The Surface of Venus As Revealed by the Venera Landings: Part II

The surface of Venus as revealed by the Venera landings: Part II

A. T. BASILEVSKY R. O. KUZMIN •The Vemadsky Institute of Geochemistry and Analytical Chemistry, the USSR Academy of Sciences, Moscow, USSR O. V. NIKOLAEVA A. A. PRONIN L. B. RONCA Department of Geology, Wayne State University, Detroit, Michigan 48202 V. S. AVDUEVSKY G. R. USPENSKY Institute for Space Studies, the USSR Academy of Sciences, Moscow, USSR Z. P. CHEREMUKHINA V. V. SEMENCHENKO V. M. LADYGIN Moscow State University, Moscow, USSR

ABSTRACT INTRODUCTION

Observations of the panoramic photographs transmitted by Ven- Part I of this research (Florensky and others, 1977a) described some eras 13 and 14 indicate that many visible rock units are layered and of the instrumentation and some of the results of the Veneras 9 and 10 that at least a portion of the fine material could be the result of in situ missions. Several interpretations were also presented. The purpose of this geomorphic disintegration. Chemical and material property experi- article is to describe some of the results obtained by Veneras 13 and 14 and ments indicate that the chemical composition of the material at the to expand or modify some of the interpretations. Figure 1 shows a locator landing sites is essentially basaltic, that the bearing strength is minimal map of all the landings. Preliminary determinations of the coordinates are (a few kilograms per square centimetre, or a few hundred kilopascals), lat. 7°30'S and long. 303° for Venera 13, lat. 13°15'S and long. 310°9' for that the density is less than 1.5 g/cm3, that the porosity is very high Venera 14. (>50%), and that the electrical resistivity is very low (<90 ohm • m). The television system of Veneras 13 and 14 was an improved version The discussion on the possible geological nature of the surface of of the system used on Veneras 9 and 10, with the difference that, for Venus presented in Part I (Florensky and others, 1977a) is extended in Veneras 13 and 14, each craft had two cameras pointing in opposite light of these new data. Six possible origins of the rock units were directions. For a description of the geometry of the field of view, of the discussed: (1) surface lava extrusion; (2) igneous intrusion, later ex- method of recording, and of the telemetry and enhancement techniques, posed by erosion; (3) pyroclastic fall; (4) impact ejection and lithifica- the reader is referred to Part I. The resolution of the photos of Veneras 13 tion; (5) sedimentary deposits llthified at depth and later exposed by and 14 (-1/5°) is considerably better than that of Veneras 9 and 10 erosion; and (6) surface metamorphism due to unique Venusian sur- (—1/3°). In addition, the cameras of Veneras 13 and 14 were able to face conditions. It is concluded that, although none of the above hy- perform several scans, allowing better computer enhancements. Some of potheses can be conclusively proven or disproven, hypothesis 6 is to be the pictures were obtained using three color filters, but the final processing advanced to the forefront. This is suggested by (1) the low density and has not yet been completed. low bearing strength, (2) the presence of layering, (3) the possible Veneras 13 and 14 also carried instrumentation for geochemical and presence of sedimentary structures in the rock units, such as cross- material-property experiments. This paper will report only the results of bedding and ripple marks, (4) the low electrical resistivity, which may the geochemical experiments and will present the material-property exper- indicate chemical alterations of surface material, and (5) the albedo of iments in more detail. the loose, fine material, which is lower than the albedo of the rock units. This again suggests chemical alterations. DESCRIPTIONS AND OBSERVATIONS Within the context of this hypothesis, the rock units are sedimentary or sedimentary-volcanic. Lithification occurred in the past due to Venera 13 Panoramas surface chemical effects. The rock units, analogous with a similar terrestrial phenomenon, can be referred to as "duricrusts" (with the Figure 2 shows the computer-enhanced panoramas. Close to the craft, understanding that the Venusian process is completely different from one can see slab-like rocks of relatively high albedo separated by loose the terrestrial process). Presently, the rock units are undergoing disin- material of lower albedo. This is due to contrast enhancement, as both tegration, indicating that changes have occurred in the surface condi- units have very low albedo (slabs, 5 to 9%; loose material, 3 to 5%) tions at the landing sites. (Selivanov and others, 1983). Scarplets with heights of a few centimetres

Geological Society of America Bulletin, v. 96, p. 137-144, 9 figs., 3 tables, January 1985.

