the cameras aboard the venera spacecraft were digital fac- · 2013. 1. 11. · 3382 garvin et...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B5, PAGES 3381-3399, MAY 10, 1984

Venus' The Nature of the Surface from Venera Panoramas

JAMES B. GARVIN, JAMES W. HEAD, MARIA T. ZUBER, AND PAUL HELFENSTEIN

Department of Geological Sciences, Brown University

Images of the surface of Venus obtained by the Soviet Venera 9, 10, 13, and 14 landers have been analyzed to provide a basis for understanding the nature of geologic processes operating there. The four spacecraft landed in the Beta-Phoebe region at median elevations in the upland rolling plains province. The landing points are each separated by distances of more than a thousand kilometers. The Venera panoramas were digitized and transformed into various perspectives in order to facilitate analysis and comparison with other planetary surfaces. Bedrock is exposed at the Venera 10, 13, and 14 sites and is characterized by semicontinuous, flat polygonal to subrounded patches up to several meters in width. The bedrock surface is often dominated by subhorizontal to horizontal layered plates with thicknesses of several centimeters and abundant linear and polygonal verticai fractui'es. Angular to subangular layered to platy blocks in the 5- to 70-cm range dominate the Venera 9 site and occur much less frequently at the other sites. Blocks appear to share many characteristics with the exposed bedrock and are interpreted to be largely derived from it. Soils (particles < 1 cm) are abundant at the Venera 9, 10, and 13 sites but are uncommon at Venera 14. Features indicative of a strong eolian influence (moats, dunes, wind tails) are not observed. A striking aspect of the Venera landing sites is their extreme similarity despite separation distances of thousands of kilometers. Several hypotheses are considered for the origin of the bedrock surfaces, and we investigate in detail the hypothesis that bedrock originated from surface lava flows. In this interpretation, the broadly platy nature of the surface is analogous to the rolling and undulating nature of terrestrial pahoehoe flows caused by the formation and deformation of a semisolid crust. The layering is interpreted to be formed by a combination of upper thermal boundary layer formation and horizontal sheets formed by cooling and sheafing during flow emplacement. Vertical fractures are at- tributed largely to joint patterns formed during cooling. This interpretation made on the basis of surface morphology is consistent with Venera 13 and 14 geochemical results which reported high potassium basalt and tholeiitic basalt compositions, respectively. If this interpretation is correct, large regions of the Beta-Phoebe area are likely to be characterized by lava flows. The relative freshness of features observed by Venera 14 suggests that some bedrock surfaces are geologically young or that erosion rates are low.

1. INTRODUCTION

Images obtained by landed spacecraft and astronauts on the moon [-Surveyor Investigation Team, 1969; Vinegrader, 1971; USSR Academy of Sciences, 1966, 1969; Swannet al., 1972; Muehlberger et al., 1972], Mars [Mutch et al., 1976a, b; Garyin et al., 1981a, hi, and Venus [Florensky et al., 1977a, b, 1982a, b] have provided fundamental information about planetary surfaces at scales from milllimeters to decameters. This infor-

mation provides a means of (1) documenting the types of geologic materials on the surface, (2) understanding the geologic processes operating to form and modify planetary surfaces, (3) clarifying the nature of geologic features and processes interpreted from orbital spacecraft images obtained at lower resolution, and (4) establishing a physical basis for the understanding of the behavior of incident electromagnetic radiation (e.g., radar) utilized in remote sensing of planetary surfaces. The purpose of this paper is to analyze the information collected by the Soviet Venera 9, 10, 13, and 14 lander spacecraft imaging systems in order to allow a systematic comparison with other planetary surfaces and to provide an improved understanding of the surface of Venus. Plate 1 displays the locations of the Venera sites on a map of the topography of the Beta- Phoebe region on Venus. Table 1 summarizes details of the Venera lander missions. In order to facilitate morphologic and morphometric analysis of surface features, the Venera images were transformed into various perspectives, including those utilized in the analysis of the martian surface by the Viking landers [Garvin et al., 1983a]. The array of images is described in terms of three basic characteristics: bedrock, fragments, and

Copyright 1984 by the American Geophysical Union.

Paper number 4B0162. 0148-0227/84/004 B-0162505.00

soil. Comparisons are then made with the surfaces of the moon• earth, and Mars to provide a basis for the interpretation of the surface of Venus.

2. IMAGING SYSTEMS AND TRANSFORMATION METHODS

The cameras aboard the Venera spacecraft were digital fac- simile scanning telephotometers [Florensky e t al., 1977b; Keldysli, 1979; Moro•, 1983; Bokshteyn et al., 1983]. The cameras were located about 90 cm above the base of the spacecraft and were pointed 50 ø downward from the horizontal plane of the spacecraft. The imaging system scans in vertical sweeps of 40 ø and views 90 ø to the left and right of the subcamera point on the surface in front of the spacecraft. The camera rotates about a fixed axis producing an inclined cylindrical projection. This particular camera orientation was used in order to allow both the near-field and the far horizon to be imaged using a single, fixed viewing geometry. This viewing geo.rnet•y results in the apparently inclined horizon visible at the far left and right of the unrectified panoramas (Figures 1-3). Objects in the near field directly in front of the lander impact ring are relatively undistorted, while objects nearest the horizon are stretched by a factor of 2-3 in the horizontal direction. Vener- as 9 and i0 were each equipped with cameras on one side of the spacecraft, while Veneras 13 and 14 were each equipped with two similar imaging •systems, located 180 ø apart. For Veneras 13 and 14 the 180 ø azimuthal COverage for each camera system allowed small regions of overlap in the lower corners of the panoramas so that, when rectified, a continuous 360 ø view of a narrow portion of the surface of Venus can • produced. Veneras 9 and 10 obtained black and white images only, while Veneras 13 and 14 obtained black and white and color images •Florensky et al., 1982a, hi. The dimensions of various spacecraft parts are listed for scale in Table 2.

3381

3382 GARVIN ET AL.' VENUS SURFACE FROM VENERA PANORAMAS

TABLE 1. Summary of Venera 9, 10, 13, and 14 Lander Mission Results

Characteristic Venera 9 Venera 10 Venera 13 Venera 14

Location 31.7 ø, 290.8 ø 16.0 ø, 291.0 ø -7.6ø, 303.5 ø - 13.2ø, 310.1ø (latitude, longitude)*

Landing date Oct. 22, 1975 Oct. 25, 1975 March 1, 1982 March 5, 1982 Surface operation time,• 50 44.5 100 59.5

min

Mean elevation,$ 2.1 +_ 0.4 1.5 _ 0.6 1.4 _ 0.3 1.0 _ 0.3 (kin > 6051) (mean ñ s.d.)

Mean roughness,$ 4.0 ø _+ 1.3 ø 3.1 ø +_ 0.8 ø 3.0 ø +_ 0.3 ø 2.9ø - 0.7ø deg rms

Mean reflectivity$ 0.10 +_ 0.03 0.10 +_ 0.03 0.15 _+ 0.08 0.14 _ 0.04 Model density, g/cm 3 1.9 1.9 2.6 2.4 Dielectric constant 3.8 3.8 5.2 4.8

Surface albedo 0.03-0.12 0.02-0•04 0.03-0.085 0.04-0.11 Surface pressure, atm ,-, 90 91 89.5 93.5 Average wind speed, ms-x 0.4-0.7 0.8-1.3 0.3-0.6 0.3-0.6 Solar zenith angle 33 ø 27 ø '" 35 ø '" 35 ø Number of full panoramasõ 1 1 11 6 Surface temperature, K 728 737 738 743

*Error on specified coordinates is 1 ø to 2 ø. •'Length of time images were transmitted from surface. :]:From Pioneer Venus measurements for regions +_ 100 km from landing coordinates. õComplete (whole) panoramas (~ 180 ø in azimuth).

