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

Wilson, Lionel and Head, James W. and Pappalardo, Robert (1997) Eruption of lava flows on Europa : Theory and application to Thrace Macula. Journal of Geophysical Research: Planets, 102 (E4). pp. 9263-9272. ISSN 2169-9100

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 (45°S, 171°W) 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 H2O 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 H2O 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 × 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.