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SMALL DOMES ON VENUS:

CHARACTERISTICS AND ORIGIN*

JAYNE C. AUBELE

Brown University, Providence, R.I., U.S.A.

and

E. N. SLYUTA

Vernadsky Institute, USSR Academy of Sciences, Moscow, USSR

Abstract. Approximately 22,000 small domes have been identified on the 25% of the surface of Venus imaged by Venera 15/16. The word dome is used to imply a broad, lens-shaped, positive topographic feature. The domes: (1) are generally circular in planimetric outline; (2) range in diameter from the effective limit of Venera resolution (32 km) to 20 km; (3) show flank slopes generally i 10” and possibly ~5”; and (4) occur in association with mottled plains units. Associated features include summit pits, radar bright surfaces, and basal topographic platforms. There are two significant areas of major dome concentrations approximately 180” in longitude apart: (1) the largest concentration occurs in the Akkruva Colles area of Niobe Planitia, centered at approximately 45” N/120” E, just north of the flanks of the Thetis Regio rise; and (2) another concentration occurs in northwestern Guinevere Planitia, centered at approximately 35” N/300” E, on the north flank of the Beta Regio rise. In addition to these major areas of concentrations, domes occur in smaller concenrations throughout the imaged area of Venus, in association with coronae, arachnoids, intermediate sized hills interpreted to be volcanic constructs, large volcanic centers and calderas. The characteristics and geologic associations of small domes are consistent with an interpretation of their origin as volcanic, and on the basis of their low slopes, individual characteristics, and geologic associations they are interpreted to represent dominantly effusive low shield volcanoes. The large number of small domes implies a large number of multiple centralized eruptions, each one of which represents a discrete, relatively small, volume of material available to build an edifice over a finite time period. Calculated modal volume is 0.73 km’ for individual edifices. Based on the number identified by Venera, the total number of small domes estimated for the entire planet 4.4 x lOh and total edifice volume over the entire planet represents a minimum volume equivalent to a layer approximately 7 m thick over the planet and representing 0.03% of the estimated crustal volume of Venus. In absolute number, size range, and distribution they appear to be similar to terrestrial oceanic seamounts. The global abundance and distribution, size frequency distribution, minimum size, and changes in these characteristics with latitude for the domes will be particularly important in understanding the way in which the domes form and their relationship to global models of tectonism and heat flow on Venus. Increased spatial resolution and coverage from Magellan data will enable a more thorough assessment of these features and associated questions, particularly where radar incidence angles are < 15”.

1. Introduction

Approximately 22,000 small dome-like hills have been recognized (Slyuta et al., 1988a,b) in Venera 1506 radar images, over the northern 25% of the surface of Venus. They were originally called ‘small domes’ by Barsukov et al. (1984, 1986), and described as radar-bright and dark positive topographic features, occurring

* ‘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: 493-532, 1990. 0 1990 Kluwer Academic Publishers. Printed in the Netherlands.

494 J. c. AUBELE AND E. N. S!-Y”TA

primarily on plains units and generally circular in planimetric outline. Barsukov et al. (1986) included features up to 15 km in diameter and commented on the possible similarity in morphology, but not necessarily in scale, of small domes on Venus to volcanic domes or cinder cones on Earth and Mars. Head and Wilson (1986), in their theoretical treatment of volcanic processes and resultant constructs on Venus briefly discussed the domes and stated that, under Venus surface conditions, effusive volcanic activity, explosive volcanic activity, or a combination of both, could produce features similar in morphology to the known characteristics of small domes. Preliminary analysis of some aspects of the distribution of small domes was published in the USSR by Slyuta et al. (1988b). Portions of the current study were previously presented at the Lunar and Planetary Science Conference (Aubele, et al., 1988; Slyuta, et al., 1988, Aubele, 1989; Sinilo and Slyuta, 1989; and Aubele, 1990). Subsequent to this study, Garvin and Williams (1990) have agreed with our interpretation of the domes as dominantly effusive shield volcanoes, have compared Venus domes with low Icelandic shields, and have ap- proached the calculation of total dome number and volume by different methods than described here.

The word ‘dome’ was used by Barsukov et al. (1986), following the lunar nomenclature, with no intent to invoke the strict volcanological usage of ‘dome’, or its terrestrial lithologic connotations. In fact, the features are neither small nor are they ‘domes’ in the general terrestrial usage of these words. The term ‘dome’ for the features on Venus will be retained in this paper and we include all circular to sub-circular positive topographic features with a range in basal diameter from the effective resolution of the Venera images (2 km) to 20 km in diameter. It is concluded in this study that most of these features are volcanic, and that they can be interpreted as dominently effusive shield volcanoes. The continued use of the term, ‘dome’ or ‘dome-like hill’ rather than ‘cone’ or ‘shield’ is intended to imply a broad, lens-shaped, geometric shape, while recognizing the fact that a small percentage of these hills may have been formed by non-volcanic processes or by a volcanic process that includes a small explosive eruptive component.

Venera 15116 images show a wide range of features and terrain types on Venus; including extensive plains-forming units, large mountain belts, localized linear ridge belts, large areas of complexly lineated terrain (tessera), and a variety of large circular features, many of which are interpreted to be volcanic in origin. Of all of these surface features, the abundant domes are the smallest resolvable features with distinguishing characteristics and the most numerous in total number. They are a ubiquitous landform recognized in every quadrangle of the Venera 15/16 data set.

Their uniformity in size and great abundance implies that they represent a widespread process occurring on the surface of Venus and that they are of potential importance in understanding the geological surface evolution of Venus. In this study, we assess the geologic characteristics, associations and distribution of the small domes and examine their implications for the regional and global character of the surface of Venus.

SMALL DOMES ON VENUS 495

2. Venera Data Characteristics

Venera 15 and 16 imaged the surface of Venus using synthetic aperture radar (SAR) of 8cm wavelength with a mean incidence angle of 10” from the local vertical and with variations across an image strip from 7 to 13” (Kotelnikov et al.,

1985). The imaged area is north of 2.5 to 30” N latitude and includes approximately 115 x lo6 km2 of the surface. In Venera images, bright features can represent surface roughness, reflectivity, and/or topographic slope: however, for the incidence angle of this instrument and the slope values typical of Venus (Pettengill er al., 1980; Mead et al., 1985), the backscatter cross-section of the surface is most strongly dominated by the topographic gradient at the l-10 km scale; and small variations in surface slopes will alter the amount of backscatter more than will variations in other aspects of the surface visible to radar. Radar scattering models (Hagfors, 1970) predict that the relative backscatter cross-section (a,,) is related to incidence angle (0), such that

fro(~) = (cos4 13 + Csin28))3’2(p0C/2) ,

where po is the Fresnel reflection coefficient at normal incidence, and C is the Hagfors or roughness parameter and is the inverse of the surface roughness. From this relationship it can be seen that for small values of 0 (~20”), such as in the Venera SAR, large changes in (TV will result from small changes in surface slope.

Consequently, small changes in surface slope will greatly change the relative backscatter cross-section in Venera images. Therefore, circular features with paired flanks showing high and low radar backscatter, are interpreted to be hills or depressions that may or may not also exhibit some surface roughness or reflectivity. The nature of the Venera radar image is such that altimetric slopes and morphologic forms associated with local topography are broadly similar to scenes produced by visible light, but some distortions are inherent in radar imaging systems generally due to the range delay in returned signal and variable back- scattering. Of particular importance in the Venera images of the small domes is distortion due to foreshortening; that is, an apparent shortening in length of radar- facing steep (2 10“) slopes (Figure la). For example, craters and depressions are compressed in the distal sector and expanded in the proximal sector (Ford, 1989), while most positive topographic features will be compressed in the proximal flank and expanded in the distal flank. The circularity and apparent diameter of features in the image can be affected by this foreshortening distortion. For this reason, diameters measured in the along-track radar range direction are subject to the least distortion. However, the actual width of the object measured is more difficult to determine in this direction because the radar is two orders of magnitude less sensitive to slope in the along-track direction. For this reason, diameter measurements of the small domes were made in the cross-track radar imaging direction while recognizing that the diameters measured probably represent minimum values (Figure lb). For the incidence angles of Venera, distortion of features by layover,

496 J. C. AUBtLE AND E. N. SLYV’IA

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Fig. 1. (a) Schematic diagram of radar backscatter associated with small dome-like hills. Range delay causes shortening of the proximal (radar-bright) slope in the radar image. (b) Schematic diagram of dome diameter (d) measurement. Distortion may occur in the image appearance of proximal and distal slopes in the cross-track direction, but slopes are more difficult to detect in the along-track direction.