The resolution of the imaging system can be defined as the angle subtended by a single pixel on the near-field surface. For Veneras 9 and 10 the resolution was 0.35 ø (10 mm per line pair), while on Veneras 13 and 14 it was ,•0.18 ø (4-5 mm per line pair in the near field). This can be compared to the Viking lander survey mode resolution of 0.12 ø (,• 3 mm per line pair) and the high-resolution mode of 0.04 ø (,• 1 mm per line pair) [Mutch et al., 1972; Huck et al., 1975].

In order to understand better the spatial relationships and morphology of the objects in the Venera panoramas, different perspectives of the standard Venera scene (Figures 1-3) were generated and interpreted. Previous work with rotations of the Venera panoramas [e.g., Keldysh, 1979; Florensky et al., 1977a, c; Bokshteyn et al., 1983] applied a "rubber stretch" algorithm in order to produce a flat horizon or a view from above. The "rubber stretch" technique effectively stretches the pixels to generate some desired geometry (e.g., flat horizon) and does not involve remapping them mathematically into a specific projection. Our approach is to use a variety of known perspectives which specify the detailed geometry of the image and minimize distortion in areas of interest. Using this method, accurate quantitative measurements of parameters such as fragment sizes can be made. While the Venera camera- viewing geometry provides minimum distortion in the field closest to the subcamera point, it results in significant distortion in the far field. The Viking lander survey mode perspective provides a view of the surface of Venus with low distortion in the far field and generates a flat horizon l'Mutch et al., 1972; Huck et al., 1975]. We have produced Viking lander survey mode perspectives of all the Venera panoramas [Garvin et al., 1983a] (Figures 4-9).

In addition, an orthographic projection known as the polar gnomonic [Deetz and Adams, 1945] has been used for the near-field sections immediately adjacent to the lander ring (Figures 10-13). This projection furnishes a view from directly above the surface and provides correct spatial orientation of geologic features of interest (i.e., especially in the lander near field). This projection has been utilized with certain Viking lander images [Levinthai and Jones, 1980], and we have produced polar gnomonic versions of the near fields of the Venera

panoramas (Figures 10-13). The combination of perspectives provides us with a variety of views of the Venusian surface from which to make systematic geologic observations and per- mits measurements to be made of the sizes, shapes, and spatial orientations of critical features. Details of the panorama transformation algorithms are summarized by Garyin et al. [1983a]. The algorithms are based entirely on straightforward rotation matrix techniques [Devich and Weinhaus, 1980, !982]. Photographic images of the surface of Venus from Ven- eras 9, 10, 13, and 14 were provided by V. Barsukov and A. T. Basilevsky at the Vernadsky Institute in Moscow, and computer-compatible digitized versions of the photographs were produced by D. Chesley of the University of Massachus- etts using an Optronics digital scanner. Eight-bit (256 gray levels) digitization was used, and the brightness of each Venera image pixel was sampled at least once. All image transformations were done assuming a planar Venusian surface. A slightly inclined horizon can be seen in the Viking lander perspective due to the tilt of the spacecraft on the Venusian surface. The degree of spacecraft tilt has not been accurately determined. No image resolution degradation has been applied to any of the panoramas. McGill et al. [1983] have degraded Viking lander images to Venera 9 and 10 resolution (,• 1 cm) while transforming them to a standard Venera perspective. For our purposes the highest possible resolution was desired.

3. OBSERVATIONS

Utilizing the original and transformed Venera images (Fig- ures 1-13), a series of observations has been compiled on the surface characteristics of Venus at the four sites. Surface obser-

vations were subdivided into three categories related to bedrock, fragments, and fines/soils.

Bedrock is defined as continuous, coherent material underlying soils and blocks. Each panorama was examined to determine the shape and extent of the areas covered by bedrock, the topography of bedrock outcrops in the near field and on the horizon, the bedrock surface texture, structure, and relative albedo, and the nature of any layering evident in the bedrock. Fragments are defined as discrete blocks or particles

GARVIN ET AL..' VENUS SURFACE FROM VENERA PANORAMAS 3383

BEHEPR- 13; OISPAõOTKA cccP' UiKC

Fig. 1. Venera 13 landing site panorama, side A (penetrometer side), in original perspective prior to transformation. See Table 2 for details of spacecraft scale.

with diameters greater than 1 cm. The areal distribution of fragments and their shapes, sizes, surface textures, fracture patterns, structure, and relative albedo were noted. Fragmen- tal material with grain sizes less than 1 cm is classified as fines/soil. The areal distribution, relative albedo, and presence or absence on the spacecraft of this material was studied. Fin- ally, the relationship between bedrock, fragments, and fines/soil was examined. The basic observations are summarized in Table 3. Previous analyses of the Venera sites can be found in the work by Florensky et al. [ 1977a, b, c, d, 1982a, b, 1983a, b, c-I.

Venera 9

There is no unambiguous bedrock unit visible in the Venera 9 panorama (Figures 3 and 4), although there are a few far- field features and some apparently highly buried fragments suggestive of a bedrock component. Fragments dominate the Venera 9 scene, from small particles at the limits of resolution to 0.7-m boulders in the near field and perhaps larger ones in the far field. More than 50% of the region is littered with angular to subangular blocks, many of which are platy or tabular. A significant number of these blocks are polygonal in outline, and many display planar fractures. Some of the intermediate-size fragments (<30 cm) appear to be layered, while others have an apparently fluted surface. Dark spots in several of the more compact blocks appear to represent circular to subcircular cavities, possibly containing soil. Various degrees of burial are suggested by the fragment orientations. A possible fillet can be observed in one of the highly inclined fragments in the near field; such a feature could be due to fine materials displaced by the turbulent gas flow from the free-fall descent of the spacecraft. A few of the lower-albedo blocks appear to be highly inclined relative to the plane of the surface and could be buried to a significant extent. Other fragments are clearly perched on the soil or on other rocks. A somewhat

imbricate relationship is suggested by the spatial distribution of many of the fragments. The most fundamental observations with regard to the fragment population at Venera 9 are that (1) many fragments are angular to subangular and have polygonal outlines and planar fractures, (2) there is a variety of surface textures represented on the blocks: ridges, ledges, layers, and pits can be observed, (3) the size distribution is bimodal [Garvin et al., 1981b; Keldysh, 1979; Florensky et al., 1983b] with one group of blocks 1-10 cm in size and another 30-70 cm (there is no strong evidence for a continuous spec- trum of fragment sizes at the site), and (4) the spatial distribution is continuous (there are no obvious zones where fragments are much less (or more) abundant than the norm).

Below the limits of resolution, the fines/soil at the Venera 9 site may have diverse characteristics. This is suggested by the variable albedo of the soil distributed between the larger fragments. The lighting at the surface of Venus is diffuse, much like that of an overcast day on earth [Keldysh, 1979]. Soviet photometric measurements at this site [Moshkin et al., 1979] identified a possible dust spurt during landing which would suggest the presence of materials less than 100/•m in diameter [Garvin, 1981].

Both a near and far horizon appear to be visible on the left-hand side of the panorama. The horizon at right does not exhibit this appearance. Although the resolution of features in the far field is poor, blocks observed out to the near horizon are comparable in size to the largest in the near field. These larger blocks appear to be inclined much like some of the lower-albedo near-field blocks.

