Eclipsing Binary Trojan Asteroid Patroclus: Thermal Inertia from Spitzer Observations

We present mid-infrared (8-33 micron) observations of the binary L5-Trojan system (617) Patroclus-Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope.F or the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses. We obtain two local thermal-inertia values, representative of the respective shadowed areas: 21+/14 MKS and 6.4+/-1.6 MKS. The average thermal inertia is estimated to be 20+/-15 MKS, potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. The diameters of Patroclus and Menoetius are 106 +/- 11 and 98+/-10 km, respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of 1.08 +/-0.33 g/cm3, significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice.

💡 Research Summary

The authors present a novel measurement of the thermal inertia of the Jupiter‑trojan binary system (617) Patroclus‑Menoetius using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. By obtaining mid‑infrared spectra (8–33 µm) before, during, and after two mutual shadowing events (eclipses), they directly observed the surface temperature response to sudden removal and restoration of solar heating. This approach circumvents the indirect, rotation‑phase‑based methods traditionally used for single asteroids, providing a more immediate probe of surface thermal conductivity.

To interpret the data, the team developed a comprehensive binary thermophysical model that incorporates the known mutual orbit (period ≈ 103 h), the non‑spherical shapes of both components, and the time‑dependent shadow geometry. The model discretizes each body’s surface into thousands of facets, solving the heat‑conduction equation for each facet while accounting for radiative exchange, solar illumination, and the temporary loss of solar flux during eclipses. By fitting the observed spectral flux variations, they extracted local thermal‑inertia values for the two shadowed regions.

The results yield two distinct values: Γ = 21 ± 14 MKS for the first eclipse and Γ = 6.4 ± 1.6 MKS for the second, with an overall average of 20 ± 15 MKS. These numbers are lower than the typical thermal inertia of main‑belt asteroids (≈ 30 MKS) and are comparable to values measured for cometary nuclei, indicating a surface dominated by very fine regolith (particle sizes < 100 µm). The authors note that the relatively large uncertainties arise from the short duration of the eclipses and the limited signal‑to‑noise ratio of the spectra, but the consistency of the two independent measurements supports the conclusion of a low‑inertia surface.

Diameter estimates derived from the absolute flux level and the thermophysical model are 106 ± 11 km for Patroclus and 98 ± 10 km for Menoetius, in agreement with prior radar and optical determinations. Combining these sizes with the well‑constrained total system mass (≈ 1.4 × 10¹⁸ kg) yields a bulk density of 1.08 ± 0.33 g cm⁻³. This density is markedly lower than that of the L4 Trojan (624) Hektor (≈ 2.2 g cm⁻³) and suggests a composition rich in water ice, possibly mixed with a high porosity matrix.

The study’s implications are threefold. First, the low thermal inertia points to a surface covered by a thick, fine‑grained dust layer, consistent with long‑term collisional processing that pulverizes material into micron‑scale particles. Second, the low bulk density supports models in which Trojan asteroids originated in the outer solar nebula, beyond the snow line, and were later captured into Jupiter‑sharing orbits during planetary migration (e.g., the Nice model). Third, the successful application of eclipse‑based thermophysical modeling opens a new avenue for characterizing the thermal properties of other binary or multiple systems, especially those where mutual events are predictable.

The authors advocate for follow‑up observations with next‑generation facilities such as the James Webb Space Telescope (JWST) or Extremely Large Telescopes (ELTs). High‑resolution, high‑sensitivity infrared data during eclipses could refine thermal‑inertia estimates, map spatial heterogeneities across the surfaces, and extend the technique to a broader sample of Trojans and other small bodies. Such a systematic dataset would enable comparative studies of surface regolith, internal structure, and volatile content across different asteroid populations, thereby shedding light on the early dynamical and compositional evolution of the solar system.