GEOLOGY OF THE VENUS EQUATORIAL REGION

FROM PIONEER VENUS RADAR IMAGING*

D. A. SENSKE

Department of Geological Sciences, Brown University Providence, RI, U.S.A.

Abstract. Pioneer Venus radar data has provided images (resolution 20. to 40.km) of approximately 50% of the total surface of Venus in a band between 45” N to 15” S. These data are used to map the broad radar characteristics of the equatorial region on the basis of radar brightness and texture. Seven radar units are defined and are used to assess the geologic character of the equatorial region. These units fall into two distinct classes, those that are radar-bright (35% of the equatorial region) which correspond to highlands and zones of intense tectonic deformation, and radar-dark units, corresponding primarily to plains (65% of the equatorial region). The correspondence between features in the IS” region of overlap between the Pioneer Venus and Venera 15/16 images is examined and used to extend units mapped in the northern high latitudes into the equatorial region. On the basis of the distribution of the radar units, properties of RMS slope, reflectivity, the scattering behavior of the surface. and topographic signature, seven physiographic units are mapped in the equatorial region and are identified by increasing complexity as plains (undivided), dark halo plains, upland rises, upland plateaus. inter- highland tectonic zones, tectonically segmented linear highlands, and tectonic junctions. The physio- graphic units are distributed in a nearly continuous interconnecting zone of volcanic rises and tectonic features that extends for nearly 360” around the equator of the planet. The distribution of large circular structures interpreted as coronae is also examined and it is concluded that the abundances of the largest structures, diameters greater than 500 km. is less than in the northern high latitudes with a notable absence of smaller coronae. The absence of small coronae may be due to the resolution !imit of the Pioneer Venus data since analyses of higher resolution Arecibo and Goldstone imagery suggests that a number of corona-like features not identified in the PV data are present.

Introduction

Radar imaging of the surface of Venus has been carried out using a variety of both earth-based (Arecibo, Goldstone) and spacecraft-based (Pioneer Venus (PV), Venera E/16) imaging systems. These data were obtained at a variety of resolutions ranging from 20- to 40-km for Pioneer Venus images (Pettengill et al.,

1980) to l- to 2-km for Venera, Arecibo, and Goldstone images (Barsukov et al.,

1986; Campbell et al. 1989a; Jurgens et al., 1988). The distribution of image data is such that the highest resolution images cover a region between approximately 30” N and the north pole, Venera 15/16 (Barsukov et al., 1986), a portion of the equatorial region between 45” N and 12” N and 270” and 20” and a portion of the southern hemisphere between latitudes lo” S and 66” S and longitudes 280” to 20”, Arecibo imaging (Campbell et al., 1989a; Campbell et al., 1990). Lower resolution PV image data provide a synoptic view of the equatorial region covering a zone of 360” in longitude between 4.5”N and 15” S. The northern 15” of this data set overlaps with higher resolution Venera 15/16 image data and allows large-scale structures and units mapped in the northern high latitudes to be extended into the

* ‘Geology and Tectonics of Venus’, special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).

Earth, Moon, and Planets 50/51: 305-327, 1990. 0 1990 Kluwer Academic Publishers. Printed in the Netherlands.

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GEOLOGY OF THt VtNUS EOUAIOKIAL REGION 301

equatorial region. These data are used to examine the geologic character of the equatorial region and to characterize the region by identifying major physiographic provinces.

Pioneer Venus Radar Image. Radar images of the surface of Venus were ob- tained by Pioneer Venus when the spacecraft altitude was less than 700 km and the radar system was operating in side-looking mode. Data were collected continuously between latitudes 50” N and 20” S. The total range in incidence angles for the image varies from 9” to 63”, depending on spacecraft altitude, with incidence angles bracketed between 35” and 63” near periapsis, 17” N, and between 9” and 33” at 700 km altitude (50” N and 20” S) (see Table Ib of Pettengill et al., 1988). The highest resolution data were obtained near spacecraft periapsis and the lowest resolution at 700 km. Due to the low resolution of imaging along the northern and southern edges of data coverage, the most useful data from which geologic interpretations can be made lies between 45” N and 15” S.

For images obtained at large angles typical of Pioneer Venus the average radar cross-section is not a strong function of incidence angle, and scattering is believed to be caused by sub-wavelength scale, less than 17 cm, facets on both the surface and in the subsurface (Ulaby et al., 1982). Regions of high surface roughness will appear bright in the image, while relatively smooth areas will appear dark. Varia- tions in the dielectric properties of surface materials will also influence the ampli- tude of the backscattered signal, with regions of high reflectivity appearing bright, and areas of low reflectivity appearing dark. The effect of changes in reflectivity are typically less dominant than roughness variations; however, in the highlands of Beta Regio, Atla Regio, and Aphrodite Terra, areas of high reflectivity become important (Pettengill et al., 1988).

