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The Mars ocean hypothesis states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet’s geologic history.〔Cabrol, N. and E. Grin (eds.). 2010. Lakes on Mars. Elsevier. NY〕〔Clifford, S. M. and T. J. Parker, 2001: (The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains ), Icarus 154, 40-79.〕 This primordial ocean, dubbed Paleo-Ocean and Oceanus Borealis,〔Baker, V. R., R. G. Strom, V. C. Gulick, J. S. Kargel, G. Komatsu and V. S. Kale, 1991: Ancient oceans, ice sheets and the hydrological cycle on Mars, Nature, 352, 589-594.〕 would have filled the Vastitas Borealis basin in the northern hemisphere, a region which lies 4–5 km (2.5–3 miles) below the mean planetary elevation, at a time period of approximately 3.8 billion years ago. Evidence for this ocean includes geographic features resembling ancient shorelines, and the chemical properties of the Martian soil and atmosphere.〔http://www.sciencedaily.com/releases/2015/03/150305140447.htm〕〔http://www.msn.com/en-us/news/technology/nasa-finds-evidence-of-a-vast-ancient-ocean-on-mars/ar-BBih9Y1?ocid=mailsignout〕〔Villanueva, G., M. Mumma, R. Novak, H. Käufl, P. Hartogh, T. Encrenaz, A. Tokunaga, A. Khayat, M. Smith. Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs. Science, 2015 DOI: 10.1126/science.aaa3630〕 Early Mars would have required a denser atmosphere and warmer climate to allow liquid water to remain at the surface.〔Read, Peter L. and S. R. Lewis, “The Martian Climate Revisited: Atmosphere and Environment of a Desert Planet”, Praxis, Chichester, UK, 2004.〕 ==History of observational evidence== Features shown by the Viking orbiters in 1976, revealed two possible ancient shorelines near the pole, Arabia and Deuteronilus, each thousands of kilometers long. Several physical features in the present geography of Mars suggest the past existence of a primordial ocean. Networks of gullies that merge into larger channels imply erosion by a liquid agent, and resemble ancient riverbeds on Earth. Enormous channels, 25 km wide and several hundred meters deep, appear to direct flow from underground aquifers in the Southern uplands into the Northern lowlands.〔〔 Much of the northern hemisphere of Mars is located at a significantly lower elevation than the rest of the planet (the Martian dichotomy), and is unusually flat. These observations led a number of researchers to look for remnants of more ancient coastlines and further raised the possibility that such an ocean once existed. In 1987, John E. Brandenburg published the hypothesis of a primordial Mars ocean he dubbed Paleo-Ocean.〔 The ocean hypothesis is important because the existence of large bodies of liquid water in the past would have had a significant impact on ancient Martian climate, habitability potential and implications for the search for evidence of past life on Mars. Beginning in 1998, scientists Michael Malin and Kenneth Edgett set out to investigate with higher-resolution cameras on board the Mars Global Surveyor with a resolution five to ten times better than those of the Viking spacecraft, in places that would test shorelines proposed by others in the scientific literature.〔 Their analyses were inconclusive at best, and reported that the shoreline varies in elevation by several kilometers, rising and falling from one peak to the next for thousands of miles.〔Malin, M. C., and Edgett, K. S., 1999. ''Oceans or Seas in the Martian Northern Lowlands: High Resolution Imaging Tests of Proposed Coastlines'', Geophys. Res. Letters, V. 26, No. 19, p. 3049—3052〕 These trends cast doubt on whether the features truly mark a long-gone sea coast and, have been taken as an argument against the Martian shoreline (and ocean) hypothesis. The Mars Orbiter Laser Altimeter (MOLA), which accurately determined in 1999 the altitude of all parts of Mars, found that the watershed for an ocean on Mars would cover three-quarters of the planet.〔Smith, D. et al. (1999). Science: 284.1495〕 The unique distribution of crater types below 2400 m elevation in the Vastitas Borealis was studied in 2005. The researchers suggest that erosion involved significant amounts of sublimation, and an ancient ocean at that location would have encompassed a volume of 6 x 107 km3. In 2007, Taylor Perron and Michael Manga proposed a geophysical model that, after adjustment for true polar wander caused by mass redistributions from volcanism, the Martian paleo-shorelines first proposed in 1987 by John E. Brandenburg,〔 meet this criterion.