M-type (metallic-type, aka M-class) asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes, [1] and are widely thought to be the source of iron meteorites. [2]
Asteroids are classified as M-type based upon their generally featureless and flat to red-sloped absorption spectra in the visible to near-infrared and their moderate optical albedo. Along with the spectrally similar E-type and P-type asteroids (both categories E and P were formerly type-M in older systems), they are included in the larger X-type asteroid group and are distinguishable only by optical albedo: [3]
Although widely assumed to be metal-rich (the reason for use of "M" in the classification), the evidence for a high metal content in the M-type asteroids is only indirect, though highly plausible. Their spectra are similar to those of iron meteorites and enstatite chondrites, [4] and radar observations have shown that their radar albedos are much higher than other asteroid classes, [5] consistent with the presence of higher density compositions like iron-nickel. [1] Nearly all of the M-types have radar albedos at least twice as high as the more common S- and C-type, and roughly one-third have radar albedos ~3× higher. [1]
High resolution spectra of the M-type have sometimes shown subtle features longward of 0.75 μm and shortward of 0.55 μm. [6] The presence of silicates is evident in many, [7] [8] and a significant fraction show evidence of absorption features at 3 μm, attributed to hydrated silicates. [9] The presence of silicates, and especially hydrated silicates, is at odds with the traditional interpretation of M-types as remnant iron cores.
The bulk density of an asteroid provides clues about its composition and meteoritic analogs. [10] For the M-types, the proposed analogs have bulk densities that range from ~3 g/cm3 for some types of carbonaceous chondrites up to nearly 8 g/cm3 for the iron-nickel present in iron-meteorites. [2] [4] [9] Given the bulk density of an asteroid and the density of the materials that make it up (aka particle or grain density), one can calculate its porosity and infer something of its internal structure; for example, whether an object is coherent, a rubble pile, or something in-between. [10]
To calculate the bulk density of an asteroid requires an accurate estimate of its mass and volume; both of these are difficult to obtain given their small size relative to other solar system objects. In the case of the larger asteroids, one can estimate mass by observing how their gravitational field affects other objects, including other asteroids and orbiting or flyby spacecraft. [11] If an asteroid possesses one or more moons, one can use their collective orbital parameters (e.g. orbital period, semimajor axis) to estimate the masses of the ensemble, for example in the two-body problem.
To estimate an asteroid's volume requires, at a minimum, an estimate of an asteroid's diameter. In most cases, these are estimated from the visual albedo (brightness) of the asteroid, chord-lengths during occultations, or their thermal emissions (e.g. IRAS mission). In a few cases, astronomers have managed to develop three-dimensional shape models using a variety of techniques (c.f. 16 Psyche or 216 Kleopatra for examples) or, in a few lucky instances, from spacecraft imaging (c.f 162173 Ryugu).
| Asteroid | Density | Radar Albedo | Method (mass, size) |
|---|---|---|---|
| 16 Psyche | 3.8 ± 0.3 [12] | 0.34 ± 0.08 [13] | Ephemeris, shape model |
| 21 Lutetia | 3.4 ± 0.3 [14] | 0.24 ± 0.07 [1] | Rosetta spacecraft flyby, direct imaging |
| 22 Kalliope | 4.1 ± 0.5 [15] [16] | 0.15 ± 0.05 [5] | Orbit of its moon Linus, shape model |
| 69 Hesperia | 4.4 ± 1.0 [17] | 0.45 ± 0.12 [1] | Ephemeris, thermal IR/radar size estimate |
| 92 Undina | 4.4 ± 0.4 [17] | 0.38 ± 0.09 [1] | Ephemeris, thermal IR/radar size estimate |
| 129 Antigone | 3.0 ± 1.0 [17] | 0.36 ± 0.09 [1] | Ephemeris, thermal IR/radar size estimate |
| 216 Kleopatra | 3.4 ± 0.5 [18] | 0.43 ± 0.10 [19] | Orbits of its two moons, shape model |
Of these, mass measurements made via spacecraft deflection or the orbits of moons are considered the most reliable. Ephemeris estimates are based on the subtle gravitational pull of other objects on that asteroid, or vice versa, and are considered less reliable. The exception to this caveat may be Psyche, as it is the most massive M-type asteroid and has numerous mass estimates. [12] Size estimates based on shape models (usually derived from adaptive optics, occultations, and radar imaging) are the most reliable. Direct spacecraft imaging (Lutetia) is also quite reliable. Sizes based on indirect methods like thermal IR (e.g. IRAS) and radar echoes are less reliable.
