Solid particles in protoplanetary discs can grow by direct vapour deposition outside of ice lines. The presence of microscopic silicate particles may nevertheless hinder growth into large pebbles, since the available vapour is deposited predominantly on the small grains that dominate the total surface area. Experiments on heterogeneous ice nucleation, performed to understand ice clouds in the Martian atmosphere, show that the formation of a new ice layer on a silicate surface requires a substantially higher water vapour pressure than the deposition of water vapour on an existing ice surface. In this paper, we investigate how the difference in partial vapour pressure needed for deposition of vapour on water ice versus heterogeneous ice nucleation on silicate grains influences particle growth close to the water ice line. We developed and tested a dynamical 1D deposition and sublimation model, where we include radial drift, sedimentation, and diffusion in a turbulent protoplanetary disc. We find that vapour is deposited predominantly on already ice-covered particles, since the vapour pressure exterior of the ice line is too low for heterogeneous nucleation on bare silicate grains. Icy particles can thus grow to centimetre-sized pebbles in a narrow region around the ice line, whereas silicate particles stay dust-sized and diffuse out over the disc. The inhibition of heterogeneous ice nucleation results in a preferential region for growth into planetesimals close to the ice line where we find large icy pebbles. The suppression of heterogeneous ice nucleation on silicate grains may also be the mechanism behind some of the observed dark rings around ice lines in protoplanetary discs, as the presence of large ice pebbles outside ice lines leads to a decrease in the opacity there.
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Acknowledgements. The authors would like to thank the referee, Joanna Dra¸żkowska, for her comments that helped us improve the paper. Helene Stal-heim and Hanna Vehkamäki are acknowledged for discussions on CNT and ice nucleation. K.R. and A.J. acknowledge funding by the European Research Council (ERC Starting Grant 278 675-PEBBLE2PLANET). A.J. was also supported by the Knut and Alice Wallenberg Foundation (Wallenberg Academy Fellow grant 2012.0150 and grant 2014.0048 “Bottlenecks for particle growth in turbulent aerosols”) and the Swedish Research Council (grant 2014-5775). I.R. and D.S. were supported by AtmoRemove.
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- Astronomi, astrofysik och kosmologi