Giant Impact Research Articles

Mixing model of Phobos’ bulk elemental composition for the determination of its origin: Multivariate analysis of MMX/MEGANE data

The formation process of the two Martian moons, Phobos and Deimos, is still debated with two main competing hypotheses: the capture of an asteroid or a giant impact onto Mars. In order to reveal their origin, the Martian Moons eXploration (MMX) mission by Japan Aerospace Exploration Agency (JAXA) plans to measure Phobos’ elemental composition by a gamma-ray and neutron spectrometer called MEGANE. This study provides a model of Phobos’ bulk elemental composition, assuming the two formation hypotheses. Using the mixing model, we established a MEGANE data analysis flow to discriminate between the formation hypotheses by multivariate analysis. The mixing model expresses the composition of Phobos in 6 key lithophile elements that will be measured by MEGANE (Fe, Si, O, Ca, Mg, and Th) as a linear mixing of two mixing components: material from Mars and material from an asteroid as represented by primitive meteorite compositions. The inversion calculation includes consideration of MEGANE’s measurement errors (EP) and derives the mixing ratio for a given Phobos composition, based on which the formation hypotheses are judged. For at least 65% of the modeled compositions, MEGANE measurements will determine the origin uniquely (EP = 30%), and this increases from 74 to 87% as EP decreases from 20 to 10%. Although the discrimination performance depends on EP, the current operation plan for MEGANE predicts an instrument performance for EP of 20—30%, resulting in 70% discrimination between the original hypotheses. MEGANE observations can also enable the determination of the asteroid type of the captured body or the impactor. The addition of other measurements, such as MEGANE’s measurements of the volatile element K, as well as observations by other MMX remote sensing instruments, will also contribute to the MMX mission’s goal to constrain the origin of Phobos.

Post-giant impact planetesimals sustaining extreme debris discs

ABSTRACT Extreme debris discs can show short-term behaviour through the evolution and clearing of small grains produced in giant impacts, and potentially a longer period of variability caused by a planetesimal population formed from giant impact ejecta. In this paper, we present results of numerical simulations to explain how a planetesimal populated disc can supply an observed extreme debris disc with small grains. We simulated a sample of giant impacts from which we form a planetesimal population. We then use the N-body code rebound to evolve the planetesimals spatially and collisionally. We adopt a simplistic collision criteria in which we define destructive collisions to be between planetesimals with a mutual impact velocity that exceeds two times the catastrophic disruption threshold, V*. We find that for some configurations, a planetesimal populated disc can produce a substantial amount of dust to sustain an observable disc. The semimajor axis at which the giant impact occurs changes the mass added to the observed disc substantially, while the orientation of the impact has less of an effect. We determine how the collision rate at the collision point changes over time and show that changes in semimajor axis and orientation only change the initial collision rate of the disc. Collision rates across all discs evolve at a similar rate.

Fluorine abundance of the lunar magma ocean constrained by experimentally determined mineral-melt F partitioning

To quantify fluorine (F) evolution during lunar magma ocean (LMO) crystallization, high-pressure, high-temperature experiments have been conducted to determine mineral-melt partitioning of F for lunar minerals (plagioclase, orthopyroxene and ilmenite). Results constrain the F abundance in the magma ocean to 21–41 ppm at the time crust-forming plagioclase started crystallizing. Forward modeling shows that 352–703 ppm F would remain in the final 1 % of magma toward the end of magma ocean solidification. This range overlaps that inferred for the urKREEP reservoir (660 ppm). Taking into account model uncertainties, from the perspective of F abundances the urKREEP reservoir can be formed at 98.9–99.5% LMO solidification, with negligible loss of F from the Moon since the onset of crust formation. Backward modeling from initial crust-forming plagioclase, an initial LMO would contain 4.2–8.5 ppm F, which is consistent with estimates of the lunar primitive mantle F content derived from melt inclusions in Apollo samples. This finding is consistent with previous suggestions that the bulk silicate Moon is depleted in F relative to the bulk silicate Earth (which contains ∼25 ppm F). A BSE-like initial LMO would yield a magma containing 122 ppm F at the onset of crust formation, significantly higher than our calculated 21–41 ppm F. Fluorine depletion could have occurred by degassing during the early LMO stages (between the onset of LMO crystallization and first crust formation), and/or prior to the LMO stage (e.g., depletion during the giant impact or vapor drainage in the protolunar disk), but seems to have ended by the time the crust started forming.

