  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.