None of the M-type asteroids have bulk densities consistent with a pure iron-nickel core. If these objects are porous (aka rubble-piles), then that interpretation may still hold; this is unlikely for Psyche, [12] because of its large size. Given the spectral evidence of silicates on most M-type asteroids, the consensus interpretation for most of these larger asteroids is that they are composed of lower density meteorite analogs (e.g. enstatite chondrites, metal-rich carbonaceous chondrites, mesosiderites), and in some cases may also be rubble piles. [20] [18] [12]
The earliest interpretation of the M-type asteroids was that they were the remnant cores of early protoplanets, stripped of their overlying crust and mantles by massive collisions that are thought to have been frequent in the early history of the solar system. [2]
It is acknowledged that some of the smaller M-type asteroids (<100 km) may have formed in this way, but that interpretation was challenged for 16 Psyche, the largest of the M-type asteroids. [21] There are three arguments against Psyche forming in this way. [21] First, it must have started as a Vesta-sized (~500 km) protoplanet; statistically, it is unlikely that Psyche was completely disrupted while Vesta remained intact. Second, there is little or no observational evidence for an asteroid family associated with Psyche, and third, there is no spectroscopic evidence for the expected mantle fragments (i.e. olivine) that would have resulted from this event. Instead, it has been argued that Psyche is the remnant of a protoplanet that was shattered and gravitationally re-accumulated into a well-mixed iron-silicate object. [21] There are numerous examples of metal-silicate meteorites, aka mesosiderites, that might be objects from such a parent body.
One possible response to this second interpretation is that the M-type asteroids (including 16 Psyche) accumulated much closer to the Sun (1–2 au), were stripped of their thin crust/mantles while still molten (or partially so), and later dynamically moved into the current asteroid belt. [22]
A third view is that the largest M-types, including 16 Psyche, may be differentiated bodies (like 1 Ceres and 4 Vesta) but, given the right mix of iron and volatiles (e.g. sulfur), these bodies may have experienced a type of iron volcanism, a.k.a. ferrovolcanism, while still cooling. [23]
In the JPL Small Body Database, there are 980 asteroids classified under the Tholen asteroid spectral classification system. [24] Of those, 38 are classified as M-type. [25] Another 10 were originally classified as X-type, but are now counted among the M-types because their optical albedos fall between 0.1 and 0.3. [26] Overall, the M-types make up approximately 5% of the asteroids classified under the Tholen taxonomy.
16 Psyche is the largest M-type asteroid with a mean diameter of 222 km, and has a relatively high mean radar albedo of suggesting it has a high metal content in the upper few meters of its surface. [13] The Psyche spacecraft was launched on october 13th, 2023 is en route to visit 16 Psyche, arriving in 2029.
21 Lutetia has a mean diameter of 100 km, [1] and was the first M-type asteroid to have been imaged by a spacecraft when the Rosetta space probe visited it on 10 July 2010. [27] Its mean radar albedo of is roughly twice that of the average S-type or C-type asteroid, and suggests its regolith contains an elevated amount of metal phases relative to other asteroid classes. [1] Analysis using data from the Rosetta spectrometer (VIRTIS) was consistent with estatitic or iron-rich carbonaceous chondritic materials. [28]
22 Kalliope is the second largest M-type asteroid with a mean diameter of 150 km. [15] A single moon, named Linus, was discovered in 2001 [29] and allows for an accurate mass estimate. Unlike most of the M-type asteroids, Kalliope's radar albedo is 0.15, similar to the S- and C-type asteroids, [5] and does not suggest an enrichment of metal in its regolith. It has been the target of high resolution adaptive optics imaging which has been used to provide a reliable size and shape, and a relatively high bulk density of 4.1 g/cm3. [15] [16]
216 Kleopatra, with a mean diameter of 122 km, is the third largest M-type asteroid known after 16 Psyche and 22 Kalliope. [19] Radar delay-Doppler imaging, high-resolution telescopic images, and several stellar occultations show it to be a contact binary asteroid with a shape commonly referred to as a "dog-bone" or "dumbbell." [19] Radar observations from the Arecibo radar telescope indicate a very high radar albedo of in the southern hemisphere, consistent with a metal-rich composition. [19] Kleopatra is also notable for the presence of two small moons, named Alexhelios and Cleoselena, which have allowed its mass and bulk density to be accurately computed. [30]
16 Psyche is a large M-type asteroid, which was discovered by the Italian astronomer Annibale de Gasparis, on 17 March 1852 and named after the Greek goddess Psyche. The prefix "16" signifies that it was the sixteenth minor planet in order of discovery. It is the largest and most massive of the M-type asteroids, and one of the dozen most massive asteroids. It has a mean diameter of approximately 220 kilometers (140 mi) and contains about one percent of the cumulative mass of the whole asteroid belt. It was thought to be the exposed core of a protoplanet, but recent observations cast doubt on that hypothesis. Psyche will be explored by NASA, with a spacecraft of the same name, marking the first time a manmade object will journey to a metallic asteroid, launched on 13 October 2023, with an expected arrival in 2029.
