Were Aqueous Ripples on Mars Formed by Flowing Brines

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