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V-type asteroid

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V-type asteroids, also known as Vestoids, are a class of asteroids whose spectral type is characterized by a strong absorption feature at wavelengths longward of 0.75 μm, similar to that of 4 Vesta, the second-most-massive asteroid in the asteroid belt.[1] These asteroids comprise approximately 6% of main-belt asteroids and are characterized by their basaltic surface composition, making them distinct from other asteroid types.[2]

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Characteristics

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Physical Properties

V-type asteroids are relatively bright objects with moderate to high albedo values typically ranging from 0.20 to 0.40.[3] They are distinguished from other asteroid types by their basaltic composition, which indicates that they originated from differentiated parent bodies that underwent volcanic or igneous processing.[4]

The mean diameter of V-type asteroids varies considerably, from sub-kilometer objects to 4 Vesta itself with a mean diameter of approximately 525 kilometers.[5] Most V-types outside the Vesta family are relatively small, with diameters typically less than 10 kilometers.

Spectral Features

The electromagnetic spectrum of V-type asteroids exhibits several diagnostic features:[6]

  • A very strong absorption feature longward of 0.75 μm attributed to Fe2+ in pyroxene
  • A second absorption feature centered near 0.9-1.0 μm, also due to pyroxene
  • Very steep red spectral slope shortward of 0.7 μm
  • A weak absorption feature at 0.506 μm due to Fe2+ spin-forbidden transitions in pyroxene

The Band I center position typically ranges from 0.90 to 0.94 μm, while the Band II center is usually located between 1.89 and 2.00 μm.[7] The ratio of Band II to Band I depths (BII/BI) typically ranges from 1.5 to 2.5 for V-type asteroids.

Composition

V-type asteroids are composed primarily of basaltic material containing pyroxene and plagioclase feldspar.[8] The pyroxene composition is typically low-calcium pyroxene (orthopyroxene) with varying amounts of high-calcium pyroxene (clinopyroxene). The visible and near-infrared spectra of V-type asteroids closely resemble those of basaltic achondrite meteorites, particularly the HED meteorites (Howardites, Eucrites, and Diogenites).[9]

Spectroscopic analysis has revealed compositional variations among V-types:[10]

  • Eucrite-like: High calcium content, consistent with basaltic eucrite meteorites
  • Diogenite-like: Low calcium content, consistent with orthopyroxenitic diogenite meteorites
  • Howardite-like: Intermediate composition, mixture of eucrite and diogenite material
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Distribution

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Vesta Family Members

The vast majority of V-type asteroids are members of the Vesta family along with Vesta itself.[11] The Vesta family is one of the largest asteroid families with more than 15,000 known members.[12] Spectroscopic studies indicate that approximately 85% of the members of the Vesta dynamical family are V-type asteroids.[13]

Mars-Crossing V-types

Several V-type asteroids have been identified as Mars-crossers, including:[14][failed verification]

Recent systematic searches have confirmed three additional V-type asteroids in the Mars crossing region through spectroscopic observations.[15]

Near-Earth V-types

Several V-type asteroids have been identified among Near-Earth objects:[16]

Non-Vesta Family V-types

There is a scattered group of V-type asteroids in the general vicinity of the Vesta family but not dynamically associated with it.[17] As of current surveys, 22 V-type asteroids have been identified outside the Vesta family in the inner asteroid belt:[18]

Middle and Outer Main Belt

Recent spectroscopic surveys have identified V-type asteroids throughout the main belt:[21]

  • Ten confirmed V-types orbiting in the middle main belt (2.5 < a < 2.82 AU)
  • Five V-types in the outer main belt (a > 2.82 AU)
  • Two V-types identified beyond 3.3 AU
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Origin and Formation

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Vesta Origin Hypothesis

The predominant theory suggests that most V-type asteroids originated as fragments of 4 Vesta's crust during large impact events.[22] NASA's Dawn mission identified two enormous impact basins on Vesta's southern hemisphere:[23]

  • Veneneia basin: ~395 km diameter, formed approximately 2.1 billion years ago
  • Rheasilvia basin: ~505 km diameter, formed approximately 1 billion years ago

These impact events excavated and ejected large amounts of basaltic material from Vesta's crust and upper mantle.[24] The ejected fragments formed the Vesta family and are thought to be the source of the HED meteorites that fall to Earth.

