Spectroscopy: matching meteorites to asteroids
Sunlight reflected from an asteroid carries the absorption fingerprints of its minerals. Comparing those spectra with meteorites in the laboratory links meteorites to the asteroids they came from.
- Calculate photon energies from wavelengths with E = hc/λ.
- Explain why olivine and pyroxene absorb near 1 μm and 2 μm.
- Describe how Vesta was linked to the HED meteorites.
Links to: A-level physics, quantum phenomena and waves: photon energy, E = hf = hc/λ, energy levels and absorption spectra.
Reflectance spectra
An asteroid's light is reflected sunlight. Dividing the asteroid's spectrum by the Sun's gives its reflectance spectrum, which shows how much light it reflects at each wavelength. Dips in the spectrum, absorption bands, are where minerals absorb photons of particular energies.
A photon of wavelength λ has energy E = hc/λ, with h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹. A photon is absorbed only if its energy matches a transition between energy levels in the material.
Iron in crystals
In olivine and pyroxene, Fe²⁺ ions sit in sites surrounded by oxygen atoms. The electric field of these neighbours splits the iron's d-orbital energy levels. Electron transitions between the split levels absorb near-infrared photons: olivine has a broad band centred near 1.05 μm, and pyroxene has two bands, near 0.9 to 1.0 μm and 1.9 to 2.0 μm. The exact band positions shift with the mineral's composition, so they reveal both which minerals are present and roughly in what proportions.
Vesta and the HEDs
In 1970 Thomas McCord and colleagues measured Vesta's spectrum and found a strong pyroxene absorption that matched basaltic eucrites in the laboratory. This, together with the discovery of small 'Vestoids' with the same spectra strung between Vesta and the 3:1 resonance, linked the HED meteorites to Vesta. The Dawn spacecraft, orbiting Vesta in 2011 and 2012, confirmed the match and found the huge Rheasilvia impact basin near the south pole, the probable source of many HEDs.
The common S-type asteroids were linked to ordinary chondrites in a similar way, a link confirmed in 2010 when the Hayabusa mission returned grains from the S-type asteroid Itokawa that matched LL chondrites.
The energy of an absorbed photon
Calculate the energy, in joules and electronvolts, of a photon at a pyroxene absorption wavelength of 0.92 μm. (1 eV = 1.60 × 10⁻¹⁹ J)
- E = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) ÷ (0.92 × 10⁻⁶).
- Numerator: 1.989 × 10⁻²⁵ J m.
- E = 1.989 × 10⁻²⁵ ÷ 9.2 × 10⁻⁷ ≈ 2.16 × 10⁻¹⁹ J.
- In eV: 2.16 × 10⁻¹⁹ ÷ 1.60 × 10⁻¹⁹ ≈ 1.35 eV.
Answer: About 2.2 × 10⁻¹⁹ J, or 1.35 eV: the energy gap between split d-orbital levels of Fe²⁺ in pyroxene.
Calculate the photon energies, in eV, for the 1.05 μm olivine band and the 1.95 μm pyroxene band. Then explain why S-type asteroid spectra appear redder and with weaker bands than freshly cut ordinary chondrites (research 'space weathering' in the glossary).
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What is a reflectance spectrum?
Dividing the observed spectrum by the solar spectrum gives how much light the surface reflects at each wavelength.
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Question 2What causes the 1 μm absorption band in olivine?
The crystal field splits the Fe²⁺ d-levels; transitions absorb near-infrared photons.
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Question 3What is the energy of a 1.24 μm photon? (hc ≈ 1.24 × 10⁻⁶ eV m)
E = hc/λ = 1.24 × 10⁻⁶ eV m ÷ 1.24 × 10⁻⁶ m = 1.0 eV.
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Question 4Which spacecraft confirmed the link between Vesta and the HED meteorites?
Dawn orbited Vesta in 2011–2012 and mapped its composition and the Rheasilvia basin.
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Question 5Which mission's samples showed that S-type asteroids are related to ordinary chondrites?
Grains returned from Itokawa in 2010 matched LL ordinary chondrites.