137

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Fractures are visible, especially in the isolated slab on the left of the upper photo. Evidence of layering can be found in several places (on the right side of the lower photo and in the bandings of the distant outcrops). In some cases, these bandings resolve into scarplets. On the whole, the rock units appear to be similar to the units photographed (at lower resolution) by Venera 10. The loose material is confined to areas between the rock units and on fractures and depressions on the rock units. The largest fragments are platy and are angular in shape. Fragments smaller than ~5 cm are somewhat rounded. Closer to the supporting ring, there is a narrow zone (—0.5 m wide) where coarser fragments appear to be more common than they are elsewhere. Large blow-ups of the photos suggest that this is a reiil effect and not a resolution-range effect. Figure 3 displays the cumulative diameter distribution of the fragments in a far field zone (curve 1) and in the two near field zones (curves 2 and 3). In the upper right corner, the per:entagc of the surface area covered by 2- to 10-cm fragments in a far field zone (column I) is compared with the same percentage in a near field zone (column II). The cumulative distribution of the fragments observed by Venera 9 is also included, although the poorer resolution of Venera 9 photos makes the comparison problematic. There is essentially no difference, at large diameters, between near and far fields, and there is an excess of small particles in the near fields. Presumably, the smaller particles in the far fields are still covered with dust which has been raised by the landing in the near fields. This is also suggested by the spectrophotometer (Moroz and others, 1982), which recorded a cloud of dust that produced a deposit 270° 280° 290° 300° 310° 320° 330° 340° seen in places on the surface of the supporting ring (a similar phenomenon Figure 1. Locator map of the Venera landings. They are located was observed by Veneras 9 and 10). on the eastern flank of an elevated belt consisting of the Beta and Phoebe Region««. Venera 14 Panoramas

can be seen. The upper surface of the rock units is subhorizontal, and Figure 4 shows the photos taken from Venera 14. The landing terrain centimetre-scale undulations are apparent (to the left of the trellis girder is composed mainly of the subhorizontal upper surfaces of rock units. and above the lens cover in the upper photo). Apparent striations occur on Unlike the landing sites of Veneras 9, 10, and 13, this site does not have the surface of the slabs (above and to the upper left of the lens cover in the any significant mantle of loose, fine material. Small accumulations of loose lower photo). material are visible in discontinuous shallow depressions, especially at the

Figure 2. Venera 13 panoramas. The teeth in the center belong to the supporting ring ;uid are 5 cm apart. The photometric color standard on the right is 40 cm long. The trellis girder (center-left of upper photo) is 60 cm long, and the view-port cover (lower photo) is 20 cm in diameter and 12 cm in height.

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Figure 3. Cumulative distribution of the fragment sizes, in Venera 13 photographs, located in the two zones next to the supporting ring (Curves 2 and 3) and in more distant zones (Curve 1). The insert (upper right) represents the percentage of the surface area covered by 2- to 10-cm fragments in the close-to-the-supporting-ring zones (Column II) and in distant zones (Column I). The difference is explained by disturbances caused by the landing. This indicates the presence of fines in the loose material, easily blown away by air currents.

edges of platelets. As in the case of Venera 13, fragments larger than ~5 cm in horizontal dimensions are platy; smaller ones are subrounded. The rock units are layered, and the layers are thin (3 cm or less) in many cases. This is apparent in the upper center of the upper photo and on the left side of the lower photo. The layers are intersected by arcuate and irregular linear fractures.

Other Experiments

Observations by Veneras 9 and 10 were made to determine the concentrations of potassium, uranium, and thorium. The findings indicate that the surface material is close in composition to that of terrestrial magmatic basic rocks (Surkov and others, 1976). Veneras 13 and 14 had a sophisticated geochemical laboratory (X-ray fluoresence spectrometer), the results of which are given in Table 1 (Barsukov and others, 1982). The surface material at the landing sites of Veneras 13 and 14 is also close in chemical composition to that of terrestrial magmatic basic rocks. Specifi- cally, for Venera 13, there is a similarity to alkaline melanocratic basal- toids; for Venera 14, the resemblance is to tholeiitic oceanic basalts (Barsukov and others, 1982). Determination of the physical and the mechanical characteristics of the surface material in the landing sites has been made from the impact dynamic loading data, recorded as the descending spacecraft made contact with the surface, and also with the help of the trellis girder (Avduevsky

Figure 4. Venera 14 panoramas. See Figure 2 caption for size of objects.

