Dust traps in protoplanetary disks 
efficiently form organic macromolecular matter

doi:10.21203/rs.3.rs-3467633/v1

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Dust traps in protoplanetary disks efficiently form organic macromolecular matter

https://doi.org/10.21203/rs.3.rs-3467633/v1

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published 30 Jul, 2024

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The chondritic building blocks from which Earth formed contain most of their elemental carbon and other volatiles in organic macromolecular matter. This substance is essential to set the chemical composition of terrestrial planets and start the emergence of life, but how it formed is unknown. We demonstrate that this organic macromolecular matter can efficiently form in dust traps – a prominent planet formation mechanism – in the solar nebula. By combining a protoplanetary disk dust evolution model with radiative transfer, we confirm that dust traps host regimes where ice-coated grains receive exceptionally large doses of radiation of several 10’s of eV molecule^-1 year^-1. This allows for the processing of approximately 4% of the total disk ice reservoir from simple molecules into complex macromolecular matter in a matter of decades. This finding shows that planet formation and the emergence of organic macromolecular matter that sets habitable conditions, are intimately linked.

Physical sciences/Astronomy and planetary science/Astronomy and astrophysics/Astrophysical disks

Physical sciences/Chemistry/Chemical origin of life

Physical sciences/Astronomy and planetary science/Planetary science/Meteoritics

Physical sciences/Physics/Chemical physics

Chondrites are a main component of meteorites and were the building blocks of the terrestrial planets that supplied their volatile elements¹. Organic macromolecules, often collectively referred to as Insoluble Organic Matter (IOM), in chondrites are the dominant carriers of the volatile elements carbon, nitrogen and noble gases² and are thought to have directly contributed to the emergence of life³. Characterization of IOM has helped suggest formation mechanisms, but it remains unclear whether this material formed in the proto-Solar Nebula or interstellar medium, and if this happened via plasma, gas-phase, liquid-phase, or solid-state reactions².

We demonstrate that IOM could efficiently form in the proto-Solar Nebula, via radiation-driven ice chemistry. Using dust evolution models of protoplanetary disks, we found for the first time that dust traps – a prominent planet-forming mechanism – provide an environment where ice-coated dust grains receive an exceptionally high radiation dose that converts simple molecules into complex macromolecular matter within decades to centuries - a tiny fraction of the typical disk lifetime (few Myr). Furthermore, we found that large quantities of ice, in the order of 4% of the total disk ice reservoir, could be processed in this way, consistent with an efficient mechanism. Our findings show a direct link between the mechanism that forms planets and the chemical process that sets their volatile element budget.

The molecular structure of IOM is characterized by small (poly)cyclic aromatic units, linked by short, highly branched aliphatic chains, furan/ether bonds, and additional functional groups, such as ketones and carboxyls, resulting in molecules of hundreds of atoms in size^4,5. Heterogeneous deuterium and ¹⁵N enhancements hint at a cold (< 20 K), radiation-rich formation environment where isotopes are readily fractionated, and molecules are frozen onto dust grains⁶. Subsequent radiation of frozen organics could enhance deuterium enrichments⁷, while heating and aqueous alteration in the parent bodies may have reduced them⁸. IOM contains high and localized concentrations of monoradicals and diradicals⁹, possibly formed during irradiation of organic molecules¹⁰. IOM also contains high concentrations of noble gases which are isotopically fractionated (Ne ~ 40%/amu, Xe ~ 1%/amu) and strongly depleted in light noble gases compared to the solar composition¹¹. Similarities in elemental composition of meteoritic IOM and refractory cometary organic matter indicates that they have the same or a similar origin in the colder environments at larger radial distances from our young Sun in the proto-Solar Nebula¹².

To summarize, many lines of evidence indicate that IOM formed from irradiated frozen molecules. Laboratory studies found that radiation doses of several 100 up to 1000 eV molecule^-1 convert interstellar or comet-like ice into a refractory substance with characteristics that resemble IOM^13–15. Such doses are much higher than what molecules in the solid phase typically experience during the star- and planet-formation cycle. UV and cosmic-ray doses received by ice in dark clouds and protoplanetary disk midplanes during their lifetimes are moderate at several 10s eV molecule^-1 (or 10^− 7 eV molecule^-1 year^-1)¹⁶. Radiation doses received by ice-coated dust grains along the protoplanetary disk atmosphere are similarly moderate and only sufficient to produce Soluble Organic Molecules (SOM)¹⁷, a class of molecules that is usually only several dozens of atoms in size. Until now, it was unknown where large quantities of ice and other frozen material can be heavily irradiated to form IOMs during the star- and planet-formation sequence.