The anomalies deep within Earth may be remnants of the giant impact that formed the Moon 4.5 billion years ago

The anomalies deep within Earth may be remnants of the giant impact that formed the Moon 4.5 billion years ago

Dynamical Interactions and Mass Loss within the Uranian System

The origin and evolution of planetary rings and moons remains an active area of study, particularly as they relate to the impact history and volatile inventory of the outer solar system. The Uranian system contains a complex system of rings that are coplanar with the highly inclined planetary equator relative to the orbital plane. Uranus also harbors five primary regular moons that play an important role in the distribution of material that surrounds the planet. Here we present the results of a dynamical simulation suite for the Uranian system, intended to explore the interaction between the five primary regular moons and particles within the system. We identify regions of extreme mass loss within 40 planetary radii of Uranus, including eccentricity excitation of particle orbits at resonance locations that can promote moonlet formation within the rings. We calculate a total dynamical particle mass-loss rate of 35% within 0.5 × 106 yr and 40% mass loss within 107 yr. We discuss the implications for postimpact material, including dynamical truncation of stable ring locations and/or locations of moon formation promoted by dynamical excitation of ring material.

Evolution of a Water-rich Atmosphere Formed by a Giant Impact on an Earth-sized Planet

The atmosphere of a terrestrial planet that is replenished with secondary gases should have accumulated hydrogen-rich gas from its protoplanetary disk. Although a giant impact blows off a large fraction of the primordial atmosphere of a terrestrial planet in the late formation stage, the remaining atmosphere can become water-rich via chemical reactions between hydrogen and vaporized core material. We find that a water-rich postimpact atmosphere forms when a basaltic or CI chondrite core is assumed. In contrast, little postimpact water is generated for an enstatite chondrite core. We investigate the X-ray- and UV-driven mass loss from an Earth-mass planet with an impact-induced multicomponent H2–He–H2O atmosphere for Gyr. We show that water is left in the atmosphere of an Earth-mass planet when the low flux of escaping hydrogen cannot drag water upward via collisions. For a water-dominated atmosphere to form, the atmospheric mass fraction of an Earth-mass planet with an oxidizing core after a giant impact must be less than a few times 0.1%. We also find that Earth-mass planets with water-dominated atmospheres can exist at semimajor axes ranging from a few times 0.1 au to a few au around a Sun-like star, depending on the mass-loss efficiency. Such planets are important targets for atmospheric characterization in the era of JWST. Our results indicate that efficient mixing between hydrogen and rocky components during giant impacts can play a role in the production of water in an Earth-mass planet.

The moon before mare

The crust of the Moon experienced a unique geodynamic evolution, beginning with its crystallization from a magma ocean, continuing through a period of heavy impact bombardment, and followed by extensive basaltic mare volcanism. All these events have left crucial records imprinted in the form of topographic features and gravity anomalies. Here, we invert gravity and topography data using a two-layer thin-shell loading model under the premise of pre-mare isostasy to investigate the global structure of the crust and solve for feldspathic crust and mare thickness, together with mare-induced flexure. The tectonic record and partially buried crater population are used to constrain the bulk of mare volcanism to have been emplaced on a 40 km elastic lithosphere, although mare within large impact basins may have formed on a thinner elastic lithosphere. The mare thickness and associated flexure are removed to calculate a map of the surface and crust of the Moon before mare volcanism. The pre-mare surface in the Oceanus Procellarum region is found to be ∼2 km lower than the surrounding nearside, and several possible explanations, including a giant impact, pore space annealing, isostatic adjustment, and crustal erosion induced by a mantle plume or thermal anomaly, are discussed. The pre-mare elevation map further sheds light on the ring structure of Imbrium, which is seen to resemble that of Orientale. Imbrium's outermost ring is observed to be at a larger radial distance to the northeast relative to the south, indicating that some level of lithospheric variability affected ring formation at the time of impact. The western part of Imbrium's ring within Oceanus Procellarum is not found in the pre-mare topography, implying that it either never formed or that some processes erased its signature from gravity and topography. The feldspathic, pre-mare, crust is found to be ∼7 km thinner within large nearside basins than in models not accounting for the high-density mare. The pre-fill floor of these basins was ∼6 km deeper than currently observed, and together with their updated crustal structure, these new insights have implications for impact simulations that try to reproduce the crustal structure of nearside mare basins.

