes. Morphologic features were chosen so that rocks of similar weathering histories, ages of exposure, or modes of emplacement could be described by a unique set of parameters. These features (or 'attributes') are presented in tabular form in Figure 1, and are described in detail below. Note that for each attribute an 'indeterminate' option is provided to prevent possible misinterpretations due to shadows, small rock size, or partial obscuration of one rock by another. The figures that follow (Viking lander images) were chosen to illustrate best the morphology attributes of Figure 1, and do not necessarily correspond to images used in collecting the data for this study. However, all of these images were analyzed using the techniques described here, and were found to better depict features more subtle in the mosaic images (Table I). Rock form describes the relation between the three dimensions of a rock, according to Folk (I 974). It incorporates sphericity, roundness, and volume to give the overall rock shape. A two-dimensional assessment of this is possible, especially if stereo pairs are available. Dobkins and Folk (1 970) have demonstrated how form can be used with basalt fragments to distinguish beach environments from fluvial ones. They found compact and compact-elongate rocks are characteristic of fluvial deposition and abrasion, while platy, very platy, and very elongate fragments are most common on beaches. Sneed and Folk (1958) devised a 'form ratio' as a numerical measure of the 'platiness' or 'elongateness' of a rock population. They defined the form ratio (FR) as (CP - CE) + 2(P - E)+ 4(VP- VE) FR = 2N ' where CP, CE, P, E, VP, and VE are abbreviations for form classes(Figure 1) and represent the numbers of rocks in each class, and N is the total size of the population (number of rocks). Positive values of FR indicate a prevalence of platy rocks; negative values indicate a dominance of elongated rocks; and values near zero suggest intermediate forms or equal quantities of platy and elongate rocks (Sneed and Folk, 1958). Using the form ratio, 360 J. B. GARVIN ET AL. entire rock populations on Mars and Venus can be compared with each other and with terrestrial rocks in terms of rock shape. Figure 2 illustrates a rock (number 1) with compact or equi-dimensional form, as well as an elongate (rod-like) rock (number 2). An example of a platy or disc-like rock is shown in Figure 3 (number 2). Combinations of these three end-members are also possible.

Fig. 2. Examples of important rock morphology attributes. Rock (1) is compact (equidimensional) while rock (2) is elongate in form; both are unburied. Rock (3) is partly buried (increasing with depth), and possesses an apron of dark sediment. Rock (4) is also buried, and has straight flutes aligned and parallel. There is a high density of flutes as well. Rock (5) shows planar fractures and facets. Rock (6) displays linear features. Rock (7) has a uniform cavity distribution. [Part ofVL-2 frame 21A164]

Roundness is important in sedimentological studies concerning transport mechanisms, as well as an indicator of the amount of abrasion that a rock has undergone. Solution weathering and insolation weathering (flaking) can also produce appreciable rounding. Ventifaction by dust-laden winds often reduces rounding in the terrestrial case in forming keels and facets (Whitney and Dietrich, 1973). Roundness is defined as the amount of rounding of the edges and corners of a two-dimensional projection of a rock or grain outlin e (Krumbein, 1941a; Krumbein and Sloss, 1953; Pettijohn eta!., 1972; Folk, 1974). The degree of roundness ranges from very angular rocks with no rounded edges, through sub-angular rocks, to rounded rocks where there are no sharp edges. Roundness charts presented in Krumbein (194la) and in Pettijohn et al. (1972) were used in our analysis. CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 361

Fig. 3. Rock (1) is an example of an unburied compact rock; it is sub-angular and has a bimodal surface texture with varying albedo. A platy rock with high albedo is shown at (2); this rock is highly buried. Rock (3) has a planar fracture (dark face), and is occulted by another rock. [Part of VL-2 frame 21Al64; area is left of Figure 2]

Rock number 3 in Figure 4 is rounded while rock number 1 in Figure 5 is angular; many of the rocks in the upper frame of Figure 6 are sub-round. Rock surface-texture homogeneity involves all of the visible surface features (markings) on a rock, from alb edo, dust, and pitting, to fractures and apparent roughness. Bimodal rocks have two dominant surface textures, while heterogeneous rocks display more than two textures. Rocks 1 and 3 in Figure 3 are examples of heterogeneous surface texture. This could be useful in indicating whether a rock is multilithic such as a breccia or conglomerate, or if varnish is present. Rocks that have rolled could also be distinguished by this attribute. Occultation is evaluated to preclude biasing due to partially visible rocks, and is a key factor in monoscopic ranging and rock size determination. The 'other' sub-category might include part of a spacecraft as in Figure 6. Cavity distribution involves the number of different types or styles of surface depressions (cavities) visible on a rock. It considers pits, flutes, holes, and their variety on a given rock. The 'no cavities' option is important here, even though resolution of lander cameras limits this assignment to rocks close (~2- 3m) to the camera. Figure 2 (number 7) illustrates a uniform cavity distribution, and rock 3 in Figure 5 has a bimodal distribution. Cavities are useful indicators of fluid-flow erosion by wind or water; they 362 J. B. GARVIN ET AL.

Fig. 4. A buried rock with large, deep cavities is shown in (1); wavy flutes are seen along its top edge. An excellent example of a faceted rock (2) also displays flutes (right side). Note the uniform cavity distribution on rock (2) in contrast to the bimodal distribution of rock (1). Rock (3) is a rounded rock with compact-elongate form. [Part ofVL-2 frame 22C197]

might also tell whether a fragment is vesicular. Grain-size information can be gleaned from a study of cavities in that rocks with large phenocrysts tend to have the less resistant phenocrysts weather out before the finer-grained matrix. Cavity size is determined re lative to all of the cavities visible on rocks in a given image or mosaic, and not to rock size. All surface depressions visible on rocks in an image are compared in evaluating the relative cavity size for a rock. We considered only Viking lander images with camera inclination angles between - 20° and - 40° from the hori­ zontal to preclude problems in interpreting distant rocks. Rock 1 in Figure 4 is an example of very large (and deep) cavities, while rock 1 in Figure 5 has small cavities. Cavity depth is the apparent depth of the predominant cavity type on a rock's surface. As with cavity size, it is important to consider images from different sun angles to properly estimate relative cavity depth. Shadowing effects can then be used in evaluating the depth. Only the most common style or type of cavity is used in depth determinations, since many rocks have non-uniform cavity distributions. Rock 1, Figure 4, is the best available example of deep cavities. Cavity shape is the two-dimensional outline of the major cavity types on a rock. Elliptical cavities are elongated or oval and may represent flutes, from which they are not distinguished. Cavities with irregular outlines may represent coalesced round and/or elliptical examples. This category could help distinguish vesicular rocks from wind-pitted and fluted ones because of the tendency for vesicles not to coalesce as in wind-formed pits (McCauley eta!., 1979). CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 363

Fig. 5. Rock (1) exhibits a variety of diagnostic features: it is elongate, angular, fluted (wavy), faceted, and has a bimodal cavity distribution. Its left end is fractured in a planar style (note the 5-sided relation). Dust can be seen deposited on its surface. Rocks (2) and (3) exhibit low albedo. Linear features (cracks?) are seen on (2), while deep cavities are visible on (3). Notice the drift to the left of rock (1). [Left side of VL-2 frame 22Cl97]

