Post contributed by Prof. Lionel Wilson, Lancaster University, UK and Dr. Peter J. Mouginis-Mark, Hawaii Institute of Geophysics and Planetology, USA.
Image 1 shows a distinctive flow deposit southwest of the Cerberus Fossae on Mars. The flow source is a ~20 m deep, ~12 x 1.5 km wide depression within a yardang field associated with the Medusae Fossae Formation. The flow traveled for ~40 km following topographic lows to leave a deposit on average 3-4 km wide and up to 10 m thick. The surface morphology of the deposit suggests that it was produced by the emplacement of a fluid flowing in a laminar fashion and possessing a finite yield strength. There is an ongoing debate about whether flows in this region of Mars are lava flows or water-rich debris flows.
Topographic data from a digital elevation model (DEM) was used to model the dynamics of the motion and the results imply that the fluid had a Bingham rheology with a plastic viscosity of ~1 Pa s and a yield strength of ~185 Pa. Although the low viscosity could be consistent with the properties of komatiite-like lava, the combination of values of viscosity and yield strength, together with the surface morphology of the deposit, suggests that this was a mud flow.
Comparison with published experimental data on sampled mud flow materials implies a solids content close to 60% by volume and a grain size dominated by silt-size particles. Further, comparison of the ~1.5 km3 deposit volume with the ~0.03 km3 volume of the source depression shown in detail in Images 2 and 3 implies that ~98% of the flow material was derived from depth in the crust. If this flow deposit is indeed a mud flow, then not only water but also a great deal of fine-grained material was brought up from the aquifer that was its source.
The non-Newtonian properties of such fluid mixtures may have implications for models of the erosion processes in martian channels currently assumed to have been eroded by initially sediment-free water. The source area of the flow is quite different from volcanic vents seen elsewhere on Mars, raising further questions about the origin of the flow. Particularly intriguing is the fact that the distal portion of the flow (Image 4) has a surface morphology that is very similar to that of other flows on Mars that are interpreted to be lava flows. This suggests that a reanalysis of certain other flows previously described as lava flows may be in order to investigate if they too are mud flows.
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