Radar Units

In order to characterize the surface in the equatorial region of Venus from the PV radar image, seven radar map units are defined on the basis of a visual assessment of the degree of radar brightness, texture, and relative contributions of bright and dark components. The uniqueness of each unit was verified by determining the range and average brightness of the pixels in each unit, with values of average brightness found to increase continuously between units. The properties of RMS slope, uncorrected reflectivity, and the abundances of diffuse (wavelength scale) scatterers, a parameter derived from the PV SAR image data (Pettengill et al., 1988; Bindschadler and Head, 1988), are used to characterize each unit. These units are classified in increasing order of brightness as: very dark (Dv), dark (D), mottled dark (Dm), intermediate (I), mottled bright (Bm), bright (B), and very bright (Bv) (Figure 1). The characteristics of the different units are summarized in Table I.

The unit identified as very dark corresponds typically with regions of low topography (elevations less than 6052.0 km); however, two broad topographic

308 D. A. SENSKE

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in Table I.

rises, Eisila Regio and Bell Regio (Figure 1; see Figure 2 for the location of specific features), are characterized by material that is very dark. In several regions, for example radar-dark regions to the west of Eisila Regio centered at 21” N, 330” (Figures 1 and 3), the radar-dark material embays adjacent areas of higher elev- ation which are composed of brighter material (Senske, 1989). This relationship suggests possible volcanic emplacement of the very dark unit. The correspondence of this unit with low lying plains regions interpreted elsewhere to be volcanic and the embayment relation with regions of higher topography, suggests that this unit is made up of smooth lava flows typically forming plains. Like the very dark unit, the dark unit is found primarily in regions of low topography. This unit is interpre- ted to be associated with volcanic deposits that are smooth at radar wavelengths.

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GEOLOGY OF THE VENUS EQUATORIAL REGION 311

Correlations with higher resolution Arecibo data (Campbell et al., 1989a) suggest that the brighter material within this unit is associated with bright, possibly rougher lava flows. These flows are often located distally from volcanic constructs, for example, dark units associated with Gula Mons (Figure 1). The mottled dark unit is found mainly on the flanks of topographic rises but is also found in topograph- ically low plains. In areas where this unit overlaps with Venera image data, it corresponds with plains containing systems of widely spaced ridges and arachnoids associated with Bezlea Dorsa (Figure 2) and an unnamed region of tessera (30” N, 5.5”) (Figure 3a) (Barsukov et al., 1986). On the basis of this correspondence, this unit is interpreted to be associated predominantly with intermediate-scale (hundreds of kilometers) tectonic features within the plains (e.g., isolated blocks of tessera which have been embayed, ridges, and arachnoids). Regions mapped as intermediate are typically found in areas of low topography (elevations less than 6051.0 km). An area mapped as intermediate located between Tellus Regio and Shimti Tessera (Figure 2) corresponds in Venera 15/16 images to a region possess- ing a large concentration of small domes interpreted to be volcanic (Slyuta et al., 1988), whose presence and associated deposits may be responsible for the mottled texture of this unit. Intermediate units are interpreted to be of volcanic origin and are associated with rough lava deposits and in some cases dome fields. Areas of mottled bright material are concentrated to the east of Beta Regio in Guinevere Planitia and to the northeast of Atla Regio (Figures 1 and 2). Units located in Guinevere Planitia correspond in high resolution data with areas of extremely bright flow features (Campbell et al., 1989a; Fisher, 1990), while units located on high topography (e.g., Tellus Regio) correspond with features mapped as tessera (Barsukov et al., 1986). In two locations, Sif Mons and Tepev Mons (Figure 1) the mottled bright unit forms a lobate pattern and presents an appearance of being superimposed on regions mapped as dark or mottled dark. Mottled bright units are interpreted to be associated with areas of intense tectonic deformation, and very bright (rough) lava flows. Regions mapped as bright correspond to areas of high topography (greater than 6052.0 km). This unit corresponds to areas mapped from high resolution data as tesserae, for example eastern Beta Regio (Campbell et al., 1989b; Senske et al., 1990), and eastern Dali Chasma which is interpreted as an extensional tectonic zone (Schaber, 1982) (Figure 1). On the basis of this correspondence, the bright unit is interpreted to be associated with regions of intense tectonic deformation (e.g., tesserae), producing ridges, troughs, and scarps. The very bright unit occurs typically on the highest topography in the equatorial region (elevations greater than 6054.0 km). This unit corresponds to Devana Chasma within Beta Regio (Figure 1) and its southern extension into Phoebe Regio. Extensions of this unit to the west into Atla Regio where it corresponds to Hecate Chasma, Ganis Chasma, and Parga Chasma are also ob- served (Figures 1 and 2). The highland of Western Aphrodite is also mapped as very bright. In addition the bright summits of a number of peaks interpreted to be volcanoes (Theia Mons, Ozza Mons, Gula Mons) are mapped as very bright. The very bright unit is interpreted to be dominantly a tectonic unit and corresponds

312 D. A. SENSKL

Fig. 3a.

to zones of chasmata and areas interpreted to be associated with intense tectonic deformation (Western Aphrodite) (Schaber, 1982; Head and Crumpler, 1987). In several locations where volcanic deposits are mapped as very bright, a correspon- dence with regions of high reflectivity is observed, suggesting part of the brightness of the unit in these areas is associated with material with high dielectric components (e.g., mafic minerals in volcanic deposits; cf. Pettengill et al., 1980).

