Mercury, the second smallest planet in the solar system, is the closest planet to the Sun. It has a diameter of 4,880 km (about 40% smaller than Earth) and an orbit of 57,910,000 km from the Sun. Mercury is also the second densest body of the solar system after Earth.

In 1974-75, Mariner 10 became the only spacecraft to approach Mercury and it mapped nearly 40-45% of the planet. Another mission to Mercury called 'Messenger' has been launched by NASA (National Aeronautics and Space Administration) in the year 2004. It is scheduled to orbit Mercury in the year 2011.


 Mercury                       Mercury          Mercury Caloris Basin
The astronomical symbol for Mercury is a circle on top of a short vertical line with a cross below and a semicircle above the circle.

Astronomical Symbol

Physical Characteristics

Mercury has a mean surface temperature of 452 K, but it ranges from 90-700 K. Sunlight on Mercury's surface is 6.5 times as intense as it is on Earth. Radar observations taken in 1992 indicated that there is frozen water ice at Mercury's North Pole. Mercury has no known satellites of its own.

Interior Composition

Mercury has a large iron core whose radius ranges from 1800 to 1900 km. Its surface comprises of approximately 70% metals and 30% silicates. Mercury's average density is marked at 5430 kg/m³ which is slightly less as compared to Earth's density (5515 kg/m3).

Geology of Mercury

Mercury was formed nearly 4 billion years ago. After its formation, it received a shower of asteroids and comets which resulted in the formation of craters across the surface of Mercury. The craters were gradually filled in with magma from inside the planet.With gradual cooling & contraction, the surface of Mercury began to crack and these cracks are still visible on top of the craters and plains.


                     Mercury                         Mercury

At present, the surface of Mercury is pocked with craters and plains. The size of the craters ranges from 100 meters to 1,300 kilometers.

The largest crater on Mercury is known by the name of Caloris basin which is nearly 1300 kilometers in diameter. The Caloris Basin was defined by Hartmann and Kuiper in 1962 as a large circular depression with characteristic concentric rings. It has now been filled with lava flows.

Mercury's Atmosphere

Although Mercury has a very thin atmosphere due to very low gravity, it keeps the temperature more or less stable. Since the intensity of sunlight on Mercury is very high, the day temperatures at its surface can reach up to 420 degrees C while at night it slides to about -180 degrees C.

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