Constraining the Surface Composition of Europa with Spatially Resolved Mid-UV Spectra

Overview. This is a project to use HST to observe Jupiter's icy moon, Europa, to obtain spatially resolved UV spectra of its leading, trailing, sub-Jovian and anti-Jovian hemispheres that include diagnostic mid-UV wavelengths that have not previously been analyzed. Current published observations of Europa include near-UV observations from 210 nm - 320 nm and far-UV observations from ~110 nm - 170 nm; however, the data in the mid-UV (170 nm - 210 nm) is lacking. Although Europa's surface consists mostly of water ice, the characteristic sharp absorption edge observed at ~165 nm in lab spectra and on the icy moons of Saturn has not been detected on Europa. Observations in the mid-UV could reveal the absorption edge at longer wavelengths, suggestive of larger ice grains on the surface. Conversely, the mid-UV spectrum could verify that the water ice absorption edge is not present on Europa. The reflectance of Europa's hemispheres drops significantly between the near- and far-UV. The spectral shape in the mid-UV will be used to constrain the surface composition across the satellite. Mid-UV observations with STIS will span the previous near- and far-UV data sets, providing a seamless UV spectrum. These observations will provide the first spatially resolved spectra of each of Europa's hemispheres in the mid- UV.

Scope. Co-I Amanda Hendrix will lead the spectral modeling effort, producing spectral models assuming various compositions, ratios, and grain sizes to compare with the Europa spectra. Her current spectral modeling routines and extensive expertise in the subject area will enable direct comparisons of spectral slope, shape, and features. These models will include tholins, hydrocarbons, sulfur, salts, and water ice grains of various sizes to attempt to replicate the observed spectrum. The models produced by Dr. Hendrix will extend into the visible and near-IR wavelengths so that features across the spectra can be compared. This task will require ~1.8 weeks.