AQUEOUS AND AEOLIAN PROCESSES IN THE NORTHERN LOWLANDS OF MARS FROM IRON MINERALOGY

The geology of the northern lowlands of Mars is dominated by dark-toned sediments. Here we present new results based on analysis of near-infrared spectra that the distribution of iron-bearing minerals in these sediments is a result of both aeolian and aqueous processes.

Dark-toned regions in the northern plains exhibit broad absorptions centered near 1 micron that are consistent with absorptions due to iron in a crystal lattice. In laboratory spectra, the shape and center position of this band is commonly used to discriminate between iron-bearing minerals. In this study, we have found that two spectral parameters most uniquely identify iron-bearing minerals: the 1 micron band center and band asymmetry. By applying these parameters to laboratory spectra of a variety of iron-bearing minerals and mineral mixtures, we have developed a quantitative system for discriminating between iron-bearing minerals and detecting mineral mixture trends in planetary surface spectra. In particular, this system allows us to detect the presence of glass in a mineral mixture, which is difficult to detect qualitatively. Furthermore, our results indicate that glass-pyroxene mixtures are spectrally indistinguishable from many olivines in the near-infrared, perhaps suggesting that glass may be more common on planetary surfaces than previously thought.

Applying these spectral analysis techniques to near-infrared Mars surface spectra from the Mars Express OMEGA imaging spectrometer has revealed new and surprising results on the composition of broad regions of the northern plains. Dark toned sediments have two dominant compositions: glass-rich and high-calcium pyroxene-rich. Glass-rich sediments cover several million square kilometers in the north polar region and Acidalia Planitia, and also exhibit spectral characteristics consistent with leached glass rinds, suggesting that the glass-enrichment is a result of acidic leaching and dissolution of other minerals. The different compositions also occur in two stratigraphically distinct units within the north polar plateau, suggesting that these units either had different sources or have undergone substantially different processes. The north polar sand sea contains a mixture of glass and pyroxene, and appears to be at least partially sourced from both units.