The evolution of the water distribution in a viscous protoplanetary disk

Abstract

Astronomical observations have shown that protoplanetary disks are dynamic objects through which mass is transported and accreted by the central star. This transport causes the disks to decrease in mass and cool over time, and such evolution is expected to have occurred in our own solar nebula. Age dating of meteorite constituents shows that their creation, evolution, and accumulation occupied several Myr, and over this time disk properties would evolve significantly. Moreover, on this timescale, solid particles decouple from the gas in the disk and their evolution follows a different path. It is in this context that we must understand how our own solar nebula evolved and what effects this evolution had on the primitive materials contained within it. Here we present a model which tracks how the distribution of water changes in an evolving disk as the water-bearing species experience condensation, accretion, transport, collisional destruction, and vaporization. Because solids are transported in a disk at different rates depending on their sizes, the motions will lead to water being concentrated in some regions of a disk and depleted in others. These enhancements and depletions are consistent with the conditions needed to explain some aspects of the chemistry of chondritic meteorites and formation of giant planets. The levels of concentration and depletion, as well as their locations, depend strongly on the combined effects of the gaseous disk evolution, the formation of rapidly migrating rubble, and the growth of immobile planetesimals. Understanding how these processes operate simultaneously is critical to developing our models for meteorite parent body formation in the solar system and giant planet formation throughout the galaxy. We present examples of evolution under a range of plausible assumptions and demonstrate how the chemical evolution of the inner region of a protoplanetary disk is intimately connected to the physical processes which occur in the outer regions.

Early aqueous activity on the ordinary and carbonaceous chondrite parent bodies recorded by fayalite

“…As mass accretion rates diminished with time, the viscous dissipation would slow, causing the snow line to migrate inwards with time. While specific details vary with the assumed disk structure and viscosity, models [44][45][46][47] suggest that the snow line would be located beyond 5 AU early in disk evolution, but is likely to have been present at 2-3 AU in the 1.8-2.5-Myr-old disk, which corresponds to the accretion ages of ordinary, CV and CO chondrite parent bodies. Thus, this would suggest that the ordinary, CO and CV chondrite parent bodies most likely accreted in the inner part of the Solar System, close to the current position of the main asteroid belt, rather than experiencing large radial excursions that may have occurred during planet migration 5 .…”

The chemistry of planet-forming regions is not interstellar

“…One potential way of explaining the increase of the water concentration (beyond what is possible with the solar abundance of oxygen -∼ 4 × 10 −4 from 34 ) is if the water has been been preferentially enriched by advection (inwards migration of icy bodies against the disk pressure gradient) 1 . Such bodies might be preferentially water-rich, if they formed outside the water snow line, but inside the CO 2 explain the high water column densities suggested by some slab models.…”

“…The elements in the material are differentially partitioned into gas and solids, and apart from a small contribution from nuclear reactions with stellar and cosmic radiation, their total abundances remain largely unchanged. Conversely, their molecular carriers, including those carrying the bulk of some elements, may change dramatically along this path, and depending on their volatility the local elemental abundances my be altered by hydrodynamic transport processes 1,2 . This rich history of pre-planetary matter is validated by strong differences in chemical content of primitive chondrites from the 3 AU region of the solar nebula 3 , comets that formed beyond 10s of AU 4 and protostellar envelopes 2 .…”

Uncertainties in water chemistry in disks: An application to TW Hydrae

“…Large scale mixing processes, dust settling, the formation of ices all can affect locally as well as globally the gas-phase C/O ratio in a disk (e.g. Ciesla & Cuzzi 2006;Hogerheijde et al 2011).…”