Asteroid spectral types

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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. [1]

Contents

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

Summary of asteroid taxonomic classes [5] :Table 2
Tholen ClassSMASSII
(Bus Class)
AlbedoSpectral Features
A AmoderateVery steep red slope shortward of 0.75 μm; moderately deep absorption feature longward of 0.75 μm.
B, F BlowLinear, 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, ChglowLinear, generally featureless spectra. Differences in UV absorption features and presence/absence of narrow absorption feature near 0.7 μm.
D DlowRelatively featureless spectrum with very steep red slope.
E, M, P X, Xc, Xe, Xkfrom 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 QmoderateReddish slope shortward of 0.7 μm; deep, rounded absorption feature longward of 0.75 μm.
R RmoderateModerate reddish slope downward of 0.7 μm; deep absorption longward of 0.75 μm.
S S, Sa, Sk, Sl, Sq, SrmoderateModerately 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 TlowModerately reddish shortward of 0.75 μm; flat afterward.
V VmoderateReddish shortward of 0.7 μm; extremely deep absorption longward of 0.75 μm.
K moderateModerately 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, LdmoderateVery 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 (S3OS2or 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 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

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. [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 parenthesis:

C-group

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

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

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

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 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):

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

Wavelengths UBV-System.png
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:

Wavelengths of the visible light
Colors violet blue green yellow orange red
Wavelengths 380–450 nm450–495 nm495–570 nm570–590 nm590–620 nm620–750 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. 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–V0.895±0.1900.973±0.1740.886±0.2130.875±0.1590.795±0.0350.777±0.091
V–R0.568±0.1060.622±0.1260.573±0.1270.553±0.1320.441±0.1220.445±0.048
V–I1.095±0.2011.181±0.2371.104±0.2451.070±0.2200.935±0.1410.861±0.090
R–I0.536±0.1350.586±0.1480.548±0.1500.517±0.1020.451±0.0590.416±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

Related Research Articles

S-type asteroid Asteroid spectral type indicating stony composition

S-type asteroids are asteroids with a spectral type that is indicative of a siliceous mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are of this type, making it the second most common after the carbonaceous C-type.

C-type asteroid Asteroid spectral type; most common variety, forming around 75% of known asteroids

C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks and minerals. Their density averages at about 1.7 g/cm3. They occur most frequently at the outer edge of the asteroid belt, 3.5 astronomical units (AU) from the Sun, where 80% of the asteroids are of this type, whereas only 40% of asteroids at 2 AU from the Sun are C-type. The proportion of C-types may actually be greater than this, because C-types are much darker than most other asteroid types except for D-types and others that are mostly at the extreme outer edge of the asteroid belt.

2934 Aristophanes, provisional designation 4006 P-L, is a carbonaceous Veritasian asteroid from the outer regions of the asteroid belt, approximately 22 kilometers in diameter. It was discovered during the Palomar–Leiden survey in 1960, and later named after ancient Greek dramatist Aristophanes.

4659 Roddenberry, provisional designation 1981 EP20, is a Nysian asteroid from the inner regions of the asteroid belt, approximately 3.6 kilometers (2.2 miles) in diameter. It was discovered on 2 March 1981, by American astronomer Schelte Bus at the Siding Spring Observatory in Australia. The likely S-type asteroid has an unsecured rotation period of 12 hours. It was named for American screenwriter Gene Roddenberry.

38 Leda

Leda is a large, dark main-belt asteroid that was discovered by French astronomer J. Chacornac on January 12, 1856, and named after Leda, the mother of Helen of Troy in Greek mythology. In the Tholen classification system, it is categorized as a carbonaceous C-type asteroid, while the Bus asteroid taxonomy system lists it as a Cgh asteroid. The spectra of the asteroid displays evidence of aqueous alteration.

251 Sophia

251 Sophia is a stony background asteroid from the outer regions of the asteroid belt. It was discovered on 4 October 1885, by astronomer Johann Palisa at the Vienna Observatory in Austria. The S-type asteroid (S/L) has a rotation period of 20.2 hours and measures approximately 28 kilometers in diameter. It was named after Sophia von Seeliger, wife of German astronomer Hugo von Seeliger (1849–1924).

The Vesta family is a family of asteroids. The cratering family is located in the inner asteroid belt in the vicinity of its namesake and principal body, 4 Vesta. It is one of the largest asteroid families with more than 15,000 known members and consists of mostly bright V-type asteroids, so-called "vestoids".

The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.