Cavity orientation addresses the possibility of a preferred orientation of cavities into rows , chains, or other identifiable patterns. This is a presence or absence category like many of those that follow. Pit chains are common on many ventifacts and evolve into grooves or flutes. Parallel rows of cavities are also suggestive of wind erosion (Whitney, personal communication, 1980). Oriented cavities can then be visually compared to indicato rs of wind directionality, such as wind tails. In some cases these parallel rows follow bedding planes in rocks, especially sedimentary ones. Tail or streak of sediment considers the presence or absence of accumulations of sediment in the lee of obstacles. Such features can be formed by wind, snow, and water (Karcz, 1968). Excellent examples of tails behind rock obstacles can be seen in Mutch (1978, Figures 44, 45 , 48 , and 49). The geometry of these obstacle shadows is deter­ mined by secondary flow patterns induced within the main flow by the rock obstacles themselves. Interference effects on the flow pattern around one rock by a nearby rock can effectively preclude tail formation. This category also includes the possibility of a rock lying within the tail of another rock. Tails are strong evidence for sediment-laden wind or water flow about rocks. w 0\ ~

... ?' Cl > :;ol z< tTl ....,

!"'>

BEHEPA-10 25. 10. 1975

Fig. 6. Venera 9 and 10 panoramas of the Venusian surface. The upper image is the Venera 9 landing site, where compact, sub-round rocks abound. Notice the apparent lack of pitting on the rocks. This may be due to resolution. The lower frame is of the Venera 10 site. Here, extremely platy, highly buried rocks(?) are most common. Cavities are visible and sediment appears to lie on many rock surfaces. [This image courtesy of NSSDC] CHARACTERIZATION OF ROCK POPULATION ON PLANETARY SURFACES 365

Sediment moats or current crescents can also be recorded as present or absent. They are arcuate troughs commonly seen as scouring effects in front of an obstacle (windward or upstream). A horseshoe shape is typical under terrestrial stream flow conditions. Rock shape, size, and orientation controls the actual position of these obstacle scours. The dimensions of moats depend on the spacing of obstacles such as rocks at a locality (Karcz, 1968). Aprons or fillets of sediment deals with the presence or absence of an apron of material lying anywhere around the base of a rock, and often lapping up the sides of a rock. Aprons are often an indicator of fluid transport by wind or water, but can also be related to in situ rock weathering. Many of the larger fragments observed at the Surveyor landing sites on the Moon are partly, or entirely, surrounded by a fillet or apron offine-grained material (Morris and Shoemaker, 1968). Morris and Shoemaker (1968) postulated that these fillets are formed by the ballistic trapping of small particles ejected from many small local impact events. Figure 2 (numbers 3 and 4) and Figure 6 show examples of this category. Fluting categories. Flutes have been described by several authors (Maxson, 1940; Sharp, 1949; Sugden, 1964; Allen, 1971 ; Morris et al., 1972, Whitney and Dietrich, 1973; Whitney, 1978; McCauley et al. , 1979) as depressions with regular outlines having sub­ parallel walls. They are commonly elongate, have flaring from one end, and are steeper towards their head. Flutes are transverse erosional marks of importance since they are common on terrestrial ventifacts. Allen (1971) has shown them to be significant in inter­ preting the type of flow regime and its behavior about an obstacle. Flute morphology is diagnostic of flow regime (wind or water), and hence the weathering process responsible for them. The flute features considered here are much like those considered in general for cavities ; their distribution, relative size and depth, shapes, and orientation. The relative density of flutes on a rock surface, and whether pits or cavities can be identified inside flutes were also included in our analysis. Examples of fluted rocks are numbers 3 and 4 (Figure 2), number 3 (Figure 3), and number 2 (Figure 4). Facets are planar, convex, or concave faces on a rock. Previous rock structure strongly affects the faceting style according to Whitney and Dietrich (I973) and Sugden (1964), and may depend on rock type as well. Morris et al. (I972) illustrate terrestrial analogues to many styles of facets (see their Figures 42, 42, 43 , 45 , and 46). Figures 2 (numbers 5 and 6), 4 (number 2), and 5 (number I) show a variety of faceted rocks, and illustrate how original rock structure might control the shape and arrangement of facets. The large block (~1m) in Figure 5 (number I) shows what appears to be five-sided columnar jointing at its leftmost end; this rock could also be described as multi-faced. Relative albedo refers to the relative brightness (with respect to a certain image and a solar elevation angle range) of a rock surface. A varying albedo may reflect dust deposits on a rock's surface, or relate to the existence of desert varnish over part of a rock. Com­ position can affect the relative albedo also. It is difficult to quantify the albedo of a rock's surface without normalizing the lighting conditions very carefully. For this reason we have chosen to evaluate a rock's brightness in a relative (qualitative) sense. 366 J. B. GAR YIN ET AL.

Fracture style classifies the most prevalent type of fracture visible on a block. Conchoidal or concave fractures and planar fractures are sought, since they serve to identify certain igneous lithologies from other types of lithologies (i.e., breccias). Planar fractures would include structures like columnar joints (Figure 5, rock 1) and some styles of facets (Figure 2, rock 5). Some rocks (Figure 3, rock 3) are good examples of planar fracture without being clearly faceted. Figure 6 shows the Venera 9 scene (upper image) from Venus in which some rocks appear to be fractured in a planar style. Dust effects describe the way in which fine particles(< 1 mm) relate to the surface of a rock. It differentiates between dust mantles on rock surfaces and dust confined to rock cavities. Figure 3 shows a rock (number 1) with dust on its upper surface. Dust in cavities provides evidence of wind or water weathering processes, since dust-laden fluids flowing into the cavities is the most likely means of its deposition. Underground rock shape, or the apparent degree of burial of a rock, is a critical but difficult parameter to assess accurately in a qualitative determination. An inferred increas­ ing width with depth implies a high degree of burial, whereas an implied decrease in width with depth represents slight burial. Unburied rocks are of importance here, for they could represent the most recently emplaced constituents of the rock population. Basal particles refer to very small fragments ( < 10 mm) lying around the base of a block. This could relate to in situ rock weathering; flaking due to insolation weathering has been observed in the dry valleys of Antarctica by Morris eta/. (I 972), and might be operating at the Viking lander sites (Binder et al., 1977; Mutch et al. , 1977). These particles may be eroded units of soil or duricrust. Fragment of a rock considers in-place rock break-up, and the possible association a rock fragment may have with a neighboring rock. Often, diurnal temperature variations cause rocks to split apart as does frost-wedging ; Ollier (1969, Figure 8) has documented this phenomenon in Australia. Such a process might also be important on Mars , so that basal fragments could indicate an extensively weathered rock. Local en vironment includes any significant environmental features that may be important in describing a rock's physical location. Mutch (1978) illustrates many local rock environments with rocks lying within troughs or depressions at the Viking lander 2 site. It should be noted that the existence of bedrock is still questionable , despite the apparently good examples from VL-1 (Figure 53 in Mutch, 1978), and Venera 10 (Figure 6) images. Flats refer to areas with a paucity of rocks as seen in some Viking lander images taken from Mutch (1978). The 'rock on rock' subcategory specifies the case of two rocks in physical contact. Linear and planar features comprise a suite of possibilities, from cracks and fractures to rock banding. All linear and planar features or markings on a rock surface are con­ sidered in this case (i.e ., grooves). Figure 5 (number 2) and Figure 6 (Venera 9 rocks to the left of center) show examples. Surveyor VII images from near Tycho on the Moon illustrate many examples as well (Morris and Shoemaker, 1968). Notch in rock relates to the style of fracturing (irregular or hackly) a rock displays. Notches are gouges or deep cuts in a rock face or surface, and are much larger and deeper CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 367

than flutes. The filleted rock (number 3) in Figure 2 is an example. Some of the irregular 'rock' outlines in the Venera 10 panorama in Figure 6 may be notches.

D. Rock morphometry attributes. The average rock diameter, computed by averaging the major and minor axis lengths for individual rocks, along with the two-dimensional sphericity are the only morphometric attributes considered in this investigation. How­ ever, both rock size and sphericity are potentially diagnostic parameters in that they strongly influence the morphologic effects of weathering, transport, airflow, pitting, fluting, and sandblast on terrestrial rocks.

