Desiccation mechanism for formation of giant polygons on Earth and intermediate-sized polygons on Mars: Results from a pre-fracture model

We present a pre-fracture incrementally non-linear elastic-hydric model to constrain the models of formation of giant (spacing of 20-300m) desiccation cracks on Earth, and possibly intermediate sized (80 to 350m) polygonal networks of cracks located in many impact craters on Mars which have been interpreted to form by desiccation as opposed to thermal contraction (El Maarry et al., 2010). The results of the model show that tensile stresses rise monotonically with desiccation. However, in soils with diffusivities below 10 ^-4m ²/s, it is not possible to generate high enough stresses to cause fracturing. On the other hand, intermediate values between 10 ^-2 and 10 ^-4m ²/s create optimum conditions for the formation of cracks at the time scales suggested for the formation of giant desiccation polygons on Earth. Our model clearly shows that for typical diffusivity values of clayey soils with a considerable amount of smectites enough stress can build up to stimulate cracking of various spatial scales. These results corroborate earlier assumptions that giant desiccation polygons on Earth occur through lowering of the water table rather than surface evaporation. Extending the model to Mars shows that soils would crack within similar diffusivity limits but in slightly longer periods of time owing to the lower gravity. Finally, a model for formation of desiccation cracks on Mars is presented that shows that two main conditions need to be fulfilled for desiccation cracks to occur under current martian climatic conditions, namely, that the thermal and soil diffusivity conditions 1) allow for the formation of an unsaturated zone with a considerable thickness while at the same time 2) limiting the growth of the permafrost downward which would disrupt the cracking process.