The origin of chondrules, and chondritic mixtures, is a long-standing problem in planetary science. We have developed a model for the formation of chondrites based on the outcomes of vaporizing collisions between planetesimals that were dynamically excited by giant planets. Initially the dynamical excitation from growing giant planets inhibited the growth and differentiation of nearby planetesimals. Giant planet migration would instigate a distinct period of intense collisional evolution between planetesimals that ended with dynamical clearing and low collision probabilities. During migration, collisional velocities exceeded the thresholds for catastrophic disruption and transiently increased the local dust-to-gas ratio. A subset of these collisions were also vaporizing, where a portion of the colliding bodies is compressed into states that expand into mixtures of dust, droplets and vapor. When water dominates the vapor, the plume is cool and ejected dust is solid. Vapor plumes expanded supersonically and drove varying strength shock waves into the nebular gas. Portions of these nebular shocks exceeded the melting temperature of silicates and melted nebular dust. The expanding flows cooled and sorted nebular debris into distinct groups of varying sizes and thermochemical histories. The subsequent hydrodynamic collapse of the plume turbulently mixed and cooled the variably processed and size-sorted droplets and dust. These mixtures have characteristics similar to chondrules and matrix, which implies that local processes could lead to aggregation. We propose that the major chondrite groups reflect varying initial locations and collisional histories. Chondritic bodies formed in the asteroid belt and in the outer solar system; the outer solar system bodies were implantation in the asteroid belt by dynamical scattering by the gas giants. Under this model framework, chondritic meteorites contain an invaluable record of nebular conditions during planet formation and the presently uncertain history of giant planet migration in the early solar system.

The processes described in our model were not occurring at the very beginning of planet formation. If there were molten spherules within the very first generation of planetesimals, then they were formed by a different set of processes. In our model, the initial planetesimals near the gas giant planets were formed of primitive dust and then processed into dust and chondrules as these bodies were destroyed and reaccreted. Collisional processing in the vaporizing regime can begin with the (rapid) formation of the proto-cores of the gas giant planets. The precursor planetesimals to chondrule formation were likely similar to CI-like asteroids Bennu and Ryugu (prior to their hydrothermal alteration).

The ages of iron meteorites indicate that there was a first generation of planetesimals, in both the inner solar system (NC group) and outer solar system (CC group), that formed quickly and became large enough to differentiate from radiogenic heating from ²⁶Al. However, small planetesimals and/or planetesimals that are dynamically excited into an erosive collision regime can remain primitive bodies comprised of the original dust components in the solar system. These first-generation, primitive planetesimals were likely similar to CI chondrites prior to hydrothermal alteration. Some bodies may be in various stages of partial differentiation and there is evidence in the REE abundances in some chondrules for a history of differentiation. The IVANS model for the formation of OC and CC chondrite groups considered the outcomes of collisions between first-generation, primitive planetesimals and vaporizing collisions between differentiated bodies will be considered in future work.

This model development focused on the dynamical influence of the gas giant planets during their growth and migration and the formation of the main chondrite groups (CC, OC, EC). These planetesimals were located in regions under the gravitational influence of these planets. Some formed in the asteroid belt region (OC) and others were implanted into the asteroid belt by scattering from the outer solar system (CC). In the outer solar system, gravitational stirring by the ice giants did not reach the onset of vaporization of ice and the formation of chondrules from impact-generated nebular shocks.

Our model is consistent with the observed isotopic dichotomy between carbonaceous chondrites (CC) and non-carbonaceous chondrites (NC). The current interpretation is that CC were formed in the outer solar system, e.g., beyond the orbit of Jupiter, and NC were formed in the inner solar system. Our model supports chondrule formation in these regions and separation in space and time is consistent with distinct chondrite groups. There are individual chondrules that are swapped between these groups (e.g., NC chondrules in CC chondrites and vice versa). Our model would generate free-floating chondrules that can be scattered into the other regions of the solar system.

There has been no agreement on the main heating mechanism to melt dust into sub-mm sized igneous spheres. The features in chondritic meteorites require a set of environmental conditions that have been hard to satisfy with what was known about the protoplanetary disk.

