page_title: Gullies on Mars
section_title: Formation
hierarchical_section_title: Gullies on Mars / Formation
caption_attribution_description: English: Group of gullies on north wall of crater in Phaethontis, Mars.
context_page_description: Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars. They are named for their resemblance to terrestrial gullies. First discovered on images from Mars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has a dendritic alcove at its head, a fan-shaped apron at its base, and a single thread of incised channel linking the two, giving the whole gully an hourglass shape. They are estimated to be relatively young because they have few, if any craters. A subclass of gullies is also found cut into the faces of sand dunes, that are themselves considered to be quite young. Linear dune gullies are now considered recurrent seasonnal afeatures.
  Most gullies occur 30 degrees poleward in each hemisphere, with greater numbers in the southern hemisphere. Some studies have found that gullies occur on slopes that face all directions; others have found that the greater number of gullies are found on poleward facing slopes, especially from 30° to 44° S. Although thousands have been found, they appear to be restricted to only certain areas of the planet.
context_section_description: After being discovered, many hypotheses were put forward to explain the gullies. However, as in the usual progression of science, some ideas came to be more plausible than others when more observations were made, when other instruments were used, and when statistical analysis was employed. Even though some gullies resembled debris flows on Earth, it was found that many gullies were on slopes that were not steep enough for typical debris flows. Calculations showed that the pressure and temperatures were not right for liquid carbon dioxide. Moreover, the winding shape of the gullies suggested that the flows were slower than what would be produced in debris flows or eruptions of liquid carbon dioxide. Liquid carbon dioxide would explode out of the ground in the thin Martian atmosphere. Because the liquid carbon dioxide would throw material over 100 meters, the channels should be discontinuous, but they are not. Eventually, most hypotheses were narrowed to involve liquid water coming from an aquifer, from melting at the base of old glaciers (or snowpacks), or from the melting of ice in the ground when the climate was warmer.
  Close-up images with HiRISE showed details that support the idea that a fluid was involved.  Images show that channels were formed a number of times.  Smaller channels were found in larger valleys, suggesting that after a valley formed another formed at a later time.  Many cases showed channels took different paths at different times.  Streamlined forms like teadrop-shaped islands were common in some channels.  The following group of pictures of gullies illustrate some of the shapes that lead researchers to think that water was involved in creating at least some of the gullies.
  However, more studies open up other possibilities; a study released in October 2010, proposes that some gullies, the ones on sand dunes, may be produced by a buildup of solid carbon dioxide during cold winter months.
  On July 10, 2014, NASA reported that gullies on the surface of Mars were mostly formed by the seasonal freezing of carbon dioxide (CO₂ ice or 'dry ice'), and not by that of liquid water as thought earlier.
  The exact cause/causes of these gullies is still under debate.  A study supported formation by the melting of ground ice or snowpack as the chief cause.  Over 54,000 CTX images were examined which covered about 85% of the surface of the planet.
caption: Group of gullies on north wall of crater that lies west of the crater Newton. The alcove and apron of one gully are labelled. These gullies are associated with moraine-like ridges at their downslope ends, which suggests they have formed at the site of a now-absent flowing ice. Note that they are cut into a mantle, which is much smoother than the rough-textured underlying material. Image taken by Mars Global Surveyor.
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