The robotic spacecraft MESSENGER found out as it looked toward the Earth during its closest approach to the Sun about three months ago.
当太空探测器信使号在三个月前到达离太阳最近的地方并回望地球时，它找到了答案。
Messenger was launched more than six years ago, travelling through the inner solar system and embarking on flybys of Earth, Venus and Mercury.
信使号于六年前发射，穿过太阳系内部，飞越地球、金星和水星。
"Despite its proximity to Earth, the planet Mercury has for decades been comparatively unexplored," said Sean Solomon, Messenger principal investigator.
信使号首席科学家Sean Solomon说：“尽管离地球比较近，但水星在几十年来没有怎么被探索过。”
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