In summary, the large abundance of fragments (from just above the limits of resolution to blocks tens of centimeters in

dimension) dominates the Venera 9 site. The variable degree of burial of fragments throughout the Venera 9 scene, coupled with their generally polygonal and tabular appearance are important observations for the local area. Of all the Venera

BEHEP-.-Ft. ! 4 .. H ... Fig. 2. Venera 14 landing site panorama, side A, prior to transformation.


Fig. 3. Venera 9 (top) and 10 (bottom) landing site panoramas prior to transformation.

sites, Venera 9 is the only one with no obvious bedrock exposures.

l/enera 10

At the Venera 10 locality there is a significant bedrock component, covering 40-60% of the visible area (Figures 3 and 5). The exposures are semicontinuous with both sharp and sinu- ous edges. Most of the bedrock is generally flat-surfaced and often polygonal in outline with some subrounded surface features. The largest bedrock exposures are a few meters across. The surfaces are typically rough at the centimeter scale with linear and irregular depressions often filled with dark fine materials. Ledgelike features and promontories with up to 10 cm of relief are observed on some of the exposures. Bedrock topography in the far field may be up to a meter in relief.

The relative albedo of the bedrock exposures is generally higher than the fine mantling materials and the patches of soils and small fragments which separate the exposures. Some of the bedrock boundaries are very gradational with the soil patches. Such boundaries are often jagged and suggest an em- bayment or onlap relationship of soil zones onto bedrock. Small soil zones occur on bedrock exposures in what appear to be depressions or undulations in the bedrock surface. The distribution of soil patches and soil accumulations on bedrock surfaces is uneven and is related to the roughness of the bedrock. On the basis of these observations the soil deposits are inferred to be thin (perhaps tens of centimeters) and the bedrock is thought to be laterally continuous below the soil. Many of the low-albedo soil deposits are irregular in outline, while others display some orientation, perhaps due to a pattern of depression on the bedrock surface. The higher-relief bedrock exposures have a lower concentration of low-albedo soil deposits. The bedrock exposure at the middle right of the panorama (see Figures 3 and 5) has a pronounced ledge or curved scarp ("cuesta") on which a relatively small number of dark soil deposits can be observed.

Fractures and dark lineaments are a prominent feature of the Venera 10 bedrock. Fractures in the bedrock plates often define boundaries between adjacent plates. Most fractures are

linear and often either parallel or perpendicular to each other. Most linear features are lower in brightness than the bedrock they traverse due to the presence of dark, fine soil. Outcrops visible in the right-hand far field (Figure 5) display linear boundaries and edge relief of several tens of centimeters. The pervasiveness of these fractures and linear boundaries strongly suggests that blocks derived from the bedrock would very likely be angular to subangular and platy in form.

A fundamental characteristic of the Venera 9 site is a lack of

visible bedrock and the predominance of fragments. In con- trast, the Venera 10 site displays extensive bedrock exposures but a paucity of fragments relative to Venera 9. A few loose fragments can be recognized in the region nearest the lander ring as well as a few examples apparently embedded in the soil materials between bedrock outcrops.

The fine materials at Venera 10 are concentrated in large deposits between the bedrock exposures as well as in small deposits in surface depressions on the bedrock surfaces. They are lower in albedo than the bedrock, have particle sizes below the identification resolution of the camera (1 cm), and contain a coarse fraction made up of centimeter-sized fragments of a higher albedo. The soil patches of the intermediate field of the Venera 10 panorama generally resemble the soils at the Venera 13 site (Figures 6 and 7) in texture and areal

TABLE 2. Spacecraft Dimensions

Spacecraft Part Dimension

Outer diameter of lander ring Pentagon on lander ring

Color test chart

(inclined 30 ø from horizontal plane of spacecraft)

Yield strength "arm" (penetrometer) Small circular structure at end

Large circular structure Lander ring "teeth"

Camera heat shield (lens covering)

1.2 m

5 cm high 5.3 cm wide

8 x 8 cm squares

60 cm long 6.5 cm diameter

9.5 cm diameter

7 cm long 4.3 cm wide at base

20 cm diameter

GARVIN ET AL.' VENUS SURFACE FROM VENERA PANORAMAS 3385

Fig. 4. Venera 9 panorama in Viking lander survey mode perspective. This is a cylindrical Mercator projection such that the horizon should appear flat if the spacecraft is not tilted. In such a projection, distortion is minimized at the horizon and increases as one approaches the spacecraft (e.g., note the appearance of the circular base of the spacecraft). See text for further details.

distribution. Some of the fines at the Venera 10.site may have been perturbed by the spacecraft landing and carried aloft in a dust cloud that later settled in the near field [Garvin, 1981]. This suggests that the soil materials are loose enough to be mobilized by a gas flow resulting from wake turbulence due to the spacecraft landing (spacecraft terminal velocity of 8-10 m/s).

Summarizing observations of the Venera 10 locality, the most characteristic features are that (1) significant bedrock exposures cover up to 60% of the visible surface area, (2) there is a paucity of discrete fragments (those fragments that are observed are at least partially embedded in the interbedrock soil patches), (3) fractures and linear features are visible on bedrock surfaces and often display orthogonal or parallel relationships with other fractures, and (4) fine materials occur in

bedrock surface depression forming regular and irregular dark spots and patches.

Venera 13

A factor of 2 improvement in resolution for Veneras 13 and 14 over that of Veneras 9 and 10 (4-5 mm versus 10 mm per line pair) allows small-scale features to be better characterized. At this higher resolution it is possible to characterize in greater detail the coarse fraction (5-10 mm) of the particles that make up the fine/soil component. In addition, small-scale features such as pits, undulations, and possible clasts can be recognized. Figures 6, 7, 10, and 11 show the Venera 13 scene in various perspectives.

As at the Venera 10 site, there is a significant bedrock component at Venera 13, with up to 50% of the visible surface

Fig. 5. Venera 10 panorama in Viking lander survey mode perspective.


Fig. 6. Venera 13 (side A) panorama in Viking lander survey mode perspective.

consisting of semicontinuous, subhorizontal exposures, many of which are polygonal in outline. Layering of bedrock exposures is suggested by the characteristics of fragments and their relationship to bedrock. The bedrock displays a variety of surface textures, some of which suggest a regular pattern of depressions, ridges, or undulations on their surfaces. Most of the bedrock plates are subhorizontal, but in the far field there are examples which appear to be more highly tilted relative to the plane of the surface. The boundaries of the bedrock exposures are of two basic types similar to those observed at Venera 10: (1) sharp and often polygonal in outline, and (2) diffuse and gradational with soil zones separating exposures. These diffuse boundaries suggest a burial effect in which bedrock is mantled by a layer of fine materials several centimeters

thick. The sharp boundaries often display a few centimeters of reliefi

The bedrock exposures or plates at the site are up to a meter in extent and have textured surfaces reminiscent in form

of those at Venera 10 (Figures 5 and 6). In addition, 5-10 cm of surface topography can be observed on the exposures, manifesting itself in the form of cuspate scarps and a variety of surface depressions. The cuspate scarps resemble the cuesta- like scarp identified at the Venera 10 locality. The depressions on the bedrock surfaces are often filled with dark, fine material producing an apparently mottled surface. Several bedrock plates have surfaces with subparallel depressions filled with deposits of dark soil. Linear fractures also occur in abundance

on the bedrock surfaces but appear to be •.•.•..?e irregular in

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Fig. 7. Venera 13 (side B) panorama in Viking lander survey mode perspective.


:.

Fig. 8. Venera 14 (side A) panorama in Viking lander survey mode perspective.

extent and orientation than those at Venera 10. The overall

roughness of the bedrock surfaces at Venera 13 is comparable to that of Venera 10, with perhaps a slightly lesser amount of surface texture and small-scale topography.

