STYLES OF CHAOTIC TERRAIN FORMATION ON EUROPA

Europa’s surface is, in places, disrupted by regions of chaos and lenticulae, generally agreed to be the youngest geological units. Within these features the surface is broken into fragments or plates, of preexisting terrain, which are surrounded by a lower albedo matrix of finer textured, hummocky material. Chaos regions can be hundreds of square kilometers in area [1], while lenticulae are smaller, generally subcircular features with a characteristic size distribution of ~15 km in diameter [2, 3]. Several models have been proposed for the formation of these terrains, but the geological evidence points towards some type of diapiric upwelling, probably of "warm" ice, perhaps interacting with a pocket of lower melting temperature material [4].

Morphological and textural variations exist between different areas of chaos and separate microchaos lenticulae [3]. A study is underway to investigate the boundaries of these disruption features with respect to their surroundings. Some, such as the Conamara Chaos, appear to have undergone significant brittle deformation, with sharp boundaries and large internal plates [1]. Others, such as Thrace Macula, appear to have disaggregated in situ, and show evidence of low albedo material embaying the surrounding terrain [5]. This may indicate a greater thermal influence during formation of this chaos feature. Lying mid-way between these endmembers is "speckled" chaos [6] in which the preexisting terrain appears to have been "softened" and subdued without undergoing full-scale disruption. In addition, a comparison of chaos and microchaos matrix material with fine-textured Europan bands will investigate whether matrix consists of disaggregated preexisting surface material, or subsurface material such as that from which bands are formed.

1. Greeley, R. et al, J. Geophys. Res., 105, 22,559 - 22,578, 2000. 2. Pappalardo, R. et al., Nature, 391, 365-368, 1998. 3. Spaun, N., Eos Trans. AGU, 79 (45), Fall Meet. Suppl., F540, 1998. 4. Collins, G. et al., J. Geophys. Res. 105, 1709-1716, 2000. 5. Kortz, B. et al., LPSC XXXI, CD-ROM 2052, 2001. 6. Prockter, L. J. Geophys. Res., 104, 16531-16540, 1999.