Chondritic meteorites are the foundation of understanding the chemistry of the solar system. They also provide key isotopic constraints on planet formation processes. However, the vast amounts of information in chondritic meteorites cannot be fully linked to planet formation until we understand how these bodies were created.

There are excellent reviews of the problem: e.g., Connolly & Jones JGR 2016, Desch et al. MAPS 2012, and Ruzicka MAPS 2012.

In this model, chondrules form and cool within the nebular shock front generated by the expanding impact vapor plume. As the shock front reaches free-floating nebular debris, typically dust clusters, the dust is slowed and coupled to the moving shock front. The size particles that couple to the shock front depends on the density of the shock-compressed gas. These coupled particles are trapped in the moving front that cools as it expands. Some dust is incorporated early while the shock front is hot enough to melt silicates; dust incorporated later experiences lower temperatures. In a typical impact event, only the downrange direction of the nebular shock front reaches the temperatures required to melt silicate dust. The other regions of the shock front trap solid particles. These cold particles are mixed in with the cooled chondrules during the collapse of the vapor plume.

In the IVANS model, the timing of chondrule formation is related to the dynamical excitement of planetesimals due to the growth of planetary embryos. A period of planet migration would increase the number of mutual collisions between a population of planetesimals. Planetesimal collisions would decrease as the main planets dynamically cleared regions of the solar system. Ultimately chondrule formation ceases by the IVANS process when the nebular gas dissipates.

Individual chondrules can be dated with radioisotope chronometry systems, including U-Pb, Al-Mg, Mn-Cr, and Hf-W. In general, the results indicate that most chondrules formed about 2 Myr after CAIs, but that chondrule production was a continuous process in the solar nebula. Recently, the details of chondrule ages have been called into question by evidence for heterogeneous Al²⁶ in the nebula (Krestianinov et al. Nat. Comm. 2023) and the need for a U isotope correction for U-Pb ages. In general, the Hf-W system supports a 2.2±0.8 Myr age for Allende chondrules (Budde et al. PNAS 2016). Mn-Cr and U-Pb ages support an extended time period of chondrule formation over the first 4 Myr for both ordinary and carbonaceous chondrites (e.g., Yin et al. ApJ 2007; Bollard et al. Sci. Adv. 2017).

These ages overlap with the time period where giant planet growth would lead to vaporizing collisions between planetesimals and the formation of new chondrules by impact-generated nebular shocks.

Calcium-aluminum-rich inclusions (CAIs) are the first condensates from the hot nebular gas, and they define the start of our planetary system. The ages of chondrules spans a few million years, and so the CAIs must be preserved until after chondrule formation to be assembled in chondrite parent bodies.

Because the IVANS model, explicitly involves collisions between primitive planetesimals, we suggest that CAIs may be stored within these bodies. When the population of planetesimals is excited to erosive collisions, the dust and CAIs within these ancestor planetesimals would be ejected as debris. This process would happen contemporaneously with chondrule formation in nebular shocks. Then, new planetesimal formation would aggregate the older CAIs and younger chondrules into a new chondritic body.

CB chondrites have previously been interpreted as impact products derived from planetesimal material. Our work has focused on the generation of OC and CC chondrites from nebular materials. The metal in CB chondrules are thought to have originated from a differentiated planetesimal. Future work will examine in more detail the origin of CB chondrites in light of the physical processes identified in this work.

In the IVANS model, as in most other chondrule formation models, the size of chondrules is controlled by a size limit due to shear with the nebular gas. Larger molten droplets would be sheared into smaller sizes.

The IVANS model does agree with the range of cooling rates inferred for chondrules. In this model, the heating event is the arrival of the nebular shock wave and the cooling rate is related to the decaying nebular shock as it expands outward from the impact. The decaying nebular shock has a much slower cooling rate compared to materials in an ejecta plume.

In the IVANS model, the formation of chondrules is coincident with an increase in collisional erosion and production of dusty ejecta. When a population of planetesimals at a given location of the solar system enters the vaporization regime (due to excitation by protoplanets), that region will be enhanced in dust. Then, impact-generated nebular shocks will form in a region that is transiently enhanced in the local dust-to-gas ratio.

