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Question 1: Evolution of the Protoplanetary Disk

This chapter addresses the history of the solar nebula, the protoplanetary disk that evolved into the solar system.¹ Our disk was formed as a by-product of star formation via the collapse of a molecular cloud composed of gas and dust. The evolution of the protoplanetary disk had four sequential, but partially contemporaneous phases: (1) the initial molecular cloud collapse and disk formation; (2) the physical and chemical evolution of the disk; (3) planetesimal formation; and (4) dispersal of the nebular gas (Figure 4-1). The processes that occurred in these phases are foundational to establishing the conditions that led to the physical components of the solar system as we know it today, from primordial presolar grains preserved in comets, to gases accreted to giant planets, to the volatile contents of Earth and the other inner solar system rocky bodies. Moreover, the history of the solar nebula provides a point of comparison for models and astronomical observations of protoplanetary disks in general, with cross-cutting relevance to exoplanet studies (see Question 12, Chapter 15).

That said, understanding this portion of the solar system’s history is challenging because the processes are complex and the forces governing disk evolution are not fully identified. Gas and dust, evolving in the protoplanetary disk, are not only coupled to one another but also to the young Sun and the surrounding star-forming region. Solids combine with one another to form small particles while being affected by a host of processes, including gas drag, turbulence within the disk, and the gravitational effects of new worlds plowing new orbital paths. Tiny spherules in meteorites called chondrules tell us that some small particles experienced sudden melting events within the disk, but how and where they did so are critical unsolved problems. Macroscopic small bodies apparently became concentrated enough within certain regions of the disk to form large planetesimals, the building blocks of the planets, but the details of this process remain elusive. The best chance we have to solve these issues is to study them from many different directions, with theory, observations, laboratory studies, and spacecraft missions all playing critical roles.

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¹ A glossary of acronyms and technical terms can be found in Appendix F.

Q1.1 WHAT WERE THE INITIAL CONDITIONS IN THE SOLAR SYSTEM?

The story of the protoplanetary disk is one of rapid evolution occurring as new material infalls from the stellar birth cluster and is transported and processed within the disk. Meteorites contain byproducts of prior stellar evolution and other energetic processes that took place millions to billions of years before the protosun formed. The most pristine samples from this epoch may be found in comets, interplanetary dust, and asteroids composed of carbonaceous chondrites. Missions that can return samples of these bodies to Earth for analysis or those that can perform detailed in situ studies provide the best chance of producing breakthroughs in our understanding of the initial conditions of the protoplanetary disk over the near future.

Q1.1a What Initiated the Collapse of the Molecular Cloud?

The solar system likely formed from the collapse of a molecular cloud core within a cluster of perhaps ~10³−10⁴ stars (Adams 2010). Dense cloud cores are supported against self-gravity by a combination of turbulent motion, magnetic fields, thermal (gas) pressure, and centrifugal forces. As turbulence within a cloud dissipates over time, magnetic fields delay the initial collapse until they are depleted by ambipolar diffusion and rapid collapse begins (Boss and Goswami 2006). Another possible trigger of collapse is a shock front from a nearby supernova (Cameron and Truran 1977), suggested by isotopic anomalies preserved in meteorites. The best chance of better constraining the events that triggered collapse is a combination of astronomical observations and modeling of disks, and measurements of the isotopic record inherited from the molecular cloud and injected nucleosynthetic materials preserved in meteorites. Regardless of the collapse mechanism, the Sun started as a protosolar core supported by thermal pressure while still undergoing infall from the surrounding cloud material.

Subsequent evolution progressed through so-called class I (protostar and gas-dust disk surrounded by infalling spherical cloud), class II (protostar obscured gas-dust disk), and class III (protostar and gas-depleted debris disk) phases. These evolutionary phases can now be observed with facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) in various star forming regions to conduct detailed studies on conditions at the initial onset of star formation (e.g., Friesen et al. 2018). During the first stages of the Sun’s formation, a >30 AU-wide solar nebula of gas (99 wt.% and mostly in the form of hydrogen and helium) and solids (1 wt.% dust) formed because of the conservation of angular momentum during molecular cloud infall. Astronomical observations of hundreds of protoplanetary disks indicate they have a large range in mass with a median of about 1 percent of the Sun’s mass and have rich substructures, including holes, gaps, and rings (Andrews 2020).

The best representative of the molecular cloud composition is probably the Sun, which contains >99 percent of the mass of the present solar system. Its composition is inferred from spectroscopic measurements, analyses of refractory elements in primitive meteorites, and direct sampling of the solar wind (Palme et al. 2014). The analysis of chondrites, however, also reveals the presence of nanometer- to submicron-sized grains with extremely variable isotopic compositions that can be related to specific types of parent stars. These data demonstrate that the molecular cloud core was not fully homogenized or had not digested its inherited interstellar components (Nittler and Ciesla 2016), and that laboratory measurements of minute isotopically anomalous materials can be used to investigate the molecular cloud collapse, inherited protosolar components, and disk evolution.

Furthermore, planetary bodies—including primitive meteorites and the inner planets—host daughter products of radioactive isotopes with half-lives shorter than the age of the solar system, the so-called extinct radionuclides (Dauphas and Chaussidon 2011). Their half-lives, denoted as t_1/2, vary over three orders of magnitude, such that they could not originate from a single stellar event but instead required contributions from several generations of stars having different properties. In particular, the presence of the aluminum isotope ²⁶Al [t_1/2 = 0.73 million years] suggests that at least one supernova exploded near the parent molecular cloud core. The supernova could have injected ²⁶Al and other radionuclides into the solar system after it had already collapsed (Adams 2010) or, if the injection event occurred earlier, the supernova could have also triggered the initial collapse of the molecular cloud core (Cameron and Truran 1977; Vanhala and Boss 2002). On the other hand, the ²⁶Al may have been sourced from one or more dying giant stars, not necessarily involving a supernova (Tang and Dauphas 2012).

Furthermore, the abundances of isotopes produced by different nucleosynthetic pathways (e.g., those of molybdenum (Mo), ruthenium (Ru), titanium (Ti), and chromium (Cr)) systematically vary among meteoritic groups. The so-called noncarbonaceous chondrites (NC), which include the ordinary and enstatite chondrite meteorites, form a group having variations in slow neutron-capture process (s-process) isotopes, whereas the carbonaceous chondrites (CC)—which include the CI, CM, CV, and CO meteorite groups—form another distinct group richer in rapid neutron-capture process (r-process) isotopes (see Figure 6-1).

The NC versus CC dichotomy in meteorites has been interpreted as resulting from the heterogeneous infall of molecular cloud core material at different radial locations in the disk and/or a different timing for this infall. This scenario is consistent with the idea that the source bodies for the NC meteorites formed in the inner solar system, while those for the CC meteorites formed in the outer solar system. The growth of Jupiter might have

then prevented subsequent mixing between NC and CC groups, although a pressure bump in the gas disk near the location of Jupiter’s formation may have provided an even earlier barricade (Kruijer et al. 2020).

One way to elucidate this problem is to analyze extinct radioactivity products in various mineral phases whose origin and formation are constrained by independent approaches. The interpretation of the available data in terms of spatial and/or temporal homogeneity of ²⁶Al in the nascent solar system, however, remains controversial (Gregory et al. 2020). At present, the samples of primitive material used to address the ²⁶Al controversy are severely constrained by the availability of natural objects that are known to have originated in select regions of the solar nebula. Knowing that two bodies formed in the same location, even if that location is unknown, would help distinguish spatial versus temporal effects. Interpretation is further complicated by the fact that some meteorite samples have been altered by either planetesimal geologic processes, entry into Earth’s atmosphere, and/or their residence time on Earth.

