Mercury

Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god of commerce and travel. It has a average (mean) distance of only 57,910,000 kilometres, and a diameter of 4,880 kilometres. Mercury's orbit is very elliptical and tilted towards the orbit of other planets. The space probe Mariner 10 was the first spacecraft to visit Mercury, in order to obtain information about Mercury, in 1974. The surface was similar to the lunar surface, and the atmosphere appears to be very thin.

Orbit, size and mass
Mercury is the closest planet in the Solar System, to the central star, Sun. The average distance to Sun is 48 million kilometres (about 70 million kilometres on furthest), as this reflects the elliptical shape of the orbit. The planet is very eccentric, in which it is one of the most elliptic orbits in the Solar System. Mercury rotates once in its axis in 59 Earth days, as Mercury orbits the Sun every 88 days. Mercury's axis of rotation is tilted 2 degrees to the plane of its orbit around the Sun.

Mercury is the smallest planet in the Solar System. With a diameter of 4,878 km, Mercury is 1/3 the size of Earth, and 1 1/2 larger than the Moon. Mercury has a mass of 3.3 x 1023 kg. This mass is contained in a volume of 60.8 billion cubic km (14.6 billion cubic miles). The mass and volume of Mercury is only about 0.055 times that of Earth. But because Mercury's small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in 98 percent of the density of our planet. Only Earth is denser. This high density in a planet that otherwise resembles the moon raises interesting questions about the composition of the planet's interior.

Structure
Mercury is the second densest planet, after Earth, with a density of 5427 kg/m3. It has a large metallic iron core with a radius of about 1,289 miles (2,074 kilometers), about 85 percent of the planet's radius. There is evidence that it is partly molten, or liquid. Mercury's crust, comparable to Earth's crust (called the mantle and crust), is only about 400 kilometers (250 miles) thick and composed of silicate rock.

Surface
Along with Venus, Earth and Mars, Mercury has many impact craters on the Mercury, similar to the Moon. The craters are called rupes. It was formed when the planet was born, about 4.5 billion years ago, the core of the planet was cooler. First images of Mercury was prepared in 1920, identifying various dark and light spots and cloud layers. The largest known crater is Caloris Planitia, or Caloris Basin, with a diameter of 1,550 kilometres. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km tall surrounding the impact crater.

Atmosphere
Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and meteoroids. Mercury's exosphere is composed mostly of oxygen, sodium, hydrogen, helium and potassium. Mercury's magnetic field, or magnetosphere, is offset relative to the planet's equator. Though Mercury's magnetic field at the surface has just one percent the strength of Earth's, it interacts with the magnetic field of the solar wind to sometimes create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface of the planet. When the ions strike the surface, they knock off neutrally charged atoms and send them to the sky.

Visibility
At its brightest, it can reach magnitude -1.9, brighter than Sirius, the brightest star in the sky. However, it more normally hovers at around magnitude 0, becoming roughly the fifth brightest object in the sky. Mercury can be seen rarely, because of its relative proximity to the Sun. It is visible only shortly before sunrise and after sunset. In Earth's latitudes, the planet can be observed most easily in the evening at spring, and in morning at autumn. However, Mercury's elongation is greatest, merely 28 degrees. Similar to the Venus and the Moon, Mercury has phases, waxings and wanings from full to new Mercury. This planet travels on a path across the solar disc, called transit, with approximately 12 times per century. A telescope would reveal Mercury as a small black disc in the front of our central star, the Sun. This is known as Mercury Transit. The next transit of Mercury will take place on 9 May 2016 (visible in Europe).