The exploration of our solar system was originally driven primarily by curiosity and the search for answers to the eternal question: Are we alone? As the fir st spacecraft began to visit our neighboring planets in the solar system, hopes of finding life quickly faded and instead the focus began to shift toward gaining a more scientific understanding of their nature and history After a generation of exploratory robotic spacecraft missions from the 1960s through the 1980s, it became clear that most planets-except Mercury-and some moons had permanent and substantial atmospheres These atmospheres can be subdivided into three large families: those composed primarily of nitrogen (Earth, Titan, Triton, Pluto), carbon dioxide (Venus, Mars), and hydrogen/helium (Jupiter, Saturn, Uranus, Neptune). On page 1.366 of this issue, Forbes etal. (1) provide fresh insights into the effects of variations in solar radiation intensity on the upper atmospheres of Mars and Earth, which have now been simultaneously observed.
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