Eruption of lava flows on Europa: Theory and application to Thrace Macula

Abstract

Abstract. Europa, on the basis of its spectral characteristics and its apparently young surface age, is likely to be at least partially resurfaced by liquid water volcanic processes; however, convincing evidence for lava flow morphology in Voyager data has not previously been found, and the lower density of liquid water relative to an overlying solid water ice ,crust suggests that extrusive volcanism could be uncommon. We examine a candidate lava-flow-like feature (Thrace Macula) and compare its characteristics with theoretical predictions for the ascent and eruption of water magma on Europa, outlining a range of conditions under which water ice magma could plausibly be erupted to the surface. Thrace Macula (45øS, 17 IøW) is a dark lobate feature extending NNW to SSE; its northern part is centered in a 140 km diameter arcuate to circular lineament, and within this zone are radial and concentric lineaments and three dark lobes similar in appearance to flow lobes extending away from a central region for distances of 40, 50, and 160 km. The 160 km lobe appears to follow local topographic features, producing lobate margins at subdued structures, broadening locally parallel to structural trends, and terminating against a gray band. Models for the present structure and state of the predominantly H20 exterior of Europa range from solid water ice to a thin ice layer overlying a liquid water layer, and these conditions may even vary as a function of space and time. Ascent and eruption of H20 magma through a less dense water ice crust means that sustained effusive eruptions are very difficult to produce if a globally continuous liquid water (or salt-water) layer exists [e.g., Crawford and Stevenson, 1988; Kargel, 1991]. For flows as extensive as the Thrace Macula feature, we thus require magma source regions sufficiently localized that excess pressure can be built up in the source region. Given the elastic properties of ice at these source depths, the most likely scenario is intrusion of silicate magma into the base of an ice crust, and melting and mobilization of water magma. With this range of magma source settings, we model plausible surface vent geometries for a water lava flow interpretation of Thrace Macula. Modeling calculations show that flow lobes with the above geometries, and thicknesses on the order of 10 m, could be formed in water eruptions on Europa involving discharge rates of-2 x 105 m3/s through dikes with widths in the range 5-10 m. These discharge rates (about 3 times larger than those of terrestrial flood basalts) should not be unusual for water eruptions on Europa under these conditions and suggest that water volcanic resurfacing could be common on the satellite if source regions meeting these criteria were readily available. The lack of abundant similar dark flows could be due to brightening with time or might imply that conditions for their formation (grounded or nearly solid ice crust) only evolved in the latter part of Europan history.

Large-scale doming on Europa: A model of formation of Thera Macula

“…Besides, the presence of two diffuse patches on the south and east borders of Thera Macula (dt in Fig. 1) could agree with infiltration of fluid material, probably brines or slurries, coming from the matrix according to the interpretation given by literature about the same patches along the neighbouring feature Thrace Macula (Wilson et al, 1997;Kortz et al, 2000;Fagents, 2003). Although this cryovolcanic event is shown on the last sketches of Fig.…”

“…This subcircular structure (140 by 80 km in diameter) associated with a similar but longer feature, Thrace Macula, is located in the south hemisphere (centred at ~ 47.7°S and 180.9°W). They were previously described and assumed to be cryomagmatic in origin by analyzing the lower resolution images acquired by Voyager imaging system (Wilson et al, 1997). On the basis of the morphological observations of Galileo images presented in section 2, we propose a plausible model of formation of Thera Macula.…”

Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life

“…Evidence of cryovolcanic activity is not common on the surface; this can largely be accounted for by the buoyancy of solid water ice and the difficulty of erupting eutectic brines through eutectic solid crust without involving exsolved gases (Wilson et al 1997, Fagents et al 2000. But some models may favor production of positively buoyant magma.…”

“…The development of density stratification during cryomagmatism depends on details of the melt generation processes and eruption mechanisms (Kargel 1991, Hogenboom et al 1995, Wilson et al 1997, Kadel et al 1998, Wilson and Head 1999, Fagents et al 2000. Equilibrium crystallization-or quenching with glass formation-might occur in some rapidly cooled cryovolcanic flows and cryoclastic deposits, resulting in frozen layers that would tend to consist of eutectic or peritectic compositions.…”

Does Europa have a subsurface ocean? Evaluation of the geological evidence

“…In fact, other configurations are preferable for some features. For example, if buoyancy mechanisms cannot be invoked to explain liquid effusions, then the requirement for pressure-driven eruption could be met by having discrete water reservoirs; it would be difficult or impossible to achieve local pressurization of a global water layer [Wilson et al, 1997]. Cryoclastic volcanism requires that a volatilebearing liquid be depressurized by the opening of a fracture.…”