Effect of nucleation on icy pebble growth in protoplanetary discs

Katrin Ros; Anders Johansen; Ilona Riipinen; Daniel Schlesinger
2019 | Astron Astrophys | 629 (A65)

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.

Cloud droplet activation of black carbon particles with organic compounds of varying solubility

Dalirian, M.; Ylisirnio, A.; Buchholz, A.; Schlesinger, D.; Strom, J.; Virtanen, A.; Riipinen, I.
2018 | Atmos. Chem. Phys. | 18 (12477-12489)

X-ray Scattering and O-O Pair-Distribution Functions of Amorphous Ices

Mariedahl, D; Perakis, F; Spah, A; Pathak, H; Kim, KH; Camisasca, G; Schlesinger, D; Benmore, C; Pettersson, LGM; Nilsson, A; Arnann-Winkel, K
2018 | J. Phys. Chem. B | 122 (30) (7616-7624)
1st-order transition , diffraction , dynamics , high-density , hyperquenched glassy water , liquid water , neutron-scattering , phases , pressure-induced amorphization , xii
The structure factor and oxygen-oxygen pair distribution functions of amorphous ices at liquid nitrogen temperature (T = 77 K) have been derived from wide-angle X-ray scattering (WAXS) up to interatomic distances of r = 23 angstrom, where local structure differences between the amorphous ices can be seen for the entire range. The distances to the first coordination shell for low-, high-, and very-high-density amorphous ice (LDA, HDA, VHDA) were determined to be 2.75, 2.78, and 2.80 angstrom, respectively, with high accuracy due to measurements up to a large momentum transfer of 23 angstrom(-1). Similarities in pair-distribution functions between LDA and supercooled water at 254.1 K, HDA and liquid water at 365.9 K, and VHDA and high-pressure liquid water were found up to around 8 angstrom, but beyond that at longer distances, the similarities were lost. In addition, the structure of the high-density amorphous ices was compared to high-pressure crystalline ices IV, IX, and XII, and conclusions were drawn about the local ordering.

Diffusive dynamics during the high-to-low density transition in amorphous ice

Perakis, F; Amann-Winkel, K; Lehmkuhler, F; Sprung, M; Mariedahl, D; Sellberg, JA; Pathak, H; Spaeh, A; Cavalca, F; Schlesinger, D; Ricci, A; Jain, A; Massani, B; Aubree, F; Benmore, CJ; Loerting, T; Grubel, G; Pettersson, LGM; Nilsson, A
2017 | Proc. Natl. Acad. Sci. U.S.A. | 114 (31) (8193-8198)
1st-order transition , amorphous ice , behavior , glass transition , glass-liquid transition , liquid-liquid transition , phases , photon-correlation spectroscopy , pressure , reorientation , speckle , supercooled water , water , x-ray photon-correlation spectroscopy
Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high(HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid-liquid transition in the ultraviscous regime.

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