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FIGURE 8.1 Montage of the major outer planet satellites, with Earth’s Moon for scale. Ganymede’s diameter is 5,262 km. The Moon’s diameter is 3,476 km. SOURCE: NASA.

theme includes the question, What were the primordial sources of organic matter, and where does organic synthesis continue today? The surfaces and interiors of the icy satellites display a rich variety of organic molecules—some believed to be primordial, some likely being generated even today; Titan presents perhaps the richest planetary laboratory for studying organic synthesis ongoing on a global scale. Europa, Enceladus, and Titan are central to another key question in this theme: Beyond Earth, are there modern habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now? Exhibiting a global methane cycle akin to Earth’s hydrologic cycle, Titan’s complex atmosphere is key to understanding the workings of the solar system theme and the question, Can understanding the roles of physics,



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