Unmasking the Dark Side of Iapetus

Overview. This is a project to take advantage of the unique capabilities of STIS in a UV- intensive series of observations to obtain high-SNR spectra of the bright and dark hemispheres of Iapetus over a critical near-UV range where Cassini instruments lack coverage (190-350 nm). The leading face of Iapetus acts a sort of canvas, painted with material from Saturn's captured moon Phoebe, itself an interloper from the Kuiper Belt, that may provide important clues to the composition and processing history of primordial material from the early stages of planet formation. This underscores the long recognized importance of identifying the composition of the dark material on Iapetus and placing it in the context of other primitive Solar System objects. In spite of extensive Cassini observations, this has proven to be an elusive goal because of the lack of unique spectral signatures of possible dark materials in the wavelengths observed by Cassini. Each proposed darkening agent (nanoiron/hematite, aromatic and aliphatic tholins, NH3) has a distinctive signature within this previously unobserved spectral window. STIS observations will provide a critical test of proposed compositions for the dark material and its origin. We will draw on our team's extensive experience with STIS observations, Cassini UVIS and VIMS spectra, Earth-based near-IR spectroscopy, and radiative transfer scattering models to compare the observed spectra with predictions for a variety of candidate materials. The Iapetus results will complement recent STIS observations of other icy Saturn satellites (Program 13694 Hendrix) and the STIS-based investigation of the mysterious redness of Saturn's rings (Program 12478 Cuzzi).

Scope. Co-I Amanda Hendrix will lead the spectral modeling effort, using radiative transfer and other calculations to compare models to the spectra, for a variety of possible substances, and compare results with other satellites and with rings. Hendrix will produce spectral models assuming various compositions and grain sizes to compare with the Iapetus spectra. Her current spectral modeling routines will enable direct comparisons of spectral slope, shape, and features. These models will include tholins, irons, ammonia and water ice grains of various sizes to attempt to replicate the observed spectra.