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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.〔Malin, M., Edgett, K. 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335.〕 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. 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°-44° S.〔 Although thousands have been found, they appear to be restricted to only certain areas of the planet. In the northern hemisphere, they have been found in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia. In the south, high concentrations are found on the northern edge of Argyre basin, in northern Noachis Terra, and along the walls of the Hellas outflow channels.〔 A recent study examined 54,040 CTX images that covered 85% of the Martian surface found 4861 separate gullied landforms (e.g., individual craters, mounds, valleys, etc.), which totaled tens of thousands of individual gullies. It is estimated that CTX can resolve 95% of gullies.〔Harrison, T., G. Osinski1, and L. Tornabene. 2014. GLOBAL DOCUMENTATION OF GULLIES WITH THE MARS RECONNAISSANCE ORBITER CONTEXT CAMERA (CTX) AND IMPLICATIONS FOR THEIR FORMATION. 45th Lunar and Planetary Science Conference. pdf〕 On the basis of their form, aspects, positions, and location amongst and apparent interaction with features thought to be rich in water ice, many researchers think that the processes carving the gullies involve liquid water. However, this remains a topic of active research. Because the gullies are so young, this would suggest that liquid water has been present on Mars in its very recent geological past, with consequences for the potential habitability of the modern surface. On July 10, 2014, NASA reported that gullies on the surface of Mars were mostly formed by the seasonal freezing of carbon dioxide (CO2), and not by that of liquid water as considered earlier. ==Formation== After being discovered, many hypotheses were put forward to explain the gullies.〔http://www.psrd.hawaii.edu/Aug03/MartianGullies.html〕 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.〔〔Forget, F. et al. 2006. Planet Mars Story of Another World. Praxis Publishing. Chichester, UK.〕 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.〔Head, J., D. Marchant, M. Kreslavsky. 2008. Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin. PNAS: 105 (36), 13258–13263.〕 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. ESP 037506 2285gullychannels.jpg|Gullies on wall of crater, as seen by HiRISE under HiWish program Location is the Mare Acidalium quadrangle. ESP 037506 2285gullychannelsclose.jpg|Close-up of gully channels, as seen by HiRISE under HiWish program. This image shows many streamlined forms and some benches along a channel. These features suggest formation by running water. Benches are usually formed when the water level goes down a bit and stays at that level for a time. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image. ESP 039753 1385gulliespits.jpg|Gullies in crater in Phaethontis quadrangle, as seen by HiRISE under HiWish program ESP 039753 1385channeldetail.jpg|Close-up of channels in gullies showing that the paths of the channels changed over time. This feature suggest formation by running water with a high load of sediment. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image in Phaethontis quadrangle. ESP 039793 1385gullies.jpg|Gullies in crater, as seen by HiRISE under HiWish program. Location is Eridania quadrangle. ESP 039793 1385channeldetails.jpg|Close up of gullies in crater showing channels within larger valleys and curves in channels. These characteristics suggest they were made by flowing water. Note: this is an enlargement of the previous image by HiRISE under HiWish program. Location is Eridania quadrangle. ESP 039793 1385channelsclose.jpg|Close up of gully network showing branched channels and curves; these characteristics suggest creation by a fluid. Note: this is an enlargement of a previous wide view of gullies in a crater, as seen by HiRISE under HiWish program. Location is Eridania quadrangle. 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.〔(NASA/Jet Propulsion Laboratory. "Study links fresh Mars gullies to carbon dioxide." ScienceDaily 30 October 2010. 10 March 2011 )〕〔 〕 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.〔Harrison, T., G. Osinski, L. Tornabene, E. Jones. 2015. Global documentation of gullies with the Mars Reconnaissance Orbiter Context Camera and implications for their formation. Icarus: 252, 236-254.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gully (Mars)」の詳細全文を読む スポンサード リンク
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