Surface Characteristics of Venus Derived From Pioneer Venus … · using a correction factor for...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. B8, PAGES 6873-6885, JULY 10, 1985

Surface Characteristics of Venus Derived From Pioneer Venus

Altimetry, Roughness, and Reflectivity Measurements

JAMES W. HEAD III, ALAN R. PETERFREUND, AND JAMES B. GARVIN x

Department of Geological Sciences, Brown University Providence, Rhode Island

STANLEY H. ZISK

Massachusetts Institute of Technology/North East Radio Observatory Corporation, Haystack Observatory, Westford

The three primary data sets for the Pioneer Venus orbiter radar experiment (topography, roughness, and reflectivity) contain important information about the geological and textural characteristics of the surface of Venus. We have subdivided the range of roughness and reflectivity values into three categories as follows: roughness, in degrees rms slope: relatively smooth (1ø-2.5ø), transitional from smooth to rough (2.5ø-5ø), and relatively rough (> 5ø); and Fresnel reflectivity: surfaces dominated by soil or porous material (<0.1), surfaces dominated by rock material (0.1-0.2), and surfaces with a significant percentage of anomalously high dielectric material (>0.2). We have analyzed each of these data sets and their relationships to each other in order to define areas of the surface that are characterized by distinctive properties (e.g., rough rocky surfaces, smooth soil surfaces). We then describe the abundance and areal distribution of such areas and locally calibrate the geological significance of some of the surface types by examining high-resolution images from spacecraft and earth-based observatories. We find that the majority of Venus is covered by regionally contiguous rock and bedrock surfaces. Many of the smooth surfaces we interpret to be of volcanic origin, most likely lava flows, while rougher surfaces are locally characterized by tectonic deformation of several types. Soil surfaces cover less than about 27% of the planet and are generally patchy in their distribution. On the basis of the distribution of these surfaces we see no evidence for the extensive preservation of an ancient global regolith or for widespread, topographically controlled erosion, lateral transport, and sedimentation. The small percentage of the surface of Venus characterized by high-dielectric material appears to originate from several processes including primary lava flows probably containing enrichments of high-dielectric materials, such as metal or metal oxides (e.g., Theia Mons in Beta Regio), and exposure of high-dielectric materials by tectonic deformation (e.g., Maxwell Montes in Ishtar Terra). These global data set correlations provide a fundamental framework for understanding the nature of the surface of Venus and will permit extrapolation of local and regional findings from future geochemical and imaging experiments to a global context.

INTRODUCTION

The nature of the venusian surface from -65øS to 78øN has

been revealed by Pioneer Venus (PV) observations to be di- verse at scales from tens to hundreds of kilometers. Three

primary data sets were derived from the PV orbiter radar (17-cm wavelength) observations: global topography, rms surface slopes (roughness), and radar reflectivity [Pettengill et al., 1980a, b, 1982; Masursky et al., 1980]. On the basis of absolute elevation and spatial association of topographic features, Masursky et al. [1980] defined several global topographic provinces. Global maps of roughness and reflectivity showing many subdivisions within the range of data have also been used to characterize the planet's surface [Pettengill et al., 1980b, 1982; Basilevsky et al., 1982; McGill et al., 1983]. The roughness and reflectivity data sets have also been evaluated statistically and correlated with elevation [Garvin et al., 1983a, b, 1984a]. Correlations of the three PV data sets have been qualitatively described [Masursky et al., 1980; McGill et al., 1983]. Previous efforts to characterize quantitatively these correlations [Basilevsky et al., 1982; Schaber et al., 1982; Davis and Schaber, 1984] have involved a variety of statistical ap- proaches incorporating various computerized clustering tech- niques.

• Now at Geophysics Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland.

Copyright 1985 by the American Geophysical Union.

Paper number 5B0197. 0148-0227/85/005B-0197505.00

In this paper we subdivide the range of values for roughness and reflectivity into a small number of categories (high, inter- medite, and low intervals), and we examine the distribution of each of these subdivisions on the surface of Venus. The subdi-

visions provide a general concept of the nature of the surface as follows: roughness expressed in degrees rms slope: lø-2.5 ø (relatively smooth), 2.5ø-5.0 ø (transitional, smooth to rough), and >5.0 ø (relatively rough); and Fresnel reflectivity: <0.1 (surfaces with a majority of porous material), 0.1-0.2 (surfaces with a majority of material comparable to terrestrial rock), and >0.2 (surfaces with a significant percentage of anomalously high-dielectric material). We then assess the degree of correspondence of the subdivisions of roughness and reflectivity with the physiographic/topographic provinces defined by Masursky et al. [1980]. Finally, we assess the degree of correspondence between the roughness and reflectivity subdivisions by mapping globally the distribution of combinations of the subdivisions of these two parameters (e.g., mapping the distribution of regions characterized by high values of both roughness and reflectivity). The relationship between radar roughness and reflectivity and the relationship of these two parameters to topography are of interest due to (1) possible variations in crustal structure and composition which may be revealed by variations in roughness and reflectivity and which may vary as a function of elevation, (2) geochemical processes that may be pressure-temperature dependent over the range of surface pressures (60 bars) and temperatures (100 K) on Venus [Florensky et al., 1978; Nozette and Lewis, 1982], and (3) sedimentation and weathering that may be a function of absolute

6873

6874 HEAD ET AL.: SURFACE CHARACTERISTICS OF VENUS

elevation and/or slopes and which may be reflected in terms of variations in surface roughenss or proportions of soil/rock/high-dielectric material.

METHOD

The three PV data sets used to produce the maps for this study include data collected and processed as of December 1982. Analyses were done utilizing the PV data mapped into a Mercator projection with a spatial resolution of løx 1 ø (equivalent to 105 km x 105 km at the equator and 53 km x 105 km at +60ø). The maps were produced using a 5 ø x 5 ø boxcar filter with uniformly weighted coefficients in order to fill in those 1 ø x 1 ø cells for which valid PV data were

unavailable. Approximately 10% of the cells required filling. Surface areas were estimated from these Mercator maps by using a correction factor for latitude. Banding observed in these maps is due primarily to absent or invalid data associated with a specific PV spacecraft revolution (e.g., orbital ground tracks) and, to a lesser extent, with orbit-to-orbit variations. Geographic place names for Venus used for reference in the following discussion can be found on the U.S. Geologi- cal Survey (USGS) 1'50,000,000 maps [USGS, 1981, 1984] and in the works by Strobell and Masursky [1983] and Mas- ursky et al. [1984] and are shown in Plate 1.

