The X-group of

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

s collects together several types with similar spectra, but probably quite different compositions.

Tholen classification

In the

Tholen classification An asteroid spectral type is assigned to asteroids based on their emission 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 X-group consists of the following types: * E-type – with high

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

(> 0.30), composed of

enstatite Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and ...

, forsterite and

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

. They are found in the inner main-belt. * M-type – the largest grouping, intermediate albedo, "metallic", composed of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

and

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...

, thought to be the progenitors of nickel–iron meteorites. They are found around 3.0 AU and in the Hungaria region (innermost main-belt). * P-type – low albedos (< 0.10) with featureless red spectra; presumably composed of

carbonaceous chondrites Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...

, and found in the outer main-belt and in the

Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange p ...

region. Since in this scheme the

is crucial in discriminating between the above types, some objects for which

information was not available were assigned an X-type. An example of this is

50 Virginia Virginia (minor planet designation: 50 Virginia) is a large, very dark main belt asteroid. It was discovered by American astronomer James Ferguson on October 4, 1857, from the United States Naval Observatory in Washington, D.C. German astronomer ...

SMASS classification

The

SMASS classification An asteroid spectral type is assigned to asteroids based on their emission 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 ...

does not use

, but several spectral types are distinguished on the basis of

spectral ''Spectral'' is a 2016 3D military science fiction, supernatural horror fantasy and action-adventure thriller war film directed by Nic Mathieu. Written by himself, Ian Fried, and George Nolfi from a story by Fried and Mathieu. The film stars J ...

features which were too subtle to be visible in the broad-band ECAS survey used for the Tholen scheme. The X-group contains the types: * core X-type containing the asteroids with the most "typical" spectra * Xe-type of asteroids whose spectra contain a moderately broad absorption band around 0.49 μm. It has been suggested that this indicates the presence of troilite (FeS). There is some correlation between this group and the Tholen E-type. * Xc- and Xk-type asteroids, which contain a broad convex

feature in the range 0.55 μm to 0.8 μm (i.e. increased

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...

in this range). These spectra tend to be intermediate between the core X-type and the C and K-type. Apart from the Xe-type, there is no significant correlation between the split into these SMASS types and the Tholen E, M, and P-types. All the types in the X-group contain a mixture of asteroids classified as either type E, M, or P.

References

* {{Small Solar System bodies Asteroid spectral classes

Tholen classification

SMASS classification

See also

References