If the collapse of the molecular cloud that made the Sun was triggered by a stellar explosion, the nature and distribution of presolar materials and extinct radionuclides could have been heterogeneous between the inner and outer parts of the solar system. The analysis of extinct radioactivity products and of presolar phases requires, however, analytical precision and spatial resolution that cannot be obtained in situ on planetary objects through missions. Thus, a clear resolution to the question of a supernova trigger versus later injection of isotopes like ²⁶Al requires laboratory-based measurements of additional primitive materials that sample different reservoirs in the nebula from sample return missions and terrestrial collection.

Q1.1b What Were the Original Elemental, Isotopic, and Molecular Compositions of Gas, Dust, and Ice Components Delivered from the Molecular Cloud to the Solar Nebula?

Measurements of the Sun’s composition (Palme et al. 2014) and chemical thermodynamics indicate that the most abundant species in the starting material of the solar system and, presumably, of the molecular cloud core were hydrogen (H₂), carbon monoxide (CO), water (H₂O), and nitrogen (N₂) (these species, along with related ones, are collectively referred to as HCON) plus the noble gas helium (He). Despite this, HCON and noble gases only make up <0.1 percent of Earth’s mass. The initial depletion on Earth and other terrestrial planets may in fact have been even more severe, with one or more post-accretional processes probably enriching them to these levels.

The relative paucity of the terrestrial planets’ initial volatile abundances arises because gaseous HCON host species could not be efficiently trapped in rocky solids during solar system formation. It can be shown that condensation in rocky solids or dissolution in molten silicate/metal are very inefficient processes for trapping volatile elements in planetary bodies, with the possible exception of carbon, and noting that it is necessary to account for the incorporation of oxygen into silicates and oxides (Lodders 2004). In fact, these processes are so inefficient at incorporating volatiles that they also cannot account for the observed low hydrogen, oxygen, nitrogen and noble gas abundances in planetary bodies or primitive meteorites. This implies that other more efficient processes took place that could concentrate and preserve volatile elements during planetary accretion.

The isotopic signatures of HCON in solar system objects and reservoirs are markedly different from those of the protosolar nebula (based on solar composition), and, by inference, from those of the parent molecular core (Figure 4-2). The origin of these large-scale isotopic heterogeneities is debated, with two main scenarios being considered: (1) solid materials grew from a heterogeneous molecular cloud hosting interstellar dust that escaped isotopic homogenization, or (2) the processes that were instrumental in the formation of HCON-bearing solid phases (e.g., photochemical and ion-molecule exchange reactions) were themselves responsible for the large isotopic variations.

The isotopic variations of HCON of solid material originating at different heliocentric distances (which may differ from their current distances) can provide insights into the processes of early solid formation and transport from the molecular cloud to the early solar system. Furthermore, these isotopic fingerprints are tracers for investigating the origin(s) of planetary material. For instance, the isotopic composition of the jovian atmosphere points to a nebular gas origin but those of the other giant planets are unknown. Contributions of cometary-like matter may also imprint significant isotopic shifts, depending on the overall mass of material added, given the large differences between comets and protosolar nebula. Likewise, the composition of the venusian atmosphere, in particular its

∆¹⁷O composition (where ∆¹⁷O ≡ δ¹⁷O − 0.56 δ¹⁸O; see Figure 4-2), would permit a test for a genetic relationship with documented solar system reservoirs, as already achieved for Earth and Mars.

The distribution of presolar grains in primitive undifferentiated meteorites (chondrites) appears heterogeneous, with CCs being richer in stellar dust than NCs. Because CCs are argued to originate from greater radial distances than NCs (e.g., Desch et al. 2018), this observation suggests a process that separated the inner and outer solar system from mixing at a critical point during early accretion, or an outward radial gradient that could be related to heterogeneous infall of molecular cloud material, as may be suggested by preserved isotopic anomalies. Testing the heterogeneity and dynamics of molecular cloud core contributions, including refractory dust and more easily altered organic matter, would require return of material originating from the outer solar system to terrestrial laboratories or definitive proof that a primitive sample originated in that region.

Heterogeneous infall of stellar material alone cannot produce all of the observed isotopic variability. The variability in HCON of several hundreds to thousands of parts per million throughout the solar system is not observed in the nonvolatile elements. For instance, isotopic variations observed in refractory elements such as Mo, Ti, Cr, and Ru are of the order 1 part per 10,000 or less. Ion-molecule exchange reactions, which occur at low temperature in dense molecular clouds (Terzieva and Herbst 2000; Aikawa et al. 2018) could potentially account for some of the deuterium (D) enrichments found in molecular cloud ices (Sandford et al. 2001; Cleeves et al. 2014) and in organic molecules such as nitrogen (¹⁵N)-rich organics (Rodgers and Charnley 2008). Isotopic variations of volatile elements across the solar system could be pristine fingerprints of molecular cloud chemistry, with the outer solar system having better preserved unprocessed (D- and ¹⁵N-rich) material than the inner regions of the solar nebula (Pignatale et al. 2018). Laboratory measurements of returned and collected samples of inner and outer solar system materials are needed to address this question.

The observation of mass-independent isotope fractionation of oxygen in planetary bodies (as opposed to mass-dependent fractionation that occurs during ordinary processes such as sublimation and condensation) indicates another possible fractionation process. In particular, self-shielding occurs when stellar photons penetrate into a cloud of gas and become progressively absorbed by photoreactions, as observed in giant molecular clouds (Bally and Langer 1982). Self-shielding of gaseous CO could represent a significant source of atomic carbon (C) and oxygen (O) that could then react with H₂ to form hydrocarbons (C_xH_y molecules) and H₂O molecules. Self-shielding could have taken place in the parent molecular cloud illuminated by nearby stars (Yurimoto and Kuramoto 2004) or at the disk surface of the solar nebula with photons from the nascent Sun (Lyons and Young 2005; Figure 4-3). This process may also account for ¹⁵N enrichments in organic molecules (Heays et al. 2014; Garani and Lyons 2020). Noble gases provide a further piece of evidence for irradiation of nebular gas. Chondritic argon (Ar), krypton (Kr) and xenon (Xe) are trapped in specific phases associated with refractory organics and present isotopic and elemental signatures different from those inferred for the solar nebula (Busemann et al. 2000). These signatures have only been reproduced in the laboratory when noble gases are incorporated into organics as ions (Frick et al. 1979; Marrocchi et al. 2011). Spatially resolved astronomical observations of isotopologues (i.e., molecules that differ only in their isotopic composition) of C- and O-bearing species in protoplanetary disks and the interstellar medium can address the role of self-shielding.

Q1.1c How Did the Compositions of the Gas, Dust, Ice, and Organic Components and the Physical Conditions Vary Across the Protoplanetary Disk?

A protoplanetary disk is intrinsically heterogeneous, with large radial gradients in temperature, pressure, and chemical compositions owing to the presence of a growing central protostar, cold interstellar medium at its edges, and nearby massive, luminous stars. Many aspects of the disk are uncertain, including the temperature, pressure, and density of the nebula, its effects on the elemental and isotopic composition of nebular gas and solids, and the petrology and crystal structure of solids. Identifying and quantifying certain species—for example, the major carriers of carbon (CO, CO₂, organics, graphite, etc.) and how these change over time and space—can provide constraints on disk chemical evolution.

In a broad sense, hot and reducing conditions are thought to have prevailed within the inner regions of the disk, whereas oxidizing, cold and volatile-rich environments dominated in the outer disk (Bergin et al. 2007). This heterogeneous distribution of volatile elements across the solar nebula has major implications for the origin of volatile elements in the terrestrial planet-forming region. For example, a key question is whether the majority of

the volatiles in Earth and Mars were indigenous to the formation regions of these worlds or were delivered from outer solar system sources.

Over timescales of several million years, organic-bearing materials and silicate solids agglomerated with variable amounts of ice to form increasingly larger planetary bodies such as asteroids, planetary embryos, and planets. During this period, HCON volatile condensation fronts (both radial and vertical) swept through the disk, changing the density and composition of the residual gas (Owen 2020). This in turn differentially influenced the elemental and isotopic compositions of volatile and refractory components depending on their accretion locations.

