M-type asteroid
Encyclopedia
M-type asteroids are asteroid
s of partially known composition; they are moderately bright (albedo
0.1–0.2). Some, but not all, are made of nickel
-iron
, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts, and are thought to be the source of iron meteorite
s. M-type asteroids are the third most common asteroid type.
There are also M-types whose composition is uncertain. For example, 22 Kalliope
has an accurately known density that is far too low for a solid metallic object or even a metal rubble pile
: a rubble pile of iron-nickel metal would need about 70% porosity which is inconsistent with packing considerations. Kalliope and 21 Lutetia
have features in their spectra which appear to indicate the presence of hydration minerals
and silicate
s, anomalously low radar albedos inconsistent with a metallic surface, as well as characteristics more in common with C-type asteroid
s. A variety of other M-type asteroids do not fit well into a metallic body picture.
M-type spectra are flat to reddish and usually devoid of large features, although subtle absorption features longward of 0.75 µm and shortward of 0.55 µm are sometimes present.
is the largest M-type asteroid, and does appear to be metallic. 21 Lutetia
, an anomalous, probably non-metallic body, was the first M-type asteroid to be imaged by a spacecraft when the Rosetta space probe visited it on July 10, 2010. Another M-type, 216 Kleopatra
, was imaged by radar by the Arecibo Observatory
in Puerto Rico
and has a dog bone
-like shape.
M type was one of three basic asteroid types in early classifications (the others being the S
and C
types), and was thought to indicate a metallic body.
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s of partially known composition; they are moderately bright (albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
0.1–0.2). Some, but not all, are made of nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts, and are thought to be the source of iron meteorite
Iron meteorite
Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...
s. M-type asteroids are the third most common asteroid type.
There are also M-types whose composition is uncertain. For example, 22 Kalliope
22 Kalliope
22 Kalliope is a large main belt M-type asteroid discovered by J. R. Hind on November 16, 1852. It is named after Calliope, the Greek Muse of epic poetry...
has an accurately known density that is far too low for a solid metallic object or even a metal rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...
: a rubble pile of iron-nickel metal would need about 70% porosity which is inconsistent with packing considerations. Kalliope and 21 Lutetia
21 Lutetia
21 Lutetia is a large main-belt asteroid of an unusual spectral type. It measures about 100 kilometers in diameter . It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name for Paris....
have features in their spectra which appear to indicate the presence of hydration minerals
Mineral hydration
Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....
and silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s, anomalously low radar albedos inconsistent with a metallic surface, as well as characteristics more in common with C-type asteroid
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
s. A variety of other M-type asteroids do not fit well into a metallic body picture.
M-type spectra are flat to reddish and usually devoid of large features, although subtle absorption features longward of 0.75 µm and shortward of 0.55 µm are sometimes present.
Observational history
16 Psyche16 Psyche
16 Psyche )is one of the ten most massive main-belt asteroids. It is over 200 kilometers in diameter and contains a little less than 1% of the mass of the entire asteroid belt. It is the most massive of the metallic M-type asteroids....
is the largest M-type asteroid, and does appear to be metallic. 21 Lutetia
21 Lutetia
21 Lutetia is a large main-belt asteroid of an unusual spectral type. It measures about 100 kilometers in diameter . It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name for Paris....
, an anomalous, probably non-metallic body, was the first M-type asteroid to be imaged by a spacecraft when the Rosetta space probe visited it on July 10, 2010. Another M-type, 216 Kleopatra
216 Kleopatra
216 Kleopatra is a trinary main-belt asteroid that was discovered by Johann Palisa on April 10, 1880, in Pola. It is named after Cleopatra, Queen of Ancient Egypt....
, was imaged by radar by the Arecibo Observatory
Arecibo Observatory
The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...
in Puerto Rico
Puerto Rico
Puerto Rico , officially the Commonwealth of Puerto Rico , is an unincorporated territory of the United States, located in the northeastern Caribbean, east of the Dominican Republic and west of both the United States Virgin Islands and the British Virgin Islands.Puerto Rico comprises an...
and has a dog bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
-like shape.
M type was one of three basic asteroid types in early classifications (the others being the S
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
and C
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
types), and was thought to indicate a metallic body.