Mercury (planet)

Mercury

Mercury in color, by MESSENGER (2008)
Designations
Pronunciation	i/ˈmɜːrkjəri/
Adjectives	Mercurian, mercurial
Orbital characteristics
Epoch J2000
Aphelion	0.466 697 AU 69,816,900 km
Perihelion	0.307 499 AU 46,001,200 km
Semi-major axis	0.387 098 AU 57,909,050 km
Eccentricity	0.205 630
Orbital period	87.969 1 d 0.240 846 yr 0.5 Mercury solar day
Synodic period	115.88 d
Average orbital speed	47.362 km/s
Mean anomaly	174.796°
Inclination	7.005° to ecliptic 3.38° to Sun's equator 6.34° to invariable plane
Longitude of ascending node	48.331°
Argument of perihelion	29.124°
Satellites	None
Physical characteristics
Mean radius	7006243970000000000♠2,439.7±1.0 km 6999382900000000000♠0.3829 Earths
Flattening	0
Surface area	7013748000000000000♠7.48×107 km2 0.147 Earths
Volume	7019608300000000000♠6.083×1010 km3 0.056 Earths
Mass	7023330109999999999♠3.3011×1023 kg 0.055 Earths
Mean density	7003542700000000000♠5.427 g/cm3
Surface gravity	7000370000000000000♠3.7 m/s2 0.38 g
Moment of inertia factor	6999346000000000000♠0.346±0.014
Escape velocity	4.25 km/s
Sidereal rotation period	7006506701440000000♠58.646 d 7006506700000000000♠1407.5 h
Equatorial rotation velocity	10.892 km/h (3.026 m/s)
Axial tilt	2.04′ ± 0.08′ (to orbit) (0.034°)
North pole right ascension	18h 44m 2s 281.01°
North pole declination	61.45°
Albedo	0.068 (Bond) 0.142 (geom.)
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	−2.6 to 5.7
Angular diameter	4.5–13″
Atmosphere
Surface pressure	trace (≲ 0.5 nPa)
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, and neon

Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System. It is named after the Roman deity Mercury, the messenger to the gods.

Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, so it can only be seen visually in the morning or the evening sky, and never exceeds 28° away from the Sun. Also, like Venus and the Moon, the planet displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, the so-called synodic period. Although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus.

Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance, and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

Mercury's axis has the smallest tilt of any of the Solar System's planets (about  ¹⁄₃₀ degree), and its orbital eccentricity is the largest of all known planets in the Solar System. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface appears heavily cratered and is similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, surface temperatures varies diurnally more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no known natural satellites.