Unlike Venera 10 there is a significant component of frag- mental material at the Venera 13 locality. The fragments at the site range in size from centimeter scale to boulders up to 0.5 m in length. The size distribution of fragments is clearly bimodal with the pebble fraction (1-5 cm) dominating the boulders (tens of centimeters) in number. The albedo of most of the fragments of all sizes at Venera 13 is comparable to that of the unmantied bedrock exposures and is higher than that of the dark, fine materials that compose the soil. Unlike the Venera 9 locality, the blocks at Venera 13 are clustered in isolated piles rather than providing a continuous cover of the surface. The smaller fragments are more continuously distributed and can be found on top of bedrock exposures, mixed

with the soil, and on the lander ring. The highest concentration of the pebble fraction occurs in an annulus around the lander ring. Many of the pebbles nearest the lander ring are unburied, perhaps due to effects associated with the spacecraft landing. However, most pebbles in other areas are at least partially buried.

In terms of general morphology, the larger fragments are predominantly platy in form, have angular to subangular outlines, are often layered, and display a tendency to fracture in a planar manner. Many of the larger fragments are strikingly similar in form to those seen at the Venera 9 site. The larger fragments appear to be distributed in clusters related to bedrock exposures at Venera 13. Surface textures similar to those visible on bedrock surfaces are typical of such large fragments. The smaller fragments are generally in the pebble size range and are more rounded in outline than the boulders, although a few are polygonal in outline like their larger counterparts.

Fig. 9. Venera 14 (side B) panorama in Viking lander survey mode perspective.

3388 GARVIN ET AL.' VENUS SURFACE FROM MENERA PANORAMAS

Fig. 10. Venera 13 (side A) polar gnomonic perspective (plan view) of near field. This is a projection from directly above the spacecraft and provides minimum distortion in the immediate near field. As one moves farther from the spacecraft base, distortion increases, so that only the near field is presented. See text for further details.

Many of the smaller fragments are somewhat platy. The pebbles are not preferentially distributed in piles and do not appear to relate to bedrock exposures. The ubiquity of the smaller fragments in the near field of the panoramas and the observation that a few of them were displaced up onto the lander ring (10 cm high) suggest that spacecraft-landing-

induced effects may have locally modified the pebble distribution at the site. Away from the near field, it is apparent that the smaller fragments are well mixed with the lower-albedo soils and are not as highly concentrated on the surface of the soil. In this regard, they appear similar to the small fragments in the interbedrock soil areas at Venera 10.

Fig. 11. Venera 13 (side B) polar gnomonic perspective (plan view) of near field.


Fig. 12. Venera 14 (side A) polar gnomonic perspective (plan view) of near field.

Evidence for in situ breakdown of large fragments and bedrock plates is widely observed at the Venera 13 site. In certain cases, several fragments can be pieced together into a single larger one (Figure 7, right-hand side; Figure 6, left-hand side). This "jigsaw puzzle" effect was observed to a lesser degree at the Venera 9 locality [Florensky et al., 1977c]. A few partially buried rocks appear to be significantly inclined relative to the

plane of the surface, as was observed at Venera 9. At least two layers can be discerned in several of the larger blocks, but layering cannot be observed in the pebble-sized fragments, possibly because the pebbles are derived from single, centimeter-thick layers [Krumbein and Sloss, 1953]. Several of the smaller fragments display curvilinear outlines.

The centimeter to meter scale roughness of this locality is

Fig. 13. Venera 14 (side B) polar gnomonic perspective (plain view) of near field.

3390 GARVIN ET AL.' VENUS SURFACE FROM MENERA PANORAMAS


TABLE 3. Observed Characteristics of Venera Lander Sites

Site Bedrock Fragments (> 1 cm) Fines/Soil (< 1 cm)

Venera 9

Venera 10

Venera 13

Venera 14

No unambiguous evidence for bedrock exposure.

Covers 40-60% of surface.

Exposed as semicontinuous, generally fiat, subrounded to polygonal patches up to several meters in width.

Surface rough; in near field, 5-10 cm of topography, up to a meter in background.

Surface texture is pitted and also con- tains a cuspate scarp.

Linear and orthogonal fractures.

Covers 20-50% of surface.

Exposed as semicontinuous generally fiat polygonal to subrounded patches.

Surface rough in near field, bedrock plate edges and surface shows 5-10 cm of topography; pits and shallow linear depressions.

Several rounded, elongate, and cuspate scarps.

Linear fractures.

Surface has a somewhat layered appearance.

Covers almost 100% of surface.

Exposed as continuous areas of interlocking, generally fiat, polygonal- plates.

Surface is rough at scale of centimeters.

Surface structure dominated by subhorizontal to horizontal layered plates with thicknesses of several centimeters; some layers show different albedo, with uppermost layers darkest;uppermost plate in near field has hole or window re-

vealing underlying layer; some surface sublayers show tonguelike overlaps.

Surface textures include pitting, wavi- ness, and elongate, cuspate scarps.

Abundant linear and polygonal fractures.

Abundance of angular to subangular layered and platy blocks in the 5-70 cm range.

Several blocks have elongate, rounded ridges and other undulatory surfaces.

Some blocks polygonal in outline; some are steeply inclined relative to horizon.

Finer fraction (1-5 cm) distributed in interblock areas.

Only a few discrete fragments > 5 cm. Located in areas dominated by fines,

not on bedrock.

Visible fragments in soil grade into roughness at the scale of resolution in intermediate and far field.

Larger fragments are angular to subangular, layered and platy; locally distributed in patches associated primarily with bedrock. Many fragment boundaries can be related to adjacent bedrock fractures.

Smaller fragments are located on and within soil/fines patches and only occasionally on bedrock. Occur in two modes: 1) in an annulus surrounding the lander ring. The most rounded particles occur here; fragments lie on top of soil layer; 2) in soil patches between bedrock exposures. Particles mostly partly buried.

Only a few discrete blocks > 10 cm. One 50-cm block with layered/striated

texture.

Fragments are angular to subangular; several can be geometrically fitted into adjacent bedrock.

In far field, fragments appear in local patches between extensive flat platy bedrock exposures.

In near field, smaller fragments have two modes of occurrence:(1) in depressions and fractures in bedrock, and (2) distributed around lander ring on arm side of spacecraft.

Apparent bimodal distribution (fines below limits of resolution, coarse, •1 cm).

Few intermediate fragments between fines and 10-cm blocks.

Distributed between blocks, little evidence of fillets around blocks.

Fines distributed in extensive patches in low areas and in small patches on bedrock surfaces.

Fines lower albedo than bedrock.

Fines grade into small fragments in size.

Fines distributed in extensive patches between bedrock exposures and in local small patches in bedrock pits and depressions.

Fines lower albedo than bedrock. Bimodal distribution ?

Soil occurs on surface of lander ring on both sides of spacecraft and on lens cover.

Distinct paucity of fines compared to other sites.

Some local accumulations in fractures

in bedrock and in front of spacecraft on arm side.

Soil occurs on surface of lander ring only on arm side of spacecraft.

most strongly influenced by the accumulations of larger fragments and the bedrock exposures that are most inclined relative to the flat surface. In general, the bedrock topography appears comparable to that of Venera 10.

