|
Penticton Crater is an impact crater in the Hellas quadrangle of Mars, located at 38.35° south latitude and 263.35° west longitude. It is 8 km in diameter and was named after a Town in British Columbia, Canada, nearby the little town of Okanagan Falls where is located the Dominion Radio Astrophysical Observatory.〔Dominion Radio Astrophysical Observatory〕 Images with HiRISE show gullies which were once thought to be caused by flowing water. Penticton is famous with Mars geologists because evidence for recent flowing liquid was found there. The Mars Reconnaissance Orbiter discovered changes on the wall of Penticton Crater between 1999 and 2004. One interpretation of the changes was that they were caused by water flowing on the surface. A further analysis, published about a year later, revealed that the deposit could have been caused by gravity moving material downslope. The slope where the deposit was sighted was close to the stability limits of dry, unconsolidated materials. 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 believed 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 which themselves considered to be quite young. On the basis of their form, aspects, positions, location and apparent interaction with features thought to be rich in water ice, many researchers believed that the processes carving the gullies involve liquid water. However, this remains a topic of active research. As soon as gullies were discovered,〔 researchers began to image many gullies over and over, looking for possible changes. By 2006, some changes were found.〔Malin, M., K. Edgett, L. Posiolova, S. McColley, E. Dobrea. 2006. Present-day impact cratering rate and contemporary gully activity on Mars. Science 314, 1573_1577.〕 Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water.〔Kolb, et al. 2010. Investigating gully flow emplacement mechanisms using apex slopes. Icarus 2008, 132-142.〕〔McEwen, A. et al. 2007. A closer look at water-related geological activity on Mars. Science 317, 1706-1708.〕〔Pelletier, J., et al. 2008. Recent bright gully deposits on Mars wet or dry flow? Geology 36, 211-214.〕 With continued observations many more changes were found in Gasa Crater and others.〔NASA/Jet Propulsion Laboratory. "NASA orbiter finds new gully channel on Mars." ScienceDaily. ScienceDaily, 22 March 2014. www.sciencedaily.com/releases/2014/03/140322094409.htm〕 With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to believe that gullies were formed from dry ice. Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon-dioxide frost and temperatures that would not have allowed for liquid water. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes.〔http://www.jpl.nasa.gov/news/news.php?release=2014-226〕〔http://hirise.lpl.arizona.edu/ESP_032078_1420〕〔http://www.space.com/26534-mars-gullies-dry-ice.html?cmpid=557882〕 In some years frost, perhaps as thick as 1 meter, Wikipenticton.jpg|Penticton Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Image:Penticton Crater New Light Toned Feature.JPG|Penticton Crater New Light-Toned Feature, as seen by HiRISE Image:Penticton Crater Gullies.JPG|Penticton Crater gullies, as seen by HiRISE ==Why are Craters important?== The density of impact craters is used to determine the surface ages of Mars and other solar system bodies.〔http://www.lpi.usra.edu/publications/slidesets/stones/〕 The older the surface, the more craters present. Crater shapes can reveal the presence of ground ice. 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「Penticton (crater)」の詳細全文を読む スポンサード リンク
|