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Question 4: Impacts and Dynamics

Much of the story of how the solar system formed and evolved can be told through the history of impacts and dynamics. Questions 1–3, Chapters 4–6, have discussed the growth of planetesimals, protoplanets, and how they merge to make the terrestrial planets and giant planet cores.¹ A poorly understood part of planetary formation, however, is its endgame. Modelers now need to consider how giant planet migration, in conjunction with planet formation processes, affected bodies throughout the solar system (e.g., Nesvorný et al. 2021).

Previous work has also led to a series of tiered tests for solar system planet formation and evolution models. The top tier is to reproduce the orbits, sizes, and angular momentum budgets of the terrestrial and giant planets, including the small size of Mars. Successful models then move on to a second tier, where they need to reproduce small body reservoirs including the asteroid and trans-neptunian (or Kuiper) belts.

Here the committee focuses on a critical third tier of constraints, namely how small bodies—including trans-neptunian objects (TNOs), comets, leftover planetesimals, and asteroids—both dynamically evolved and bombarded the solar system worlds over the past 4.5 billion years. While the impacts discussed here often involve a limited amount of mass compared to planetary-scale collisions (e.g., the Earth–Moon forming impact, Question 3), they are crucial to deciphering critical questions, such as the nature of different episodes of giant planet migration, the primordial histories of solar system worlds, the size and orbital distributions of small body populations, the collisional histories of said populations, and the astrobiological history of Earth and other abodes of life affected by impacts.

Q4.1 HOW HAVE PLANETARY BODIES COLLISIONALLY AND DYNAMICALLY EVOLVED THROUGHOUT SOLAR SYSTEM HISTORY?

The major set pieces of early solar system evolution—namely, planet formation and giant planet migration—depleted early small body reservoirs, created new ones, and reconfigured most through mutual collisions between planetesimals (e.g., see Questions 2 and 3; Nesvorný 2018). The final stages of these processes left us with small

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

body populations both in relatively stable zones and on unstable orbits. Some of the latter found their way onto orbits where they could strike the planets and satellites (e.g., Zahnle et al. 2003; Dones et al. 2009; Bottke and Norman 2017). In addition, all these populations experienced mutual collisions before, during, and after these events. By using models constrained by the bombardment histories of all worlds, we can glean insights into how the planets, dwarf planets, and small body populations formed and evolved.

Many questions and gaps in our knowledge, however, still obscure our understanding of these fundamental planet-building processes. For example, the initial size distributions of planetesimals across the solar system are poorly understood, as are the physical processes that govern small body disruption. Many small body reservoirs might also be created/depleted by different phases of giant planet migration, but the details and precise timing are uncertain. Particular small body populations are recorded in the bombardment history of worlds with ancient surfaces (e.g., Mercury, Moon, Mars, and Iapetus), yet the nature of those populations at different times is unsettled and is difficult to disentangle from subsequent geological or cratering events. Impacts in the asteroid belt produce a collisional cascade of fragments, with many smaller bodies having their physical evolution affected still further by nongravitational forces, yet the relative importance of these processes in breaking down small bodies is unclear (e.g., Bottke et al. 2015). Many small bodies also escape their reservoirs and approach the Sun closely enough to shed mass or disrupt, yet the mechanisms describing this behavior are still enigmatic (Jewitt et al. 2015).

Q4.1a How Were the Last Surviving Planetesimal Populations in Regions Now Populated by Planets Affected by Collisional and Dynamical Evolution?

Planet formation models suggest that there were three major sources of early solar system bombardment (Bottke and Norman 2017). The first was the primordial Kuiper belt, a massive disk of ice-rich planetesimals that formed beyond the original orbits of the giant planets. At an unknown time, Neptune migrated outward through this population and scattered most of the planetesimals onto planet-crossing orbits, thereby triggering the dynamical instability of the giant planets (see Question 2; Tsiganis et al. 2005; Nesvorný and Morbidelli 2012; Nesvorný 2018). These former Kuiper belt planetesimals produced most early impacts on outer solar system worlds, and they were a strong component in the initial bombardment of terrestrial planets and asteroid belt as well. The impact signatures left behind also show intriguing signs that the primordial Kuiper belt population was affected by collisional evolution before, during, and after the giant planet instability (Morbidelli et al. 2021). Key unknowns in deducing the nature and early evolution of the primordial Kuiper belt are its initial size and orbital distributions, the timing of the giant planet instability, and how ice-rich planetesimals are disrupted by various mechanisms.

The bombardment produced by the dispersal of the primordial Kuiper belt is also ongoing. Many ice-rich bodies were left on long-lived but unstable orbits or were placed into the Oort cloud. Over billions of years, some of these bodies have escaped onto giant planet-crossing orbits, where they were able to hit numerous worlds. As shorthand, the committee refers to these escaped bodies as comets, although only some may ultimately develop tails. Accordingly, knowledge of what happened at early times is constrained by what is known of the present-day comet populations, what is known of craters and impact events on outer solar system worlds, and modeling work (Zahnle et al. 2003; Dones et al. 2009; Nesvorný 2018).

The second source of early solar system bombardment are the small bodies from the terrestrial and giant planets zones that were left on highly eccentric and inclined orbits by interactions with the growing planets, which are classified here as leftover planetesimals (Bottke et al. 2007; Morbidelli et al. 2018). These bodies experienced extensive collisional and dynamical evolution, and only a small number lasted for more than a few tens of millions of years. Some impactors even moved to new regions, with an unknown fraction of giant planet zone planetesimals moving into the inner solar system. The survivors, however, had high collision probabilities with the worlds in/near their formation zones, and this may allow them to dominate other sources of early bombardment. Their impacts may also have left behind critical traces of the unknown planetesimals that made the terrestrial planets and giant planet cores.

