Thermal inertias of pebble-pile comet 67P/Churyumov-Gerasimenko

TL;DR

This study estimates the thermal inertias of comet 67P/Churyumov-Gerasimenko to infer that its building blocks are cm- to dm-sized pebbles, supporting the idea of planetesimal formation through pebble accretion.

Contribution

It provides a novel calculation of thermal inertias and skin depths linking comet surface data to pebble sizes, advancing understanding of comet and planetesimal formation.

Findings

Thermal inertias are consistent with cm- to dm-sized pebbles.

Supports hierarchical dust aggregate model of comet surface.

Indicates formation of icy planetesimals via pebble accretion.

Abstract

The Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided new data to better understand what comets are made of. The weak tensile strength of the cometary surface materials suggests that the comet is a hierarchical dust aggregate formed through gravitational collapse of a bound clump of small dust aggregates so-called ``pebbles'' in the gaseous solar nebula. Since pebbles are the building blocks of comets, which are the survivors of planetesimals in the solar nebula, estimating the size of pebbles using a combination of thermal observations and numerical calculations is of great importance to understand the planet formation in the outer solar system. In this study, we calculated the thermal inertias and thermal skin depths of the hierarchical aggregates of pebbles, for both diurnal and orbital variations of the temperature. We found that the thermal inertias of the comet 67P/Churyumov-Gerasimenko are consistent with the hierarchical aggregate of cm- to dm-sized pebbles. Our findings indicate that the icy planetesimals may have formed via accretion of cm- to dm-sized pebbles in the solar nebula.