Global Expansion of Ganymede Derived from Strain Measurements in Grooved

Introduction: Grooved terrain on Ganymede records an episode of extensional tectonics and possibly cryovolcanism [1], but the cause of groove formation remains uncertain. With little evidence for surface contraction to balance out extension, various mechanisms have been proposed to increase the interior volume of Ganymede and thus cause global expansion. Global expansion may be caused by the interior differentiation of Ganymede, which could produce as much as 3.2-3.6% expansion in radius [2,3], though Galileo gravity data [4] appears to constrain this maximum radial expansion to < 3%. Volume expansion of up to 2% (radial expansion of 0.7%) may also occur as a result of melting and warming Ganymede's ice mantle during an eccentricity-driven thermal runaway event [5]. Observational constraints derived after the Voyager encounters set the maximum areal expansion of Ganymede at 1% (corresponding to a radial expansion of 0.5%). McKinnon based his estimate on Galileo Regio being an intact spherical cap [6] and Golombek based his estimate on the strain represented by the grooved terrain [7]. In the latter study, a portion of Uruk Sulcus was examined and extensional strains were assigned to the different ages of grooves to calculate the percent area increase. The assumed strains of a few percent were based on the assumption that the grooves are graben with small displacements. Golombek then applied the areal expansion from Uruk Sulcus to the whole body, assuming a 50-50 mix of bright and dark terrain. The simple graben model does not describe the characteristics of grooved terrain at high resolution [1]. Strain measurements in grooved terrain based on fault geometry [8] and deformed impact craters [9] show that tens of percent extension is not uncommon in grooved terrain. This begs the question of just how common large extensional strains are, and whether they imply either (a) global expansion in excess of that possible through volume change mechanisms, or (b) the existence of large-scale contractional deformation which has so far eluded observation. I will address this question by combining recent strain measurements with a global database of grooves on Ganymede [10]. Linking strain to groove morphology: Because of the limited extent of Galileo high-resolution observations, there are only a few areas in which we can measure the strain across swaths of grooved terrain. The challenge in estimating global strain is to link local strain observations from images at the 10 100 m/pixel scale with groove morphology Figure 1: Example global-scale images of grooved terrain where the strain has been constrained. Each image is 100 km on a side. Morphological types include (a) type I, (b,c) isolated type I grooves in dark terrain, (d) type II, (e) type III, and (f) type V.