Analysis of the Region of Venera 1916 and Pioneer Venus Image Overlap

Examination of the 15” overlap region between the Venera and PV images (north of 30” N) has allowed a comparison between large-scale features in the two data sets to be made (Figure 3). On the basis of this correspondence and the character- istic radar properties of these features, it is possible to extend units mapped in the northern hemisphere into the equatorial region. A major unit found in both

313

Fig. 3a.

data sets is the tectonic unit mapped as tesserae (Barsukov et aE., 1986). Regions of tesserae are characterized as high topographic plateaus with interconnecting ridges and valleys, high roughness, low values of uncorrected reflectivity, are highly diffusely scattering, and are mapped typically as mottled bright to bright. In several locations, for example Bell Regio where small isolated blocks of tesserae are embayed by plains material, they are mapped as mottled dark (Figures 1 and

314 0. A. SENSKE

Fig. 3b.

Fig. 3. (a) Venera units overlaid on the PV radar image. (b) Correspondence between volcanic and tectonic features mapped from Venera imaging and extended into the equatorial region. The white line corresponds to the southern limit of Venera image coverage. T = tessera, R = ridge belts, C = coronae, U = large dome-like uplands. Primed letters correspond to Venera units extended into the

equatorial region.

3). The largest area of tesserae in the overlap region is Tellus Regio (Figures 2 and 4). This highland is characterized by a plateau region of elevated topography exhibiting 2.0km of relief relative to the surrounding plains (Figure 4b). In ad- dition three smaller regions of tessera, Shimti Tessera, Kutue Tessera, and an unnamed region of tessera (29.5”N, 124”) are found in the plains to the east of Tellus (Figures 2 and 4).

On the basis of radar properties, a mottled radar texture, and topographic

GEOLOGY OF THE VENUS EQUATORIAL REGION 315

signature, additional tessera-like highland features have been identified in the region extending south from Tellus Regio into the northern part of Western Aphrodite Terra. Included in this region are a series of isolated elevated blocks trending to the southwest from the block of the unnamed tessera (Figure 4). These tessera-like features, labeled ‘A’, intersect a large plateau, labeled ‘B’, which extends along the northern flank of Thetis Regio and possesses tessera-like proper- ties (Figure 4). Other areas of tessera-like material within the equatorial region are located along the eastern edge of Beta Regio, which in high resolution Arecibo radar images possesses a complex ridged texture (Campbell et al., 1989b; Senske et al., 1990), north of Asteria Regio, and at Phoebe Regio (Figure 2). The interpretation of these features as tessera-like is consistent with an analysis by Bindschadler et al. (1990) which predicts these regions to have a high probability of being tessera. In a recent study by Ford and Senske (1990) which compared the scattering behavior of eastern Beta Regio, Tellus Regio, and the northern flank of Thetis Regio, it was found that the latter two regions were similar to each other but were different from eastern Beta. These results suggest that regions of tesserae can appear different when imaged at large incidence angles.

Additional tectonic features corresponding between the two image data sets include units mapped as ridge belts, coronae, and large dome-like uplands (Figure 3). Mapping of ridge belts from PV radar data show them to be characterized by units ranging from mottled dark to very bright. Topography associated with ridge belts varies, with some belts located on high topography and others on slopes or in depressions. Due to the variability in both radar signature and topography, the continuation of the mapping of ridge belts into the equatorial region has been limited to two areas where radar-bright ridges form a linear belt extending south into the equatorial region. These patterns are most evident at Bezlea Dorsa and south of the large concentration of ridge belts in Atalanta Planitia, between latitudes of 20” N and 40” N and longitudes 135” and 200” (Figure 3). Coronae in the overlap region possess radar-bright, typically discontinuous rims which corre- spond with an annulus of closely spaced concentric ridges mapped in high resol- ution images (Pronin and Stofan, 1988; Stofan and Head, 1990), have radar-dark interiors, and exhibit positive local topographic relief. Typical examples include, Nefertiti (semi-corona), and an unnamed corona (26”N, 16”) to the north of Sappho (Figure 3). Two large features showing characteristics typical of coronae, a locally elevated ring structure with a discontinuous rim and raclw-dark interior, are Pavlova (525 km x 370 km), located in eastern Eisila kegio and a circular structure (840 km diameter) associated with Heng-o Chasma which has been im- aged by PV, Goldstone, and Arecibo (Senske and Head, 1989a; Arvidson et al., 1990; Campbell et al., 1979) (Figure 3). The last tectonic unit corresponding with PV imaging, identified as large dome-like uplands, possesses two distinct signatures in the PV data, radar-bright and radar-dark. The highlands of Beta Regio and Ulfrun Regio are characterized by radar-bright material, are very diffusely scat- tering and rough at radar wavelengths. Two other features in the equatorial region

316 D. A. SENSKE

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Fig. 4a.

showing similar characteristics are Aphrodite Terra and Atla Regio. In contrast to Beta and Ulfrun, the upland of Bell Regio is characterized by radar-dark material and is interpreted to be smooth at radar wavelengths. On the basis of topography and radar characteristics, the highland of Eisila Regio shows similarit- ies to the dome-like upland of Bell.

