Asteroid spectral types

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An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo (reflectivity). 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. There are a number of different taxonomic systems such as the Tholen, SMASS and Bus–DeMeo classification.

Present-day classifications[edit]

The present-day classification was initiated by Clark R. Chapman, David Morrison, and Ben Zellner in 1975 with three categories:[1] C for dark carbonaceous objects, S for stony (silicaceous) objects, and U for those that did not fit into either C or S. This classification has since been expanded and clarified.

A number of classification schemes are currently in existence,[2] and while they strive to retain some mutual consistency, quite a few asteroids are sorted into different classes depending on the particular scheme. This is due to the use of different criteria for each approach. The two most widely used classifications are described below:

Overview Tholen and SMASS[edit]

Summary of asteroid taxonomic classes[3]
Tholen Class SMASSII
(Bus Class)
Albedo Spectral Features
A A moderate Very steep red slope shortward of 0.75 µm; moderately deep

absorption feature longward of 0.75 µm.

B, C, F, G B, C, Cb, Ch, Cg, Chg low Linear, generally featureless spectra. Differences in UV absorption features and presence/absence of narrow absorption feature near 0.7 µm.
D D low Relatively featureless spectrum with very steep red slope.
E, M, P X, Xc, Xe, Xk from low (P)
to very high (E)
Generally featureless spectrum with reddish slope; differences in subtle absorption features and/or spectral curvature and/or peak relative reflectance.
Q Q moderate Reddish slope shortward of 0.7 µm; deep, rounded absorption feature longward of 0.75 µm.
R R moderate Moderate reddish slope downward of 0.7 µm; deep absorption longward of 0.75 µm.
S S, Sa, Sk, Sl, Sq, Sr moderate Moderately steep reddish slope downward of 0.7 µm; moderate to steep absorption longward of 0.75 µm; peak of reflectance at 0.73 µm. Bus subgroups intermediate between S and A, K, L, Q, R classes.
T T low Moderately reddish shortward of 0.75 µm; flat afterward.
V V moderate Reddish shortward of 0.7 µm; extremely deep absorption longward of 0.75 µm.
K moderate Moderately steep red slope shortward of 0.75 µm; smoothly angled maximum and flat to blueish longward of 0.75 µm, with little or no curvature.
L, Ld moderate Very steep red slope shortward of 0.75 µm; flat longward of 0.75 µm; differences in peak level.
O Peculiar trend, known so far only for asteroid 3628.

Tholen classification[edit]

The most widely used taxonomy for over a decade has been that of David J. Tholen, first proposed in 1984. This classification was developed from broad band spectra (between 0.31 μm and 1.06 μm) obtained during the Eight-Color Asteroid Survey (ECAS) in the 1980s, in combination with albedo measurements.[4] The original formulation was based on 978 asteroids.

The Tholen scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types. They are, with their largest exemplars:[citation needed]

and the small classes:

Inconsistent data[edit]

For inconsistent spectral data, the Tholen classification scheme uses the letter "I", which is not a formal spectral type. An example is the Themistian asteroid 515 Athalia, which had a spectrum of a stony, and an albedo of a carbonaceous asteroid at the time of classification.[5]

Multiple types[edit]

When the underlying numerical color analysis was ambiguous, objects were assigned two or three types rather than just one (e.g. "CG" or "SCT"), whereby the sequence of types reflects the order of increasing numerical standard deviation, with the best fitting spectral type mentioned first.[5] In case a qualifying flag for an unusual spectrum is used (see below), the Tholen taxonomy may then encompass up to four letters (e.g. "SCTU").

Flags[edit]

The Tholen taxonomy uses the following notations, appended to the spectral type:[5]

  • U → asteroid has an unusual spectrum, which falls far from cluster center
  • :  → noisy spectral data
  • :: → very noisy spectral data

For example, the Mars-crosser 1747 Wright has an AU: class, which means that it is an A-type asteroid, though with an unusual and noisy spectrum.