Diameter measurement may be minimum diameter if flank slopes vary.


and the subsequent development of true radar shadows, is unlikely to occur except in the case of slopes much greater than the radar incidence angle.

Where two or more data sets overlap to image the same feature or features on Venus, especially at similar resolution, there is a significant advantage to be attained in the identification of characteristics based on radar properties and in geologic interpretation of specific features. Overlap of Venera 15116 and Earth- based Arecibo and Goldstone radar images occurs in a few areas within a band 40”N and 40” S of the Venus equator and within these latitudes there is some overlap in coverage of areas of small dome concentration. Individual radar images obtained at the Goldstone tracking station exhibit a range of incidence angles between 0” and 8”. At these incidence angles, Hagfors law predicts that the radar is sensitive to changes in topography. Radar bright features in this data set, therefore, can be interpreted in the same manner as in the Venera data set. Recent high resolution images acquired by Arecibo are comparable to Venera resolution (Campbell et al., 1989; Senske et al., 1990). The incidence angle varies across the image, increasing wih latitude from the sub-radar point which falls in a band between 9” N and 9” S. For the range of incidence angles that occur, the Hagfors law predicts that the radar is sensitive to changes in surface roughness. Radar bright features in this data set, therefore, are interpreted to exhibit some surface roughness that varies from the surrounding terrain. Small domes in the overlapping coverage areas have been examined in both Venera and Arecibo images. Al- though, the domes are occasionally difficult to identify in the Arecibo images, the presence of significant radar signal return from a number of small domes in all three data sets suggests that detection of small domes can, in many cases, be based on flank characteristics that differ in radar backscatter properties from the surrounding plains as well as by a change in topographic slope.

On Venera, a nadir-pointed altimeter acquired data simultaneously with the radar. Due to the size of the altimeter footprint on the surface, reliable information is only available for surface relief features greater than 40-50 km wide (Barsukov et al., 1986) and information cannot be obtained for the small domes. However, radarclinometric measurements by the Institute of Radio Engineering and Elec- tronics, USSR Academy of Sciences, have produced slope and height information for a few small domes (Sinilo and Slyuta, 1989). A radar image presents data on surface relief in differential form, and topography can be produced from a radar image by a process that is similar to photoclinometry (Wildey, 1986). In order to calculate radarclinometric measurements the scattering function and reflectivity are assumed to be constants over the area to be profiled. This is an arguable assumption when dealing with features interpreted to be volcanic edifices; and the resultant radarclinometric data must be interpreted with this in mind.

3. Characteristics of Venus Small Domes

The characteristics of individual small domes include: (1) circular to sub-circular in planimetric outline and ranging in diameter from the effective limit of Venera

498 J. C. AUBELE AND E. N. SLYUTA

resolution (32 km) to 20 km; (2) slopes generally G 10” and flank surface properties that promote radar backscatter; and (3) individually associated features including summit pits, radar bright terrain, and low topographic rises.

3.1. CIRCULARITY AND DIAMETER

At the Venera 15/16 resolution, the domes exhibit generally circular to sub-circular planform outlines. Distortion due to foreshortening and the small size of the domes makes the quantification of circularity imprecise. Very few of the domes show an elongation in planimetric shape: and in most of these cases, an apparent elongation can be resolved as two or more circular to sub-circular domes that are closely spaced or adjoining. A few examples show an apparent interference in circularity of adjoining features (Figure 2b) that we have interpreted as evidence of a sequential emplacement of adjacent domes. A few larger domes show more irregular planform outlines, for example Figure 2d (~20 km) appears to consist of a long ridge-like flank. This may indicate an evolution of planform outline from smaller circular to larger non-circular domes or the detailed planform outline of smaller domes may not be visible at Venera resolution.

The small domes in this study occur in a limited diameter range. The minimum diameter observed is dependent upon the Venera resolution, only those features that showed paired high and low backscatter (bright and dark) sides were interpreted to be domes and included in the data set described in this study. In the areas of best resolution, minimum diameter is =2 km. Small bright spots, without a dark side, occur frequently in the radar images and may represent (1) loss of information; or (2) small areas of unusual radar backscatter due to surface roughness or reflectivity. However, some of these bright spots may represent a population of small domes that are less than 2 km in diameter, or have exceptionally low slopes but rough surfaces, and cannot be unambiguously resolved in the Venera images. The maximum diameter included in this study is 20 km. This is a somewhat arbitrary cutoff because similar appearing features occur on Venus in the 20 to 100 km size range (Slyuta and Kreslavsky, 1990). These features exhibit similar circular to sub-circular planimetric outlines, slopes and associated features such as summit pits and radar bright patches; however, the defined 20 km maximum reflects a significant change in the overall number of edifices; that is, 22,000 small domes (620 km) and only 800 similar features occuring in the diameter range from 20 to 100 km in the Venera coverage.

A typical cluster of small domes in Tethbs Regio (Figure 3) was selected as a type locality and examined in detail in order to determine the diameter frequency distribution. Measuring in the cross-track direction from the near edge of the bright proximal flank to the far edge of the dark distal flank of all identified domes in this cluster, best estimate modal basal diameter was determined (Aubele et al., 1988). These measurements may represent minimum diameters since radar response is dominated by large-scale slope effects on the order of meters and it is difficult to locate the precise position of the base of a slope, particularly if the


Fig. 2. Examples of small dome-like hills with summit pits. (A) Subquad 4-31, SE of Otau corona, 64” N, 307”; (B) sub-quad 12-33, NE of region of arachnoids, 46” N, 24”; (C) sub-quad 22-23, 32”N. 15”; (D) sub-quad 6-23, between Fortuna and Meshkenet Tessera, 71”N, 96”. Note double vents and interference of circularity (B). Quad 22 exhibits a larger than usual number of domes with summit pits, as illustrated by (C). Bar scale is 100 km. The summit pit diameters of small Venus domes range from 15 to 33% of basal diameter. Average pit/dome diameter ratio for Venus small domes is 0.29.

lower slopes are gradational or if there is an abrupt change in slope on the flanks of the feature. The 443 domes identified in this cluster ranged in basal diameter from 2-8 km, with a predominant size range of 2-5 km and a mode of 3-4 km, although average or modal basal diameter may not be accurate because of the unknown population of domes smaller than 2 km.

The dome diameter frequency distribution data from this type locality is shown in log-log and semi-log plots fitted to power law and exponential distributions in Figure 4a,b and Table I. In general, a power-law distribution predicts an infinitely increasing number of features with decreasing size, an assumption common to impact crater statistics, while an exponential distribution predicts a finite number


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502 J. C’. A”l3ELt AND L. N. SLYU-IA

TABLE I

Estimate of parameters for exponential and power law models of size distribution for representative sample of Venus small domes and Earth seamounts

Small Domes (Tethus Regio) EEZ Seamounts

Exponential Power Law n = n,,e -t,r 1, = n,,r- ’

0 t111 ? 141

1.1 1.39 x l@’ 4.8 9.32 x 10”

0.4 4.7 x 10’ 2.1 I.09 x lo”

of small domes less than 2 km in diameter and a finite smallest-size diameter. Binning by square root of 2, which is standard procedure for impact crater statistics, has not been used. This standardized statistical binning is useful for the comparison of impact crater populations between different planetary terrain units and is associated with assumptions of crater population and relative surface age, and therefore would not be useful in this case. Although the fit to a power- law and exponential distribution appears similar in the center of the curve, the exponential curve is linear to smaller diameters than is the power-law. This suggests that the number of small diameter domes is finite and that an exponential distribution can be used to predict total number of domes. The implications of this are further discussed in Section 4.3. For another approach to calculating total global number of the Venus domes see Garvin and Williams, 1990.

3.2. SLOPE, FLANK CHARACTERISTICS AND ESTIMATED VOLUME

Based on constraints on the appearance of features imaged by Venera 1506, in particular the general lack of layover distortion caused by slopes much greater than the SAR incidence angle, the majority of small domes appear to have flank slopes ~7-13”. A modal height of 600 m can be derived from simple geometric models, assuming: (1) triangular cross-section; (2) measured basal diameters of the Tethus Regio domes; and (3) assumed slope of lo”. In this simple geometric model, increased basal diameter would correspond to increased height; however, this relationship is undoubtedly modified in reality by variable slopes on the dome flanks, the presence of flat or concave summits, or the presence or absence of summit pits as discussed later.

