Bright Rayed Craters

This image shows two prominent craters (upper right) with bright halos on Mercury. The craters are about 40 kilometers (25 miles) in diameter. The halos and rays cover other features on the surface indicating that they are some of the youngest on Mercury. (Credit: Calvin J. Hamilton, and NASA; FDS 275)

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 (-300°F).

Mercury Statistics:

Mass (kg) 3.303e+23
Mass (Earth = 1) 5.5271e-02
Equatorial radius (km) 2,439.7
Equatorial radius (Earth = 1) 3.8252e-01
Mean density (gm/cm^3) 5.42

Mean distance from the Sun (km) 57,910,000
Mean distance from the Sun (Earth = 1) 0.3871
Rotational period (days) 58.6462
Orbital period (days) 87.969
Mean orbital velocity (km/sec) 47.88

Orbital eccentricity 0.2056
Tilt of axis 0.00°
Orbital inclination 7.004°

Equatorial surface gravity (m/sec^2) 2.78
Equatorial escape velocity (km/sec) 4.25

Visual geometric albedo 0.10
Magnitude (Vo) -1.9
Mean surface temperature 179°C
Maximum surface temperature 427°C
Minimum surface temperature -173°C
Atmospheric composition
Helium 42%
Sodium 42%
Oxygen 15%
Other 1%
imum surface temperature 427°C
Minimum surface temperature -173°C
Atmospheric composition
Helium 42%
Sodium 42%
Oxygen 15%
Other 1%