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TABLE 1. COMPOSITION OF THE MATERIAL AT THE LANDING SITES (wti)

Oxide Venera 13 Venera 14

Si02 45.1 ± 3.0 48.7 i 3.6 rio2 1.59 ± 0.45 1.25 ± 0.41 ALJOJ 15.8 ±3.0 17.9 ± 2.6 Fe in the form of FeO 9.3 ±2.2 8.8 ± 1.8 MnO 0.2 ±0.1 0.16 ± 0.08 MgO 11.4 ±6.2 8.1 ±3.3 CaO 7.1 ± 0.96 10.3 ± 1.2

K2O 4.0 ± 0.63 0.2 ±0.07

and others, 1983). In order to measure the impact characteristics, a set of sensors mounted near the center of mass of the craft (two piezoelectric transducers and an inertial sensor) and a computer (consisting of the control unit and the unit for information storage and conversion) were used. The system is a more refined version than that used by Veneras 13 and 14 (Avduevsky and others, 1982). The results of the measurements were obtained as discrete points, with a time interval of 1 millisecond (ms) between them. To assure a reliable interpretation, Earth-based experiments were also performed, making it possible to obtain reference data of the shock processes gener- ated when an object was dropped upon surfaces intended to be analogous to those on Venus. The surface of the planet was imitated by sites made of loose or packed sintered sand (in some cases, crushed rock 30 to 80 mm in Figure 5. Strain-stress diagram. The stress, on the vertical axis, is size was used) or of foam concrete, intended to be analogous to vesicular in kg/cm. The strain, on the horizontal axis, is not normalized and is in basalt. actual centimetres. Curves 1 and 2 are caused by the landings of In these test!, use was made of a full-sized spacecraft mock-up that Veneras 13 and 14, respectively. Curve 3 is produced by the mock-up had the same inertial mass and rigidity characteristics as those of the actual described in the text. Veneras. The surface conditions of Venus do not affect the inertial mass, and they have insignificant effects on the rigidity. Possible effects of the atmospheric viscosity were ignored, as they were assumed to be minimal during the 70 ms of measurement. stresses

strated that the toroidal supporting ring remains practically undeformed at where G is the effective weight of the landing craft, nx(t) is the load landing. Accordingly, in the mathematical analysis of landing, the craft function, and S(t) is a function having the dimension of a surface and was treated as a solid object interacting with pliable material making up a describing the time dependence of the area of the landing contact w: th the

nearly horizontal landing surface. The measured impact load function nx(t) surface material. This was calculated using the a(t) dependence, that is, the was introduced in the right-hand side of the equation penetration depth of the craft into the material. After that, canceling time from the functions e(t) and cr(t), we have constructed the curves of e(a)

a^-=-g$nx(t) + g$ shown in Figure 5, which are in good agreement with the mathematical model proposed by S. S. Grigoryan (1959) to describe the properties of the

where gg is the fres-fall acceleration at the surface of Venus, and Vx is the rocks. vertical velocity. At the moment of contact with the surface, the velocities The character of the e(a) function is determined by physical and of the crafts, Venera 13 and Venera 14, were equal to 7.96 m/s and 7.50 mechanical properties of the rocks [such as the initial density of th e un- m/s, respectively. packed material, the density of the packed (deformed) material, the bear- After integration of the equation (at the known initial impact veloci- ing capacity, and so on]. The coincidence of functions, then, in the

ties), the time functions of the crafts' velocities at the impact Vx(t), and the identical conditions of dynamic load of the different materials, suggests

