GLOBAL ROUGHNESS: A PRELIMINARY ANALYSIS. James B. Garvin, James W. Head, Dept. of Geol. Sci., Brown Univ., Providence, RI 02912, Stanley Zisk, , Westford, MA 01886, and Gordon Pettengill, Dept. Earth and Planet. Sci., Mass. Inst. of Tech., Cambridge, MA 02139. The Pioneer orbiter radar instrument obtained data for over 93% of the surface of Venus and from these data, three properties of the surface could be derived: 1) elevation, 2) reflectivity, and 3) surface roughness (1-3). The roughness of a planetary surface can be an important measure of the type and . variety of geologic processes operating to form and modify the planet. Local and regional variations can indicate areaspresently undergoing active erosion. In regions where image resolution is low, roughness data can aid in the inter- pretation of geologic structures (1-9). In order to provide a framework for the interpretation of local and regional roughness data, we have analyzed the global roughness data set. The purpose of this paper is to report on a quan- titati ve correlation of gl obal radar roughness and elevation. The PV radar altimeter data give a statistical measure of the angular distribution of scatterers or facets in the 0.1-10 m range. Roughness can be described in terms of the Hagfors C-factor (C), a dimensionless parameter which is related to rms slope in degrees by: rms slope = 180/~r&. The alti- meter data set consists of approximately 200,000 surface radar measurements. As previously described (3) these are fitted to 16 Hagfors C-factor templates to derive the first estimate of C-factor. A second level of fitting, an interpolation between the template values of the initial fit, is then attempted in order to improve the C-factor estimate. In 60% of the cases, the detailed structure of the data did not permit a second-level fit, We then subdivided the altitude range into 256 bins and plotted the dis- tribution of C-factor in each elevation bin as a function of elevation, We found that each altitude bin shows a wide range of C-factor values and be- cause of this we chose to average the C-factor of all the data points in each bin, keeping separate the first and second-level fitted data sets. Compari- sons of the distributions of these two averages showed that below about 2 km elevation the second-level fit data consistently showed lower C-factor values (rougher), but mimicked the variations with altitude of the first-level fitted averages. Above about 2 km, the averages general ly coincided. Therefore, in order to describe the general trends in the correlation of global roughness and elevation, we display the averaged C-factor data as a function of eleva- tion for the combined data set (Fig. 1-2). On the basis of this preliminary analysis of the global data sets we make the following observations : 1) The global Venus roughness average is appreciably lower than that measured for or the Moon at comparable radar wavelengths. The Venus roughness distribution is skewed heavily towards the very smooth end of the spectrum, and only 7% of the planet approaches the lunar highlands or martian fresh volcanic plains in terms of very high roughness (3,8,9). Over 67% of Venus is less rough than the smoothest lunar mare or martian smooth plains (8,9). 2) A wide range of surface roughness is observed at each elevation level. 3) There is a definite trend of increasing average roughness with increasing a1 ti tude. 4) The increase in average roughness as a function of altitude is not monotonic. The mean C-factor data display as a series of relatively discrete plateaus, transitions, and peaks. 5) The elevation boundary between the rolling plains and highlands province (1) (+1 .5 km) coincides with a change in the distribution of roughness values.

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Garvin, J. B. et al.

6) The boundary between the rolling plains and lowlands (-0.5 km) does not correlate with any major change in roughness distribution. 7) The 1owest elevations appear anomalously rough, perhaps reflecting the nature of the troughs and chasmae (2). However, we have not ruled out the possibility that this roughness may be an artifact. Highest elevations (> 4.5 km) show high roughness values (1). References: 1) Masursky H. et a1 . (1980) JGR, 85, 8232. 2) Schaber, G. G. (1981) NASA TM-84211, 429. 3)ttengill,. H. et al. (1980) -JGR, 85, 8261. 4) Pettengill, G. H. (1978) Ann. Rev. Astron. Astrophys. 16, 265. 5) Pettengill, G. H. et al. (1980) IEEE Trans. Geo. Rem. Sens. Gr18, 28. 6) Pettengill, G. H. mal.(1982) Bull. AAS, 14, 739. 7) Garvin, J. 8. and Head, J, W. (1982) ~Gm3(in press). 8) Moore, H. J, et al. (1980) Geol. Surv. Prof. PZ1046-B (Sec B, 34-41), 78 pp. 9) Simpson, R. A, and Tyler, G. L. (1980) -JGR, 85, 6610.

I I I I -3.00 -150 000 150 300 450 6D3 7.50 900 10.50 12.00 ALTITUDE ( KM) Figure 1, kan C-factor versus altltude in km for combined Venus data set. Altitude is relative to mean planetary radius of 6051.5 km. In general, C-factors belw 200 are rough, while those above 1000 are smooth.

ALTITUDE ( KM 1 Figure 2. Data density (no. of radar points) versus altitude for cabined Venus data set. Ver- tical axis is in tens of the nu&er of data points. Note that most of the planet falls between -1.5 and t1.5 km.

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