Contact conductivity K_C of a packed bed of grains of size R (from yellow to dark red) as a function of its porosity p at temperature T = 80 K and emissivity ϵ = 1. Left panel: crystalline ice form. Right panel: amorphous ice form. Curves correspond to Watson theory (W, full line; loose contacts), Gusarov model for porosity coupled with JKR contact theory (G+JKR, dashed line; tight contacts) and Gundbach and Blum (GB, dotted line; tight contacts) models.