Eclipsing Binary Trojan Asteroid Patroclus: Thermal Inertia from Spitzer Observations

TL;DR

This study uses Spitzer mid-infrared observations of the binary Trojan asteroid Patroclus to directly measure its thermal inertia, revealing a surface covered in fine regolith and suggesting a composition rich in water ice.

Contribution

First direct thermal inertia measurement of a Trojan asteroid using real-time eclipse observations with a new thermophysical model.

Findings

Thermal inertia of 20+/-15 MKS indicates fine regolith surface.

Surface heterogeneity suggested by differing local thermal inertia values.

Bulk density implies a water ice dominated composition.

Abstract

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.