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A new hypothesis attributes the Martian "wave of darkening" to soil frost phenomena. Diurnal thawing and freezing of the ground, which uses moisture transported by the atmosphere from the melting polar cap, can produce various minute, frost-heaved, soil surface features. These microrelief features result in a complex porous surface structure, which causes optical darkening. The boundary at which the wave of darkening terminates on the winter hemisphere correlates with the latitude at which the diurnal peak surface temperature drops below 0 degrees C. The hypothesis is examined in terms of known properties of the Martian atmosphere and surface and the availability of water.
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Otterman et al. (1966) studied this question.
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