MERCURY
Numbers
Type: Rocky planet
Size (diameter): 4880 km, or about 38% the size of Earth
Mass: 3.301 × 1023 kg, or 1/20th the mass of Earth
Length of a year (orbital period): 88 days
Number of moons: 0
Average distance from the Sun: 57.9 million km
Temperature Between : -173 and 427 degrees Celsius
Historical Review
Mercury is appropriately named for the swiftest of the ancient Roman gods.
Atmosphere
Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. Mercury's exosphere is composed mostly of oxygen, sodium, hydrogen, helium, and potassium.
Surface
Mercury's surface is similar to Earth's Moon, with numerous impact craters and features named after famous figures. Large impact basins, such as Caloris and Rachmaninoff, were created by asteroid impacts. The planet's surface appears greyish-brown to the human eye, with bright streaks called "crater rays" formed when an asteroid or comet hits. Mercury's extreme temperatures can reach 800 degrees Fahrenheit during the day and minus 290 degrees Fahrenheit at night due to its lack of an atmosphere. Water ice may exist at its north and south poles inside deep craters, but only in regions in permanent shadows.