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Mercury was named by the Romans after the messenger of the gods because it seemed to move more quickly than any other planet. Mercury is the innermost planet in our solar system and is the second smallest one. Pluto is the smallest. Both Saturn and Jupiter have moons that are larger than Mercury, such as Titan and Ganymede. Jupiter's moons Io, Europa, and Callisto are very close in size to Mercury.
Mercury resembles our moon with lunar-like terrain but differs with respect to its density. Mercury has a density of 5.43 gm/cm3 which is similar to the density of the Earth. This density indicates that its core has an iron composition like the Earth. The core probably takes up about 70% to 80% of the planet's radius with the outer region largely composed of silicate rocks.
Mercury has almost no atmosphere. The atmosphere on Earth helps keep a uniform temperature from day to night. On Mercury, due to its closeness to the Sun, the temperature rises to over 400 C (750 F) during the day. At night, because of the lack of atmosphere to help retain heat, the temperature drops to -180 C (-300F).

      MEAN RADIUS:                      2439.7 km
      MASS:                             0.055 (Earth=1)
      DENSITY:                          5.43 (gm/cm)
      GRAVITY:                          0.376 (Earth=1)
      ORBIT PERIOD:                     87.97 (Earth days)
      ROTATION PERIOD:                  58.65 (Earth days)
      SEMIMAJOR AXIS OF ORBIT:          0.387 au (earth=1 au)
      ECCENTRICITY OF ORBIT:            0.206 (Earth=0.017)
      EQUATORIAL INCLINATION (deg):     0.0 (Earth=23.44)
      SURFACE PRESSURE:                 ~10-15 bar (0.001 picobar)
      AVERAGE TEMPERATURE:              440 K (590-725 K, sunward side)
      WIND SPEEDS:                      0.3 to 1.0 m/s (surface)

      Atmospheric composition: 98% Helium (He), 2% Hydrogen (H2)