Dynamical Evolution

V-type asteroids ejected from Vesta have undergone complex dynamical evolution:[25]

  • Fragments initially formed a collisional family near Vesta
  • Yarkovsky effect and YORP effect caused slow orbital drift
  • Interaction with mean-motion and secular resonances dispersed fragments
  • Some fragments entered the 3:1 and ν₆ resonances, allowing delivery to Earth-crossing orbits

Multiple Parent Body Hypothesis

Recent research indicates that V-type asteroids in the middle and outer main belt are unlikely to have originated from Vesta.[26] Extensive numerical simulations demonstrate the lack of efficient dynamical routes to transport Vesta fragments beyond 2.5 AU.[27]

The asteroid 1459 Magnya provides compelling evidence for multiple differentiated parent bodies:[28]

  • Located at 3.14 AU, beyond plausible Vesta ejecta dispersal
  • Spectroscopic differences from Vesta suggest distinct parent body
  • May represent remnant of destroyed differentiated asteroid

Classification Methods

Photometric Identification

V-type asteroids can be identified through various observational methods:[29]

  • Visible photometry using SDSS filters (u, g, r, i, z)
  • Near-infrared colors from 2MASS and WISE surveys
  • Combined visible and near-infrared spectroscopy

Spectroscopic Confirmation

Definitive classification requires spectroscopic observations covering the 0.4-2.5 μm range to identify characteristic pyroxene absorption bands.[30] Key diagnostic parameters include:

  • Band I center position (0.90-0.94 μm)
  • Band II center position (1.89-2.00 μm)
  • Band area ratio (BAR = Band II area/Band I area)
  • Spectral slope

J-type Subclassification

A J-type classification has been proposed for asteroids exhibiting particularly strong 1 μm absorption bands similar to diogenite meteorites, with Band I centers >0.95 μm.[31] These objects likely sample deeper crustal or upper mantle material from differentiated parent bodies.

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Notable Examples

4 Vesta

4 Vesta is the archetype of the V-type class and the only intact differentiated asteroid accessible to detailed study.[32] Key characteristics:

  • Mean diameter: 525.4 ± 0.2 km
  • Bulk density: 3.456 ± 0.035 g/cm³
  • Differentiated structure with metallic core (~220 km diameter)
  • Basaltic crust thickness: 12–20 km

1459 Magnya

1459 Magnya represents the most significant non-Vestoid V-type asteroid:[33]

  • Semi-major axis: 3.14 AU
  • Diameter: ~17 km
  • Spectroscopic properties distinct from Vesta
  • Possible fragment of destroyed differentiated asteroid

2579 Spartacus

2579 Spartacus shows unusual spectroscopic properties suggesting deep origin:[34]

  • Enhanced olivine content
  • May sample mantle material
  • Located at 2.71 AU
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Significance

Solar System Evolution

V-type asteroids provide crucial constraints on early Solar System processes:[35]

  • Timeline of planetesimal differentiation (first ~5 Myr)
  • Extent of igneous processing in the asteroid belt
  • Number and distribution of differentiated parent bodies
  • Collisional evolution of the asteroid belt

Meteorite Connections

V-type asteroids are the likely source of HED meteorites, providing ground-truth for asteroid composition studies.[36] This connection enables:

  • Laboratory analysis of asteroid material
  • Calibration of remote sensing techniques
  • Understanding of space weathering processes
  • Chronology of asteroid belt evolution

Future Research

Ongoing and future research priorities include:[37]

  • Spectroscopic surveys to identify additional V-types
  • Detailed compositional studies of non-Vestoid V-types
  • Dynamical modeling of V-type distribution
  • Search for olivine-rich V-types sampling mantle material
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See also

References

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