THE GROWTH OF EUROPA’S ICY SHELL: CONVECTION AND CRYSTALLIZATION

A central unanswered question in Europa science is the thickness of its outer icy shell. Until an instrument with the capability of directly measuring this thickness is successfully brought to the satellite, the task of estimating this thickness lies within estimates made based upon the physical conditions in operation. We propose that the crystallization of the shell from a liquid ocean in the presence of subsurface thermal convection largely controls the shell thickness. We model this crystallization assuming that the ice shell is in the stagnant lid convection regime with a conductive surface lid and a simplified one-dimensional layer of subsurface thermal convection. The presence of this convective layer leads to a wide range of possible shell thicknesses, when variable tidal heating is taken into account, with our model finding shell thicknesses in the range of 5 to 28 km possible on the satellite. On the other hand, the thickness of the conductive lid (or the “lithosphere”) was consistently uniform (within a kilometer of 3 km) no matter what input parameters we used. The time required for the ice shell to crystallize was also much shorter than we expected (less than 1 million years for all model cases). Looking at a possible global state of the shell leads us to the possibility that there may be constant shell melting in the polar regions of the satellite, which would greatly facilitate ocean-ice exchange. Also, the rapidity of shell melting and re-crystallization suggests that maybe the ice shell is so young because it completely melted and recrystallized within the last 100 million years.