水星(Mercury)

太阳系中最靠近太阳的行星。距太阳的平均距离约5,800万公里，但由於其轨道呈大椭圆形，因此它距离太阳最近和最远时可与先前资料相差1,200万公里。它是太阳系中第二小的行星（最小的是冥王星），其直径为4,870公里，重量是地球的八分之一。它的公转周期最短（只有88个地球日），而轨道速度最快（48公里／秒），因此以罗马神话中脚程最快的信使命名。它的自转很慢，其自转速度同其他星球相比，相当於59个地球日，而受其公转期的影响，它的一个太阳日（两个日出之间的间隔时间）是176个地球日。它的表面是厚重的地壳，最大的特点是它的半径达1,300公里的卡路里盆地，是由一个巨大的陨石撞击而成。水星上还有陡峭的悬崖，绵延几百公里。在盆地的附近发现的引力场证明它有一个大的铁质内核，其密度同地球相当。它的大气可被忽略不计，地表重力约为地球1/3，只有很薄的一层大气。地表温度变化很大，面对太阳的地方温度高达402℃，而夜晚温度则只有零下173℃。

English version：

Mercury

Innermost planet of the solar system. Its average distance from the sun is about 36 million mi (58 million km), but its highly elliptical orbit carries it 7.5 million mi (12 million km) nearer to and farther from the sun. It is the second-smallest major planet (after Pluto), having a diameter of about 3,050 mi (4,870 km) and a mass about one-eighteenth of earth's. With the shortest period of revolution (only 88 earth days) and the highest average orbital speed (30 mi/second, or 48 km/second) of any planet, it is aptly named after the fleet-footed Roman messenger god. It rotates very slowly, making one complete rotation relative to the stars every 59 earth days, while its solar day (from one sunrise to the next) is 176 earth days, owing to its revolution around the sun. Its surface is heavily cratered. Its most impressive feature is perhaps the 800-mi (1,300-km) Caloris Basin, formed by a huge meteorite impact. Mercury also has steep cliffs that extend for hundreds of miles. The discovery of a magnetic field in its vicinity suggests it has a large iron core, which would account for a mean density almost as high as earth's. Its atmosphere is negligible; its surface gravity, about one-third that of earth's, holds little more than a thin layer of gases. Temperatures at its surface change dramatically, ranging from a high of about 756°F (402°C) on the side facing the sun to a low of about −279°F (−173°C) at the end of its night.

参考文章

• 为什么水星上没有水？天文天体