Volcanic features mapped from Venera imaging as smooth plains and rolling plains (Figure 3) correspond with broad regions mapped typically as mottled dark to very dark in the PV image overlap area and are characterized by low topogra- phy, at or near the mean planetary radius, are smooth at radar wavelengths, and possess low to intermediate values of reflectivity (0.02 to 0.2), suggesting the presence of some soil with substantial rock exposure. These characteristics are pervasive throughout the low lying part of the equatorial region, suggesting that an estimated 65% of the surface in this part of the planet can be be classified as plains of volcanic origin.

Physiographic Subdivisions of the Venus Equatorial Region

On the basis of observations from topography data, previous studies of the Venus equatorial region have established three topographic provinces: lowlands, rolling

GLOLOGY OF THE VENUS EQUATORIAL REGION 317

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Fig. 4. (a) Pioneer Venus image of Tellus Regio and northern Aphrodite Terra. Tellus has been mapped from Venera imaging as tessera. Tellus Regio and several isolated blocks of tessera, Shimti tessera (S), Kutue tessera (K), and an unnamed region of tessera (LJ) in the PV image show a correspondence with the Venera image. The white line indicates the approximate location of the southern extent of Venera image coverage. Areas possessing tessera-like properties outside of Venera image coverage are labeled ‘A’ and ‘B’ and are indicated by the arrows. (b) Pioneer Venus topography of Tellus Regio and the northern part of Western Aphrodite Terra. Features corresponding with units mapped as tessera exhibit elevated topography with 0.6 to 2.4 km of relief relative to the surrounding plains. Features corresponding with tessera in Venera images are indicated by the dotted pattern. Tessera-like features in the PV image are indicated by the stippled pattern, are labeled ‘A’ and ‘B’, and are located to the southwest of a region of the unnamed region of tessera labeled ‘U’, and along the northern flank of Thetis Regio. The black dotted line indicates the southern extent of Venera

15/16 data coverage.

plains, and highlands (Masursky et al., 1980). On the basis of morphology, topogra- phy, radar properties derived from the PV data, and the units identified above, this general characterization has been extended and seven major physiographic provinces within the the units defined by Masursky et al. (1980) are established. In addition the distribution of features interpreted as coronae and individual volcanoes is examined. The physiographic subdivisions are defined as: undivided

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319

plains (planitia), dark halo plains (planitia), upland rises (Bell Regio), upland plateaus (Tellus Regio), tectonically segmented linear highlands (Western Aphro- dite Terra), inter-highland tectonic zones (Eastern Aphrodite Terra), and tectonic junctions (Beta Regio) (Figure 5).

Undivided plains (Planitia). Undivided plains (Figure 5) correspond with areas identified by Masursky et al. (1980) as rolling plains and lowlands, low lying areas with an elevation less than 6052.0 km. A variety of isolated domes and mountains with varying radar properties (radar-bright summits with radar-dark flanks to mountains that are radar-dark with no associated bright deposits) are present within this unit. The undivided plains cover the largest percentage of the equa- torial region, 65% of the surface imaged by PV. RMS slope data show the plains to be typically smooth (1.0” to 2.5”), although a large region east of Beta Regio (Guinevere Planitia) is transitional in roughness with values between 2.5” and 5.0”. Reflectivity values typically fall in the low to internmediate range of 0.02 to 0.2 while diffuse scattering data show the plains to be composed of less than 10% diffuse scatterers. These properties suggest the presence of a rocky surface with little soil. The radar signature of this unit varies from predominantly radar-dark to mottled bright. The plains extending from near longitude 150” eastward to Western Eisila Regio (345”) are dominated by units which are intermediate to mottled bright. Areas which overlap with both high and low resolution Arecibo radar image data suggest that the brightness and mottled texture in this region is associated with the presence of bright lobate flow features (Fisher, 1990). The plains extending from Western Eisila Regio (345”) east to 150” are characterized by units that are uniquely radar-dark (very dark to mottled dark). Unlike the brighter units, there is no evidence of pervasive lobate flows in overlapping high resolution data.

