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资料：A high-precision NASA radar instrument left a NASA airport in Southern California for Iceland on January 28 to create detailed maps of how glaciers move in the dead of winter. This will help scientists around the world better understand some of the most basic processes involved in melting glaciers, which are major contributors to rising sea levels. NASA used the same airborne instrument in June 2012 to map the summer flows of two Icelandic ice caps. The ice caps — large areas of permanent snow and ice cover — encompass multiple glaciers flowing in different directions and at different speeds. By mapping the same ice caps now, in winter, when the surface remains frozen all day, and then comparing the winter and summer velocities, the researchers will be able to isolate the effects of melt water. Above is a view of a small part of the Hofsjkull ice cap, which encompasses several glaciers. The fan at upper left is part of a glacier called Mlaj?kull. For more on the research, see this NASA press release. What does the underlined word permanent （paragragh 2） mean?