GEOPHYSICAL RESEARCH LETTERS, VOL. 13, NO. 13, PAGES 1415-1418, DECEMBER 1986 VEGA LANDING SITES: VENERA 15/16 UNIT ANALOGS FROM PIONEER VENUS REFLECTIVITY AND RMS SLOPE DATA Duane L. Bindschadler and James W. Head III Department of Geological Sciences, Brown University James B. Garvin NASA Goddard SpaceFlight Center, Geophysics Branch Abstract. Pioneer Venus radar data on surface properties have been used to compare the Vega spacecraft landing sites with the northern 1/4 of Venus mapped by the orbitersVenera 15 and 16. The regionssurrounding both landing sites possess surface reflectivity and small-scale roughness properties most similar to those of mapped volcanoes and volcanic plains regions and different surfaceproperties than thoseof mappedtectonic units. Regions analogous to the Vega 1 site are relatively rare, covering 2.8% of the mapped surface. Vega 2 analogs are much more common and cover22.6% of the surface. Neither landing siteis representative of the nearby highlands of Aphrodite, but the Vega 2 landing site is similar to much of the northernplains of Venus. Introduction The Vega 1 and 2 spacecraft landed on Venus in June of 1985. Bothlandingsites are located in Rusalka Planitia, near the northern flanks of Aphrodite Terra. Vega 1 landed at 8.05ø4- 1.0 ø N, 176.89ø4-1.0 ø E, Vega 2 at 7.51ø4- 1.5 ø S, 179.8ø4- 1.5ø E [V.L. Barsukov, personal communication 1985]. Gamma-ray spectrometry at the Vega 1 site and gamma- and X-ray fluorescence (XRF) spectrometry at the Vega 2 site have been interpreted to indicate gabbroic rock compositions [Surkov et al., 1986a,b]. Basaltic compositions of surface materials were measured by XRF and gamma-ray spectrometery in the Beta-Phoebe region by the Venera 9,10 [Surkov et al., 1976; Surkov, 1977], 13, and 14 landers [Surkov et al., 1982; Moroz, 1983]. Synthetic aperture radar (SAR) images of the northern 1/4 of Venus have also been obtained by the Venera 15 and 16 orbiters [Kotelnikov et al., 1984]. On the basis of surface morphology, much of the surface of northern Venus has been inferred to consist of volcanic plains,likely to be basaltic in nature [Barsukov et al., 1986]. In addition, the close proximity of the landing sites to the highlands of Aphrodite makes it important to see whether these sites possess surface properties similar to Aphrodite.Thus a comparison of the Vega landing sites and the geologicunits mapped by Venera 15 and 16 would add to our understanding of how representative the landingsites are of the rest of Venus. Method The Pioneer Venus (PV) reflectivity and rms slope data [Pettengill et al., 1980, 1982] provide a method for Copyright 1986 by the American Geophysical Union. Paper number 6L7052. 0094-8276/86/006L-7052503. O0 comparison of the Vega landing sitesand the portion of the northern high latitudes covered by Venera 15 and 16. Reflectivity and rms slope data yield information on the materialproperties (dielectric constant, porosity, etc.) and the 0.5 to 10 meter scale roughness of the surface, respectively [Pettengill et al., 1980, 1982; Head et al., 1985, Garvin et al., 1985]. The estimated landing error noted above for each of the landers was used to determine the area over which surfaceproperties would be analyzed. Thus all PV reflectivity and rms slopedata within 1.0 ø of the Vega 1 site and within 1.5 ø of the Vega 2 site were averaged to obtain a measure of the surface properties of the landing sites. Both the original reflectivity data set [Pettengill et al., 1982] and a version corrected for the effects of diffuse scattering [Ford and Pettengill, 1984] were used as part of the characterization of the landing sites. Only the uncorrected data were usedin analysis and comparison of the Venera 15/16 unit map because the corrected data is limited by the latitudinal extent of Pioneer Venus SAR coverage: from 15ø S to 45 ø N [Pettengill et al., 1980]. Venera 15/16 data extendssouthward from 90 ø N only as far as 30 ø N [Kotelnikov et al., 1984]. The Venera 15/16 geologic unit map [Barsukov et al., 1986] has been digitizedand placed into an equal area map projection along with the PV data [Bindschadler, 1986]. The geologic unit map consists of six plains units, six tectonic units, and a volcanoes unit. [Barsukov et al., 1986; Basilevsky et al., 1986; Bindschadler, 1986]; unit names are given in Table 1. Locationsof regionswith reflectivityand rms slope within 1 standarddeviationof values for the Vega landing site regions have been noted. These analogous regions are shownin Figures l a and lb on a simplifiedversion of the Venera unit map. To find the Venera unit mostanalogous in surface properties to the Vega sites, the percent area covered by analogous regions was measured for each of the Venera units (Table 1). Because of the large difference in the total area coveredby Vega 1 and Vega 2 analogous regions, a normalized residual percent was also calculated (Table 1) to allow a direct comparison of similarityor dissimilarity of unitsto landing sites. This residual is calculated by taking the difference between the percent Vega 1 or 2 analogs within a given unit and the percent area expected if the analogs were randomly distributed. This difference is then normalized to the random distribution percentage. A residualof zero thus indicates no more than the expected percent area covered by landing site analogs, while a positive value indicates more than expected coverage. Discussion The 2 ø by 2 ø region surrounding the Vega 1 site contains 3 PV measurements of rms slopeand reflectivity. 1415