Dark halo plains (Planitia). Dark halo plains (Figure 5) are broad, extremely radar-dark (mapped as very dark) quasi-circular regions located in low lying areas possessing very little topographic relief and extend for distances of hundreds of kilometers; they are observed exclusively in rolling plains and lowland areas (Pettengill et al., 1979; Masursky et al., 1980; Stofan et al., 1987; Senske and Head, 1988a). The dark material commonly appears to embay surrounding topo- graphically higher areas of radar-bright material suggesting possible emplacement by lava flooding. A radar-bright circular feature or ring, which itself has a radar- dark center, is frequently found within the zone of dark material. In regions covered by high resolution data these circular features correspond to craters, several of which have surrounding lobate flow deposits and are interpreted to be volcanic (Campbell et al., 1989a). In several locations, the central radar-bright features are on topographic rises which exhibit a maximum of 400 m of relief (Senske, 1989). The most extensive regions of dark halo plains are located west of Eisila Regio and west of Atla Regio (Figure 5). Additional areas of dark halo plains are also observed in the northern high latitudes (Campbell and Burns, 1980). The presence of interior elevated radar-bright crater(s) and associated

320 I). A. StNSKL

deposits, the embayment relations expressed by the very dark material, and radar properties suggest that these regions are volcanic plains with the craters being possible source regions.

Upland rises (Bell Regio). Upland rises (Figure 5) are broad, radar-dark, topo- graphic highs containing individual peaks expressing 1 .O- to 3.0-km of relief which are typically radar-bright. The detailed radar signatures of the peaks possess a complex integrated texture, containing material that is both mottled bright and very bright (Senske and Head, 1988a). Two regions designated as upland rises include: (1) Eisila Regio, with its associated peaks, Sif Mons, Gula Mons, and Sappho and (2) Bell Regio with its peaks Nefertiti, Tepev Mons, and an unnamed peak (Figures 2 and 5). The upland rises show no evidence of rift valleys or chasmata; however, features interpreted to be associated with extension are ob- served in Bell Regio (Janle et al., 1987). Gravity anomalies associated with these features have amplitudes of +35 mgal. Estimates of apparent depth of compen- sation suggests these features are compensated at large depths similar to Beta Regio and Atla Regio (Esposito et al., 1982; Janle et al., 1987; Senske and Head, 1989b), interpreted to suggest a deep mantle source for topographic support (e.g., a hot spot or mantle plume).

Upland plateaus (Teffus Regio ). Upland plateaus are broad mottled bright to bright plateaus covering areas of hundreds of square kilometers typically bounded by steep scarps standing l- to 2-km above the surrounding plains. Few individual peaks are present or they are entirely absent and there is no evidence of a rise crest similar to that observed in Aphrodite Terra (Head and Crumpler, 1987). Troughs similar to those associated with Beta, Alla, and Aphrodite are not ob- served. Specific upland plateaus are located north of Asteria Regio, on the eastern flanks of Beta Regio, at Phoebe Regio, adjacent to Ovda Regio and Thetis Regio, in the southern hemisphere at Alpha Regio (outside the coverage of PV imaging but exhibits characteristics typical of upland plateaus), and at Tellus Regio (Figure 5). Venera 15/16 imaging of Tellus Regio shows it to be characterized by a complex tectonic structure of intersecting valleys and ridges mapped as tessera. Additional high resolution Arecibo images of Alpha Regio shows it to be characterized by a similar complex tectonic pattern (Campbell er al., 1989b). Radar properties indi- cate that both upland plateaus and tessera are characterized by high roughness (RMS slopes of 2.5” to 5.0”) and low uncorrected reflectivity (0.02 to 0.1) and that they contain a large percentage of wavelength-scale (5-50 cm) diffuse scatterers (Bindschadler and Head, 1988). On the basis of radar properties and correspon- dences with Venera imaging, it is suggested that the areas identified as upland plateaus are tectonic units similar to tesserae.

Tectonically segmented linear highlands (Western Aphrodite Terra). Western Aphrodite Terra (Ovda Regio and Thetis Regio) is the only feature classified as a tectonically segmented linear highland (Figure 5). The morphology and structure of this highland are characterized by an elongated (east-west) topographic high that is somewhat rectangular in plan view. Average topography reaches an elevation of 3.0 km on a plateau that is bounded to both the north and south by steep scarps;

