| Abstract: Evidence for geologically-active planetary bodies beyond our Solar System has evaded astronomical detection to date. Here, by analogy to the most active body in the Solar System, Io, I describe the possible Na & K spectral signatures imprinted by the activity occurring on an extrasolar Io. I briefly review calculations that show tidal theory can allow active exomoons to be dynamically stable around extrasolar giant planets. As the geometry and column density of an ejected Na & K cloud depends strongly on the plasma interaction, I provide order of magnitude expressions to probe the influence of an exomoon. I demonstrate that an optically thin cloud of Na or K sourced by an exomoon, is roughly consistent with alkali observations at exoplanets to date. Furthermore, the measured extrasolar Na/K ratios, useful in probing physical processes, are highly non-solar contrary to current giant planet formation scenarios, and more consistent with the ratios observed at differentiated silicate and icy bodies across the Solar System. With the advent of high-resolution transmission spectra, modeling these possibly active systems is now warranted. While the sheer presence of an exomoon around a close-in planet would illuminate our understanding of tidal gravitation, the added utility of a spectral signature may be our first step in detecting the presence of extrasolar activity. Moreover, the Na & K signatures driven by the concomitant heavy ion plasma could be an indication of an exoplanetary magnetic field. |