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curve 3 (packed sintered sand). This suggests the presence of a soft surface layer and a somewhat stronger basement. The following model is proposed: The upper surface layer (-10 cm thick) is a weak porous material with a density between 1.15 and 1.2 g/cm3 and a bearing strength of ~2 kg/cm2 (200 kilopascals). The characteristics of the underlying material are close to those of the material at the landing site of Venera 13 (density of -1.5 g/cm3, bearing strength of 4 to 5 kg/cm2 (400 to 500 kilopascals). These estimates indicate that the material encountered at the landing sites of Veneras 13 and 14 have an extremely low strength. At densities of 1.2 to 2.5 g/cm3, their porosity is estimated to be 50% to 60%. For these rocks, an irregular shape of the constituent particles, with point contacts between them, is probably typical; aggregates of spherical particles are incapable of reaching such high values of porosity, even with a uniform grain size and a packing due to free filling. In Part I of this research (Florensky and others, 1977a), measurements by Veneras 9 and 10 were quoted as indicating a high density of the surface material (as much as 2.8 g/cm3). This should not be interpreted as indicating that Veneras 13 and 14 happened to measure the properties of the loose, fine material, and that Veneras 9 and 10 measured solid rock, because the Venera 14 site is practically devoid of loose material. Whether this discrepancy is due to real differences between landing sites or is due to error cannot be said at present. The bearing strength and the conductivity of the surface material at the contact between the sensor and the surface have also been estimated using the trellis girder (Kemurdzhian and others, 1983). For the layered material of Venera 13, the bearing capacity ranges from 2.6 to 10 kg/cm2 (260 to 1,000 kilopascals), in good agreement with our loading estimates. Agreement, however, does not occur in the case of Venera 14. The trellis Figure 6. Comparison between the landing effects of Venera 14 girder experiment gives an estimate of bearing strength equal to 65 to 250 2 (Curve 1) and the mock-ups on different materials (Curves 2, 3, 4; kg/cm (6,500 to 25,000 kilopascals). This is, in our opinion, due to the see text). fact that the camera port-hole cover interfered with the proper function of the trellis girder (Murphy's Law). The electrical resistivity of the surface material was also measured that their physical and mechanical characteristics are also similar. For the (Kemurdzhian and others, 1983) and was found to be very low (73 to 89 Venera 13 landing (Fig. 5), our Earth-based experiments allowed us to ohm • m). On Earth, geological material can have such a low resistivity select the counterpart rock (packed sintered sand) for which the strain- only when water-bearing. Most of the rock-forming silicates, when dry, are stress diagram best coincides with the calculated dependence. insulators. As the low resistivity of the Venusian surface material cannot be From the above analysis, one can conclude that at the landing site of due to water, some other process must be responsible (see below). Venera 13, the surface material had physical and mechanical properties similar to those of packed sintered sand, namely, initial density = 1.4 to 1.5 DISCUSSION AND INTERPRETATION g/cm3, and bearing capacity = 4 to 5 kg/cm2 (400 to 500 kilopascals). In the case of the Venera 14 landing, the interaction between the craft The photographs transmitted by Veneras 13 and 14, as well as by and the surface material was more complicated. From the analysis of the Veneras 9 and 10, show essentially the presence of two items: (1) consoli- strain-stress diagram (Fig. 5), it can be seen that upon reaching a pressure dated, mostly layered rock units (the outcrops of Veneras 10, 13, and 14, of 2.5 kg/cm2 (250 kilopascals), there occurred a rapid packing of the and the talus slabs of Venera 9), and (2) dark, loose, fine material (occur- material; a further increase in pressure caused an insignificant increase in ring to a greater or lesser extent at each site). strain. No vertically homogeneous material could be found which was able The loose, fine material will be discussed first. Evidence suggests that to duplicate the Venera 14 strain-stress diagram. at least a portion of the loose material is the result of in situ disintegration For an analysis of the landing conditions of Venera 14, the loading of the rock units (Florensky and others, 1977c). This is indicated by the functions presented in Figure 6 have been used. Curve 1 represents the presence of loose material in fractures and in front of scarplets and by the axial impact loading at the landing of Venera 14 (V = 7.5 m/s). Curve 4 is sequence of fragment sizes and shapes, from angular and platy to sub- the curve obtained for the case in which the mock-up was dropped upon rounded. It is difficult to speculate on the nature of the degradational foam concrete. process. Active chemical compounds may still be undetected in the lower The following model of interaction between the spacecraft and the atmosphere, or they may appear sporadically as a result of volcanism or surface material is suggested. In the initial stage of penetration of the impact. It must be remembered that no reliable data exist for the near- spacecraft into the surface material (down to a point S, which corresponds surface atmosphere. The lowest Soviet gas-chromatograph data are for to 2.5 kg/cm2, or 250 kilopascals), the character of the interaction is elevations 3.5 to 5 km above the surface, and doubts exist on the Pioneer analogous to the craft being slowed down by loose sintered sand (curve 2). Venus mass spectrometer data due to possible H2SO4 droplets. At the point S, the interaction drastically changes it character. The An important characteristic of the fine material in all four landscapes further penetration of the spacecraft into the surface rocks is described by is the absence, on its surface, of visible ripple marks and microdunes. This

