Fred J. Ciesla

The evolution of the water distribution in a viscous protoplanetary disk

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.

The formation conditions of chondrules and chondrites.

Chondrules, which are roughly millimeter-sized silicate-rich spherules, dominate the most primitive meteorites, the chondrites. They formed as molten droplets and, judging from their abundances in chondrites, are the products of one of the most energetic processes that operated in the early inner solar system. The conditions and mechanism of chondrule formation remain poorly understood. Here we show that the abundance of the volatile element sodium remained relatively constant during chondrule formation. Prevention of the evaporation of sodium requires that chondrules formed in regions with much higher solid densities than predicted by known nebular concentration mechanisms. These regions would probably have been self-gravitating. Our model explains many other chemical characteristics of chondrules and also implies that chondrule and planetesimal formation were linked.

Organic Synthesis via Irradiation and Warming of Ice Grains in the Solar Nebula

Space Organics The origin of the organic compounds found in meteorites and interplanetary dust particles is a matter of debate. Laboratory experiments suggest that these organics were inherited from the interstellar medium and predate the existence of the solar system. By using particle-tracking models, Ciesla and Sandford (p. 452, published online 29 March; see the Perspective by Nuth and Johnson) explored the possibility that the organics could have been produced within the outer reaches of the protoplanetary disk from which the solar system planets originate. Grains within a protoplanetary disk follow irregular paths; such orbital histories subjected model particles to ultraviolet radiation and temperatures that have been shown in laboratory experiments to generate organic compounds. A mechanism for production of carbon compounds in the interstellar medium could also operate in protoplanetary disks. Complex organic compounds, including many important to life on Earth, are commonly found in meteoritic and cometary samples, though their origins remain a mystery. We examined whether such molecules could be produced within the solar nebula by tracking the dynamical evolution of ice grains in the nebula and recording the environments to which they were exposed. We found that icy grains originating in the outer disk, where temperatures were less than 30 kelvin, experienced ultraviolet irradiation exposures and thermal warming similar to that which has been shown to produce complex organics in laboratory experiments. These results imply that organic compounds are natural by-products of protoplanetary disk evolution and should be important ingredients in the formation of all planetary systems, including our own.

Two-dimensional transport of solids in viscous protoplanetary disks

Abstract Large-scale radial transport of solids appears to be a fundamental consequence of protoplanetary disk evolution based on the presence of high temperature minerals in comets and the outer regions of protoplanetary disks around other stars. Further, inward transport of solids from the outer regions of the solar nebula has been postulated to be the manner in which short-lived radionuclides were introduced to the terrestrial planet region and the cause of the variations in oxygen isotope ratios in the primitive materials. Here, both outward and inward transport of solids are investigated in the context of a two-dimensional, viscously evolving protoplanetary disk. The dynamics of solids are investigated to determine how they depend on particle size and the particular stage of protoplanetary disk evolution, corresponding to different rates of mass transport. It is found that the outward flows that arise around the disk midplane of a protoplanetary disk aid in the outward transport of solids up to the size of CAIs s and can increase the crystallinity fraction of silicate dust at 10 AU around a solar mass star to as much as ∼40% in the case of rapidly evolving disks, decreasing as the accretion rate onto the star slows. High velocity, inward flows along the disk surface aid in the rapid transport of solids from the outer disk to the inner disk, particularly for small dust. Despite the diffusion that occurs throughout the disk, the large-scale, meridonal flows associated with mass transport prevent complete homogenization of the disk, allowing compositional gradients to develop that vary in intensity for a timescale of one million of years. The variations in the rates and the preferred direction of radial transport with height above the disk midplane thus have important implications for the dynamics and chemical evolution of primitive materials.