Radarclinometric profiles (Sinilo and Slyuta, 1989) for seven small domes in Tethus Regio, ranging in basal diameter from 7.2 to 19.2 km, are shown in Figure 5 and Table II. The profiles show features with low slopes and aspect ratios (h/d) of approximately 0.02, where height is small in comparison to basal diameter. This confirms the assumptions based on the appearance of the features as imaged by the Venera radar. Sinilo and Slyuta (1989) have interpreted the domes as showing an average profile that is slightly convex upward near the summit, rela-


0 6

7

Fig. 5. Radarclinometric profiles (after Sinilo and Slyuta, 1989) for Venus small dome-like hills in the Tethus Regio area. Numbers correspond to Table II.

TABLE II

Radarclinometry data for representative sample of small dome-like hills on Venus based on Venera 15/16 radar images

Coordinates Basal Height Slopes latitude longitude diameter max (km) min (km) proximal distal

W-4 flank flank

1 6tY46’ 112”23’ 7.2 0.155 0.155 2.2” 2 61”03’ 117”17’ 8.8 0.300 0.104 1.5”

3 6WO6’ lll”O3’ 9.6 0.195 0.115 1.5” 1.0”

4 60”35 ’ 112”02’ 11.2 0.205 0.167 1.3” 5 60”38’ 117”29’ 14.4 0.652 0.256 5.2”

1.6” 6 60”49’ 116”44’ 16.8 0.304 0.229 1.6”

0.9” 1.3”

7 60”30’ 116”59’ 19.2 0.545 0.213 1.4 5.0 2.5” 1.0

3.1” 4.9” 3.6 1.6” 3.1” 1.5” 2.9” 4.4” 1.7” 4.4” 2.5” 0.6” 3.0” 1.6”

* ‘Proximal flank’ refers to radar-facing flank: ‘distal flank’ refers to flank facing away from the radar. Multiple slope values for each flank of each dome are listed in consecutive order (from top to bottom in the column) from flank summit to base.

tively straight on the flank, and slightly concave near the base on the basis of their radarclinometric study. Mean height is 340 m, the maximum slope appears to be approximately 5” and the minimum slope is approximately 1”. Using a flank slope of 5” and a simple triangular cross-section, the modal volume of the domes in the Tethus Regio dome cluster is calculated to be 0.73 km3.

Where domes occur in an area of overlapping coverage of Venera and Arecibo, only large domes (2 10 km) in the Venera image are detectable in the correspond-


ing Arecibo image, and only the radar bright slope in the Venera image can be correlated with features in both images (Aubele, 1990). This characteristic makes identification of a dome solely on the basis of the Arecibo image difficult; however, the presence of significant radar signal return from some domes at the high incidence angles of Arecibo implies the presence of flank surface properties that promote radar backscatter, such as surface roughness that varies at the wavelength of the Arecibo radar. Although small domes are primarily observed in low incidence angle radar images, their presence may be detected on images produced by radar at higher incidence angles depending on their small scale surface characteristics.

3.3. ASSOCIATED FEATURES

In the Venera data set, approximately 2% of the domes have visible summit pits (examples are shown in Figure 2). Presumably there is an unknown number of domes with summit pits smaller than the limit of Venera resolution, although the summit pits are probably not a ubiquitous feature of all domes. The visible pits appear to be circular and occur near the center of the domes at their apparent topographic summit. Average pit/dome diameter ratio for Venus small domes with visible and measurable summit pits is 0.29.

A few isolated examples of radar bright surfaces surrounding domes, occur in the Venera data set. An example is shown in Figure 6a where small domes can be identified within radar bright patches occurring on a mottled plains surface in Sedna Planitia, southeast of Clotho Tessera. In most cases, these radar bright patches do not appear to have a topographic component and are interpreted to result from increased backscatter due to changes in surface roughness or reflectivity relative to that of the surrounding plains. In one instance the origin of a bright surface associated with a dome can be readily interpreted. Figure 6b, located in Guinevere Planitia, shows an associated bright feature that appears to be a volcanic flow; and it is the only example in the Venera coverage that can be readily interpreted as such in close association with a dome. This dome is approximately 18 km in diameter and the flow-like bright feature extends 100 km and appears to bifurcate into two segments each approximately 10 km wide. The dome and flow are clearly visible in the recent high-resolution Arecibo data (Campbell et al., 1989; Aubele, 1990) where variations in surface roughness at meter scales tend to dominate the backscatter.

A major association of small domes with radar bright aureoles or halos occurs in Loukha Planitia, northeast of Atalanta Planitia. This area has been discussed briefly by Kryuchkov and Basilevsky (1989) and Klose (1990). The domes in this area occur on a mottled plains unit that is located between two areas of ridged terrain. The plains unit shows extreme variation in both radar bright and radar dark surfaces. The radar bright surfaces, which do not appear to have a topographic expression, range in diameter from 10 to 35 km, with adjoining patches producing bright areas up to 100 km in length (Figure 7). The bright aureoles gradually merge into mottled plains to the south and east where irregular patches of dark


(4

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Fig. 6. (a) Image of domes and associated bright patch, Sedna Planitia, sub-quad 11-23. Small domes occur at far right of elongated radar-bright patch (lower arrow) in mottled plains and in center of radar-bright patches that appear to have some topographic relief (upper arrow). Bar scale is 100 km. (b) Image of small dome and associated volcanic flow feature, Guinevere Planitia, sub-quad 20-14. Flow-like feature appears to flow toward an area that is a topographic low, in the Pioneer Venus topography. between the dome and a large volcanic center that can be partially seen at the left margin

of the figure. Bar scale is 100 km.

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terrain can be mapped. Small domes in this area predominantly occur on the radar bright patches, either as single features or in groups, and commonly near the center of the bright surfaces. While there is a general correlation between their occurrence and that of the bright patches, there are some radar bright patches that exist without visible associated domes, at least at the limiting resolution of Venera, and a small number of domes that occur without associated radar bright patches. In general, however, the radar bright surfaces on this plain seem to be related to the occurrence of small domes and we interpret the aureoles as consisting of material formed or deposited on the surface in association with the development of the domes.

In the absence of topographic slopes, bright surfaces in the Venera images would imply an increase of surface reflectivity and/or roughness at the 8 cm scale. The general Atalanta region is an area of high radar reflectivity in the Pioneer Venus data set (Garvin et al., 1985) so that the bright aureoles in this area may simply be related to some regional variation in reflectivity, perhaps a variation in the composition of material available at the surface or in the subsurface, as previously suggested by Bindschadler and Head (1989). Abramov et al., (1989) have produced generalized maps of RMS slope and reflectivity from the Venera data that show low to average values averaged over the mottled plains unit on which the aureoles occur. This result is probably due to the patches of dark terrain that also occur in this plains unit. Kryuchkov and Basilevsky (1989) suggest that the major component of the aureole brightness is due to roughness and interpret them to be young, rough lava flows, although in this general region their figures show a comparable range of values for RMS slope and reflectivity of both the bright aureoles and the surrounding plains. The low elevation of the Atalanta region may have contributed to an increased possibility of vulcanian eruptions. If the aureoles do represent significant pyroclastic activity, since they extend 10’s of km from postulated dome source vents, then they imply the presence of volatile rich magmas, at least in a few areas. Garvin et al. (1982) Calculate 2-4 wt% required volatiles.

The distinct boundary between the bright halo and the plains and their apparent near-circularity (although foreshortening may alter their appearance) are observations that are difficult to explain with either a pyroclastic or young flow origin. An alternative volcanic interpretation, that we suggest, is that the bright aureoles represent older surface materials surrounded by younger and smoother lava plains. In this interpretation the circularity is a result of topographic gradients away from a central vent region. Subsequent to the formation of the domes and their associated flows, later plains-forming material flooded and surrounded the lower flanks at a uniform topographic contour level.

In a few instances, a low basal topographic rise or platform appears to be associated with a single dome or with a small group but this does not appear to be a common association. The basal platforms extend 2 or 3 diameters beyond the domes, and may be material associated with the formation of the feature, such


as lower dome flanks with an abrupt change in slope, or they may represent pre- existing topography.