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Figure 7. A detail of Venera 14 photographs. The thinness of the layers is clearly shown by the stratigraphie window. A fracture, restricted to the underlying layer, is visible to the left.

Figure 8. A detail of Venera 14 photographs. Some of the Figure 9. Details of Venera 13 photographs. Photo A shows a surface features inay indicate "pinching-out" of layers; others could slab, fractured in silu, and undulations on the surface of the rock units be ripple marks. which may indicate ripple marks. Photo B shows striations which may be due to fines deposited in depressions of ripple marks.

is rather surprising, as the thick Venusian atmosphere seems to be the The data show evidence that fines exist in the loose material. The perfect medium for aeolian transportation (Iverson and others, 1976; presence of the narrow zone close to the supporting ring on Venera 1.3 (see Ronca and Green, 1970; Greeley and others, 1984). Wind velocities at a Fig. 3 for the particle size distribution) can be explained by the blowing height of — 1 m above the surface have been measured to be between 0.3 away of fines by the turbulence caused by the landing. This is confirmed and 1 m/sec (Avduevsky and others, 1976), greater than those necessary by the cloud of dust recorded by the spectrophotometer (Mora/, and to strip a small panicle from the surface (Ronca and Green, 1970; Greeley others, 1982). and others, 1984). The maximum grain size that can be transported by Another aspect of the fine material is its albedo, which is lower than saltation and traction is 0.5 to 1 mm. On the other hand, particles smaller that of the rock units. Generally, a powder has an albedo higher than that than -0.02 mm can be transported by suspension and so are unlikely to of the rock from which it is derived. Interpreting this to indicate a distant produce the typical aeolian geomorphologies. origin of the fines may be tenuous, however, because of the previously

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TABLE 2. PHYSICAL AND MECHANICAL PROPERTIES OF QUATERNARY BASALTIC ROCKS OF VARIOUS POROSITY

Porosity Number of samples Density* (g/cm3) Compressive strength* (kg/cm2)

(*) Tolbachik Armenia Tolbachik Armenia Tolbachik Armenia

0-3 50 2.85 1945 2.78-3.0 1400 - 2600

3-10 60 330 2.68 2.73 1650 1480 2.58-2.72 2.62-2.88 1000- 2300 750-2250

10-20 60 200 2.45 2.45 910 750 2.29-2.61 2.26-2.66 350-1570 350-1250

20-30 50 50 2.20 2.16 620 420 2.07-2.32 2.05-2.34 170-1220 150 - 700

30-40 30 5 1.84 2.03 360 270 1.72-1.98 1.98-2.06 150-710 140 - 350

50-60 20 1.30 155 1.09-1.42 80-260

•Above the line = average value; below (he line = range of values

Number of Chemical composition, wt %

determinations Si02 Ti02 A1203 Fe(FeO) CaO MgO

Tolbachik 46 49.6-51.8 1.13-1.72 14.16-17.06 9.22-10.46 8.05-10.25 4.45-8.4 1.6-08 2.6-3.55 0.1 Armenia 19 45.8-56.9 1.0-1.9 15.4-18.9 3.7-11.9 5.7-10.0 2.3-9.1 1.0-3.1 2.4-4.8 0.1-3.8

Note: data in this table are from V. M. Ladygin.

