Plasma ion composition measurements for Europa

摘要

Jupiter magnetospheric interactions and surface composition, both important to subsurface ocean detection for the Galilean icy moons Europa, Ganymede, and Callisto, can be measured using plasma ion mass spectrometry on either an orbiting spacecraft or one designed for multiple flybys of these moons. Detection of emergent oceanic materials at the Europa surface is more likely than at Ganymede and Callisto. A key challenge is to resolve potential intrinsic Europan materials from the space weathering patina of iogenic species implanted onto the sensible surface by magnetospheric interactions. Species-resolved measurements of pickup ion currents are also critical to extraction of oceanic induced magnetic fields from magnetospheric interaction background dominated by these currents. In general the chemical astrobiological potential of Europa should be determined through the combination of surface, ionospheric, and pickup ion composition measurements. The requisite Ion Mass Spectrometer (IMS) for these measurements would need to work in the high radiation environment of Jupiter's magnetosphere between the orbits of Europa and Ganymede, and beyond. A 3D hybrid model of the moon-magnetosphere interaction is also needed to construct a global model of the electric and magnetic fields, and the plasma environment, around Europa. Europa's ionosphere is probably usually dominated by hot pickup ions with 100-1000 eV temperatures, excursions to a "classical" cold ionosphere likely being infrequent. A field aligned ionospheric wind driven by the electron polarization electric field should arise and be measurable.