243 Ida is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. Later telescopic observations categorized Ida as an S-type asteroid, the most numerous type in the inner asteroid belt. On 28 August 1993, Ida was visited by the uncrewed Galileo spacecraft while en route to Jupiter. It was the second asteroid visited by a spacecraft and the first found to have a natural satellite.
S-type asteroids are asteroids with a spectral type that is indicative of a siliceous mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are of this type, making it the second-most common after the carbonaceous C-type.
1620 Geographos is a highly elongated, stony asteroid, near-Earth object and potentially hazardous asteroid of the Apollo group, with a mean diameter of approximately 2.5 km (1.6 mi). It was discovered on 14 September 1951, by astronomers Albert George Wilson and Rudolph Minkowski at the Palomar Observatory in California, United States. The asteroid was named in honor of the National Geographic Society.
C-typeasteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks and minerals. They have an average density of about 1.7 g/cm3.
P-type (primitive-type) asteroidss have low albedo and a featureless reddish spectrum. It has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their interior. P-type asteroids are found in the outer asteroid belt and beyond. There are about 33 known P-type asteroids, depending on the classification, including 46 Hestia, 65 Cybele, 76 Freia, 87 Sylvia, 153 Hilda, 476 Hedwig and, in some classifications, 107 Camilla.
D-type asteroids have a very low albedo and a featureless reddish spectrum. It has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors. D-type asteroids are found in the outer asteroid belt and beyond; examples are 152 Atala, 944 Hidalgo and most Jupiter trojans. It has been suggested that the Tagish Lake meteorite was a fragment from a D-type asteroid, and that the Martian moon Phobos is closely related.
21 Lutetia is a large M-type asteroid in the main asteroid belt. It measures about 100 kilometers in diameter. It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name of Paris.
216 Kleopatra is a large M-type asteroid with a mean diameter of 120 kilometers and is noted for its elongate bone or dumbbell shape. It was discovered on 10 April 1880 by Austrian astronomer Johann Palisa at the Austrian Naval Pola Observatory, in what is now Pula, Croatia, and was named after Cleopatra, the famous Egyptian queen. It has two small minor-planet moons which were discovered in 2008 and later named Alexhelios and Cleoselene.
25143 Itokawa (provisional designation 1998 SF36) is a sub-kilometer near-Earth object of the Apollo group and a potentially hazardous asteroid. It was discovered by the LINEAR program in 1998 and later named after Japanese rocket engineer Hideo Itokawa. The peanut-shaped S-type asteroid has a rotation period of 12.1 hours and measures approximately 330 meters (1,100 feet) in diameter. Due to its low density and high porosity, Itokawa is considered to be a rubble pile, consisting of numerous boulders of different sizes rather than of a single solid body.
(53319) 1999 JM8 is an asteroid, slow rotator and tumbler, classified as a near-Earth object and potentially hazardous asteroid (PHA) of the Apollo group, approximately 7 kilometers (4 miles) in diameter, making it the largest PHA known to exist. It was discovered on 13 May 1999, by astronomers of the Lincoln Near-Earth Asteroid Research at the Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico.
92 Undina is a large main belt asteroid. The asteroid was discovered by Christian Peters on 7 July 1867 from the Hamilton College Observatory. It is named for the eponymous heroine of Undine, a popular novella by Friedrich de la Motte Fouqué.
97 Klotho is a fairly large main-belt asteroid. While it is an M-type, its radar albedo is too low to allow a nickel-iron composition. Klotho is similar to 21 Lutetia and 22 Kalliope in that all three are M-types of unknown composition. Klotho was found by Ernst Tempel on February 17, 1868. It was his fifth and final asteroid discovery. It is named after Klotho or Clotho, one of the three Moirai, or Fates, in Greek mythology.
22 Kalliope is a large M-type asteroid from the asteroid belt discovered by J. R. Hind on 16 November 1852. It is named after Calliope, the Greek Muse of epic poetry. It is orbited by a small moon named Linus.
(6178) 1986 DA is a metallic asteroid, classified as near-Earth object of the Amor group, approximately 3 kilometers in diameter. It was discovered on 16 February 1986, by Japanese astronomer Minoru Kizawa at Shizuoka Observatory, Japan.
337 Devosa is a large Main belt asteroid. It was discovered by Auguste Charlois on 22 September 1892 in Nice. The asteroid is orbiting the Sun at a distance of 2.38 AU with a period of 3.68 years and an eccentricity (ovalness) of 0.14. These orbital elements are similar to that of the large asteroid 4 Vesta. The orbital plane of 337 Devosa is tilted at an angle of 7.85° to the plane of the ecliptic.
An asteroid spectral type is assigned to asteroids based on their reflectance spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiated, the surface and internal compositions are presumably similar, while large bodies such as Ceres and Vesta are known to have internal structure. Over the years, there has been a number of surveys that resulted in a set of different taxonomic systems such as the Tholen, SMASS and Bus–DeMeo classifications.
The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.
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