The landscape polewards of around 30 degrees latitude onMars has a distinctively different appearance to that nearer the equator, and is said to have undergoneterrain softening. Softened terrain lacks the sharp ridge crests seen near the equator, and is instead smoothly rounded. This rounding is thought to be caused by high concentrations of water ice in soils. The term was coined in 1986 bySteve Squyres and Michael Carr from examining imagery from theViking missions to Mars.
Below 30 degrees of latitude, impact craters have steep walls; well-defined, sharp rims; and flat or smoothly bowl-shaped floors. Ridges on intercrater plains come to similarly well-defined, pointed crests. However, above this latitude, these same features appear very different. The crests seen on ridges and crater rims appear strongly rounded and much more poorly defined. Therelief (height) of features is somewhat reduced. Small craters are noticeably less common. In other words, terrain which elsewhere looked sharp here looks "soft".[1][2][3] This texture has also been described as "smooth", or "rolling".[4] Softened craters are also commonly infilled withconcentric patterns on their floors.[2]
On Earth,diffusive creep of soils is associated with rounded hillslopes.[5][6] Squyres and Carr thus attributed the softened texture to accelerated viscouscreep in shallow soils near the surface, and went on to associate this accelerated creep with the presence of ground ice at these latitudes.[1] This conclusion has been largely borne out by subsequent research.[2][7] In the late 1980s some attempts were made to link terrain softening with dust andaeolian processes,[8][9] though this hypothesis has largely been superseded by more recent observations.[2]
Terrain softening is one of a suite of features seen in the midlatitudes of Mars—also includinglobate debris aprons,lineated valley fill,concentric crater fill,latitude dependent mantle,patterned ground,viscous flow features,arcuate ridges,recurring slope lineae, andgullies—whose form and distribution strongly suggest theabundance of ice at the surface.[2]
