Asteroid spectral types

Source: Wikipedia, the free encyclopedia.
For the spectral type of centaurs and TNOs, see
Distant object spectral type
.
Distribution of asteroid spectral types by distance from the Sun

An asteroid spectral type is assigned to

SMASS and Bus–DeMeo classifications.[1]

Taxonomic systems

In 1975, astronomers

Clark R. Chapman, David Morrison, and Ben Zellner developed a simple taxonomic system for asteroids based on color, albedo, and spectral shape. The three categories were labelled "C" for dark carbonaceous objects, "S" for stony (silicaceous) objects, and "U" for those that did not fit into either C or S.[2] This basic division of asteroid spectra has since been expanded and clarified.[3] A number of classification schemes are currently in existence,[4]
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 of Tholen and SMASS

See also: Category:Asteroid spectral classes
Summary of asteroid taxonomic classes[5]: Table 2 
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, F B low Linear, generally featureless spectra. Differences in UV absorption features and presence/absence of narrow absorption feature near 0.7 μm.
C, G 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 for very few asteroids.

S3OS2 classification

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.[1] 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.[1]

Bus–DeMeo classification

The Bus-DeMeo classification is an asteroid taxonomic system designed by

Stephen Slivan in 2009.[6] 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.[6]

Tholen classification

The most widely used taxonomy is that of

albedo measurements.[7] 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 (also see § Overview of Tholen and SMASS
above). The types are, with their largest exemplars in parentheses:

C-group

Asteroids in the
1 Ceres) are much rarer. Other low-albedo classes are the D-types (624 Hektor), typically seen in the outer asteroid belt and among the Jupiter trojans, as well as the rare T-type asteroids (96 Aegle
) from the inner main-belt.

S-group

Asteroids with an
silicaceous (or "stony") objects. Another large group are the stony-like V-type (4 Vesta), also known as "vestoids" most common among the members of the large Vesta family, thought to have originated from a large impact crater on Vesta. Other small classes include the A-type (246 Asporina), Q-type (1862 Apollo), and R-type asteroids (349 Dembowska
).

X-group

The umbrella group of
Hungaria family
in the innermost region of the asteroid belt.

Taxonomic features

The Tholen taxonomy may encompass up to four letters (e.g. "SCTU"). The classification scheme uses the letter "I" for "inconsistent" spectral data, and should not be confused with a spectral type. An example is the Themistian asteroid 515 Athalia, which, at the time of classification was inconsistent, as the body's spectrum and albedo was that of a stony and carbonaceous asteroid, respectively.[8] 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.[8] The Tholen taxonomy also has additional notations, appended to the spectral type. The letter "U" is a qualifying flag, used for asteroids with an "unusual" spectrum, that falls far from the determined cluster center in the numerical analysis. The notation ":" (single colon) and "::" (two colons) are appended when the spectral data is "noisy" or "very noisy", respectively. 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.[8]

SMASS classification

This is a more recent taxonomy introduced by American astronomers

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. As for the Tholen taxonomy, the majority of bodies fall into the three broad C, S, and X categories, with a few unusual bodies categorized into several smaller types (also see § Overview of Tholen and SMASS
above):

  • C-group of carbonaceous objects includes the C-type asteroid, the most "standard" of the non-B carbonaceous objects, the "brighter" B-type asteroid largely overlapping with the Tholen B- and F types, the Cb-type that transition between the plain C- and B-type objects, and the Cg, Ch, and Cgh-types that are somewhat related to the Tholen G-type. The "h" stands for "hydrated".
  • S-group of silicaceous (stony) objects includes the most common S-type asteroid, as well as the A-, Q-, and R-types. New classes include the K-type (181 Eucharis, 221 Eos) and L-type (83 Beatrix) asteroids. There are also five classes, Sa, Sq, Sr, Sk, and Sl that transition between plain the S-type and the other corresponding types in this group.
  • X-group of mostly metallic objects. This includes the most common X-type asteroids as well as the M, E, or P-type as classified by Tholen. The Xe, Xc, and Xk are transitional types between the plain X- and the corresponding E, C and K classes.
  • Other spectral classes include the T-, D-, and V-types (4 Vesta). The Ld-type is a new class and has more extreme spectral features than the L-type asteroid. The new class of O-type asteroids has since only been assigned to the asteroid 3628 Božněmcová.

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.[citation needed]

Color indices

For non-classical asteroids, see
Distant object color indices
.
For the taxonomic classes BB, BR, IR and RR, see
Distant object spectral type
.
Wavelengths

The characterization of an asteroid includes the measurement of its

characterize distant objects
in addition to classical asteroids, the three basic filters are:

  • U: passband for the ultraviolet light, (~320-380 nm, mean 364 nm)
  • B: passband for the blue light, including some violet, (~395-500 nm, mean 442 nm)
  • V: passband sensitive to
    visible light
    , more specifically the green-yellow portion of the visible light (~510-600 nm, mean 540 nm)
Wavelengths of the visible light
Colors violet blue cyan green yellow
orange
 red
Wavelengths 380–450 nm 450–495 nm 495–520 nm 520–570 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

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.[10]

Mean-color indices of
outer Solar System[10]
: 35 
Color index Plutinos
Cubewanos
Centaurs
SDOs
 Comets Jupiter trojans
B−V 0.895±0.190 0.973±0.174 0.886±0.213 0.875±0.159 0.795±0.035 0.777±0.091
V−R 0.568±0.106 0.622±0.126 0.573±0.127 0.553±0.132 0.441±0.122 0.445±0.048
V−I 1.095±0.201 1.181±0.237 1.104±0.245 1.070±0.220 0.935±0.141 0.861±0.090
R−I 0.536±0.135 0.586±0.148 0.548±0.150 0.517±0.102 0.451±0.059 0.416±0.057

Appraisal

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).[citation needed]

Correlation with meteorite types

Some groupings of asteroids have been correlated with

meteorite types:[citation needed
]

See also

References

  1. ^
    doi:10.1016/j.icarus.2004.06.006
    . Retrieved 22 December 2017.
  2. doi:10.1016/0019-1035(75)90191-8
    .
  3. ISBN 978-1-10-709684-4
    , p.163, Asteroid Taxonomy
  4. ISBN 978-0-8165-2281-1
    .
  5. doi:10.2307/j.ctv1v7zdn4.48
    . Retrieved 27 October 2017.
  6. ^
    doi:10.1016/j.icarus.2009.02.005. Archived from the original on 17 March 2014. Retrieved 28 March 2018. (Catalog at PDS
    )
  7. ISBN 978-0-8165-1123-5
    .
  8. ^ a b c David J. Tholen. "Taxonomic Classifications Of Asteroids – Notes". Retrieved 6 January 2019.
  9. doi:10.1006/icar.2002.6856
    .
  10. ^
    S2CID 12844258
    .

External links
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