M-type asteroid
M-type asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases than other asteroid classes, and are widely thought to be the source of iron meteorites.
Definition
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, they are included in the larger X-type asteroid group and are distinguishable only by optical albedo:Characteristics
Composition
Although widely assumed to be metal-rich, 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, and radar observations have shown that their radar albedos are much higher than other asteroid classes, consistent with the presence of higher density compositions like iron-nickel. 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.High resolution spectra of the M-type have sometimes shown subtle features longward of 0.75 μm and shortward of 0.55 μm. The presence of silicates is evident in many, and a significant fraction show evidence of absorption features at 3 μm, attributed to hydrated silicates. The presence of silicates, and especially hydrated silicates, is at odds with the traditional interpretation of M-types as remnant iron cores.
Bulk density and porosity
The bulk density of an asteroid provides clues about its composition and meteoritic analogs. For the M-types, the proposed analogs have bulk densities that range from ~3 for some types of carbonaceous chondrites up to nearly 8 for the iron-nickel present in iron-meteorites. Given the bulk density of an asteroid and the density of the materials that make it up, 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.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. If an asteroid possesses one or more moons, one can use their collective orbital parameters 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 of the asteroid, chord-lengths during occultations, or their thermal emissions. In a few cases, astronomers have managed to develop three-dimensional shape models using a variety of techniques or, in a few lucky instances, from spacecraft imaging.
| Asteroid | Density | Radar Albedo | Method |
| 16 Psyche | 3.8 ± 0.3 | 0.34 ± 0.08 | Ephemeris, shape model |
| 21 Lutetia | 3.4 ± 0.3 | 0.24 ± 0.07 | Rosetta spacecraft flyby, direct imaging |
| 22 Kalliope | 4.1 ± 0.5 | 0.15 ± 0.05 | Orbit of its moon Linus, shape model |
| 69 Hesperia | 4.4 ± 1.0 | 0.45 ± 0.12 | Ephemeris, thermal IR/radar size estimate |
| 92 Undina | 4.4 ± 0.4 | 0.38 ± 0.09 | Ephemeris, thermal IR/radar size estimate |
| 129 Antigone | 3.0 ± 1.0 | 0.36 ± 0.09 | Ephemeris, thermal IR/radar size estimate |
| 216 Kleopatra | 3.4 ± 0.5 | 0.43 ± 0.10 | 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. Size estimates based on shape models are the most reliable. Direct spacecraft imaging is also quite reliable. Sizes based on indirect methods like thermal IR 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, then that interpretation may still hold; this is unlikely for Psyche, 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, and in some cases may also be rubble piles.
Formation
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.It is acknowledged that some of the smaller M-type asteroids may have formed in this way, but that interpretation was challenged for 16 Psyche, the largest of the M-type asteroids. There are three arguments against Psyche forming in this way. First, it must have started as a Vesta-sized 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 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. 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 accumulated much closer to the Sun, were stripped of their thin crust/mantles while still molten, and later dynamically moved into the current asteroid belt.
A third view is that the largest M-types, including 16 Psyche, may be differentiated bodies but, given the right mix of iron and volatiles, these bodies may have experienced a type of iron volcanism, a.k.a. ferrovolcanism, while still cooling.