Mercury (planet)

Mercury

Mercury photographed by MESSENGER spacecraft in 2008
Designations
Pronunciation	/ˈmɜrkjəri/ (listen)
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
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 diameter = 4,880 km
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
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]

1. ↑ "Mercurian". Oxford University Press. Archived from the original on 2016-12-20. Retrieved 2016-12-15.
2. ↑ ^{2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09} "Mercury Fact Sheet". NASA Goddard Space Flight Center. November 30, 2007. Archived from the original on 2015-11-06. Retrieved 2008-05-28.
3. ↑ "The MeanPlane (Invariable plane) of the Solar System passing through the barycenter". 2009-04-03. Archived from the original on 2009-05-14. Retrieved 2009-04-03. (produced with Solex 10 Archived 2003-04-13 at Archive.today written by Aldo Vitagliano; see also Invariable plane)
4. ↑ Yeomans, Donald K. (April 7, 2008). "HORIZONS System". NASA JPL. Archived from the original on 2020-05-23. Retrieved 2008-04-07.
5. ↑ ^{5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7} Munsell, Kirk; Smith, Harman; Harvey, Samantha (May 28, 2009). "Mercury: Facts & Figures". Solar System Exploration. NASA. Archived from the original on 2002-11-19. Retrieved 2008-04-07.{{cite web}}: CS1 maint: multiple names: authors list (link)
6. ↑ ^{6.0 6.1} Seidelmann, P. Kenneth; Archinal, B. A.; A’hearn, M. F.; et al. (2007). "Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006". Celestial Mechanics and Dynamical Astronomy. 90 (3): 155–180. Bibcode:2007CeMDA..98..155S. doi:10.1007/s10569-007-9072-y. ISSN 0923-2958. S2CID 122772353. Archived from the original on 2019-05-19. Retrieved 2007-08-28.{{cite journal}}: CS1 maint: multiple names: authors list (link)
7. ↑ Margot, L.J. (2007). "Large Longitude Libration of Mercury Reveals a Molten Core". Science. 316 (5825). Peale, S.J.; Jurgens, R.F.; Slade, M.A.; Holin, I.V.: 710–714. Bibcode:2007Sci...316..710M. doi:10.1126/science.1140514. PMID 17478713. S2CID 8863681. Archived from the original on 2017-09-02. Retrieved 2009-08-03.
8. ↑ "Mercury Facts | Information, History, Location, Size & Definition". nineplanets.org. July 2020 [last update]. Archived from the original on 24 November 2015. Retrieved 17 August 2020.
9. ↑ "BBC Solar System - Mercury: A tortured world close to our blazing Sun". bbc.co.uk. 2012 [last update]. Archived from the original on 1 October 2011. Retrieved 7 March 2012.
10. ↑ Pluto used to be the smallest, but, as of 2006, Pluto is now known as a dwarf planet.
11. ↑ ^{11.0 11.1} Murchie, Scott L.; Vervack Jr., Ronald J.; Anderson, Brian J. (March 2011), "Space Science: Journey to the Innermost Planet", Scientific American, vol. 304, no. 3, New York, pp. 26–31
12. ↑ "Venus year is 224". Archived from the original on 2010-06-07. Retrieved 2010-08-26.
13. ↑ How it Works Book of Space. Imagine Publishing. 2010. p. 36-37. ISBN 9781906078829.
14. ↑ "NASA - NSSDC - Spacecraft - Details". Archived from the original on 2009-07-30. Retrieved 2010-08-26.
15. ↑ "Mercury magnetic field". C. T. Russell & J. G. Luhmann. Archived from the original on 2010-07-19. Retrieved 2007-03-16.
16. ↑ "Background Science". European Space Agency. Archived from the original on 2010-01-17. Retrieved 2008-05-23.
17. ↑ Dunne, J. A. and Burgess, E. (1978). "Chapter One". The Voyage of Mariner 10 — Mission to Venus and Mercury. NASA History Office. Archived from the original on 2011-05-24. Retrieved 2009-08-03.{{cite book}}: CS1 maint: multiple names: authors list (link)
18. ↑ Duncan, John Charles (1946). Astronomy: A Textbook. Harper & Brothers. pp. 125. The symbol for Mercury represents the Caduceus, a wand with two serpents twined around it, which was carried by the messenger of the gods.
19. ↑ "CBBC Newsround". British Broadcasting Corporation. Archived from the original on February 24, 2012. Retrieved February 4, 2012.