Disk processes are recorded in the volatile, organic, and silicate compositions of small bodies and planets, which in turn can be used to help constrain the provenance of the planets’ accretionary material over time. Ground- and space-based observations of primitive bodies across the solar system provide constraints on preserved volatile components from the disk. Comparisons to resolved maps of disks at various evolutionary stages will inform compositional tracers of evolution that enable detailed modeling of the presolar disk.

Strategic Research for Q1.1

• Constrain the variation in physical conditions and distribution of gas and dust components across the nebula through in situ and orbital measurements of the elemental and isotopic composition of the surface, and, where relevant, atmospheres of bodies formed from different nebular reservoirs (especially Uranus, Neptune, and Mercury, but also Venus, asteroids, Centaurs, and Saturn), laboratory analyses of returned samples of comets and terrestrially collected interplanetary dust and meteorite samples, and ground- and

• space-based telescopic observations of the composition (gas, ice, dust) of comets, Kuiper belt objects, and protoplanetary disks.
• Address the timing and role of injection of supernova material on the formation of the solar nebula, and distinguish materials that retain a presolar heritage from the products of nebular and parent body processes through return of comet samples and laboratory isotopic analyses of returned and terrestrially collected samples.
• Constrain the role of self-shielding on the formation of O and N isotopic reservoirs in the solar nebula through spatially resolved astronomical observations of isotopologues C- and O-bearing species in protoplanetary disks and the interstellar medium using ground- and space-based telescopes (e.g., ALMA and the James Webb Space Telescope).

Q.1.2 HOW DID DISTINCT RESERVOIRS OF GAS AND SOLIDS FORM AND EVOLVE IN THE PROTOPLANETARY DISK?

Conventional wisdom in previous decades held that most asteroids formed within the main asteroid belt, and that evolution in the protoplanetary disk between 2 and 3 AU was ultimately responsible for major isotopic differences found between the different meteorite classes. More recently, a new model has been advanced that suggests that the isotopic differences between noncarbonaceous and carbonaceous chondrite meteorites are so pronounced that they likely represent materials formed in vastly different parts of the solar system (e.g., Kruijer et al. 2020).

This idea is provocative, in that it suggests samples from planetesimals that formed in the terrestrial planet, giant planet, and trans-neptunian zones may already be in our possession in the form of meteorites or could be obtained from highly accessible near-Earth objects. It also raises a number of challenging issues, such as ascertaining the degree of mixing that took place in the solar nebula prior to the isolation of the noncarbonaceous and carbonaceous zones, determining the mechanism(s) that brought about the isolation of the inner and outer solar system, and identifying the formation locations of meteoritic materials in our collections. Testing these ideas will require a combination of theory, observations, laboratory studies, and spacecraft missions.

Q1.2a When and How Did the First Macroscopic Solids Form?

The primary rock record for constraining the nature of the first macroscopic solids in the protoplanetary disk are chondrites. Chondrites are largely composed of chondrules, refractory inclusions including calcium aluminum-rich inclusions (CAIs) and finer-grained matrix materials.

CAIs are the oldest known macroscopic solar system solids and contain minerals that are thermodynamically predicted to be the first to condense from a gas of approximately solar composition (Krot 2019). They define the cosmochemical time zero for the solar system at 4,567.3 ±1 Ma (Amelin et al. 2010) or perhaps 4,568.2 ±0.2 Ma (Bouvier and Wadhwa 2010). CAIs are variably solidified melts, condensates, and/or agglomerates (MacPherson et al. 2012; Krot 2019). Some authors have argued based on petrography, abundances in rare-Earth elements (REE) and relative ages derived from aluminum-magnesium (Al-Mg) isotopic systematics that at least some of the nonigneous CAIs may be older than those that are igneous, with the nonigneous potentially being the precursors to the igneous ones (e.g., MacPherson et al. 2012). Some refractory inclusions can be as simple as melted spheres of the minerals spinel (MgAl₂O₄) and hibonite (CaMg_xTi_xAl_12−2xO₁₉), whose precursors were likely direct condensates from a gas phase.

Individual minerals such as platy-hibonite crystals found in the matrices of chondrites are generally accepted as condensates, having model ages equal to that of other CAIs. It can be argued that small refractory inclusions (30–100 µm) in carbonaceous CO₃ chondrites likely represent the earliest stages of coagulation of refractory dust to form refractory inclusions, with larger refractory inclusions forming on the order of 40,000 years after the smaller ones (Liu et al. 2019). Thus, CAIs may record a progression from almost pure condensation to the agglomeration of larger, cm-sized objects. This may explain why some refractory minerals have ages younger than 4,567.3 ±1 Ma, which is either attributed to longer durations of agglomeration and formation or to disturbances in the initial isotope values owing to processing post-formation—either pre- or post-accretion.

Chondrules are tiny, 0.1–1 mm igneous rocks and are the main structural component of chondrites. Various models have been proposed to explain the transient heating necessary for formation of chondrules from their precursor dust particles. For example, bipolar outflows and X-ray flares, observed around young stellar objects, have been postulated as possible mechanisms for CAI and chondrule formation because they are transient and can redistribute dust in the disk (Shu et al. 2001). Nebular shock waves, such as bow shocks produced by planetesimals plowing through nebular gas, is the most developed of the ideas (Boley et al. 2013). Transient heating during impacts between planetesimals has also been recently studied (Johnson et al. 2015). Other putative chondrule formation mechanisms include lightning within the disk and magnetic reconnection events (Joung et al. 2004). These models could be distinguished by measuring chondrule cooling rates, the strength of the ambient magnetic field during chondrule formation, and determining the ages of chondrules relative to that of planetesimals and the nebular lifetime.

Chondrules are generally considered younger than CAIs by at least 1 million years based on Al-Mg isotopic systematics (Kita et al. 2010) and have ages distributed over at least 3 million years. With that said, absolute model ages obtained by lead-lead (Pb-Pb) isotopic systematics of individual chondrules from one class of chondrite yield no gap between the two types of objects (Connelly et al. 2012; Bollard et al. 2017). Resolving this issue is important for two reasons. First, it will elucidate the different formation mechanisms for CAIs. Second, it is unclear how to maintain CAIs and chondrules in the disk until they can be incorporated into chondrites, an interval of several million years, given how fast these small objects can reach the Sun by gas drag, although pressure bumps may inhibit this inward transport (Desch et al. 2018) (see Q1.2b). A related issue is that if chondrules and CAIs are incorporated into large planetesimals very early, the planetesimals could heat up and alter or even destroy these grains. As such, further research is needed to detail the absolute ages of chondrules and CAIs.

As described above, some solar system minerals originally condensed from the gas phase within the nebula (Grossman 1972; Krot 2019) and were likely the precursors to some CAIs. The complex relationships between the condensation of refractory minerals, rocks, and igneous CAIs remain unclear (Yoneda and Grossman 1995; Ebel and Grossman 2000; Connolly et al. 2001; Bollard et al. 2015). Chondrules and some CAIs are igneous, recording high temperatures (>2,000°C) for seconds to minutes. This evidence indicates these objects experienced transient melting events within the disk. The majority of them also experienced cooling rates on the order of 5–100°C per hour, relatively slow when compared to expectations of how fast a molten droplet would cool if it was sitting in empty space. That suggests chondrules, after formation, were surrounded by gas that could allow slow cooling. Another major constraint on the melting of chondrules, and to a lesser extent CAIs, is that they were processed multiple times and preserve petrologic and geochemical evidence of remelting (Connolly and Jones 2016). Satisfying all of these constraints is a challenge for any formation model.

Continued investigation of chondrules and igneous CAIs is important for placing their formation in context with that of other rocky solar system bodies (Connolly et al. 2018). An important implication of such studies is that any igneous rocks in chondrites that formed from some kind of collision mechanism would be byproducts of the growth of planetesimals. By comparison, igneous rocks formed while free-floating in the disk before accretion were on the direct pathway to forming planetesimals. Such mechanisms could be potentially distinguished with measurements of the ambient magnetic field strength (Weiss et al. 2021) and of extinct radioactivity products within chondrules.

