Wavelength of Reticulate Bedforms on the Tharsis Montes, Mars: Spacing Control by Atmospheric Density

Images of the Tharsis Montes and surrounding areas from the High Resolution Imaging Science Experiment (HiRISE) document numerous ridges at the scale of several meters that have been interpreted as aeolian ripples composed of dust aggregates [Bridges et al., 2009]. Dominant morphologies are linear/accordion and honeycomb, with the former common on the volcano flanks and plains and the latter in calderas and other closed depressions. We have recently measured the wavelength of reticulate bedforms in nine HiRISE images on Olympus, Ascraeus, and Pavonis Mons over an elevation range of 3.4 to 21.6 km to determine if there is dependence of wavelength on local atmospheric pressure. This elevation range is greater than that elsewhere on Mars and given the probable similar composition of the bedforms, allows us to critically examine any wavelength dependence on pressure. Three of the images are dominated by honeycomb bedforms, with the other images having mainly linear or accordion. Within each image, 4 sub-regions were selected. Within these sub-regions, 10 traverses were made in which the wavelengths of 4-7 ripple sets were measured, each along a parallel line. This resulted in 200-234 ripple wavelength measurements per image. The results show that reticulate bedform wavelengths are inversely proportional to local atmospheric pressure (power law fit of -0.9, with R2 of 0.6). This suggests that atmospheric density is controlling the wavelength of ripple formation. At present, we are uncertain of the precise physical mechanism that may exert such a control and will present several hypotheses in our poster. We note that the threshold speed is inversely proportional to the square root of density. Ripple wavelength, in turn, is proportional to the square of the excess shear velocity [Pelletier, 2009], and, if this is proportional to the friction speed, wavelength should be proportional to inverse density, consistent with the observations. Variations from a perfect fit may be reflective of local wind conditions, threshold speeds, and particle properties. Bridges, NT. et al. (2009), Icarus, doi:10.1016/j.icarus.2009.07.035 Pelletier, J.D. (2009), Geomorphology, 105, 322-33.