The third source, referred to here as asteroids, are bodies that formed or were captured near/within the main asteroid belt between Mars and Jupiter (Morbidelli et al. 2015). The effects of giant planet migration moved some asteroids onto planet-crossing orbits over a wide range of timescales, with some objects in mildly unstable

zones hitting the Moon and terrestrial planets over a billion years later. Depending on the impact signatures left behind on various worlds, the size and nature of the initial asteroid populations can be discerned by modeling the dynamical process by which these bodies are transported out of the main asteroid belt. While these escaping bodies experienced a lesser degree of collisional evolution than leftover planetesimals, mainly because their collision probabilities are lower than objects that reside closer to the Sun, their net probabilities of striking inner solar system worlds are smaller as well (Bottke et al. 2007).

All three populations struck worlds in the inner solar system, although the degree to which different populations dominated at different times is uncertain. The signatures they produced may be found in a variety of early bombardment traces (e.g., the size distributions of ancient craters and remnant small body populations, meteorite shock degassing ages, impactor fragments, and trace compositions found within terrestrial and extraterrestrial samples).

Q4.1b How Has Collisional and Dynamical Evolution Affected Small Body Populations Now Found in Stable Reservoirs Within the Inner and Outer Solar Systems?

The events involved with planet formation and giant planet migration (see Questions 2 and 3; Chapters 5 and 6, respectively) left the solar system with two primary small body reservoirs within our observational reach: the asteroid and trans-neptunian belts, and several smaller ones, such as those populations captured in mean motion resonance with giant planets (e.g., the Trojan asteroids of Jupiter and Neptune and the Hilda asteroids). It also created a reservoir of comets beyond our observational reach—namely, the Oort cloud of comets (see Q2.6b)—and populations of bodies on planet-crossing orbits that are steadily replenished by the stable reservoirs, such as the near-Earth objects, the ecliptic comets (which include the Jupiter-family comets and the Centaurs), and the nearly isotropic comets (i.e., long-period comets). The committee focuses here on the asteroid and trans-neptunian belts.

For the main asteroid belt, numerous bodies were likely captured from nearly every solar system region (Bottke et al. 2006; Walsh et al. 2011; Morbidelli et al. 2015; Vokrouhlický et al. 2016; Nesvorný et al. 2017; Raymond and Izidoro 2017; see Question 3). This implies that some bodies experienced collisional and physical evolution prior to entering the main belt region. Once in the main belt, asteroids were potentially struck at early times by each other and external impactors, although the precise number of main belt bodies shattered or disrupted at early times is uncertain (e.g., Bottke et al. 2015).

Once the planets settled into their quasi-final configurations ~4 to 4.5 Ga, the asteroid belt was left with a population comparable to the current one. It can be argued that only a few tens of the largest asteroids (diameter, D >100 km) have been disrupted over the past several billions of years (Bottke et al. 2015). These events provided enough smaller fragments to keep the asteroid belt size distribution in a quasi-steady state, with its overall structure controlled by the nature of the disruption laws affecting different asteroid types. Evidence for this behavior may be found in the crater size distributions identified on ancient asteroid surfaces (Marchi et al. 2015).

Asteroids gain mobility in the main belt via collisions, encounters with larger asteroids, and, most importantly, by the Yarkovsky effect, defined as a nongravitational thermal force produced by the anisotropic reemission of energy from sunlight (see also Q4.1c). Over time, the Yarkovsky effect allows many asteroids smaller than a few tens of kilometers to “drift” into dynamical resonances with the planets, where they can be pushed onto planet-crossing orbits (Vokrouhlický et al. 2015). A small fraction then go on to strike the terrestrial planets.

Small asteroids can also have their spin rates and obliquities modified by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, a net torque produced by solar photons that are absorbed and reemitted from a body (Vokrouhlický et al. 2015). Those with an asymmetric shape can have their rotation rates accelerated or decelerated, potentially causing spin rates on the orders of minutes for bodies tens of meters in size, or such slow spin rates that some asteroids enter tumbling rotational states. YORP torques also tilt asteroid spin vectors toward 0 degrees or 180 degrees, which maximizes how fast they migrate by the Yarkovsky effect. The combined behavior of the Yarkovsky/YORP effects make it possible to estimate the timing of many asteroid breakups from the dynamical evolution of their fragments. Accordingly, it is now possible to connect the evidence for putative impact surges on Earth, the Moon, and other terrestrial planets with specific asteroid breakup events from the past (e.g., Terada et al. 2020). This opens new areas of investigation for how changes in the history of various worlds, including life itself, might be linked to impact surges.

For the trans-neptunian belt, the extent of collisional evolution is dependent on the nature of the primordial population, when it was dynamically dispersed by a migrating Neptune (via the giant planet instability; see Questions 2 and 3), how it was collisionally bombarded by external populations, such as the numerous planetesimals residing in the giant planet zone, and the disruption law controlling these bodies (e.g., Morbidelli et al. 2021). Collision probabilities decrease as bodies move away from the Sun, though, so it seems likely the present-day size distribution for bodies larger than a few tens of kilometers was mainly set by planetesimal formation and dynamical depletion processes (see Q1.3b). It is possible, though, that collisional evolution occurring prior to the dynamical excitation of the primordial Kuiper belt influenced the shape of the present-day size distribution.