In the last decade, our understanding of planet formation has been revolutionized¹⁸, following the first detection of a dust trap in the protoplanetary disk IRS 48¹⁹. Dust traps are localized pressure bumps in protoplanetary disks where the radial drift of dust is reduced or stopped, and subsequently material piles up. As a result, grains can efficiently grow to form pebbles and planetesimals²⁰. Dust traps are now regularly found in protoplanetary disks with different properties (e.g., disk ages from < 1 to 10 Myr) and in the form of rings and crescents in millimeter dust continuum observations with ALMA¹⁸. Dust traps are thought to have played a fundamental role in the formation of the Solar System, as they were likely present in the primordial disk, even beyond the current orbit of Uranus²¹.

Dust traps have been observationally shown to contain large amounts of ice-produced molecules likely inherited from the early dark cloud out of which the star formed²². Species such as SO, H₂CO, CH₃OH, and CH₃OCH₃ have recently been detected in the gas-phase co-spatial with the dust traps^23–26. Their detection can be explained by irradiation and vertical transport of ice-coated grains while the ice thermally desorbs, whereas complex molecules likely remain locked up in the ice in many other disks, and undetectable in the gas-phase^27–29. Irradiated dust trap walls thus provide favourable conditions for the formation of macromolecules.

In this study, we use the output of a state-of-the-art dust evolution model of a protoplanetary disk with a dust trap³⁰ and calculate the radiation-dose rate in ice-coated grains throughout the disk. We demonstrate that dust traps contain a heavily irradiated region where IOM can efficiently form.

The dust and gas distribution predicted by the dust evolution model for a 1 Myr old protoplanetary disk are shown in Fig. 1.A and 1.B (details in the Methods). The modeled dust grain sizes range from 10^− 7 to 10^− 1 m, but grains smaller than several tens of µm dominate at elevated heights in the dust trap (Fig. S1.). The assumption is made that each dust grain is covered by 100 monolayers (ML) of ice and other frozen molecules, where 1 ML = 10¹⁵ molecules cm^-2. UV photons emitted by the protostar penetrate throughout the disk (Fig. 1.C) and the photon flux is calculated as a vertical (that is, normal to the disk midplane) radiation field, which is attenuated by the gas and dust throughout the disk. At each point, the optical depth is determined following $\varvec{\tau } (\varvec{r}, \varvec{z}) = {\int }_{\varvec{z}}^{\varvec{\infty }}\varvec{\rho } (\varvec{r}, \varvec{z}) \varvec{\kappa }\varvec{d}\varvec{z}$ where the column of material at radius R above height Z is multiplied with the average opacity κ (cm² g^-1) of this material. The UV flux is calculated with the Beer-Lambert law $\varvec{F}\left(\varvec{r}, \varvec{z}\right)={\varvec{F}}_{0 }{\varvec{e}}^{-\varvec{\tau }(\varvec{r},\varvec{z})}$, where F₀ is the number of incoming photons. In the fiducial model, we use F₀ = 1000 G₀, where G₀ is a UV flux of 10⁸ photons cm⁻² s⁻¹. Various models of protoplanetary disks demonstrate that this is a flux that is readily achieved in the surface layers of disks, due to the irradiation of the central star³¹. The mean opacity is set to κ = 10 cm² g⁻¹, which is in the range of values that are commonly used for protoplanetary disks³².

The grain photon flux (that is, the product of the photon flux and the combined grain surface area) calculated for the fiducial model is shown in Fig. 1.D and a close-up is presented in Fig. 1.E. It reveals that the ice-coated dust grain reservoir receives the highest photon flux in the dust trap at Z = ± 10 astronomical units (au) and R = 45 au. In the dust trap at Z = 5–20 au, grain temperatures generally exceed ∼50 K, while sub-µm grain temperatures surpass even ∼100 K in specific regions (see Fig. 1.F and Fig. S2.). Therefore, most grains in the dust trap retain a luke-warm ice film that is dominated by H₂O, CH₃OH, CO₂, NH₃, PAHs a number of trapped volatile species such as CH₄ and CO, and smaller quantities of organic molecules³³. The combination of high dust concentrations, moderately elevated temperatures, and radiation makes the dust trap wall a hotspot for luke-warm radiation-driven ice chemistry, which stimulates the formation of complex molecules³⁴.

