即使在麦哲伦号航天器（Magellan）还未看穿金星（Venus）的云层覆盖之前，天文学家（astronomer）便很有把握地相信金星上没有水。金星的表面很热，至少有450摄氏度。它的温度超过了水星（Mercury），你也许认为水星会比金星热，因为它离太阳更近。然而，水星没有保温的大气层。
金星大气层中的二氧化碳（Carbon dioxide）把太阳的热量保留了下来。这和温室能够保存太阳的热量好让植物生长类似。太阳的光线能够穿过云层到达金星表面，但其热量却无法逃逸。天文学家们懂得,水在如此炽热的环境下是无法以液体的形式存在的。然而，它附近的两颗行星——地球和火星（Mars）——却存在着水或冰。金星过去是否也曾有过水？
麦哲伦号航天器发回的照片没有提供曾有水存在的迹象。金星的表面有很多像河一样的长长的纹路，有些长达数百公里。但这些纹路似乎是熔岩（melted rock）造成的，而不是水造成的。
金星上没有古海洋的迹象，也没有山峦被水冲刷的痕迹。一位天文学家说，金星太热，水星太冷，而地球恰好适合水的存在。
Even before the Magellan spacecraft saw through Venus’s cloud cover, astronomers were confident that Venus had no water. The surface of Venus is very hot, at least 450 degrees Celsius. It’s hotter than Mercury, which you might expect to be hotter than Venus because it’s closer to the Sun. Mercury, however, has no insulating atmosphere.
The Carbon dioxide in Venus’ atmosphere traps the sun’s heat. This is similar to how a greenhouse preserves the sun’s heat to allow plants to grow. The sun’s rays can pass through the clouds to reach the surface of Venus, but its heat cannot escape. Astronomers knew that water could not exist as a liquid in such hot conditions. However, two nearby planets, Earth and Mars, have water or ice. Did Venus once have water?
Photos from the Magellan spacecraft provide no evidence that water ever existed. The surface of Venus is lined with long river lines, some hundreds of kilometers long. But these lines seem to be caused by lava, not water.
There are no signs of an ancient ocean, or mountains washed away by water. One astronomer says Venus is too hot, Mercury is too cold, and Earth is just right for water.