At the Venera 15/16 resolution, the individual domes appear to lack visible radial or elongate patterns, summit grooves or graben, or other indications of associated tectonic features. In many cases, small domes occur on plains that show bright striations interpreted to be tectonic lineaments (upper right, Figure 6a), but these striations generally disappear in the vicinity of the domes and appear to be covered by plains material associated with the domes. The lack of visible tectonic features at Venera resolution does not preclude the association of the domes with some regional. structural control. Local and regional spatial patterns of distribution have been examined in order to answer this question and are discussed in the next section.

4. Distribution

Preliminary analysis of some aspects of dome distribution was published in the USSR by Slyuta et al. (1988b) who produced density contour maps of dome concentration. They classified local distribution patterns of equidimensional groups of several tens of domes within areas of lo3 km2, clusters consisting of lo-20 groups within areas of lo5 km2, and dome concentrations consisting of large numbers of clusters. They recognized one such dome concentration in the general area of 60”N, 120” E. In this paper, we elaborate on this discussion and present new data that shows two major areas of concentration of dome clusters separated by approximately 180” in longitude.

The domes in local groups and clusters occur in equidimensional patterns showing no dominant orientation or alignment. A representative cluster in the Tethus Regio region (shown in Figure 3) shows an overall density of approximately 0.4 per 100 km2 and maximum density of 1 per 100 km’.

For the purposes of this study’s spatial pattern analysis of domes, a sub-set of Venus small domes has been defined as ranging in diameter from 5 km to 20 km. The lower diameter limit was chosen so that spatial distribution patterns were not due to variations in the resolution of individual radar images and quadrangle mosaics. The domes were mapped on the Venera Lambert conformal projection, their latitude/longitude positions digitized, and the point locations reprojected onto an azimuthal equal area polar projection (Figure 8). The resulting plots of dome density by equal area latitudinal bands and 15” longitudinal wedges are shown in Figure 9a,b. The area within which domes could be recognized is approximately 88% of the total area north of 30”N latitude due to gaps in the Venera coverage. Quadrangle 1 and 27 were not included due to poor overall image resolution. The resultant data set totals 7171 small domes. This subset of domes provides a statistically significant population with which to acquire information regarding spatial patterns and regional density.

The polar projection dome map and number versus longitude plot show two major areas of concentration of dome clusters occurring approximately 180” in


longitude apart. The number versus latitude plot may also reflect these two major concentrations; however, it also indicates a greater number of domes at lower latitudes.

The largest concentration coincides with a major dome density concentration in the Akkruva Colles area of Niobe Planitia centered at approximately 45” N/120” E. This area is east of Tellus Regio and north of the flanks of the Thetis Regio rise and includes Ananke, Kutue and Shimti Tesserae and Uni Dorsa. Major dome clusters within this general area of dome concentration are located in Tethus Regio (65” N, 110” E), Atalanta Planitia (60” N, 155” E), Ananke Tessera (55” N, 138” E), and Akkruva Colles (from Niobe Planitia, 35” N, 130” E, to Allat Dorsa, 65” N, 70” E). The Akkruva region cluster appears to be made up of several smaller clusters aligned in a significantly elongated area oriented NW-SE. The second

Fig. 8. Dome location map. All domes larger than 5 km plotted on an azimuthal equal area polar projection. Domes primarily occur in rolling plains terrain. Two major areas of dome concentration are north of Thetis Regio in the Akkruva Colles area and northeast of Beta Regio in Guinevere Planitia. Secondary concentrations occur in Ganiki, Bereghinya, Niobe, and Snegurochka Planitias.

Total number of domes plotted is approximately 7000.

I. C. AUBELE AND E. i-4. SLYUTA

240 300 0

(4

(b) Fig. 9. (a) Number of domes per lo6 km2 plotted by (a) longitude and (b) equal area latitude bands. The two major dome concentrations are shown in (a) and may be shown in (b) which also indicates a

greater number of domes at lower latitudes.

significant concentration occurs in northwestern Guinevere Planitia centered at approximately 35”N/300” E. This area includes Lachesis Tessera and the north flank of Beta Regio, the latter is an area that was previously identified by Stofan er al. (1989) as having a relatively large population of small domes.

The Akkruva concentration is centered at approximately 45”N, 120” in quadrangle 14; and the Beta concentration is centered at approximately 35” N, 300” in Quadrangle 20. Because the concentrations extend to the limits of Venera image coverage, the extent of these regional dome concentrations is known only for their north, east and west margins. Recent Arecibo images (Campbell et al., 1989)


cover part of the Beta concentration, however a complete distribution of the domes in this area is difficult because of variation in incidence angle and difficulty in identifying domes in images produced by high incidence angle radar (Aubele, 1990).

The Akkruva and Beta areas of dome concentration are characterized by intermediate elevation (6051-6053 km) plains and generally coincide with a rolling plains surface (Masursky et al., 1980) and with the Plains-Corona-Tessera, Plains, and Plains-Corona Assemblages (Head, 1989), which have been characterized in a tectonic sense as having been formed by predominantly vertically directed deformation. The surface of both regions shows a relatively prominent regional orthogonal tectonic pattern analogous to that associated with tessera but distin- guishable from the disrupted and elevated surface of adjacent areas of true tessera and the surrounding regions of relatively smooth low-lying plains. In a few dome clusters within these areas of concentration there is a suggestion of this orthogonal pattern in the alignment of dome groups (Venera Quadrangles 14 and 22). This pattern is rare overall, since most of the domes in local groups and clusters occur in equidimensional patterns showing no dominant orientation or alignment. Pioneer Venus radar properties analysis (Head et al., 1985) describes these two areas as smooth to transitional roughness and low to intermediate reflectivity. The latter is interpreted to imply a predominant surface material ranging from porous material such as soil to a material comparable to terrestrial rock. Both of the regional dome concentrations occur off slope, or in proximity to, significant regional topography that correlates with positive anomalies in the gravity potential field (Bills et al., 1987).

The mechanism of formation of small domes has resulted in their occurrence throughout the northern latitudes of Venus; however, there has been an enhance- ment in the concentration of domes in two specific regions that are approximately 180” in longitude apart. The observed concentration in two major zones suggests that the process responsible for their formation may operate at global or near- global scales.

5. Terrain and Geologic Associations

In order to further assess the potential origins of small domes and their regional concentration, we have examined the detailed terrain characteristics and the geologic environment of individual regions of small domes. The surface of Venus consists of several characteristic types of topographic and physiographic terrains as identified on the basis of Venera, Arecibo, and Pioneer-Venus (Masursky et al., 1980) radar data sets. These include: (1) extensive areas of lowland plains, generally interpreted to be volcanic; (2) highlands, generally plateau-like areas several kilometers above surrounding lowland plains, topographically rough, and associated with complex tectonic processes; and (3) rolling plains, consisting of intermediate elevation. Small domes preferentially occur in large numbers on the intermediate elevation rolling plains units; and the domes predominantly occur on


(a)

lb)

Fig. 10. (a) Dome density contours on azimuthal equal area polar projection produced from dome location map (see Figure 8). Density contours show number of domes per IO6 km*. Dome contour interval is 50. Total number of domes represented is approximately 7000. (b) simplified Venera 15/16 terrain map (adapted from Barsukov, 1986) on polar projection. A comparison of (a) and (b) shows that dome concentrations occur on plains units in areas dominated by coronae. arachnoids and small patches of tessera; dome concentrations do not occur in areas dominated by ridge belts or in the

general area of Ishtar Terra.


plains units in the areas of Venus that are dominated by coronae, arachnoids and small areas of tessera (Figures lOa,b).