1.1.2. Data Analysis A. Histograms. Several graphical approaches have been adopted for the display of the data collected for Martian and Venusian rocks. Histograms comparing feature percent­ ages for two different populations are useful in delineating general trends; it is easy to spot major differences or similarities from histograms such as these (Garvin eta/., 1980). Morphology comparisons of attributes associated with the presence or absence of a single feature on rocks of one population can be presented in histogram form. In this type of histogram (see Figure 7), the horizontal axis is calibrated with respect to the percentage of rocks with a specified attribute, so that values greater than 1.0 (right of the vertical 'unity line') show an association of an attribute for the absence of the specified attribute, while values less than 1.0 (left of the 'unity line') show a corre­ lation for the presence of the specified attribute. In other words, these histograms can be read using a 'lever rule' in which absence of a feature in association with some other features means the histogram bar falls to the right of the 'unity line'. Histograms are used as a vehicle for comparison here, and not for the presentation of raw data (see Tables II, III, IV). They are intended for the pictorial display of morphology data only.

B. Size-frequency curves. Rock size distribution are best illustrated with frequency curves that represent smoothed histograms in which a continuous curve replaces the dis­ continuous bars. These curves are independent of the rock size intervals used, and have been shown to be the best method available for dividing mixed populations into their separate normal distributions (Folk, 1974). From size frequency curves it is possible to identify the mode as the highest point on the curve. The mode is the most frequently occurring rock diameter. Sometimes several modes occur. When such is the case, the rock size distribution is polymodal, reflecting multiple modes of emplacement (Folk, 1974).

C. Statistical measures. Several important statistical parameters can be derived from cumulative frequency plots of rock size data. The sigma phi value is the standard deviation of the rock size in phi units, and serves as a measure of sorting. This is the inclusive graphic standard deviation, and is derived from the cumulative size-frequency plot. Values of sigma phi greater than 1.0 suggest poor sorting. 368 J. B. GARVIN ET AL.

A. 'l. ROCKS NOT FLUTED / % RO CKS FLUTED

TOTAL OCCURRENCE

FACETS

ANGULAR

W IN 0

UNIFORM

APRONS

ELONG II RREG CAYS

NON-UNIFORM CAYS

B. %ROCKS NOT FLU T ED/'\ ROCKS FLUTED

TOTAL OCCURRENCE

FACETS

ANGULAR

WI NO FEATURE

COMPACT

UNIFORM CAVITIES

APRONS

CAYS

NON-UNIFORM CAYS

Fig. 7. Morphology comparison of attributes associated with fluted and non-fluted rocks at both VL-1 (A) and VL-2 (B) landing sites. Attribute values to the right of 1.0 show preference for non­ fluted rocks, while those left of the 1.0 line are more commonly associated with fluted rocks (e .g., those. with non-uniform cavities). 'Non-uniform cavs.' are bimodal and heterogeneous cavity distri- butions; 'elong./irreg. cavs.' are elongate and/or irregular shaped cavities. Wind features include tails and sediment moats. CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 369

The mean size of a rock population is another useful measure. It represents the central tendency or average size of rocks in one locality. It is clearly a function of the size distri­ bution of the available source materials. The mean sphericity is one way of quantitatively measuring the overall shape of rocks in a population and reflects overall lithology (Folk, 1974). The modality index devised by Sahu (1964) is an objective way for quantitatively assessing the gross character of a size frequency curve. Values around 2.0 suggest a bimodal distribution, while those nearer 1.0 are more normal (unimodal). The inclusive graphic skewness measures the asymmetry inherent in a frequency distribution, and marks the position of the mean with respect to the median. Negative skewness values indicate that coarser (larger) fragments dominate the distribution. The form ratio is a measure of the average rock shape of a rock population, and is useful in comparing populations of isotropic rocks. Negative values indicate elongation, while positive values indicate platiness. Average roundness values are useful in assessing the types of transport mechanisms that might have been acting at a very local scale. For further details see Folk (1974), Sneed and Folk (1958), and Dobkins and Folk (1970).

D. Statistical clustering. A good approach for identifying 'clusters' or groupings in data sets made up of individuals described by a set of parameters or attributes is the use of a divisive multi-parameter clustering algorithm. We make use of such an algorithm in seeking to categorize the rocks described by the attributes of Figure 1. Gill and Tipper (1978) describe this technique and how it can be applied to numerous geologic data sets. The version of the algorithm we have implemented is based on Tipper (1979), and requires a binary (0 and 1 values) data set of N individuals (rocks) and M variables (attributes). It is a straightforward process to convert our qualitative morphology data into a binary format by creating new attributes for every subcategory. Quantitative data such as rock sizes can be made binary by generating one new attribute for each rock-size class of relevance (Krumbein and Sloss, 1953). The unique feature of this clustering technique is that it is 'divisive'; it starts with the entire population as a whole and attempts to split it into the two most dissimilar groups that are possible. A dissimilarity measure based on the 'information content' of an individual or group of individuals is used. Once the original population is divided into two subgroups, the process begins anew, by considering each subgroup as a new population and subdividing that in a similar manner to the original population. By making use of more than one attribute at a time in the process of subdividing, it is possible to discover those attributes which are most useful in separating groups of rocks. This is easily accom­ plished by checking the percentage occurrence of a desired attribute in a subgroup. If the attribute is important by its presence, it will have a percentage occurrence of over 50%; if it is significant by its absence, it might have a percentage occurrence of less than 10%. By choosing a set of attributes (any subset of those in Figure 1) based on some classification hypothesis, one is able to test the validity of the hypothesis by examining the percentages of the attributes in the subgroups produced by the clustering. TABLE II Comparisons of martian and venusian rock morphology. All attribute percentages greater than 20% of the entire site sample are shown. w_, N is the size of each rock population. 0 ATTRIBUTE VL-1 VL-2 Venera 9 Venera 10 (N = 240) (N= 210) (N = 63) (N = 30)

Elongate (22%) Compact/Elongate (30%) Compact (30%) Very platy (33%) FORM Compact/ Compact (23%) Elongate (22%) Platy (23%) Elongate (20%) Elongate (21 %) Sub-angular ( 43%) Sub-angular (54%) Sub-round (43%) Sub-angular (47%) ROUNDNESS Sub-round (32%) Angular (22%) Sub-angular (33%) Angular (23%) Angular (21%) Sub-round (20%) Angular (21%) Sub-round (23%) SURFACE Uniform (58%) Uniform (56%) Uniform (67%) Uniform (50%) TEXTURE Bimodal (41%) Bimodal (42%) Bimodal (32%) Bimodal (50%) HOMOGENEITY :- ~ Bimodal (33%) CAVITY Uniform (46%) Bimodal (42%) Indet. (56%) 0 Uniform (27%) >- DISTRIBUTION Bimodal (40%) Uniform (39%) ;>:l Indet. (27%) < RELATIVE Medium (40%) z Medium (46%) Medium (54%) Indet. (48%) tT1 CAVITY Indet. (23%) ..., Small (36%) Small (32%) Medium (24%) SIZE Large (20%) >- t"' RELATIVE Moderate (45%) Moderate (57%) Indet. (48%) Moderate (43%) CAVITY Shallow (41%) Shallow (24%) Indet. (30%) DEPTH CAVITY Round and Round and Indet. (54%) Indet. (23%) SHAPE Elliptical (25%) Elliptical (37%) Absent (49%) CAVITY Absent (51%) Indet. (70%) Indet. (53%) Indet. (31 %) ORIENTATION Indet. (31%) Absent (28%) Absent (34%) Present (20%) Absent (47%) Indet. (59%) Absent (77%) TAILS Present (31%) Absen t (81%) Absent (41 %) Indet. (20%) Indet. (22%)

