POSSIBLE IMPACT ORIGIN FOR CHAOS TERRAIN ON EUROPA: EVIDENCE FROM SHAPE, SIZE, AND GEOGRAPHIC DISTRIBUTION

Chaos areas on Europa—regions where the surface has been disrupted, with rafts of remnant crust set in a hummocky matrix of slushy appearance—do not look like classic impacts; but several lines of evidence suggest they may be sites where impactors fully penetrated the ice crust. In this model, the matrix represents the boiled and refrozen surface of the underlying water layer, and the embedded plates are wreckage of the fragmented crust.

The size and shape of chaos areas are significantly correlated. Large chaos areas tend to have complex, irregular borders whereas smaller ones are generally equidimensional with simpler boundaries. There is also a correlation between chaos shape and raft count: the more irregular the chaos shape, the more rafts it tends to have. These relationships match predictions from impact experiments into ice over water, which show a relationship between impact energy and the shape of the resultant hole. The large jagged openings reflect wide-field fragmentation of the ice crust above the water layer, which is greater at higher energy (or in thinner crust).

Chaos areas are concentrated at low latitude, consistent with the expectation for impacts on a tidally locked satellite. Almost 60% of chaos areas (653 of 1092 features imaged at 230 m/pixel or better) are clustered between 30° north and south. Likewise, the percent surface area occupied by chaos terrain is greatest at low latitudes, and decreases with distance from the equator. The chaos distribution echoes that of non-penetrating craters on Europa, which also show low-latitude clustering (14/28 craters > 4km diameter are within 30° of the equator, and only 5/28 are at >60°).

Existing data hint at some apex-antapex asymmetry, but we present this tentatively because longitudinal coverage in the Galileo data is limited. There are slightly more chaos areas (both absolutely and per degree longitude) in the leading hemisphere REGMAP strip than in the corresponding trailing hemisphere strip; and the area occupied by chaos is 11% in the trailing in comparison with 21% in the leading hemisphere strip. Although strong conclusions cannot be drawn because of the lack of body-wide coverage, these data are consistent with predicted impact asymmetry.