Indication of Insensitivity of Planetary Weathering Behavior and Habitable Zone to Surface Land Fraction

It is likely that unambiguous habitable zone terrestrial planets of unknown water content will soon be discovered. Water content helps determine surface land fraction, which influences planetary weathering behavior. This is important because the silicate-weathering feedback determines the width of the habitable zone in space and time. Here a low-order model of weathering and climate, useful for gaining qualitative understanding, is developed to examine climate evolution for planets of various land-ocean fractions. It is pointed out that, if seafloor weathering doesnotdependdirectlyonsurfacetemperature,therecanbenoweathering-climatefeedbackonawaterworld.This would dramatically narrow the habitable zone of a waterworld. Results from our model indicate that weathering behavior does not depend strongly on land fraction for partially ocean-covered planets. This is powerful because it suggests that previous habitable zone theory is robust to changes in land fraction, as long as there is some land. Finally, a mechanism is proposed for a waterworld to prevent complete water loss during a moist greenhouse through rapid weathering of exposed continents. This process is named a “waterworld self-arrest,” and it implies that waterworlds can go through a moist greenhouse stage and end up as planets like Earth with partial ocean coverage. This work stresses the importance of surface and geologic effects, in addition to the usual incident stellar flux, for habitability.

Oxygen Isotopic Composition of the Sun and Mean Oxygen Isotopic Composition of the Protosolar Silicate Dust: Evidence from Refractory Inclusions

Preliminary analysis of the oxygen isotopic composition of the solar wind recorded by the Genesis spacecraft suggests that the Sun is 16O-rich compared to most chondrules, fine-grained chondrite matrices, and bulk compositions of chondrites, achondrites, and terrestrial planets (Δ17O = –26.5‰ ± 5.6‰ and –33‰ ± 8‰ (2σ) versus Δ17O ~ ±5‰). The inferred 16O-rich composition of the Sun is similar or slightly lighter than the 16O-rich compositions of amoeboid olivine aggregates and typical calcium-aluminum-rich inclusions (CAIs) from primitive (unmetamorphosed) chondrites (Δ17O = –24‰ ± 2‰), which are believed to have condensed from and been melted in a gas of approximately solar composition (dust/gas ratio ~ 0.01 by weight) within the first 0.1 Myr of the solar system formation. Based on solar system abundances, 26% of the solar system oxygen must be initially contained in dust and 74% in gas. Because solar oxygen is dominated by the gas component, these observations suggest that oxygen isotopic composition of the solar nebula gas was initially 16O-rich. Due to significant thermal processing of the protosolar molecular cloud silicate dust (primordial dust) in the solar nebula and its possible isotope exchange with the isotopically evolved solar nebula gas, the mean oxygen isotopic composition of the primordial dust is not known. In CO self-shielding models, it is assumed that primordial dust and solar nebula gas had initially identical, 16O-rich compositions, similar to that of the Sun (Δ17O ~ –25‰ or –35‰), and solids subsequently evolved toward the terrestrial value (Δ17O = 0). However, there is no clear evidence that the oxygen isotopic compositions of the solar system solids evolved in the direction of increasing Δ17O with time and no 16O-rich primordial dust have yet been discovered. Here we argue that the assumption of the CO self-shielding models that primordial dust and solar nebula gas had initially identical 16O-rich compositions is incorrect. We show that igneous CAIs with highly fractionated oxygen isotopic compositions, fractionation and unidentified nuclear effects (FUN), and fractionation (F) CAIs, have Δ17O ranging from –0.5‰ to –24.8‰. Within an individual FUN or F CAI, oxygen isotopic compositions of spinel, forsterite, and pyroxene define a mass-dependent fractionation trend with a constant Δ17O value. The degree of mass-dependent fractionation of these minerals correlates with the sequence of their crystallization from the host CAI melt. These observations and evaporation experiments on CAI-like melts indicate that FUN and F CAIs formed by melting of solid precursors with diverse Δ17O values in vacuum (total pressure 50☉) ejecta. The 16O-depleted compositions of chondrules, fine-grained matrices, chondrites, and achondrites compared to the Suns value reflect their formation in the protoplanetary disk regions with enhanced dust/gas ratio (up to 105× solar).

