Ganymede Craters: Relationships between Spectral Properties and Crater Retention Age

Introduction: Both bright and dark craters occur on Jupiter's satellite Ganymede. The Near Infrared Mapping Spectrometer (NIMS) onboard the Galileo spacecraft performed spectroscopic measurements of Ganymede in the spectral range of 0.7 to 5.2 microns [1]. Observations of Ganymede's craters acquired at sufficiently high spatial resolution coordinated with Galileo SSI (Solid State Imaging) observations and additionally with medium-and low-resolution SSI and Voyager observations were used for investigating relationships between the spectral properties, morphology and relative retention age of Ganymede's impact crater material, and of substrate material. Data: The analysis was conducted on NIMS high-resolution observations which cover the areas around the craters Amon, Mir, Antum, Tammuz, Melkart, Harakthes and Osiris located on Ganymede's leading hemisphere and the craters Kittu, Hershef, Ta-urt, Shu and Tashmetum on the trailing side. Based on the high-resolution Galileo SSI images, the stratigraphic position of Kittu, Melkart and Harak-thes, and their relative and absolute ages have been determined by measuring the superimposed crater frequencies on their continuous ejecta blankets and crater floors. If no high-resolution images were available, the superimposed crater frequency and the relative age of the specific crater was estimated using a method described by Wagner et al. (1999) [2]. Currently, two conflicting impact cratering chronology models to estimate absolute cratering model ages exist: Model I by Results: Bright ray craters like Osiris and dark ray craters Kittu, Mir and Antum were found to be formed fairly recently [5,6,7]. The dark ray material of Kittu (0.5°lat, 334.5°lon), Antum (5°lat, 219°lon) and Mir (3.5°lat, 230.2°lon) shows the highest concentration of non-ice component(s) found so far on the surface of Ganymede. Kittu's cratering model age could be defined at a maximum of 800 Myr (Model I) and 20 Myr (Model II) respectively, the youngest crater age determined in this analysis. The young age of Kittu could account for the color variations in Voyager images described by Schenk & McKinnon [6], if there are processes that support " reddening " of surface material with time. NIMS spectra provide no obvious spectral differences between the dark ejecta of Kittu and the other dark ray craters (Fig. 1). If the color difference of