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Olympia Undae is a vast dune field in the north polar region of the planet Mars. It consists of a broad "sand sea" or erg that partly rings the north polar plateau (Planum Boreum) from about 120° to 240°E longitude and 78° to 83°N latitude. Stretching about across〔JPL Photojournal. http://photojournal.jpl.nasa.gov/catalog/PIA10948〕 and covering an area of 470,000 km2,〔Lancaster, N.; Greeley, R. (1990). Sediment Volume in the North Polar Sand Seas of Mars. ''J. Geophys. Res.,'' 95(B7), p. 10,924.〕 Olympia Undae is the largest continuous dune field on Mars. It is similar in size to the Rub' Al Khali in the Arabian Peninsula, the largest active erg on Earth.〔Tsoar, H.; Greeley, R.; Peterfreund, A.R. (1979). Mars: The Northern Polar Sand Sea and Related Wind Patterns. ''J. Geophys. Res.,'' 84(B14), p. 8167.〕 Olympia Undae lies within the informally named Borealis basin (also called the north polar basin〔Head, J. et al. (1999) Possible Ancient Oceans on Mars: Evidence from Mars Orbiter Laser Altimeter Data. ''Science,'' 286, pp. 2134–2137.〕), the largest of three topographic basins that occur in the northern lowlands of Mars.〔Tanaka, K.L. ''et al.'' (2008). North Polar Region of Mars: Advances in Stratigraphy, Structure, and Erosional Modification. ''Icarus,'' 196, p. 321.〕 The average elevation in Olympia Undae is about 4,250 m below datum (martian "sea" level).〔JMARS MOLA elevation dataset. Christensen, P.; Gorelick, N.; Anwar, S.; Dickenshied, S.; Edwards, C.; Engle, E. "New Insights About Mars From the Creation and Analysis of Mars Global Datasets;" American Geophysical Union, Fall Meeting 2007, (abstract #P11E-01).〕 The 19-km-diameter crater Jojutla lies near the geographic center of Olympia Undae at 81.63°N latitude and 169.65°E longitude.This crater was named by Andres Eloy Martinez Rojas, Mexican astronomer and science writer.〔USGS Gazetteer of Planetary Nomenclature. Mars. http://planetarynames.wr.usgs.gov/.〕 ''Unda'' (pl. ''undae'') is a Latin term meaning water, particularly water in motion as waves.〔Simpson, D.P. (1968). ''Cassell's New Latin Dictionary;'' Funk & Wagnalls: New York, p. 623.〕 The International Astronomical Union (IAU) adopted the term to describe "undulatory," dune-like features on other planets.〔Russell, J.F.; Snyder, C.W.; Kieffer, H.H. (1992). Appendix: Origin and Use of Martian Nomenclature; In ''Mars,'' H.H. Kieffer ''et al.,'' Eds. University of Arizona Press: Tucson, 1992, p. 1313.〕 Olympia Undae contains a variety of dune forms and wind-related (aeolian) depositional features, including sand sheets,〔Tanaka, K.L. ''et al.'' (2008). North polar region of Mars: Advances in stratigraphy, structure, and erosional modification. ''Icarus,'' 196, p. 346.〕 transverse dunes, simple barchan dunes, mega-barchans, and complex barchanoid ridges.〔Breed, C.S.; Grolier, M.; McCauley, J.F. (1979). Morphology and Distribution of Common "Sand" Dunes on Mars: Comparison with the Earth. ''J. Geophys. Res.,'' 84(B14), 8187.〕 All of these dune types occur on Earth too. Barchans are isolated, crescent-shaped dunes with horns that point downwind. They occur is areas where sand supply is moderate to low.〔Zimbleman, J.R.; Williams, S.H. (2007). Eolian Dunes and Deposits in the Western United States as Analogs to Wind-Related Features on Mars, in ''The Geology of Mars: Evidence from Earth-Based Analogs,'' Chapman, M. Ed. Cambridge University Press: Cambridge, p. 237.〕〔McKee, E.D. (1979). Introduction to a Study of Global Sand Seas. USGS Professional Paper 1052, pp. 1-19.〕 Small simple barchan dunes and large mega-barchans are common at the margins of Olympia Undae and in areas where the sand cover is thin.〔Greeley, R. ''et al.'' (1992). Martian Aeolian Processes, Sediments, and Features in'' Mars,'' H.H. Kieffer ''et al.,'' Eds. University of Arizona Press: Tucson, AZ, p. 750.〕 Barchanoid ridges are broad linear to sinuous sand accumulations.〔Zimbleman, J.R.; Williams, S.H. (2007). Eolian Dunes and Deposits in the Western United States as Analogs to Wind-Related Features on Mars, in ''The Geology of Mars: Evidence from Earth-Based Analogs,'' Chapman, M. Ed. Cambridge University Press: Cambridge, p. 235.〕 They form through the lateral coalition of individual barchans and indicate increasing sand supply. Where sand is abundant, transverse dunes occur; they are commonly defined as long barchaoid ridges with fairly straight segments that are perpendicular to the wind direction.〔Breed, C.S.; Grolier, M.; McCauley, J.F. (1979). Morphology and Distribution of Common "Sand" Dunes on Mars: Comparison with the Earth. ''J. Geophys. Res.,'' 84(B14), p. 8187.〕 The majority of dunes in Olympia Undae are transverse dunes. Their spacing ranges from 200 to 800 m apart crest to crest, and comparison to terrestrial dunes with similar spacing indicates that they are 10 to 25 m high.〔Greeley, R. ''et al.'' (1992). Martian Aeolian Processes, Sediments, and Features in'' Mars,'' H.H. Kieffer ''et al.,'' Eds. University of Arizona Press: Tucson, AZ, p. 750-751.