Editing M-type asteroid

{{short description|Asteroid spectral type}}
[[File:Rosetta_triumphs_at_asteroid_Lutetia.jpg|thumb| Image of the M-type asteroid [[21 Lutetia]] taken by the ESA [[Rosetta (spacecraft)|Rosetta Spacecraft]] during a flyby in 2010]]
'''M-type''' (metallic-type, aka M-class) asteroids are a [[Asteroid spectral types|spectral class]] of [[asteroid]]s which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes,<ref name="Shepard et al 2015"/> and are widely thought to be the source of [[iron meteorite]]s.<ref name="Bell_AsteroidsII"/>

==Definition ==
[[Asteroid]]s are classified as M-type based upon their generally featureless and flat to red-sloped absorption [[Absorption spectroscopy#Applications|spectra]] in the visible to near-infrared and their moderate optical [[albedo]]. Along with the spectrally similar [[E-type asteroid|E-type]] and [[P-type asteroid|P-type]] asteroids (both categories E and P were formerly type-M in older systems), they are included in the larger [[X-type asteroid|X-type]] asteroid group and are distinguishable only by optical albedo:<ref name="Tholen_Barucci"/>
:{|
|-
| [[P-type asteroid|P-type]] || [[albedo]] < 0.1
|-
| '''M-type'''  || [[albedo]] in 0.1 ... 0.3
|-
| [[E-type asteroid|E-type]]  || [[albedo]] > 0.3
|}

==Characteristics==
===Composition===
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 meteorite]]s and [[enstatite chondrite]]s,<ref name="Gaffey_asteroidsII"/> and radar observations have shown that their [[Albedo#Radar Albedo|radar albedo]]s are much higher than other asteroid classes,<ref name="Magri2007"/> consistent with the presence of higher density compositions like iron-nickel.<ref name="Shepard et al 2015"/> Nearly all of the M-types have radar albedos at least twice as high as the more common [[S-type asteroid|S-]] and [[C-type asteroid|C-type]], and roughly one-third have radar albedos ~3× higher.<ref name="Shepard et al 2015"/>

High resolution spectra of the M-type have sometimes shown subtle features longward of 0.75&nbsp;[[μm]] and shortward of 0.55&nbsp;μm.<ref name="BusBinzel"/> The presence of silicates is evident in many,<ref name="Ockert-Bell"/><ref name="Lupishko82"/> and a significant fraction show evidence of absorption features at 3&nbsp;μm, attributed to hydrated silicates.<ref name="Rivkin00"/> The presence of silicates, and especially hydrated silicates, is at odds with the traditional interpretation of M-types as remnant iron cores.

{{multiple image
| align = right
| direction = vertical
| header=Possible meteorite analogs for M-type asteroids.
| total_width = 220
| image1=TolucaMeteorite.jpg |caption1=An [[Iron meteorite|iron–nickel meteorite]] with characteristic [[Widmanstätten pattern]].
| image2=Vaca_muerta_mesosiderite.jpg|caption2=A [[mesosiderite]] showing a mixture of metals and silicates.
| image3=ETypeChondrite-AbeeEH4-RoyalOntarioMuseum-Jan18-09.jpg|caption3=An [[enstatite chondrite]] displaying a mix of metals and silicates (enstatite).
| image4=Gujba_meteorite,_bencubbinite_(14785860604).jpg|caption4=A metal-rich [[carbonaceous chondrite]], or bencubbinite.
|image5=Imilac_pallasite.jpg||caption5=A stony-iron [[pallasite]], composited of iron-nickel and [[olivine]].
}}

===Bulk density and porosity===
The [[bulk density]] of an asteroid provides clues about its composition and meteoritic analogs.<ref name="Britt_AsteroidsIII"/> For the M-types, the proposed analogs have bulk densities that range from ~3 {{val|u=g/cm3}} for some types of [[carbonaceous chondrite]]s up to nearly 8 {{val|u=g/cm3}} for the iron-nickel present in [[Iron meteorite|iron-meteorites]].<ref name="Bell_AsteroidsII" /><ref name="Gaffey_asteroidsII" /><ref name="Rivkin00" /> 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.<ref name="Britt_AsteroidsIII"/>

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.<ref name="Pitjeva2018"/> If an asteroid possesses one or more [[Minor-planet moon|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 [[Albedo#Optical or Visual Albedo|visual albedo]] (brightness) of the asteroid, chord-lengths during [[Occultation#Asteroids|occultations]], or their thermal emissions (e.g. [[IRAS|IRAS mission]]). In a few cases, astronomers have managed to develop three-dimensional shape models using a variety of techniques (cf. [[16 Psyche#Shape and Spin Pole|16 Psyche]] or [[216 Kleopatra#Size and Shape|216 Kleopatra]] for examples) or, in a few lucky instances, from spacecraft imaging (cf. [[162173 Ryugu#Shape|162173 Ryugu]]).

