In English, it is named after the Roman god Mercurius ( Mercury), god of commerce, communication and the messenger of gods. Like Venus, Mercury orbits the Sun within Earth's orbit, making it appear in Earth's sky only as a "morning star" or "evening star" that's relatively close to the Sun. Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄ 30 of a degree), and its orbital eccentricity is the largest of all known planets in the Solar System. Counterintuitively, due to Mercury's slow rotation, an observer on the planet would see only one Mercurian solar day (176 Earth days) every two Mercurian solar years (88 Earth days each).
Mercury orbits the Sun in a 3:2 spin–orbit resonance, meaning that relative to the background stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimal and rock vaporization.īecause Mercury is very close to the Sun, the intensity of sunlight on its surface is between 4.59 and 10.61 times the solar constant (amount of the Sun's energy received at 1 astronomical unit, which is roughly the distance between Earth and the Sun). At Mercury's poles though, there are large reservoirs of water ices that are never exposed to direct sunlight, which has an estimated mass of about 0.025–0.25% the Antarctic ice sheet. Having almost no atmosphere to retain heat, Mercury has surface temperatures that change wildly during the day, ranging from 100 K (−173 ☌ −280 ☏) at night to 700 K (427 ☌ 800 ☏) during sunlight across the equator regions. Mercury has a dynamic magnetic field with a strength about 1% of that of Earth's and has no natural satellites.Īccording to current theories, Mercury may have a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. Despite being the smallest planet in the Solar System with a mean diameter of 4,880 km (3,030 mi), 38% of that of Earth's, Mercury is dense enough to have roughly the same surface gravity as Mars. It is a terrestrial planet with a heavily cratered surface due to the planet having no geological activity and an extremely tenuous atmosphere (called an exosphere).
Mercury is the first planet from the Sun and the smallest planet in the Solar System.
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