Terrain softening (Mars)
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The landscape polewards of around 30 degrees latitude on
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
has a distinctively different appearance to that nearer the equator, and is said to have undergone terrain 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 by
Steve Squyres Steven Weldon Squyres (born January 9, 1956) is an American geologist and planetary scientist. He was the James A. Weeks Professor of Physical Sciences at Cornell University in Ithaca, New York. His research area is in planetary sciences, with a f ...
and Michael Carr from examining imagery from the
Viking Vikings ; non, víkingr is the modern name given to seafaring people originally from Scandinavia (present-day Denmark, Norway and Sweden), who from the late 8th to the late 11th centuries raided, pirated, traded and se ...
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. The
relief Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term '' relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that th ...
(height) of features is somewhat reduced. Small craters are noticeably less common. In other words, terrain which elsewhere looked sharp here looks "soft".Squyres, Steven W., and Michael H. Carr. "Geomorphic evidence for the distribution of ground ice on Mars." Science 231.4735 (1986): 249-252.Carr, Michael H. The surface of Mars. Vol. 6. Cambridge University Press, 2006. This texture has also been described as "smooth", or "rolling". Softened craters are also commonly infilled with concentric patterns on their floors. On Earth, diffusive creep of soils is associated with rounded hillslopes.Roering, Joshua J., James W. Kirchner, and William E. Dietrich. "Evidence for nonlinear, diffusive sediment transport on hillslopes and implications for landscape morphology." Water Resources Research 35.3 (1999): 853-870. Squyres and Carr thus attributed the softened texture to accelerated viscous creep in shallow soils near the surface, and went on to associate this accelerated creep with the presence of ground ice at these latitudes. This conclusion has been largely borne out by subsequent research. In the late 1980s some attempts were made to link terrain softening with dust and
aeolian processes Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit mate ...
,Clifford, S. M., and J. R. Zimbelman. "Softened Terrain on Mars: The Ground Ice Interpretation Reconsidered." Lunar and Planetary Institute Science Conference Abstracts. Vol. 19. 1988. though this hypothesis has largely been superseded by more recent observations. Terrain softening is one of a suite of features seen in the midlatitudes of Mars—also including lobate debris aprons,
lineated valley fill Lineated valley fill (LVF), also called lineated floor deposit, is a feature of the floors of some channels on Mars, exhibiting ridges and grooves that seem to flow around obstacles. Shadow measurements show that at least some of the ridges are sev ...
,
concentric crater fill A concentric crater fill (CCF) is a landform where the floor of a crater is mostly covered with many parallel ridges. It is common in the mid-latitudes of Mars, and is widely believed to be caused by glacial movement. Areas on Mars called Deutero ...
,
latitude dependent mantle Much of the Martian surface is covered with a thick ice-rich, mantle layer that has fallen from the sky a number of times in the past. In some places a number of layers are visible in the mantle. Image:Layered mantle in Icaria Planum.JPG, Laye ...
, patterned ground, viscous flow features, arcuate ridges,
recurring slope lineae Seasonal flows on warm Martian slopes (also called recurring slope lineae, recurrent slope lineae and RSL) are thought to be salty water flows occurring during the warmest months on Mars, or alternatively, dry grains that "flow" downslope of at le ...
, and
gullies A gully is a landform created by running water, mass movement, or commonly a combination of both eroding sharply into soil or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble lar ...
—whose form and distribution strongly suggest the abundance of ice at the surface.


Contrast between softened and unsoftened terrain

;Unsoftened terrain Image:Zumba Crater.JPG, Zumba crater, 29° S Image:Saheki Crater.jpg, Small craters inset within
Saheki crater Saheki is a crater on Mars, located in the Iapygia quadrangle at 21.75° S and 286.97° W. It measures approximately 82 kilometers in diameter and was named after Tsuneo Saheki, a Japanese amateur astronomer (1916–1996). The naming was adopted ...
, 22° S Image:Bright Slope Streaks PIA03587.jpg, Crater in
Arabia Terra Arabia Terra is a large upland region in the north of Mars that lies mostly in the Arabia quadrangle, but a small part is in the Mare Acidalium quadrangle. It is densely cratered and heavily eroded. This battered topography indicates great age ...
, 13° N Image:Radau Crater.jpg, Sharp terrain in Radau crater, 17° N
;Softened terrain Image:Crater floor in Eridania, Mars.jpg, Part of crater rim and concentric crater fill in the Eridania quadrangle, 39° S Image:ESP 028214 1435cratermesa.jpg, Unnamed crater, 36° S Image:28415facewide.jpg, Unnamed crater with concentric crater fill, 41° S Image:Concentric Crater Fill Wide-view.jpg, Unnamed crater with concentric crater fill, 38° S


See also

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References

{{Geography of Mars, geology Geology of Mars Surface features of Mars Geomorphology Patterned grounds Permafrost