716 Berkeley is a background asteroid from the central regions of the asteroid belt. It was discovered by Austrian astronomer Johann Palisa at the Vienna Observatory on 30 July 1911. The stony S-type asteroid has a rotation period of 15.6 hours and measures approximately 21 kilometers in diameter. It was named after the city of Berkeley, California, where the discoverer's colleague Armin Otto Leuschner (1868–1953) was the director of the local observatory.

743 Eugenisis is a minor planet orbiting the Sun that was discovered by German astronomer Franz Kaiser in 1913.

779 Nina is a large background asteroid, approximately 80 kilometers in diameter, located in the central region of the asteroid belt. It was discovered on 25 January 1914, by Russian astronomer Grigory Neujmin(1886-1946) at the Simeiz Observatory on the Crimean peninsula. The metallic X-type asteroid with an intermediate albedo has a rotation period of 11.2 hours. It was named after the discoverer's sister, Nina Neujmina (Tsentilovich) (1889–1971).

814 Tauris is a dark and very large background asteroid, approximately 109 kilometers in diameter, located the outer regions of the asteroid belt. It was discovered on 2 January 1916, by astronomer Russian Grigory Neujmin at the Simeiz Observatory on Crimea. The carbonaceous C-type asteroid has a longer-than average rotation period of 35.8 hours. It was named after the ancient name of the Crimean peninsula where the discovering observatory is located.

913 Otila is a bright Flora asteroid from the inner regions of the asteroid belt. It was discovered by German astronomer Karl Reinmuth at the Heidelberg Observatory on 19 May 1919. The stony S-type asteroid has a short rotation period of 4.9 hours and measures approximately 12 kilometers in diameter. It was named after a common German female name unrelated to the discoverer's contemporaries, that was taken from the almanac Lahrer Hinkender Bote.

943 Begonia

943 Begonia is a large, carbonaceous background asteroid, approximately 70 kilometers in diameter, from the outer region of the asteroid belt. It was discovered by German astronomer Karl Reinmuth at the Heidelberg Observatory on 20 October 1920 and given the provisional designations A920 UA and 1920 HX. The dark C-type asteroid (Ch) has a rotation period of 15.7 hours. It was named after the genus of popular houseplants, Begonia.

1020 Arcadia, provisional designation 1924 QV, is a stony Agnia asteroid from the central regions of the asteroid belt, approximately 11 kilometers in diameter. It was discovered on 7 March 1924, by German astronomer Karl Reinmuth at the Heidelberg Observatory in Heidelberg, Germany. The asteroid was named after the Greek region of Arcadia.

7803 Adachi, provisional designation 1997 EW2, is a stony Agnia asteroid from the middle region of the asteroid belt, approximately 6.4 kilometers (4 miles) in diameter. The asteroid was discovered on 4 March 1997, by Japanese amateur astronomer Takao Kobayashi at the Ōizumi Observatory in central Japan. It was named for Japanese amateur astronomer Makoto Adachi. The S-type asteroid has a rotation period of 5.2 hours.

1580 Betulia

1580 Betulia, provisional designation 1950 KA, is an eccentric, carbonaceous asteroid, classified as near-Earth object of the Amor group, approximately 4.2 kilometers in diameter. It was discovered on 22 May 1950, by South African astronomer Ernest Johnson at the Union Observatory in Johannesburg. The asteroid was named for Betulia Toro, wife of astronomer Samuel Herrick.

2423 Ibarruri, provisional designation 1972 NC, is an eccentric, tumbling and rare-type asteroid, classified as slow rotator and sizable Mars-crosser from the inner regions of the asteroid belt, approximately 5 kilometers in diameter.

1759 Kienle, provisional designation 1942 RF, is a stony background asteroid from the central regions of the asteroid belt, approximately 7 kilometers in diameter. It was discovered on 11 September 1942, by astronomer Karl Reinmuth at the Heidelberg-Königstuhl State Observatory in southwest Germany. The S-type asteroid has a longer-than average rotation period of 29.3 hours. It was named for German astrophysicist Hans Kienle.

3198 Wallonia, provisional designation 1981 YH1, is a stony asteroid and sizable Mars-crosser from the inner regions of the asteroid belt, approximately 7.1 kilometers (4.4 miles) in diameter. It was discovered on 30 December 1981, by Belgian astronomer François Dossin at the Haute-Provence Observatory in France. The S/K-type asteroid has a rotation period of 7.5 hours. It was named after the French speaking region of Wallonia in Belgium.

References

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