Absent (39%) Indet. (73%) MOATS Present (38%) Absent (93%) Absent (80%) Absent (27%) Indet. (23%) Absent (44%) Indet. (57%) Present (68%) Present (70%) APRONS Present (36%) Present (22%) Absent (21%) lndet . (30%) Indet. (20%) Absent (21%) Present (46%) Present (45%) Indet. (64%) Absent (50%) () FLUTES Indet. (32%) Absent (28%) :r: Present (24%) Indet. (47%) ;> Absent (22%) Indet. (27%) :>:l ;> Present (48%) Absent (43%) lndet. (57%) Absent (73%) () FACETS ..., Indet. (25%) Present (31 %) Absent (27%) Indet. (27%) tT1 :>:l Varying (40%) ;::; RELATIVE Medium (38%) Medium (40%) Medium (47%) High (25%) ;> ALBEDO Varying (31%) Varying (35%) High (30%) ..., Medium (21%) 0 %) z FRACTURE Indet. (33%) Indet. (34%) lndet. (51%) Indet. (63 0 STYLE Planar (27%) Planar (29%) Planar (33%) Planar (27%) 'TJ :>:l On surface (43%) Indet. (27%) In pits & 0 DUST lndet. (63%) () In pits & In pits & on surface (43%) ;:.: EFFECTS On surface (33%) on surface (31%) on surface (23%) On surface (33%) ., 0., Slight (40%) DEGREE Slight (46%) Slight (40%) High (60%) c Unburied (34%) t"' OF BURIAL High (35%) Unburied (37%) Indet. (23%) ;> High (22%) ..., 0 Absent (42%) z BASAL Absent (54%) Indet. (48%) Present (50%) [f) Present (30%) PARTICLES Present (26%) Present (41%) Indet. (37%) 0 lndet. (28%) .,z Indet. (38%) lndet. (41%) t"' FRAGMENT Present (54%) Present (50%) ;> Absent (37%) Absent (34%) z OF ROCK Indet. (44%) Indet. (43%) tT1 Present (25%) Present (25%) ..., Average (51%) Bedrock (50%) ;> LOCAL Average (42%) :>:l Average (85%) Rock Rock -< ENVIRONMENT Flats (20%) [f) Contact (46%) Contact (33%) c :>:l Present (40%) Absent (46%) 'TJ LINEAR Absent (45%) Present (47%) ;> Absent (37%) lndet. (30%) () FEATURES Present (36%) lndet . (30%) tT1 lndet. (23%) Present (24%) [f) Absent (42%) Absent (49%) lndet. (37%) Present (53%) NOTCH Present (34%) lndet. (26%) Present (35%) Indet. (27%) IN ROCK -lw Indet. (24%) Present (25%) Absent (28%) Absent (20%) ..... 372 J. B. GARVIN ET AL.

TABLE III Comparisons of martian and venusian rock morphometry parameters. Note that form ratios and average roundness values are based on subjective data. Modes were deter­ mined from size frequency curves shown in Figure 10. Rock size classes are expressed in percent of total rock population at a given site. PARAMETERS VL-1 VL-2

No. of Rocks 240 210 Range from Spacecraft 2.12 m--4.06 m 1.84 m--4.87 m Rock Diameter 17.9 mm- 212.6 mm 12.7 mm- 250.5 mm I Mean Rock Diameter 56.8 mm (- 5.83¢) 64 .5 mm (- 6.0¢) Sigma (4>) 0.687¢ 1.037¢ Skewness - 0.174¢ - 0.057¢ Sorting Coefficient t 1.45 1.65 Mode(s) t 40 mm, 82 mm(?) 43 mm, 86 mm, 178 mm Mean Sphericity 0.634 0.631 Form Ratio* - 0.25 - 0.30 Average Roundness * Sub-angular Sub-angular Large Cobbles 2.7% 16.6% (128- 256mm) Small Cobbles 27.4% 28.4% (64- 128mm) Very Large Pebbles 55 .3% 35.5% (32- 64mm) Large Pebbles 14.6% 18.3% (16- 32mm) Medium Pebbles 0% 1.2% (8- 16mm)

t derived from rock size frequency curves. * from subjectively collected data on form and roundness.

TABLE IV Comparisons of martian and venusian rock shape parameters. Form ratios and average roundness values are based on subjective data; average sphericity values were calcu­ lated from measurements of rock major and minor axes (in mm). Note that Viking lander rock populations have similar overall shapes. PARAMETER VL-1 VL-2 Venera 9 Venera 10

N 240 210 63 30 FORM RATIO* - 0.25 - 0.30 - 0.17 + 0.45 AVERAGE sub- sub· sub- sub- ROUNDNESS angular angular angular angular AVERAGE .634 .631 N/A SPHERICITYt N/A

* Values < 0 are elongated or rod-like. Values > 0 are platy or disc-like. t N/A means not available. CHARACTE RIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 373

able to test the validity of the hypothesis by examining the percentages of the attributes in the subgroups produced by the clustering. The algorithm can be halted at any stage in its subdividing process: after 2 groups, 4 groups, 8 groups, 16 groups, and so on. A 95% level of dissimilarity between groups is maintained at every stage. In essence, this technique enables one to classify individuals described by a set of attributes in a top-down fashion. Most typical clustering algorithms work from the 'bottom up' by attempting to combine single individuals into groups that are eventually merged until the original population is generated. As an analogy, consider a simplified classification of basalts from a petrologic point of view. Recall that Ne-normative basalts are like most other basalts except for the presence

of nepheline in the norm. Most basalts have between 45- 49% Si02 , so other criteria had to be found before basalts themselves could be reasonably subdivided. If our divisive clustering algorithm were applied to a data set consisting of the normative analyses of basalts, it would first divide all basalts based on the most significant normative minerals. For example, after the first stage, a separation between tholeiites and alkali basalts might occur, with hypersthene and augite serving to separate the groups. Note that both groups would have augite present, but that the alkali basalt group would have a higher percentage than the tholeiite group. The same situation occurs when rock morphologies are considered and classified. The algorithm is inherently enumerative and thus time consuming. We found that classifications with up to 23 parameters at a time were the most practical in terms of computer time and results achieved. Figures 8 and 9 are simplified dendrograms produced by using the divisive multi-parameter clustering technique on the martian and venusian rock morphology data sets. In Figure 8A the primary separation criteria is the presence or absence of flutes. However, groups A and B combined only have 55% flutes. This is because some non-fluted rocks as well as some rocks in which flutes were indeterminate were allocated to groups A and B based on other criteria they share in common, such as aprons.

2. Rock Characteristics and Associated Features

2.1 . POPULATIONS 2.1.1. Size Distributions. Frequency curves for the rock size distributions measured for VL-1 and VL-2 are shown in Figure 1 0. From these curves it is not difficult to see there are differences in the rock distributions at the two sites. A summary of the critical morphometric parameters for these sites is given in Table III. The differences in the size distributions are most apparent when the sigma phi values are compared. At VL-1 the value of 0.687<1> represents moderately well sorted fragments, whereas the VL-2 value of 1.037<1> indicates poorly sorted rocks (Folk, 1974). Both sites are coarsely skewed, but VL-1 is more so than VL-2. Examination of the rock-size class percentages serves to further identify differences 374 J. B. GARVIN ET AL.

