TRAVERSING THE AQUEOUS HISTORY OF MARS THROUGH ANALOGS

The canonical aqueous history of Mars is that the earliest period (Noachian and earlier) when water was most abundant was followed by a period that showed a substantial decline in water activity leading to the long-lived era of a very cold and dry climate dominated by a gas-solid water cycle. Analysis of data from high spectral resolution visible to mid-infrared imaging spectrometers (TES, OMEGA, CRISM) has revealed that the igneous and aqueous history recorded in the mineralogy changed with time (Bibring et al., Science, 2006). However many important details of this evolution remain to be examined. We are investigating basaltic terrains to characterize the mineralogy, chemistry and spectroscopic signatures of analogs that may have experienced the same conditions that characterized Mars. The analogs are weatherered Deccan basalts in India, weathered and hydrothermally altered basaltic lavas in Iceland, and weathered Ferrar dolerite in Antarctica. For the Deccan basalts, we sampled a weathering profile that includes saprolite and laterite. The kaolinite-dominated laterite horizon is spectrally similar to kaolinite-bearing units on Mars, except the analog terrain also includes abundant ferric oxides not observed in association with the kaolinite deposits on Mars. The clay-dominated saprolite shows weak spectral affinities to Mars observations. The degree of weathering in our Iceland analog site, quantified in terms of percentage of alteration minerals and geochemical fractionation, is less than in India, but the nature of smectite clay spectroscopic signatures show a strong affinity to what has been observed on Mars. In particular the Iceland alteration assemblages show Fe-Mg smectite clays with diagnostic absorptions at 2.3 µm comparable to those observed on Mars. In Antarctica a distinctive oxidation rind has developed on outcrops of the Ferrar Dolerite, mineralogic analog to Mars meteorites. The contrast between the rind and interior spectroscopic signatures is analogous to that seen in the northern basaltic plains of Mars. Thus far analogs with alteration under low water-rock ratios (Iceland, Antarctica) provide the best correspondence to Mars observations. This presentation will discuss similarities and differences between these analog studies of terrestrial basaltic environments on Mars.