Global subdivision of topography into provinces has been previously proposed by Masursky et al. [1980] as follows' (1) lowlands were defined as regions with altitude <6051.0 km radius (i.e., <0 km elevation), (2) rolling plains as regions between 6051.0 and 6053.0 km (0.0-2.0 km), (3) highlands (>6053.0 km). We further subdivide the highlands of Mas- ursky et al. [1980] into highlands (regions between 6053.0 and 6055.5 km; 2.0-4.5 km), and mountainous regions (>6055.5 km' >4.5 km) (Plate 2). The mean planetary radius of 6051.2 km [Pettengill et al., 1980b] (updated to 6051.9 km for the December 1982 data set by Garyin et al. [1984a]) is within the rolling plains unit, which covers •75% of the planet. The subdivisions of Masursky et al. [1980], amended by us, define topographic provinces on Venus that are spatially distinct and serve to outline specific geographic regions.

Subdivisions for radar roughness and reflectivity were chosen on the basis of standard interpretations of radar roughness and reflectivity measurements (reviewed by Pet- tengill et al. [1980b, 1982] and Garyin et al. [1983a, b, 1984a] and summarized here). The rms surface slope is a measure of the small-scale (0.5-100 m) roughness averaged over the radar resolution element. The rms slope is derived from a model based on the Hagfors scattering law for the quasi-specular radar return from a planetary surface as follows [Ha•;lfors, 1970]'

ao(O) = (Po C/2) (cos'* 0 + C sin 2 0)-3/2 (1)

where ao is the radar cross section per unit surface area at angles of incidence 0, Po is the Fresnel reflection coefficient at normal incidence angle, and C is the Hagfors parameter. In Hagfors' original model calculations [Haqfors, 1964], the rms slope of the reflecting (specular) surface facets ())wavelength) was found to be equal to 180/7rC •/2 for low to moderate roughness (C))100). As the calculations are based on a model, however, the resulting values for rms slope are only an indication of the angular distribution of scattering objects on the surface. The larger the rms slope, the greater the amount of surface undulation or surface block cover. The mean rms

slope for Venus is 2.65ø+ 0.75 ø [Pettengill et al., 1980a; Garyin et al., 1984a] where the scale length for the roughness

measurement is approximately 0.5 m to tens of meters [Pet- tengill et al., 1980b]. In comparison with similar radar measurements for the moon and Mars (see reviews by Pet- tengill [1978] and Ostro [1983]), Venus appears to be relatively smooth. Three subdivisions in rms slope were chosen: (1) smooth, 1ø-2.5 ø, typical of the smoothest regions of Mars, (2) transitional from smooth to rough, 2.5ø-5.0 ø , typical of the lunar maria, and (3) relatively rough, > 5.0 ø, typical of lunar highlands and the roughest surfaces on Mars [Simpson et al., 1984] (Plate 3). The transitional range comprises -,•46% of the observed surface area of the planet. It could either be inhomogeneous, possibly containing a mixture of both smooth and rough elements, or could represent a distinct surface mor- phology.

Reflectivity values are derived from the scattering model by fitting the Fresnel reflection coefficient Po to the data [Pet- tengill et al., 1982]. The reflectivity is a function of the complex dielectric constant e, where

/9 0 = + (2) The entire Hagfors theory [Hagfors, 1964], including this equation, concerns only the quasi-specular surface component and none of the diffuse (random scattering) component. It is possible, therefore, that if a surface is covered by a large fraction of random-scattering elements, the reflectivity calculated from the remaining quasi-specular echo will be less than the Fresnel reflectivity of the surface materials. The complex dielectric constant is a characteristic of the surface material and

includes a dependence on the volume conductivity and the bulk density (porosity) as well as rock chemistry. Krotikov [1962] and Krotikov and Troitsky [1963] have measured dielectric properties of a variety of dry terrestrial rocks ranging in density from pumice, p = 1000-1800 kg m -3, to dunites, p- 3300 kg m -3 (including basalts, glasses, and granites). They found an approximate relationship between density and dielectric constant. For dry, nonconducting terrestrial materials, the bulk density d (in kilograms per cubic meter) can be approximated by

d • x/•-I 0.5

X 10 3 (3)

where the constant 0.5 was empirically derived for measurements at PV radar wavelengths [Krotikov and Troitsky, 1963]. Campbell and Ulrichs [1969] also measured the dielectric constant and loss tangent for a variety of geologic materials, including both solid rocks and powdered samples of identical rocks. At least for the solid rock samples it is observed that (3) remains a good approximation of bulk density [Garvin et al., 1985]. Hence for terrestrial materials the observed dielectric constant e for solid rock exhibits a lower limit of about 4

(reflectivity of 0.11). Observed values of reflectivity lower than this are likely to be due to a significant fraction of porous materials (e.g, soils), with the expected fraction of such material increasing as the observed reflectivity decreases. For example, the lunar surface is dominated by a porous regolith with a mean reflectivity of 0.07 [Tyler and Howard, 1973].

