Mean orbital velocity: 29.7 miles (47.9 km) per second
Inclination of axis: 0 í
Orbital eccentricity: 0.206
Orbital inclination: 7.0 í
Diameter at equator: 3,033 miles (4,879 km)
0.382 x Earth's diameter
Oblateness: 0
Mass: 3.30 x 10^23 kg
0.055 x Earth's mass
Volume: 6.07 x 10^10 km^3
0.056 x Earth's volume
Density: 5.43 g/cm^3
0.984 x Earth's density
Surface gravity: 2.78 m/s^2
0.38 x Earth's gravity
Escape velocity: 2.6 miles (4.3 km) per second
Mean surface temperature: 662 F / 350 C / 623 K (day)
-274 F / -170 C / 103 K (night)
Maximum apparent diameter as seen from Earth (seconds of arc): 12".9
Albedo: 0.1
Number of known satellites: 0
Mean diameter of Sun as seen from the planet: 1 22' 40"
Planet composition
Mercury's radius is 2440 km and its density is 5.43 g/cm^3. These two facts, in addition to information about its magnetic field, lead scientists to believe that Mercury has a relatively large iron core composed of 65 to 75% iron. Surrounding the core is a mantle probably composed of iron and magnesium silicates and a crust thought to be composed of calcium, sodium, and other silicates. However, it is possible that Mercury does not have a chemically distinct crust. If the hypothesis about the sizes and compositions of the core and mantle are correct, Mercury is approximately twice as rich in iron as any other planet in the solar system.
Core: radius of 1800 km
Mantle: 460 km thick
Crust: 180 km thick
Atmosphere
The planet has only a trace atmosphere, composed mainly of argon, neon, and helium, and its surface pressure is one trillionth of the pressure on Earth.
Mercury boasts the largest surface temperature variance in the solar system, ranging from -298 F / -183 C / 90 K on the night side to 872 F / 467 C / 740 K on the day side.
Magnetic fields and gravity
Mercury possesses a magnetic field, but its strength is only one-hundredth the strength of the Earth's. Possible explanations for the magnetic field include a dynamo in Mercury's large iron core, a "fossil" field which once existed and magnetized the iron and rocks, and an induced field caused by interactions with the Sun. However, none of these theories completely explains the measured field. The magnetic field causes a magnetosphere similar to the Earth's but without radiation belts.
The surface gravity on Mercury is 0.38 times the surface gravity on Earth.
Orbit and rotation
Obit has a mean radius of only 36 million miles (58 million km), which means it's usually too close to the sun to be seen. Mercury spins only three times on its axis in the course of two of its years. With the exception of Pluto, Mercury has the most eccentric orbit of all the planets.
Topography
Mercury is difficult to observe from Earth because it's never seen against a dark background (the best observation times being near sunrise and sunset, when the same feature are always pointed toward Earth). Most of our knowledge of Mercury is a result of the 1974 Mariner 10 flyby.
Craters are the main topographical feature on the planet. Most were created approximately 4 to 4.5 million years ago and range from 100m to 1,000 km in diameter. Mercury also has plains, which researchers believe were formed later in Mercury's evolution. Other features include scarps and troughs, which are probably the result of tectonic activity.
The planet's northwest quadrant is dominated by the Caloris Basin (1,300 km in diameter), the largest structural feature on Mercury. The northeast quadrant contains a smooth plain (Borealis Planitia) and a large lava-flooded basin, 310 km in diameter. There are also several prominent ray craters, including Rodin; a double ring basin surrounded by other major craters. The southwest quadrant consists mainly of inter-crater plains. The largest crater in the region is Beethoven (625 km wide; the second-largest known crater on Mercury). The southeast quadrant is the most heavily cratered and probably the oldest. This region also hosts three relatively young ray craters (Coply, Snorri, and Kuiper) and eight of 16 named scarps on the planet.
Moons and/or rings
There are no known moons or rings associated with Mercury.
Mythology
This planet was known to the Egyptians and Greeks, who named it after Hermes (Mercury), the messenger of the Gods and deity of science, eloquence, and cunning. In ancient astrology, a child born under Mercury loved learning, travel, and art. Wednesday was dedicated to Mercury, which was known to influence the kidneys of the human body.
Discovery and exploration
The first Mercurian observer is believed to be Johannes Hevelius in the 17th century. First maps of Mercury were created in Milan by G. V. Schiaparelli beginning in 1821. The only exploratory probe to visit Mercury was Mariner 10. Its flyby in March 1974 provided most of what we know about Mercury today.
