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Martian geysers are putative sites of small gas and dust eruptions that occur in the south polar region region of Mars during the spring thaw. "Dark dune spots" and "spiders" are the two most visible types of features ascribed to these eruptions. They are unlike any terrestrial geological phenomenon. The reflectance (albedo), shapes and unusual spider appearance of these features have stimulated a variety of hypotheses about their origin, ranging from differences in frosting reflectance, to explanations involving biological processes. However, all current geophysical models assume some sort of geyser-like activity on Mars.〔〔〔 Their characteristics, and the process of their formation, are still a matter of debate. These features are unique to the south polar region of Mars in an area informally called the 'cryptic region', at latitudes 60° to 80° south and longitudes 150°W to 310°W;〔Titus T. N. et al. (2003) Third Mars Polar Science Conference, Abstract #8081.〕〔Kieffer, H. H. (2001) Second International Conf. On Mars Polar Sci. and Exploration, no. 1057.〕〔Kieffer, H. H. (2003), Sixth International Conference on Mars, no. 3158.〕 this 1 meter deep carbon dioxide (CO2) ice transition area —between the scarps of the thick polar ice layer and the permafrost— is where clusters of the apparent geyser systems are located. The seasonal frosting and defrosting of carbon dioxide ice results in the appearance of a number of features, such dark dune spots with spider-like rilles or channels below the ice, where spider-like radial channels are carved between the ground and the carbon dioxide ice, giving it an appearance of spider webs, then, pressure accumulating in their interior ejects gas and dark basaltic sand or dust, which is deposited on the ice surface and thus, forming dark dune spots.〔〔〔〔〔〔 This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars.〔 However, it would seem that multiple years would be required to carve the larger spider-like channels.〔 There is no direct data on these features other than images taken in the visible and infrared spectra. ==History== The geological features informally called dark dune spots and spiders were separately discovered on images acquired by the MOC camera on board the Mars Global Surveyor during 1998–1999. At first it was generally thought they were unrelated features because of their appearance, so from 1998 through 2000 they were reported separately on different research publications (〔 and -respectively), with the first "jet" or "geyser" models starting to be proposed and refined from 2000 onwards.〔〔 The name 'spiders' was coined by Malin Space Science Systems personnel, the developers of the camera. One of the first and most interesting spider photos was found by Greg Orme in October 2000.〔 〕 The unusual shape and appearance of these 'spider webs' and spots caused a lot of speculation about their origin. The first years' surveillance showed that during the following Martian years, 70% of the spots appear at exactly the same place, and a preliminary statistical study obtained between September 1999 and March 2005, indicated that dark dune spots and spiders are related phenomena as functions of the cycle of carbon dioxide (CO2) ice condensation and sublimation.〔 It was also initially suggested that the dark spots were simply warm patches of bare ground, but thermal imaging during 2006 revealed that the temperature of these structures is as cold as the ice that covers the area,〔〔 indicating they were a thin layer of dark material lying on top of the ice and kept chilled by it.〔 However, soon after their first detection, they were discovered to be negative topographical features – i.e. radial troughs or channels of what today are thought to be geyser-like vent systems.〔〔〔〔〔〔〔 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「Geyser (Mars)」の詳細全文を読む スポンサード リンク
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