The fine materials at Venera 13 are generally lower in albedo than any of the other materials at the site and are distributed in soil patches between bedrock exposures, as is

the case for the Venera 10 site. In addition, fine materials have been deposited in depression on bedrock surfaces. The edges of bedrock exposures throughout the landing site are sometimes diffuse, suggesting a cover of soil on top of such bedrock. It is clear from the panoramas of the Venera 13 site that the fine materials ( < 1 cm) making up the soil patches are variable in size. Fine materials are also observed on the lander

3392 GARVIN ET AL.: VENUS SURFACE FROM VENERA PANORAMAS

ring on both sides of the spacecraft, and a change in the accumulation of dust on the lander ring as a function of time on the surface of Venus was also reported [Selivanov et al., 1982]. The fines on the spacecraft landing ring probably represent the dust fraction of the fine materials and are likely to be < 100 #m (0.1 mm) in diameter [Garvin, 1981]. A few pebbles can also be observed on the lander ring (Figure 7). These fragments are likely to have been transported to their present position by means of landing-induced effects such as turbulent eddies. This same effect may have caused transport of small fragments onto the soil and bedrock surfaces.

In summary, (1) the Venera 13 site is characterized by a fractured, somewhat platy bedrock surface very similar to that observed at Venera 10 and a bimodal distribution of fragments, (2) the larger fragments are concentrated in local mounds and are generally similar in morphology to fragments observed at the Venera 9 site, and (3) dark, fine materials mixed with small fragments occur in zones separating bedrock exposures, as observed at Venera 10.

Venera 14

The most striking feature of this locality is the predominance of bedrock, which occurs over nearly 100% of the surface visible in the panoramas (Figures 8, 9, 12, and 13). Dis- crete fragments and some fine materials at the boundaries of bedrock plates can be recognized amidst the continuous exposures of platy and layered bedrock. There are no continuous soil patches mantling the bedrock exposures as seen at Veneras 10 and 13. There are no extensive blocky coverings as seen at Venera 9 and no block clusters as seen at Venera 13.

The most abundant bedrock exposures at Venera 14 are typically interlocking subhorizontal polygonal plates which have variable surface texture. Bedrock surface textures include

shallow cuplike depressions or pits, elongate depressions re- sembling flutes or grooves, cuspate scarps somewhat similar to those at Venera 13 in morphology, wavy and linear undulations, and linear fractures often filled with low-albedo fine materials. Several areas of more irregularly textured bedrock occur within the regions of platy bedrock. In some cases the irregular texture is dominated by small lobate or tonguelike layers and a somewhat ropy texture (Figure 8, right-hand side). In other cases the irregular texture appears to be a series of small, slightly disrupted plates occurring between the larger, more continuous bedrock plates (Figure 9, left-hand side). The extensive bedrock plates at Venera 14 share many of the characteristics of bedrock exposures at Veneras 10 and 13.

One of the most distinctive aspects of the bedrock at Venera 14 is the presence of horizontal to subhorizontal layered plates with thicknesses of several centimeters. Layer thicknesses are variable, ranging from a few to tens of centimeters. Sub- layering can also be recognized. The most prominent example of layering occurs in the middle left of Figure 9 where a low- albedo layer clearly overlies a higher-albedo layer. A small hole in the low-albedo layer reveals the presence of the underlying layer and indicates the thinness and continuity of the upper layer. The lateral continuity of individual layers is diffi- cult to determine, but several extend over several meters distance. In numerous cases, bedrock layers are observed to overlap, particularly in the more irregularly textured bedrock regions, where tonguelike overlaps are sometimes observed (Figure 9).

Fractures in the Venera 14 bedrock are abundant and range from several extensive linear fractures similar to those seen at

Veneras 10 and 13 to a host of polygonal fractures which break up the surface bedrock into smaller in situ plates. In some cases, fractures appear not to extend through the uppermost layer. The extensive polygonal fracttire patterns at the scale of tens of centimeters are unlike the more widely spaced fracture patterns of Veneras 10 and 13. They may, however, be the type of fracturing that would aid in the production of the jigsaw-puzzle-like exposures at Venera 13 and the extensive polygonal blocks observed at Venera 9.

There are extremely few discrete fragments larger than 10 cm in diameter at the Venera 14 site. of the few fragments that can be identified, most are platy, angular to subangular in outline, and variable in albedo. One 0.5-m-long tabular block in the near field displays a layered or striated surface. In some cases, the smaller fragments can be geometrically fitted into adjacent bedrock. In general, fragments appear to cluster in local patches between platelike bedrock exposures. This is similar to the rock clusters observed at Venera 13. In the near

field, pebble-sized fragments occur either in depressions or fractures between bedrock plates or in a diffuse zone around the lander ring on the penetrometer side of the spacecraft (Figure 8). In the far' field, fragments appear predominantly in local patches between extensive flat platy bedrock exposures. In one 'case, a fragment dislodged from bedrock has exposed a high-albedo surface on the underlying layer and the underside of the dislodged fragment.

There is a distinct paucity of fine-grained material at the Venera 14 site relative to other sites. Low-albedo fines appear to be preferentially deposited in the linear and polygonal fractures which define bedrock plate boundaries. These fine materials have a grain size less than the resolution of the camera and probably represent a dust fraction, similar in properties to the finest materials at the other Venera sites. Some of this dust

material was perturbed by the spacecraft landing enough to be carried at least 10 cm off the surface and onto the lander

impact ring. In summary, the Venera 14 locality is characterized by (1)

the dominance of continuous, flat, multilayered bedrock exposures, (2) a paucity of fines and fragments, and (3) the variable albedo of bedrock layers with the uppermost layer in one area having a low albedo.

4. SUMMARY AND DISCUSSION

The following points summarize the geological observations derived from the Venera panoramas at Veneras 9, 10, 13, and 14 (this work and Florensky et al. [1977a, b, c, d, 1982a, b, 1983a, b, c]) and present additional observations which might be relevant to the interpretation of geological processes operating at each site.

Bedrock

Panoramas of the sites reveal that bedrock exposures dominate the Venera 14 site and comprise at least a third (Venera 10) to one half (Venera 13) of two of the other sites. This is truly remarkable considering that the landing sites are separated by several thousand kilometers. On the moon, bedrock exposures are almost unknown, and on earth they are relatively rare. On Mars, they are also likely to be uncommon [Kieffer et al., 1977], although some bedrock appears to be exposed at the Viking lander 1 site [Binder et al., 1977; Mutch et al., 1978].

The surface topography ranges from several tens of centimeters (Veneras 10 and 14) to possibly several meters (Venera

GARVIN ET AL.: VENUS SURFACE FROM MENERA PANORAMAS 3393

13). The surface morphology of the bedrock outcrop shows multiple plates with horizontal dimensions of centimeters to meters often bounded by vertical fractures and having a surface texture which includes layers, lobes, pits, flutes, and small arcuate ridges. The centimeter scale layering observed at Ven- eras 14, 13, and 10 is discontinuous and closely related to the platy nature of the surface. Some layers appear to be trunca- ted by others at low angles, but systematic, well-developed cross lamination is not observed. Potential processes to account for the layering include (1) depositional (sediments of erosional or pyroclastic origin, [see Florensky et al., 1983c]), (2) duricrust and/or autometamorphism [Florensky et al., 1983c], (3) sheeting due to unloading, and (4) lava flow due to primary flows, flow dynamics (lobes, deformation of plastic surface, crystal segregation, etc.), cooling history (cooling units), or some combination of these lava flow mechanisms.

Fractures are apparent in bedrock at all sites. They are vertical, form orthogonal and polygonal patterns, and, in at least one place at Venera 14, do not penetrate the uppermost layer. No evidence for lateral offset is seen. Potential processes to account for the fractures include (1) landing-induced (appears unlikely because of the pervasiveness of the fractures and the angularity of the blocks), (2) thermal cycling and des- iccation (appear unlikely at present because of relatively constant surface temperatures and lack of water (although these may be candidates in previous periods of the history of Venus)), (3) volatile loss or degassing of ash layers, (4) lava flow cooling (deformation of surface plates and longer-term cooling and production of joints), and (5) tectonic (fractures associated with stress release or active deformation).

