Evidence of surface heterogeneity on active asteroid (3200) Phaethon

Abstract

Thermal infrared emission and thermophysical modeling techniques are powerful tools in deciphering the surface properties of asteroids. The near-Earth asteroid (3200) Phaethon is an active asteroid with a very small perihelion distance and is likely the source of the Geminid meteor shower. We estimate and interpret the thermal inertia of this extraordinary asteroid using observations that span ten distinct sightings. The variation in thermal inertia over these sightings is inconsistent with the expected temperature-dependent thermal inertia theorized from radiative heat transfer within the regolith. Thus, we test whether the variation in thermal inertia can be explained by modeling a regolith layer over bedrock and two spatially heterogeneous scenarios. We find that the model in which Phaethon's north and south hemispheres have distinctly different thermophysical properties can sufficiently explain the thermal-inertias determined herein. In particular, we find that a boundary located between latitudes −30 • and +10 • separates fine-grained southern latitudes from a northern hemisphere that is dominated by coarse-grained regolith and/or a high coverage of porous boulders. We discuss the implications related to Phaethon's activity and potential association with 2005 UD.

Asteroid (3200) Phaethon: results of polarimetric, photometric, and spectral observations

“…The difference in scattering properties of regolith of different regions of Phaethon's surface may provide large variations of polarization degree over the surface at large phase angles. McLennan et al (2022) also detected the variations in thermal inertia across the Phaethon surface. For explanation, the authors proposed a two-component hemispherical model, in which Phaeton's northern and southern hemispheres have different thermophysical properties due to different material covering a northern (coarse-grained regolith) and southern hemispheres (fine-grained regolith).…”

“…2018) andMcLennan et al (2022) both argue that Phaethon is an insufficient match to Pallas on dynamical and spectroscopic grounds.…”

Thermal decomposition as an activity driver of near-Earth asteroid (3200) Phaethon

“…Phaethon's mid-infrared spectrum does not exhibit any transparency features that may be expected from volumetric scattering of fine-grained silicates [48,49,52,64] and the large spectral emissivity contrast of ∼8% is likely a result of a large thermal gradients from a coarse-grained or boulder-rich surface [62]. The absolute reflectivity of CY chips at 0.55 µm of around 10% are systematically brighter than their powdered counterparts [65,66] which is consistent with Phaethon's geometric albedo of 11% [29].…”

“…6. Using an effective diameter of 5.4 km [62], we estimate Phaethon's surface area to be nearly 100 km 2 . Gas production estimates integrated over the entire surface can be on order of ∼ 10 8 times more than the values in Fig.…”

“…The Spitzer-IRS fluxes [34] are divided by modeled continuum fluxes to generate Phaethon's emissivity spectrum. Continuum fluxes are generated from the TPM by setting the rotation phase for the midpoint time of the observation and use the nominal best-fit values (D eff = 4.2 km, Γ = 480 J K −1 m −2 s −1/2 , and low default roughness) from MacLennan et al (2022) [62] to the IRS fluxes. We employ the loess (locally estimated scatterplot smoothing) algorithm in R to generate a smoothed function for use in the PCA (see below); a 2nd degree polynomial with a span of 0.7 are used.…”