GEOLOGY OF ,Ht VtNUS LQUAlOKlAL KLGION 321

the highest point in Aphrodite is at an elevation of 5.4 km. The topographic rise of Aphrodite possesses a central trough that is offset right laterally by a series of cross-strike structural and topographic discontinuities (CSD’s) which are perpend- icular to the trend of the rise, forming an en echelon pattern (Head and Crumpler, 1987; Crumpler and Head, 1988a). Head and Crumpler (1987) have interpreted these features to be similar to terrestrial fracture zones and suggest that spreading may be occurring in Aphrodite. Topography data reveal a number of individual peaks along the rise crest interpreted to be volcanoes (Senske, 1989). Reflectivity and RMS slope data show Western Aphrodite to be characterized by values of reflectivity ranging from 0.1 to 0.5 indicating the presence of high dielectric ma- terial and high values of RMS slope (2.5” to 10.00). Additional information from diffuse scattering data shows Aphrodite to be characterized by a high percentage (greater than 25%) of diffuse scatterers, interpreted to be associated with the complex tectonic deformation identified by Head and Crumpler (1987). Although Western Aphrodite possesses the highest topography in the equatorial region, its gravity anomaly is only approximately half that of Beta and Atla (+25 to +28 mgal at a spacecraft altitude of 200 km). This suggests that the mechanism supporting topography in Western Aphrodite is different from that in either Beta or Atla. The primary differences between Western Aphrodite and Beta and Atla are the en echelon rift pattern, the lack of converging rifts (Senske and Head, 1988a), the steep bounding scarps and the lower amplitude gravity anomaly. Modeling of spreading centers under Venus conditions by Sotin et al. (1989) suggests that both gravity and topography in part of Western Aphrodite (Ovda Regio) is consistent with a model of spreading and crustal thickness variations.

Inter-highland tectonic zones (Eastern Aphrodite Terra). Inter-highland tectonic zones (Figure 5) are linear to arcuate regions of elevated topography with lengths of thousands of kilometers which rise typically 0.8- to 2.0-km above the surround- ing plains. A central trough with flanking individual peaks or chains of mountains is commonly present, for example Dali Chasma (Eastern Aphrodite), Ganis Chasma, Hecate Chasma, northern Ulfrun Regio, southern Devana Chasma (southern ex- tension from Beta Regio), and Parga Chasma (Figures 2 and 5). In two locations, east of Atla Regio and adjacent to Phoebe Regio, a central trough is not evident. These regions are mapped as bright to very bright with few resolvable details. Roughness data show Dali Chasma, Ganis Chasma, and Hecate Chasma to be dominated by surfaces typically transitional in roughness with values between 2.5” and 5.0”. Reflectivity data show that Dali Chasma and Ganis Chasma are characterized by low values of reflectivity (0.02 to 0.1) while Hecate Chasma possesses intermediate values of reflectivity (0.1 to 0.2). In general, these two regions are characterized by high roughness and low reflectivity suggesting the presence of rocky surfaces. Diffuse scattering data shows both Dali Chasma and Ganis Chasma to possess a large percentage of wavelength-scale scatterers with the greatest concentration corresponding to the central trough interpreted to be associated with tectonic deformation.

The inter-highland tectonic zone located at northern Ulfrun Regio is charac-

322 D. A. SENSKE

terized from Venera 15/16 images as a broad elliptical topographic rise with a central graben structure, showing characteristics similar to the graben at Beta Regio, but unlike Beta the graben is not topographically distinct in PV altimetry data. Several large peaks interpreted to be volcanoes are present on the crest of the topographic rise (Barsukov et al., 1986). Radar properties show Ulfrun to have low to intermediate values of roughness (1.0’ to 5.0’) and reflectivity along with a high abundance (40% to 50%) of wavelength-scale scatterers, suggesting this region possesses a rough rocky surface.

The extension of Parga Chasma from Atla Regio into the southern hemi- sphere forms a zone of disrupted plains which extends to Themis Regio, corre- sponding to a region interpreted by Schaber (1982) as a zone of lithospheric weakness. In some places troughs are present which are often flanked by peaks and domes. RMS slope data show the unit to be typically rough (2.5” to 5.0”), along with possessing low values of uncorrected reflectivity (0.02 to 0.1). In the vicinity of Atla this unit is characterized by a high percentage of wavelength-scale diffuse scatterers (35%).

The inter-highland tectonic zones associated with Parga Chasma, Ganis Chasma, and Hecate Chasma have been interpreted to be associated with zones of extension (Schaber, 1982) while Dali Chasma is suggested to be a site of crustal spreading (Crumpler and Head, 1988b). Radar properties show these regions to be very diffusely scattering and are interpreted as rough at radar wavelengths with signifi- cant rock exposure. Map patterns (Figure 5) show the inter-highland tectonic zones to form spoke-like patterns, converging at regions mapped as tectonic junctions. In areas where the inter-highland tectonic zones do not link up at tectonic junctions, they die out into the surrounding plains, for example the northern extension of Ganis Chasma, and Ulfrun Regio.

Tectonic junctions (Beta Regio). Tectonic junctions (Figure 5) are radar-bright highlands located at the convergence of three or more inter-highland tectonic zones, and are centers of volcanism possessing individual mountains which are typically located at the crest of a domal topographic rise. Radar-bright lobate deposits on the flanks of the peaks (e.g., Theia Mons) are interpreted to be lava flows. Features mapped as tectonic junctions include Beta Regio, Atla Regio, Asteria Regio, northern Phoebe Regio, the region of convergence of Ulfrun Regio and Hecate Chasma (15’ N, 225”), and an elevated region southeast of Atla (5” S, 220”) (Figure 5).

