Mercury; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

The innermost planet in the solar system, Mercury has the most extreme characteristics of the terrestrial bodies. Daytime temperatures on the planet reach 427 degrees Celsius (801 degrees Fahrenheit)-hot enough to melt zinc. At night, however, the lack of an atmosphere lets the temperature plunge to -183 degrees C, which is cold enough to freeze krypton.

Mercury is also unusually dense. To account for its density of 5.44 grams per cubic centimeter (0.20 pound per cubic inch), astronomers believe the planet must have a relatively huge core that is unusually iron-rich. The core probably takes up 42 percent of Mercury's volume; in comparison, Earth's core is only about 16 percent, and Mars's, about 9 percent.