VL- 1 ROCKS A. (N = 240) I I I Flutes No Flutes (N = 194) (N = 46) I I I I GROUP A. GROUP B. GROUP C. GROUP D. Planar Not Planar Elongate Facets (N = 137) (N = 57) (N = 36) (N = 10)

GROUP A : 76% ap ron s GROUP C : 58% aprons 52% facets 56% e l ongate 47% planar 47% sub-round

GROUP B: 100 % flutes GROUP D : 100 % facet s 33% a prons 50% aprons 26% linear 50% sub-round

VL - 2 ROCKS B. (N=21 0) I I El o ngate No t Elongat e (N=132) (N=78) I I I I I I GROUP A. GROUP B. GROUP C. GROUP D . Apr ons No Aprons Plan a r Rounded (N=11 3) Flutes (N:58) No Flutes (N=19) (N:20)

GROUP A : 67% a prons GROUP C: 69 % not buri e d 60% linea r 53 % planar 54% flut es 43 % fa ce t s 29% f acets 35% flut es

GROUP B: 100% elongate GROUP D: 80 % r ou nd e d 7 4% flutes 40% not buried 42% not buried 15 % facets 26% f acets NO flutes NO ap r o n s

Fig. 8. Dendrograms and subdivisions of VL-1 (A) and VL-2 (B) rocks. N is the size of each rock population or subpopulation. The percentage of rocks in each group with specific attributes is also given. See text for complete details of their interpretation.

between the rock populations at the Viking lander sites. A deficiency of large cobbles (rocks with diameters 128- 256 mm) at VL-1 is not seen at VL-2 (2. 7% vs 16.6%). Most of the rocks at VL-1 are pebble size (16- 64mm), while there are fewer pebbles at VL-2 (70% vs 55%) and many more cobbles. The unimodal appearance of the VL-1 frequency curve in Figure 10 can be tested by applying Sahu's (1964) modality index measure. CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 375

V ENERA 9 ROCK S A. (N= 6 3 ) I I Rounded An gul ar (N=50 ) ( N = 1 3 ) I I I I GROUP A. GROUP B. GROUP C. Flutes No Flutes An gul ar (N = 38) Planar Pl anar (N = 1 2) (N = 13)

GROUP A: 53% round ed GROUP C: 100% angul ar 37% r ock contac t 38% plan ar 34% not buri ed 38% ro c k co nt ac t 29% flutes 31% not buried 3 1% lin ear GROUP B : 100% p la na r 83% r ock co nt act 6 7% r ou nd ed 50% no t buried 42% a pro ns NO flut es

VE NERA 10 ROCKS B. (N= 3 0) I I I Not Very Pl a t y Ver y Pl a ty (N=2 0) (N = 1 0 ) I I I I GROUP A. GROUP B. GROUP C. Elongated Sub- Round Very P laty ( N = 1 5) (N= 5 ) Large Cavities (N = 1 0)

GROUP A: 67% aprons GROUP C: 100% ver y platy 53% e longated 80% aprons 4 7% lin ear 70% linear 4 7% rock contact 60% larg e cavities 27% angular 60% planar

GROUP B: 100% sub-round 60% apron s 40% planar 40% ro ck cont act 40% pla t y

Fig. 9. Dendrograms and subdivisions of Venera 9 (A) and Venera 10 (B) rocks. Format same as Figure 8.

The modality index for VL-1 is 1.0, assuming the small spike in the curve to be within the error bar. At VL-2, however, three modes can be read from the peaks in the curve. The modality index comes to be~ 2.0, confirming the polymodal nature of the rock size distribution at the site. There appears to be significance to the population of large cobbles at VL-2, along with the very large pebble class that dominates both VL-1 and VL-2. Another means of comparing rock populations involves the mean sphericity. Sur­ prisingly, the values for both VL-1 and VL-2 are identical (0.63) and represent sub­ elongate rocks. This value is less than 0.65 which Dobkins and Folk (1970) found as the 376 J. B. GARVIN ET AL.

20 VL-1 15 (/) ~ (.) 10 0 a: c w 5 ....1 a.. ::E <( (/) 10 100 1000 u. 0 w "<( 15 VL-2 1-z w (.) a: 10 w a.. 5

10 100 1000 ROCK DIAMETER (mm)

Fig. 10. Rock size frequency curves for VL-1 and VL-2 sites on Mars. Note the polymodal nature of the VL-2 curve, and the essentially unimodal character of the VL-1 plot. dividing line between beach and fluviatile pebbles on Tahiti. To test the significance of this overall similarity of shapes of pebble and cobble sized rocks on Mars, form ratios were computed for the rock populations at VL-1 and VL-2. Table IV summarizes the ave rage sphericity and form ratio data for the Viking sites. As can be seen, the form ratios agree quite well with the mean sphericities. This demonstrates that the average shapes of rocks at the two martian sites are remarkably similar. The roundness values for VL-1 and VL-2 average out to fall into the same class as well. Comparisons of form ratios for the Venera 9 and 10 sites delineate major differences in the populations of rocks at the two sites in terms of their overall shapes. Venera 10 rocks are platy, while those at Venera 9 are somewhat elongate. Table IV serves to compare the populations of the martian and venusian landing sites in terms of average roundness and form ratios (i.e., shape). The fact that all four sites have an average round­ ness that corresponds to sub-angular rock is surprising, but digital image resolution effects could serve to promote such an effect. In summary, we observe a trend for the average shapes of pebbles and cobbles on Mars to become elongated to the same degree. On Venus, no such trend is observed from the limited data available and the Venera sites are markedly different in terms of overall rock shape. CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 377

2 .2. ROCK CHARACTERISTICS A summary of the characteristics of the rocks at the four landing sites is presented in Table II. From this table, inter-site and interplanetary rock population morphologies can be compared. Some of the key comparisons are illustrated in Figure 11. It is apparent that there is a tendency for VL-1 rocks to be less elongate and more rounded than their more angular and rod-like VL-2 counterparts. Facets of all styles are more prevalent at VL-1 than at VL-2. In terms of rock surfaces, there are more rocks at VL-1 with uniform cavity distributions than at VL-2, where bimodal cavity distributions are the most common. A significant percentage of VL-1 rocks have small, shallow cavities while medium, moderately deep cavities are most common on VL-2 fragments. In general, the Lrnportant rock characteristics for the VL-1 site on Mars can be summarized as follows: (1) Rocks are somewhat elongated and mostly sub-angular; (2) uniform surface textures and cavity distributions are the most common; (3) small, shallow cavities of all shapes are significant on the rocks; (4) flutes and facets are quite common(> 46% of the rocks have them); (5) linear features in the form of cracks or fissures occur on 36% of the fragments ; (6) notches frequently occur; (7) flutes are commonly associated with compact rocks having non-uniform cavity distri­ butions, and rarely with rocks having uniform cavity distributions or aprons; (8) facets are seen preferentially on compact, somewhat angular rocks often having planar fractures; (9) all rocks are pebble or cobble size in the sample area studied. For the VL-2 site on Mars , we observe the following general rock characteristics:

(1) Rocks are commonly angular to sub-angular and rod-like ; (2) bimodal cavity distributions are more common than uniform ones; (3) medium cavities of moderate depth with round or elliptical shapes are most typical; ( 4) flutes are as common as at VL-1, but facets are much less so (31 % vs 48%); (5) linear features in the form of grooves, fissures, and cracks are significant; (6) flutes are rarely seen on angular, faceted rocks; (7) unlike VL-1, flutes are not associated with compact rocks; (8) flutes are common on fragments with non-uniform cavity distributions; (9) all rocks are pebbles or cobbles in the areas studied.

