Word: perihelion
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Dates: during 1970-1979
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...distance from the sun. The Martian north pole is currently tilted toward the sun only when the planet is also at its greatest distance, or aphelion, from the sun. In contrast, the southern hemisphere is tilted toward the sun at the planet's closest approach, or perihelion. As a result, the southern polar cap gets warm enough to evaporate almost completely each summer, releasing most of its dry ice into the atmosphere. The northern polar region, however, remains cold enough during summer to retain a large and relatively thick polar cap throughout the year...