Hafnium-tungsten evolution with pebble accretion during Earth formation

We combine calculations of pebble accretion and accretion by large and giant impacts to quantify the effects of pebbles on the hafnium-tungsten system during Earth formation. Our models include an early pebble accretion phase lasting 4–6 Myr with a global magma ocean and core segregation, a 20–50 Myr phase of large impacts, and a late giant impact representing the Moon-forming event. We consider various mass additions during each accretion phase, vary the metal-silicate partition coefficient for tungsten over a wide range, and track Hf180, Hf182, W182 and W184 in proto-Earth and impactor models over time using standard chondritic values for these isotopes in the pebbles. We find that an early phase of pebble accretion is compatible with the tungsten anomaly of Earth's early mantle as well as the present-day Hf/W ratio, but under restricted conditions. In particular, the pebble mass of proto-Earth is limited to 0.7 Earth masses or less, the average metal-silicate partition coefficient for tungsten is 30–50, and because the metal-silicate equilibration efficiency for giant impacts is low, the equilibration efficiency must be high for the large impactors.

Painting’s liveness

Sarah Kate Wilsons’s article ‘Painting’s liveness’ is written from her vantage point, as an artist/curator/researcher/collaborator/educator. Here she reveals how painting’s relationship with performance, particularly during the second part of the twentieth century, has led to painting’s liveness. This, she asserts, is apparent in paintings by Daniel Buren, Robert Rauschenberg, Yoko Ono, works by the Gutai Art Association and performances by Ei Arakawa. The inauguration of a performance programme at Bauhaus in Germany, Black Mountain College in America and the formation of the Gutai Art Association in Japan are highlighted by Wilson as important milestones. Writing by RoseLee Goldberg and Peggy Phelan on performance, J. L. Austin’s speech act theory as well as Satori, an expression from Zen Buddhism meaning enlightenment, are woven into this text. David Joselit’s declaration that the medium of painting is live and ‘On Air’ is drawn into her argument for painting’s liveness, whilst Catherine Wood’s curatorial project A Bigger Splash: Painting After Performance (2012) Tate Modern, London and her own curatorial project Painting in Time (2015–16) set the stage for this text.

Masses and Densities of Dwarf Planet Satellites Measured with ALMA

We have used the Atacama Large Millimeter/submillimeter Array to measure precise absolute astrometric positions and detect the astrometric wobble of dwarf planet Orcus and its satellite Vanth over a complete orbit. We also place upper limits to the astrometric wobble induced by Dysnomia on dwarf planet Eris around its orbit. From the Vanth–Orcus barycentric motion, we find a Vanth–Orcus mass ratio of 0.16 ± 0.02—the highest of any known planet or dwarf planet. This large ratio is consistent with the hypothesis that Vanth is a largely intact impactor from a giant collision in the system and that the system has likely evolved to a double synchronous state. We find only an upper limit of the barycenter motion of Eris, which implies a 1σ upper limit to the Dysnomia–Eris mass ratio of 0.0085, close to the modeled transition region between giant impact generated satellites, which are largely intact remnants of the original impactor and those which form out of reaccreted disk material left over postimpact. The low albedo of Dysnomia leads us to marginally favor the intact impactor scenario. We find that Dysnomia has a density of <1.2 g cm−3, significantly lower than the 2.4 g cm−3 of Eris.

Moon’s high-energy giant-impact origin and differentiation timeline inferred from Ca and Mg stable isotopes

Mass-dependent stable isotopic variations recorded in lunar samples provide novel resolution to the formation and differentiation history of the Moon. In this study, we report new high-precision Ca-isotope measurements for lunar rocks and minerals. Ca-isotope data and modeling of the lunar magma ocean together demonstrate indistinguishable mass-dependent Ca isotopic compositions of the bulk silicate Earth and Moon. This implied Earth-Moon isotope equilibration is consistent with the Moon’s high-energy giant-impact (Synestia) origin and not readily compatible with the traditional giant-impact models. Moreover, a cross-comparison between Ca and Mg isotopic data for an important anorthosite sample (60025) consistently clarifies its formation near the completion of the lunar magma ocean crystallization. Therefore, the various existing radiometric dating for 60025 sets the lunar magma ocean to have fully solidified by either 4.51 or 4.38 billion years ago, constraining the two respective lunar differentiation timescales to <30 (short-lived) or ~130–150 (long-lived) million years.

