Regolith Heterogeneity on (3200) Phaethon and Implications for Activity

The near-Earth asteroid (3200) Phaethon is classified as an active asteroid and is one of the largest objects with a perihelion within the orbit of Mercury. A dust tail has been observed during each of the last 3 closest approaches with the Sun. The activity, which is likely not driven by volatile sublimation, strongly suggests that Phaethon is the most likely parent body of the annual Geminid meteor shower. Phaethon is the main target for the JAXA DESTINY+ mission that will measure the dust environment, among other goals.

Regolith properties in the near-surface of asteroid can be inferred from the thermal inertia, Γ = √kρc. The effective thermal conductivity, k, is the most influential controlling factor in thermal inertia and can be expressed as a sum of the radiative conductivity and solid conductivity, which are controlled by the grain size and porosity of the regolith: specifically, the conduction through contacts through the regolith grains and the radiative transfer within the pores between them. Gundlach and Blum (2013) presented a thermal conductivity model that accurately models both these effects for planetary regoliths.

Using a radar-derived shape model and thermal infrared observations from 10 observing epochs, we estimate Phaethon’s thermal inertia independently for each epoch. We find that Phaethon’s thermal inertia increases with decreasing heliocentric distance. Using the regolith thermal conductivity model presented by Gundlach and Blum (2013), we model the thermal inertia as a function of heliocentric distance for various grain sizes and porosities. We find that larger grain sizes are consistent with smaller heliocentric distances and smaller grain sizes are consistent with larger heliocentric distances.

We consider two scenarios for Phaethon's regolith: 1) a depth-dependent, two-component model (fine-grained regolith over bedrock) and 2) a two-component hemispherical model (different grain sizes for the northern and southern hemispheres). The effective thermal inertia of the layered regolith model exhibits a stronger dependence on heliocentric distance, but does not match the observed thermal inertia estimate for each epoch. The hemispherical regolith model is very consistent with the observed thermal inertias, from which we conclude that the northern hemisphere consists of larger grains compared to Phaethon’s southern hemisphere.

We propose that Phaethon’s dust tail, which occurs at perihelion, may be caused by landslide activity triggered by changing illumination conditions during perihelion passage that help to charge large grains that exist on the northern latitudes.