The IVANS model can explain how molten chondrules are cooled and brought into contact with primitive matrix dust that was not heated to high temperatures. As the nebular shock propagates, the amplitude of the shock decays and the corresponding temperatures decay. As the vapor plume itself cools and the gas components recondense, variously processed materials are brought into closer proximity. The collapsed plume is a dusty cloud of materials with very different thermal histories. We suggest that a chondrule-matrix chemical complementarity can be established in a collapsed vapor plume. In addition, some warm clusters of chondrules may form in this process. Our 2025 paper does not include a detailed study of the evolution of the collapsed plume. Our initial estimates suggest that many collapsed clouds will be dispersed into the solar nebula and then a clear signature of complementarity would not form. Therefore, the IVANS model currently predicts that a complementary signature would not be a universal signature in chondritic bodies.

In the IVANS model, the free-floating nebular dust is transformed into chondrules via the impact-generated nebular dust. Generating chondrules involves many planetesimal-planetesimal collisions that process the nebular dust in a given region of the solar system. OC chondrites have a much smaller matrix fraction compared to CC chondrites. In the context of our model, we suggest that a larger fraction of planetesimals and nebular dust experienced vaporizing collisions in the OC formation region compared to the CC formation region. Different planet growth and migration patterns have different dynamical effects on nearby regions of the solar system. This effect is calculated Carter and Stewart PSJ 2020, as shown in their Figure 7. For example, in the Grand Tack scenario, a larger fraction of planetesimals in the asteroid belt experienced mutual collisions compared to planetesimals originally located beyond Jupiter. In this case, in a short amount of time, a large fraction of nebular dust could be thermally processed in the asteroid belt. Subsequent formation of new planetesimals from debris in the main belt would have a different chondrule to matrix fraction compared to the outer solar system.

Figure 7. Fraction of planetesimals that experienced collisions as a function of their initial location within the disk. These collisions could have occurred anywhere in the disk. The collision fractions are averaged across similar simulations. The top panel shows the results from in situ growth simulations, the middle panel migration-only simulations, and the bottom panel Grand Tack simulations. The area below each panel indicates the location and migration of Jupiter (black) and Saturn (gray). Solid triangles indicate the fixed positions of the giant planets, open triangles indicate initial positions from which the giant planets followed the path indicated by the arrows. Note that the solid histogram is used for easier comparison and generally shows the lowest values; it should not be considered the nominal case.

Chondrules have a range of FeO-bearing phases that are divided into two groups: Type I with FeO ≤10 wt% and Type II with FeO >10 wt%. There is evidence that some chondrules were reprocessed in environments that changed the amount of FeO. In the IVANS model, the nebular shock front is very dusty, which is a more oxidizing system compared to the bulk solar nebula, which is extremely reduced. In addition, the boundary between the nebula and a water-vapor plume front is initially a sharp contrast in composition and redox states and chondrules may flow across this interface. The IVANS model generates these transient environments during the expansion of the vapor plume and nebular shocks. Each impact event could have different oxygen fugacities, depending on the compositions (dust-to-gas ratios) of the nebular dust and vapor plume. Therefore, the IVANS model provides a means to generate a variety of redox conditions for the formation and reprocessing of chondrules.

• Many planetesimal-planetesimal collisions from giant planet growth and migration (Carter & Stewart PSJ 2020).
• Impact plumes are water vapor + cool dust.
• Supersonic vapor plumes make nebular shocks.
• Chondrules are nebular dust melted by nebular shocks.
• Chondrules cool as nebular shock decays with distance.
• Nebular shock size sorts the nebular debris (reverse sieve).
• Nebular shock decay, size sorting, and plume collapse makes chondritic mixtures.

Previous collision models focused on generating chondrules from the planetesimal material. Here, we focus on chondrules created by the nebular shocks generated by impact vapor plumes. The nebular shock is the transient environment for chondrule formation from free-floating debris. New to our model is the idea that the chondritic mixture of thermally processed and unprocessed material is aggregated by the collapsing vapor plume.

