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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Desiccation mechanism for formation of giant polygons on Earth and intermediate-sized polygons on Mars: Results from a pre-fracture model
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Desiccation mechanism for formation of giant polygons on Earth and intermediate-sized polygons on Mars: Results from a pre-fracture model

机译：地球上巨型多边形和火星上中等尺寸多边形形成的干燥机制：预断裂模型的结果

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摘要

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~(-4)m~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~(-4)m~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.
机译：我们提出了一种预断裂增量非线性弹性水力模型，以约束地球上巨大的（20-300m间距）干燥裂缝形成模型，以及可能存在于许多地方的中等大小（80至350m）的多边形裂缝网络撞击火星的陨石坑被解释为是由干燥而不是热收缩形成的（El Maarry等，2010）。模型的结果表明，拉伸应力随着干燥而单调上升。然而，在扩散率低于10〜（-4）m〜2 / s的土壤中，不可能产生足够高的应力来引起破裂。另一方面，介于10〜（-2）和10〜（-4）m〜2 / s之间的中间值为在地球上形成巨型干燥多边形建议的时间尺度上的裂缝形成创造了最佳条件。我们的模型清楚地表明，对于具有大量蒙脱石的黏性土壤的典型扩散系数值，可以建立足够的应力来刺激各种空间尺度的开裂。这些结果证实了先前的假设，即地球上巨大的干燥多边形是通过降低地下水位而不是表面蒸发而发生的。将模型扩展到火星表明，土壤将在相似的扩散极限内开裂，但由于重力较低，开裂时间会稍长一些。最后，提出了一种在火星上形成干燥裂纹的模型，该模型表明在当前火星气候条件下要发生干燥裂纹需要满足两个主要条件，即热和土壤扩散条件1）允许形成火星。具有相当大厚度的不饱和区域，同时2）限制了永久冻土的向下生长，这将破坏裂化过程。

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