Pressure–temperature evolution of primordial solar system solids during impact-induced compaction

Prior to becoming chondritic meteorites, primordial solids were a poorly consolidated mix of mm-scale igneous inclusions (chondrules) and high-porosity sub-μm dust (matrix). We used high-resolution numerical simulations to track the effect of impact-induced compaction on these materials. Here we show that impact velocities as low as 1.5 km s−1 were capable of heating the matrix to >1,000 K, with pressure–temperature varying by >10 GPa and >1,000 K over ~100 μm. Chondrules were unaffected, acting as heat-sinks: matrix temperature excursions were brief. As impact-induced compaction was a primary and ubiquitous process, our new understanding of its effects requires that key aspects of the chondrite record be re-evaluated: palaeomagnetism, petrography and variability in shock level across meteorite groups. Our data suggest a lithification mechanism for meteorites, and provide a ‘speed limit’ constraint on major compressive impacts that is inconsistent with recent models of solar system orbital architecture that require an early, rapid phase of main-belt collisional evolution.

Tracing the ingredients for a habitable earth from interstellar space through planet formation

Significance With the rapid pace at which exoplanets are being discovered, many efforts have now been dedicated to identifying which planets are expected to have the ingredients necessary for the development of life. In this work we explore the relative disposition of the essential elements carbon and nitrogen in each stage of star and planet formation, using the Earth and our solar system as grounding data. Our results suggest that planets like the Earth are readily supplied with these key elements, but their relative amounts on the surface and in the atmosphere will be highly variable. We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites, and terrestrial planets; we include an updated estimate for the bulk silicate Earth (C/N = 49.0 ± 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macromolecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of ∼1–12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N bulk silicate Earth ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable.

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

Chronology of aqueous activity on chondrite parent bodies constrains their accretion times and thermal histories. Radiometric (53)Mn-(53)Cr dating has been successfully applied to aqueously formed carbonates in CM carbonaceous chondrites. Owing to the absence of carbonates in ordinary (H, L and LL), and CV and CO carbonaceous chondrites, and the lack of proper standards, there are no reliable ages of aqueous activity on their parent bodies. Here we report the first (53)Mn-(53)Cr ages of aqueously formed fayalite in the L3 chondrite Elephant Moraine 90161 as Myr after calcium-aluminium-rich inclusions (CAIs), the oldest Solar System solids. In addition, measurements using our synthesized fayalite standard show that fayalite in the CV3 chondrite Asuka 881317 and CO3-like chondrite MacAlpine Hills 88107 formed and Myr after CAIs, respectively. Thermal modelling, combined with the inferred conditions (temperature and water/rock ratio) and (53)Mn-(53)Cr ages of aqueous alteration, suggests accretion of the L, CV and CO parent bodies ∼1.8-2.5 Myr after CAIs.

The early impact histories of meteorite parent bodies

We have developed a statistical framework that uses collisional evolution models, shock physics modeling, and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies. It is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces— compacting, heating, and mixing the outer layers; it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers. The first 10-20 Myr were the most important time for impacts, both in terms of the number of impacts and the increase of specific internal energy due to impacts. The model has been applied to evaluate the proposed impact histories of several meteorite parent bodies: up to 10 parent bodies that were not disrupted in the first 100 Myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the CB chondrite parent; around 1-5% of bodies that were catastrophically disrupted after 12 Myr sustained impacts at times that match the heating events recorded on the IAB/winonaite parent body; more than 75% of 100 km radius parent bodies, which survived past 100 Myr without being disrupted, sustained an impact that excavates to the depth required for mixing in the outer layers of the H-chondrite parent body; and to protect the magnetic field on the CV chondrite parent body, the crust would have had to have been thick (approximately 20 km) to prevent it being punctured by impacts.