Observational Selection Effects in Europa Image Data: Identification of Chaotic Terrain

The identification of terrains on images of Europa is highly dependent on the image resolution and illumination angle, both of which are quite variable due to the observational selection constraints of the Galileo mission. Higher resolution images allow for the identification of smaller patches of chaotic terrain, which are indistinguishable from surrounding tectonic terrain in most other Galileo imagery. The incidence angle of light also introduces 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 terrain, and the geologic history of the satellite: At ∼200 m/pixel 30% of Europa's surface appears as chaotic terrain, but with the effects of lighting 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 earlier thick-ice model; "mottled terrain" and "lenticulae" appear to be the same as chaotic terrain, even though they have been mapped separately in the past; and accounting for how observational bias affects the freshness of appearance of chaotic terrain and the identification of tectonic modification of chaotic terrain demonstrates that the formation of tectonic and chaotic terrain have been concurrent processes throughout Europa's surface history.