For Venus the observed mean reflectivity is 0.13, which suggests that either the fraction of porous material is much lower on the Venusian surface than on the moon or that the domi-

nant materials have a remarkably higher bulk dielectric constant [Pettengill et al., 1982]. In light of the observed range in reflectivity over the planet's surface, we have subdivided the

HEAD ET AL.: SURFACE CHARACTERISTICS OF VENUS 6875

data set into three parts: (1) <0.1, to be less than any measured terrestrial solid rock, low dielectric constant (e < 4), almost certainly incorporating a significant fraction of porous material, (2) 0.1-0.2, moderate dielectric constant, probably comprising a large fraction of consolidated material if of terrestrial or lunar mineralogy [01hoeft and Strangway, 1975], and (3) >0.2, at or beyond the upper end of the range of terrestrial rocks, high dielectric constant (e > 9), indicating mainly consolidated material with perhaps some anomalously high-dielectric material representing mineralogies probably enriched in FeS2, Fe, and/or Ti oxides (Plate 4). The moderate range, which comprises 67% of the observed surface area of the planet, is likely to include a significant proportion of rocks or bedrock with only a minor contribution from porous ("soil") fractions. Even soil derived from high-dielectric minerals should make only a minor contribution, since Campbell and Ulrichs [1969] determined that the dielectric constants of powdered rock samples are universally low (e.g., e < 4), inde- pendent of the properties of the original rock. As has been noted elsewhere, this surface type, the largest fraction of the Venusian surface, is unlike the unconsolidated soil typical of the moon and much of Mars.

The map distribution of correspondences between roughness and elevation subdivisions is shown in Plate 5, between reflectivity and elevation subdivisions in Plate 6, and between roughness and reflectivity subdivisions in Plate 7. Note that the three parameters (roughness, reflectivity, and elevation) are continuous over the entire data range. Therefore, although the resulting surface units are defined within a specific range of values (for example, regions of low roughness and intermediate reflectivity have values of 1ø-2.5 ø rms slope and 0.1-0.2 reflectivity, indicating that the terrain is likely to be smooth and composed of a large fraction of rock material), they are in fact gradational with each other because roughness, reflectivity, and elevation represent a spectrum of values. Thus, as is common with terrestrial geologic unit definition, the units are transitional at their boundaries.

RESULTS

Elevation Versus Roughness

Spatial correspondences of three divisions in roughness and four divisions in elevation are shown in Plate 5. Using hue to distinguish divisions in elevation and intensity to distinguish divisions in roughness, a pattern of increasing roughness with elevation is apparent (also compare Plates 2 and 3). As noted in previous studies [Pettengill et al., 1980b; Masursky et al., 1980; McGill et al., 1983; Garyin et al., 1983a], highest regions are generally roughest, and lowest regions are relatively smooth, although not necessarily the smoothest regions found on Venus. In detail, however, noteworthy deviations from this trend are observed.

Most lowlands (<0.0 km elevation) are smooth or transitional from smooth to rough. Within the lowlands the regions of low and moderate roughness (darker hues) are spatially well defined (i.e., occur as clusters of løx 1 ø cells). In contrast, high-roughness units in the lowlands occur in isolated patches (i.e., single løx 1 ø cells of high roughness surrounded by smoother surfaces). Small topographic depressions (<10 • km 2) tend to have moderate roughness values, while broader lowlands are both smooth and transitional. The pattern of roughness in the broader lowlands shows regional clustering. In Sedna and Guinevere planitiae, for example, areas toward Beta Regio show intermediate roughness values, while those

toward Ishtar Terra show low roughness values. Atalanta Pla- nitia, in contrast, is dominated by widespread regions of low roughness.

The rolling plains (0.0-2.0 km) display broad regions of both smooth and transitional roughness. Transitional roughness areas often flank highland regions, most notably around Beta-Phoebe Regio, but also in narrower, less continuous zones around Ishtar and Aphrodite Terra. Broad circular features, such as the Nightingale-Earhart region of eastern Tethus Regio, show intermediate roughness values. Many of the small elevated plateaus (e.g., Tellus, Alpha Regio) contained within this topographic province have transitional to high roughness values. Rough units occur as either small regional clusters or isolated features. Chasmata contained within Aphrodite Terra, while poorly resolved at this spatial resolution, appear to be relatively smooth.

Most highlands (2.0-4.5 km) appear transitional in roughness. Smooth regions can be observed in Lakshmi Planum, northwest and west of Maxwell Montes, west of Akna Montes, and in isolated sections of Aphrodite. Rough regions occur adjacent to the mountainous terrain within the highlands, as seen, for example, in the western and central highlands of Aphrodite and eastern Ishtar Terra. Mountainous regions (> 4.5 km) are mostly transitional or rough, with only isolated occurrences of smoother surfaces.

Elevation Versus Reflectivity

Spatial correspondences of radar reflectivity and elevation are shown in Plate 6 (also compare Plates 2 and 4). Elevations are represented by hues and reflectivity subdivisions by intensity variations. A less distinct relationship exists betwen elevation and reflectivity than that observed between elevation and roughness from a statistical standpoint [Garvin et al., 1984a] as well as from general map unit trends. While a general trend of increasing reflectivity with elevation is apparent, for a given elevation range a wide range of reflectivity map units are observed (Plate 6).

The majority of lowlands are moderate in reflectivity, suggesting a predominantly rock surface. Lower-reflectivity units dominate Lavinia Planitia and occur as distinctive patches in the otherwise intermediate-reflectivity Guinevere and Sedna planitiae. The most distinctive occurrences of high-reflectivity patches are in Atalanta Planitia. On the basis of these observations it is clear that there is a diversity of reflectivity units both within and between planitia (e.g., within Sedna and between Sedna and Atalanta planitiae).

Rolling plains are also mostly moderate in reflectivity, implying a predominance of surface materials with dielectric properties similar to terrestrial rocks. Low-reflectivity units tend to be adjacent to highland regions, such as Beta and Aphrodite Terra, but are not evenly distributed around the entire highland perimeters. Several regiones are also dominated by low reflectivity (Alpha, Tellus) as is Lada Terra, the rolling plains area at high southern latitudes. Chasmata, in general, appear to contain material with low reflectivity. High- reflectivity units are widely distributed but occur mostly in isolated small areas. A notable exception to this is the region west of Atalanta Planitia (the Nightingale-Earhart region of eastern Tethus Regio).

