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Solar System > Planets > Mercury v



Mercury
    

The closest planet to the Sun.

Prior to 1965, astronomers believed that Mercury's sidereal rotation matched its orbital period of 88 days (synchronous rotation). This belief was reinforced by the chance coincidence of six 58.65-day rotation periods (352 days) closely matching the synodic period of Mercury's maximum elongation (350 days). Therefore, observers saw the same features at every opportunity for several years in a row. However, in 1965, G. Pettengill and R. Byce bounced radar signals off the planet using the 300-meter Arecibo Radio Observatory in Puerto Rico and discovered a rotation rate of 59 days, proving that Mercury was actually in a 3:2 resonance such that Mercury's day is exactly 2/3 of its 88-day year.

Mercury has been visited by only one spacecraft. Mariner 10 flew by three times in 1974-1975. Unfortunately, the orbital geometry was such that Mariner 10 photographed the same face all three times.

Because of torques from the Sun, Mercury advances its perihelion at a rate of 5,600 arcseconds per century. According to observations, there was an extra 43 arcseconds per century which could not be accounted for. This discrepancy was resolved in 1915, when the theory of general relativity Eric Weisstein's World of Physics predicted an additional perihelion precession of exactly 43 arcseconds per year.

Because of its proximity to the Sun, Mercury has 700 K days, while temperatures on its night side are a frigid 100 K. Mercury has a heavily cratered surface. Kuiper, the largest crater, is 60 km across, and crater Copley has bright rays which extend for 400 km. Scarps also abound on Mercury, the largest being Discovery Rupes (550 km long). Although Mercury has plains such as Tir and Odin, it has peculiar hilly terrain on the side of the planet opposite the Caloris Basin, where a large meteorite is assumed to have impacted. Mercury has no atmosphere, no moons, and a magnetic field only times as strong as Earth's. Its crust and mantle are thought to be composed of silicates, and its core is thought to be nickel-iron. Mercury is twice as iron-rich as any other planet.

Slade et al. (1992) detected permanently shaded ice at the poles of Mercury using synthesis imaging of transmitted radar. This discovery was completely unexpected, but subsequent thermal modelling shows that it is indeed possible for ice to persist stably for billions of years in the shaded portion of craters near the poles.

Mercury transits (passes in front of the Sun as seen from the Earth) 5-7 times every 46 years. The transits occur during daylight hours around November 10 or May 9 in the following years (where * indicates a May transit):

1677     1690   1697 1707* 1710
1723     1736 1740* 1743 1753* 1756
1769   1776 1782 1786* 1789 1799* 1802
1815   1822   1832* 1835 1845* 1848
1861   1868   1878* 1881 1891* 1894
1907   1914   1924* 1927   1940
1953 1957* 1960   1970* 1973   1986
1999 2003* 2006   2016* 2019   2032

The transits repeat roughly every 46 years after between 5 and 7 transits, the intervals being either 3.5, 7, 9.5 or 13 years. 145 Mercury synodic orbits equal 46 solar years plus 1.1 days. Transits of Mercury can last up to 6 hours.

Earth, Jupiter, Mars, Neptune, Pluto, Saturn, Uranus




References

Arnett, W. "The Nine Planets: Mercury." Nine Planets.

"Mercury's Cool Surprise." Sky & Telescope, pp. 35-36, Jan. 1992.

Baum, R. and Sheehan, W. In Search of Planet Vulcan: The Ghost in Newton's Clockwork Universe. New York: Plenum, 1997.

Cross, C. A. and Moore, P. The Atlas Of Mercury. New York: Crown Publishing, 1977.

Davies, M. E. et al. Atlas of Mercury. Washington, DC: NASA Scientific and Technical Information Office, 1978.

Murray, B. and Burgess, E. Flight to Mercury. New York: Columbia, 1977.

Shklovskii, I. S. and Sagan, C. "Mercury and Venus: Environmental Biology." Ch. 22 in Intelligent Life in the Universe. New York: Dell, pp. 312-325, 1966.

Slade, M. A., Butler, B. J., and Muhleman, D. O. "Mercury Radar Imaging: Evidence For Polar Ice." Science 258, 635-640, 1992.

Strom, R. G. "Mercury: The Forgotten Planet." Sky & Telescope, 256-260, Sept. 1990.

Vilas, F; Chapman, C. R.; and Matthews, M. S. (Eds.). Mercury. Tucson, AZ: University of Arizona Press, 1988.

Weisstein, E. W. "Books about Mercury." http://www.ericweisstein.com/encyclopedias/books/Mercury.html.