The fate of icy pebbles undergoing sublimation in protoplanetary discs

Abstract

Icy pebbles may play an important role in planet formation close to the water ice line of protoplanetary discs. There, dust coagulation is more efficient and re-condensation of vapor on pebbles may enhance their growth outside the ice line. Previous theoretical studies showed that disruption of icy pebbles due to sublimation increases the growth rate of pebbles inside and outside the ice line, by freeing small silicate particles back in the dust reservoir of the disc. However, since planet accretion is dependent on the Stokes number of the accreting pebbles, the growth of planetesimals could be enhanced downstream of the ice line if pebbles are not disrupting upon sublimation. We developed two experimental models of icy pebbles using different silicate dusts, and we exposed them to low-temperature and low-pressure conditions in a vacuum chamber. Increasing the temperature inside the chamber, we studied the conditions for which pebbles are preserved through sublimation without disrupting. We find that small silicate particles (< 50 µm) and a small quantity of ice (around 15 % pebble mass) are optimal conditions for preserving pebbles through sublimation. Furthermore, pebbles with coarse dust distribution (100 − 300 µm) do not disrupt if a small percentage (10 − 20 % mass) of dust grains are smaller than 50 µm. Our findings highlight how sublimation is not necessarily causing disruption, and that pebbles seem to survive fast sublimation processes effectively.

Are there any pristine comets? Constraints from pebble structure

“…As already mentioned in Section 2.2.3, the pebbles are not expected to crumble during the sublimation process, despite losing some of their internal volume. As long as the rock/ice mass fraction is high (which is also the case here, taking after comet 67P/C-G), various studies show that the pebbles do not desiccate when the ice sublimates away (Haack et al 2021b,a;Spadaccia et al 2021). Under certain conditions this might even be true for ice-rich pebbles (Spadaccia et al 2021).…”

“…Our choice to employ the solution only at the beginning of the calculation originates from the fact that even as the comet warms and volatiles migrate out and freeze in the cold outer layers, while it can change the density profile, the pressure by self-gravity remains similar throughout the calculation. Additionally, neither does vapour transport desiccate the pebbles structure or undermine its stability (Haack et al 2021b,a;Spadaccia et al 2021), as will be further discussed in Section 4.3.…”