Highland regions generally show a pattern of highest reflectivity adjacent to mountainous terrain, with decreasing reflectivity away from these peaks. This pattern is well illustrated in the western and central highlands of Aphrodite and in northern Beta Regio. In eastern Ishtar Terra the pattern of reflec-

6876 HEAD ET AL.' SURFACE CHARACTERISTICS OF VENUS

Subdivision

TABLE 1. Summary of Roughness-Reflectivity Subdivisions

rms Slope, deg Reflectivity

Surface Type Region Area, (N Latitude, Interpreted

% Longitude) Characteristics

A 1.0ø-2.5 ø 0.02-0.1 11.5

B 2.50-5.0 ø 0.02-0.1 13.5

C 5.0ø-10.0 ø 0.02-0.1 2.2

D 1.0ø-2.5 ø 0.1-0.2 36.5

E 2.50-5.0 ø 0.1-0.2 28.6

F 5.0ø-10.0 ø 0.1-0.2 2.3

G 1.0ø-2.5 ø 0.2-0.5 2.1

H 2.50-5.0 ø 0.2-0.5 2.3

I 5.0ø-10.0 ø 0.2-0.5 1.0

Lavinia Planitia smooth; soils d- rock (-40 ø , 340 ø ) Alpha Regio transitional; soils d- rock (- •5 ø, 5 ø) Tellus Regio rough' soils d- rock ( + 35 ø, 80 ø ) NW of W Aphrodite smooth; rock d- soil

Terra ( + 30 ø, 50 ø) Beta Regio transitional' rock d- soil ( + 30 ø, 285 ø) Rhea Mons rough' rock d- soil ( + 33 ø, 282 ø ) W Atalanta Planitia smooth; high dielectrics -/-_ rock ( +60 ø, 150 ø) SW Atalanta Planitia transitional' high dielectrics d- rock ( + 55 ø, 135 ø) Maxwell Montes rough; high dielectrics d- rock ( + 63 ø, 4 ø )

tivity units follows a more regional distribution than the pattern observed around Beta and Aphrodite Terra. Much of eastern Ishtar is dominated by low reflectivity values in contrast to the dominance of intermediate values in the Lakshmi

Planum region of western Ishtar Terra. Mountainous regions, as noted by Pettengill et al. [1982],

contain most of the highest-reflectivity material observed on Venus. These high-reflectivity values are in excess of what would be expected based on the bulk density-reflectivity relationship (equation (3)) and as such are inferred to indicate the presence of a considerable percentage of high-dielectric material. The PV radiometric observations of exceptionally low emissivity at these locations support the presence of such high-dielectric materials [Ford and Pettengill, 1983]. Moun- tainous regions showing these high values include Maxwell, some peaks within Akna and Freyja Montes, Theia Mons, and the highest terrain within Aphrodite. Not all mountainous terrains, however, are characterized by high reflectivity values; for example Rhea Mons and most of Akna and Freyja Montes have surface material with moderate reflectivity.

Roughness Versus Reflectivity

Roughness-reflectivity comparisons are shown in Plate 7 with nine subdivisions, increasing in reflectivity along the hori- zontal axis and in roughness along the vertical axis. Ninety percent of the surface is contained within four of the nine defined roughness-reflectivity units. Two low-reflectivity units, A and B, are smooth and transitional in roughness, respec- tively, and account for • 25% of the observed Venusian surface. Two moderate-reflectivity units, D and E, are also smooth and transitional and make up • 65% of the observed surface. Table 1 presents a summary of the unit parameters, their distribution, and characteristics of the individual units discussed below.

Unit A (light green) contains surfaces that are smooth and have a low reflectivity, implying that a majority of the surface is likely to be covered by porous and unconsolidated material. Thus we refer to unit A as the smooth soil unit. This unit is

slightly more prevalent proportionally (with respect to areal divisions of global topography) in lowlands and rolling plains than in highlands and mountains. The occurrence of the unit in all topographic regions is as small patches (< 104 km 2) or

isolated cells. Significant examples of this unit are located in Lavinia Planitia and the rolling plains to the east (Lada Terra) as well as adjacent to mountains within Ishtar Terra and along the flanks of eastern Ishtar Terra.

Unit B (green) contains surfaces that are transitional in roughness and have a low reflectivity. We refer to this as the intermediate-roughness soil unit. This unit is proportionally less prevalent in lowlands and more common in highlands. These surfaces appear closely associated with the edges of highlands (Aphrodite, Beta), but the units do not fully circum- scribe the highlands. Many of the small elevated plateaus (regiones) are characterized by this unit. The most predominant occurrences of this unit in the highlands are in eastern Ishtar and Akna and Freyja Montes.

Unit C (dark green) is the third of the low-reflectivity surfaces and is characterized by high radar roughness. We refer to this as the rough soil unit. The unit occurs predominantly in areas adjacent to and within highlands. Occurrences within lowlands are isolated. Most of the chasmata are contained within this unit. Other occurrences of the unit include Tellus

and Phoebe regiones and the highlands in eastern Ishtar. Unit D (light blue) is characterized as smooth and of mod-

erate reflectivity and is the most widespread surface unit (Table 1). It is probably dominated by surface materials with rocklike dielectric properties, and we therefore refer to it as the smooth rock unit. This unit occurs predominantly in the lowlands and rolling plains with only isolated occurrences in the highlands and is virtually nonexistent in the mountains. In the lowlands and rolling plains the unit usually occurs in contiguous patches, a major patch occurring in Atalanta Planitia. A very large irregular region of this unit is seen at southern mid- and high latitudes extending from about 120 ø to 330 ø longitude. A distinctive linear patch is seen extending from just south of Maxwell Montes eastward to Tethus Regio, a distance of over 3000 km. In the highlands, notable examples of occurrence include Lakshmi Planum and Hathor Mons.