The matrices of chondrites are composed of a variety of materials that include mineral grains and organic phases and are enriched in volatile elements compared to chondrules and CAIs (Ehrenfreund and Charnley 2000; Weisberg et al. 2006; Glavin et al. 2021). The origin of organic phases is an issue that requires considerable further research. It is unclear how much of the diversity in the organic matter observed across chondrites and cometary dust samples is owing to differences in hydrothermal parent body processing and how much reflects differences in the materials accreted to different parent bodies. Moreover, the range of formation locations, temperatures, and mechanisms spans accretion as icy mantles on anhydrous minerals in the outer nebula, accretion to metal grains and chondrules in the inner nebula, and as direct nebular condensates (Alexander et al. 2017). Laboratory studies of the organic matter in newly returned comet and asteroid samples from known bodies, in addition to organics in terrestrially collected samples, are needed to deconvolve nebular and parent body histories and terrestrial alteration artifacts.

Q1.2b How, Where, and When Did Radial Mixing and Segregation of Solids and Gas Occur in the Nebula?

During the protoplanetary disk phase as the Sun continued to accrete, the first solids formed and planetesimals assembled and agglomerated into protoplanets. Along the way, some planetesimals experienced large-scale melting, collisional evolution and disruption, and gas-driven orbital migration. At the same time that all of these processes were occurring, the gas and solids of the nebula experienced radial mixing and were likely transported both inward and outward. Inward transport of volatile solids may have led to volatile evaporation at their respective snow lines (e.g., the distances from the young sun at which temperatures are at the condensation temperatures of ices), which in turn could have produced changes in the local nebular gas composition. Outward transport of refractory solids, on the other hand, could have enriched the dust content of planetesimals.

Such mixing played a fundamental role in determining the size, bulk composition, internal structures, and orbital architecture of the solar system today. For example, it may have contributed to Mercury’s iron-rich and reduced, moderately volatile-rich composition (Kruss and Wurm 2018; Nittler and Weider 2019). It may have also led to the enhancement of heavy elements and noble gases in the giant planets relative to the bulk composition of the nebula (Mandt et al. 2020) and the presence of chondrule-like and CAI-like grains in comets and interplanetary dust particles (Wooden et al. 2017). Mixing has presumably also influenced the formation of more distal objects like Centaurs and Kuiper belt objects for which we currently have few compositional constraints from remote sensing and no in situ measurements or known samples.

It has long been recognized that gas drag leads to the inward drift of solids, particularly for meter-sized bodies (Weidenschilling 1977a). It has also long been predicted that the disk experienced viscous spreading owing to magnetohydrodynamic and/or purely hydrodynamic turbulence as well as owing to laminar, nonturbulent processes (Weiss et al. 2021). Turbulence from the magnetorotational instability (MRI) and/or torques in a laminar disk owing to a large-scale toroidal field are thought to have transported material radially within the disk plane (Armitage 2015). Meanwhile a magnetized, laminar disk wind may have thrown material upward and outward. Whether these magnetic mechanisms dominantly transported inward or outward was influenced by the relative orientation of the rotation direction of the solar system relative to the direction of the mean vertical magnetic field. A diversity of hydrodynamic instabilities that generate turbulence have also been theorized to transport angular momentum (Fromang and Lesur 2019).

All of these processes are thought to have occurred at the time disk substructures may have formed. Such substructures may have formed as the result of these disk transport mechanisms or alternatively by the accretion of giant planets, and whether planets or substructures came first is unclear. In either case, the formation of substructures themselves would have then influenced subsequent disk evolution, possibly serving as a barrier to further transport in their locations or instead filtering the grain sizes of transported materials. For example, after the giant planets grew to a sufficiently large mass, they should have opened up a gap in the disk that would have inhibited radial transport (Johansen and Lambrechts 2017). Also, snow lines serve as barriers to inward transport of volatile ices (Pontoppidan et al. 2014). Last, magnetized disk winds can also open gaps in the disk and transport disk materials outward.

Evidence for transport of dust outward in the solar system is provided by the presence of crystalline silicates including chondrule-like and refractory inclusions in comets and interplanetary dust particles (Bockelée-Morvan et al. 2002; Joswiak et al. 2017). The presence of clasts, chondrules, and refractory inclusions formed with isotopic compositions distinct from their bulk meteorites provides evidence for dust transport in both directions (Brennecka et al. 2020; Schrader et al. 2020; Williams et al. 2020), although it is possible that some of these isotopic variations could be explained by temporal changes in the nebula’s composition. Evidence for inward transport of ices and evaporative concentration of volatiles in the gas phase at snow lines may be provided by enrichments of heavy elements and noble gases in giant planet atmospheres (Mandt et al. 2020).

There are several key unsolved questions relating to disk evolution. First, and foremost, the radial extent of mixing and transport is uncertain. For example, there is a lack of clarity about which materials formed close to the Sun (<0.1 AU), or in the terrestrial planet and asteroid belt region (1–3 AU), and were transported outward to 15 AU and beyond. Similarly, it is unclear which materials formed outside the terrestrial planet region and

were transported inward. As a result of this bidirectional transport, many regions of the solar nebula may have served as the source materials for the terrestrial planets, the giant planets, and small bodies like asteroids, comets, Centaurs, and Kuiper belt objects. The issue is to quantify the magnitude of the contribution and the degree of mixing from each zone.

Second, the nature of the mechanisms driving such transport are uncertain. Several of the above processes (or others) were responsible, but the role of hydrodynamic processes and magnetic torques are debated. If magnetic fields were important, certain magnetic mechanisms (MRI, laminar toroidal field, and/or magnetized disk wind) controlled transport at different distances from the Sun, but their strength is unknown. The spin pole and vertical magnetic field of the nebula are also mysterious, and it is unclear whether they were aligned or anti-aligned. If hydrodynamic turbulence was important, certain conditions enabled this, and the mechanisms that generated such behavior need to be explored.

Third, despite astronomical evidence for the presence of holes, rings, and gaps in other protoplanetary disks, it is unknown whether the solar nebula formed such substructures. Accepting that it did, their geometry, location, and timing are critical to understanding planet formation processes, as are the mechanisms that influenced their formation. We also need to know how they may have inhibited or filtered the transport of gas and/or dust in the nebula. Progress on these questions can be achieved with chemical, isotopic, petrographic, and paleomagnetic studies of nebular materials in meteorites and returned samples, in situ isotopic and compositional measurements of planetary and small bodies at a range of distances from the Sun, and astronomical observations of the density, velocity structure and magnetism of disks.

Q1.2c How, Where, and When Were Gas and Solids Processed in the Nebula and During Accretion (by Heat, Aqueous Alteration, and Electromagnetic and Particle Irradiation)?

In addition to constraints from chondrules and CAIs (see Q1.2a), astronomical observations and analyses of meteorites tell us that irradiation also played an important role in the evolution of dust and gas in the nebula. For example, stellar and galactic cosmic rays can cause spallation—fragmentation of larger nuclides (e.g., carbon, oxygen, magnesium, and iron, or C, O, Mg, and Fe, respectively) into smaller ones (e.g., lithium, beryllium, and boron, or Li, Be, B, respectively)—which can result in detectable changes in the composition of dust. Evidence for energetic particle irradiation of nebular dust comes from Be- and B-isotope signatures of CAIs, which are consistent with live ¹⁰Be (t_1/2 = 1.5 million years), a short-lived radionuclide associated with exposure to cosmic ray irradiation.

