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Yardangs are common in some regions on Mars, especially in what's called the "Medusae Fossae Formation." This formation is found in the Amazonis quadrangle and near the equator.〔(SAO/NASA ADS Astronomy Abstract Service: Yardangs on Mars )〕 They are formed by the action of wind on sand sized particles; hence they often point in the direction that the winds were blowing when they were formed.〔http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Yardangs_on_Mars〕 Because they exhibit very few impact craters they are believed to be relatively young.〔http://themis.asu.edu/zoom-20020416a〕 The easily eroded nature of the Medusae Fossae Formation suggests that it is composed of weakly cemented particles, and was most likely formed by the deposition of wind-blown dust or volcanic ash. Yardangs are parts of rock that have been sand blasted into long, skinny ridges by bouncing sand particles blowing in the wind.〔http://www.uahirise.org/ESP_039563_1730〕〔http://www.uahirise.org/ESP_039563_1730 〕 Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking, Mars Global Surveyor, and HiRISE photos. Images from spacecraft show that they have different degrees of hardness probably because of significant variations in the physical properties, composition, particle size, and/or cementation. File:Medusae Fossae Formation.jpg|Medusae Fossae Formation as seen with Mars Odyssey's THEMIS. Notice elongated formations called yardangs. File:Yardangs in Medusae.jpg|Yardangs in Medusae Fossae Formation with caprock labeled, as seen by HiRISE. Location is Aeolis quadrangle. WikiESP 035295 1885yardangs.jpg|Yardangs of various sizes, as seen by HiRISE under HiWish program. WikiESP 035295 1885yardangstop.jpg|Yardangs of various sizes, as seen by HiRISE under HiWish program. WikiESP 039563 1730yardangs.jpg|Yardangs formed in light-toned material and surrounded by dark, volcanic basalt sand, as seen by HiRISE under HiWish program. WikiESP 039563 1730yardangsclose.jpg|Close-up image of yardangs, as seen by HiRISE under HiWish program. Arrows point to transverse aeolian ridges, TAR's, a type of dune. Note this is an enlargement of the previous image from HiRISE. WikiESP 036429 1925yardangscrater.jpg|Yardangs near a crater, as seen by HiRISE under HiWish program Location is in the Amazonis quadrangle. WikiESP 035558 1830yardang.jpg|Yardangs, as seen by HiRISE under HiWish program Location is near Gordii Dorsum in the Amazonis quadrangle. These yardangs are in the upper member of the Medusae Fossae Formation. Wikiesp036510 1735layersyardang.jpg|Yardangs showing layers, as seen by HiRISE under HiWish program Location is Aeolis quadrangle. ==See also== *Climate of Mars *Geology of Mars *Yardang 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「Yardangs on Mars」の詳細全文を読む スポンサード リンク
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