Unit E (sky blue) is characterized by moderate roughness and reflectivity values, and we refer to this as the intermediate- roughness rock unit. It occurs in the same proportion at all elevation levels. In the lowlands the unit occurs as a substan-

tial portion of Guinevere Planitia and as isolated cells in most other lowland areas. With'?. the rolling plains, two broad re-

HEAD ET AL.: SURFACE CHARACTERISTICS OF VENUS 6877

gions fall within the intermediate-roughness rock unit. The first, which occurs to the east of Beta Regio, is particularly notable in that it contains all of the Venera landing sites for which images are available [Florensky et al., 1977, 1983]. Radar characterisics for the areas surrounding these specific sites have been derived from PV observations [Garvin et al., 1984b] and are consistent with values contained within this unit (roughness= 30-4 ø and reflectivity=0.10-0.15). The other major occurrences of this unit are in regions to the north and east of Atla Regio. Within the highlands the unit is common in Metis Regio, Vesta Rupes, and Beta Regio and in much of Aphrodite. Isolated occurrences of the unit are found in most mountainous regions.

Unit F (dark blue) contains rough surfaces of moderate reflectivity, and we refer to it as the rough rock unit. Only isolated occurrences of the unit are found in lowlands and

rolling plains. Within the highlands the unit is observed near mountains in Aphrodite, north of Maxwell Montes, and in the southern part of Tethus Regio. Significant occurrences of the unit within mountainous terrain include Rhea Mons, Akna and Freyja Montes, and a band east of Maxwell Montes.

Unit G (yellow) is characterized by smooth surfaces and high-reflectivity material. As seen in Plate 4, reflectivity values are not as high as observed in the mountains (unit I) but are such that a significant percentage of high-dielectric material is likely to be present. The most distinctive occurrence of this unit is in a region east of Tethus Regio surrounding Earhart and Nightingale and extending into Atalanta Planitia. Other occurrences are more localized with significant clusters southwest of Akna Montes and as patches in the widespread unit D at southern mid- to high latitudes.

Unit H (orange) is transitional in roughness and contains materials with high radar reflectivity. This unit is proportionally more common in highlands and mountainous regions. Occurrences within lowlands and rolling plains are limited to small patches and isolated occurrences, with well-defined examples in the Atalanta region, southeast of Aphrodite Terra, and west of Alpha Regio. In the highlands this unit occurs in small patches adjacent to summit areas. Examples within the mountains include Rhea Mons and part of Maxwell Montes.

Unit I (red) occurs predominantly in highlands and mountainous regions and contains surfaces that are rough and have a high reflectivity. Isolated occurrences of this unit are found, however, in lowlands and rolling plains. Maxwell Montes in Ishtar Terra, Theia Mons in Beta Regio, and Ovda, Thetis, and Atla regiones within Aphrodite are the most extensive examples of this unit.

DISCUSSION

The analysis and subdivision of roughness and reflectivity data have permitted us to plot the distribution of these subdivisions on the surface of Venus (Plates 3 and 4), to examine the altitude distribution of these subdivisions (Plates 5 and 6), and to assess the distribution of units defined by combinations of roughness and reflectivity subdivisions (Plate 7). The basis for the subdivisions of roughness and reflectivity data, the transitional nature of the boundaries of these subdivisions, and thus the nature of the units shown in Plate 7 have been

discussed in previous sections. The units resulting from the combination of these subdivisions do not uniquely define distinctive geological processes because they map (1) variations in surface roughness and (2) properties which can be interpreted in terms of relative proportions of soil, rock, and high- dielectric materials on the surface. Obviously, for example, a

rock surface can be produced by a wide range of geologic processes including volcanism, tectonism, erosion, and sedimentation. The subdivisions and units do provide, however, an important characterization of the surface properties of Venus which permits an initial assessment of classes of geologic processes that might be operating to form and modify the surface of Venus. In this section we first examine imaging data which allow us to identify the geological processes occurring within some regions of our mapped units. We then review the distribution of these subdivisions and units and develop a series of interpretations and predictions concerning the nature of the surface of Venus.

Moderate-reflectivity units contain surface materials with dielectric constants consistent with terrestrial rocks. The pau- city of a soil cover could be indicative of extensive bedrock exposures or a high percentage of large fragmental rocks. For example, the Venera lander sites for which images are available all exhibit blocky or bedrock surfaces. The Venera 9 area contains a mixture of blocks and soil, while at Venera 10 the surface is characterized by a bedrock pavement overlain by patches of soil [Florensky et al, 1977]. The region surrounding these two sites is characterized by average reflectivity values lying at the lower end of the intermediate (predominantly rock) range [Garvin et al., 1984b]. The region surrounding the Venera 13 and 14 sites is characterized by average reflectivity values which lie in the middle of the intermediate (predominantly rock) range. Images of these sites show bedrock pavement at both locations, with bedrock comprising essentially 100% of the surface viewed by the Venera 14 spacecraft [Flor- ensky et al., 1983; Gart)in et al., 1984c].

Regional bedrock units could represent volcanic flow surfaces, consolidated sediments of various origins, or eroded and exposed rock of various origins; the radar reflectivity data alone do not distinguish between these possibilities. Earth- based radar data [Campbell and Burns, 1980; Campbell et al., 1983, 1984a] provide information on possible origins in some areas. For example, the region in the rolling plains southeast of Lakshmi Planum is dominated by a sequence of lava flows [Campbell et al., 1984b-I covering several hundred thousand square kilometers. This particular region is characterized primarily by intermediate reflectivity units of low to intermediate roughness (units D and E). High-resolution radar images of Beta Regio [Campbell et al., 1984] confirm the presence of large volcanic structures and extensive flowlike deposits associated with the Devana Chasma rift zone. These volcanic

deposits lie predominantly in the moderate reflectivity (rock) units (E and F). In addition, recent orbital high-resolution images from Venera 15/16 show that Lakshmi Planum, a broad region of intermediate reflectivity in Ishtar Terra, is characterized by two major calderas and widespread lava flows [Barsukov et al., 1984]. Thus we conclude that volcanic processes characterize portions of the intermediate reflectivity (rock) units (D, E, and F) and may account for some of the larger expanses of this unit in the lowlands and rolling plains such as Guinevere, Sedna, Helen, and Atalanta planitiae.

