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

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  • 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 (0.2). 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 ma- jority 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, topo- graphically 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 frame- work 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 sur- face slopes (roughness), and radar reflectivity [Pettengill et al., 1980a, b, 1982; Masursky et al., 1980]. On the basis of abso- lute 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: 0.2 (surfaces with a significant percentage of anoma- lously high-dielectric material). We then assess the degree of correspondence of the subdivisions of roughness and reflec- tivity with the physiographic/topographic provinces defined by Masursky et al. [1980]. Finally, we assess the degree of correspondence between the roughness and reflectivity subdi- visions 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) sedi- mentation 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 associ- ated with a specific PV spacecraft revolution (e.g., orbital ground tracks) and, to a lesser extent, with orbit-to-orbit vari- ations. 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 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 rela- tively 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.