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 lenticulae on Europa. One mechanism involves explosive venting of a gas-particle spray derived from a subsurface body of volatile-bearing water. Assuming the dark features represent cryoclastic deposits erupted from a medial or central vent, modeling the dynamics of the eruptive plumes indicates that eruption velocities of 30 to 250 [ms.sup.-1] and volatile contents of 0.1 to 20 wt% are required to produce deposits having the observed dimensions. Calculated heights for ballistic plumes range from [is less than] 1 to ~25 km. As an alternative 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 enrichment of nonice material. An analytical model describing a constant-temperature, infinite heat source places lower bounds on the time scales required to produce a detectable feature. A numerical treatment of more realistic geometries and boundary conditions allows cooling of the heat source to be modeled. 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 ~[10.sup.4] to [10.sup.5] years. The formation of a sublimation lag deposit proceeds rapidly once the ice temperature exceeds a threshold of ~130 K. However, for triple bands measuring up to 25 km in width, additional heat sources, such as repeated or multiple intrusions, are necessary to reproduce the observed dimensions. This is consistent with the observation that well-developed triple bands typically have multiple medial ridge sets, each of which may represent a discrete tabular heat 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. Key Words: Europa; volcanism; satellites of Jupiter; surfaces, satellite; ices.