Observational selection effects in Europa image data: Identification of chaotic terrain

The identification of terrains on images of Europa is highly dependent on t he image resolution and illumination angle, both of which are quite variabl e due to the observational selection constraints of the Galileo mission. Hi gher resolution images allow for the identification of smaller patches of c haotic terrain, which are indistinguishable from surrounding tectonic terra in in most other Galileo imagery. The incidence angle of light also introdu ces an observational bias in the identification of chaotic terrain: Images acquired closer to Europa's terminator generally reveal more chaos features than images taken with higher Sun. Proper accounting for selection effects has implications for the size distribution and chronology of chaotic terra in, and the geologic history of the satellite: At similar to 200 m/pixel 30 % of Europa's surface appears as chaotic terrain, but with the effects of l ighting and resolution taken into account, the portion is 40% or more; bias correction eliminates the 10-km-size peak that had been the basis of an ea rlier thick-ice model; "mottled terrain" and "lenticulae" appear to be the same as chaotic terrain, even though they have been mapped separately in th e past; and accounting for how observational bias affects the freshness of appearance of chaotic terrain and the identification of tectonic modificati on of chaotic terrain demonstrates that the formation of tectonic and chaot ic terrain have been concurrent processes throughout Europa's surface histo ry. (C) 2001 Academic Press.