Mercury (Planet)

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Mercury is the first planet from the Sun and the smallest in the Solar System. It is a terrestrial planet with a heavily cratered surface due to overlapping impact events. These features are well preserved since the planet has no geological activity and an extremely tenuous atmosphere called an exosphere. 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, Mercury is dense enough to have roughly the same surface gravity as Mars. Mercury has a dynamic magnetic field with a strength about 1% of that of Earth's and has no natural satellites.

According to current models, Mercury may have a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. Having almost no atmosphere to retain heat, Mercury has surface temperatures that change wildly during the day, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during sunlight across the equator regions. At Mercury's poles 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. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimal and rock vaporization.

Because Mercury is very close to the Sun, the intensity of sunlight on its surface is between 4.59 and 10.61 times the Sun's typical energy received by the Earth: the solar constant. 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. 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'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.

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 is relatively close to the Sun. In English, it is named after the Roman god Mercurius (Mercury), god of commerce, communication and the messenger of gods. Mercury is the most difficult planet to reach from Earth because it requires the greatest change in a spacecraft's velocity. Only three spacecraft have visited Mercury as of 2023: Mariner 10 flew by in 1974 and 1975; MESSENGER , launched in 2004, flew by Mercury first in 2008 and orbited it over 4,000 times between 2011 and 2015; the BepiColombo spacecraft flew by Mercury first in 2021, and it is scheduled to make its final arrival at Mercury in 2025, where it will then insert two orbiters.