17099

A recent analysis of HST-STIS images has revealed the presence of water vapor in the atmosphere of Ganymede comma the largest moon of Jupiter and in the entire solar system. The derivation of H2O was through emissions from oxygen comma using the relative brightness of far-UV multiplets at 1304 A and at 1356 A. The findings constitute the first detection of sublimated H2O in the outer solar system. This atmosphere likely collapses when Ganymede is eclipsed by Jupiter since surface temperature and sublimation flux are expected to decrease considerably. Here we propose follow-up observations to confirm the detection of H2O in the atmosphere and characterize the response of the H2O component to eclipse. For this comma we use the succesful STIS FUV spectral imaging mode to monitor and map the diagnostic 1304 A and at 1356 A emissions from before to after a virgul3-hour eclipse passage of Ganymede. During another eclipse occasion comma we use COS and the G230L mode to measure OH aurora from dissociative excitation of H2O for the first time. This visit provides an independent confirmation of the H2O atmosphere. Ganymede is a primary target of ESA.s JUpiter Icy Moon Explorer (JUICE) and NASA.s Europa Clipper mission. An improved understanding of the moon.s neutral environment delivers important information for optimal science planning and addressing the habitability goals for these milestone missions.