5.1. TERRAIN ASSOCIATIONS

The major areas of dome concentration, described above, occur on mottled plains units, designated as “rolling plains, interpreted to be of volcanic origin” on the Venera 15/16 geologic map (Barsukov et al., 1986) and as primarily hummocky plains on the USGS-USSR map (Sukhanov et al., 1989). The common interpretation of volcanic origin for the plains is based on the fact that light and dark mottling appears to be a common characteristic of radar images of volcanic flows of varying radar reflectivity and small scale roughness. The major areas of dome concentration also coincide with the “plains-corona-tessera, plains, and plains- corona” assemblages described by Head (1989), who has characterized these assemblages in a tectonic sense as having formed predominantly from vertically directed deformational forces as opposed to significant lateral deformation known to occur elsewhere on Venus (Crumpler et aE., 1986; Head, 1990; Vorder Bruegge and Head, 1990). Clusters frequently occur on plains units at the margins of tessera terrain or in plains between areas of tessera, while small groups occasionally occur in intra-tessera plains within large tessera units. These plains frequently appear to embay tessera terrain imaged by Venera, and plains and their associated domes frequently appear to have been formed by resurfacing or flooding that post-dates the formation of the tessera terrain. Areas of buried topography at the margins of tessera and in intra-tessera plains can be readily identified and distinguished from small domes by their elongated and aligned features. The partially buried tessera terrain presents a different appearance than do clusters of small domes which are characterized by large numbers, randomly scattered occurrence, consistent circularity, and the general lack of linear alignment.

5.2. GEOLOGIC FEATURE ASSOCIATIONS

A survey of the geologic associations of domes in every sub-quadrangle of the Venera 1506 data set illustrates that dome groups or clusters of groups are associated with the following specific geologic features: (1) coronae, (2) arachnoids, (3) intermediate-sized hills (20-100 km in diameter) interpreted to be volcanic constructs, (4) large volcanic centers (2100 km in diameter), and (5) calderas. This association of domes and these specific geologic features is ubiquitous, even where one of these geologic features occurs in an area that has a low overall density of small domes (Aubele, 1989).

Groups of domes occur predominantly inside the annular concentric ridges of coronae, while smaller groups occur on the surrounding plains. Flow-like features and domes in the interior of corona structures have been previously interpreted as evidence of volcanic activity (Barsukov et al., 1986: Pronin and Stofan, 1990).

Aruchnoids occur in clusters in lowland regions; and have been described as “central domes (~10 to over 30 km in diameter, commonly with central pits) surrounded by rings and linear features interpreted to be tectonic in origin” (Stofan

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and Head, 1988). The central domes have been interpreted to be volcanic in origin (Stofan and Head, 1988). Groups of small domes occur in the areas of lineations on the plains surrounding and between adjacent arachnoids. Infrequently, small groups occur in arachnoid-like depressions spatially associated with arachnoids.

Intermediate-sized hills (20-100 km in diameter), frequently exhibiting summit pits and associated radial or lobate flow features, and interpreted to be volcanic edifices, are commonly spatially associated with groups or clusters of small domes. These intermediate sized hills are fewer in absolute number than the small domes and generally occur as individual isolated features on plains units near groups or clusters of small domes. Groups and clusters of small domes occur predominantly on the lower flanks, or beyond the distal edges (Figures 11 and 12), of the bright radial patterns associated with large volcanic centers (2100 km). This may imply that the spatially associated small domes pre-date final volcanic eruptions at these centers, although some domes may be present but difficult to detect in the radar bright flanks.

Venus domes are predominantly associated with terrain units and geologic features that have either been interpreted to be volcanic or to be associated with the process of volcanism. In general, the areas of Venus that are dominated by linear ridge belts (Frank and Head, 1990) contain a very low density of small domes, although small groups of domes occasionally occur at the ends. or along the margins of individual ridge belts, such as the group of domes occurring along one margin of Ausra Dorsa. This particular group shows an unusual uniformity in basal diameter that is not generally observed in other dome groups. The other significant area of dome depletion is in the Ishtar Terra region. Small groups of domes occur on the rims and periphery of the calderas, Collette and Sacajawea (Magee and Head, 1988) and domes occur on the plains to the north and south of Ishtar Terra; but Lakshmi Planum and the horizontal compressional fold belts of Akna and Freyja Montes (Crumpler et al., 1986) contain a very low density of small domes.

6. Discussion

6.1. SUMMARY OF SMALL DOME OBSERVATIONS

Small domes have specific individual and general characteristics and associations that are summarized in the following points and in Figure 13. These observations represent minimum criteria that must be explained by any interpretation of their origin. It is anticipated that increased resolution and variations in incidence angle in images produced by the Magellan mission will both reenforce and add to this list:

(1) The domes occur in groups or clusters of groups. Where two or more domes are closely spaced or adjoining, an interference in circularity implies sequential emplacement and growth and suggests a constructional origin. The domes range in diameter from the effective limit of resolution to 20 km, with a mode of 3-4 km

SMALL DOMES ON VENUS

A h=- 1 Al t. 2km l-l

s -50 A”

I’

I------ r=l-10hv----+~ Fig. 13. Schematic diagram showing summary of observations describing morphology and associated

characteristics of small domes.

for a typical dome cluster. The generally circular planimetric outline implies a process of formation in which material is relatively evenly distributed within centralized discrete edifices. The large number of edifices ~20 km implies that a characteristic and recurring finite and specific volume of material is involved in the formation of each small dome.

(2) Small domes are characterized by low topographic slopes and formed by a relatively small volume of material. Radar-clinometric measurements of domes indicate heights of lo* m, and slopes ~5”. Estimated modal individual volume is 0.73 km3. The domes are radar bright in the Venera images predominantly due to topographic slope changes. Areas of buried topography, tessera protruding through plains-forming material, at the margins of tessera and in intra-tessera plains can be readily identified and distinguished from small domes. Based on the comparison of domes as imaged by both Venera and Arecibo, there is a contribution of surface roughness to the flanks of some domes. However, not all of the domes visible in a Venera image are identifiable in the overlapping Arecibo image; and, in most cases, the range of backscatter, due to non-topographic effects, of the domes and their immediate surroundings seems to be generally comparable to the range of backscatter of the larger plains surface on which they are located.

(3) Approximately 2% of the small domes have summit pits visible in the Venera images. It is not clear whether this reflects a low frequency of occurrence of summit pits or whether summit pits are, in general, much smaller than the 2 km limiting resolution of the Venera images so that only a small percentage are large enough to be detected. Average summit pit to dome diameter ratio is 0.29. There are occasional associations of radar bright surfaces with domes, and at Venera incidence angles it is not surprising that, even if associations with radar bright surfaces are common, only a few associated non-topographic radar bright surfaces are visible in areas of dome concentration.

518 J. C. AUBELE AND t. N. SLYUTA

(4) The domes preferentially occur on plains units that have been interpreted to be volcanic. Based on Arecibo images, the range of backscatter of domes and the range of backscatter of the plains with which they are associated seems to be generally comparable indicating that the domes and their immediate surroundings may be composed of the same material.

(5) The mechanism of formation of small domes has resulted in their occurrence throughout the northern latitudes of Venus; however, there has been an enhance- ment in the concentration of domes in two specific regions that are approximately 180” in longitude apart. The observed concentration in two major zones suggests that the process responsible for their formation may operate at global or near- global scales.

(6) The dominant geologic association of Venus small domes is with features that have been interpreted to be volcanic; and the domes predominantly occur on plains units in the areas of Venus that are dominated by coronae, arachnoids and tessera.

(7) The areas of Venus that are dominated by linear ridge belts and the Ishtar Terra region contain a very low density of small domes. Many of the ridge belts, and the erogenic belts around Lakshmi Planum, have been interpreted to be compressional in origin. Major dome concentrations seem to occur in regions of dominantly vertical, perhaps extensional, tectonics. Elevation and crustal thickness in the Ishtar region are two potentially distinguishing characteristics of this area that could be related to a paucity of domes.

6.2. POTENTIAL ORIGINS AND ANALOGS

Based on observed number density and distribution, small domes are as abundant on Venus as are impact craters on many of the smaller terrestrial planets. Because they are so numerous, the origin and mechanism of formation of the small domes of Venus is a significant question that has implications not only for the geologic interpretation of much of the surface, but also for heat transport and surface evolution of the planet as a whole.

Hypotheses for the origin of the small domes on Venus include the following: (1) non-volcanic processes: (a) topographic highs with intervening flooded lows; (b) domical uplifts, including all endogenic mechanisms by which the surface can be deformed; (c) concentrations of surficial materials, including all exogenic mechanisms by which the surface can be shaped into hills; or (2) volcanic processes: (a) dominantly Icelandic-type centralized shield eruptions; (b) dominantly pyro- elastic Strombolian or Plinian-type eruptions with or without associated flows; (c) some combination of (a) and (b); (d) extrusive domes produced by relatively viscous material; and (e) domical uplift produced by near-surface laccolithie in- trusion.