For closer inspection, the dust trap hotspot is divided into regions at intervals of 10^− 3, 10^− 2, and 10^− 1 times the peak grain flux (F_gr,peak). By dividing the incoming photon flux with the amount of available ice and by assuming an average UV photon energy of 6 eV³⁵, the average energetic input is determined in eV molecule^-1 s^-1. At its peak position (within 10^− 1 F_gr,peak), the fiducial model shows that the dose rate is 36 eV molecule^-1 yr^-1, for each molecule in the ice mantle. A dose of 1000 eV molecule^-1 is achieved within 30 years, which is sufficient to convert the ice mantle to a macromolecular substance^13–15. Even in the extended region, where the dose rate decreases to 2 eV molecule^-1 yr^-1, a 1000 eV molecule^-1 dose is obtained in approximately 500 years. This timescale corresponds to ~ 1.7 local Keplerian periods (around a Solar mass star). These timescales are well within the expected lifetime of a dust trap²⁰.

A substantial quantity of ice is contained in the hotspot region, with 2.6%, 2.1%, and 3.5% of the total disk ice reservoir in the three regions, respectively. Material in the dust trap is subject to vertical stirring, which results in the loss of ice-coated grains to the disk atmosphere. For this model, approximately half of the grains are lost (see Methods for details), which means that the other half of the now processed ice reservoir (4% of the disk total) settles to the disk midplane, where it is available as planet-forming material. Due to this large conversion percentage, IOM formation in dust traps can currently best explain the ~ 6% formation efficiency of IOM as derived from atomic C/Si ratios². The ice-conversion factor is likely higher in a dynamic disk where fresh ice-coated grains are continuously replenished in the dust trap hot spot due to vertical mixing. Dust trap locations and opacities may differ from disk to disk and will critically influence the dose rates and above derived conversion fraction. For opacities ranging from 5–40 cm² g^-1, the average dose rate does not alter significantly for this model (Fig. 2.A.), but the amount of processed frozen material increases with decreasing κ, see Fig. 2.B. This is the result of photons penetrating deeper into the disk, that is, closer to the midplane, and accessing regions where the concentration of dust grains is larger.

The following scenario for IOM formation is suggested (Fig. 3.). In dense interstellar clouds, dust grains first accumulate refractory carbon and PAHs before acquiring ice mantles of simple species enriched in minor isotopes^36,37. Noble gases are efficiently trapped in these mantles³⁸ and moderate chemical processing result in minor production of SOM³⁹. The protoplanetary disk inherits ice-coated grains from its host cloud²². As grains migrate into the dust trap hot spot, the ice warms up and UV flux increases, resulting in a rapid conversion of the mantle material into macromolecular matter. Noble gases in the ice mantles may be incorporated into the macromolecular matter during this process⁴⁰, and isotopic signatures are carried over and further modified by e.g., radiation^7,10.

The question remains whether IOM is formed in a water-rich or -poor environment. Ice mixtures in irradiation experiments that produce IOM analogues only contain minor amounts of water^14,15. Water-rich ice is expected to lead to oxygen-rich macromolecular material, inconsistent with IOM. Its formation may therefore rely on a preceding step where SOM is formed, and subsequently, water-ice is removed (e.g., by thermal desorption), and then heavy irradiation to form complex refractory macromolecules^13,41. In our model, some of the smallest dust grains, which dominate the dust composition in the hot spot, reach temperatures at which solid-state water readily sublimates. If a dust trap is located closer to the protostar, grains of larger sizes will be heated up to higher temperatures and the loss of water-ice becomes more prominent. A thermal heating cycle also explains why noble gases detected in natural IOM are depleted in the lighter species compared to the solar composition, since He and Ne desorb at significantly lower temperatures compared to Kr and Xe.

IOM shows a range of chemical characteristics, which can be explained by our model. Disks are turbulent environments, where turbulence counteracts the trapping of particles in a pressure bump²⁰, which creates variations of photon flux and grain surface temperature. Protoplanetary disks can have multiple dust traps at different radii with different shapes, amplitudes, and around stars with different properties (such as age and luminosity)¹⁸, which further help enhance the diversity of macromolecular products and its characteristics. For example, radiation fractionates deuterium⁷ and different radiation flux will result in different levels of fractionation. Mixing of material that experienced different degrees of alteration of these pathways explains the heterogeneous isotope nature of IOM on (sub)µm scales⁴². Small (sub)µm ice-coated dust grains are preferentially stirred up into the hotspot region [Fig. S1.], but trajectories and residence times may differ. This affects the transformation of pristine material into organic macromolecular matter and, in turn, its chemical composition and the texture of the material. For example, nanoglobules, small spherical and sometimes hollow inclusions⁴³, have been suggested to form by UV irradiation of ice mantles of (sub)micrometer-sized grains due to wall thickness of 100–200 nm, which matches the penetration depth of UV radiation⁴⁴.

