Fractures, Scarps, and Lineaments on Callisto and Their Correlation with Surface Degradation

Introduction: Tectonic features on Callisto are found (1) preferentially in large multi-ring structures, or (2) in several areas in its dark, cratered plains units outside the large multi-ring structures [1, 2]. In this latter case, tectonism is represented by fractures, scarps, and albedo lineaments. In this paper, we will mainly concentrate on tectonic features outside the large multi-ring structures and investigate (1) their spatial distribution, as seen on high resolution images returned by the Galileo SSI camera, (2) their preferential orientations, (3) their correlation to tectonic features found on lower resolution Voyager images, (4) their time-stratigraphic position, and (5) their correlation to surface degradation processes. Target areas: High-resolution views of Callisto's surface which essentially reveal tectonic features studied in this paper were obtained during Galileo's orbits C9, C10, and C20. In these target areas, stereo data could be used. Red-blue anaglyph images were constructed to aid in the evaluation of these data. Geologic units and crater ages: In orbit C10, an area originally termed smooth plains [3] was targeted at 270 m/pxl and 68 m/pxl (SSI target areas 10CSSMTHPL02 and 01). The area appears bright and smooth in Voyager-1 images. It was interpreted as either a volcanically resurfaced area [3], or as a degraded palimpsest [1]. The higher resolution SSI data showed a rough and knobby surface instead. No evidence is found for volcanic resurfacing in this area, but some more or less concentric scarps indicate an impact origin , most likely a degraded palimpsest. From measurements of the crater distributions, cratering model ages are 4.2±0.05 Gyr in model I [4] and 4.3 Gyr (uncertainty range 4.56 Gyr to 2.1 Gyr) in model II [5], while the surrounding cratered plains have ages of about 4.2 to 4.25±0.05 Gyr in model I, or 4.3 to 4.5 Gyr in model II. Hence the stratigraphic position of the degraded impact structure is close to the base of the system defined by the Asgard impact [6]. In orbit C20, a dark patch interpreted as dark volcanic flow [1] was targeted for high resolution at 430 m/pxl and 108 m/pxl (SSI target areas 20CSDRKFLO02 and 01). Both observations could not reveal features unequivocally indicative of volcan-ism [7]. The dark " flow " is located close to a bright area which appears smooth at Voyager resolution but turned out to be rough and knobby in the SSI frames, comparable to the C10 " smooth plains ". …