Evolution of lineaments on Europa: Clues from Galileo multispectral imaging observations

Four distinct classes of lineaments can be described on the basis of Galileo's improved spectral and spatial coverage of Europa: (1) incipient cracks are narrow (<1.6 km wide) and are generally colorless fractures which tend to crosscut other lineament types, (2) ridges have raised relief, tend to be wider (3-6 km) than the cracks, and are distinct in color from the icy plains they transect, (3) triple bands have dim medial stripes similar in color and width to the ridges and broad (8-12 km wide) diffuse low-albedo margins tainted by a red-brown non-ice contaminant, and (4) ancient bands are generally similar in width to ridges and triple bands and have colors intermediate between those of triple bands and the undisturbed icy plains. They are only slightly darker than the plains at visible wavelengths-hence, they were largely undetected by Voyager-but are distinctly brighter than the plains in the infrared, precluding the possibility that they have simply faded from their formerly darker appearance. The morphologies, spectral properties, and orientations of the lineaments vary systematically with age, suggesting that these four classes represent different stages of development in an evolutionary sequence. Lineament formation appears to be the dominant resurfacing mechanism on Europa, and every landscape that has escaped erasure by heating from below is imprinted with generation after generation of intersecting ridges at various scales and orientations, Relatively recent fractures expose coarse-grained, clean ice in the shallow subsurface, possibly accounting for the distinctive color of the satellite in comparison to the other icy moons of Jupiter, The process of lineament formation may be continuing today; the bright band Agenor Linea is among the leading candidates for current activity. (C) 1998 Academic Press.