Based on their area1 and size-frequency distribution, geologic associations, and individual characteristics as summarized in Section 6.1, the majority of small domes are interpreted to be dominantly effusive shield volcanoes constructed by multiple centralized eruptions.


In order to aid in the assessment of the morphometric characteristics of the Venus small domes, comparisons can be made with concentrations of similar morphologic features on Earth. Dominantly effusive volcanism occurs on Earth in the form of shield volcanoes and domes, and basaltic shield volcanoes appear to be common on many planetary surfaces. Lunar domes (Head and Gifford, 1980) terrestrial low shields such as the Snake River plains shields (Greeley and King, 1977) and the low small shields of Iceland (Pike, 1978; Pike and Glow, 1981; Garvin and Williams, 1990) appear to be similar, in terms of generalized morphology/morphometry, basal diameter range and flank slope comparisons to Venus small domes, but they occur in much smaller numbers.

There is another type of effusive volcano on Earth which occurs in much larger numbers. The most abundant volcanic features on Earth occur on the seafloor in the form of seamounts (Abers, 1988; Batiza, 1982; Smith and Jordan, 1987, 1988). Very large, intraplate seamounts appear to be composites of effusive flows, pillow lavas, sheet flows, and hyaloclastic eruption products (Bonatti and Harrison, 1988). These are large edifices, with relatively steep slopes and are numerically a minor fraction of the observed oceanic seamounts. The more numerous seamounts orig- inating near oceanic spreading axes are generally smaller, low slope edifices interpreted to be dominantly effusive lava constructs composed of pillow lavas and sheet lavas. They are generally irregularly circular in planimetric shape, although Vogt and Smoot (1984) have suggested that oceanic volcanoes follow a progression from small circular simple edifices to large star-shaped edifices dominated by flank eruptions and multiple summits. Seamounts frequently exhibit constructional volcanic features and summit depressions ranging from craters to large complex calderas (Taylor et al., 1980; Fornari et al., 1984). They commonly show a conical profile with occasional truncated tops which apparently represent summit collapse and infilling (Searle, pers. comm. 1989). Basal diameters are generally calculated by measuring bathymetric height and assuming a fixed height to radius ratio, and are estimated to be in the range of 2 to 30 km; although the limiting resolution due to technique and small scale surface topography on the seafloor presents a problem comparable to the limiting resolution of most planetary geology data sets. Densities of 0.1 to 0.4 per 100 km2 have been calculated for the Pacific (Jordan et al., 1983; Batiza, 1982; Smith and Jordan, 1987). Assuming an exponential distribution and extrapolating from a statistical subset, these submerged oceanic volcanoes are estimated to number lo6 (Smith and Jordan, 1988) in the Pacific alone, although estimates of total abundance vary over an order of magnitude due to limited sampling and the variable density of ship tracks. Seamounts have been examined by a variety of limited data sets acquiring bathymetry, petrologic samples, wide-beam sonar and Sea Beam (multi-narrow beam) topographic information, GLORIA sonar images, and Seasat geoid information. There is no complete data set that can supply size frequency distribution, crater to basal diameter ratio, control of location and distribution, relationship to seafloor plains, or formation dominantly on-ridge or off-ridge.

In order to compare seamount data with the Venus small dome data in as similar


a format as possible, volcanic seamounts from the GLORIA sonar imagery (EEZ- SCAN 84 Scientific Staff, 1986) in the East Pacific were counted and measured. The Geological Long-Range Inclined Asdic (GLORIA) system is a sidescan sonar, developed at the Institute of Oceanographic Sciences, U.K., that is capable of mapping a large area of the ocean floor on a single pass of the ship. Along-track

4 6 8 10 12

Basal Diameter (km)

EEZ Pacific Volcanic Seamounts

Basal Diameter (km)

Fig. 14. Histograms of basal diameter frequency of the dominantly effusive seamounts within the area covered by GLORIA compared with the basal diameter frequency of the small domes within the

Tethus Regio dome field near Meshkenet Tessera.


resolution varies from 200 m to 700 m. Cross-track resolution is 50m. The ship track was fixed by satellite navigation and bathymetric contours from various sources were added. The resultant sonar images cover a total area of approximately 850,000 km2 off the west coast of the U.S. The system was specifically designed to map the morphology and texture of sea floor features in the deep ocean. Features are illuminated from two different directions. Uncovered volcanic lava has a high acoustic impedance and a surface roughness visible in the GLORIA images (Searle and Hey, 1983). Relatively young volcanic material, generated on the seafloor, will produce very bright features in the sonar images, while older, sediment covered areas will be darker in the image. A total of 366 seamounts were identified from the total area covered by sonar mosaics and their accompanying bathymetry and geologic interpretation maps (EEZ-SCAN 84 Scientific Staff, 1986). Only those bright features interpreted to be of definite volcanic origin by the EEZ-SCAN staff were included in the data set. Measurements were made directly from the sonar mosaic and checked against the apparent diameter of the contours when bathymetry was available. Basal diameters of volcanic seamounts visible in the sonar imagery in this area range from 0.7 to 18.0 km, with a predominant size range from 1 to 9 km and a mode of 2 to 3 km.

Figure 14 shows basal diameter frequency distribution of a comparable number of dominantly effusive seamounts in comparison with the basal diameter frequency of the Venus small domes within the Tethus Regio dome cluster. The size range and distribution of the ocean floor volcanic features and the dome field studied in Tethus Regio are relatively similar, although this dome field is anomalously de- ficient in domes larger than 8 km.

6.3. ESTIMATEDNUMBER OFDOMES AND VOLUME CONTRIBUTION

In general, seamount size data fit an exponential distribution (Abers et al., 1988), and this statistical approach is generally used to estimate total number of seamounts. Using this technique and the size frequency distribution data of the domes in Tethus Regio (see Figure 4a), it is possible to estimate the number of domes smaller than 2 km not visible in this region. Assuming a minimum diameter of 0.5 km, 1063 domes, a factor of 2 times the number visible, may be estimated for this area of approximately 1.1 X lo5 km2. Assuming a similar distribution slope planetwide, the total number, 20.5 km, possible over the entire planet is calculated to be 4.4 X 106. This suggests that small domes will be quantitatively the most abundant feature occurring on the surface of Venus.

Using a modal basal diameter of 4 km from the Tethus Regio data and an average slope of S’, modal volume for edifices in the Tethus Regio area is 0.73 km3. If we assume that (1) this modal volume is an estimate for volume contributed by each individual edifice over the whole planet, with the assumption that this diameter would contribute a larger percentage of volume than would any other diameter, and (2) the total number of domes over the whole planet, 20.5 km in


diameter, calculated from an exponential distribution is 4.4 x 106; then, the total volume represented by the total estimated population of domes would be 3.2 x lo6 km3, equivalent to a layer 7 m thick distributed over the entire planet. This volume is 0.03% of the estimated crustal volume of 1 X 10” km3 (Grimm and Solomon, 1987) proposed for Venus. Batiza (1982) has presented a similar calculation for seamounts in the Earth’s oceans, calculating a minimum volume equivalent to a thickness of 100 m distributed only over the ocean basins.

The small domes and their associated plains may have been formed by a related process of regional volcanism. In detail, this interpretation suggests that the plains could consist essentially of overlapping wide and low effusive shields in much the manner envisioned for the Snake River plains on Earth (Greeley and King, 1977). This suggestion is supported in particular by the geometric problems of fitting within the observed area of plains the overall observed number and abundance of small domes predicted on the basis of the exponential size-freqency distribution. Although estimated volumes of individual domes are relatively small, the values represent only the surface edifice and do not include the accompanying intrusive volume or associated distal flows that do not contribute to the construction of the edifice but could contribute to a significant volume of plains if the vertical geologic section consists of layering of overlapping shields. Assuming that associated extrusive flows represent 5 times the volume of the shield edifice and that the associated intrusive component represents 10 times the total extrusive flow and edifice volume, then total volume for the 4.4 x lo6 domes possible on the planet would be 1.6 x lo8 km3 or a layer 350 m thick distributed over the entire planet. This value is 1.6% of the estimated crustal volume of 1 x 1O”‘km’ (Grimm and Solomon, 1987). It should be noted that this calculation only takes into account domes calculated to be on the present surface of the plains and calculations of magma production rate would be dependent on the age of the present surface. For another approach to calculating volume of the Venus domes see Garvin and Williams, 1990.

