TRACING THE AQUEOUS ALTERATION HISTORY IN THE REGION BETWEEN ISIDIS AND HELLAS PLANITIAE ON MARS

We initiated our study of the morphology and mineralogy of aqueous outcrops at Libya Montes (LM) at the southern rim of the Isidis impact basin. The Noachian to Amazonian aged rocks there result from a complex history of impact, volcanic, tectonic, fluvial, and aeolian processes. Our work involved coordinated studies using HRSC, CRISM, HiRISE, and CTX data that resulted in extensive base mapping, age dating, detailed spectro-morphological mapping of a focused region at LM, spectral analyses of various mineral outcrops, and 3D topographic analyses using combinations of HRSC DTMs and CRISM data. We observed a diversity of mineral assemblages, suggesting that the nature of aqueous alteration at LM varied in space and time. This mineralogy, together with geologic features, indicates a transition from Noachian impact-induced hydrothermal alteration through Hesperian evaporative processes to Amazonian resurfacing.

More recently, we extended our investigations southward to include wide areas of Tyrrhena Terra (TT) toward the rim of the Hellas Basin. The entire TT region hosts a fascinating diversity of aqueously altered materials accompanied by unaltered mafic rocks. It is cut by fluvial channels, is overprinted by numerous impact craters, and includes various lobate and fan-shaped deposits. Similar to LM, the combinations of these different landforms provide evidence of a variable history of erosion and deposition. Current work is defining the effects of aqueous processes and impacts on mineral alteration and weathering. Again, we began with extensive geomorphological mapping of the entire TT region and age dating of selected units based on HRSC and CTX data. This geomorphological map was then coordinated with a map of aqueous minerals derived from the analysis of CRISM multispectral imagery together with focused CRISM hyperspectral analyses at selected sites to assess whether or not there are spatial trends in the relationships between specific geomorphological units and mineral occurrences throughout TT. We found clear correlations between certain mineral types and their proximity to one of the major impact basins. In particular, the Hellas impact event appears to have produced more variable temperatures and water chemistries, leading to increased mineral variability near its rim.