Abstract We have developed a computer program, MatchMod, which inverts the NEWMOD® forward modeling program of Reynolds (1985a) and provides a “match” for a given experimental X-ray diffraction (XRD) pattern. As used here, a forward model is a set of equations, based on first principles, which can predict the state of a system from the variables which control that system. An inverse model, on the other hand, is one which estimates the control variables from some state of the system. Reynolds' series of tools (NEWMOD®, CLAYS®, MIXER®; Reynolds, 1985 a, b, c) have the potential for producing a quantitative clay mineral analysis based on die 001 peaks of oriented clay mineral aggregates because they account for most instrumental factors, preferred orientation, sample length, a host of mineral compositional and structural variables as well as mass absorption coefficient and unit cell volume. However, in the practice of pattern-matching (inverse modeling) these tools are extremely time-consuming and tedious to apply, primarily because of the large number of variables involved in the trial-and-error procedure.

ABSTRACT The discovery in 2004 by Mars exploration rover Opportunity of sedimentary rocks with centimeter-scale trough cross-bedding is one of the compelling lines of evidence for flowing water on the Martian surface. The rocks contain a significant evaporite component mixed with weathered mafic silicates, suggesting that the aqueous fluid in contact with the sediments must have been of very high ionic strength because dissolution features are not observed. Recent thermodynamic modeling indicates that these brines could have had higher densities (by up to a factor of 1.3) and significantly higher viscosities (by up to a factor of 40) than pure water. Because fluid density and viscosity can significantly affect sediment transport mechanics, herein we analyze whether ripples could have been stable bed forms under flowing Martian brines. To this end, we compiled bed form stability diagrams with an emphasis on those studies that have considered high-viscosity fluids. For the case of viscous Martian brines, we find that ripples are stable under modest Shields numbers and low particle Reynolds numbers. These conditions translate into sediment sizes ranging from sand to gravel, and they are substantially coarser than sediment sizes for equivalent ripple-forming flows in freshwater. It is likely that ripples might also form in silt sizes under viscous brines, but these conditions (i.e., particle Reynolds numbers < 0.1) have not yet been explored in flume experiments, motivating future work. Using flow-resistance equations and assuming steady uniform flow, we calculate that Marian brines must have had flow depths ranging from 0.01 to 1 m and flow velocities of 0.01 to 1 m/s, and been driven by gravity on slopes of 10 −4 to 10 −2 in order to generate the bed stresses necessary to produce ripples. These conditions seem reasonable given the interdune environment that has been proposed for the Burns formation. In addition to the potential for ripples in much coarser sediments, ripples formed by viscous brines also might be larger in height and wavelength than their freshwater counterparts by as much as a factor of 12. Thus, large (>10 cm heights) and fine-grained (<1 mm particle diameter) cross strata would be compelling physical evidence for flowing brines in the Martian past, provided that independent evidence could be provided for a subaqueous (i.e., not eolian) origin of the cross-stratification. Smaller centimeter-scale ripples can also be formed by brines due to flow-depth limitations or lower-viscosity fluids, and therefore the physical sedimentological evidence in support of brines versus freshwater flows may be ambiguous in these cases.