Several areas characterized by moderate reflectivity (rock) units are interpreted to be dominated by tectonic activity, including the banded terrain in Akna and Freyja Montes of Ishtar Terra [Campbell et al., 1983], the region between Ut and Vesta Rupes, and portions of the Devana Chasma rift zone in Beta Regio [Campbell et al., 1984a, hi. These areas tend to be characterized by intermediate to high roughness. Deformational processes (tectonics) appear to be responsible at least in part for the local development of the intermediate-


reflectivity units either by local surface disruption or the gen- eration of regional topographic slopes.

The low-reflectivity units on Venus most likely contain surfaces with a majority of porous and unconsolidated fine materials (soil). The rougher low-reflectivity units may contain a sufficient fraction of diffuse-scattering elements such that their reflectivity values have been artificially lowered by 10-15% [Pettengill et al., 1982; Ford and Pettengill, 1984]. Possible origins of the soil include impact-generated regolith, volcanic pyroclastics, in situ weathering of bedrock, and depositional products of lateral mass transport of sediments. Although the exact areal extent of soil on the surface of Venus is difficult to

ascertain, surface units consistent with a soil interpretation (units A, B, and C) cover only about 27% of Venus. The distribution of these units can be an aid in limiting the various models of soil formation. The two smoothest low-reflectivity units (Plates 4 and 7) comprise less than 25% of the surface, are usually spatially contiguous, occur at all elevations, and often occur in small patches. On the moon a global impact- generated regolith dominates the surface to depths in excess of several meters. The regional clusters of soil units on Venus argue against an origin related to a global impact-generated ancient regolith, although remnants of ancient regolith cannot be ruled out. If soil formation and accumulation (not necessarily by impact comminution) scales simply as a function of time, then areas of extensive soil development, such as eastern Ishtar Terra, Lada Terra, and Tellus Regio, may be some of the oldest terrain on the planet.

Soil units might also represent processes of weathering, degradation, and lateral downslope movment of sediment, driven by gravity. On earth these processes, assisted by the atmosphere and hydrosphere, result in regional sediment and soil units in low-lying areas such as abyssal plains, continental interiors, and annuli surrounding elevated regions. A first- order observation for Venus is that soil units (units A, B, and C) are only slightly more common proportionally in the lowlands and rolling plains than in the highlands. Comparison of Plates 2, 4, 6, and 7 shows that soil units do not form extensive, continuous parts of the lowlands (for example, the floor of Sedna, Guinevere, or Atalanta planitiae). We thus conclude that large-scale lateral movement of sediment and soil in this mode is not a dominant process in shaping the observed surface of Venus. Sharpton and Head [1984], in an analysis of regional slopes on Venus and earth, find that Venus has a deficiency of the very low slopes typical of continental plat- forms and abyssal plains on earth.

There is evidence, however, that soil units occur adjacent to some highland regions on their flanking slopes, or adjacent upland rolling plains, particularly in northern Beta Regio, southwest of Ishtar Terra, and adjacent to portions of Aphro- dite Terra (Plates 4 and 6). This suggests that at least some processes of weathering and downslope movement may be taking place adjacent to highland regions, although the asym- metric development of soil units in these areas hints at com- plexity. Soil formation might also preferentially occur in areas of steep slopes and tectonic disruption due to fracturing, gravity-induced movement and abrasion, or simple preferential accumulation in topographic traps. For example, on earth, mountains or ridges formed by tectonic activity in both extensional (Basin and Range) and compressional (Appala- chian Valley and Ridge) environments are characterized by intervening areas of sediment fill. On Venus, soil units are associated with the tectonic banded terrain in Ishtar Terra

[Campbell et al., 1983] and with the floor of Artemis Chasma in Aphrodite Terra lehmann and Head, 1983]. Thus other

areas of regional soil development, such as eastern Ishtar Terra or Lada Terra, may be areas of tectonic activity.

Eolian processes represent an additional possible mechanism of soil transport on Venus [White, 1981; Greeley et al., 1984]. Evidence for local erosion of bedrock and collection of soil in intervening low areas is seen in the Venera lander panoramas !-Florensky et al., 1977], although the role of eolian processes at this scale has not been firmly established. The possibility exists that some of these regional soil units may be eolian deposits. Under the present environmental conditions, surface temperature is relatively constant, surface winds have relatively low velocity, and there is a lack of seasonal or lati- tudinal variations in environment that might enhance eolian processes. Therefore it is difficult to predict the global patterns of units that might result from eolian activity, although altitude variations may be important because of the variation in temperature and pressure. Higher-resolution data are required in order to identify potential eolian deposits and to assess their relation to these soil units.

Pyroclastic activity offers another mechanism for the formation of soil units. Extensive pyroclastic deposits are considered unlikely in the present environment because of the influence of high atmospheric pressures !-Garyin et al., 1982; Head and Wilson, 1982]. In addition, soil units are not directly associated with the distinctive volcanic deposits and constructs recognized on Venus thus far [Campbell et al., 1984a, b; Head et al., 1985]. However, a contribution to the distribution of soil units by pyroclastic activity cannot be ruled out.

In the southern high latitudes a vast region is dominated by the soil units. Topographically, this soil-rich terrain is associated with a broad elevated region (Lada Terra) within the rolling plains that extends to the southern edge of the Pioneer Venus coverage. Soil units are also common in some regions (Tellus, Alpha, eastern Ishtar) while uncommon in others (Beta Regio), suggesting possible differences in origin or surfaces of differing age.