Although the bedrock surfaces have distinctive morphologic features, there is no compelling evidence for the presence of clasts or grains distinguishable within the bedrock outcrops. Any clasts, grains, or phenocrysts would either be below the limits of resolution or indistinguishable from the matrix. If the fragments seen at the Venera sites were incorporated into a

,

deposit and lithified into bedrock, it is likely that they would be observed because of their size and distinctiveness. This

argues against the simple conversion (e.g., lithification) of observed erosional products into the type of bedrock seen at the Venera sites.

Fragments

Although some fragments (> 1 cm) are visible at all sites, the abundance is variable from site to site and within a given site. Fragments dominate the Venera 9 area and are sparsely distributed at Veneras 14 and 10. At Veneras 13 and 14 there

is evidence for redistribution of fragments around the lander ring from the influence of the spacecraft descent and landing. At Venera 14, additional fine fragments occur in fractures in the bedrock and in low complex areas between major bedrock plates. Small fragments occur within the soil patches between bedrock exposures at Veneras 10 and 13. Larger fragments can often be fitted into adjacent bedrock, as at Venera 14, or form small mounds where the fragments occur in a jigsaw puzzle fashion, as at Venera 13. In the latter case, the fragments appear to have formed in situ from bedrock.

The vast majority of the larger fragments at the Venera sites are platy, angular to subangular, show no evidence for discrete grains or clasts, are textured, and occasionally are layered. The similarity of fragments from site to site and the similarity to bedrock characteristics is striking. These similarities strongly suggest that the fragments are derived from underly-

ing bedrock and that most of those observed in the images have not been subjected to extensive transport. The similarities of Venera 14, 13, and 10 fragments to those observed at Venera 9, where no bedrock is seen, strongly suggest that the bedrock in the Venera 9 region is similar to that at the other Venera sites. In addition, the angularity of Venera 9 fragments indicates that the bedrock source is in the near vicinity of the landing site [Florensky et al., 1977a, b, c, d].

Fines

The fine materials (< 1 cm) have a low albedo and are abundant at Veneras 9, 10, and 13. They occur evenly distributed between blocks (Venera 9), as irregular patches in low areas between bedrock (Veneras 9 and 10), and in pits and patches on bedrock surfaces (Veneras 10 and 13). There is no evidence for major transport of fines by atmospheric processes (ripples, scour, wind tails, etc.), although such transport cannot be ruled out. Fines may be locally derived by the chemical and physical degradation of bedrock and fragments. The lack of fines at Venera 14, the landing site with the least degraded bedrock characteristics, and the abundance of fines at Veneras 10 and 13, where bedrock is more degraded, are consistent with the local derivation of a soil component.

Continuity Between Landing Sites

The Venera sites are separated from each other by thousands of kilometers (Plate 1), and the four sites are spread over an area comparable to the continental United States, the Tharsis region on Mars, or the largest of the lunar maria, Oceanus Procellarum. The similarity of bedrock, fragment, and soil characteristics at the sites is extraordinary. Any process proposed to account for the formation of bedrock must operate over extensive regions, at least in the Venusian plains.

Albedo Characteristics and Variations

The albedo of the materials at the Venera 9 and 10 sites is

low to very low, comparable to basalt [Florensky.et al., 1977a, b, c, d]. Veneras 13 and 14 appear to have a similar low albedo. At Venera 14, the uppermost centimeter-thick layer (Figure 9) in one area is characterized by a lower albedo than underlying bedrock and appears to be the only layer with a distinctively different albedo. On the basis of the presence of the "window" in this unit and the nature of the layer edges, the layer appears to be undergoing erosion. Since Venera 14 is the least degraded site, this low-albedo latter may represent the uppermost layer or layers of fresh, relatively uneroded bedrock.

In other layers at Venera 14, displaced blocks expose two types of surfaces. One dislodged block broken apparently at an angle to layering (Figure 8, right center) reveals the internal structure of a bedrock unit and shows the albedo to be com-

parable to the adjacent surface. Elsewhere in the same panorama (Figure 8, left side) two blocks have been dislodged parallel to layering and at least one has been overturned. These surfaces display a higher albedo than surrounding bedrock surfaces. If the three fragments were dislodged by spacecraft landing, then there is some evidence for a higher albedo along planes separating layers. If the fragments dislodged at different times, then surface modification effects may be important (e.g., fresh surfaces have a higher albedo but have the albedo lowered by exposure to the Venusian environment). In either case, these examples appear to differ from the low-albedo layer observed on the other side of the spacecraft.

3394 GARVIN ET AL..' VENUS SURFACE FROM MENERA PANORAMAS

Compositional Information

Gamma ray spectroscopy experiments on board the Venera 9 and 10 spacecraft provided information on the K, U, and Th contents of materials at the sites. Surkov et al. 1-1977a] and Florensky et al. 1-1977c, d] compare the values at Veneras 9 and 10 with terrestrial basalts. X ray fluorescence spec- trometer experiments on board Veneras 13 and 14 analyzed material drilled from below the lander ring and transported inside the spacecraft. Venera 13 results suggest a high- potassium basalt composition and Venera 14 a tholeiitic basalt composition l-Surkov et al., 1983, 1984; Barsukov et al., 1982].

Material Properties

Material properties of the substrate can be measured and estimated from a variety of observations. At Venera 10 the gamma ray densitometer yielded a density of 2.8 _ 0.1 g/cm 3 l-Surkov et al., 1977b] for bedrock, while observations of the influence of dropping the 2-kg gamma ray spectrometers (on a rock at Venera 9 and bedrock at Venera 10) indicate that the material is hard competent rock l-Florensky et al., 1977c]. Esti- mates of the dynamic tensile strength of the Venusian surface at Veneras 9 and 10 [Keldysh, 1979] indicate a wide range extending from 40 to 300 bars. This range includes materials ranging in tensile strength from welded tuffs (tens of bars) to massive basalts (over 200 bars). Values less than 10 bars are typical of soils. At Venera 13 the penetrometer appears to have impacted a bedrock outcrop or blocks. Surkov et al. [1984] report dynamic tensile strengths of 2.6-10 bars based on their interpretation of the penetrometer measurements. At Venera 14 a similar measurement in bedrock (perhaps influenced by the lens covering which fell beneath the penetrometer) was 65-250 bars. On the basis of the dynamics of the impact of the landing module, the values of 4-5 bars [Surkov et al., 1984] reported from Veneras 13 and 14 were suprisingly similar given that Venera 13 appears to have landed largely on soil and Venera 14 on bedrock. If the fragments surrounding the lander ring we•'e largely broken by the spacecraft landing, then Garyin et al. 1-1983b] find that the dynamic tensile strength of the fragment source rock is in the range of tens of bars and no greater than 100 bars. If the fragments were merely displaced by the spacecraft landing, then 100 bars is a lower limit. Values less than 100 bars are typical of weakly indurated rock material, while higher values represent more highly consolidated or crystalline rocks [Nafe and Drake, 1968]. It is important to note, however, that impact-related fragmentation and the possible effect of weathering could act to decrease the strength of near-surface layers. It is therefore unclear whether the strength of the sampled region is typical of Venusian bedrock. The above material properties measurements should thus be considered a lower limit.