6.4. CHARACTERISTICS OF VOLCANISM IN THE VENUS ENVIRONMENT

On the basis of their observed size distribution, geologic and terrain associations, and individual characteristics, small domes on Venus are interpreted to be dominantly effusive shield volcanoes. In particular, in abundance and distribution, they appear to be similar to dominantly effusive small seamounts. Because the Venus surface and Earth seafloor surface environments are different, it might reasonably be questioned whether these differences preclude the formation of volcanic edifices of similar morphology.

The morphology of volcanic edifices is a function of differences in several fundamental properties, including: (1) the original chemical composition and volatile content of the magma; (2) the tectonic environment; (3) the surface environment under which they are erupted; and (4) eruption volume, rate and duration.


In order to interpret the morphology of the Venus small domes, we must consider the influence of Venus geologic evolution on the petrology of probable melts, and the Venus environment as it would affect eruption conditions. Hess et al. (1989) have discussed a range of probable Venus melts from MORB type tholeiites to ferro basalts and conclude that viscosity values for tholeiite may adequately de- scribe the characteristic rheology of a typical Venus melt. Garvin and Bryan, 1987, compared Venera 14 and 13 Lander compositional data with terrestrial data and found close similarities to oceanic tholeiitic and alkalic magmas, respectively. Pyroclastic volcanism is possible on Venus, but unless volatile content is large, it is unlikely that edifices produced by dominantly pyroclastic activity would exceed average terrestrial diameters and pyroclastic ejecta would be expected to produce deposits of shorter radius from the source vent than is typical on Earth (Garvin et al., 1982; Head and Wilson, 1986). Wood (1979) discussed the effects of Venus temperature and pressure on volcanic style and landform, and concluded that effusive volcanism would produce long flows and effusive edifices would be broad with low relief.

It has been previously suggested that terrestrial ocean floor lavas are good analogs of Venus lavas in terms of flow lengths (Wood, 1979) and vesiculation (Garvin et al., 1982). In terms of the overall abundance, widespread distribution, individual size range, and association with widespread plains units, small domes are interpreted to be most similar to the characteristics of dominantly effusive seamounts on the seafloor of Earth. Considering the great differences in the environment of eruption of seamounts and Venus domes (cool, deep seawater versus hot, high pressure COZ), we may question whether their similarities in size and geologic characteristics is a coincidence, or whether the processes and mechanisms of formation of these features are similar. Head and Wilson (1986) evaluated the difference in eruption conditions and heat flux for a lava flow under terrestrial atmospheric conditions of eruption on Earth and compared the results with surface conditions of eruption on Venus, and assessed the influence of these two environments on the development of lava flows. Their results showed that, in general, the rate of thermal loss from an open channel lava flow on Venus would be greater than that of an open channel subaerial flow on Earth for early time in the cooling of a given flow, but because the flow continued to cool to a lower background temperature, the rate of thermal loss and crust development remained higher for much longer on Earth. As a consequence, short duration flows would cool faster and flow for shorter periods (shorter distance) on Venus than would subaerial flows on Earth; and long duration flows on Venus would tend to flow for much longer periods (greater distance) than would subaerial flows of similar size on Earth.

In order to assess the similarity between seamount and small dome morphometry/morphology, we assumed that domes are composed primarily of repetitive effusive flows and evaluated the conditions of the eruption of lava flows, as a function of rate of heat loss (cooling rate), on Earth’s seafloor, using Head and


Wilson’s methods. We can then compare the way in which the environment influences the solidification rate of basaltic lavas in the terrestrial subaqueous and in the Venus subaerial cases. The results are illustrated in Figure 15. In terms of radiative, natural convective and total thermal flux, and, therefore, probable rates of crustal thickening, the conditions of lava flow eruption on the seafloor are more similar to those on Venus than are the conditions of typical terrestial subaerial eruption. This suggests that, in general, we might expect some effusive flows on Venus to resemble seafloor lavas more than Earth subaerial lavas. In particular for short duration flows, the rates of cooling are greater on Venus than they are for short flows in the Earth subaerial case, which suggests that something similar to ‘pillow lavas’ may commonly occur in the near-vent regions of Venus effusive edifices. For long duration flows, Venus lavas would flow for longer periods and would form broader and lower effusive edifices than would similar lava flows in the Earth subaerial case. These calculations assume open channel flows, and in a comparison with similar lava flows in the Earth seafloor case, the higher thermal loss rates at lower temperature of seafloor lava flows may cause them to crust over even more quickly than do terrestrial subaerial flows and to develop long tube-fed lava flows. Detailed conditions would undoubtedly vary for individual flows, but it is probable that, due to rapid lava tube development, long duration seafloor lava flows could travel similar distances from the source vent as would typical lava flows on Venus.

The results of these calculations indicate that the conditions of eruption on the seafloor are relatively comparable to those on the surface of Venus. On the basis of the behavior of flows as a function solely of heat loss, the process of formation of dominantly effusive edifices could result in generally similar morphologic features on Earth’s seafloor and Venus.

6.5. RELATIONSHIP TO GLOBAL THERMAL AND TECTONIC CONDITIONS

A major question regarding the small domes is whether they represent regional volcanism associated with thermal anomalies or whether they are the result of globally higher heat flow and extensive melting at the base of the lithosphere. Origin of the domes by distributed regional volcanism implies that the global heat flow is great enough and the lithosphere is sufficiently thin enough to globally produce small batches of magma and many small volcanic edifices in many plains units throughout the northern hemisphere. We observe domes almost everywhere in the northern hemisphere north of 30” N at a relatively uniform low density; however, there are two major concentrations of domes that cannot be explained by a model of globally constant heat flow and lithospheric thickness. The significance of these two regions of dome concentration depends on where the domes have been formed in relation to major global tectonic processes.

If the two areas of dome concentration formed in association with regional hot spots in the absence of horizontal motion, then these areas are or were the sites of major regional hot spots and they are relevant to the regional thermal history


Temperat we/K

300 900 1300

Temp/K

200

/ Ea

1

100

L&l

/ / t

I I , I

300 900 1300

Temp/K

Fig. 15. Relative values of (a) natural convective, (b) radiative, and (c) total heat loss with respect to surface temperature of lava flows on Venus and Earth. Thermal losses from natural convection are similar for the seafloor and higher altitudes on Venus. Convective loss rates in Venus lowlands are more than 10 times terrestrial subaerial rates; and convective loss rates on the seafloor are more than 5 times subaerial rates. VL = Venus lowlands, VH = Venus highlands, Es = Earth seafloor, E, = Earth subaerial. Earth subaerial and Venus data from Head and Wilson, 1986. Seafloor data calculated for this study using Head and Wilson’s equations. Average typical values of sea water parmeters at a

depth of 1000 m used in seafloor calculations.

526 I. C. AUBELE AND E. N. SLYUTA

and, perhaps, global mantle convection characteristics of the planet. On Earth, new analysis of global surface topography, after correction for shallow density variations within the lithosphere, shows a degree 2 pattern with two highs centered 180” apart and well correlated with geoid highs, positive gravity anomalies and hotspot concentrations (Cazenave et al., 1989). This long wavelength pattern is interpreted to represent the dynamic response of Earth’s surface to mantle convection. The presence of numerous small effusive volcanoes on Venus may be indica- tive of the regional and global distribution of advective heat. Two regional concentrations of numerous effusive volcanoes may imply two regional areas of greater than average heat flow similar to the two clusters of hot spots centered in the Pacific and Africa/North Atlantic regions on Earth.

Evidence has been cited for the formation and lateral movement of crust on Venus analogous to that occurring near Earth’s oceanic ridges (Head and Crumpler, 1987) and for the creation of mountain belts by lateral compression (Crumpler et al., 1986; Head, 1990). If the domes formed uniformly, in time and space, on the surface of Venus but in the presence of lateral movement of the lithosphere, then number and density of domes would increase with distance from the spreading axis (with age of the surface) until burial or resurfacing occurred. In this case, the two major dome concentrations would represent regions of old lithosphere.

