The surface of Io: Geologic units, morphology, and tectonics

Abstract

A prelimanary geologic map, representing 26.5% of the surface of Io, has been compiled using best-resolution (0.5 to 5 km/line pair) Voyager 1 images and (as a base) a preliminary pictorial map of Io. Nine volcanic units are identified, including materials of mountains (1.9% of total area), plains (49.6%), flows (31.1%), cones (0.1%), and crater vents (4.0%), in addition to seven types of structural features. Photogeologic evidence indicates a dominantly silicate composition for the mountain material, which supports heights of at least 9 ± 1 km. Sulfur flows of diverse viscosity and sulfur-silicate mixtures are thought to compose the pervasive plains. Pit crater and shield crater vent wall scarps reach heights of 2 km and layered plains boundary scarps have estimated heights of 150 to 1700 m; such scarps indicate a material with considerable strenght. A cumulative, volcanic crater size-frequency distribution plot has been prepared using 170 mapped Ionian vents with diameters > 14 km; the shape and slope of the curve are like those for impact craters on other bodies in the solar system, attesting to a similar nonrandom distribution to crater diameters and a surplus of small craters. Io's equatorial zone has six times the number of vents per unit area as the south polar zone. No craters of unequivocal impact origin have been identified on Io to date. A total of 151 lineaments and grabens are recognized with four dominant azimuthal trends forming two nearly orthogonal sets spaces 110° apart (N 85° E, N 25° W and N 45° E, N 55°W). The mapped area lies within the longitudinal zone (250 to 323°) of least-abundant SO 2 frost, indicating that other sulfurous components dominate the upper surface layers in this area.