Chaos and Lenticulae on Europa: Structure, Morphology and Comparative Analysis

Introduction: Based on Voyager imaging of Jupiter’s moon Europa, Lucchita and Soderblom [1] classified the surface of this icy satellite into two major terrain types: plains and mottled terrain. Mottled terrain was defined as consisting of isolated small spots, hummocky topography, and sometimes broken-up lineae. One of the primary goals of the Galileo mission was to determine the origin and particular characteristics of mottled terrain. Recent Galileo images of Europa reveal areas of mottled terrain that are typified by an abundance of ovoidal features ranging from 5 to 20 km in diameter known as lenticulae [2] and larger disrupted features comprised of blocky material and hummocky matrix described as chaos [3,6]. Lenticulae have been divided into three major classes [4,5]: domes (upraised domical features which commonly do not affect the texture of pre-existing terrain), spots (smooth low albedo areas which subdue or conceal preexisting terrain), and pits (lenticulae that have disrupted the pre-existing terrain and sometimes contain a chaos-like matrix material). A diapiric origin for lenticulae has been suggested by Pappalardo et al. [2]. The first recognized chaos region on Europa was Conamara Chaos (8 ̊N, 274 ̊W), imaged at 180 meters/pixel. Carr et al. [3] describes it as an irregularly shaped region with a discrete inward facing cliff-like boundary. Images of Conamara Chaos obtained during the Galileo Europa Mission at 54 m/pixel have allowed us to discern the two major terrain units comprising chaos: fragmented and dislocated polygonal blocks of background plains and a hummocky matrix of finer textured material [6,7]. Carr et al. suggested that chaos formed through a thermal upwelling [3]. We suggested that chaos may originate by coalescence of the surface effects of diapirs [6]. In this analysis, we address the questions: 1) what range of morphology characterizes the internal texture of chaos, and 2) is there a relationship between chaos internal texture and the micro-chaos within pits? The answers to these questions will help to constrain possible mechanisms for the origin of chaos. What range of morphology characterizes the internal texture of chaos? The 54 m/pixel high resolution imaging of Conamara Chaos allows us to classify the individual units which comprise chaos: linear-textured polygons and matrix material. Linear-textured polygons (yellow) are angular fragments exhibiting recognizable linear textures (ridges, troughs, and bands) typical of background plains. The matrix can then be subdivided into angular polygons, micropolygonal blocks, peaks, and hummocky matrix. These subunits are smaller in scale and differ in texture from lineartextured polygons, and/or are more affected by the hummocky material. Angular polygons (green) are steep-sided angular fragments of linear textured polygons with a steep tilt towards one side and a high cliff on the opposite side; these appear as if they may have been tilted relative to the plane of the Europan surface. Micro-polygons (blue) are smaller plate-like structures that show very faint evidence of linear texture, appear highly degraded, and are sometimes at a lower topographic elevation that the linear-textured polygons. Peaks (red) are individual ovoidal features usually less than 2 km across; these show very little or no linear texture and tend to be isolated peaks standing above the topographic level of the matrix, sometimes appearing to stand higher than the lineartextured polygon margins. Hummocky material (white) is a rough textured surface that is distributed between other matrix units and includes jumbled surface blocks from ~1 km in diameter down to the resolution limit of the image, ~200 m. We have found most or all of these units present in the 3 regions that we have studied. We performed a reconstruction of the linear-textured polygons within Conamara Chaos to determine the movement and nature of blocks [6,7]. We found that linear textured polygons account for 40% of the chaos region; thus implying the destruction, removal, and/or heavy modification of more than half of the pre-existing terrain. Is there a relationship between chaos texture and the micro-chaos within lenticulae? To address this question, we selected an area of Europa which exhibits both micro-chaos within pits and also chaos texture. The Galileo nominal mission obtained images (E11 Regional Map) of an area of mottled terrain centered at (5 ̊S, 237 ̊W) with a resolution of 220 meters/pixel. We applied our chaos subunit classification scheme to the structure and morphology of micro-chaos found in some lenticulae. We asked the questions: what are the similarities and differences in morphology and scale, and what can this reveal about modes of formation and the possible relationship between lenticulae and chaos? We also mapped structural features, such as troughs, domes, and escarpments, to study their possible relationships to chaos features. We located and mapped 61 features containing materials that fit the definition of chaos/micro-chaos texture