Cryomagmatic mechanisms for the formation of Rhadamanthys linea, triple band margins, and other low-albedo features on Europa

We investigate cryomagmatic mechanisms for the formation of the diffuse low -albedo margins of triple bands and haloes associated with elliptical lenti culae on Europa. One mechanism involves explosive venting of a gas-particle spray derived from a subsurface body of volatile-bearing water. Assuming t he dark features represent cryoclastic deposits erupted from a medial or ce ntral vent, modeling the dynamics of the eruptive plumes indicates that eru ption velocities of 30 to 250 ms(-1) and volatile contents of 0.1 to 20 wt% are required to produce deposits having the observed dimensions. Calculate d heights for ballistic plumes range from <1 to similar to 25 km, As an alt ernative mechanism, we consider conductive heating of lithospheric ice by a subsurface heat source (e.g., a warm ice diapir or cryomagmatic intrusion) , which promotes enhanced sublimation of surface ice, producing local enric hment of nonice material. An analytical model describing a constant-tempera ture, infinite heat source places lower bounds on the time scales required to produce a detectable feature. A numerical treatment of more realistic ge ometries and boundary conditions allows cooling of the heat source to be mo deled. We find that the thermal model satisfactorily explains features with the dimensions of minor lineaments and lenticulae haloes (a few kilometers ) over time intervals of similar to 10(4) to 10(5) years, The formation of a sublimation lag deposit proceeds rapidly once the ice temperature exceeds a threshold of similar to 130 K, However, for triple bands measuring up to 25 km in width, additional heat sources, such as repeated or multiple intr usions, are necessary to reproduce the observed dimensions, This is consist ent with the observation that well-developed triple bands typically have mu ltiple medial ridge sets, each of which may represent a discrete tabular he at source. Although neither the volcanic nor intrusive/thermal model can be ruled out at this stage, the latter is a more satisfactory explanation for some characteristics of the low-albedo features analyzed in this study, (C ) 2000 Academic Press.