Lobate craters are an indication of the existence of fluids in the Martian megaregolith. The emplacement of the ejecta could be similar to debris flow dynamics. In this paper we suggest that the lobate ejecta result of an instability mechanism during the surface flow. Indeed, recent experimental studies on injection of heavy viscous fluid in a lighter one show that the wavelength of the instability depends on the viscosity and density contrast between the two fluids. An extension of this 2D experiment to a cylindrical geometry has been carried out. In that case we demonstrate that a distribution of wavelengths is related to viscosity contrast. Those results have been applied to 250 single layer craters in Lunae Planum and Sinai Planum to measure the variation of viscosity with depth. The results are in agreement with a possible 1 km thick volatile rich layer above a less porous one.