Mercury is the innermost planet  in the Solar System . It is also the smallest, and its orbit is the most eccentric  (that is, the least perfectly circular) of the eight planets.[a]  It orbits  the Sun  once in about 88 Earth days, completing three rotations about its axis for every two orbits. The planet is named after the Roman  god  Mercury , the messenger to the gods.
Mercury's surface is heavily cratered and similar in appearance to Earth's Moon , indicating that it has been geologically inactive for billions of years. Due to its near lack of an atmosphere to retain heat, Mercury's surface experiences the steepest temperature gradient of all the planets, ranging from a very cold 100 K  at night to a very hot 700 K during the day. Mercury's axis has the smallest tilt  of any of the Solar System's planets, meaning that there are no seasons on its surface. Mercury and Venus  can each make appearances in Earth's sky both as a morning star and an evening star (because they are closer to the Sun than the Earth ), and at times Mercury can technically be regarded as a very bright object when viewed from Earth; however, its proximity in the sky to the Sun makes it more difficult to see than Venus.
Mercury is one of four terrestrial planets  in the Solar System , and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius  of 2,439.7 km.  Mercury is even smaller —albeit more massive—than the largest  natural satellites  in the Solar System, Ganymede  and Titan . Mercury consists of approximately 70% metallic  and 30% silicate  material.  Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth’s density of 5.515 g/cm3.  If the effect of gravitational compression  were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm3 versus Earth’s 4.4 g/cm3. 
Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core , Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron. 
- Book: Mercury
- Book: Solar System
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- Mercury in astrology
- Mercury in fiction
- Colonization of Mercury
- Exploration of Mercury
Research with space probes
Reaching Mercury from Earth poses significant technical challenges, since the planet orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers into the Sun’sgravitational  potential well . Mercury has an orbital speed  of 48 km/s, while Earth's orbital speed is 30 km/s. Thus the spacecraft must make a large change in velocity  (delta-v ) to enter a Hohmann transfer orbit  that passes near Mercury, as compared to the delta-v required for other planetary missions. 
The potential energy  liberated by moving down the Sun’s potential well  becomes kinetic energy ; requiring another large delta-v change to do anything other than rapidly pass by Mercury. To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. Aerobraking  is ruled out because the planet has very little atmosphere. A trip to Mercury requires more rocket fuel than that required to escape  the Solar System completely. As a result, only two space probes have visited the planet so far.  A proposed alternative approach would use a solar sail  to attain a Mercury-synchronous orbit around the Sun. 
Impact basins and craters
Craters on Mercury  range in diameter from small bowl-shaped cavities to multi-ringed impact basins  hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity. 
The largest known crater is Caloris Basin , with a diameter of 1,550 km.  The impact that created the Caloris Basin was so powerful that it caused lava  eruptions and left a concentric ring over 2 km tall surrounding the impact crater . At the antipode  of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around the planet, converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface.  Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode. 
There are two geologically distinct plains regions on Mercury.   Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces,  predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter.  It is not clear whether they are of volcanic or impact origin.  The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.[citation needed ]