Giant Impact Events for Protoplanets: Energetics of Atmospheric Erosion by Head-on Collision

Numerous exoplanets with masses ranging from Earth to Neptune and radii larger than Earth have been found through observations. These planets possess atmospheres that range in mass fractions from 1% to 30%, reflecting the diversity of atmospheric mass fractions. Such diversities are supposed to be caused by differences in the formation processes or evolution. Here, we consider head-on giant impacts onto planets causing atmosphere losses in the later stage of their formation. We perform smoothed particle hydrodynamic simulations to study the impact-induced atmosphere loss of young super-Earths with 10%–30% initial atmospheric mass fractions. We find that the kinetic energy of the escaping atmosphere is almost proportional to the sum of the kinetic impact energy and self-gravitational energy released from the merged core. We derive the relationship between the kinetic impact energy and the escaping atmosphere mass. The giant impact events for planets of comparable masses are required in the final stage of the popular scenario of rocky planet formation. We show it results in a significant loss of the atmosphere, if the impact is a head-on collision with comparable masses. This latter fact provides a constraint on the formation scenario of rocky planets with substantial atmospheres.

Internal Structure of the Lunar Mantle: Matching of Geochemical and Geophysical Models

There are serious contradictions between the geophysical and geochemical classes of models of the chemical composition and internal structure of the Moon, associated with the assessment of the abundance of the main oxides. The search for a potential consensus between the models was carried out on the basis of a set of geophysical and geochemical data using the Monte-Carlo method using the Markov chain scheme in combination with a method of minimization of the Gibbs free energy. The influence of the chemical composition and mineralogy of several conceptual models on the internal structure of the Moon has been studied. Two classes of chemical composition models are considered—the E models with terrestrial values of Al2O3 and CaO and M models with their higher content, as well as two classes of the most popular geochemical models, the Taylor Whole Moon (TWM) and Lunar Primitive Upper Mantle (LPUM) models, with ~45 wt % SiO2, but with different concentrations of refractory oxides and FeO. In both classes of E and M models, the lunar mantle is enriched in silica (~50 wt % SiO2) and FeO (11–13 wt %, Mg# 79–81) relative to the bulk composition of the silicate Earth (BSE, ~45 wt % SiO2, ~8 wt % FeO, Mg# 89). Such high concentrations of SiO2 and FeO become the determining factors for understanding the features of the mineral, velocity, and density structure of the lunar mantle. For the E and M models and geochemical models TWM and LPUM, the speed of sound and the density of stable phase associations are calculated. For E and M models, good agreement was obtained between the velocities of P- and S-waves and seismic sounding data from the Apollo program, which supports the idea of a silica-rich (olivine-pyroxenite) upper mantle. Unlike the Earth’s upper mantle, the dominant mineral in the Moon’s upper mantle is low-calcium orthopyroxene, not olivine. In contrast, the sound velocities of silica-unsaturated compositions, both FeO and Al2O3 enriched (TWM) and depleted (LPUM) models, do not match the seismic signatures. Thermodynamically justified restrictions on the chemical composition, mineralogy, and physical characteristics of the mantle based on the E and M models make it possible to eliminate some contradictions between the geochemical and geophysical classes of models of the internal structure of the Moon. Simultaneous enrichment in ferrous iron and silica is difficult to reconcile with the hypothesis of the formation of the Moon as a result of a giant impact from the substance of the Earth’s primitive mantle or from the substance of a shock body (bodies) of chondrite composition. Limitations on lunar concentrations of FeO and SiO2 probably correspond to the parent bodies of some achondrites.

Sublimation of refractory minerals in the gas envelopes of accreting rocky planets

Protoplanets growing within the protoplanetary disk by pebble accretion acquire hydrostatic gas envelopes. Due to accretion heating, the temperature in these envelopes can become high enough to sublimate refractory minerals which are the major components of the accreted pebbles. Here we study the sublimation of different mineral species and determine whether sublimation plays a role during the growth by pebble accretion. For each snapshot in the growth process, we calculate the envelope structure and the sublimation temperature of a set of mineral species representing different levels of volatility. Sublimation lines are determined using an equilibrium scheme for the chemical reactions responsible for destruction and formation of the relevant minerals. We find that the envelope of the growing planet reaches temperatures high enough to sublimate all considered mineral species when M ≳ 0.4 M⊕. The sublimation lines are located within the gravitationally bound envelope of the planet. We make a detailed analysis of the sublimation of FeS at around 720 K, beyond which the mineral is attacked by H2 to form gaseous H2S and solid Fe. We calculate the sulfur concentration in the planet under the assumption that all sulfur released as H2S is lost from the planet by diffusion back to the protoplanetary disk. Our calculated values are in good agreement with the slightly depleted sulfur abundance of Mars, while the model over predicts the extensive sulfur depletion of Earth by a factor of approximately 2. We show that a collision with a sulfur-rich body akin to Mars in the moon-forming giant impact lifts the Earth’s sulfur abundance to approximately 10% of the solar value for all impactor masses above 0.05 Earth masses.