At the Venera 9 site near the Beta-region on Venus, we can observe the following general rock characteristics: (1) Rocks are mostly compact and somewhat rounded; (2) cavities appear to be absent or rare; (3) elongate features suggestive of flutes are present; ( 4) facets are not discernable; (5) dust or sediment lies on the surfaces of some rocks; 378 J. B. GARVIN ET AL. A. 8 0 VL- 1 ..J 6 0 0. " 4 0 "' 2 0 " 0 "0 2 0

4 0 VL-2 ( N = 2 1 0 ) 0 6 0

0 a: "' 8 0 w 0 0 "' z ::> ' w ::> ...... a: 0 "z ::> "'... ""' "'... ., z 0. ..."' "' "'"z ... 0 0 ., ., "o a: "' z ::> ::> o_. " 0 0. 0 uw "' z ;. "' "' "' " B.

w 6 0 ..J 0. MAR S ( N = 4 5 0 ) ""' 2 0 "' 0 " 2 0 "0 VENUS a: 4 0 ( N : 9 3 )

0 "' :::> z ' W :::> ...... "z 0 "' a: " ""' ..."' "'w z "'0. o.z 0 "'' "'" w ... ., .,' "o :::> a: :::> :::> 0" O..J " ..J 0. "' "' " uw ..."' ... "'

Fig. 11. Morphological comparisons of (A) VL-1 and VL-2 rocks; (B) Mars and Venus rocks. The total percentage of the rock sample with each attribute is presented for each histogram.

(6) nodules(< 1 em) are common about the bases of rocks; (7) many rocks appear to be fragments of each other; (8) rocks are often in physical contact with their neighbors; (9) notches are significant in the Venera 9 fragments.

The Venera 10 landing site on Venus is difficult to describe in terms of its rock popu­ lation. Pebble and cobble size fragme nts are uncommon (Keldysh, 1979). Of the 'rocks' visible in the panorama, it can be observed that they are very platy, pitted, and 'dust' covered. Most of the rocks appear to be part of bedrock exposures (Keldysh, 1979), and linear cracks and fissures can be seen transecting their upper surfaces (Figure 6, lower frame). CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 379

2.3. ASSOCIATED FEATU RES Morphologic features such as obstacle scours (tails and moats), aprons, degree of burial, and local environment can be related to the environment's response to a rock. Often the rock serves as an obstacle to the transport of sediment. In this section we examine the associated rock features characteristic of each of the four landing sites. At Vlr 1 in Chryse Planitia, the following observations were gathered: (1) Tails and moats are commonly visible around rocks; (2) aprons or fillets of sediment are ubiquitous around the bases of rock fragments; (3) most rocks are at least slightly buried; ( 4) very few rocks are located in local environments different from the general site character.

For the Vlr2 locality 48° N of the martian equator the following features can be observed: (I) Obstacle scours are not very common, but do exist; (2) aprons or fillets are far less common than at VL-1 but are not rare ; (3) there is a significant population of unburied fragments; ( 4) few rocks appear to be highly or deeply buried; (5) troughs with few rocks in them can be seen.

Venera lander camera resolution makes detailed morphological analysis of rocks difficult except for immediately in front of the lander (middle portion of Figure 6 images) where ~ 1 em resolution is possible. In general, obstacle scour features are not observed at the Venera sites. Aprons or collars of sediment can be seen around most of the Venera 10 'rocks', and around a few Venera 9 fragments. Rocks at both sites appear to be in physical contact with each other. Venera 9 blocks are slightly buried if at all, while the Venera 10 rocks appear to be highly buried. The rocks at Venera 10 resemble bedrock exposures.

2.4. MORPHOLOGIC GROUPINGS The preliminary results of our application of cluster analysis to rock morphology data sets for the Viking and Venera landing sites are presented in dendrogram format in Figures 8 and 9. The set of parameters selected for the objective classification of these rock populations was chosen to include the widest variety of commonly occurring features. Other classifications, based on parameters related by their genetic significance (i.e., erosional features), have been carried out, but are not presented here due to their specific nature. All of the 19 attributes used in the clustering process that produced the dendrograms shown in Figures 8 and 9 have over 20% occurrence at the Viking sites on Mars, or at the Venera sites on Venus. The dendrograms generated by the divisive cluster­ ing algorithm display the sizes of the subgroups and percentages of attributes for each subgroup at each level. 380 J. B. GARVIN ET AL.

The attributes involved in the process were first converted to a presence/absence or binary format and are listed below: (1) Form: platy, elongate, very platy, very elongate; (2) roundness: angular, sub-round, round; (3) rock surface texture: heterogeneous; (4) cavity distribution: no cavities; (5) cavity size: large; ( 6) oriented cavities; (7) tails ; (8) apron or fillet; (9) flute presence (any style); (10) facet presence ; ( 11) planar fracture; (12) not buried; (13) rock on rock (contact); (14) linear features ; By examining the results of the objective classification as summarized in the dendro­ grams, it is possible to identify those parameters that were of the most use in separating the populations into morphologic groups. From the dendrogram for VL-1 in Figure 8A it is apparent that aprons are significant in all subgroups, while features such as flutes and facets are important within single subgroups. Groups A and B contain all of the fluted rocks from the population, but there are rocks without flutes as well. This is because the clustering process had to consider all 19 parameters when constructing the subgroups, and there must have been some non-fluted rocks that were so similar morphologically to the others in groups A and B that not being fluted was statistically unimportant. The dendrogram suggests that there is a morphologic class of rocks without flutes that are somewhat rounded and lacking planar fractures. Another class of rocks shows appreciable fluting along with linear features and planar-style fractures. The VL-2 classification demonstrates that there are real differences between the rock populations at the two Viking landing sites in terms of their morphologies. In this case, only one subgroup of rocks contains rocks with apron features, and only a small (N = 20) class of rounded fragments is non-fluted. The degree of burial is an important morpho­ logic class separator at VL-2, unlike VL-1. It is interesting to see how rock shape (elongate vs not elongate) is the most important morphologic rock group separator at VL-2 , while an erosional marking (flutes) is the most significant at VL-1. Morphologic rock groups at the Venera sites on Venus are displayed in Figure 9. Form­ and roundness-related attributes play the most dominant roles in the classification of venusian rocks, possibly due to lower resolution of the Venera cameras. Notice, however, that fluting separates subgroups at Venera 9, while non-burial is significant in all three subgroups (Figure 9A). Presence of aprons does not appear important at this site, whereas they are ubiquitous at Venera 10 much like the situation at VL-1. The key observation CHARACTERIZATION O F ROCK POPULATIONS ON PLANETARY SURFAC ES 381

that can be made from the Venera rock classifications is that there are two morphologically distinct populations of rocks at the Venera 9 lander site based on differences in roundness. At Venera 10, one subgroup of rocks is distinguished by the presence of relatively large and irregular cavities. These rocks are also seen to be extremely platy or tabular. In conclusion, we observe that a simple multi-parameter classification of rocks visible in images of the martian and venusian surfaces identifies meaningful morphological sub­ groups. Erosional markings, rock form, and roundness are seen to be the most important criteria for morphologic group separation.

3. Speculations

In attempting to characterize the rock populations on planetary surfaces, there are three fundamental questions we must ask. First, what can be inferred about the rock types from the morphometric and morphological data that has been collected? Second, can anything be said regarding the mode or modes of emplacement of the fragments? Finally, what types of modifications have the rocks undergone; what sorts of weathering processes have affected the fragments?