In a variety of characteristics, the Venus domes are similar to terrestrial seamounts, which are dominantly formed at or near divergent oceanic plate boundaries on Earth. Origin of the domes at or near divergent plate boundaries would imply small areas of high heat flow, thin lithosphere, and a lower global thermal budget in comparison to that necessary for the distributed regional volcanism model. If the small domes commonly formed in the restricted environment limited to areas within 100-1000 km of a divergent boundary, implications for the global thermal budget, thermal structure of the lithosphere, and distribution of melting anomalies differs considerably from that based on the interpretation that they formed in situ without significant lateral transport. If the small domes are associated with the relatively thin lithosphere and enhanced melting associated with crustal spreading, then the conditions for their occurrence (thin lithosphere, anomalous melting, and high regional heat flow) are relevant to a relatively limited area of the surface near spreading centers. If domes form only on very young lithosphere along the axis of spreading, then their number density would be uniform with distance from the spreading axis, or would decrease with distance if there was subsequent modification of the surface. The two major dome concentraions could only occur in this case if there were regions of enhanced dome formation along the spreading axis. In general, this would imply bilaterally symmetric patterns of density variation with respect to time (distance from axis) and position along-axis and might imply that the Akkruva and Beta concentrations are either symmetrically located around a spreading axis at their centers or that each area is one-half of a pair of areas of dome concentration, the other half of which has not yet been imaged.


Using the Venera data set, no specific characteristics of dome occurrence enables these models to be distinguished. It is the presence or absence of horizontal transport that is the discriminant between the regional hot spot and global crustal spreading models. The two prominent global concentrations of small domes occur north of two areas of the Equatorial highlands (Beta Regio and Thetis Regio) interpreted, in both the stable and mobile global tectonic models, to be potential regional hot spots. The regional and global temporal distribution of domes may be useful in this respect in distinguishing the presence or absence of “tracks” associated with movement of lithosphere in relation to large scale hot spots and thermal anomalies.

The combination of temporal and spatial distribution of dome formation has implications for models of global heat flow. If all of the domes within both regional areas of concentration formed generally contemporaneously, in either hot spot or crustal spreading models, their large number would imply anomalous heat flow in the regions where they formed. If the domes formed over a long time period, then their large number may not imply unusually high or unusually variable global heat flow in either model.

In order to fully understand the relationship between the small domes and models of global heat transport, and the significance of this large number of shield volcanoes for the formation and evolution of the crust, it is necessary to know the complete global abundance of domes, their spatial distribution, size frequency distribution, minimum size, and changes in these characteristics with latitude. Increased resolution and increased area1 coverage from MAGELLAN data should enable a more thorough assessment of these questions: however, the MAGEL- LAN data set will require careful analysis.

6.6. IMPLICATIONS AND QUESTIONS FOR MAGELLAN

MAGELLAN SAR operating characteristics (Dallas and Nickle, 1987) will produce radar images of high spatial resolution with a range (vertical) resolution of 110-270 m and an azimuth resolution of 120-150 m; however, the incidence angle will vary from 52” to 19” in a direct relationship with nominal spacecraft altitude (and therefore, in general, with increasing latitude). Because of this high range of incidence angles, the domes will be visible on the MAGELLAN images near the equator only when they vary in surface roughness from the surrounding terrain; and they will be increasingly visible as topographic features with increasing latitude. Although individual small domes and their associated characteristics will be ideal targets for Magellan’s high resolution, recognition and mapping of their complete global distribution during the nominal mission, especially in the equatorial regions, will only be possible to the extent that they exhibit a change in backscatter properties as well as a change in topographic slope from their surroundings. Therefore, an apparent latitudinal dependence of small domes (fewer in the low latitudes) might be expected to be observed during the primary Magellan

528 .I. c. AUBELE AND E. N. SLYClTA

global mapping cycle. In order to determine the global distribution of small domes accurately, a global mapping cycle using incidence angles < 15” will be important.

The dome global abundance and distribution, size frequency distribution, minimum size, and changes in these characteristics with latitude are presently unknown and will be particularly important in understanding the way in which the domes form and their relationship to global models of tectonism and heat flow on Venus. Questions that remain for Magellan include the following: (1) What is the total global abundance and distribution of the small domes, and change of these quantit- ies with latitude, on Venus? (2) Are the major dome concentrations described in this paper an artifact of the limited area and limited resolution currently available for Venus?; (3) Are there other areas of enhanced volcanic effusive activity, particularly in the southern hemisphere, and what characteristics do they have in common with the Beta and Akkruva areas of dome concentration?; (4) What is the number and size frequency distribution of small domes in large volcanic regions, such as Aphrodite Terra? (5) What are the detailed planform outlines of domes, the diameter of the smallest sized domes, the number and distribution of summit pits, and the evidence for regional structural control such as fissures and local dome spatial and temporal patterns?: and (6) Are there distinct morphologic types or classes of domes related to different eruption styles or regional volcanic processes?

7. Conclusions

Small domes are the most abundant feature on the surface of Venus imaged by Venera, and are at least as common as craters are on the smaller terrestrial planets. Because of their numbers and widespread occurrence, regardless of origin, they will be important in the local and global geologic interpretation of the surface. Based on this study’s examination of their area1 and size-frequency distribution, abundance, individual characteristics, and geologic and regional associations in the Venera data set, we conclude that:

(1) Small domes (2 to 20 km in diameter) occur in numbers of 10J on the northern quarter of the surface of Venus. Dome size-frequency appears to fit an exponential distribution, which suggests that the number of small domes aO.5 km in diameter may number 10” within the northern quarter of the surface. Assuming a planet-wide distribution similar to that of the northern quarter of the planet, the total global number of domes ~0.5 km in diameter is calculated to be 4.4 X 10h. This suggests that small domes will be quantitatively the most abundant feature occurring on the surface of Venus.

(2) Small domes are interpreted to be predominantly low shield volcanoes and to represent multiple centralized effusive eruptions of discrete volumes over finite periods. Using a modal basal diameter of 4 km, and an average slope of Y, modal volume for edifices in the Tethus Regio area is 0.73 km’. Assuming 4.4 x 10”


total domes, the minimum total volume represented by the domes, taking into consideration only the volume of the edifices, would be 3.2 x 1Oh km3, or a layer approximately 7 m thick distributed over the entire planet. Adding assumed intrusive and extrusive components, the total volume would be equivalent to a layer approximately 350 m thick over the entire planet and representing 1.6% of the estimated total crustal volume of Venus.

(3) Small domes occur throughout the northern latitudes, but attain maximum regional concentrations in two areas, one located north of Thetis Regio in Akkruva Colles, and another located on the northeast flanks of Beta Regio in Guinevere Planitia. Both areas are characterized by intermediate elevations, large regional positive gravity anomalies, and large scale regional background orthogonal fabrics. Small domes occur preferentially on plains units in areas on Venus that are dominated by coronae, arachnoids, large volcanic centers and small areas of tessera terrain. They are least abundant in the vicinity of linear mountain belts interpreted to be of compressional origin and in the areas of major linear ‘ridge belts’, also commonly interpreted as compressional.

(4) The presence of two global concentrations implies that magma production and eruption was enhanced within these two areas. An important question that remains to be answered is whether these are global hot spots similar to those responsible for large scale concentrations of hot spots within the mobile lithosphere on Earth, or whether they represent stable areas of anomalously thinned lithosphere and associated vertical hot spot-related tectonism. To fully understand their relationship with mechanisms of heat transport on Venus, it will be necessary to know the complete global dome abundance and distribution.

Acknowledgements

This work was funded by NASA Grant NGR-40-002-088 and NAGW-713 (J. W. Head, Principal Investigator). We gratefully acknowledge the travel aid provided by the Brown-Vernadsky agreement and the Mellon Foundation.

We thank Academician V. L. Barsukov and A. T. Basilevsky of the Vernadsky Institute, USSR Academy of Sciences, for help in obtaining the data and for facilitating interactive scientific work on this study.

We especially thank J. W. Head, who made it possible for this joint study to be accomplished, and provided thoughtful critiques and discussions.

We gratefully acknowledge reviews by J. B. Garvin and C. A. Wood; as well as the many pertinent discussions we have had with D. B. Bindschadler, L. S. Crumpler, S. Frank, T. H. Jordan, M. A. Kreslavsky, 0. V. Nikolaeva, K. P. Magee Koberts, G. G. Schaber, R. Searle, V. P. Sinilo, E. R. Stofan and L. Wilson. Joel Plutchak and Peter Neivert provided assistance in the preparation of figures.


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