Classification type of a class of astronomical objects
Distribution of asteroid spectral types by distance from the Sun
An
asteroid spectral type
is assigned to
asteroids
based on their
reflectance 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
classifications.
[1]
Taxonomic systems
[
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]
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
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]
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
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]
The
Small Solar System Objects Spectroscopic Survey
(S
3
OS
2
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
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]
The Bus-DeMeo classification is an asteroid taxonomic system designed by
Francesca DeMeo
,
Schelte Bus
and
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
[
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]
The most widely used taxonomy is 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.
[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
[
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]
- Asteroids in the
C-group
are dark,
carbonaceous
objects. Most bodies in this group belong to the standard
C-type
(e.g.,
10 Hygiea
), and the somewhat "brighter"
B-type
(
2 Pallas
). The
F-type
(
704 Interamnia
) and
G-type
(
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
[
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]
- Asteroids with an
S-type
(
15 Eunomia
,
3 Juno
) are
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
[
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]
- The umbrella group of
X-type asteroid
can be further divided into three subgroups, depending on the degree of the object's reflectivity (dark, intermediate, bright). The darkest ones are related to the C-group, with an albedo below 0.1. These are the "primitive"
P-type
(
259 Aletheia
,
190 Ismene
), which differ from the "metallic"
M-type
(
16 Psyche
) with an intermediate albedo of 0.10 to 0.30, and from the bright "enstatite"
E-type asteroid
, mostly seen among the members of the
Hungaria family
in the innermost region of the asteroid belt.
Taxonomic features
[
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]
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
[
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]
This is a more recent taxonomy introduced by American astronomers
Schelte Bus
and
Richard Binzel
in 2002, based on the
Small Main-Belt Asteroid Spectroscopic Survey
(SMASS) of 1,447 asteroids.
[9]
This survey produced spectra of a far higher resolution than ECAS
(see
Tholen classification
above)
, 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. 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 Bo?n?mcova
.
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
[
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]
Wavelengths
The characterization of an asteroid includes the measurement of its
color indices
derived from a
photometric system
. This is done by measuring the object's brightness through a set of different, wavelength-specific filters, so-called passbands. In the
UBV photometric system
, which is also used to
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)
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
. For asteroids, 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.
[10]
Mean-color indices of
dynamical groups
in the
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
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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
[
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]
Some groupings of asteroids have been correlated with
meteorite types
:
[
citation needed
]
See also
[
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]
References
[
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]
- ^
a
b
c
Lazzaro, D.; Angeli, C. A.; Carvano, J. M.; Mothe-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. (May 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
.
doi
:
10.2307/j.ctv1v7zdn4.48
. Retrieved
27 October
2017
.
- ^
a
b
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
.
- ^
a
b
c
David J. Tholen.
"Taxonomic Classifications Of Asteroids ? Notes"
. Retrieved
6 January
2019
.
- ^
Bus, Schelte J.; Binzel, Richard P. (July 2002). "Phase II of the Small Main-Belt Asteroid Spectroscopic Survey. A Feature-Based Taxonomy".
Icarus
.
158
(1): 146?177.
Bibcode
:
2002Icar..158..146B
.
doi
:
10.1006/icar.2002.6856
.
- ^
a
b
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
:
0704.0350
.
Bibcode
:
2007Icar..190..622F
.
doi
:
10.1016/j.icarus.2007.03.033
.
S2CID
12844258
.
External links
[
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]