Asteroid spectral types
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 surface and internal compositions are presumably similar, while large bodies such as Ceres and Vesta are known to have internal structure. Over the years, there has been a number of surveys that resulted in a set of different taxonomic systems such as the Tholen, SMASS and Bus–DeMeo classification.
The present-day[clarification needed] classification was initiated by Clark R. Chapman, David Morrison, and Ben Zellner in 1973 with three categories: 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, 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
|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.|
The Small Solar System Objects Spectroscopic Survey (S3OS2 or S3OS2, also known as the Lazzaro classification) observed 820 asteroids, using the former ESO 1.52-metre telescope at La Silla Observatory during 1996–2001. This survey applied both the Tholen and Bus–Binzel (SMASS) taxonomy to the observed objects, many of which had previously not been classified. For the Tholen-like classification, the survey introduced a new "Caa-type", which shows a broad absorption band associated indicating an aqueous alteration of the body's surface. The Caa class corresponds to Tholen's C-type and to the SMASS' hydrated Ch-type (including some Cgh-, Cg-, and C-types), and was assigned to 106 bodies or 13% of the surveyed objects. In addition, S3OS2 uses the K-class for both classification schemes, a type which does not exist in the original Tholen taxonomy.
The Bus-DeMeo classification is an asteroid taxonomic system designed by DeMeo Bus and Stephen M. Slivan in 2009. It is based on reflectance spectrum characteristics for 371 asteroids measured over the wavelength 0.45–2.45 micrometers. This system of 24 classes introduces a new "Sv"-type and is based upon a principal component analysis, in accordance with the SMASS taxonomy, which itself is based upon the Tholen classification.
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. 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:
- C-group dark carbonaceous objects.
- S-type (15 Eunomia, 3 Juno) silicaceous (or "stony") objects.
and the small classes:
- A-type (246 Asporina)
- D-type (624 Hektor)
- T-type (96 Aegle)
- Q-type (1862 Apollo)
- R-type (349 Dembowska)
- V-type (4 Vesta)
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.
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. 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").
The Tholen taxonomy uses the following notations, appended to the spectral type:
- U → asteroid has an unusual spectrum, which falls far from cluster center
- : → noisy spectral data
- :: → very noisy spectral data
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. 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:
- 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.
- 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 the Bus and Binzel SMASS scheme only a single type was assigned to any particular asteroid.
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
|Wavelengths||380–450 nm||450–495 nm||495–570 nm||570–590 nm||590–620 nm||620–750 nm|
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 V–R, V–I and R–I indices, where the photometric letters stand for visible (V), red (R) and infrared (I), are also used. A photometric sequence such as V–R–B–I can be obtained from observations within a few minutes.
|B–V||±0.190 0.895||±0.174 0.973||±0.213 0.886||±0.159 0.875||±0.035 0.795||±0.091 0.777|
|V–R||±0.106 0.568||±0.126 0.622||±0.127 0.573||±0.132 0.553||±0.122 0.441||±0.048 0.445|
|V–I||±0.201 1.095||±0.237 1.181||±0.245 1.104||±0.220 1.070||±0.141 0.935||±0.090 0.861|
|R–I||±0.135 0.536||±0.148 0.586||±0.150 0.548||±0.102 0.517||±0.059 0.451||±0.057 0.416|
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:
- C-type – Carbonaceous chondrite meteorites
- S-type – Stony meteorites
- M-type – Iron meteorites
- V-type – HED meteorites
- Lazzaro, D.; Angeli, C. A.; Carvano, J. M.; Mothé-Diniz, T.; Duffard, R.; Florczak, M. (November 2004). "S3OS2: the visible spectroscopic survey of 820 asteroids" (PDF). Icarus. 172 (1): 179–220. Bibcode:2004Icar..172..179L. doi:10.1016/j.icarus.2004.06.006. Retrieved 22 December 2017.
- 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.
- Thomas H. Burbine: Asteroids – Astronomical and Geological Bodies. Cambridge University Press, Cambridge 2016, ISBN 978-1-10-709684-4, p.163, Asteroid Taxonomy
- Bus, S. J.; Vilas, F.; Barucci, M. A. (2002). "Visible-wavelength spectroscopy of asteroids". Asteroids III. Tucson: University of Arizona Press. p. 169. ISBN 978-0-8165-2281-1.
- 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.
- DeMeo, Francesca E.; Binzel, Richard P.; Slivan, Stephen M.; Bus, Schelte J. (July 2009). "An extension of the Bus asteroid taxonomy into the near-infrared" (PDF). Icarus. 202 (1): 160–180. Bibcode:2009Icar..202..160D. doi:10.1016/j.icarus.2009.02.005. Archived from the original on 17 March 2014. Retrieved 28 March 2018. (Catalog at PDS)
- Tholen, D. J. (1989). "Asteroid taxonomic classifications". Asteroids II. Tucson: University of Arizona Press. pp. 1139–1150. ISBN 978-0-8165-1123-5.
- David J. Tholen. "Taxonomic Classifications Of Asteroids – Notes". Retrieved 24 August 2017.
- 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.
- Fornasier, S.; Dotto, E.; Hainaut, O.; Marzari, F.; Boehnhardt, H.; De Luise, F.; et al. (October 2007). "Visible spectroscopic and photometric survey of Jupiter Trojans: Final results on dynamical families". Icarus. 190 (2): 622–642. arXiv: . Bibcode:2007Icar..190..622F. doi:10.1016/j.icarus.2007.03.033.
- Asteroid spectrum classification using Bus-DeMeo taxonomy, Planetary Spectroscopy at MIT (2017)