A race against the clock: Constraining the timing of cometary bombardment relative to Earth’s growth

Comets are considered a potential source of inner solar system volatiles, but the timing of this delivery relative to that of Earth’s accretion is still poorly understood. Measurements of xenon isotopes in comet 67P/Churyumov-Gerasimenko revealed that comets partly contributed to the Earth’s atmosphere. However, there is no conclusive evidence of a significant cometary component in the Earth’s mantle. These geochemical constraints would favor a contribution of comets mainly occurring after the last stages of Earth’s formation. Here, we evaluate whether dynamical simulations satisfy these constraints in the context of an Early Instability model. We perform N-body simulations of the solar system’s dynamic evolution considering an outer disk of 10000 comets, different initial conditions for the inner solar system and assuming that the giant planet instability happened within ≃10 million years of the start of planet formation. Out of 30 simulations, 13 meet a list of criteria for success. We calculate the probability of collision between comets and Earth analogs component embryos through time and estimate the total cometary mass accreted in Earth analogs as a function of time. We determine that the cumulative cometary mass accreted by the Earth analogs during the first 100 million years ranges between 4 ×1021 and 5 ×1024 g. While our results are in excellent agreement with geochemical constraints, we also demonstrate that the contribution of comets on Earth might have been delayed with respect to the timing of the instability, due to a stochastic component of the bombardment. More importantly, we show that it is possible that enough cometary mass has been brought to Earth after it had finished forming so that the xenon constraint is not necessarily in conflict with an Early Instability scenario. Indeed, 2 of the 13 successful simulations provide enough comet delivery after a last giant impact happening later than 30 Myr. However, it appears very likely that a few comets were delivered to Earth early in its accretion history, thus contributing to the mantle’s budget. Finally, we compare the delivery of cometary material on Earth to Venus and Mars. These results emphasize the stochastic nature of the cometary bombardment in the inner solar system.

An improved model of metal/silicate differentiation during Earth's accretion

We improved the algorithm presented in Rubie et al. (2015) to model the chemical evolution of Earth driven by iron/silicate differentiation during the planet's accretion. The pressure at which the equilibration occurs during a giant impact is no longer a free parameter but is determined by the smooth particle hydrodynamic (SPH) simulations of Nakajima et al. (2021). Moreover, impacting planetesimals are now assumed to be too small to cause melting and differentiation and thus their materials are stored in the crystalline upper mantle of the growing planet until a hydrostatically relaxed global magma ocean forms in the aftermath of a giant impact, whose depth is also estimated from Nakajima et al. (2021). With these changes, not all dynamical simulations lead to a satisfactory reproduction of the chemical composition of the bulk silicate Earth (BSE). Thus, the latter becomes diagnostic of the success of dynamical models. In the successful cases also the BSE abundances of W and Mo can be reproduced, that were previously hard to fit (Jennings et al., 2021).

Pluto's Sputnik basin as a peak-ring or multiring basin: A comparative study

Pluto's Sputnik basin is the only large impact basin in the outer Solar System that resembles basins in the inner Solar System in morphology and scale, with dimensions of ∼1800 × 1000 km and a depth of ∼2.5 km. Sputnik basin has been compared to giant impact basins without prominent topographic rings, such as the Hellas basin on Mars and South Pole-Aitken basin on the Moon. However, a north-south trending arcuate chain of water-ice mountain blocks within the western half of the basin resembles a peak-ring structure as found in impact basins on terrestrial bodies, suggesting that Sputnik basin may be more analogous to a peak-ring or multiring basin. Here, we perform a comparative analysis between Sputnik basin and 23 impact basins in the inner Solar System. We reconstruct Sputnik basin's topography prior to being partially filled with an N2-rich ice deposit. In our analyses, we measure key characteristics such as average radial topography, ring spacing and crustal/shell thinning beneath the basins. We find that Sputnik basin is morphologically, topographically, and structurally consistent with the peak−/multiring basins, having most similarities to Apollo and Humboldtianum basins on the Moon, rather than giant impact basins without prominent topographic rings. Sputnik's consistency with a peak−/multiring structure has implications for aspects of its formation, evolution, and structure. In particular, this new interpretation of the basin structure decreases the size of both the impactor and the possible mass anomaly contained within the basin by half relative to previous interpretations.