The largest tectonic junction, Beta Regio, mapped as bright to very bright, is located at the convergence of Devana Chasma, which itself is located at the crest of the topographic rise, Hecate Chasma, and an unnamed chasma. Topograph- ically, Beta rises to an average elevation of 3.4 km. Two radar-bright peaks located on the crest of the topographic high, Theia Mom and Rhea Mons, rise 4.4 km and 3.8 km above a datum of 6052.0 km and are interpreted to be volcanoes (Campbell et al., 1984). Roughness data show Beta to possess RMS slopes between 2.5” and 5.0” which are classified as transitional in roughness by Head et al.,

GEOLOGY OF THE VENUS EQUATORIAL RtGlON 323

(1985). Uncorrected reflectivity data shows the region to be characterized by values between 0.1 and 0.5, suggesting surface materials comparable to terrestrial rock with some material having a high dielectric component present, associated with Theia Mons and its deposits. PV LOS gravity data shows Beta to be charac- terized by the highest value of gravity on the planet, +73 mgal at a spacecraft altitude of 185 km. The anomaly shows a positive correlation with topography and exhibits a ‘bulls-eye’ structure and is interpreted from modeling to indicate the presence of a hot spot or plume (Esposito et al., 1982).

Atla Regio, characterized by radar-bright map units, is located at the conver- gence of Parga Chasma, Ganis Chasma, and Dali Chasma (Figure 2). The broad topographic high of Atla rises to an average elevation of 3.1 km above the sur- rounding plains. Two peaks, Ozza Mons and Maat Mans, are located at the convergence of the chasmata, rise 5.0 km and 6.0 km in elevation respectively, and are interpreted to be volcanoes similar to Theia Mons. However, due to the low signal to noise ratio of this part of the image data it is not possible to resolve systems of lava flows associated with these peaks. Values of RMS slope, 2.5 to 10.0 (transitional to rough), and reflectivity (0.1 to 0.5) at Atla are similar to those measured at Beta (Head et al., 1985). The highest values of reflectivity are associated with Ozza Mons which are interpreted to be associated with mafic minerals in volcanic deposits. Atla is characterized by an LOS gravity anomaly with a peak amplitude of +64 mgal at a spacecraft altitude of 220 km. Like Beta Regio, Atla Regio is interpreted to be associated with a mantle thermal anomaly (Senske and Head, 1988b).

Two smaller tectonic junctions that are broadly similar to Beta and Atla are located at Asteria Regio and northeast of Phoebe Regio (Figure 5). Like Beta and Atla, these features are located at the convergence of tectonic structures, are characterized by the presence of individual peaks, are radar bright, possess a high degree of roughness (RMS slopes 2.5” to 5.0”), have a diffuse scattering component in the range of 15% to 30%, but have low to intermediate values of uncorrected reflectivity (0.02 to 0.2). Unlike Beta and Atla the morphology of these units is not domal, gravity anomalies have amplitudes in the range of only +20 to +25 mgal, and peaks interpreted to be volcanoes are not associated with high dielectric material. In order to establish a correspondence between Beta and Atla and these regions, higher resolution images are needed and gravity modeling is necessary to better determine if the difference in gravity anomalies is due to differences in topography.

An additional tectonic junction is located at the western end of Hecate Chasma where it intersects southern Ulfrun Regio. This zone is characterized by elevated topography, l.O-km, and is a broad region of radar-bright material. There is no evidence of a broad dome like those observed at Beta and Atla. A number of radar-bright peaks are present, but details at the resolution of the PV image cannot be resolved.

The largest tectonic junctions, Beta Regio and Atla Regio, have been interpre-

324 1). A. SENSKE

ted to be associated with deep mantle thermal anomalies (Esposito et al., 1982; Senske and Head, 1988b; Stofan et al., 1989). The tectonic junctions located at Asteria Regio and northern Phoebe Regio possess characteristics similar to Beta and Atla and may have formed in similar manners, but are smaller. It is found that the combination of inter-highland tectonic zones, tectonic junctions, and upland rises forms an interconnecting network extending for nearly 360” around the circumference of the planet.

In addition to the physiographic units, features interpreted as coronae and volcanic mountains have been mapped (Figure 5). Four structures interpreted to be coronae are observed in the PV image data, two of which are located in the overlap region with the Venera data (Figure 2). These features have diameters ranging from 210- to 840-km, correspond with local areas of elevated topography, are characterized by a narrow, discontinuous radar-bright ring, and are concen- trated in the vicinity of Eisila Regio and Bell Regio. Two structures found outside of high resolution Venera images are Heng-o and Pavlova (Senske, 1989; Senske and Head, 1989a). In comparison with the northern high latitudes, the number of coronae per unit area with diameters greater than 500 km is less than in the northern high latitudes (Nikolaeva et al., 1986). There is an absence of smaller coronae, diameters less than 300 km, in the equatorial region. This may be related to the inability to resolve the rings of such features in the PV data, whose presence is suggested from higher resolution Arecibo and Goldstone data (Stofan et al., 1990; Arvidson et al., 1990).

