Mercury

Mercury is the chemical element with the symbol Hg and atomic number 80. It is a dense, silvery liquid metal at room temperature—the only metal that is liquid under ordinary conditions. Its melting point is -38.83 C and its boiling point 356.73 C, with a density of about 13.534 g/cm³. Mercury forms amalgams with many metals and is highly toxic if inhaled or ingested, posing health and environmental risks. Historically, it has been used in thermometers, barometers, and dental fillings; today its use is restricted and regulated in many applications. The name derives from hydrargyrum, from Greek hydr- water and argyros- silver.

Mercury is the closest planet to the Sun and the smallest in the Solar System since Pluto’s reclassification. Its radius is about 2,439.7 km and its mass about 3.30 × 10^23 kg. It orbits the Sun every 87.97 Earth days with a highly elliptical orbit (eccentricity about 0.2056) at an average distance of 0.39 astronomical units. Mercury has a heavily cratered, moon-like surface and is cut by large tectonic scarps formed as it cooled. Its rotation is in a 3:2 resonance, so a solar day is about 176 Earth days. The planet has a tenuous exosphere rather than a substantial atmosphere, and a weak magnetic field likely generated by a partially molten core. Evidence from spacecraft indicates possible water ice in permanently shadowed polar regions. Missions include MESSENGER, which mapped the planet, and the ongoing BepiColombo mission.