TABLE 3. VARIATION IN PHYSICAL AND CHEMICAL CHARACTERISTICS OF ARMENIAN DAC1TES

Zones Density Porosity Compressive Si02 TiOj AI2O3 Fe MgO CaO Na20 K20 n.n.n. (g/ctn3) m strength (FeO) (kg/cm2)

1 2.52 3.3 2 000 68.43 0.67 13.11 3.54 0.33 2.77 3.80 4.36 1.74 II 2.47 3.6 990 63.93 0.70 14.81 4.23 0.80 2.44 5.0 2.99 3.87 111 2.37 8.4 920 IV 2.3 12.5 870 64.54 0.66 14.19 4.64 1.01 2.42 3.10 4.27 3.81 V 2.12 19.1 330 65.67 0.67 12.36 4.24 1.87 4.40 1.55 4.30 3.76 VI 1.61 39.0 20 61.70 0.70 13.72 3.81 2.63 2.17 2.20 1.91 10.02

Note: these dacites were hydrothermally altered from fresh varieties (zone I) to montmorillonite clay (zone VI) [21],

mentioned lack of aeolian features. Perhaps the process of disintegration of The pros and cons of each hypothesis will not be repeated here. the rock units is accomplished by chemical/mineralogical changes. The Although no hypothesis can be proven to be true by the new information, low electrical resistivity, mentioned previously, may indicate the presence the authors believe that hypothesis 6 is to be advanced to the forefront. of some "coating" on the particles. Specifically, the authors find support for the hypothesis that the rock units The rock units will be discussed next. In Part I (Florensky and others, either are sedimentary or are volcanic-sedimentary rocks (transported or 1977a), the evidence for the presence of layering was not firm. Venera 14, ejected loose material which has been lithified by some surface metamor- on the other hand, showed unquestionable evidence of layering (Fig. 7 phic process). This is warranted by the following. shows a specific detail). The thinness of the layers is clearly shown by a 1. The low density and the low bearing strength of the surface mate- stratigraphic window which exposes the underlying layer. To the left, rial. Table 2 shows the yield strength measured in some terrestrial lavas of where the underlying layer is exposed, it is possible to see a fracture which different porosities, and Table 3 shows the effects of hydrothermal altera- disappears under the overlying layer, indicating that it does not encompass tions (Ladygin and others, 1983). Although comparisons between Venu- more than one layer. This indicates not only the presence of layers, but also sian and terrestrial rocks should be done with great care, because unknown the lack of dynamic coupling between them. Figure 8 shows that some of variables may be present, it is evident that the Venusian rocks have me- the layers may be at an angle to the surface and that the layers themselves chanical characteristics very different from terrestrial basaltic lavas, even are truncated or "pinch-out." Figure 9 (upper photo) shows undulation or when extremely altered. striations. The lower photo shows striations that could be caused by fine 2. The presence of layering. Venera 9 landed on a slope where material deposited in the low portions of undulations. slab-like rocks and fine material were mass-wasting downhill. As these In Part I (Florensky and others, 1977a), six hypotheses on the origin items have been moved and are not in situ, strictly speaking, the slabs and of the rock formations were discussed. These are: (1) surface lava extru- fines are a sedimentary complex. Of more interest is the origin of the slabs sion, (2) igneous intrusion later exposed by erosion, (3) pyroclastic fall, themselves. They are clearly the result of the breaking up of a layered (4) impact ejection and lithification, (5) sedimentary deposits lithified at structure. Some of the slabs also show the presence of layering. Venera 10 depth and later exposed by erosion, and (6) surface metamorphism due to landed in a plain consisting of "islands" of consolidated outcrops in a "sea" unique Venusian conditions. of fine material. The outcrops are apparently layered, and there is the