Curiously, cratering evidence on Pluto’s moon, Charon, and the Kuiper belt object (486958) Arrokoth (Figure 2.20) suggests a paucity of impactors between a few tens of meters and 1 km (Singer et al. 2019). The reasons for this deficit are debated; perhaps larger disruptions produce few bodies in this size range, or the disruption law for ice-rich planetesimals produces a collisional equilibrium consistent with such trends. If the latter scenario holds, the size distribution for comets and TNOs would likely become substantially steeper for sizes smaller than a few tens of meters (Morbidelli et al. 2021). Testing this may require visits to additional large TNOs to see primary craters not influenced by superposed secondary impacts (i.e., the impact of fragments of rock or ice ejected during the formatter of larger craters), determination of how many small comets that strike Jupiter and other giant planets, or detection of small TNOs with serendipitous stellar occultations.

Q4.1c What Are the Life Cycles (Physical States and Rotational Properties) of Small Bodies in the Solar System and How Are They Affected by Collisions, Thermal Changes, and Nongravitational Forces?

Collisions are a primary geologic process for small body populations. They break down worlds and create new fragments that can also be disrupted by subsequent impacts. Collisional by-products include craters, asteroid satellites, fragments with rubble-pile structures, and so on. Major collisions also create swarms of fragments on similar orbits, called families, that can tell us about the nature of large-scale impact events occurring in the past. Using models, these kinds of constraints can be used to glean insights into how individual bodies and populations have evolved from their primordial states (e.g., Bottke et al. 2015).

For asteroids smaller than a few tens of kilometers, the evolutionary effects of impacts and gravitational perturbations are expanded to include the Yarkovsky and YORP effects (see Q4.1b) (Vokrouhlický et al. 2015). Concerning YORP, as a body’s spin state evolves, its coupled rotational and translational motions become increasingly complex to model. The rubble pile nature of small bodies further complicates their evolution, allowing them to be spun fast enough that they reshape themselves or even fail (e.g., rotational mass shedding). If caught in the act of mass shedding, these bodies are referred to as active asteroids (Jewitt et al. 2015). These effects can lead to the formation of satellites that can evolve into long term stable systems. In turn, these systems are subject to complex internal dynamics driven by their mutual gravity and exogenous nongravitational effects akin to Yarkovsky and YORP (Margot et al. 2015; Walsh and Jacobson 2015).

A full understanding of how rubble pile asteroids evolve in response to Yarkovsky and YORP effects is lacking. While the specific YORP and Yarkovsky effects have been clearly documented—as best exemplified by OSIRIS-REx observations of (101955) Bennu (e.g., Farnocchia et al. 2021)—there are several aspects that are not understood yet that can dominate the eventual evolution of the mechanical state of these bodies.

There are hypothetical stable end states that such bodies may evolve to, preserving their mechanical structure and dynamical state over long-time spans. Conversely, there are other hypothesized effects which ultimately cause rubble piles to disaggregate into their constituent boulders and grains, which would then further evolve as monolithic bodies. The existence and efficiency of these different processes are unknown, yet are crucial to understanding the nature and age of the small bodies in the solar system, and the current rate at which they are created through catastrophic impacts.

Another set of active asteroids, sometimes called main belt comets, show sporadic cometary activity in the main belt (Jewitt et al. 2015). The best-known ones, such as 133P/Elst-Pizarro, are associated with asteroid families, and they show repeated activity near their perihelion. It is possible these bodies were formed by the disruption of a carbonaceous chondrite-like body with an ice-rich mantle or subsurface ocean. This might make some of the

resultant family members agglomerations of ice and rock, thereby allowing near-surface ice to sublimate when exposed by impacts, YORP spin-up, or some other process.

It is often challenging to identify the specific process that led to mass loss on a given asteroid. Not only are there several mechanisms to choose from (e.g., rapid spins, released volatiles, impacts, and subdued surface activity for inactive comets), but in some cases, the active asteroid is too small to be resolved using existing telescopes. At present, dust-signature analysis and long-term observations of repeated activity are the primary tools available to distinguish between different mass loss mechanisms (Jewitt et al. 2015).

In parallel to asteroids, the spins and orbits of cometary bodies, are also driven in complex ways by nongravitational effects. For cometary bodies, these effects are dominated by outgassing and mass ejection, which in general is more effective than solar photons (i.e., Yarkovsky effect) at changing an object’s spin state or orbit. Owing to differences in their mechanical properties and the strength of the effects, the response of cometary bodies to rapid rotation and rotational fission are often markedly different than seen in rubble pile asteroids. For example, wide-binary comets are unknown and thought unlikely, even though observed comets include many contact-binary shapes (Keller et al. 2015). On the other hand, rings have been detected around sizable Centaurs (see Question 8, Chapter 11). The exploration of stable orbiting material around ice-rich small bodies is a fascinating problem.

How cometary bodies preserve these morphologies over their lifetimes and yet are susceptible to rapid dissolution requires improved understanding of the mechanics of these primitive bodies. Although these evolutionary effects may be unique to the current epoch of the solar system, similar physics and processes undoubtedly were present, and perhaps persistent, at earlier epochs of the solar system.

Our knowledge of the life cycles of other small body populations—including Trojans, Centaurs, and small trans-neptunian objects—are far more speculative than for asteroids and comets because they have not been as thoroughly explored by robotic missions, and telescopic observations can be more challenging owing to their distance. Small trans-neptunian objects may have comparatively tame life cycles, as the YORP and Yarkovsky effects are not strongly effective far from the Sun, and outgassing likely ceased long ago. This implies that Arrokoth, a ~20 km world with a flattened bilobate shape and enigmatic geologic features, may be largely a byproduct of planetesimal formation rather than post-formation processes (see Question 1, Chapter 4). It remains to be seen if Trojans and Centaurs, also potentially derived from the primordial Kuiper belt, are similar or whether their shapes are dominated by processes connected to Kuiper belt collisions and/or solar-driven outgassing (e.g., volatile outbursts, comet splitting events, and spin up effects).