The dust trap in our model is located at 45 au, following average radii of observed dust traps in protoplanetary disks. This is beyond the water sublimation front and in a region where comets are thought to form. Meteoritic IOM and refractory organics in Solar System comets, which bear similarities in elemental composition¹², could both originate from this environment. As grain mantles are processed in the dust trap hotspot, they cycle back to the disk midplane. Dust traps efficiently coagulate grains to form pebbles and boulders and, in this process, a mixture of pristine ice and macromolecular matter is created, in line with the fact that chondrites accreted significant amounts of ice⁴⁵. It is possible that the resulting objects lose their icy content during migration in the Solar System and in this way a link is created between the IOM in comets and that found in asteroids such as Ryugu⁴⁶. However, since multiple dust traps can occur in a protoplanetary disk, as well as in the proto-Solar Nebula (inside or outside the water ice line), it is also possible that cometary refractory organics and IOM in meteorites and asteroids are formed at different radii and times. In both cases, the heavy irradiation of grain mantles results in the production of similar macromolecules.

Over the last years, high-resolution observations of protoplanetary disks have demonstrated that disks have dust traps across different disk and stellar properties. We show that there are heavily irradiated hot spots in these dust trap walls, where the mantle material on dust grains can efficiently be converted to macromolecular IOM. As IOM was essential in setting the elemental carbon and nitrogen abundances of terrestrial planets in the Solar System, this suggests that the mechanism to form planets is intimately linked with setting the chemical conditions from which life can emerge.

The dust evolution models are performed using the Dustpy code⁴⁷. The disk is assumed to be around a Solar-mass star, with a gas surface density that assumes a critical radius at 80AU and an initial disk mass of 5% of the Sun³⁰. The model assumes a fragmentation velocity of ν_f = 10 m s^-1, a gas viscous evolution parameter, radial diffusion, turbulent mixing, and vertical settling/stirring all set to 10^− 4. All grains are initially small between 0.1-1 microns in size, with a power law distribution as $\varvec{n}\left(\varvec{a}\right)\propto {\varvec{a}}^{-3.5}$. The models include the dust growth and dynamics of particles. The model output gives the density distribution of the dust grains (ρ_dust, Fig. 1.A) and of the gas (ρ_gas, Fig. 1.B) at 1 Myr of evolution. The data is portrait in a radial grid from 1 to 300 au (only 1 to 100 au shown in Fig. 1) with 300 logarithmically spaced cells. A logarithmically spaced grain size distribution between the minimum grain size of 0.5 micron to 10^− 1 m in 127 steps is used. A gap centered at 40 AU is assumed in the disk³⁰, which yields to a pressure bump at around 45 AU. Smaller and lighter dust grains are more easily stirred up in the disk and therefore dominate the particle sizes at greater disk heights, whereas larger particles concentrate around the disk midplane [Fig. S1.].

From the dust density distribution, the grain surface area is calculated by assuming that all grains are spherical and have a density of $\rho$_grain = 1.65 g cm^-3. This allows us to calculate the particle mass for each grain size and convert the dust density (g cm^-3) into a particle density (n cm^-3). Next, the particle density is multiplied by the grain area to find the total grain area per volume, which gives:

$${A}_{grain}=\frac{\left(3 \cdot {\rho }_{dust}\right)}{\left({\rho }_{grain}\cdot {r}_{grain}\right)}$$

where r_grain is the grain size (that is, the radius). Using the grain area, the available amount of ice is calculated by assuming that each grain is covered with 100 monolayers (1 ML = 10¹⁵ molecules cm^-2) of ice and by multiplying this value with the available grain surface area.

The thermal structure of the disk is modelled with RADMC-3D⁴⁸, assuming a vertical grid of 180 cells over a semicircle following Z = R · cos(θ), were Z is the disk height and R the radius, and following the same procedure as³⁰. The grain sizes that dominate the dust trap hotspot have sizes of 0.1 to several tens of µm and generally have temperatures greater than 50 K. However, only sub-µm grains are heated above 100 K, and only in specific regions of the hotspot [Fig. S2.]