Ultraviolet and X-ray irradiation contributes to ionization of nebular gas and formation of complex molecules on the surfaces of dust and ice grains. Such irradiation may have influenced the isotopic composition of light elements (HCON) in nebular gas and dust. For example, ultraviolet dissociation of CO has been proposed to explain mass-independent O-isotope variations observed in meteorites and ionization from X-rays may facilitate ion-molecule reactions that could explain enrichment in deuterium in organic material in chondrites (see Q1.1b). Studies of cosmogenic systems using a wide range of planetary material along with observational analyses and models of irradiation from young stellar objects are necessary to further develop our understanding of the chemical evolution of the solar nebula.

Nebular dust is also affected by thermal metamorphism and fluid alteration associated with the accretion of planetesimals. Evidence for thermal processing of dust in planetesimals includes observations of a range in metamorphic grades and an inverse correlation between the ages and cooling rates for chondrites with the same bulk composition. Thermal processing of nebular dust within chondrite parent bodies includes dehydration and recrystallization of matrix minerals, modification of organic material, destruction of presolar grains, chemical equilibration of minerals, and recrystallization of glassy material (e.g., Quirico et al. 2014; Floss and Haenecour 2016).

Early thermal metamorphism of planetesimals is largely controlled by the abundance of ²⁶Al, the time of planetesimal formation, the size of the body formed, and its ratio of ice to rock. Planetesimals that form earlier end up with more active ²⁶Al, larger bodies are better able to insulate materials warming up in the interior, and the temperatures of ice-rich bodies are buffered by the ice’s latent heat of melting. Fluid alteration of nebular solids is largely associated with water–rock reactions in chondrite parent bodies. Evidence for these processes include the

occurrence of secondary minerals that require precipitation from fluid and varying degrees of fluid alteration for chondrites with the same bulk composition. Melting of accreted ice to facilitate these water-rock reactions likely stemmed from heating from the decay of ²⁶Al. Fluid alteration occurred 2–5 million years after CAIs, based on age dating of secondary minerals such as fayalite and carbonates (Jilly-Rehak et al. 2017). Additionally, thermal metamorphism and fluid alteration of nebular solids within chondritic planetesimals could have also been affected by impacts, which can both heat bodies (through deposition of the impactor’s energy) and cool them by unroofing or excavating deep materials in the body.

Deconvolution of parent body and nebular processing enables the identification of common parent bodies for groups of meteorites, and a more comprehensive understanding of the conditions that gave rise to the chemical signatures in planetary materials, including the nature of materials before incorporation into parent bodies. The composition of the accreted ice and that of the fluids associated with water–rock alteration in chondrite parent bodies is still unclear, but necessary for a comprehensive picture about secondary processing of nebular components.

Fundamental questions regarding all forms of nebular processing remain to be answered. For example, the full range of heating mechanisms for processing solids in the nebula is unknown, and there is no consensus on the heating mechanisms for melting refractory inclusions (see Q1.2a). We know that gas, ice, and organics were affected by heating and irradiation in the disk, but the degree is uncertain and could vary with location and other factors. There is also the issue of distinguishing nebular, parent body, and impact processes, all which have shaped the composition of planetary materials. Ideally, we would like to isolate specific effects both to trace conditions in the nebula and to better understand the original composition of dust, gas, ice, and organic matter in the disk.

Progress on these topics will require an interdisciplinary approach that combines astronomical observations of young stellar objects, numerical modeling, and geochemical analyses of chondrites and samples returned from primitive bodies. Observational surveys of disks for detailed studies of density structure that span an appropriate range of environmental and evolutionary states could be used to work out the mechanics of key evolutionary processes. Such studies are now possible with the sensitivity, angular and spectral resolution now offered through millimeter interferometers such as ALMA (Andrews 2020).

Strategic Research for Q1.2

• Constrain the predominant driving forces for, and the radial extent of, nebular mixing and transport through spacecraft elemental, isotopic, and magnetic measurements of—most importantly comets, Centaurs, and Mercury—and also of Saturn, Venus, or Kuiper belt objects; return of samples from comet surfaces and, with a lower priority, from asteroids; disk transport modeling; and laboratory petrologic, isotopic, and paleomagnetic analyses of returned and terrestrially collected samples, and telescopic observations of protoplanetary disks.
• Determine if, how, when, and where gaps, rings, or holes developed in the nebular disk through spacecraft isotopic and elemental measurements of gas, dust, ice, and organic components in outer and inner solar bodies; return of asteroid and comet surface samples; disk transport modeling; ground- and space-based astronomical measurements of protoplanetary disks; and laboratory petrologic, isotopic, and paleomagnetic analyses of returned and terrestrially collected samples.
• Constrain the original compositions and processing histories of dust, gas, ice, and organic matter in the solar nebula through return of asteroid and comet surface samples; astronomical observations of young stellar objects and outer solar system volatiles; modeling of heating and radiation processing; and laboratory petrographic, elemental, and isotopic analyses of returned and terrestrially collected asteroid and comet samples.
• Determine the timing and range of formation mechanisms of the earliest solids in the solar system, including gas phase condensation, irradiation, and transient heating by collisional, X-ray, magnetohydrodynamic, or other means, through return of asteroid and comet surface samples; astronomical observations (e.g., ALMA) of dust in young stellar objects; and laboratory petrographic, elemental, isotopic, and paleomagnetic analyses of returned and terrestrially collected asteroid and comet samples.

Q1.3 WHAT PROCESSES LED TO THE PRODUCTION OF PLANETARY BUILDING BLOCKS—THAT IS, PLANETESIMALS?

A common assertion in classical planet formation models is that the initial size of planetesimals—that is, planetary building blocks, was approximately a kilometer, and that they grew from pairwise collisions between smaller objects. As awareness grew that meter-size bodies could not last long in the protoplanetary disk, however, new physics was incorporated, and new accretion models were considered that made the aerodynamic gas-particle processes in the protoplanetary disk a critical component of planetesimal formation. Current models describe how small particles, often called pebbles, can become highly concentrated in zones within the solar nebula. Their mutual gravity can then lead to collapse and the agglomeration of objects that are commonly ~100 km in diameter. The formation and physical properties of planetesimals made this way can be constrained by the small body populations, although some signatures of formation may have been erased by subsequent collisional and thermal evolution.

In addition, we also do not yet understand the role of chondrules—tiny molten droplets found within chondritic meteorites—in creating planetesimals across the solar system. Indeed, chondrule formation may postdate the formation of many planetesimals. While chondrules have been studied in detail for decades, they do not yet have a broadly accepted mode of formation, and their numerous constraints have long confounded understanding. This limitation hinders our ability to interpret what meteorites are actually telling us about planetesimal and planet formation.

Q1.3a How and When Did the First Grains Aggregate and Form Centimeter-Scale Objects?

Clues to how the earliest accretion of solids occurred are recorded by chondrites and other primitive planetary materials. They suggest that small grains came together in the protoplanetary disk under low velocity collisions, where they stuck together. From there, the generally accepted scenario suggests that dust accreted to form millimeter- to centimeter-sized objects that melted and cooled. These bodies may have then accreted with dust and other materials to form centimeter- to meter-sized bodies (see discussion in Q1.3b), which then accrete into ~100-km-size bodies by some concentration mechanism in the protoplanetary disk (e.g., Morbidelli et al. 2009).

With that said, there are several unknowns about the processes that aggregated grains into centimeter-sized objects. First, we do not know what processes are responsible for enabling the first submicron-sized grains to stick together, with van der Waal’s forces, electrostatic attraction, and ferromagnetic attraction forces all being possibilities (Dominik et al. 2006). We also do not understand how chondrites and planetesimals were lithified (i.e., turned into coherent, rock-like bodies) and if the lithification process was linked with the accretion process.

Additionally, we do not know the full extent to which the millimeter- to centimeter-scale objects found in these meteorites formed before or after meter-sized to kilometer-sized to ~100-km-scale accretion occurred. Although it is generally argued that CAIs formed, melted, and cooled before the accretion of planetesimals, the same cannot be said for chondrules. Indeed, the measured formation ages of some iron meteorites indicate that some large, differentiated bodies formed contemporaneously with some chondrules (Connelly et al. 2012; Kruijer et al. 2020). If the formation of all igneous objects within chondrites occurred before planetary accretion began in earnest, then their formation is on the pathway to creating larger solid bodies.

