Eclipsing binary Trojan asteroid Patroclus: Thermal inertia from Spitzer observations

We present mid-infrared ( 8 – 33 μ m ) 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. For 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 J s - 1 / 2 K - 1 m - 2 and 6.4 ± 1.6 J s - 1 / 2 K - 1 m - 2 . The average thermal inertia is estimated to be 20 ± 15 J s - 1 / 2 K - 1 m - 2 , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (