Mercury (planet)

Mercury is the smallest planet in the Solar System.^[8][9][10] It is the closest planet to the sun.^[11] It makes one trip around the Sun once every 87.969 days.^[2][12] Mercury is bright when it is visible from Earth, ranging from −2.0 to 5.5 in apparent magnitude. It cannot be easily seen as it is usually too close to the Sun. Because Mercury is normally lost in the glare of the Sun (except during a solar eclipse), Mercury can only be seen in the morning or evening twilight.^[13]

Mercury (Hermes)

Mercury
Designations
Pronunciation	/ˈmɜrkjəri/ (listen)
Adjectives	Mercurian, Mercurial[1]
Orbital characteristics[4]
Epoch J2000
Aphelion	69,816,900 km 0.466 697 AU
Perihelion	46,001,200 km 0.307 499 AU
Semi-major axis	57,909,100 km 0.387 098 AU
Eccentricity	0.205 630[2]
Orbital period	87.969 1 d (0.240 846 a)
Synodic period	115.88 d[2]
Average orbital speed	47.87 km/s[2]
Mean anomaly	174.796°
Inclination	7.005° to Ecliptic 3.38° to Sun’s equator 6.34° to Invariable plane[3]
Longitude of ascending node	48.331°
Argument of perihelion	29.124°
Known satellites	None
Physical characteristics
Mean radius	2,439.7 ± 1.0 km[5][6] 0.3829 Earths
Flattening	< 0.0006[6]
Surface area	7.48×107 km² 0.147 Earths[5]
Volume	6.083×1010 km³ 0.054 Earths[5]
Mass	3.3022×1023 kg 0.055 Earths[5]
Mean density	5.427 g/cm³[5]
Surface gravity	3.7 m/s² 0.38 g[5]
Escape velocity	4.25 km/s[5]
Sidereal rotation period	58.646 day 1407.5 h[5]
Equatorial rotation velocity	10.892 km/h (3.026 m/s)
Axial tilt	2.11′ ± 0.1′[7]
North pole right ascension	18 h 44 min 2 s 281.01°[2]
North pole declination	61.45°[2]
Albedo	0.119 (bond) 0.106 (geom.)[2]
Surface temp. min mean max 0°N, 0°W 100 K 340 K 700 K 85°N, 0°W 80 K 200 K 380 K
Apparent magnitude	up to −1.9[2]
Angular diameter	4.5" – 13"[2]
Atmosphere
Surface pressure	trace
Composition by volume	42% Molecular oxygen 29.0% sodium 22.0% hydrogen 6.0% helium 0.5% potassium Trace amounts of argon, nitrogen, carbon dioxide, water vapor, xenon, krypton, & neon[2]

Compared to what is known about the other planets in the Solar System, little is known about Mercury. Telescopes on the Earth show only a small, bright crescent. The first of two spacecraft to visit the planet was Mariner 10,^[14] which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which mapped another 30% during its trip to Mercury on January 14, 2008. MESSENGER made one more pass by Mercury in 2009, and is now orbiting the planet to complete the mapping.

Mercury looks a lot like Earth's Moon. It has many craters with areas of smooth plains, no moons around it and no atmosphere as we know it. However, Mercury does have an extremely thin atmosphere, known as an exosphere.^[11] Unlike Earth's moon, Mercury has a large iron core, which gives off a magnetic field about 1% as strong as that of the Earth.^[15] It is a very dense planet due to the large size of its core. Surface temperatures can be anywhere from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F),^[16] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Known sightings of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers thought that Mercury was two different objects: one able to be seen only at sunrise, which they called Apollo; the other that was only able to be seen at sunset, which they called Hermes.^[17] The English name for the planet is from the Romans, who named it after the Roman god Mercury, which they thought to be the same as the Greek god Hermes. The symbol for Mercury is based on Hermes' staff.^[18]

Even though Mercury is the closest planet to the Sun, it is not the warmest. This is because it has no atmosphere, so any heat that the Sun gives to it quickly escapes into space.^[19]