3 .1. ROCK TYPE (LITHOLOGY) The mean sphericity and form ratio data provide us with a measure of the average rock shape of a population. Folk (1974) has noted that sphericity of pebble and cobble size fragments is partly controlled by lithology (Sneed and Folk, 1958; Folk, 1974). Drake (1970) studied the shapes of rock fragments and concluded there was a systematic relation between the texture of a rock type and the shapes of fragments produced by fracturing it. In his investigation, Drake found that there is a tendency for aphanitic and glassy rocks to fragment into bladed or rodlike shapes, while coarse textured rocks break up into compact or spherical fragments. Isotropic rocks (i.e., those with no foliation or small scale joint patterns) followed the trend outlined above better than anisotropic ones (i.e ., schists, porphyries, conglomerates, etc.). Drake showed that as grain size decreases, pebble and cobble size fragments become less spherical and more bladed or elongate. From these results, we can make generalizations about the rock types that occur on Mars and Venus. Form ratios for the Viking lander sites are indicative of elongate or rodlike rocks . This is in agreement with aphanitic or glassy isotropic fragments (Drake, 1970). Since the best Viking camera resolution is 1 mm per pixel and grains cannot be seen in the rocks visible in the images taken on Mars, we conclude the rocks there are isotropic with aphanitic or glassy textures. The most common terrestrial rocks with these charac­ teristics are quartz, felsite , basalt, and obsidian (Drake, 1970). It is unlikely there are cobble-size fragm ents of quartz everywhere at the landing sites, since not all rocks are conchoidally fractured, and may have low albedo surfaces. Likewise, obsidian or volcanic glass would have reflective or high albedo surfaces and would tend to fracture conchoidally. From Table II, however, it is evident that planar fractures are more prevalent than conchoidal ones. Based on these factors, it appears that basalts and felsites could be the dominant rock types at the Viking lander sites. This hypothesis is based 382 J. B. GARVIN ET AL. entirely on the data and arguments outlined above. However, additional constraints on rock morphology may also be inferred from Viking orbiter images. Greeley eta!. (1977) identified numerous wrinkle ridges within Chryses Planitia (VL-1) that are morphologically similar to lunar mare ridges. If this comparison is justified, then basaltic rocks at VL-1 appear most likely although the possible existence of a rhyolite flow in Arcadia Planitia ( 40° N, 150° W), identified by Fink (1980), does not permit the preclusion of felsitic rocks. In addition, Binder et al. (1977) and Mutch et al. (1977) have inferred from rock morphology at the two Viking lander sites that terrestrial basaltic rocks are most con­ sistent with the observed martian rock lithologies. Form ratios for aphanitic basalt localities on Earth support this theory (Garvin, unpub­ lished data) and rock morphometry and morphology data for the martian localities fit well with a basalt lithology. The mean sphericities for the sites (0.63) are suggestive of bladed or rod-like rocks. The form ratio as well as the mean form values indicate an overall trend to elongation. Fracture styles show evidence for columnar jointing (see Figure 5, rock 1). Felsites and basalts from the Dry Valleys of Antarctica become pitted and fluted in a manner similar to many of the martian blocks (Morris et al., 1972). Even if the pits in the rocks are not due to windblast by dust or sand, vesicles are common in many basalts or felsites. Since there is no morphometric data for the Venera sites, and owing to the lower resolution of the Venera cameras, it is difficult to speculate on the types of rocks at these localities. The form ratio data for the Venera 9 locality does indicate that most rocks there are somewhat elongate, but less so than their martian counterparts. The grain size of the fragments at this site could be anywhere from ~ 1 em to less than 1 mm (i.e., from coarse to aphanitic). A significant portion of the Venera 9 blocks display planar fractures, and some even possess linear features resembling contraction cracks. The sub-rounded nature of many of the rocks is probably a result of modification. There is some evidence, therefore, that the fragments at the Venera 9 site are isotropic, fine grained, and elongate. This suggests there are basalts or felsites at the site, but other lithologies are probably also represented. A significant portion of the rocks are compact and not elongate at all. These blocks could be anisotropic, coarse grained, and possibly foliated (Drake, 1970). We hypothesize a mixture of lithologies at the Venera 9 locality, with basalts or felsites, and some coarser-grained igneous rocks (granites?) as the most plausible rock types. The Venera 10 site is even more beguiling than Venera 9 due to the paucity of identifiable rock fragments in the panorama describing its vicinity. The platy or disc-like character of the rocks at this locality, combined with their pitted and dust mantled nature makes them difficult to describe in terms of lithology. Keldysh (1979) has pointed out that the gamma ray densitometer aboard both the Venera 9 and 10 landers indicates basaltic compositions for these areas. If Venera 10 landed on a younger basalt flow, then it must be one that has yet to fragment into blocks.

3.2. MODE OF EMPLACEMENT The primary characteristics of a fragment that influence its settling velocity in any fluid

(water, air, or C02 ) and thus its conditions of transportation are its size, sphericity, CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 383 roundness, and specific gravity (Morris, 1957). The mode of transportation also depends on the laws of fluid flow governed by the velocity, viscosity, and turbulence of the fluid. In assessing the mode or modes of emplacement responsible for the rocks on any planetary surface, one must consider their size distribution, sphericity, and roundness as well as their morphology. From the morphology data, it might be possible to determine whether a fragment was emplaced from afar or has remained in place since its formation from a lava flow. From Viking orbiter images, multiple modes of rock emplacement appear to be likely at both sites. Theilig and Greeley (1979) identified several episodes of fluvial deposition and volcanic plains emplacement in Chryses Planitia that probably resulted in sediments from Lunae Planum being deposited at VL-1 in addition to rocks derived from local flows. Arvidson et al. (1979) interpreted many of the impact craters close to VL-1 to be relatively fresh and consequently should still possess parts of their ejecta blankets. Because the majority of these craters are smaller than the 2 km threshold diameter for fluidized ejecta deposits (Boyce, 1979), ballistically emplaced ejecta blocks may well be present at VL-1. Conversely, the ejecta deposits at VL-2 are believed to be dominated by ground-flow materials from the crater Mie (Masursky and Crabill, 1976; Mutch et al., 1977). Since VL-2 lies approximately four crater radii from the center of Mie (100 km in diameter), both ballistic and ground-flow ejecta should be present at this range for a crater the size of Mie (Mouginis-Mark, 1979). Masur sky and Crabill ( 197 6) interpret the extensive areas of fractured ground to the south and west of the lander to be old lava flows, suggesting that volcanic rocks may be prevalent within the ejecta at VL-2. Average roundness values for VL-1 and VL-2 are similar, suggesting common exposure ages or modes of transport. The scarcity of large cobbles (rocks > 128 mm in diameter) at VL-1 in comparison with VL-2 implies more active or rigorous processes emplaced the VL-2 cobbles, or that the VL-2 site is less mature than VL-1. The fragments at VL-1 are mostly all of one size class (i.e., 70% pebbles, rock diameters < 64 mm) and could be the end result of a single emplacement process that sorts its load very well. The sigma phi values (i.e., sorting) indicates that the rocks at VL-1 are moderately well sorted, while those at VL-2 are poorly sorted. Based on these sigma values, we conclude there are no rocks characteristic of terrestrial mudflows or glacial till at either VL-1 or VL-2 (Folk, 1974). Both sites have sigma values that fall into the same range as terrestrial river pebbles (0.4 to 2.5¢); the mean sphericities and roundness values for the sites are also similar. The sorting differences between VL-1 and VL-2 may reflect exposure ages more than emplacement processes, since other data useful in characterizing such processes show both sites to be very much alike. We conclude, therefore, that the modes of emplacement for the rocks at the Viking lander sites on Mars are similar, and that differences in the rock populations at these localities reflected in their degree of sorting can be attributed to differences in ages of exposure. The rocks at both sites appear to reflect multiple modes of emplacement with in-place break-up of lava flows and ejecta from nearby craters the most likely sources. The sub-angular and elongate character of the rocks at both sites bears this out. 384 J. B. GARVIN ET AL. Dobkins and Folk (1970) studied basalt pebbles and cobbles in the same size range (16-256 mm) as our martian rocks from terrestrial river and beach environments. They discovered that for isotropic rocks such as basalts, a sphericity zone from 0.65 to 0.66 served to distinguish pebbles shaped by surf action from those affected by fluvial transport. Since sphericity has been shown to depend on inherent abrasional properties of different rock types (Sneed and Folk, 1958), it is little affected by selective sorting. The sphericity value of 0.63 derived from our studies of VL-1 and VL-2 rock populations suggests that fluvial processes were not responsible for the emplacement of these rocks if terrestrial analogues are valid for Mars (Dobkins and Folk, 1970). This is because 0.63 is less than the 0.65 to 0.66 zone and is thus an indicator of a beach environment. The process that transported the pebbles on Mars must have had an intensity at least as great as surf action on terrestrial beaches. Whether prolonged wind-blast by dust and sand can abrade rocks in such a manner is an unanswered question. If so, there is still the possibility that fluvial processes transported pebbles to the Viking lander sites. For isotropic rocks like basalt, chert, or quartz, fluvial transport tends to produce elongate or rodlike forms. This is in agreement with the observed characteristics of martian rocks and supports our rock type hypothesis that basalts or felsites are common at the lander sites no matter how they were emplaced. Identical mean sphericity values for two localities 6000 km apart implies a similarity of source or parent rocks for the fragments (Folk, 1974). Detailed studies of the morphometry and morphology of eolian pebble populations as well as of rock populations around impact craters are required before more definitive hypotheses can be formulated. For the Venera 9 site, the trend towards elongation combined with the sub-round nature of the fragments makes it difficult to propose theories explaining how these fragments were emplaced. Keldysh (1979) concludes that the lander rests on a talus slope near the base of some tectonic rise . Many of the rocks at this locality are unburied and in physical contact with their neighboring fragments, unlike the Viking sites. Also , quite a few of the fragments appear to be pieces of each other. These factors along with the significance of planar fractures at the site suggests the Venera 9 lander could have landed in a blocky basalt field or on an ejecta deposit from a nearby crater. The average roundness and form ratio values for Venera 9 agree with results obtained by Dobkins and Folk (1970) for basalt pebbles and cobbles on beaches. In general, as roundness increases, sphericity decreases and rocks become more elongate (Krumbein, 1941 b). The sub round and somewhat elongate nature of the rocks at Venera 9 on Venus demonstrates this agreement. Abrasion of rock fragments by the viscous, dust-laden venusian atmosphere could be similar in effect to the action of surf on pebbles in terrestrial beach environments. The Venera 10 locality is almost devoid of rock fragments, and hypotheses concerning the mode of emplacement for the tabular rock bodies visible in the panorama are not attempted here.