〕 On Earth, dunes are produced by saltating grains of sand. The requirement that dunes are produced by saltation allows scientists to determine the likely grain size for the particles making up the dunes in Olympia Undae and other martian dune fields. On Mars, the particle size most easily moved by wind is about 100 μm in diameter (fine sand).〔Greeley, R. ''et al.'' (1992). Martian Aeolian Processes, Sediments, and Features in'' Mars,'' H.H. Kieffer ''et al.,'' Eds. University of Arizona Press: Tucson, AZ, p. 733.〕 The sand in Olympia Undae is extremely dark in color and probably consists of basaltic rock fragments. The surface of Olympia Undae has a strong TES Type 2 spectral signature,〔Tanaka, K.L. et al. (2008). North Polar Region of Mars: Advances in Stratigraphy, Structure, and Erosional Modification. ''Icarus,'' 196, 347.〕 indicating that the surface materials consist of basaltic andesite or weathered basalt and/or basaltic glass.〔Wyatt, M., McSween, H., Tanaka, K., Head, J., 2004. Global Geologic Context for Rock Types and Surface Alteration on Mars. ''Geology,'' 32, pp. 645–648.〕 In 2005, the OMEGA instrument on the Mars Express orbiter detected high concentrations of gypsum in the eastern portion of Olympia Undae (centered at 244.5°E, 80.2°N).〔Langevin, Y. et al. (2005). Sulfates in the North Polar Region of Mars Detected by OMEGA/Mars Express. ''Science,'' 307(1584), .〕 CRISM data from the Mars Reconnaissance Orbiter (MRO) suggests that the gypsum is more concentrated along the crests of dunes than in the interdune hollows.〔Roach L.H.; Mustard J.F.; Murchie S.; Langevin Y., Bibring J-P., ''et al.'' (2007). CRISM Spectral Signatures of the North Polar Gypsum Dunes. Lunar Planet. Sci. Conf., 38th, Abstr. 1970.〕 The source of the gypsum is uncertain. Gypsum is an evaporitic mineral that precipitates from saline water; thus, its presence may indicate conditions different from today’s martian environment.〔Byrne, S. (2009). The Polar Deposits of Mars. Annu. Rev. Earth Planet. Sci., 37, pp. 551–552.〕 The mineral may have formed through the melting of acidic snow, or the melting and discharge of sulfur-rich water from the base of the polar ice cap.〔Fishbaugh, K.E. et al. (2006). Formation of the Martian North Polar Gypsum Deposit During the Amazonian. Fourth International Conference on Mars Polar Science and Exploration, Abstract #8041. http://www.lpi.usra.edu/meetings/polar2006/pdf/8041.pdf.〕 However, the presence of gypsum does not necessarily require large surface water bodies (e.g., playa lakes). The mineral could have formed in volcanically heated groundwater in the shallow subsurface and later been exposed and concentrated by wind erosion and winnowing ("eolian mining").〔Tanaka, K.L. (2006). Mars’ North Polar Gypsum: Possible Origin Related to Early Amazonian Magmatism at Alba Patera and Aeolian Mining. Fourth International Conference on Mars Polar Science and Exploration, Abstract #8024. http://www.lpi.usra.edu/meetings/polar2006/pdf/8024.pdf.〕 The term Olympia Undae can be the source of some confusion among Mars researchers. The term is used to describe 1) the geographical area described above and the type area for 2) a stratigraphic or geologic map unit (e.g. formation) called the Olympia Undae unit. As a stratigraphic unit, Olympia Undae describes materials that make up the geographic Olympia Undae as well as other sand sheets and dune fields encircling Planum Boreum (e.g., Abalos Undae). The Olympia Undae unit is Amazonian in age.〔Tanaka, K.L. ''et al.'' (2008). North Polar Region of Mars: Advances in Stratigraphy, Structure, and Erosional Modification. ''Icarus,'' 196, 318–358.〕 To address some of this confusion, the stratigraphic term Olympia Undae unit has recently been renamed to simply "undae unit," since it encompasses other named dune fields (undae) around Planum Boreum.〔Tanaka, K., USGS, personal communication, September 8, 2010.〕 Another possible source of confusion is the distinction between Olympia Undae and Olympia Planum (formerly, Olympia Planitia). As a geographic area, Olympia Undae refers to the erg that covers a large fraction of Olympia Planum between longitude 120° and 240°E. Olympia Undae and Olympia Planum are not interchangeable terms. Olympia Planum is a broad, plain (and topographic bench) adjacent to Planum Boreum. It is half-domed shaped in profile (cross-section) and slopes southward into the Vastitas Borealis. The Olympia Undae erg covers both the bulk of southern Olympia Planum and part of the northern Vastitas Borealis. Image:Dunes in Olympia Planum.jpg|Olympia Undae dunes, as seen by HiRISE. Two sets of transverse dunes are visible, indicating varying wind direction. Olympia Undae is also known at the North Polar Erg. Image:Olympia Undae Gypsum PIA10948.jpg|CRISM spectral signal for gypsum (polyhydrated sulfates) in Olympia Undae dunes. ==References== 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「Olympia Undae」の詳細全文を読む スポンサード リンク
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