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vague appearance of cross-bedding. The photographs from Venera 13 and ACKNOWLEDGMENTS from Venera 14 show convincing evidence of layering. Although it is possible to produce layering by other processes, some of the characteristics The first author of the first paper (Part I), K. P. Florensky, is de- of the layering strongly suggest a sedimentary origin. Specifically, these are ceased. The authors of this paper have deeply missed the contributions of the thin layering, the fracturing which does not cross layer boundaries, and this scientist. the pinching out of beds. It must be stressed, however, that many of these The authors are grateful to the creators of the photo-television system structures could lie the result of lava flow. which has made possible a detailed study of the surface of Venus; and also 3. The presence of undulations and/or striations on the surface of to G. S. Krivoplyasov, G. V. Nesterenko, A. I. Polyakov, O. D. Rode, V. I. some of the rock units. A possible explanation for these features is that Sval'nova, M. Ya. Frenkel, A. Ya. Sharas'kin, and A. A. Yaroshevsky for they are ripple marks, although they could possibly be flow festoons. helpful consultations on the character of layering in magmatic arid sedi- 4. The low electrical resistivity of the surface material and the albedo mentary rocks. of the fine material, which is lower than that of the rock units. These Thanks are also extended to the Academy of Sciences USSR, to the properties suggest chemical alteration of surface material. In Part I (Flo- National Academy of Sciences (USA), and to Wayne State University for rensky and others, 1977a) and in Florensky and others (1977b, 1977c), supporting the exchange visits of L. B. Ronca to the Vernadsky Institute. several geochem:!cal scenarios were discussed. As nothing new can be

