PLANETARY S URFACES: T ECHNIQUES A NDA PRELIMINARY A NALYSIS OF MARS A ND V ENUS

Characteristics of rock populations on the surfaces of Mars and Venus can be derived from analyses of rock morphology and morphometry data. We present measurements of rock sizes and sphericifies made from Viking lander images using an interactive digital image display system. The rocks considered are in the gravel size range (16-256 mm in diameter). Mean sphericities, form ratios, and roundness factors are found to be very similar for both Viking lander sites. Size distributions, however, demonstrate differences between the sites; there are significantly more cobble size fragments at VL-2 than at VL-1. A model calling for aphanitic basalts emplaced as ejecta or lava flows at the Viking sites is supported by the rock shape, size, and roundness data. Morphologic features pertaining to the modification history of a rock are considered for Mars and Venus. A multi-parameter clustering algorithm is utilized to objectively categorize martian and venusian rocks in terms of various criteria. Erosional markings such as flutes are demonstrated to be most important in separating VL-1 rock morphologic groups, while rock form (i.e., shape) represents the primary separator of subpopulations at VL-2 and the Venera landing sites. Fillets are common around VL-1 and Venera 10 fragments. Obstacle scours occur frequently only at VL-1. Cavities in rocks are ubiquitous at all lander sites except Venera 9. Eolian processes, possibly assisted by local solution weathering, are a strong candidate for the origin of cavities and flutes in martian rocks. Rock populations on Earth typically contain morphological information p ertinent to their compositions, modes of origin, emplacement styles, and subsequent weathering histories (Ollier, 1969; Folk, 1974). Terrestrial analogues for the Martian environment (Morris etal., 1972, McCauley etal., 1979), permit some inferences to be made about the evolution of the Martian surface. Previous studies of block fields and fine particles on Mars (Moore etal., 1977, Evans and Adams, 1979; Strickland, 1979), and Venus (Florensky et al., 1977; Keldysh, 1979), have identified rock subpopulations within the fields of view of the Viking and Venera landers, but have lacked the large-scale data base required for multiple-parameter morphological analysis. In this report, we provide an overview of a data collection and analysis scheme that has been developed for the interpretation of rock morphology from lander images (Garvin et al., 1980). Emphasis is placed here on our approach to solving the problem of how to best characterize rock populations on planetary surfaces. It involves the collection of quantitative data such as rock size and sphericity, as well as qualitative information regarding morphological features. Full descriptions of the morphological attributes chosen with specific rock examples are presented. Data analysis techniques are also