Q4.1d Which Bodies and Processes Produce the Smallest Particles and How Do the Particles Evolve?

The smallest bodies in the solar system are often grouped as “dust,” although the term is not well defined. For dynamical purposes, dust may be used as a stand-in for monolithic grains, allowing the definition to range from micron-sized shards to cm-sized pebbles, depending on the application. Across this size range, different nongravitational effects can affect their dynamics: relativistic Poynting-Robertson drag moves the smallest grains toward the Sun or, when created within a giant planet system, toward the giant planet in the system; larger grains are affected by YORP torques and Yarkovsky thermal drift forces. It is important to note that monolithic grains can spin very fast (much faster than an equivalently sized rubble pile), owing to their internal strength.

The existence and cataloged distribution of dust in the solar system has expanded greatly over time. They run the gamut from the identification of the Zodiacal dust cloud to the detection of fine dust on the surface of many asteroids and comets, to the discovery of ejected particles from (101955) Bennu’s surface by the OSIRIS-REx mission (e.g., Jenniskens 2015; Lauretta et al. 2019).

Dust originating within the solar system can be produced by many mechanisms, including cratering and catastrophic impacts, thermal disruption events produced by a body’s proximity to the Sun, electrostatic levitation, asteroidal rotational instability, and cometary outgassing. The subsequent orbital migration and accretion of these smallest grains, however, is largely unknown except for the simplest of circumstances. Further, to produce ejected dust grains at (101955) Bennu, fundamental questions remain on whether they are primarily owing to thermal processing, meteoroid impacts, or a synergetic combination of both processes (Lauretta et al. 2019).

The primary source of interplanetary dust particles (IDPs) in the inner solar system is thought to be from Jupiter-family comets that disrupt at relatively small perihelion distances (Nesvorný et al. 2008). (Jupiter-family comets are often defined as comets with short orbital periods, less than ~20 years, whose dynamics are dominated by Jupiter.) Smaller yet important IDP contributions come from nearly isotropic comets and asteroids (Nesvorný et al. 2008). Recent disruption or mass shedding events among small bodies, such as those associated with comet 2P/Encke or asteroid (3200) Phaethon, lead to meteor streams that can strike Earth, the Moon, and other bodies at specific times (Jenniskens 2015).

The meteoroid population produced by these sources is a major source of bombardment for spacecraft and celestial bodies. Dust particles can strike objects at high orbital speeds and are capable of breaking down rocks and producing regolith on airless bodies. On Mercury and the Moon, meteoroid impacts also create exospheres detectable by remote and/or in situ observations (e.g., Colaprete et al. 2015; Killen et al. 2015; Benna et al. 2019; Jauches et al. 2021). Micrometeoroid bombardment can also contribute to space weathering (see Q5.5). Understanding the full role of dust impacts in the evolution of airless planets and small body surfaces will require additional in situ observations and sample return missions.

In the outer solar system, many IDPs originate from collisions among TNOs, with active Centaurs, nearly isotropic comets, and the interstellar medium also making contributions (e.g., Poppe et al. 2019). For giant planet systems, one needs to also consider irregular satellites as a source of dust. For example, dust from Saturn’s distant irregular satellite Phoebe likely explains the extreme albedo differences seen on Iapetus. In fact, if the irregular satellite populations were much larger in the past, as suggested by modeling work (e.g., Nesvorný 2018), their debris may explain why dark carbonaceous chondrite-like material coats the surfaces of ancient terrains on worlds ranging from Callisto to the uranian moons (Bottke et al. 2013).

Taken together, dust can not only tell us much about the nature of the original planetesimals in the solar system but also about exoplanetary disks and giant molecular clouds evolving within our galaxy.

Strategic Research for Q4.1

• Determine how meteoroid bombardment can alter the surfaces and potentially produce exospheres on airless worlds by characterizing dust populations across the solar system and determining their impact effects through laboratory studies, observations, and numerical experiments.
• Determine the nature of impactors striking the most ancient regions of the Moon to constrain early bombardment populations by returning samples of soils/breccias from lunar farside regions where ancient materials have been recently excavated by impacts.
• Find evidence for the earliest terrestrial impactors by searching for and analyzing impact spherule beds in early Earth terrains that tell us about massive impacts from the Hadean, Archean, and Proterozoic eons.
• Constrain the early impact populations striking Mars by identifying the oldest basins and impact structures (including basins that have been potentially erased by other geologic processes) and determining their age.
• Determine the Kuiper belt size distribution for objects smaller than 30 meters, and by proxy constrain the production population striking the giant planet satellites, by obtaining high resolution images of large (>50 km) KBOs, where secondaries and sesquinaries (i.e., those craters formed on planetary satellites by the impact of debris ejected into planetary orbit from larger impacts) are not factors, and by expanded TNO surveys from ground and space-based telescopes.
• Determine the size-frequency distribution for comets smaller than a few tens of meters in diameter, thereby constraining the nature of the comet disruption law, by monitoring impact flashes/events on the giant planets from ground-based observations or orbiting spacecraft.
• Constrain the early dynamical history of the asteroid belt, the nature of impactor populations whose members were captured within existing small body reservoirs, and the nature of existing asteroid families by characterizing the sizes, orbits, and compositions of small body populations within stable reservoirs and on planet-crossing orbits with a combination of ground- and space-based observations.

• Benchmark the ages of asteroid families and the nature of family-forming events by observing asteroid family members in situ, counting craters on their surfaces, and comparing their model ages to dynamical evolution models of how the family members evolve.
• Determine how carbonaceous chondrite asteroids and comets disrupt as they approach the Sun by observing primitive asteroids or comets at low perihelion and tracking how they evolve.
• Determine the processes that create active asteroids and compare their relationship to comets by in situ observations of active or recently active asteroids.

Q4.2 HOW DID IMPACT BOMBARDMENT VARY WITH TIME AND LOCATION IN THE SOLAR SYSTEM?

The history of impacts on different bodies varies across the solar system and is dependent on impactor populations that evolved over time. The fingerprints of these different impactor populations are recorded in the cratering record of planetary surfaces, so interpreting these crater populations can tell us about small body populations that might no longer exist. Additionally, untangling the temporal evolution of impact bombardment on different bodies has scientific value beyond impact studies, because the accumulated crater populations on landforms is the only way of estimating their age without returning samples and analyzing them in laboratories on Earth.

Despite the usefulness of crater records, there are many potential sources of error—including observational errors and errors caused by geologic disruptions—that need to be better understood if impact crater populations are to be used for reliable age determinations. Most critically, the contribution of secondary craters and sesquinary craters—together with the processes that degrade, relax, or remove craters—needs to be understood to fully analyze planetary crater populations (e.g., Zahnle et al. 2003). Additional analysis is needed to understand these factors across the solar system.

For the Moon, a provisional sample-calibrated impactor rate for objects smaller than a few hundred meters has been established for 3.9 to 3.0 Ga, but the rate of impact bombardment outside this epoch and projectile size range remains uncertain (e.g., Wilhelms 1987; see Bottke and Norman 2017) (Figure 7-1). It is common practice to use models to extrapolate lunar surface ages to other planets and moons, although this practice has yet to be tested by direct sample analysis. It is evident from observations of many planetary bodies that the first 0.5–1 billion years of solar system history had an impact bombardment rate that was much higher than it is at present. However, the sources and forms of this higher early impact flux remain in dispute. More robust geochronological measurements on the Moon and other worlds (e.g., Mercury, Mars, and large main belt asteroids) are needed to fill this gap.

Q4.2a What Small Body Populations Dominated the Early Bombardment of Worlds in the Inner and Outer Solar Systems?

As discussed in Q4.1a, there are three major populations that dominated early bombardment of solar system worlds: comets, leftover planetesimals, and asteroids. Their relative importance depends on how the impact flux from those populations changed with time as well as the context of the worlds in question, namely when they formed and what happened to them when they were struck.

Starting with comets, their impacts dominate the bombardment of outer solar system worlds, with the magnitude of the early flux dependent on the timing of giant planet migration and the size of the primordial Kuiper belt (see Question 2, Chapter 5). With that said, many large impacts may have taken place before certain worlds could achieve a stable crust that could record an impact over geologic time. It is also possible that large impact basins can be removed from icy worlds with high heat flow or near-surface oceans via viscous relaxation of topography (e.g., Marchi et al. 2016). Taken together, it is possible that the largest observed basins on Mercury, the Moon, Mars, large asteroids like Vesta and Ceres, and giant planet satellites did not form until many tens to hundreds of millions of years after accretion.

Early impacts, produced by comets liberated by the depletion of the primordial Kuiper belt (see Question 2) and leftover planetesimals in the terrestrial planet region (see Q4.1), had the potential to scramble and disrupt

main belt asteroids still warm from being heated by the decay of the aluminum isotope ²⁶Al (Q1.2c) (Bottke et al. 2015). Discriminating these primordial impact events from billions of years of subsequent impacts produced by main belt asteroids is a challenge that has yet to be met.

The signatures of cometary impacts on the terrestrial planets is unclear, except perhaps from constraints provided by noble gas and volatile isotope delivery, which would tell us about the very largest impactors (see Question 3, Chapter 6). If cometary impact basins exist in the inner solar system, the most accessible place to study them may be the ancient farside of the Moon—which has the oldest, most heavily cratered terrains. For leftover planetesimals, it is expected that their flux undergoes a steep decline as the most unstable leftovers are dynamically removed from the system and as the leftovers themselves undergo collisional evolution (Bottke et al. 2007; Nesvorný et al. 2017; Bottke and Norman 2017; Morbidelli et al. 2018). Despite this, some leftovers may be able to strike worlds after hundreds of millions of years of evolution. This suggests some ancient craters identified on Mercury, the Moon, and Mars may come from such impacts. It is also possible that the nature of these projectiles may be discerned from trace materials found within meteorites, lunar/martian samples, and ancient terrestrial rocks.

A poorly characterized component of the inner solar system impact flux is the leftover planetesimal population scattered in from the giant planet zone (Walsh et al. 2011; Morbidelli et al. 2015; Raymond and Izidoro 2017). The size and orbital distributions of these carbonaceous chondrite-like bodies will depend on models of giant planet formation, migration, and evolution (see Questions 2 and 3; Chapters 5 and 6, respectively). While this leftover component may explain volatile delivery to the terrestrial planets, the best existing constraint on this population may come from the quantity of carbonaceous chondrite-like bodies ostensibly captured within the asteroid belt (see Question 3).

Eventually, as the other bombardment sources fade, main belt asteroids will begin to dominate the impact flux of the terrestrial planets (e.g., Nesvorný et al. 2017). This transition for the production of km-sized and smaller craters is thought to take place between ~3 and 3.5 Ga. Even then, however, there are indications that meaningful impact surges took place in this early era, with compelling evidence provided by studies of impact spherules beds on Earth (e.g., Bottke and Norman 2017).

Q4.2b Was There a Late Heavy Bombardment Approximately 4 Billion Years Ago, and If So What Caused It and What Was Its Magnitude?

Shock age estimates on Apollo samples based on the ⁴⁰Ar-³⁹Ar isotopic systems show clustering around 4.1 to 3.8 Ga (see Figure 7-1). This led to the so-called late heavy bombardment (LHB) hypothesis, which suggests that the lunar surface was heavily bombarded during this short time window (see Wilhelms 1987; Bottke and Norman 2017). To constrain whether the LHB occurred, it is crucial to understand the full impact record of the Moon, given that the Moon is the only planetary object that provides a strong connection between absolute and relative ages based on age dating of lunar rock samples and crater counting. The intensity over time of the Moon’s early impact record determines our understanding of the impact history of the solar system.

The LHB hypothesis has been supported by other isotopic measurements, including Rb-Sr, Sm-Nd, and U-Pb. If the LHB was a real uptick in the impact flux hundreds of millions of years after the end of planet formation, the late timing needs an explanation, because this is not expected from the declining remnants of planetesimals (Nesvorný et al. 2017; Morbidelli et al. 2018). This led, in part, to the formulation of the so-called Nice model. The Nice model originally proposed that the giant planets experienced an instability that led to the ejection of numerous comets and asteroids from their stable reservoirs near the proposed time interval (e.g., Gomes et al. 2005; Tsiganis et al. 2005; Nesvorný 2018; see Questions 2 and 3). While the timing of the instability is a free parameter in the model, and recent work favors an instability taking place shortly after the dissipation of the solar nebula (i.e., within a few million years to perhaps a few tens of millions of years; Nesvorný et al. 2018, 2021), a late instability could potentially explain the LHB, if it exists. Even so, there are substantial uncertainties in the magnitude, timing, and nature of the LHB, and a major challenge has been establishing reliable absolute age estimates for the large, early impact basins on the Moon and other bodies.

One of the arguments against the LHB is that the Imbrium basin could have distributed ejecta across much of the lunar nearside, and thus be dominating the observed impact record from the Apollo samples (which are all from the lunar nearside, potentially contaminated by Imbrium). Consequently, the apparent shock age clustering may represent mostly the Imbrium-forming impact, instead of multiple basin-forming events. Additionally, Boehnke and Harrison (2016) suggest that partial resetting of the ⁴⁰Ar–³⁹Ar system may have led to the observed clustering of ages in that system, which may not be traceable to a specific, narrow basin-forming period.

These challenges and uncertainties in the record of early lunar impact basins have implications across the inner solar system because the lunar crater chronology is used as a basis for understanding the absolute timing of events elsewhere. There are LHB-era constraints from other terrestrial bodies besides the Moon, but their interpretation is challenging (see Bottke and Norman 2017). For example, interpreting Earth’s surface condition during the LHB is extremely limited because there are no preserved crustal rocks older than 4.03 billion years to date. The only accessible materials are detrital zircons, which are extremely resilient minerals that formed as long ago as 4.4 Ga. Whether these zircons tell us anything about Earth’s ancient impact record, however, has been actively debated. The crater size distributions on Mars and the ⁴⁰Ar–³⁹Ar shock degassing ages of meteorites from asteroids provide further constraints, but whether they record the LHB remains unclear. New measurements of the ages of ancient basins on the Moon (particularly from the lunar farside, far from the contamination by Imbrium) and other worlds would be valuable to test the LHB hypothesis. Such measurements could be achieved either with new sample return missions or in situ geochronology.

Q4.2c Can Absolute Chronologies Be Derived for the Timing of Events Across the Solar System?

Determining the absolute timing of when events occurred on planets is important for inferring planets’ geologic and geophysical histories, as well as the rate of planetary processes. Presently, we have a range of certainty about the timing of events in different settings, because we assess the age of materials and landforms using techniques with widely varying fidelity. Our understanding of planetary histories is often limited by uncertainty in the ages of major events or landforms. Examples include the unknown ages of large ancient impact basins like the South Pole–Aitken (SPA) basin on the Moon and Hellas basin on Mars, giant impact scars on large icy bodies, as well as the unknown average age of Venus’s crust.

The cornerstone of geochronology is radiogenic dating of planetary materials in terrestrial laboratories. For lunar samples, ages derived using radiogenic techniques have been tied directly to observations of the crater populations on surfaces to create a model for the absolute cratering rate. With such a crater chronology model in hand, the ages of events and rates of processes can be assessed with crater statistics (i.e., how many craters are found that post-date a landform or geologic unit). For the Moon, the chronology model is arguably well-constrained from ~3.9 to 3 Ga, but is uncertain for impact basins older than Imbrium, a critical knowledge gap for understanding its early history (e.g., Q4.2b) (e.g., Wilhelms 1987; Stöffler and Ryder 2001). For the period younger than 3 Ga, the absolute chronology of lunar surfaces is also less well-established because few geologic units from that period have been dated with samples (e.g., Wilhelms 1987; Stöffler and Ryder 2001).

On planetary bodies besides the Moon, a direct connection between the radiogenic ages of samples and the observed crater population has yet to be established (Zahnle et al. 2003). There have been two primary strategies employed to assess absolute chronology in the absence of this absolute age calibration.

The first is to extrapolate the lunar absolute calibration to other planetary surfaces using models. At least in the inner solar system, the impactor population that affects planets is thought to be a common one, so the observed lunar impact flux can be corrected to determine an absolute chronology by accounting for differences in impact velocity, gravity, and target properties like porosity. The reliability of this extrapolation of the lunar record remains uncertain, however, and needs to be tested.