The high-reflectivity units indicate the presence of a significant amount of high-dielectric material near the surface. On earth, common high-dielectric materials include liquid water, free metal, and certain minerals (for example, futile, magnetite, ilmenite, hematite, spinel, pyrite, pyrolusite). The geologic environments in which high-dielectric materials are found on earth are quite variable. Liquid water is ubiquitous, free metal is extremely rare, and high-dielectric minerals are common in igneous and metamorphic rocks in relatively low concentrations. For example, the highest concentration of TiO: in typical terrestrial lava flows is in the 2-3 wt % range and occurs in continental rift zones [Carmichael, 1982]. Although direct measurements have not been made, these concentrations are likely to be in the radar reflectivity range of intermediate, rather than high, values. Concentrations of iron and titanium oxides in lunar lava flows are often considerably higher (10-12%). Olhoeft and Strangway [1975] have shown that Apollo 11 high-titanium basalts have dielectric constants in the range of 10-12, which would produce radar reflectivity values of 0.25-0.30, within the high-reflectivity unit mapped in this study. Therefore high concentration of these high- dielectric minerals occurring in primary volcanic rocks is a possible geologic environment for high-reflectivity units on Venus. It is recognized that many of these minerals may change their dielectric properties with increased temperature and pressure [Parkhomenko, 1967]. Detailed data on mineral stability and dielectric properties under Venusian conditions are required before specific candidates can be identified. Ero- sion and tectonic activity could expose subsurface high-

HEAD ET AL..' SURFACE CHARACTERISTICS OF VENUS 6879

reflectivity materials of igneous or metamorphic origin. Other possible origins for high-reflectivity units on Venus include elevation-dependent (temperature-pressure) chemical or physical weathering [Nozette and Lewis, 1982; McGill et al., 1983], which might produce or concentrate high-reflectivity materials, and other various erosional mechanisms, such as eolian activity, which might concentrate materials on the basis of their density.

In an analysis of Venus global surface radar reflectivity, Pettengill et al. [1982] pointed out that highly conducting metallic sulfides display high-dielectric values but would be unstable to atmospheric exposure according to the calculations of Nozette and Lewis [1982]. Pettengill et al. [1982] proposed that the regions of high reflectivity on Venus are rocks containing a significant amount of conducting materials as inclusions, favoring FeS2 (pyrite) as the inclusion. They suggest that pyrite may be widespread in original crustal rock, but that it lies in radar view only at higher elevations where new surfaces are constantly being exposed by mechanisms of chemical erosion and lateral sediment movement into sur-

rounding lows, as envisioned by Nozette and Lewis [1982]. High-reflectivity materials are concentrated in five major

areas: central Beta Regio, Maxwell Montes in Ishtar Terra, the Tethus Regio-eastern Atalanta Planitia region, Ovda, Thetis, and Atla regiones in eastern Aphrodite Terra, and areas south of -30 ø to the east and west of Lada Terra

(Plates 4, 6, and 7). The highest reflectivity values in this unit occur associated with Theia Mons, Maxwell Montes, and Atla Regio. These areas also show very high roughness values. The location, elevation, and geologic characteristics of these regions provide some constraints on the consideration of hypotheses for the origin of the high-dielectric material. First, we make the simplifying assumption that the high-dielectric material forms from a single specific process and is contained in the rock materials, and we investigate the implications of this assumption for elevation, mode of occurrence, and age. One clear manifestation of altitude dependence would be the temperature/pressure-dependent chemical reactions described by Florensky et al. [1977], Khodakovsky et al., [1979], Barsu- kov et al., [1980a, b], and Nozette and Lewis [1982]. However, the occurrence of high-dielectric materials at several elevations (Beta/Maxwell and Atalanta Planitia for example) argues against this as the sole factor under this assumption. High- dielectric materials on Venus [Pettengill et al., 1982; Ford and Pettengill, 1983] have several modes of occurrence in a geological context. The concentration at Theia Mons is directly associated with a relatively young shield volcano [Campbell et al., 1984a] while the concentration at Maxwell Montes appears to be more related to the extensive deformation associated with the banded texture [Campbell et al., 1983], although a volcanic origin for the structure cannot be ruled out [Mas- ursky et al., 1980]. Thus the mode of occurrence may be consistent with both primary emplacement and exposure by fault- ing and erosion associated with steep slopes. The very high roughness values associated with these regions is also consistent with these observations. A third consideration is age and the effects of weathering as a function of time. Although Theia Mons is interpreted by virtually all workers to be of volcanic origin [Saunders and Malin, 1977; McGill et al., 1981; Camp- bell et al., 1984a], it is not the only volcanic feature or structure on Venus. Preliminary analyses of Venera 15/16 data show evidence for widespread flows at several altitude levels in the northern hemisphere in regions estimated to have an average age of one billion years [Barsukov et al., 1984]. If the high-dielectric materials are associated with primary volcanic

rocks, perhaps they occur only in the freshest volcanic deposits and in materials exposed by recent tectonic/erosional activity (e.g., Maxwell Montes). Again, the observed high roughness values for these areas are consistent with this interpretation. Following this sequence of logic, one might favor a model in which high-dielectric material is exposed during the emplacement of lava flows but is diluted and modified by weathering processes operating on the surface of the flows as a function of time. Where there is intense tectonic activity, the underlying fresh material is brought to the surface and high- dielectric material is exposed. In this model the occurrence of high-dielectric material at high elevations is related to the fact that Theia Mons is being constructed on top of the Beta rise and that tectonic deformation tends to produce high topography, as in the case of Maxwell Montes.

If this simple model is the case, then what high-dielectric materials might be candidates for the observed units? The range of reflectivity in the high-reflectivity unit could be consistent with average terrestrial basaltic rock compositions but with relatively high concentrations (10-15 wt %, for example) of minerals with metallic elements such as Fe, Ti, Mn, or Pb. Such rock types would be typical of the reflectivity values mapped in the western Atalanta Planitia region, for example. However, Theia Mons, Maxwell Montes, and Ovda and Atla regiones are characterized by reflectivity values that are above the range of values typical of terrestrial rocks (e.g., from 0.25 to 0.45, or e from 9 to 27) [Campbell and Ulrichs, 1969], lunar basalts [Olhoeft and Strangway, 1975], and basaltic achon- drites [Campbell and Ulrichs, 1969]. One possible explanation is the presence of very high concentrations (> 12-15 wt %) of metallic oxides of Ti, Fe, and Mn (such as ilmenite, FeTiO3). Another possible explanation might be the presence of ultra- mafic rocks, many of which are enriched in Fe and Ti (e.g., Nyirangonga mafic volcanics of the East African Rift [Bell and Powell, 1969]). In addition, local extremely high concentrations of high-dielectric materials, such as the recently discovered seafloor hydrothermal activity, could average out over the larger Pioneer Venus footprint to yield very high anomalies.

