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Ice Coverage of Dust Grains in Cold Astrophysical Environments PHYSICAL REVIEW LETTERS 124, 221103 (2020) Editors' Suggestion Featured in Physics Ice Coverage of Dust Grains in Cold Astrophysical Environments Alexey Potapov,1,* Cornelia Jäger,1 and Thomas Henning2 1Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany 2Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany (Received 24 February 2020; revised manuscript received 14 April 2020; accepted 1 May 2020; published 5 June 2020) Surface processes on cosmic solids in cold astrophysical environments lead to gas-phase depletion and molecular complexity. Most astrophysical models assume that the molecular ice forms a thick multilayer substrate, not interacting with the dust surface. In contrast, we present experimental results demonstrating the importance of the surface for porous grains. We show that cosmic dust grains may be covered by a few monolayers of ice only. This implies that the role of dust surface structure, composition, and reactivity in models describing surface processes in cold interstellar, protostellar, and protoplanetary environments has to be reevaluated. DOI: 10.1103/PhysRevLett.124.221103 Introduction.—Dust grains play a central role in the laboratory (see, e.g., Ref. [9]). These densities vary physics and chemistry of interstellar and circumstellar dramatically from object to object. The column density environments. They influence the thermal properties of of water ice in prestellar cores and protostellar envelopes the medium by absorption and emission of stellar light, determined from the 3 μm band (the cleanest measure of provide surfaces for chemical reactions responsible for the the H2O ice column density) ranges from synthesis of a major part of important astronomical 0.3 × 1018 to 12 × 1018 cm−2 [10–12]. Assuming that ice molecules from the simplest ones such as H2 and H2O covers a compact dust core and taking a typical value of to complex organic molecules, e.g., sugars and alcohols, 1015 molecules per 1 cm2 per monolayer (ML), we can and they are building blocks of comets, asteroids, plane- determine the coverage of grains by water ice to be in the tesimals, and planets. range between 300 and 12 000 monolayers depending on It is well known that dust grains in cold cosmic the grain size and the physical conditions (temperature, environments such as molecular clouds and protostellar pressure) of the astrophysical environment. Physical and envelopes and planet-forming disks beyond the snow line chemical processes occurring in such ices are barely are mixed with molecular ices. Typical sketches [1,2] show sensitive to the properties of the dust surface. a compact dust core surrounded by a thick ice layer. The On this basis, the majority of the laboratory experiments dust consists mainly of carbon- and silicate-based com- modeling physical and chemical processes on the surface of pounds [3,4] and the ice includes H2O (the main constituent cosmic dust grains have been performed on thick multilayer accounting for more than 60% of the ice in most lines of ice mixtures (molecular solids) covering standard labora- sight [5]), CO, CO2,NH3,CH4,CH3OH, and a number of tory substrates, which might not be characteristic of cosmic minor components [6,7]. There is a remarkable diversity in dust grains. Adsorption, desorption, and reactivity of the mixing ratios of molecular ices in interstellar and different molecules and radicals in molecular solids have circumstellar environments. Abundances for some species been studied extensively during the past decades. We refer with respect to H2O range within more than an order of the reader to a number of review papers on these topics magnitude. The same is true for cometary ices [8]. [1,13–16]. Ice column densities are calculated from the band Compared to molecular solids, there is a handful of areas of their astronomically observed vibrational transi- studies of the physics and chemistry on the surface of dust tions using appropriate band strengths measured in the grain analogs. We refer to recent papers on the formation [17,18] and desorption [19] of molecules. Summarizing the findings of previous studies, it has been shown that (i) the efficiency of molecule formation depends on the morphol- Published by the American Physical Society under the terms of ogy of the grain surface, (ii) the binding energies of species the Creative Commons Attribution 4.0 International license. can be quite different for different grain surfaces and on the Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, grain surfaces compared to molecular solids, (iii) functional and DOI. Open access publication funded by the Max Planck groups and