Near-Earth asteroid (3200) Phaethon exhibits activity during its perihelion passage at 0.14 au from the Sun and is the likely parent body of the annual Geminid meteor shower. Its low albedo and featureless B-type reflectance spectrum indicate a primitive composition, but a definitive meteorite analogue is currently indeterminate. Here we analyse a mid-infrared emissivity spectrum of Phaethon and find that it most closely matches the Yamato group (CY) of carbonaceous chondrites. The CY chondrites experienced aqueous alteration and recent thermal metamorphism in which extreme temperatures caused mineral decomposition, resulting in the production of gas species. Temperatures within Phaethon during its close approach to the Sun are conducive to the thermal decomposition of carbonates, iron sulfides and phyllosilicates that release CO2, S2 and H2O gas, respectively. Spectral detection of these minerals strongly implies that gas release from mineral decomposition is capable of triggering dust ejection. The planned flyby of Phaethon by the DESTINY+ spacecraft in 2028 will allow us to verify this hypothesis.
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All datasets analysed in this study are publicly available. The mid-infrared flux spectrum of Phaethon measured by the Spitzer Space Telescope’s Infrared Spectrograph instrument (AOR 4890624) is publicly available on the Spitzer Heritage Archive (https://sha.ipac.caltech.edu/). The reduced spectrum was published in ref. 26 and shared with the author(s) by Joshua P. Emery, a co-author of that study. Laboratory spectra of meteorites and minerals (Supplementary Table 2) can be downloaded from the RELAB database via the PDS Geosciences Node Spectral Library (https://pds-speclib.rsl.wustl.edu/), ASU (https://speclib.asu.edu) and CRISM spectral libraries (http://speclib.rsl.wustl.edu/).
The orbTPM code is available on GitHub at https://github.com/cosmicdustbeing/asteroid-thermal. R packages (stats) are publicly available for download.
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This work is based in part on observations made with the Spitzer Space Telescope, operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. We thank J. P. Emery for sharing the previously published Spitzer-IRS spectrum of Phaethon.
The authors declare no competing interests.
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Principal components (PC#) of mid-infrared spectra of carbonaceous chondrites. Datapoints are labeled and colored by their meteorite group with ‘CMh’ indicating naturally heated carbonaceous chondrites and ‘Mur’ corresponding to samples of the Murchison meteorite heated to the indicated temperature. Phaethon is indicated by the red star.
Fraction of decomposed material (carbonate, phyllosilicate, or sulfide in panels a., b., and c., respectively) after one perihelion passage as a function of depth beneath the surface, expressed in terms of the thermal skin depth (ls)42.
Extended Data Fig. 3 Comparison between the visible and near-infrared reflectance spectra of Phaethon and CY meteorites.
Visible and near-infrared reflectance spectra of Phaethon and two CY meteorites ground into different size fractions or left as whole rock ‘chips’. The 1σ uncertainty for each reflectance value are shown as orange vertical bars. The blue line represents the average near-infrared (0.75 − 2.5 μm) spectral slope of 2005 UD, as observed by Kareta et al. (2021)20.
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MacLennan, E., Granvik, M. Thermal decomposition as the activity driver of near-Earth asteroid (3200) Phaethon. Nat Astron (2023). https://doi.org/10.1038/s41550-023-02091-w