Sublimation of buried cometary ice

The sublimation of ice covered by dust is a common way of outgassing of comets. The rate of this process depends on the thermal conductivity and permeability of dust, as well as the properties of the ice itself. It can be calculated using the Hertz-Knudsen equation, corrected for temperature dependent sublimation coefficient αs(T) (Gundlach et al., 2011; Kossacki et al., 1999; Kossacki & Leliwa-Kopystynski, 2014; Kossacki et al., 2017) and the Knudsen equation for the permeability of dust. In this work we present direct experimental verification of this set of equations using sand with different granulation: coarse (grain radii rg = 0.5 − 1 mm), medium (rg = 0.25 − 0.5 mm), fine (rg = 0.125 = 0.25 mm) and unsorted (rg = 0 − 1 mm, rg, av = 0.5 mm). We have found that at T < 220 K the measured rate of recession of the surface is correctly reproduced if αs(T) is the same as determined for the uncovered ice (Kossacki & Leliwa-Kopystynski, 2014). The characteristic radius of pores in the Knudsen equation is approximately equal to, or greater than the arithmetic average of the largest and the smallest radii of grains.