The amount of photons throughout the disk is calculated with the Beer-Lambert law (see main text). The impinging photon flux is fixed to F₀ = 1000 G₀ (G₀ = 10⁸ photons cm^-2 s^-1), but we note the (grain) photon flux throughout the disk scales linearly with the value chosen for F₀. Therefore, the dose rate can be scaled in the same way. The amount of photons absorbed by the mantle depends on its thickness and the UV photon absorption cross section. Assuming the mantle consist entirely of water-ice, then a film of 100 ML does not fully absorb the received flux⁴⁹. The simplified division of the photon energy input by the column density to yield eV molecule^-1 s^-1 is therefore a rough assumption. However, since the photons that penetrate the ice mantle still hit the underlying grain, we assume that the energy is contributed to the overall system and therefore the simplified division holds.

Gravitational attraction causes grains to settle in the disk midplane, while turbulent stirring can move particles towards or away from the midplane. The velocities of these processes can be calculated⁵⁰ and in turn be used to assess how the dust is vertically distributed.

The equation to determine the velocity with which dust settles to midplane is given as:

$${\text{v}}_{\text{s}\text{e}\text{t}\text{t}}= \text{z} {{\Omega }}_{\text{k} }{c}_{S},$$

where z is the vertical height in meter and ${{\Omega }}_{\text{k} }$ is the Keplerian frequency:

$${{\Omega }}_{\text{k}}= \sqrt{\frac{G {\text{M}}_{\text{☉}}}{{R}^{3}} },$$

with G the gravitational constant, ${\text{M}}_{\text{☉}}$ the Solar mass in kg, and R the radius in m, and ${c}_{S}$ is the isothermal sound velocity:

$${\text{c}}_{\text{S}}= \sqrt{\frac{{k}_{B}{T}_{gas}}{{\mu }_{gas}{m}_{proton}}},$$

with ${T}_{gas}$ the gas temperature (${T}_{gas}$= 50 K, the average dust temperature at the trap location and assuming T_gas = T_dust), ${\mu }_{gas}$ the mean molecular mass of the gas (${\mu }_{gas}$ = 2), and ${m}_{proton}$the proton mass in kg.

To calculate the vertical stirring velocity, the equation

$${\text{v}}_{stir} = \sqrt{3 {{\delta }}_{\text{t} }ST {c}_{S}}$$

is used, where δ_t is a turbulent mixing parameter (δ_t = 10^− 4) and ST the Stokes parameter, calculated following:

$$ST = \frac{{r}_{grain} {\rho }_{grain}}{{\varSigma }_{g}}\frac{\pi }{2}$$

where ${\varSigma }_{g}$ the gas surface density.

For the model results used in this work, we find that v_stir ≫ v_sett for the µm-sized particles that reside at greater height in the dust trap. Therefore, stirring determines which direction material migrates. Since vertical stirring can point into two directions, namely away from the midplane (up) or towards the midplane (down), we assume that the likelihood of up- or downward motion is equal. This means that 50% of material in the dust trap hotspot goes to the disk atmosphere and 50% moves towards the midplane. We note that this situation holds for a static snapshot of the disk model, but in a dynamic and evolving environment the loss and settle fractions may be different.

Acknowledgments: The authors thank Eva G. Bøgelund and Maria Drozdovskaya for insightful discussions.

Funding: NFWL is supported by the Swiss National Science Foundation (SNSF) Ambizione Grant 193453. PP acknowledge support from the UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee from ERC. MEIR is supported by the SNSF Ambizione grant 193331.

Author contributions:

Conceptualization: NFWL, NvdM, AB, JTvS

Methodology: NFWL, PP,

Investigation: NFWL, PP, NvdM, JTvS, AB, CMOA, MEIR

Visualization: NFWL, NvdM, PP

Writing – original draft: NFWL, PP, NvdM, JTvS, AB, CMOA, MEIR

Writing – review & editing: NFWL, PP, NvdM, JTvS, AB, CMOA, MEIR

Competing interests: Authors declare that they have no competing interests.

Data and materials availability: Data and code are available upon request

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FigureS1.pdf
Fig. S1. Dust density distribution for grains of 0.1, 1, 10, and 100 μm in size. The black dashed lines indicate the total dust density distribution contours at 10^-19 and 10^-16 g cm^-3.
FigureS2.pdf
Fig. S2. Dust temperatures for grains of 0.1, 1, 10, and 100 μm in size. The white dashed lines indicate the 100 K temperature contour, while the black dashed lines indicate the total dust density distribution contours at 10^-19 and 10^-16 g cm^-3.

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Dust traps in protoplanetary disks efficiently form organic macromolecular matter

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