The second strategy requires determining the cratering rates from observed populations based on impactor dynamics and the physics of impact cratering. Where the lunar record is not expected to be representative, particularly in the outer solar system, only this second strategy can be employed to establish an absolute cratering chronology.

Substantial progress on these issues would be aided by directly measuring the absolute ages of additional samples from the Moon and Mars, either using returned materials or in situ dating methods. A requirement for

this type of investigation is to obtain material that can be placed in geologic context; ages derived from lunar or martian meteorites are useful but cannot be fully exploited because they have an unknown provenance. Additional pathways to enhance current knowledge would be to obtain robust techniques for measuring planetary crater populations accurately, improving our understanding of how small body populations evolve and the impact cratering process itself, and establishing independent constraints on outer solar system chronologies.

Q4.2d When Does Recorded History Start and When Did Major Geological and Geophysical Events Occur on Different Worlds?

The dating of major events on planetary bodies is fundamentally important, whether constraining the initial stability of an ancient surface or finding the timing and duration of an internal activity such as volcanism. Ages of these events are usually estimated by the comparison of the spatial density of craters and estimated projectile fluxes, calibrated where possible by lunar sample ages and various theoretical constraints of how the impactor rates vary with time and location in the solar system.

For the Moon, determination of the oldest preserved terrains or events is possible if the right samples can be obtained. For example, SPA basin is likely the oldest distinct impact basin from a stratigraphic standpoint (Bottke and Norman 2017). Samples that date SPA would thus answer whether SPA formed shortly after the Moon formed, implying much of the Moon’s early history (and early solar system bombardment history) can be constrained, or that SPA formed hundreds of millions of years later, suggesting that much of the Moon’s early history has been erased. Reliable age determinations of lunar impact features in poorly sampled eras would substantially improve our calibration of the impact flux and thus ages of major events across the Moon and solar system.

Model ages of events on Mercury, Venus, and Mars are presently dependent on extrapolation of the lunar (and terrestrial) cratering record, which limits our ability to probe their early and more recent histories. This approach depends on theoretical assumptions and is not yet calibrated by independent constraints (e.g., lunar and Mars meteorite samples have yet to be conclusively linked to a given terrain). Accordingly, like SPA, the oldest basins on Mercury and Mars have indeterminate ages; they could be nearly as old as the planets themselves or hundreds of millions of years younger. For Venus, there is considerable debate about how to interpret the distribution of sizable craters across its surface, which are indistinguishable from random. They either tell a story of catastrophic resurfacing within the past billion years or regional volcanism that is continually erasing swaths of craters (Smrekar et al. 2018). Near-term progress on the chronology of these worlds will likely come from strategically acquired samples from the Moon and Mars, in situ dating of terrains on the Moon and Mars, and/or confirmation that certain dated meteorites come from particular source terrains.

For a given surface, impacts increase the crater population but eventually start erasing existing ones. When the rate of crater production eventually balances with that of crater erasure, the crater population apparently remains in a steady state, referred to as crater saturation equilibrium (e.g., Melosh 1989). This process can make it difficult or even impossible to determine accurate surface ages. For the Moon, researchers debate whether the global crater population up to 100 km in diameter on the most ancient surfaces is in crater saturation equilibrium, while it is in general agreed that craters less than ~200 m in diameter may be at crater saturation equilibrium at local scales on the lunar maria. To quantify when, where, and how crater saturation equilibrium occurs, cross-disciplinary studies may be needed (i.e., a combination of modeling, experimental, and geomorphological approaches).

Determining the ages of the oldest terrains and major events in the outer solar system is even more problematic (e.g., Zahnle et al. 2003; Dones et al. 2009). Impact rates on icy satellites and within the TNO region remain uncalibrated by sample-derived ages. Independent constraints on the impact flux over time mainly come from the theoretical modeling results linked to craters on outer solar system worlds and present-day observations of various small body populations. Even there, crater counting and telescopic observations are limited at smaller sizes by data resolution and completeness.

Interpretations of the impact records for outer solar system worlds are full of challenges. There is a growing recognition that many icy satellites may not retain a full record of early impact basins. As demonstrated by Ceres, worlds with subsurface oceans or zones of weakness may allow viscous relaxation to erase large basins (e.g., Marchi et al. 2016). Impact disruption and/or heating events may also contribute to basin erasure. A second

problem for planetary satellites stems from how to interpret the contributions from impactors that orbit the planet (planetocentric), rather than the Sun. Planetocentric impactors can arise from disruption events, basin ejecta, or other processes (e.g., Zahnle et al. 2003; Dones et al. 2009). These populations may dominate small crater production on many icy satellites, yet they are both poorly understood and may be different in each planetary system.

Ultimately, to interpret the earliest history of outer solar system worlds and reduce uncertainties, we need improved models of small body evolution, crater production, crater retention, and new observations of surfaces that have only been poorly explored to date.

Q4.2e What Is the Current Impact Flux on Planetary Worlds, and Has the Flux Changed Substantially over the Past Several Billion Years?

The current impact flux of large bodies across the solar system is largely determined by dynamical models constrained by the direct observations of small bodies. Ground- and space-based surveys have now discovered most km-size asteroids on planet-crossing orbits within the inner solar system and have made considerable progress on understanding comet populations in the outer solar system as well.