Modification Processes at Venera Sites

A number of factors (fragments, soil, rounding of edges, exposure of layers, presence of an apparently erosional window, pitting of surfaces, etc.) point to the probability that the Venera sites are presently undergoing erosion of bedrock layers. At the same time, deposition of fragments and fine materials is taking place. The lack of any features related to atmospheric transport of material (ripples, dunes, wind tails, scour marks, etc.) suggests that these processes are not domi- nant in the panorama areas. The dark soils may thus be locally derived from the bedrock materials. At the Venera 10 and 13 sites, soil is concentrated in lows between bedrock

exposures. At least local transport is required to concentrate the soil in lows. Examination of the topography of the essentially soil-free Venera 14 site shows that if soil were to fill the low-lying areas, the site would be remarkably similar to Ven- eras 13 and 10 (compare Figures 4 and 5 to Figures 7 and 8). On the basis of the state of degradation (sharpness of features, abundance of soil, etc.) the Venera 14 site is believed to be the least degraded and Veneras 10 and 13 to represent more degraded bedrock terrain. The Venera 9 site is anomalously blocky in comparison to Veneras 10, 13, and 14 and does not appear to fit in a simple degradation sequence because of the angular nature of its blocks. The angularity and fresh appearance of the bedrock at Venera 14, combined with the lack of soil cover, suggest that the surface may be geologically young or that erosion rates are low.

Comparisons to Other Planetary Surfaces

On the moon, impact-generated regolith of several meters thickness dominates the surface. Only at the Apollo 15 site along the edges of Hadley Rille have surface photographs of bedrock been obtained. In this case, an 8-m-thick outcrop with 12 exposed layers of basaltic lava flows were photo- graphed along the far wall of the rille, approximately 1.5 km away. Several of the more massive layers (1-3 m thick) contain less well-defined internal layering or parallel banding. A number of thinner layers less than a meter thick occur together and separate the thicker layers from each other. The thinner layers weather out distinctively. Vertical to near-vertical fractures are observed. Some fractures cut through both massive and thin layers, while others terminate within the layered sequence sometimes at a thinner layer. Other outcrops display discontinuous thin layering or parting (averaging about 0.3 m) and rare columnar jointing l-Swann et al., 1972; Howard and Head, 1972]. Resolution restrictions preclude establishing the presence or absence of centimeter scale layering.

On Mars, several processes have apparently been active in production of the Viking 1 and 2 landscapes, including volcanism, impact, thermal cycling, catastrophic flooding, and eolian mechanisms I-Mutch et al., 1976a, b; Binder et al., 1977; Garyin et al., 1981a, b]. Some areas of bedrock are visible at Viking 1 in midfield views l-Mutch et al., 1978]. These are interpreted to be exposures of the Chryse basin lava flows [Binder et al., 1977]. The midfield views show a platy surface interrupted by linear scarps giving the bedrock exposure a steplike appearance. The linear scarps are interpreted to be the boundaries of steeply dipping fractures in the lava flows [Binder et al., 1977]. A diverse population of blocks is observed at both sites. The two Viking lander sites appear to be much more variable within a given site and between sites than do the Venera 10, 13, and 14 sites. At Venera 9 the 35% block cover is more extensive than either of the two Viking sites [Garvin et al., 1981b].

On earth, multiple geologic processes combine to produce complex geology at the scale of surface panoramas l-Garvin et al., 1981a, b; Garyin, 1982]. Some environments on earth, however, are regionally and/or temporally continuous enough to produce wide expanses of terrain dominated by a single process. These areas/environments include desert sand seas, areas such as Mount St. Helens dominated by pyroclastic ash flow/fall, regions such as Hawaii dominated by basaltic lava flows, the sedimentary basins of the deep sea, and others. The striking similarity of the Venera panoramas to each other and their separation distances measured in thousands of kilometers argue for the dominance of either a single geologic pro-

GARVIN ET AL.' VENUS SURFACE FROM VENERA PANORAMAS

Pahoehoe lavas (flood basalts)

Elephant Mountain Flow (Schmincke 1967)

Vesicular zone

Platy jointing

Small columns

hackly jointing

Vesicle

cylinders

Pipe vesicles

Buckled top

upper colonade

Abrupt zone

Entablature

Gradational zone

blocky jointing Basal colonade

Flow direction

Lower Yakima Flows (Diery-McKee, 1969)

Entablature

Colonade

Vesicular top

sheeting

Vesicular zone

horizontal tier

Fan jointing

Hackly to brickbat joint•ng

Undulating columns

Platy jointing

P•llow-palagonite breccia

Flow direction

Aa to Blocky lava flows

Aa basaltic andesite Blocky latite Obsidian

Extruded lava Blocks

Scoria Spinose

Abrupt Platy joint•ng Gradat•onal

Blocky to columnar Flow-layered jointing compact curved columns interior

Platy joint•ng Abrupt scoria, Abrupt breccia breccia

Breccia

(blocks and pumice)

Breccia lens

Contorted layering

Abrupt breccia

•] Flow direction ½-'-] Flow direction ••] Flow direction Fig. 14. Cross-sectional features of terrestrial lava flows [after Hammond, 1974]. Note buckled top and platy jointing of

pahoehoe flood basalts. See text for comparison with features visible in Venera panoramas.

cess (such as lava extrusion) or the presence of an atmospheric environment that would disperse materials (such as sediments or pyroelastics) in a widespread and even manner.

Regional Nature and Global Extent

The Vcnera 9, 10, 13, and 14 spacecraft landed in the Beta- Phoebe region EBazilevski et al., 1982] at elevations within a range of 2 km above mean planetary radius (Plate 1 and Table 1). On the basis of earth-based radar images and altimetry, Saunders and Malin [1977] and McGill et al. [1981] have interpreted the Beta region as a riff structure populated with shield volcanoes such as Rhea and Theia Mons. Recent Are-

cibo radar backscatter images (1.5-2.0 km radar resolution) (D. Campbell et al., unpublished manuscript, 1983) strongly support a volcanic origin for these features and have shown that flowlike structures extend for a distance of more than 400 km from the summit area of Theia Mons, down the flanks of Beta Regio. Garyin and Head [1983] have examined the

roughness and reflectivity values of the regions surrounding each of the Venera sites as measured by the Pioneer Venus radar experiment at approximately 100-km horizontal scales. They find a positive correlation between radar roughness and roughness as observed in the panora.mas. The smoothest site (Venera 14) is near the mean roughness for the planet. Thus large areas of Venus may be even smoother at this scale than the Venera 14 region. Reflectivity values [Pettengill et al., 1982] can be used to relate dielectric constant to bulk density [Krotikov, 1962; McGill et al., 1983]. Model densities for the Venera 9 and 10 regions are 1.9 g/cm 3, much less than the 2.8 + 0.1 g/cm 3 determined for bedrock at the Venera 10 site [Surkov et al., 1977b]. This suggests that the regional site values (• 300 km 2 areas) are lowered from the bedrock values by the inclusion of the less dense soil component observed over large areas in the panoramas. Venera 14, the site with a paucity of soil, has a model density of 2.4 g/cm 3 for the surrounding region. Thus 2.4 g/cm 3 may be a lower limit for the


Fig. 15. Lava flow exposures from the Snake River Plain (from Greeley and Kin•t [1977]; photos by J. King and J. Karlo). A blocky talus pile is exposed at the edge of a series of lava flows. Such blocky surfaces, morphologically similar to Venera 9, are common along vent and rille walls. Elsewhere in the image, analogs to bedrock plates and polygonal fractures observed in Venera panoramas can be seen.

rocks at Venera 14 if any soil is exposed in the approximately 300-km 2 area surrounding the site. Reflectivity values in the Venera 9, 10, 13, and 14 range are very common on Venus, although areas of higher and lower values exist.

