ICARUS zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 61, 175-184 (1985) Patterns of Fracture and Tidal Stresses Due to Nonsynchronous Rotation: Implications for Fracturing on Europa PAUL HELFENSTEIN AND E. M. PARMENTIER Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 Received August 3, 1984; revised November 26, 1984 This study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body. The incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived, and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on Europa. The observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement. Tension fracturing can occur for a small displacement of the tidal axis, so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface. c 1985 Academic Prea. Inc. INTRODUCTION Global lineaments on Europa, observed in Voyager images of the surface (Smith et al., 1979a,b), have been interpreted as frac- tures in an icy crust. A variety of lineament types have been identified (Lucchitta and Soderblom, 1982; Pieri, 1981), and the lin- eaments appear to form a systematic pat- tern on the surface (Smith et al., 1979a; Helfenstein and Parmentier, 1980; Luc- chitta et al., 1981; Pieri, 1981). The purpose of this study is to consider possible implica- tions of surface fracturing for the internal and orbital evolution of Europa. Several different mechanisms for the for- mation of global fracture systems have been proposed. Finnerty et al. (1981) and Rans- ford et al. (1981) suggested that surface cracking is a consequence of stresses due to internal expansion caused by heating and dehydration of silicate minerals combined with stresses due to subsolidus convection. We have previously examined the hypothe- sis that fractures may result from tidal de- formation and internal volume change (Helfenstein and Parmentier, 1980, 1983). Many mechanisms may cause stresses at the surface of a planetary body so that it is important to consider the resulting pattern of tectonic features as a constraint on the origin of stresses. The observed pattern of fractures on Europa does not appear to be consistent with stresses due to tidal despin- ning. However, for a synchronously rotat- ing body, the pattern of fractures observed could be produced by a combination of stresses due to orbital recession, orbital ec- centricity, and internal contraction. Stresses due to orbital recession and in- ternal evolution are expected to accumulate over long times. However, the small num- ber of identifiable impact craters and, espe- cially, the estimated time to produce ob- served photometric differences between leading and trailing hemispheres (Squyres et al., 1983) suggest that the icy surface is very young. It is therefore interesting to consider other mechanisms of tidal distor- tion that may cause relatively rapid accu- mulation of stresses and recent fracturing of the icy crust. Greenberg and Weiden- schilling (1984) have recently pointed out that the forced eccentricity of Europa’s or- bit may result in nonsynchronous rotation. Since the tidal torque averaged over one orbital period is nonzero for an eccentric orbit, they argue that some minimum per- manent asymmetry in the mass distribution of the satellite is required to maintain syn- I75 0019-1035185 $3.00 Copyright Q 1985 by Academic Press. Inc. All rights of reproduction in any form reserved.