A key issue that could help us quantify this scenario is to determine when CAIs, chondrules, and mineral grains formed. Additional constraints on planetesimal formation and evolution would come from the timing of post-accretional aqueous and thermal metamorphic events on their parent bodies. These events can potentially be constrained using radiometric dating with short-lived (e.g., aluminum-magnesium, magnesium-chromium, hafnium-tungsten, and iodine-xenon) and long-lived (e.g., uranium-lead) radionuclides (see Q1.3b). Furthermore, observations of protoplanetary disks by ALMA and the next-generation Very Large Array can provide constraints on the growth of up to centimeter-scale particles.

Q1.3b How and When Did Grains Grow from Centimeter-Size Objects to ~100 Kilometer-Size Planetesimals?

The formation of planetesimals is a necessary step to making the terrestrial planets and giant planet cores, but precisely how this happened is still debated. We start by considering constraints from small bodies, namely the size-frequency distributions of the asteroid and Kuiper belt objects. Both show local maxima for objects ~100 km in diameter. This feature is likely a fossil from their primordial size distributions rather than a signature of collisional evolution over the past several billions of years (e.g., Bottke et al. 2015). This supports the idea that planetesimals were born big, with sub-meter bodies jumping to objects tens to hundreds of kilometers without passing through intermediate stages (Morbidelli et al. 2009; Johansen et al. 2015).

The mechanisms leading to the formation of such bodies does not appear to be pairwise accretion, as suggested by many studies of the past. The first problem is that particles do not reliably stick together during collisions, but instead may bounce off one another (Zsom et al. 2010). Mechanisms have been proposed to enable particles to jump this so-called “bouncing barrier” (Musiolik et al. 2016; Steinpilz et al. 2020), and future progress may be obtained by determining the grain size, composition, ratios of ice to rock, electrostatic forces, and remanent magnetism of primitive accretional grains.

The second problem, however, is more fundamental. Even if bodies find a way to achieve decimeter sizes to objects larger than a meter, the gas drag they experience as they orbit within the nebula leads to two negative effects: (1) it increases their mutual collision velocities with other solids, leading to fragmentation (Birnstiel et al. 2011), and (2) they begin to rapidly spiral in toward the Sun, with timescales of <1,000 years from 1 AU in a nonturbulent disk (Weidenschilling 1977b).

One way to overcome the meter-sized barrier is to concentrate sufficient particles in a small region of the disk so that they undergo gravitational collapse and form an aggregate body. Perhaps the best-studied concentration mechanism in the solar nebula is the streaming instability (SI), which describes how aerodynamic forces cause small particles to collect in regions where the solid-to-gas ratio is enhanced over solar abundances (Youdin and Goodman 2005; Johansen et al. 2007). The pressure-supported gas orbits at a somewhat slower azimuthal velocity than would a particle at the same orbital radius on a purely Keplerian orbit. As such, particles orbiting in the gas experience a “head wind” as they encounter the slower orbiting gas, which causes the particle orbits to lose energy and drift radially inward. If as particles drift inward a local concentration forms, this concentration will accelerate the local gas somewhat, lessening the rate of the concentration’s inward drift. The concentration can then continue to grow by accreting outer particles that are drifting inward more rapidly, and as the concentration grows, its effects on the gas strengthen, allowing its drift to slow further and its growth to continue. If the local spatial density of solids becoming sufficiently high, the concentration can rapidly gravitationally collapse to form ~100-km-class planetesimals directly from pebbles. Other proposed concentration mechanisms for particles include turbulent eddies (Cuzzi et al. 2008) and pressure bumps (e.g., associated with volatile snow lines, forming giant planets, and a variety of hydrodynamic or magnetohydrodynamic instabilities [Johansen et al. 2014]). Alternatively, some studies that consider a distribution of velocities and higher sticking efficiencies for ices find pairwise accretion might still form planetesimals. Further progress on this idea will require better models, and expanded studies of sticking in low-temperature, low-pressure environments.

To test different planetesimal formation mechanisms, it is useful to consider constraints from bodies relatively undisturbed by collisional or dynamical evolution. For example, a common outcome of a gravitationally bound clump of particles undergoing collapse is the formation of an equal-sized well separated binary. Many binaries of this nature are found in the Kuiper belt beyond Neptune, with most residing in the cold classical Kuiper belt region. Most of these binaries (~80 percent) have prograde rotation relative to their heliocentric orbits, a prediction fully consistent with the streaming instability, where prograde clumps are more likely to become gravitationally bound than retrograde clumps (Nesvorný et al. 2019, 2021). Individual binary components also have similar colors, again consistent with being formed simultaneously during planetesimal formation (Noll et al. 2020).

Other SI predictions are that primitive planetesimals are often porous, low-strength composites of ~mm–cm size grains with possibly limited evidence of processing by hypervelocity and catastrophic impacts. This is consistent with high-resolution Rosetta images of comet Churyumov-Gerasimenko’s surface and measurements of

the high porosity and weak strength of comets and Kuiper belt objects, particularly Arrokoth (see Figure 2-20), a contact binary in the cold classical Kuiper belt with modestly flattened lobes (22 × 20 × 7 km and 14 × 14 × 10 km) that likely formed by the gravitational collapse of a “pebble cloud” formed by the SI or a related aerodynamic concentration mechanism (Blum et al. 2017; McKinnon et al. 2020). Last, SI predictions appear to broadly match the inferred primordial size distributions of the asteroids and Kuiper belts (Johansen et al. 2015).

Q1.3c Which Reservoirs and Materials (Gas, Rock, Ice, Organics) Formed into Planetesimals?

Isotopic and elemental measurements of planetary objects can constrain the provenance of their source materials and the temperatures at which these bodies and their constituents formed. The abundances and isotope compositions of HCNO (see Figure 4-2) and noble gases are among the best available tracers for investigating the origins of planetary material.

The building blocks of the inner planets, as exemplified by the compositions of Earth, Mars, to a lesser extent the venusian atmosphere, and of primitive and differentiated meteorites did not acquire HCNO directly and unaltered from the protosolar nebula, but instead from material that was isotopically distinct. Such heterogeneity might have existed in the interstellar medium or in the parent molecular cloud during the formation of icy, organic, and silicate-bearing dust (see Figure 4-3). Trapping of isotopically processed HCNO into solids might have maintained strong isotopic heterogeneities relative to the nebular gas composition. Processed grains were transported and distributed in the disk where they accreted into forming planetesimals.

The isotopic compositions of HCNO make it possible to trace genetic relationships between the different families of meteorites and the inner planets like Earth and Mars. Earth was mainly sourced by material akin of the so-called NC chondrites, especially the enstatite chondrite group (Kleine et al. 2020), whereas material originating from more volatile-rich, and presumably more distant material akin to CC meteorites might have supplied volatile elements to an initially dry proto-Earth (Alexander et al. 2012; Marty 2012; see Chapter 6). The analysis of cometary matter, especially during the Rosetta mission, strongly suggests that such bodies also contributed volatiles to the terrestrial atmosphere and oceans, although in more limited amounts (Marty et al. 2017).

The case of Mars is more difficult to investigate, partly because the composition of volatile elements in the martian mantle is poorly known owing to lack of samples, but also because the martian atmosphere and hydrosphere evolved drastically as they lost volatiles to space. The combination makes it difficult to identify the original building blocks of Mars (Carr and Head 2003). The composition of venusian atmosphere, crust, and mantle is unknown except for a few low precision measurements of atmospheric stable isotopes and noble gases (Donahue et al. 1982).