atoms of the grain surface can participate Society. directly in surface reactions, (iv) the grain surface has a 0031-9007=20=124(22)=221103(7) 221103-1 Published by the American Physical Society PHYSICAL REVIEW LETTERS 124, 221103 (2020) catalytic effect, (v) desorption kinetics and yields of volatile fractal dimension of 1–2 and the porosity of more than molecules are different for different grain surfaces and 90%. In protoplanetary disks, such particles are believed compared to molecular solids, (vi) water molecules can be to stick to each other due to molecular forces forming trapped on the grain surface at temperatures above the kilometer-sized planetesimals that, in turn, form planets due desorption temperature of water ice. to their gravity. The formation of planets is often accom- Thus, dust grains play an important role in processes panied by the formation of debris disks around a central star occurring on their surfaces. If the ice coverage of grains in [33]. In debris disks, a new, top-down mechanism comes molecular clouds, protostellar envelopes, and planet-form- into play. Small dust grains can be formed through colli- ing disks is much thinner than what is typically assumed, sional cascades started by collisions of planetesimals. the role of the grain surfaces in the physical and chemical The high porosity of interstellar and protoplanetary dust processes in cold astrophysical environments is under- grains is also suggested by the analysis of cometary dust estimated and the results obtained in the laboratory studies particles. These particles should be the same materials that, for molecular solids have to be evaluated in their astro- mixed with ices, formed comets when the Solar System physical implications. formed [34]. Analysis of radar [35] and in situ [36–39] Porosity of dust grains.—Analysis of cometary dust measurements of cometary comas, modeling of the light- particles, dust evolution models, and laboratory experi- scattering properties of the cometary dust [40], and analysis ments have indicated that grains in interstellar clouds, of the samples collected and returned by the Stardust protostellar envelopes, and protoplanetary disks may be mission [41] showed that cometary dust particles should very porous, meaning the existence of a very large surface. present a mixture of dense and fluffy, highly porous There are a number of mechanisms responsible for the aggregates. The fluffy particles are typically considered formation of dust grains in interstellar and circumstellar as fractal aggregates of nanometer-sized grains that may be environments. First, nanometer-sized grains are formed in linked to interstellar dust. Such fractal particles can have outflows of evolved stars and then distributed into the microporosity of more than 99% [42]. interstellar medium (ISM) [3]. Dust extinction observations Thus, there are indications of high porosity of dust grains and models indicate a very efficient growth of dust grains in in interstellar clouds, protostellar envelopes, and planet- the ISM (see, e.g., Ref. [4] for a review), especially in dense forming disks. If this is true and the dust grains in these molecular clouds, where grains grow to fractal aggregates environments are highly porous particles, they have a of submicron-micron sizes having a porosity of up to 80% corresponding large surface. The surface area of their [20]. Dust analog materials with the morphological char- laboratory analogs cannot be measured precisely but can acteristic of interstellar dust can be produced in the be estimated. laboratory by gas-phase condensation of nanometer-sized Surface area of laboratory analogs of cosmic grains.— amorphous carbon or silicate grains and their subsequent Our recent laboratory experiments on cosmic dust grain deposition onto a substrate, where they aggregate (see analogs produced by gas-phase condensation and sub- Ref. [21] for a detailed review). The morphology of such sequent deposition onto a substrate have demonstrated that grain deposits, both carbon and silicate, can be understood the surface area of aggregates of nanometer-sized grains as porous layers of fractal agglomerates having the porosity can be several hundred times larger than the nominal of up to 90% [22,23]. surface area of the dust layer on a substrate, which can In the ISM, dust grains can be completely destroyed by be seen as the surface area of an imaginary compact dust supernova shocks. Estimations showed that only a few core. Our aggregates can be considered as analogs of dust percent of the dust produced by stars survive in the ISM grains in dense molecular clouds, protostellar envelopes, [24]. The second mechanism of the dust grains
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