![]() Mercury photographed by MESSENGER spacecraft in 2008 | |||||||||||||
Designations | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Pronunciation | /ˈmɜrkjəri/ (![]() | ||||||||||||
Adjectives | Mercurian,[1] | ||||||||||||
Orbital characteristics[4] | |||||||||||||
Epoch J2000 | |||||||||||||
Aphelion | 69,816,900 km 0.466 697 AU | ||||||||||||
Perihelion | 46,001,200 km 0.307 499 AU | ||||||||||||
57,909,100 km 0.387 098 AU | |||||||||||||
Eccentricity | 0.205 630[2] | ||||||||||||
87.969 1 d (0.240 846 a) | |||||||||||||
115.88 d[2] | |||||||||||||
Average orbital speed | 47.87 km/s[2] | ||||||||||||
174.796° | |||||||||||||
Inclination | 7.005° to Ecliptic 3.38° to Sun’s equator 6.34° to Invariable plane[3] | ||||||||||||
48.331° | |||||||||||||
29.124° | |||||||||||||
Known satellites | None diameter = 4,880 km | ||||||||||||
Physical characteristics | |||||||||||||
Mean radius | 2,439.7 ± 1.0 km[5][6] 0.3829 Earths | ||||||||||||
Flattening | < 0.0006[6] | ||||||||||||
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] | ||||||||||||
3.7 m/s² 0.38 g[5] | |||||||||||||
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) | ||||||||||||
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] | ||||||||||||
| |||||||||||||
up to −1.9[2] | |||||||||||||
4.5" – 13"[2] | |||||||||||||
Atmosphere | |||||||||||||
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] | ||||||||||||
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 we can see it from Earth. It has an apparent magnitude ranging from −2.0 to 5.5. It cannot be seen easily because it is usually too close to the Sun. Because of this, Mercury can only be seen in the morning or evening twilight[13] or when there is a solar eclipse.
Less is known about Mercury than about other planets of our Solar System. Even with telescopes only a small, bright crescent can be seen. It is also hard to put a satellite in orbit around it. Two spacecraft have visited Mercury. The first one was Mariner 10.[14] It only made a map of about 45% of the Mercury's surface from 1974 to 1975. The second is the MESSENGER spacecraft, which finished mapping Mercury in March 2013.
Mercury looks like Earth's Moon. It has many craters and smooth plains. It has no moons and little atmosphere as we know it. However, Mercury does have an extremely thin atmosphere, known as an exosphere.[11] Mercury has a large iron core. Because of this Mercury has a magnetic field about 1% as strong as that of the Earth.[15] It is a very dense planet because its core is large.
Temperature at the surface 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: The one that they were only able at sunrise, they called Apollo; the other one that they were only able to see at sunset, they called Hermes.[17] The English name for the planet is from the Romans, who named it after the Roman god Mercury. The symbol for Mercury is based on Hermes' staff.[18]
Even though Mercury is the closest planet to the Sun, it is not the hottest. This is because it has no greenhouse effect. The heat that the Sun gives it, quickly escapes into space. The hottest planet is Venus.[19]
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The symbol for Mercury represents the Caduceus, a wand with two serpents twined around it, which was carried by the messenger of the gods.