Knowledge of the composition of volatile-bearing bodies, such as comets or primitive asteroids not yet sampled in our collections, will permit a major advance in our understanding of the origin of inner planet material as well as of dynamic processes that led to the distribution of dust during solar system formation. New observations of primitive bodies and sample return mission that clarify genetic relationships between meteorite and IDPs already in our collections and specific bodies would be particularly valuable.

In the case of Jupiter, the Galileo probe found that the most volatile elements (e.g., noble gases and nitrogen) appear to be just as enriched in Jupiter’s atmosphere as are less-volatile elements (e.g., carbon, sulfur, and phosphorus). These enrichments are relative to the abundance ratio of the element of interest to that of hydrogen in the Sun (Q2.1). The Galileo probe results support the core accretion model and led to the idea that the building blocks of Jupiter formed at very low temperatures (and thus perhaps farther out in the nebula than 5 AU). Otherwise, the noble gases and nitrogen (primarily as N₂) would not have been trapped in solid form nearly as efficiently as carbon, and especially sulfur and phosphorus. Accretion of Jupiter’s building blocks at larger orbital distances may introduce greater constraints on accretion efficiency needed to form Jupiter prior to nebular dispersal.

The Juno mission has subsequently confirmed Jupiter’s nitrogen enrichment, with measurements of the deep-water abundance in Jupiter’s equatorial zone suggesting that oxygen also shows a similar enrichment (by a factor of ~3). Interpreting the apparently uniform element enrichments in terms of temperature-controlled volatile trapping in solids could indicate that the building blocks of Jupiter formed in a more distant region of the

protoplanetary disk, where water ice would have been in an amorphous form and cold enough that the trapping efficiency of volatiles was uniform.

Alternatively, Jupiter’s elemental enrichments could be telling us more about the evolution of nebular gas rather than the thermal history of small particles and planetesimals. This would apply if Jupiter captured gas at a time when the gas in the protoplanetary disk was already enriched. The enrichment process may have involved a depletion of hydrogen (H₂) in the disk owing to the great difficulty of retaining it in amorphous ice during gas loss. The preferential removal of hydrogen from the disk would make other elements, whose volatiles can be trapped in amorphous ice, appear enriched with respect to hydrogen. The continual removal of hydrogen over time also may produce larger enrichments until the nebular gas is fully dissipated. This scenario would imply that elemental enrichments may provide chronological information that can be linked to disk evolution. Determining the elemental enrichments in the atmospheres of the other giant planets is key to generalizing and discriminating between different scenarios of disk conditions and evolution that led to giant planet formation. The noble gases are particularly useful in this respect (see Q2.2).

Nonradiogenic argon-36 (³⁶Ar) was measured in Titan’s atmosphere by the Huygens probe (Niemann et al. 2010). The expectation was that ³⁶Ar would have a high abundance (relative to atmospheric N₂) if Titan formed at low temperatures. Instead, a very low ³⁶Ar abundance was measured, suggesting Titan’s building blocks had a relatively warm origin. This putative warm environment was likely an offshoot of the protoplanetary disk, one that formed a protosatellite disk around Saturn (see Q2.3 and Question 8; Chapters 5 and 11, respectively). Other bodies that are likely to have formed directly in the protoplanetary nebula and have N₂-rich atmospheres that are amenable to this test are Triton and Pluto. Voyager 2 and New Horizons were unable to measure their argon abundances or their isotopic compositions. But if the ³⁶Ar/N₂ ratio were determined by a future mission, Triton or Pluto could provide another valuable snapshot of temperatures in a distant region of the solar system (i.e., the primordial Kuiper belt).

Isotopic measurements can provide powerful constraints on the specific chemical forms of elements that existed in the protoplanetary disk and were incorporated into large bodies. In the outer solar system, the two isotopic systems that have proven to be most useful are hydrogen and nitrogen, owing to relatively large differences in isotopic compositions between various hydrogen and nitrogen- bearing materials. The practical consequence is that a robust conclusion can often be reached, even if in situ or remote-sensing data lack high precision.

Returning to the previous examples, the high ¹⁵N/¹⁴N ratio in Titan’s nitrogen supports an alternative source as the most important N-bearing constituent in Titan’s building blocks. This isotopic measurement is consistent with the aforementioned warm origin of Titan’s building blocks that would have prevented the trapping of both ³⁶Ar and N₂. Plausible alternative sources for Titan’s nitrogen are ammonia-bearing ices or salts and N-bearing organic matter (Miller et al. 2019).

The deuterium/hydrogen or D/H ratios in water, the dominant hydrogen reservoir on icy worlds, have been measured for certain bodies (Saturn’s midsize icy satellites; Waite et al. 2009; Clark et al. 2019). Values appear to be similar to or slightly above that of Earth’s ocean water, and they could help disentangling different sources of water and organic compounds that contributed to the bulk inventory of hydrogen on these bodies and on Titan as well.

For Jupiter, the ratios of both D/H and ¹⁵N/¹⁴N have been measured. They are found to be indistinguishable from the protosolar values, as expected for the dominant inheritance of H₂ and N₂ from the protoplanetary disk. At the other giant planets, higher values of D/H are expected to reflect increasing proportions of water relative to H₂, and this is generally observed with the ice giants having noticeably higher D/H ratios. However, the ¹⁵N/¹⁴N ratio remains to be measured in Saturn, Uranus, and Neptune, which could help constrain the deviation from protosolar values and potential contributions of N-bearing organics or ices.

Jupiter-family and nearly isotropic (i.e., Oort cloud) comets have consistently demonstrated heavy nitrogen isotope ratios. These large enrichments can be explained by interstellar chemistry theories involving ion-molecule ¹⁵N fractionation at 10 K (Charnley and Rodgers 2002; Rodgers and Charnley 2008) that has been observed in dark cold cores and modeled in disks. The similarity of the ¹⁴N/¹⁵N ratios found in comets and interplanetary dust particles strengthens a possible link to interstellar chemistry as the origin of isotopically anomalous organic particles in comets.

Strategic Research for Q1.3

• Infer the compositions and locations of nebular source reservoirs by return of samples from, especially, comet surfaces as well as from asteroids; measuring the elemental and stable isotopic compositions of refractory and volatile elements with lander and orbiter missions (especially for the ice giants, Centaurs, and Mercury and also Venus, comets, Saturn, and Kuiper belt objects); and ground- and space-based telescopic observations of atmospheric and/or sublimated volatiles toward small bodies in outer solar system, planets across the solar system and their moons; and laboratory petrological, elemental, and isotopic analyses of returned and terrestrially collected samples.
• Clarify the mechanisms that enabled accretion of objects beyond the “fragmentation barrier” size (~1 m) through determination of the structure, porosity, magnetization, and size and shapes of grains on small bodies by return of comet surface samples; in situ imaging, strength, and gravity measurements of comets, Centaurs, or Kuiper belt objects; ground- and space-based telescopic observations; accretion modeling; and laboratory petrological and paleomagnetic analyses of returned and terrestrially collected samples.
• Test models of accretion of micrometer-to-centimeter-scale objects by determining the relative ages of crystallization of chondrules, CAIs, and mineral grains, and thermal and aqueous alteration events using return of comet surface samples; and laboratory radioisotopic analyses of returned surface samples from comets and asteroids and terrestrially collected samples.
• Constrain accretion processes in protoplanetary disks by resolved studies of the volatile composition as well as of the composition, sizes and shapes of grains using ground- and space-based telescopic observations of protoplanetary disks.
• Understand the processes of accretion, fragmentation, and deformation associated with grain and particle collisions through laboratory grain accretion experiments, observations of collisions in dense giant planet rings, and modeling.

Q1.4 HOW AND WHEN DID THE NEBULA DISPERSE?