The presence of volcanic mountains in the equatorial region has been established from high resolution Arecibo radar imaging (Campbell et al., 1984). On the basis of the presence of lobate flow-like features and structures interpreted to be summit calderas (e.g., Sif Mom and Sappho) (Stofan, 1985; Senske and Head, 1988a), fourteen volcanic mountains have been mapped from lower resolution PV image data. In addition, fifty-five mountains inferred to be volcanic on the basis of the presence of radar-bright deposits interpreted to be systems of lava flows along with possessing properties similar to volcanic mountains in the region of overlap between PV and Venera imaging have also been mapped (Figure 5). From this analysis it is observed that the density and total number of large volcanic peaks in the equatorial region is greater than that in the northern high latitudes with the peaks concentrated near highland structures.

Conclusions

On the basis of mapping from the PV radar image two distinct classes of radar units are established: (1) radar-dark which are made up of very dark, dark, mottled dark, and intermediate units and (2) radar-bright, composed of mottled bright, bright, and very bright units. A general correlation between map units and surface features suggests that radar-dark units correspond to smooth volcanic plains which cover approximately 65% of the equatorial region, a percentage comparable to

the northern high latitudes. Radar-bright units covering the remaining 35% of the surface are predominantly associated with tectonic features, correspond to chasmata, tesserae, the largest equatorial highland structures (Beta Regio, Atla Regio, Aphrodite Terra) and in some places plains (Guinevere Planitia). Corre- lations with higher resolution Arecibo data (Campbell et al., 1989b; Fisher 1990) suggest that some of the bright and mottled bright units in the zone between Eastern Beta Regio and Western Eisila Regio correspond to bright lava flows. The characteristics of plains are shown to vary with longitude, and can be divided into two distinct zones. The first extends eastward between longitudes 34.5” and 150” and is made up of units that are very dark to mottled dark, interpreted as smooth plains and characterized by a notable absence of flows in regions that overlap with higher resolution data (Campbell et al., 1989a). The second region covers the remaining 195” of longitude and is made up of intermediate to bright units which are interpreted as rough plains in which individual lava flows are often observed.

Analysis of the 15” region of overlap between the PV and Venera 15/16 images has shown that it is possible to establish a correspondence between features in both data sets. On the basis of these correlations, it is possible to extend units mapped in the northern high latitudes into the equatorial region. It is found that tectonic units possessing tessera-like properties are present in association with the equatorial highlands of Beta Regio and Aphrodite Terra, suggesting that tesserae may be a significant unit making up the highlands in the equatorial region. The Venera unit of large dome-like uplands can be divided into two sub-classes, radar- bright highlands and radar-dark highlands. Coronae and ridge belts are less well defined in the PV image, but there is evidence that at least locally they are present in the equatorial region.

Seven physiographic provinces are mapped in the equatorial region and are used to establish the tectonic character of this part of the planet. It is found that the equatorial highlands form a near- equatorial encompassing network of volcanic rises (upland rises), tectonic junctions. and interconnecting tectonic zones com- posed of several distinctive terrain types. The relationship between tectonic junc- tions and inter-highland tectonic zones suggests that the junctions are regions of mantle upwelling acting as nodal points of the network. The inter-highland tectonic zones which extend to the north and do not connect with tectonic junctions die out in this direction. In some places in the equatorial network crustal spreading may be occurring (inter-highland tectonic zones, tectonically segmented linear highlands) (Crumpler and Head, 1988b: Head and Crumpler. 1987) whereas at other places (upland rises. tectonic junctions) hot spots and thermal uplift activity are apparently occurring. These characteristics and correlations suggest that both thermal uplift and lateral movement are occurring in the Venus equatorial high- lands. Two structures interpreted to be coronae have been identified, Pavlova and Heng-o. The distribution of these large coronae in the equatorial region appears to be less than in the northern high latitudes. In comparison with the northern

326 0. A. StNSKt

high latitudes which are interpreted to be associated with broad zones of com- pression forming highlands and erogenic belts (Crumpler et al., 1986; Vorder Bruegge and Head, 1989; Head, 1990), the equatorial region is characterized by interconnecting zones of extension linked at tectonic junctions.

Acknowledgments

I thank Jim Head and Ellen Stofan for helpful discussions. The Venera unit map was digitized by Sharon Frank. Reviews by J. Plaut and R. Kozak are gratefully acknowledged. I would like to thank Peter Neivert for preparing photographs for this paper. Paul Fisher and Rob DeMillo provided software for the analysis of the digital image and topography data. Thanks to Angel Hillard and Laura Demanski for assistance in drafting figures. This work was carried out with support from NASA Grants NGR-40-002-116, NAGW-713, The William F. Marlar Foundation, and under a NASA Goddard Spaceflight Center graduate student research pro- gram fellowship (NTG-SO 147).

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