  Comet C/2013 US10 (Catalina) was an dynamically new Oort cloud comet whose
apparition presented a favorable geometry for observations near close Earth
approach (~0.93au) at heliocentric distances ~2au when insolation and
sublimation of volatiles drive maximum activity. Here we present mid-infrared
spectrophotometric observations at two temporal epochs from NASA's
Stratospheric Observatory for Infrared Astronomy and the NASA Infrared
Telescope Facility. The grain composition is dominated by dark dust grains
(modeled as amorphous carbon) with a silicate-to-carbon ratio ~0.9, little of
crystalline stoichiometry (no distinct 11.2um feature attributed to Mg-rich
crystalline olivine), the submicron grain size distribution peaking at ~0.6um.
The 10um silicate feature was weak, ~12.8% above the local continuum, and the
bolometric grain albedo was low (~14%). Comet Catalina is a carbon-rich object.
This material, which is well-represented by the optical constants of amorphous
carbon is similar to the material that darkens and reddens the surface of comet
67P/Churyumov-Gerasimenko. We argue this material is endemic the nuclei of
comets, synthesizing results from the study of Stardust samples, interplanetary
dust particle investigations and micrometeoritic analyses. The atomic
carbon-to-silicate ratio of comet Catalina and other comets joins a growing
body of evidence suggesting the existence of a C/Si gradient in the primitive
solar system.
