Dust Recycling and Icy Volatile Enhancement (DRIVE): A Novel Method of Volatile Enrichment in Cold Giant Planets

TL;DR

This work addresses the puzzle of enhanced giant-planet metallicities despite cold, carbon-poor disk gas by introducing the DRIVE mechanism. By coupling 3D hydrodynamics, radiative transfer, Monte Carlo dust tracking, and DustPy fragmentation in a disk with a Jupiter-mass planet at 75 AU, the study shows that meridional flows lift small grains above the CO snow surface, where CO ice sublimates into the gas and enriches the local gas phase that the planet accretes. The results quantify potential CO enhancements up to $\sim3\times$ solar (and up to $\sim14.5\times$ under certain fragmentation and stirring conditions) and highlight the role of the dust cycle in setting planetary atmospheric composition. DRIVE offers a robust, late-stage enrichment pathway for cold giant planets that complements existing pebble- and planetesimal-based scenarios and has implications for interpreting C/O and metallicity in exoplanet atmospheres, including Jupiter.

Abstract

Giant planet atmospheres are thought to reflect the gas phase composition of the disk when and where they formed. However, these atmospheres may also be polluted via solid accretion or ice sublimation in the disk. Here, we propose a novel mechanism for enriching the atmospheres of these giant planets with volatiles via pebble drift, fragmentation, and ice sublimation. We use a combination of 3D hydrodynamic simulations, radiative transfer, and particle tracking to follow the trajectories and resulting temperatures of solids in a disk containing an embedded planet forming outside the CO snowline. We show that small dust can become entrained in the meridional flows created by the giant planet and advected above the disk midplane where temperatures are well above the sublimation temperature of CO. This transport of small grains occurs over 10 kyr timescales, with individual micron-sized grains cycling between the midplane and surface of the disk multiple times throughout the planetary accretion stage. We find that this stirring of dust results in sublimation of CO gas above the snow surface in the dust trap created exterior to the giant planet, leading to super-solar CO abundances in the pressure bump. This mechanism of Dust Recycling and Icy Volatile Enhancement in cold giant planets, which we call the DRIVE effect, may explain enhanced metallicities of both wide separation exoplanets as well as Jupiter in our own Solar System.

Figures (6)

• Figure 1: Azimuthally averaged gas density (left) and gas radial velocity relative to the planet (right) for our FARGO3D disk model containing a Jupiter mass planet at 75 au. The CO sublimation surface at 30 K recovered from our RADMC-3D model is indicated in both panels by the red dashed line. The region near the embedded planet is heated from a combination of accretion heating onto the planet, disk viscous heating, and radiative heating from the star, resulting in higher temperatures at the midplane in the gap. Gas advection relative to the local sound speed is also indicated by white arrows. Gas advection is primarily outward along the midplane near the planet, with significant vertical flows away from the midplane at the gap edge and in the pressure bump at 100 au.
• Figure 2: Dust distribution as a function of size and radius from DustPy simulations showing the initial conditions of the dust size distribution (top row) and after 1 Myr of evolution (center row) for a disk containing a Jupiter mass planet located at 75 au. We examine disks with particle fragmentation velocities of 1 m s$^{-1}$ (left column) and 10 m s$^{-1}$ (right column). The grain size distribution in the pressure bump at 101 au is shown in the bottom row. Regardless of dust fragmentation velocity, we see an enhancement of small dust less than St = $10^{-3}$ (dotted line) in the pressure bump. The total dust-to-gas mass ratio, $\varepsilon$, is labeled in each plot, where the initial dust-to-gas is $\varepsilon$ = 0.01 in the outer disk. The dust-to-gas mass ratios for grain with St $\leq 10^{-3}$, $\varepsilon_{sm}$, and St $> 10^{-3}$, $\varepsilon_{lg}$ are also shown. For the case with larger fragmentation velocity, while the total dust mass is higher and grains reach much larger sizes, the total mass of dust with St $\leq 10^{-3}$ is about 0.01 in both cases.
• Figure 3: Cartoon schematic of the DRIVE effect enriching the atmosphere of a giant planet with volatiles. Larger pebbles trap volatiles from the outer disk and transport them to the pressure bump created by the giant planet via radial drift. Here, fragmentation creates fine dust, which can be lofted away from the midplane, leading to sublimation of ice mantles on the dust and enriching the gas above the snow surface with volatiles. This volatile rich gas can then be accreted onto the planet, resulting in a volatile enriched atmosphere. The dust, due to the lower gas density at the surface of the disk, decouples from the gas and can settle back to the midplane as a bare grain, fractionating the volatile elements from the more refractory solids. Here, the color shows the azimuthally averaged gas density from our FARGO3D simulations. The solid black contour outlines the region of the disk where total dust-to-gas mass ratio is equal to 0.01 in our DustPy simulations, with higher dust-densities below this contour. The transport of icy pebbles is represented by the black and red solid arrows, while black arrows represent bare grains and red arrows represent CO gas. Meridional flows onto the planet including entrained CO gas are illustrated by the red and black dashed arrows. The approximate location of the CO snowline at 30 K is also included.
• Figure 4: The maximum temperature reached by particles of different sizes, ranging from 1 mm to 1 µm (right column) for particles starting at the midplane at 100 au. The colormap shows the number of particles out of 1000 total that have reached a given temperature as a function of time. Also indicated in each plot by a solid line is the 50th percentile for maximum temperature reached as a function of time. The dashed lines show the 25th and 75th percentiles, and dotted line show 10th and 90th percentiles. While large particles are not heated much above the midplane temperature, solids below 10 µm in size (St $\lesssim 10^{-3}$ at the midplane experience heating. All of these dust particles experience temperatures above 30 K by 1 Myr.
• Figure 5: (Top) Particle trajectory of the tracer particle reaching the maximum temperature, with the hottest point labeled with a red dot. The background color and arrows show the azimuthally averaged gas density and advection. This particle is lofted to high altitude following the meridional gas circulation in the disk, reaching temperatures up to 55 K. The particle's temperature along its trajectory is shown by the color of line. The particle temperature is shown over 20 kyr to demonstrate the particle's trajectory before and after reaching the hottest point at 0.3 Myr. This particle's height above the midplane (middle) and temperature (bottom) are also shown as a function of time over the full 1 Myr integrated by the red lines. Examples of other particles are also shown in black, indicating the highlighted particle's trajectory and temperature are typical of other similarly sized particles.