JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

Perhaps the most fundamental issue regarding any planetary body is the means by which it loses its heat. The style and rate of Europa's heat loss is a vital issue in understanding the satellite's geological history, and is of particular importance when considering whether the satellite has a global subsurface ocean today. High resolution Galileo images reveal that Europa's latter-stage geology is dominated by circular to elliptical pits, domes, and dark reddish spots ~10 km in diameter, collectively known as "lenticulae," which upwarp, disrupt, and embay the preexisting ridged plains of Europa. Pappalardo et al. [1998] interpret the features as the surface manifestation of geologically recent diapirs which have risen buoyantly through the subsurface to locally upwarp and disrupt the pre-existing surface. Such diapirs are plausibly the expression of active solid-state convection within Europa's icy shell. Much of the uncertainty involved in past modeling of Europa's heat loss derives from uncertainties in the rheology of cold ice, the nature of likely ice impurities within Europa's ice shell, and the rheology of ice that contains impurities. Recent laboratory deformation experiments by Goldsby and Kohlstedt [1997a,b] have elucidated the flow of cold ice. Based on Galileo multispectral data, it has been reported that Europa's endogenic features, including its lenticulae, show absorption bands indicative of heavily hydrated minerals. A good fit to the spectral data provided by salts, notably MgSO4*XH2O, where X=6 [McCord et al., 1998]. We propose a study that will link observations of Europa's surface geology with models of its composition, interior rheology, and convective state, through an integrated approach of geomorphological, laboratory, and theoretical research. First, we shall use regional-scale and high-resolution Galileo SSI data to examine the morphology and distribution of Europa's surface features plausibly linked to diapirism and convection within the satellite's icy shell, in order to constrain their origin, evolution, and implications for the satellite's interior. This phase of the study will include mapping, analysis, and documentation of the morphology, morphometry, and distribution of lenticulae imaged on Europa during the Galileo prime mission. We will investigate the possible genetic relationships among lenticulae to better constrain their origin and evolution, and we will examine the possible relationship of lenticulae to regions of chaotic terrain [Carr et al., 1998] on Europa. Second, we will perform a series of laboratory creep experiments on MgSO4/ice alloys as the logical first step in understanding the effect of planetologically significant salts on the rheology of polycrystalline ice. In particular, we will investigate the effect that alloying of ice with MgSO4 has on the two ice creep mechanisms that are experimentally accessible at laboratory conditions: dislocation creep, and dislocation-accommodated grain boundary sliding (GBS). These data will be used to construct a constitutive law for MgSO4/ice, similar to that derived by Goldsby and Kohlstedt [1997a,b] for pure ice. By determining the effect of salt impurities on the rheology of ice, these experiments will enable modeling of Europa's interior state and evolution, notably its convective state and thermal evolution. Third, we will analyze the effects of sophisticated ice flow laws on models of solid-state convection within Europa's ice shell. The constitutive law for pure ice I of Goldsby and Kohlstedt [1997a,b] will be adapted for use in convection modeling. Moreover, we will investigate the effects of the rheology of ice-salt mixtures on convection modeling, as derived through our ice-salt rheological studies. We will examine the effects of these rheologies on the temperature distribution of the convecting lithosphere, its convective vigor and rate of heat transport, and effects on plume morphometry. Our modeling will include analysis of the freezing rate of Europa's ocean for both pure ice and salt-ice compositions. Derived thermal profiles will be used to model the strength of Europa's icy shell with depth. The results of this integrated study will be relevant to: 1) the geological history of Europa's surface, 2) the evolution and present state of Europa's icy shell, 3) the likelihood of a subsurface ocean on Europa today, and 4) the plausibility of detecting a subsurface ocean on Europa with an orbiting spacecraft. The proposed research is related to, but separate from, activities performed of the PI through affiliation with the Galileo SSI team during the spacecraft's prime mission, which involved initial documentation and validation of the SSI data (see reference below). This proposal involves new collaborators and significantly broadens participation in the analysis of the Galileo data. Although we have not submitted a separate Education and Public Outreach proposal, the PI plans to continue his extensive public outreach and education activities, including public presentations of Galileo's discoveries in the jovian system. Pappalardo, R. T., J. W. Head, R. Greeley, R. J. Sullivan, C. Pilcher, G. Schubert, W. B. Moore, M. H. Carr, J. M. Moore, M. J. S. Belton, and D. L. Goldsby, 1998. Geological evidence for solid-state convection in Europa?s ice shell, Nature, 391, 365-368.