In the IVANS model, the nebular shock front sorts free-floating debris with a reverse sieve effect. The vapor plume and surrounding nebular shock has a cone-like shape in the principal impact direction (largest relative velocity to the background gas). The leading face of the cone is a nebular shock front that creates a sudden change in gas velocity. As the shock front encounters different sized debris, the smallest debris is coupled to the fast-moving gas behind the shock front due to gas drag effects. Larger debris is not slowed by the moving gas. The nebular shock front expands by many orders of magnitude compared to the original colliding bodies, and over this great distance the leading face of the vapor plume collects the smallest size fraction. The most significant parameter controlling the size sorting is the density of the nebular gas. For the expected range of nebular gas densities, our calculations of the maximum size particles caught in this reverse sieve agrees well with the size of particles observed in chondritic meteorites (generally less than order mm).

An example of a reverse sieve that sorts small liquid particles from larger ones can be found here.

During giant planet migration, most of the impact ejecta did not generate new chondrules; the bodies were eroded or disrupted with limited melting. The ejected debris scattered into the nebula. See 2025 LPSC poster by Cashion and Stewart. Planetesimals comprised of dust-sized components would be disrupted into variously sized dust aggregates as seen in laboratory experiments of erosive collisions (e.g., Nakamura et al. Icarus 2024)

In the IVANS model, new chondrule formation and new planetesimal formation would have involved the debris from previous collisions. For chondrule-forming events, only a fraction of the mass of the colliding bodies was the source of the vapor in the ejecta plume. The rest of the bodies was dispersed as cold nebular debris.

Most studies of planetary collisions have neglected the presence of any surrounding nebular gas. In addition, the length scales of the shocked nebular regions are orders of magnitude greater than the size of the original colliding bodies. Therefore, a shock physics code must be able to handle the dynamic range of the problem, which requires an adaptive meshing feature in a grid-based hydrocode. In Smoothed Particle Hydrodynamics (SPH) methods, the very low density of the surrounding gas translates to a large separation between particles, which means that these nebular shock features would not be resolved. In the IVANS model, we used both a grid-based Lagrangian 1D code (pyKO) and a adaptive mesh Eulerian hydrocode (CTH) to study the impact process and nebular shocks.

One previous paper observed the very beginning of the nebular shock wave from a planetesimal-planetesimal collision (Hood et al. MAPS 2009).

There is not a simple single answer to this question because it depends on multiple parameters. First, the composition and internal structure (e.g., porosity) of the colliding bodies determines the minimum impact velocity to generate a vapor plume. In general, collisions greater than about 1 km/s will produce some water vapor. Second, the relative velocity between the plume source and the peak shock pressures determine the amplitude of the nebular shock. With no initial relative velocity, a shock pressure of about 40 GPa in water ice can generate a nebular shock that can melt silicates. These tradeoffs are discussed in detail in the paper.

In the impact jetting model for chondrule formation (Johnson et al. Nature 2015), chondrules originate from molten jets of material near the contact points in collisions between planetesimals or onto protoplanets. The molten jets are sheared by the surrounding nebular gas to break the liquid into chondrule-sized particles. Note that the production of molten silicates in a jet will be accompanied by the formation of some vapor, which was not examined in this work.

In the IVANS model, most OC and CC chondrules are created from free-floating nebular dust that is heated by an impact-generated nebular shock.

The impact velocities for the formation of molten jets and volatile vapor plumes overlap and both processes should be acting together during planetesimal-planetesimal collisions. In future work, we will examine the relative amounts of material that could be converted to chondrules via these processes to compare to the meteoritic record.

The set of 2019 LPSC abstracts were motivated by the realization that many planetesimal-planetesimal impacts would reach the vaporization regime. At that time, we observed the formation of nebular shocks from the impact vapor plume; however, those abstracts suggested that chondrules formed primarily from silicates that originated in the colliding bodies. As we continued to study the problem, we realized that many more nebular shocks would form by lower velocity impact events that vaporized volatiles within the planetesimals. In these cases, the silicate fraction is modestly heated but not melted. In addition, the nebular dust to gas ratio would rise in a collisional environment. As a result, we changed our focus to the formation of chondrules in the nebular shocks from free-floating dust/debris in the solar nebula. The 2025 paper is the first to define the IVANS (Impact Vapor And Nebular Shocks) model.