SMASS classification[edit]

This is a more recent taxonomy introduced by Schelte J. Bus and Richard P. Binzel in 2002, based on the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) of 1,447 asteroids.[6] This survey produced spectra of a far higher resolution than ECAS, and was able to resolve a variety of narrow spectral features. However, a somewhat smaller range of wavelengths (0.44 μm to 0.92 μm) was observed. Also, albedos were not considered. Attempting to keep to the Tholen taxonomy as much as possible given the differing data, asteroids were sorted into the 26 types given below. The majority of bodies fall again into the three broad C, S, and X categories, with a few unusual bodies categorized into several smaller types:

  • C-group of carbonaceous objects including:[citation needed]
    • B-type largely overlapping with the Tholen B and F types.
    • C-type the most 'standard' of the non-B carbonaceous objects
    • Cg Ch Cgh somewhat related to the Tholen G type
    • Cb transition objects between plain C and B types.
  • S-group of silicaceous (stony) objects including:
  • X-group of mostly metallic objects including:
    • X-type the most 'standard' of the X group including objects classified by Tholen as M, E, or P-type.
    • Xe, Xc, and Xk transition types between plain X and the appropriately lettered types.
  • T-type
  • D-type
  • Ld-type: a new type with more extreme spectral features than the L-type
  • O-type a small category (3628 Božněmcová)
  • V-type (4 Vesta)

A significant number of small asteroids were found to fall in the Q, R, and V types, which were represented by only a single body in the Tholen scheme. In this Bus and Binzel SMASS scheme only a single type was assigned to any particular asteroid.

A few near-Earth objects have spectra that differ strongly from any of the SMASS classes. This is presumably because these bodies are much smaller than those detected in the asteroid belt, and as such may have younger less-altered surfaces or be composed of a less varied mix of minerals.[citation needed]

Color indices[edit]

In a photometric system, the brightness of an object can be measured through a set of different, wavelength-specific filters (passbands). In the UBV photometric system, three different filters are used:

  • U: passband for the ultraviolet light
  • B: passband for the blue light
  • V: passband sensitive to visible light, more specifically the green-yellow portion of the visible light

In an observation, the brightness of an object is measured twice through a different filter. The resulting difference in magnitude is called the color index. The U–B or B–V color indices are the most common ones. In addition, the R–I index, where the photometric letters stand for "red" (R) and infrared (I), is also used.

Wavelengths of the visible light
Colors violet blue green yellow orange red
Wavelengths 380–450 nm 450–495 nm 495–570 nm 570–590 nm 590–620 nm 620–750 nm

Appraisal[edit]

These classification schemes are expected to be refined and/or replaced as further research progresses. However, for now the spectral classification based on the two above coarse resolution spectroscopic surveys from the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).

Some groupings of asteroids have been correlated with meteorite types:

See also[edit]

References[edit]

  1. ^ Chapman, C. R.; Morrison, D.; Zellner, B. (1975). "Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry". Icarus. 25 (1): 104–130. Bibcode:1975Icar...25..104C. doi:10.1016/0019-1035(75)90191-8. 
  2. ^ Bus, S. J.; Vilas, F.; Barucci, M. A. (2002). "Visible-wavelength spectroscopy of asteroids". Asteroids III. Tucson: University of Arizona Press. p. 169. ISBN 0-8165-2281-2. 
  3. ^ Cellino, A.; Bus, S. J.; Doressoundiram, A.; Lazzaro, D. (March 2002). "Spectroscopic Properties of Asteroid Families" (PDF). Asteroids III: 633–643. Bibcode:2002aste.book..633C. Retrieved 27 October 2017. 
  4. ^ Tholen, D. J. (1989). "Asteroid taxonomic classifications". Asteroids II. Tucson: University of Arizona Press. pp. 1139–1150. ISBN 0-8165-1123-3. 
  5. ^ a b c David J. Tholen. "Taxonomic Classifications Of Asteroids – Notes". Retrieved 24 August 2017. 
  6. ^ Bus, S. J.; Binzel, R. P. (2002). "Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy". Icarus. 158 (1): 146–177. Bibcode:2002Icar..158..146B. doi:10.1006/icar.2002.6856.