A Scorched Story: JWST Reveals Phaethon's Dehydrated Surface Composition and Thermal History

Asteroid (3200) Phaethon is a unique near-Earth asteroid with a perihelion of 0.14 AU. It experiences extreme temperatures exceeding 1000 °C during its close solar approaches. The asteroid follows a highly eccentric orbit (e ~ 0.89) and is notable as the parent body of the Geminid meteor shower (a rare case where its origins come from an asteroid rather than a comet [1]). Observations from the Solar Terrestrial Relations Observatory (STEREO) have shown sodium emissions from Phaethon at perihelion [2]. These emissions suggest that the asteroid’s surface undergoes active and potentially transformative processes under intense solar heating. Spectroscopic data from the NASA Infrared Telescope Facility (IRTF) and the James Webb Space Telescope (JWST) show no detectable 3-µm absorption feature, indicating that Phaethon’s surface is dehydrated [3][4]. Additionally, JWST GTO Program 1245 observed Phaethon with the Mid-Infrared Instrument (MIRI), providing insights into compositional properties in this wavelength region. This presentation will discuss in-depth research on Phaethon’s thermal history and surface evolution.To model Spitzer IRS observations of (3200) Phaethon, Maclennan & Granvik (2024)[5] looked for meteorite analogs and corresponding minerals for the asteroid from the RELAB database[6]. Their work’s analysis suggests that CY carbonaceous chondrites are good meteorite analogs, with olivine samples of varying forsterite percentages as good mineral analogs. Starting materials for their linear mixing model were selected to represent primitive, aqueously altered compositions (carbonaceous chondrites) and minerals that are products of Phaethon’s thermal evolution (secondary olivine, enstatite). We build on this study with our higher resolution and signal-to-noise JWST MIRI data [4], starting with similar carbonaceous chondrites and olivine samples in our analysis.The meteorite analog and olivine (fo40 to fo80) mid-infrared data from RELAB were compared to the collected asteroid spectrum to begin constraining the mineralogical signatures observed in the JWST MIRI data. Both qualitative and quantitative spectral matching techniques were applied, and the results indicate that CY carbonaceous chondrite Y-86720 [4] appears to be the best match to Phaethon’s mid-infrared spectrum (Fig. 1). In addition, fo68 was the best-fit match to the olivine present on the surface, which differs from the previously best-fit forsterite percentage found in the previously published work [5]. This is likely due to the SNR of our data allowing for viewing of specific features like those in the 17 micron region.With these constraints, we used a linear mixing model to combine mineral spectra and compare to the MIRI spectrum. We use mineral spectra from the RELAB database, including minerals relevant to olivine hydration and thermal alteration pathways. Troilite (FeS) was included in the mixing model to account for potential sulfide phases, though its use was minimized in the fits as troilite is expected to become unstable and devolatilize at the temperatures that Phaethon reaches near perihelion [7]. A mean absolute error (MAE) calculation was performed across the wavelength range, providing a quantitative assessment of how effectively the model reproduced the mid-infrared spectra. This modeling approach was compared to previous efforts [5] that utilized the Spitzer spectrum. A diagnostic feature at 17.5 µm, corresponding to the Christensen feature associated with phyllosilicate vibrations, is notably flat in the JWST MIRI data. This behavior suggests a high degree of dehydration, as this diagnostic feature disappears with thermal alteration of the phyllosilicates. The updated JWST-based modeling (Fig. 2) provides a more refined view of Phaethon’s surface composition, indicating dehydration and thermal processing from experiencing the high temperatures with close passes to the Sun that was not fully captured by the Spitzer observations. The model analysis suggests that Phaethon’s surface likely includes enstatite and secondary olivine that would have formed alongside the enstatite during thermal processing.We are also interested in how solar heating alters the asteroid’s surface and subsurface, as remote observations only give us surface-level data. To explore this, we employed a thermal conduction model (KRC) to simulate the diurnal and annual heating cycles. This allowed for examination of how deeply high temperatures penetrated into the asteroid’s subsurface. The takeaway from this work is that primitive and aqueously altered rock is likely not found too deep under the surface because such materials are less affected by the intense heat of perihelion. Our thermal models show a dropoff of temperature as two or three skin depths are passed.We will present our in-depth research into Phaethon’s mid-infrared spectrum, linear mixing model, and thermal model. Our work, which uses new JWST data and expands on previous studies, will yield a better understanding of the composition of Phaethon and its thermal history.  References:[1] https://blogs.nasa.gov/parkersolarprobe/2023/06/14/scientists-shed-light-on-the-unusual-origin-of-a-familiar-meteor-shower/[2] Qicheng Zhang et al 2023 Planet. Sci. J. 4 70[3] Takir, D., Kareta, T., Emery, J.P. et al. Near-infrared observations of active asteroid (3200) Phaethon reveal no evidence for hydration. Nat Commun 11, 2050 (2020). https://doi.org/10.1038/s41467-020-15637-7[4] https://doi.org/10.48550/arXiv.2505.00692[5] MacLennan, E., Granvik, M. Thermal decomposition as the activity driver of near-Earth asteroid (3200) Phaethon. Nat Astron 8, 60–68 (2024). https://doi.org/10.1038/s41550-023-02091-w[6] This research utilizes spectra acquired from Ralph E. Milliken, Tim Glotch, Don Lindsay, and others with the NASA RELAB facility at Brown University[7] https://www.hou.usra.edu/meetings/metsoc2024/pdf/6409.pdf