offered, they will not be repeated here. REFERENCES CITED

Let us expand the hypothesis. The rock units, at least the layered Avduevsky, V. S., Semenchenko, V. V,, Uspensky, G. R., and Cheremukhina, Z. P.. 1982, Venusian slra!osphi:re from the ones, are the result of previously existing, loose, fine material (erosional or data of accelerometric measurements by Venera 11 and Venera 12 [in Russian]: Kosmicheskie issledovaniya, v. 20, no. 6, p. 225-231. pyroclastic) which has been transported, deposited, and lithifled At least a Avduevsky, V. S., Godnov, A. G., Zakharov, Yu. V., Petrosyvan, L. V., Uspensky, G. R., Semenchenko, V. v., Suklysh- kin, I. I., and Cheremukhina, Z. P., 1983, Estimation of physical and mechanical characteristics of tie Venusian portion of the present loose, fine material, on the other hand, is the result of ground [in Russian]: Koimicheskie issledovaniya, v. 21, no. 3, p. 331-339. in situ disintegration of the local rock units. A necessary corollary of this Bareukov, V. L., Surkov, Yu. A., Moslutleva, L. P., Shcheglov, O. P., Perminov, V. G., Kharyukova, V. P., and vlanvelyan, O. S., 1982, Geological studies of the Venusian surface by Venera 13 and Venera 14 [in Russian]: Geokhimiya, interpretation is that surface conditions at the landing sites must have v. 7, p. 899-919. Donahue, T. M., Hoffman, J. H., Hodges, R. R., Jr., and Watson, A. J., 1982, Venus was wet: A measurement of the ratio changed through time. Sometimes in the past, loose fines were transported of deuterium to hydrogen: Science, v. 216, p. 630-633 and deposited by processes involving air currents—such as winds—due to Esposito, L. W., 1984, Sulfur dioxide: Episodic injection shows evidence for active Venus volcanism: Science, v. 223, p.1072-1074. climatic effects, by volcanic or impact events, or by turbidity currents Florensky, K. P., Ronca, L. B., Basilevsky, A. T., Burba, G. A., Nikolaeva, O. V., Pronin, A. A., Trakhtnian, A. M., Volkov, V. P., and Zazetsky, V. V., 1977a, The surface of Venus as revealed by Soviet Venera 9 and 10: Geological resulting from tectonic instabilities (the Venusian atmosphere is sufficiently Society of America Bulletin, v. 88, p. 1537-1545. dense to allow turbidity currents; Ronca and Green, 1970). Subsequently, Florensky, V. P., Ronca, L. B., iind Basilevsky, A. T., 1977b, Geomorphic degradation on the surface of Ven js: Science, v. 196, p. 869-870. these fines were lithified, and the environment changed into one permitting Florensky, V. P., Basilevsky, A. T., Burba, G. A., Nikolaeva, O. V., Pronin, A. A., Volkov, V. P., and Ronca, L. B., 1977c, in Proceedings, 8th Lunar and Planetary Science Conference, Houston, Texas, p. 2655-2664. disintegration of the lithified material, but with essentially no transporta- Florensky, K. P., Basilevsky, A. T., Kryuchkov, V. P., Kuzmin, R. O., Pronin, A. A., Nikolaeva, O. V., Cheniaya, I. M., tion (except for the mass-wasting of Venera 9). Recent observations (Espo- Tyuflin, Yu. S., Selivanov, A. S., Naraeva, M. K., and Ronca, L. B., 1983, Venera 13 and 14: Sedimentary rocks on Venus?: Science, v. 221, p. 57-59. sito, 1984) indicate that volcanism may be common, suggesting the Greeley, R., Iversen, J., Leach, R„ Marshall, J., White, B„ and Williams, S., 1984, Windblown sand on Ve:ius: Icarus, v. 57, p. 112-124. possibility of environmental changes. Grigoryan, S. S., 1959, On the general equation of soil dynamics [in Russian]: Akademiya Nauk SSSR, Dokli dy, v. 114, no. 2, p. 35-42. Recently, from data on the enrichment of deuterium of sulphuric acid Iversen, J. D., Greeley, R., and Pollak, J. B., 1976, Windblown dust on Earth, Mars and Venus: Journal of A:mospheric drops in the aerosol of Venusian clouds (Donahue and others, 1982), Sciences, v. 33, no. 12, p 2425-2429. Kemurdzhian, A. L„ Brodsky, I'. N„ Gromov, V. V., Grushin, V. P., Kiselev, I. E„ Kozlov, G. V., Mitzke\ich, A. V., speculations have arisen that in the past, Venus could have had liquid Sologub, P. S., and Yudkin, E. N., 1983, Preliminary results of the physical-mechanical properties of the Venus soil [in Russian]: Kosmicheskie issledovaniya, v. 21, no. 3, p. 323-330. water, even wate:: basins, on its surface. The authors see no evidence of Ladygin, V. M., Sokolov, V. N., Shlykov, V. G., and Gvozdeva, I. P., 1983, Hydrothermally-altered volcanites of northern Armenia and their physical and mechanical properties [in Russian]: Vestnik Moscow State Universit Geology water transportation. N 3. The process of lithification is not understood. The low strength of the Moroz, V. I., Moshkin, B. E„ Elonomov, A. P., Golovin, Yu. M„ Gnedykh, V. ]., and Grigoriev, A. V., 1982, The spectrophotometric experiment aboard Venera 13 and 14 [in Russian]: Pis'ma, v. 8, no. 7, p. 404-410. rocks suggests ths.t it could have been caused by point-contact adhesion Ronca, L. B., and Green, R. R., 1970, Aeolian regime on the surface of Venus: Astrophysics and Space Science, no. 8, p. 59-65. due to chemical reactions. The well-pronounced, layer-by-layer destruc- Selivanov, A. S., Gekfin, Yu. M., Gerasimov, M. A., Nosov, B. I., Naraeva, M. K., Panfilov, A. S., Titov, A. S., Fokin, tion of these roclts and the presence of curvilinear fractures resembling A. B., and Chemodanov, V. P., 1983, Continuation of the television study of the Venus surface from landers [in Russian]: Kosmicheskie iisledovaniya, v. 21, no. 2, p. 176-182. mud cracks, which are often restricted to individual layers, suggest that Surkov, Yu. A., Kirnozov, F. F., Glazov, V. N., Dunchenko, A. G., andTatsii, L. P., 1976, Content of natural radioactive elements in Venusian rocks from the data of Venera 9 and Venera 10 [in Russian]: Kosmicheskie issledovaniya, lithification could have occurred layer-by-layer, each layer having been v. 14, no. 5, p. 704-709. lithified before the; next layer was deposited. Woolnough, W. G., 1930, The influence of climate and topography in the formation and distribution of products of weathering: Geological Magazine, v. 67, p. 123-132. Surface material which was indurated in the past by weathering and which is presently undergoing disintegration has been called "duricrust" by Woolnough (1930). With the understanding that the Venusian situation is geochemically completely different from the terrestrial case, the term "duricrust" has been informally used to describe this hypothetical surface MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 3, 1983 REVISED MANUSCRIPT RECEIVED MAY 10, 1984 induration on Venus (Florensky and others, 1983). MANUSCRIPT ACCEPTED MAY 18, 1984

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