3.3. MODIFICATIONS The rock morphologic features most useful in identifying the processes responsible for modifying rocks on planetary surfaces are rock form, roundness, and the presence or CHARACTERIZATION OF ROCK POPULATIONS ON PLANETARY SURFACES 385 absence of erosional markings such as pits, flutes, and grooves. Obstacle scours such as tails and moats are also valuable in this regard. Erosion of rock surfaces by fluid-borne particles (dust, sand, or other rocks) manifests itself in different ways by altering the rock shape, and by creating erosional marks often indicative of the nature of the fluid­ flow regime. Solution :-veathering can also result in certain erosional markings, but will never produce obstacle scours. The changes in form, roundness, and sphericity have been discussed above, so attention is given here to the morphologic features. Flutes are erosional features on rock surfaces caused by particle-laden currents of wind or water on Earth. Such features hint of wind or water weathering processes on Mars by their high frequency of occurrence on martian rocks. Objective clustering of the morphology data for the VL-1 and VL-2 fragments has demonstrated their signifi­ cance. Differences in the style of flutes between VL-1 and VL-2 (i.e., rill vs wedge­ shaped) suggests different ages of exposure for the rocks at these localities, and may also reflect varying degrees of solution weathering. However, from the mere presence of flutes on large subgroups of martian rocks at two sites separated by 6000 km, we conclude erosional processes on Mars operate on a global scale. We believe the flutes to be strong evidence for wind erosion because of the following factors: (1) The current lack of running water on Mars means wind is the only available fluid for transport of the particles (dust) necessary to erode flutes; (2) flutes are seen on lee surfaces of rocks ; (3) flutes are most common on rocks with bimodal or heterogeneous cavity distributions; ( 4) the style of flutes on martian rocks is unlike water-carved flutes seen on Earth (Maxson, 1940); (5) flutes at VL-1 most frequently occur on compact rocks; Sharp (1949) observed this same relationship with wind-fluted boulders from Wyoming. Other features support the hypothesis that wind erosion is the primary modifying agent at the rock-scale on Mars. The abundance of tails and moats around VL-1 pebbles provides further evidence for an active eolian modification regime on Mars. Rock morphologic evidence for wind-flow related features at both VL-1 and VL-2 (such as flutes, grooves, deep pits) is strong; one need not assume the rocks were originally vesicular to explain these markings. Likewise, solution weathering of large phenocrysts or inclusions in rocks at the Viking sites does not explain why there is no evidence (at high resolution or 1 mm/pixel) for these inclusions in any rocks. However, morphologic evidence for solution (i.e., water assisted) weathering at VL-1 and VL-2 exists. The sinuous depressions seen on some VL-1 rock fragments (as well as the deep, terraced cavities occurring on several VL-2 rocks) supports this hypothesis. On Earth such features are characteristic of solution weathering assisted by wind-related removal of material. Note, however, that the wedge-shaped flutes common on VL-2 rocks are not associated with solution weathering in terrestrial environments. Facets are another form of evidence for weathering at the rock-scale. limestone fragments can be faceted by both solution and wind in terrestrial arid regions (Maxson, 1940). The facets seen on many VL-1 rocks morphologically resemble wind-carved 386 J. B. GARVIN ET AL. facets (Whitney and Dietrich, 1973). Facets on several VL-2 rocks, however, appear to reflect fractured pieces of columnar joints (see Figure 5, rock 1), and probably are not related to wind sculpture. Differences in the styles of facets between the two Viking lander sites on Mars demonstrates there are fundamental variations in the rocks at the two sites, at least in terms of how they are modified by eolian processes. It is likely the strength of the eolian regime at VL-1 exceeds VL-2, but neither site is in equilibrium at present. From the limited data set for Venus, it is evident that fewer venusian rocks are sub­ angular or possess wind-related features than on Mars. Also, the overall rock shape for the venusian sites as measured by the form ratio(- 0.17 to 0.45) implies there are differ­ ences between the two localities and between Mars and Venus. These relationships appear to indicate basic variations in the erosional regimes or modes of rock emplacement on the two planets, or a primary difference in rock lithologies. These speculations demonstrate that while differences in rock types from two sites on Mars and two sites on Venus appear to exist, similarities in weathering and emplacement processes across all of these localities can be identified. Consequently, we conclude that a combination of rock morphology and morphometry data can be used to discover rock modification process characteristics on different planets, and that a classification scheme for rocks based on such data can be devised . Detailed studies of terrestrial block fields are currently in progress with the objective of permitting a more detailed comparison between Mars, Venus and Earth.

Acknowledgements

This research was supported by NASA Grant NSG-7569 of the Mars Data Analysis Program. The authors gratefully acknowledge Sam Merrell for his excellent photographic work, Ed Robinson for his programming support, Farouk El-Baz for helpful discussions concerning desert analogs, R. Dietrich for valuable advice on ventifacts, and Alton Brown for his ideas and useful critiques. Also, we wish to thank Paul Lucey, Dave Grinspoon, Mark Cintala, and Paul Helfenstein for comments, advice, and criticism. Special thanks go to Nancy Christy and Sally Bosworth for their outstanding manuscript preparation. The Venera 9 and 10 photography was provided by the National Space Sciences Data Center.

References

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