5. INTERPRETATION

Interpretation of the geological processes responsible for the features observed in the Venera panoramas must account for the range of data outlined above, specifically the extreme similarities between sites, the compositional data, and the platy surfaces, layers, fractures, and structures of the bedrock units. The soil component at these sites is largely below the limits of resolution. For this reason and the fact that physical and chemical weathering processes on Venus are poorly under- stood, we emphasize here the interpretation of bedrock units and fragments.

On the basis of observations at Veneras 9 and 10, Florensky et al. [1977c] outlined six possible origins for the bedrock outcrops, fragments and fines:

1) surface lava extrusion; 2) igneous intrusion later exposed by erosion; 3) pyroclastic fall; 4) impact lithification; 5) sedimentary rock lithified at depth and later exposed by erosion; and 6) lithification (or metamorphism) of loose material by atmospheric action at the surface.

Florensky et al. tentatively concluded that "Hard material is occasionally formed at the surface, either by lithification [of fines] through atmospheric processes or by volcanic falls." Analysis of the Venera 13 and 14 panoramas by Florensky et al. [1983c] led them to conclude that the bedrock is of sedimentary origin, deposited in the past as clastic sediments (either erosional products from highlands or pyroclastic deposits) which may have been lithified into layers by a "duricrust" phenomenon. A major factor in this interpretation is the observed layering, its thinness, "fracturing which does not

cross bed boundaries ..... and pinching out of beds" [Flor- ensky et al., 1983c].

On the basis of our assessment, two possibilities seem most likely: (1) homogeneous layered tephra, and (2) thin, platy basalt flows. Previous analyses of pyroclastic volcanism on Venus [Garvin et al., 1982] have demonstrated the theoretical requirement for unusually high volatile contents (> 2 wt %) in magma in order to produce explosive eruptions in the present Venus environment. Thus, in this analysis we pursue the second hypothesis, that is, that the bedrock originated from surface lava flows. We therefore consider the possibility that the bedrock at the Venera 10, 13, and 14 sites has originated by the first of the processes suggested by Florensky et al. [1977c]. Specifically, we believe that the broadly platy nature of outcrops at Veneras 10, 13, and 14 may be comparable to the rolling and undulating nature of terrestrial pahoehoe flows caused by the formation and deformation of a semisolid crust. The rougher interplate areas appear analogous to deformation within and between plates to produce festooning and ropy texture, and pressure ridges with associated jumbled crust, squeeze-ups, and small lava tongues (see, particularly, the middle right-hand portion of Venera 14 (A), Figures 2 and 8). The jigsaw puzzle nature of the blocky mounds at Venera 13 suggests small platy pressure ridges which have been somewhat eroded (Figure 7). The extensive layering observed at the site is unlikely to be due to individual primary flows because of the thinness of layers. Calculations on lava flow cooling efficiency in the present Venus environment [Wood, 1979; Head and Wilson, 1982; Garvin et al., 1982] indicate that thin flows on the scale of observed layering would not be widespread. The layering is interpreted to be analogous to that layering commonly observed in terrestrial lava flows, which is a combination of an upper thermal boundary layer (crust) and horizontal sheets formed generally parallel to the top of the flow due to cooling and shearing during flow emplacement. Cross sections of a variety of lava flows (Figure 14) show the


Fig. 16. Exposure of a slab pahoehoe flow surface about 4000 years old in the Snake River Plain (from Greeley and King [1977]; photos by J. King and J. Karlo). Abundant fractured plates are reminiscent of bedrock exposures at the Venera 10, 13, and 14 sites.

nature and distribution of various types of horizontal layering (platy jointing, sheeting, and flow layering). Macdonald [1972, p. 93] describes the formation of sheets in block lava flows, noting that the

moving liquid higher up tends to separate into a series of sheets slipping over each other like a series of cards in a deck when the deck is bent. The same sort of motion (laminar flow) is present in aa and pahoehoe flows, but the separation into sheets shearing over each other is less pronounced. The movement of the sheets is predominantly nearly parallel to the underlying surface, and in solidified flows the shear surfaces are visible as planes of separation (joints) essentially parallel to the top and bottom of the flow. The sheets may be very thin. Sometimes they are only a fraction 'of an inch thick, and then the lava resembles the platy sedimentary rock, shale.

than the present terrestrial subaerial environment. The high Venusian upper crustal and surface temperatures would favor greater effusion rates and less efficient lava cooling, both of which are factors that would produce long extensive flows relative to comparable terrestrial eruptions [Wood, 1979; (;arvin et al., 1982; Head and Wilson, 1982; Wilson and Head, 1983]. Although many earth analogs [cf. Green and Short, 1971; Greeley, 1974; Greeley and King, 1977] can be found which show similarities to the Venera panoramas (Figures 15 and 16), we are presently concentrating on an analysis of the flow and cooling behavior of lavas with compositions of the Venera site materials, under Venus conditions, in order to provide a framework for further investigation of terrestrial analogs in the subaerial and subaqueous environment.

Macdonald further notes that occasionally shear planes may bend upward at the front or top of the flow.

Vertical fractures, or joints, are ubiquitous in lava flows (Figure 14) and are due primarily to tensional stresses associated with volume reduction due to cooling. The nature of jointing commonly changes at subunit boundaries, and vertical joints often stop abruptly at these boundaries (Figure 14). The patterns of joints may range from rectangular to polygonal and blocks derived from jointed flows are commonly angular in form. The extensive vertical fractures seen at Venera 14, and the shape of bedrock exposures at Veneras 10 and 13, are interpreted to be related to joint patterns. The angularity of blocks visible at Venera 9 may also be related to jointing.

Thus we find a number of strong morphological similarities between the bedrock and blocks at the Venera 9, 10, 13, and 14 sites and terrestrial basaltic lava flows. We believe that this

interpretation based on morphologic characteristics is supported by geochemical analyses indicating basaltic compositions, the low albedo of surface rocks, comparison to surface images of outcrops of known (moon) or suspected (Mars) volcanic origin, the high density, the extreme similarity between sites separated by thousands of kilometers, and the strong likelihood that extrusive volcanism has been a significant mechanism of heat transfer throughout the history of Venus [Solomon and Head, 1982; Morgan and Phillips, 1983], and in the Beta-Phoebe region [Saunders and Malin, 1977; McGill et al., 1981; D. Campbell et al., unpublished manuscript, 1983].

The present Venus environment is considerably different in terms of temperature, pressure, and atmospheric composition

Acknowledgments. The authors are grateful to Soviet Scientists at the Vernadsky Institute of Geochemistry (V. I. Barsukov and A. T. Basilevsky) and the Institute of Cosmic Research (L. V. Ksanfomality) in Moscow, who kindly provided the Venera photographs and neces- sary literature on the panoramic camera systems used in this study. Sam Merrell produced the transformed Venera panorama mosaics; Jeff Tingle wrote most of the software used to display the digitized Venera photographs. The manuscript was prepared by Mary Ellen Murphy. Helpful comments and suggestions by Richard Grieve, Mark Cintala, Peter Mouginis-Mark, Robert Sharp, R. S. Saunders, and Alan Peterfreund are gratefully acknowledged. Duncan Chesley of the University of Massachusetts produced the digital versions of the Venera photographs using an Optronics scanner. 'Special thanks to Harold Masursky of the U.S. Geological Survey for providing Plate 1 and to R. Greeley of Arizona State University for providing prints of Figures 15 and 16. This research was supported by NASA grant NSG-7569 for which the authors are most grateful. One of the authors (J.B.G.) was supported by a fellowship from the William F. Marlar Memorial Foundation.

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(Received July 28, 1983; revised January 10, 1984;

accepted January 17, 1984.)

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