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Niger Vallis
Niger Vallis is a valley on Mars that appears to have been carved by water. It has been identified as an outflow channel.〔Carr, M.H. (2006), The Surface of Mars. Cambridge Planetary Science Series, Cambridge University Press.〕 It merges with Dao Vallis which runs southwestward into Hellas Planitia from the volcanic Hadriacus Mons. Like Dao, it was formed around the Late Noachian and Early Hesperian Epochs.〔 It is named after the Niger River in Africa.
== Ice-rich mantle ==
Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.〔Head, J. et al. 2003. Recent ice ages on Mars. Nature:426. 797-802.〕 This ice-rich mantle, a few yards thick, smoothes the land, but in places it has a bumpy texture, resembling the surface of a basketball. Because there are few craters on this mantle, the mantle is relatively young. An image below shows a good view of this smooth mantle around Niger Vallis, as observed with HiRISE.
Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water comes back to ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. Water vapor will condense on the particles, then fall down to the ground due to the additional weight of the water coating. When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.〔MLA NASA/Jet Propulsion Laboratory (2003, December 18). Mars May Be Emerging From An Ice Age. ScienceDaily. Retrieved February 19, 2009, from http://www.sciencedaily.com /releases/2003/12/031218075443.htmAds by GoogleAdvertise〕
Image:Niger Vallis hirise.JPG|Niger Vallis with features typical of this latitude, as seen by HiRISE. Click on image to see chevron pattern and mantle.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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