A more complicated and perhaps more realistic model would allow for various factors to govern the origin and distribution of the high-dielectric material. Such a model might allow for the primary emplacement of high dielectrics in lava flows and subsequent weathering to obscure the high- reflectivity signature, the preferential emplacement of flows with high dielectrics in certain geological environments such as rift zones, as is seen on earth, and exposure of high dielectrics by tectonic activity. Secondary occurrences of high dielectric materials might arise from erosion and concentration in lag deposits, the production of high-dielectric material through various simple reactions [e.g., Florensky et al., 1977; Nozette and Lewis, 1982; Garyin et al., 1984d], and the possibility of widespread rock coatings of high-dielectric material such as Fe and Mn, analogous to desert varnish or deep-sea nodules. At the present time there are insufficient data to es- tablish or rule out any of these factors. Although we believe that age may be an important factor, we must await additional geochemical and high-resolution imaging data in order to assess age and other factors.

CONCLUSIONS

Pioneer Venus data sets for global topography, surface roughness, and reflectivity provide important information on the geological and textural characteristics of the surface of Venus. We have analyzed each of these data sets and their


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270 ø 0 ø 90 ø 180 ø Plate 1. Reference map of Venus showing geographic location of features cited in the text.

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Plate 2. Topography of Venus. Four divisions in topography are shown in a Mercator projection of the planet at 1 ø x 1 ø resolution. The four divisions are expressed in kilometers relative to a planetary radius of 6051.0 km: (1) < 6051.0 kin, lowlands (purples), (2) 6051.0-6053.0 km, rolling plains, (blues); (3) 6053.0-6055.5 km, highlands (yellows), and (4) > 6055.0 km, mountainous regions (reds).

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Plate 3. Radar roughness of the surfacc of Venus. Three divisions in PV radar roughness arc shown in the same projccfio• as Plate 2. The three divisions, given in degrees rms slope, arc (l) 1.0ø-2.5 ø, smooth (blues), (2) 2.5ø-5.0 ø, transitional (yellows), and (3) $.0ø-10.0 ø, rough (reds).

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Plate 4. Radar reflectivity of the surface of Venus. Three divisions in PV reflectivity are shown in the same projection as Plates 2 and 3. The three divisions are (1) 0.02-0.1 (blues), predominantly porous material such as soil, (2) 0.1-0.2 (yellows), predominantly material comparable to terrestrial rock, and (3) 0.2-0.5 (reds), material with a significant percentage of a high-dielectric component.

6881

6882 HEAD ET AL.' SURFACE CHARACTERISTICS OF VENUS

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Plate 5. Map of elevation versus roughness. Twelve subdivisions arc defined on the basis of the divisions of elevation and roughness shown in Plates 2 and 3 and discussed in the text. Huc is used to show divisions in elevation and intensity to show divisions in roughness.

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2

, 6 2 18 2-0

Plate 6. Map of elevation versus reflectivity. Twelve subdivisions are defined on the basis of the divisions of elevation and reflectivity shown in Plates 2 and 4 and discussed in the text. Hue is used to show divisions in elevation, and intensity to show divisions in reflectivity.

HEAD ET AL.' SURFACE CHARACTERISTICS OF VENUS 6883

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6884 HEAD ET AL..' SURFACE CHARACTERISTICS OF VENUS

relation to each other and 'have locally calibrated the geological significance of some of the distinctive surface types by examining high-resolution images from Venera lander spacecraft and the Arecibo Observatory. These global data set correlations provide a fundamental framework for the understanding of the nature of the surface of Venus and will permit extrapolation of local and regional findings of future geochemical and imaging experiment results to a global context. Our present conclusions and interpretations are as follows:

1. Regional rock and bedrock surfaces are extremely widespread on Venus, covering the majority of the planet. We interpret the relatively smooth rock and bedrock surfaces to be of volcanic origin, most likely representing lava flows, particularly over wide areas of the lowlands and rolling uplands. Rougher rock and bedrock surfaces are locally related to tectonic deformation.

2. Porous and unconsolidated fine materials (soil) cover less than about 27% of the surface of Venus. The soil surfaces

are generally patchy in their distribution, are not preferentially located around major volcanic complexes, are to some degree localized along the flanking slopes of the highlands, and do not preferentially occur in the lowlands. We interpret this to mean that major contributions to soil formation are not impact-produced regolith or extensive pyroelastic mantles but rather local weathering and small amounts of lateral transport. If the formation of soil is related to age of surfaces, then soil-rich areas such as Lada Terra, at southern high latitudes, may be relatively old.

3. A small portion of the surface of Venus is characterized by materials with a significant amount of high-dielectric material near the surface. Although occurring predominantly at high altitudes, these surfaces do not appear to be solely related to temperature/pressure-dependent chemical reactions because of their distribution at other elevations. We investigate several hypotheses for the origin of these materials and interpret some occurrences (e.g, Theia Mons in Beta Regio) to represent primary volcanic rocks enriched in such components as metals or metallic oxides, and others (Maxwell Montes in Ishtar Terra) to represent high-dielectric material exposed by tectonic deformation. A major difficulty in interpreting the origin of the components producing the high dielectric signatures is the lack of laboratory measurements for a wide range of geological materials at these wavelengths for Venus conditions. These data are absolute essential for a more thorough understanding of the primary and secondary geological processes operating to form and modify the surface of Venus.

Acknowledgments. We are particularly grateful to Gordon Pet- tengill and Peter Ford of MIT for providing updated versions of the Pioneer Venus data. We wish to acknowledge the help of M. E. Murphy in manuscript preparation; E. Robinson and J. Tingle for computer programming support; and R. Grieve, B. Jakosky, R. Simp- son, and two anonymous reviewers for manuscript review. This research was carried out under NASA grant NGR-40-002-088 to J.W.H.

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(Received December 7, 1983; (revised February 7, 1985; accepted March 5, 1985.)

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