Contents — P through Z Methods for Detecting Current Geological Activity on Europa

Introduction: Jupiter's satellite Europa could be the site of recent, or current, geological activity. Future missions to the Jovian system, such as the proposed Jupiter Icy Moons Orbiter, should search for evidence of such activity as a way to determine the potential for liquid water, and perhaps the astrobi-ological potential, of Europa. Searches for geological activity can include change detection, plume searches, thermal methods, and magnetospheric effects. Change Detection: Comparisons of images of the same portion of Europa's surface taken on subsequent orbits or missions can be used to search for any differences due to geological activity. A search for any changes over the 20 years between the Voyager and Galileo spacecraft missions to the Jovian system revealed no evidence of geological activity [1]. However , that initial search was hampered by the low resolution of the Voyager images, and other differences in the two datasets including phase angle and wavelength. By choosing to acquire images that match pre-existing datasets, a future mission could optimize the conditions for change detection and maximize the chances of finding any changes on Europa's surface due to current or recent geological activity, if they exist. A future mission such as JIMO would have a great advantage over Voyager-Galileo comparisons because images could be targeted to match those obtained by Galileo. High-resolution images could also be compared, which could detect changes that are much smaller in spatial scale. The criteria for effective comparisons include filters at comparable wavelengths and bandpasses to Galileo (and perhaps Voyager as well, if possible), and images of regions observed by Galileo taken at the same phase angle, wavelength, and resolution. Plume Search: Although the possibility of a plume detection was raised from Voyager images of Europa, it appears likely that this was just an artifact of the Voyager vidicon imaging system [2]. A high-phase plume search was performed by the Galileo spacecraft, but no plumes were detected [1]. A future mission to Europa should perform periodic searches for material vented off Europa's surface, including high-resolution imaging sequences at locations where the periodic stresses are most favorable for plume activity [3], and global-scale images that include the entire limb of the planet for possible plume detection. A search for high-albedo plume deposits can also be made using the techniques described in the " Change Detection " section above. Due to the scattering properties of small vented particles, short-wavelength filters in the …