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.

Abstract Some knowledge of fluid dynamics is essential to an adequate understanding of sediment movement. We’re not assuming any previous knowledge on your part. Chapters 1, 3, and 5 present a very selective treatment of some of the topics in fluid dynamics that are important in the study of sediment movement. This material is not a substitute for a more substantial background in fluid flow, but it allows a level of discussion of many of the important ideas in the mechanics of sediment transport that would not otherwise be possible. Fluids are substances that deform continuously and permanently when subjected to forces that vary in magnitude or direction from point to point. The nature of the relationship between the deforming forces and the geometry of deformation varies from fluid to fluid; as discussed in this chapter, the relationship is a simple linear one for air and water, the two fluids most important in sedimentology Liquids and gases differ greatly in their structure on the molecular scale. How is it that the macroscopic motions of these two kinds of fluid need not be considered separately? The answer is that fluids can be treated as if they were continua--as if their constituent matter, which is actually distributed discontinuously as atoms and molecules, were smeared uniformly throughout space. The justification for this approach is that it works extremely well for fluid flows on scales that are much larger than the intermolecular spacing. This includes most problems in sediment movement down to the range of colloida1 sizes (fractions of a micrometer). In this chapter we’ll develop some ideas in fluid mechanics by looking at three kinds of flow: shearing of fluid between parallel plates, flow past a sphere, and flow down an inclined plane. The last two are of direct importance to sedimentology, and we’ll build upon the results in later chapters.

Abstract This chapter deals with some basic ideas about settling of sediment particles through fluids. This is a good topic in sediment transport to start with in these notes, because settling is an important aspect of sediment transport, and Chapter 1 has provided enough background for substantial progress. But complexities that require greater understanding of fluid flow will soon arise, and Chapter 3 is therefore devoted to several important topics in fluid flow. Chapter 4 is a continuation of material on settling. If placed in suspension in a viscous fluid, a sediment particle will settle toward the lower boundary of the fluid, provided that the weight of the particle is not much smaller than the random forces exerted on the particle by bombardment by the fluid molecules in thermal motion. All mineral particles larger than colloidal sizes of hundredths of a micrometer are in this category. When such a particle is released from rest in a still fluid, it accelerates in response to the force of gravity, but as its velocity increases, the oppositely directed drag force exerted by the fluid grows until it equals the weight of the particle. When the weight and the drag are in balance the particle no longer accelerates but falls at its terminal velocity, called the fall velocity or settling velocity. Particles of sand size and smaller attain terminal velocity over very short times and distances. With respect to natural sedimentary environments, the settling of a sphere in a still fluid is obviously a great oversimplification with respect both to particle shape and to the state of motion of the fluid, but it will lead to development of some important ideas and point the way toward consideration of nonspherical particles and flowing fluids.

Abstract So far we’ve been able to cover a lot of ground with a minimum of material on fluid flow. At this point we need to present some more topics in fluid dynamics before returning in the next chapter to flow past spheres at Reynolds numbers higher than the Stokes range. We’ll look at inviscid fluid flow, the Bernoulli equation, turbulence, boundary layers, and flow separation. This material will also provide some of the necessary background for discussion of dynamics of sediment movement in Chapter 6.

Abstract In Chapter 2 we outlined the basic law for settling of a spherical grain through a still, viscous fluid. This was done by establishing, partly on theoretical grounds but mainly from dimensional analysis and experimentation, a relationship between drag coefficient and Reynolds number for a sphere moving through a viscous fluid. it was seen that the nature of the relationship changes as the Reynolds number increases, corresponding to a change from a viscous regime of flow to a regime in which the motion of the particle results in the formation of a turbulent wake. In this chapter we will examine a little more closely the phenomenon of wake formation, and then consider the modifications of settling behavior that arise from changes in shape and concentration of the settling grains.