The mechanisms and timescales of solar nebular dispersal remain fundamental open questions. One interpretation of meteorite constraints is that planetesimal formation came to an end when the solar nebula dispersed. With that said, the solar nebula is big enough that this could mean modestly different timescales for the endgame of planetesimal formation in the inner and outer solar systems, and perhaps within those regions as well. For example, photoevaporation of the solar nebula from the outside-in may limit the formation of distant planetesimals in the primordial Kuiper belt, potentially explaining why Neptune’s outward migration ground to a halt prior to passing through the most distant indigenous population of icy planetesimals in the so-called cold classical Kuiper belt. The end of the solar nebula also means the termination of gas processes that can damp planetary eccentricities and inclinations. This could set the stage for a period of violent upheaval for the orbits of the giant planets (see Question 2, Chapter 5). The dispersal time of the nebula also affects the final composition of planetary objects by truncating the condensation and accretion sequence at a particular location-dependent temperature and pressure.

Q1.4a What Was the Lifetime of the Solar Nebula?

A key parameter influencing the final architecture of the solar system and the composition and structure of the planets is the lifetime of the nebula. The dispersal time of the nebula sets the timescale for stellar accretion, the formation of the gas giants, and the epoch of gas-driven planetary migration and has major implications for dust dynamics and disk structure (Takeuchi and Artymowicz 2001), the final sizes and eccentricities of the terrestrial planets (Kominami and Ida 2004), and the viability of hypothesized chondrule and planetesimal formation mechanisms involving nebular gas or nebular magnetic fields. For example, disk gravitational instabilities could in principle have formed the giant planets in <0.1 million years, while so-called core accretion is favored by longer (several to perhaps >10 million years) timescales (Helled et al. 2014; see Question 2). The evolving composition of the gas resulting from progressive condensation of increasingly more volatile elements can in turn influence

the final compositions of the terrestrial planets (Grossman 1972), giant planets (Guillot and Hueso 2006; Monga and Desch 2015) and their moons (Glein 2017).

The lifetime of the solar nebula is poorly constrained (Figure 4-4) through limited direct and indirect evidence. Astronomical observations of the abundance of dust and gas and of active accretion onto protostars indicate that protoplanetary disks have estimated lifetimes from <1 to ~20 million years with a mean value of

2 million years (Mamajek 2009). However, these bounds are themselves uncertain owing to uncertainties in the model ages of pre-main sequence stars that host these disks. Other estimates of the mean lifetime range up to ~6 million years (Bell et al. 2013). Also, there is the additional uncertainty associated with where in this distribution the solar system lies.

Other than inferences inferred from astronomical observations lifetimes of protoplanetary disks, the lifetime of the solar nebula has been constrained mainly using observations of chondrites. The presence of agglomeratic olivine chondrules in CR chondrites has been interpreted as evidence that the nebular dust disk persisted until at least the formation time of CR chondrules (Schrader et al. 2018) (i.e., at >3.46 million years). It has also been proposed that the formation of chondrules in CH and CB chondrites requires the presence of nebular gas under the hypothesis that they are impact melt sprays from planetesimal collisions. Such gas could enable planetesimals to reach the high relative velocities that can produce such impacts (Johnson et al. 2016). This may indicate that the solar nebula persisted until the ~4–6 million years formation age of CB chondrules.

Because the sustenance of magnetic fields requires the existence of a conducting medium, the dispersal of the nebula would have led to dissipation of the nebular field. Therefore, under the assumption that the field is present whenever there is gas (and this might not necessarily be the case), the dispersal time of the nebula could be estimated by establishing when the solar nebula magnetic disappeared as inferred from the absence of paleomagnetism in meteorites younger than a certain age. In fact, paleomagnetic measurements of several meteorite groups indicate that the solar nebula dispersed sometime between 1.2 and 3.9 million years after CAI-formation in the region where ordinary chondrites formed and between 2.5 and 4.9 million years after CAI-formation in the region where carbonaceous chondrites formed (Weiss et al. 2021). The presence of certain chondrules in CR chondrites has been interpreted as evidence that at least the nebular dust disk persisted until at least 3.5 million years after CAI-formation (Schrader et al. 2018). It has also been proposed that the formation of certain metal-rich chondrules may indicate the presence of nebular gas until ~4–6 million years after CAI-formation (Morris et al. 2015; Johnson et al. 2016; Garvie et al. 2017).

Several uncertainties limit our ability to accurately date the nebular lifetime. Current constraints are derived from measurements on only a half dozen meteorite groups with a narrow range of ages. The locations in the nebula for which these constraints apply are highly uncertain (up to tens of AU). Also, the uncertainties from these measurements on the dispersal times have large uncertainties relative to the mean age of dispersal. There are very few direct constraints on the gas density itself, with most indicating simply whether a solar nebula analogous to that observed for actively accreting protoplanetary disks is present or absent. Last, there are virtually no constraints on the evolution of the composition of the residual gas.

Q1.4b What Mechanisms Dispersed the Nebula?

The processes by which the protoplanetary disk dispersed are uncertain. The steady accretion of the Sun fed by viscous disk spreading would have progressively depleted the disk. However, astronomical observations indicate that after slowly evolving for several million years, most disks abruptly disperse at a rate an order of magnitude faster than their earlier accretion rates (Ercolano and Pascucci 2017).

This two-timescale evolution has motivated two theorized disk dispersal mechanisms. A leading candidate is photoevaporation, in which the central star and/or neighboring stars heat the disk atmosphere increasing the velocity of the gas to above the escape velocity (Owen et al. 2010; Weiss et al. 2021). Depending on the source of the radiation and its spectrum, this can lead the disk to disperse from the inside out, outside in, and/or to form gaps (Gorti et al. 2009). Alternatively, the solar wind could be launched by magnetic mechanisms (Shadmehri and Ghoreyshi 2019). If it has a sufficiently high flux, this wind could potentially dominate over photoevaporation in depleting the disk. The role of magnetized disk winds versus photoevaporation could be constrained by paleomagnetic measurements of meteorites, as discussed here. Also, constraints on the gas density as a function of time and distance from the Sun and young stellar objects using meteorite studies and astronomical observations could determine the direction of dispersal (inward, outward, or with gaps), which in turn could distinguish between the stellar sources for photoevaporation and the role of giant planet formation.

Several key unsolved questions thus remain. We do not yet know with confidence how rapidly the solar nebula dispersed, or whether it evolved over different timescales in the inner and outer solar systems. It is also uncertain whether the dispersal of the nebula was associated with long-lived disk substructures. Last, we do not

yet understand the role of hydrodynamic winds and/or magnetic fields in a dispersing gas in different regions of the nebula, nor do we know how the composition of the gas and the ratio of gas to dust evolved with time in the same regions. Answers to these questions would enable us to constrain which mechanisms (e.g., photoevaporation, magnetized disk winds, or other mechanisms) led to dispersal of the solar nebula and how the accretion of planets was influenced by the dispersal process.

Strategic Research for Q1.4

• Constrain the temporal and spatial evolution of the composition of nebular gas with in situ measurement of the volatile elemental compositions (noble gases and hydrogen) of Saturn, Uranus, and Neptune.
• Constrain the temporal and spatial evolution of the composition of nebular solids with return of samples from comet surfaces and laboratory petrologic, elemental, and isotopic studies and analyses of returned and terrestrially collected samples.
• Measure the intensity of the solar nebula magnetic field as a function of space and time with return of asteroid and comet surface samples; in situ magnetic measurements at asteroids, comets, Centaurs, and Kuiper belt objects; and laboratory paleomagnetic measurements of returned and terrestrially collected samples.
• Measure the temporal and spatial evolution of the density, composition, and magnetism of protoplanetary disks using optical, infrared, millimeter, and radio measurements of nearby young stellar objects.

SUPPORTIVE ACTIVITIES FOR QUESTION 1

• Expanded terrestrial-based extraterrestrial sample collection (especially ANSMET, the Antarctic Search for Meteorites).
• Expanded laboratory instrumentation development and acquisition beyond the support associated with active sample return missions.
• Laboratory observations of returned samples from Ryugu, Bennu, and Phobos.
• Telescopic observations that support cross-disciplinary studies relevant to early solar system processes, particularly protoplanetary disks.

REFERENCES