The X-group of
asteroid
An asteroid is a minor planet of the Solar System#Inner solar system, 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 o ...
s collects together several
types with similar spectra, but probably quite different compositions.
Tholen classification
In the
Tholen classification the X-group consists of the following types:
*
E-type – with high
albedo (> 0.30), composed of
enstatite,
forsterite and
feldspar. 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, thought to be the progenitors of
nickel–iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron met ...
s. 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, 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 poin ...
region.
Since in this scheme the
albedo is crucial in discriminating between the above types, some objects for which
albedo 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 differentiat ...
does not use
albedo, but several spectral types are distinguished on the basis of
spectral 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
spectral feature in the range 0.55 μm to 0.8 μm (i.e. increased
flux 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.
See also
*
Asteroid spectral types
*
L-type asteroid
L-type asteroids are relatively uncommon asteroids with a strongly reddish spectrum shortwards of 0.75 μm, and a featureless flat spectrum longwards of this. In comparison with the K-type asteroid, K-type, they exhibit a more reddish spectrum at ...
*
S-type asteroid
*
K-type asteroid K-type asteroids are relatively uncommon asteroids with a moderately reddish spectrum shortwards of 0.75 μm, and a slight bluish trend longwards of this. They have a low albedo. Their spectrum resembles that of CV and CO meteorites. A larger K ty ...
References
*
{{Small Solar System bodies
Asteroid spectral classes