The Junocam Effect: Global & High-resolution Topographic Mapping of Io, Europa, Ganymede, Callisto

The four major Galilean satellites, two of which at least are ocean worlds or active, are important exploration targets for two current flight missions (JUICE and Europa Clipper).  Despite their importance, our understanding of their global shapes and topographic constraints of geologic features and ice shell (or lithospheric) variability are shocking limited.  Recent acquisition of new imaging data from Junocam (1,2,3) at Ganymede, Europa and Io have provided important new constraints on global shape, local topography and cartography, requiring rederivation of high-resolution topography to line up with the revised coordinate systems for all 4.  Here we review the state of those efforts, including new insights into ice shell variability on Europa and Ganymede, mountain heights on Io, and topography of Callisto, also an important exploration target.Current control networks and derived shape models of the Galilean satellites have been constructed by several groups, including these authors. Each has been hampered by the incomplete coverage of the Voyager, Galileo, New Horizons imaging data.  Each has problems that must be understood in constructing reliable cartographic control networks, from Voyager camera distortions to Galileo radiation noise and severe resolution differences in all data. Further, USGS control for Europa was found to have excessive numbers of misregistered points derived from automated tools. To mitigate these issues, we use similar automated match point selection tools from which local radius values are derived to construct the shape model, but with more comprehensive filters and sifting tools for rejection of ‘bad’ points.  New Junocam (JC) imaging supplements these in northern subjovian regions for Ganymede and Europa, and near global for Io (1,2,3).  Indeed, no new shape models can be contemplated without the JC data.  However, test shape models derived from the 4 missions combined showed higher residuals and shape distortions in the JC coverage areas. A minor adjustment in the JC camera model reduces these significantly and further refinement is under way.High resolution topography is derived from stereogrammetry (SG) which is limited to as little as ~20% globally for Callisto or Ganymede, and as high as 75% for Io, and from low-Sun shape-from-shading (SfS) mapping which is higher coverage but inappropriate for regional scale characterization.An updated shape model is under construction from Voyager (plus modest Galileo) coverages as of this writing.  Stereo coverage of Valhalla confirms there is no demonstrable relief across the basin except outer graben typically