Intra-system uniformity: a natural outcome of dynamical sculpting

ABSTRACT There is evidence that exoplanet systems display intra-system uniformity in mass, radius, and orbital spacing (like ‘peas in a pod’) when compared with the system-to-system variations of planetary systems. This has been interpreted as the outcome of the early stages of planet formation, indicative of a picture in which planets form at characteristic mass scales with uniform separations. In this paper, we argue instead that intra-system uniformity in planet sizes and orbital spacings likely arose from the dynamical sculpting of initially overly packed planetary systems (in other words, the giant impact phase). With a suite of N-body simulations, we demonstrate that systems with random initial masses and compact planet spacings naturally develop intra-system uniformity, in quantitative agreement with observations, due to collisions between planets. Our results suggest that the pre-giant impact planet mass distribution is fairly wide and provide evidence for the prevalence of dynamical sculpting in shaping the observed population of exoplanets.

Spin of protoplanets generated by pebble accretion: Influences of protoplanet-induced gas flow

Context. In the pebble accretion model, protoplanets accrete millimeter-to-centimeter-sized particles (pebbles). When a protoplanet grows, a dense gas envelope forms around it. The envelope affects accretion of pebbles and, in particular, the spin angular momentum transfer at the collision to the planet. Aims. We aim to investigate the spin state of a protoplanet during the pebble accretion influenced by the gas flow in the gravitational potential of the protoplanet and how it depends on the planetary mass, the headwind speed, the distance from the host star, and the pebble size. Methods. We performed nonisothermal three-dimensional hydrodynamical simulations in a local frame to obtain the gas flow around the planet. We then numerically integrated three-dimensional orbits of pebbles under the obtained gas flow. Finally, assuming uniform spatial distribution of incoming pebbles, we calculated net spin by summing up specific angular momentum that individual pebbles transfer to the protoplanet at impacts. Results. We find that a protoplanet with the envelope acquires prograde net spin rotation regardless of the planetary mass, the pebble size, and the headwind speed of the gas. This is because accreting pebbles are dragged by the envelope that commonly has prograde rotation. As the planetary mass or orbital radius increases, the envelope is thicker and the prograde rotation is faster, resulting in faster net prograde spin. When the dimensionless thermal mass of the planet, m = RBondi/H, where RBondi and H are the Bondi radius and the disk gas scale height, is larger than a certain critical mass (m ≳ 0.3 at 0.1 au or m ≳ 0.1 at 1 au), the spin rotation exceeds the breakup one. Conclusions. The predicted spin frequency reaches the breakup one at the planetary mass miso,rot ~ 0.1 (a/1 au)−1/2 (where a is the orbital radius), suggesting that the protoplanet cannot grow beyond miso,rot. It is consistent with the Earth’s current mass and could help the formation of the Moon with a giant impact on a fast-spinning proto-Earth.

The Role of Giant Impacts in Planet Formation

Planets are expected to conclude their growth through a series of giant impacts: energetic, global events that significantly alter planetary composition and evolution. Computer models and theory have elucidated the diverse outcomes of giant impacts in detail, improving our ability to interpret collision conditions from observations of their remnants. However, many open questions remain, as even the formation of the Moon—a widely suspected giant-impact product for which we have the most information—is still debated. We review giant-impact theory, the diverse nature of giant-impact outcomes, and the governing physical processes. We discuss the importance of computer simulations, informed by experiments, for accurately modeling the impact process. Finally, we outline how the application of probability theory and computational advancements can assist in inferring collision histories from observations, and we identify promising opportunities for advancing giant-impact theory in the future. ▪ Giant impacts exhibit diverse possible outcomes leading to changes in planetary mass, composition, and thermal history depending on the conditions. ▪ Improvements to computer simulation methodologies and new laboratory experiments provide critical insights into the detailed outcomes of giant impacts. ▪ When colliding planets are similar in size, they can merge or escape one another with roughly equal probability, but with different effects on their resulting masses, densities, and orbits. ▪ Different sequences of giant impacts can produce similar planets, encouraging the use of probability theory to evaluate distinct formation hypothesis.