〔Zuber, Maria T., 2007: Planetary Science: Mars at the tipping point, ''Nature'', 447, 785-786.〕 The model indicates that these undulating Martian shorelines can be explained by the movement of Mars's spin axis. Because spinning objects bulge at their equator, the polar wander could have caused the shoreline elevation to shift in a similar way as observed.〔 Their model does not attempt to explain what caused Mars's rotation axis to move relative to the crust. Research published in 2009 shows a much higher density of stream channels than formerly believed. Regions on Mars with the most valleys are comparable to what is found on the Earth. In the research, the team developed a computer program to identify valleys by searching for U-shaped structures in topographical data. The large amount of valley networks strongly supports rain on the planet in the past. The global pattern of the Martian valleys could be explained with a big northern ocean. A large ocean in the northern hemisphere would explain why there is a southern limit to valley networks; the southernmost regions of Mars, farthest from the water reservoir, would get little rainfall and would develop no valleys. In a similar fashion the lack of rainfall would explain why Martian valleys become shallower from north to south. A 2010 study of deltas on Mars revealed that seventeen of them are found at the altitude of a proposed shoreline for a Martian ocean.〔DiAchille, G and B. Hynek. (2010). Ancient ocean on Mars supported by global distribution of deltas and valleys. nat. Geosci. 3, P. 459-463, doi:10.1038/ngeo891〕 This is what would be expected if the deltas were all next to a large body of water.〔DiBiasse, A. Limaye, J. Scheingross, W. Fischer, and M. Lamb. (2013). Deltic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars. Journal of Geophysical Research Planets 118, 1285-1302〕 Research published in 2012 using data from MARSIS, a radar on board the Mars Express orbiter, supports the hypothesis of an extinct large, northern ocean. The instrument revealed a dielectric constant of the surface that is similar to those of low-density sedimentary deposits, massive deposits of ground-ice, or a combination of the two. The measurements were not like those of a lava-rich surface.〔Mouginot, J., A. Pommerol, P. Beck, W. Kofman, S. Clifford. (2012). Dielectric map of the Martian northern hemisphere and the nature of plain filling materials. ''Geophysical Research Letters'', Vol. 39, L02202, doi:10.1029/2011GL050286〕 In March 2015, scientists stated that evidence exists for an ancient volume of water that could comprise an ocean, likely in the planet's northern hemisphere and about the size of Earth's Arctic Ocean.〔Villanueva G. L., Mumma M. J., Novak R. E., Käufl H. U., Hartogh P., Encrenaz T., Tokunaga A., Khayat A., and Smith M. D., Science, Published online 5 March 2015 ()〕 This finding was derived from the ratio of water and deuterium in the modern Martian atmosphere compared to the ratio found on Earth and derived from telescopic observations. Eight times as much deuterium was inferred at the polar deposits of Mars than exists on Earth (VSMOW), suggesting that ancient Mars had significantly higher levels of water. The representative atmospheric value obtained from the maps (7 VSMOW) is not affected by climatological effects as those measured by localized rovers, although the telescopic measurements are within range to the enrichment measured by the ''Curiosity'' rover in Gale Crater of 5-7 VSMOW.〔Webster, C.R. et al., 2013. Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere. Science 341(6), pp. 260–263.〕 For how long this body of water was in the liquid form is still unknown, considering the high greenhouse efficiency required to bring water to the liquid phase in Mars at a heliocentric distance of 1.4-1.7 AU. It is now thought that the canyons filled with water, and at the end of the Noachian Period the Martian ocean disappeared, and the surface froze for approximately 450 million years. Then, about 3.2 billion years ago, lava beneath the canyons heated the soil, melted the icy materials, and produced vast systems of subterranean rivers extending hundreds of kilometers. This water erupted onto the now-dry surface in giant floods.〔 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「Mars ocean hypothesis」の詳細全文を読む スポンサード リンク
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