{| class="wikitable"
|-
! Asteroid
! Density
! [[Albedo#Radar Albedo|Radar Albedo]]
! Method (mass, size)
|-
| [[16 Psyche]]
| 3.8 ± 0.3<ref name="Elkins-Tanton_2020" />
| 0.34 ± 0.08<ref name="Shepard_2021" />
| Ephemeris, shape model
|-
| [[21 Lutetia]]
| 3.4 ± 0.3<ref name="Sierks2011" />
| 0.24 ± 0.07<ref name="Shepard et al 2015" />
| [[Rosetta (spacecraft)|''Rosetta'' spacecraft]] flyby, direct imaging
|-
| [[22 Kalliope]]
| 4.1 ± 0.5<ref name="Vernazza2021" /><ref name="Ferrais2021" />
| 0.15 ± 0.05<ref name="Magri2007" />
| Orbit of its moon [[Linus (moon)|Linus]], shape model
|-
| [[69 Hesperia]]
| 4.4 ± 1.0<ref name="Carry2012" /> 
| 0.45 ± 0.12<ref name="Shepard et al 2015" />
| Ephemeris, thermal IR/radar size estimate
|-
| [[92 Undina]]
| 4.4 ± 0.4<ref name="Carry2012" /> 
| 0.38 ± 0.09<ref name="Shepard et al 2015" /> 
| Ephemeris, thermal IR/radar size estimate
|-
| [[129 Antigone]]
| 3.0 ± 1.0<ref name="Carry2012" />  
| 0.36 ± 0.09<ref name="Shepard et al 2015" /> 
| Ephemeris, thermal IR/radar size estimate
|-
| [[216 Kleopatra]]
| 3.4 ± 0.5<ref name="Marchis_Kleopatra" />
| 0.43 ± 0.10<ref name="Shepard_Kleopatra" /> 
| Orbits of its [[216 Kleopatra#Moons|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.<ref name="Elkins-Tanton_2020" /> 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 pile|rubble-piles]]), then that interpretation may still hold; this is unlikely for Psyche,<ref name="Elkins-Tanton_2020" /> 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 chondrite]]s, metal-rich [[carbonaceous chondrite]]s, [[mesosiderite]]s), and in some cases may also be rubble piles.<ref name="Descamps_Kalliope" /><ref name="Marchis_Kleopatra" /><ref name="Elkins-Tanton_2020" />

==Formation==
The earliest interpretation of the M-type asteroids was that they were the remnant cores of early [[protoplanet]]s, 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.<ref name="Bell_AsteroidsII" />

It is acknowledged that some of the smaller M-type asteroids (<100&nbsp;km) may have formed in this way, but that interpretation was challenged for [[16 Psyche]], the largest of the M-type asteroids.<ref name="Davis_1999" /> There are three arguments against Psyche forming in this way.<ref name="Davis_1999" /> First, it must have started as a Vesta-sized (~500&nbsp;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.<ref name="Davis_1999" /> There are numerous examples of metal-silicate meteorites, aka [[mesosiderite]]s, 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.<ref name="Scott"/>

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.<ref name="Johnson"/>

==Notable examples==
In the [[JPL Small-Body Database|JPL Small Body Database]], there are 980 asteroids classified under the [[Asteroid spectral types#Tholen classification|Tholen asteroid spectral classification system]].<ref name="SBDB_Tholen"/> Of those, 38 are classified as M-type.<ref name="SBDB_Tholen_M"/> 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.<ref name="SBDB_Tholen_XM"/> Overall, the M-types make up approximately 5% of the asteroids classified under the Tholen taxonomy.

===(16) Psyche===
[[16 Psyche]] is the largest M-type asteroid with a [[mean diameter]] of 222&nbsp;km, and has a relatively high mean [[Albedo#Radar Albedo|radar albedo]] of <math>\hat{\sigma}_{OC}=0.34 \pm 0.08 </math> suggesting it has a high metal content in the upper few meters of its surface.<ref name="Shepard_2021"/> The [[Psyche (spacecraft)|Psyche spacecraft]], launched on October 13, 2023, is en route to visit 16 Psyche, arriving in 2029.

===(21) Lutetia===
[[21 Lutetia]] has a mean diameter of 100&nbsp;km,<ref name="Shepard et al 2015"/> and was the first M-type asteroid to have been imaged by a spacecraft when the [[Rosetta space probe]] visited it on 10&nbsp;July 2010.<ref name="Schulz" /> Its mean radar albedo of <math>\hat{\sigma}_{OC}=0.24 \pm 0.07 </math> is roughly twice that of the average [[S-type asteroid|S-type]] or [[C-type asteroid]], and suggests its [[regolith]] contains an elevated amount of metal phases relative to other asteroid classes.<ref name="Shepard et al 2015"/> Analysis using data from the Rosetta spectrometer (VIRTIS) was consistent with estatitic or iron-rich carbonaceous chondritic materials.<ref name="Coradini"/>

===(22) Kalliope===
[[22 Kalliope]] is the second largest M-type asteroid with a mean diameter of 150&nbsp;km.<ref name="Vernazza2021" /> A single moon, named [[Linus (moon)|Linus]], was discovered in 2001<ref name="MargotBrown2003" /> 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,<ref name="Magri2007" /> 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 {{val|u=g/cm3}}.<ref name="Vernazza2021" /><ref name="Ferrais2021" />

===(216) Kleopatra===
[[216 Kleopatra]], with a mean diameter of 122&nbsp;km, is the third largest M-type asteroid known after 16 Psyche and 22 Kalliope.<ref name="Shepard_Kleopatra"/> 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."<ref name="Shepard_Kleopatra" /> Radar observations from the Arecibo radar telescope indicate a very high radar albedo of <math>\hat{\sigma}_{OC}=0.43 \pm 0.10 </math> in the southern hemisphere, consistent with a metal-rich composition.<ref name="Shepard_Kleopatra" /> 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.<ref name="Descamps_Kleopatra"/>

==See also==
* [[Asteroid spectral types]]

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}} <!-- end "refs=" -->

{{Asteroids}}
{{Small Solar System bodies}}

[[Category:Asteroid spectral classes]]
[[Category:M-type asteroids (Tholen)|*]]
[[Category:X-type asteroids|*]]