XXXIV Lunar and Planetary Science Conference CRUSTAL SPREADING ON EUROPA: INFERRING TECTONIC HISTORY FROM TRIPLE JUNCTION ANALYSIS

Introduction: Europa, one of the Galilean moons of Jupiter, is 1565 km in diameter with a rocky core and an outer layer of H 2 O approximately 170 km thick [1]. Galileo magnetometer data suggest that a large portion of the H 2 O layer consists of liquid water [2], and that it is capped by an ice layer between ~1 and 20 km thick [3]. This outer shell is constantly under time-varying stress fields as a result of di-urnal tides induced by the moon's proximity to Jupiter [4], nonsynchronous rotation of the decoupled outer shell [5], and possibly polar wander [6]. The effect of these fields is to crack and warp the ice shell and subsequently break it into plates. Images obtained by the Voyager and Galileo space-crafts have shown that Europa displays evidence for some morphologies associated with terrestrial plate tectonics [3,4,7,8]. These include (but may not be limited to) spreading centers and transform faults. Here we expand on a preliminary analysis [8] of a set of triple junctions in the south polar region of Europa whose boundaries appear to be characterized by ridges that have been pulled apart and explore its implications for determining relative spreading velocities, stability, and tectonic history. Triple Junctions: Triple junctions are the intersection of three tectonic plates. On Earth, the boundaries of the plates that intersect can exhibit any of three types of boundary interaction: divergent (Ridge), convergent (Trench), or transform (Fault). Considering all possible combinations of these boundaries as well as differences in the sense of subduction along a trench and shear motion along a transform boundary, the total number of unique boundary combinations is sixteen [9]. Of these, two can be considered always unstable (FFF and RRF), one always stable (RRR), and the rest stable under certain geometric conditions. If we assume that plates act rigidly on Europa (as they typically do on Earth) and that all plate motions are circular then the relative motions between the plates that form a triple junction would not be independent [9]. This implies that at any instant in time the addition of the magnitude of the velocity vectors between each boundary in the junction must equal zero and the velocity vectors would form a closed loop (typically a triangle) [9]. Velocity diagrams that satisfy the above assumptions can be used to constrain the relative velocities and motion of all boundaries in a triple …