For inner solar system worlds, modeling results predict that most impacts come from asteroids; comets are only a minor contributor (Granvik et al. 2018). When asteroids smaller than a few tens of kilometers are formed, they begin to drift inward toward and outward away from the Sun by Yarkovsky/YORP thermal forces, with direction controlled by the body’s obliquity (Q4.1b; Vokrouhlický et al. 2015). The process is slow enough that collisional evolution and mass shedding via YORP spin-up processes often disrupt these bodies, although the surviving fragments will continue to migrate via Yarkovsky/YORP forces. Eventually, these collisional cascades will deliver bodies to a main belt escape route, such as a powerful resonance with the giant planets, where they can be driven onto orbits from where they can strike the terrestrial planets. This process keeps the planet-crossing asteroid population fairly steady over time, and it helps explain why the main belt, planet-crossing asteroids, and terrestrial crater populations have similar size frequency distributions (e.g., Bottke et al. 2015).

Large asteroid breakup events in the asteroid belt near powerful resonances can change the terrestrial planet impact flux, but only if the fragments can get out of the asteroid belt before they are decimated by collisional evolution. One example of this is the destruction of the L-chondrite parent asteroid that sent numerous small particles to Earth ~470 Ma (e.g., Terfelt and Schmitz 2021). Other breakups produce so many fragments that they can substantially increase the delivery rate of asteroids to the terrestrial planets for many hundreds of million years. Estimates of lunar crater ages and terrestrial impact spherule ages suggest that several impact surges may have taken place over the past few billions of years (e.g., Terada et al. 2020).

Direct detection of fresh impacts into Earth’s atmosphere or onto other nearby worlds is also now possible. For example, repeat imaging has enabled discovery of new impact craters on the Moon and Mars, allowing estimates of the present-day impact flux. Telescopic monitoring for impact flashes on the lunar near-side has also enabled detection of craters in the process of formation. Ongoing telescopic and/or space-based detection of impact flashes during the next decade would be useful to refine the impact flux, better understand the impact process, and would be potentially synergistic with other geophysical observations (e.g., seismology).

The heliocentric impact flux on the giant planets and their satellites is largely set by the escape rate of comets from the TNO region, with a small fraction reaching giant planet-crossing orbits over time. While they keep the ecliptic comet populations in a short-term steady state, this population has been steadily decreasing over billions of years (Nesvorný 2018).

A critical issue today is how to use these model results to estimate absolute surface ages on icy worlds when constraints are limited (Zahnle et al. 2003). Existing model benchmarks include the observed comet populations and the observed present-day impact flux on the giant planets. Unfortunately, at present, these constraints yield results that differ by a factor of ~10 (e.g., Zahnle et al. 2003; Dones et al. 2009; Nesvorný et al. 2019). The reasons are unclear, but it could be because cometary activity makes it difficult to estimate comet nuclei sizes. Two solutions for the next decade would be to find and characterize additional icy bodies far from the Sun, where activity is limited, and to monitor bolide impacts into giant planet atmospheres or any other large witness plates (e.g., Saturn’s rings).

With that said, though, a good impact flux model will still leave us with challenges in estimating absolute surface ages. For example, as discussed below, comets striking giant planet satellites often produce enormous ejecta showers, which in turn produce numerous secondary and sesquinary craters. The contribution of these types of craters to small crater populations on a wide range of terrains is uncertain, partly because impact events produce variable ejecta size distributions but also because our understanding of small comet populations is limited. New observational data and laboratory/modeling work are needed to determine more reliable surface ages in the outer system from the spatial densities of small craters.

Strategic Research for Q4.2

• Determine the age of the SPA basin to determine the beginning of recorded bombardment on the ancient lunar farside by dating samples formed from or excavated by the SPA basin forming event.
• Determine a precise absolute chronology for lunar impactors that can be applied to other worlds by measuring radiometric ages for terrains likely to be much older than 3.9 Ga and younger than 3 Ga, counting superposed small craters on D >10 km craters, and calculating model ages for those craters.
• Determine the present-day lunar impact rate and better understand the nature of impact mechanics by coupling seismic monitoring to lunar observations of impact flashes and fresh impact craters.
• Determine impactor sizes, impactor compositions, and impact ages for all terrestrial impact structures by identifying and characterizing past impact structures, analyzing samples that contain telltale traces of the projectile, and dating material affected by the impact event.
• Determine the absolute age of a martian basin or well-defined surface and use it to calibrate the timing of early martian bombardment by dating a surface whose age can be determined by in situ methods or returned samples.
• Use crater counts on the ancient surfaces of Kuiper belt objects to constrain the populations of the primordial Kuiper belt and determine the start of the giant planet instability (and post-nebula giant planet migration) by mapping craters on larger (>100 km) KBO and using them to constrain early bombardment models.
• Pursue crater counting on icy bodies of the outer solar system to characterize and compare how projectile populations vary radially from the Sun (especially focused on gaps in the observational record on Europa, Ganymede, in the uranian system, and on additional >100 km trans-neptunian objects) by observing icy satellite surfaces that have yet to be imaged and at higher resolutions than existing images.
• Determine the nature of early bombardment and the primordial asteroid belt by observing large intact asteroids that may still have some record of impacts/craters from early bombardment phases, counting craters, and modeling their crater size distributions.
• Constrain the integrated bombardment history of the asteroid belt over the past few billion years by performing in situ dating of the largest basins, such as those for which we have samples (e.g., Vesta’s Rheasilvia impact basin) whose model age can also be derived from superposed crater records.

Q4.3 HOW DID COLLISIONS AFFECT THE GEOLOGICAL, GEOPHYSICAL, AND GEOCHEMICAL EVOLUTION AND PROPERTIES OF PLANETARY BODIES?

Collisional and accretional events were an integral aspect of the formation of planetary bodies and exercised a controlling influence on their physical and chemical states throughout their evolution (Melosh 1989). In the context of modern-day Earth, collisions and impacts are often viewed as destructive events. Yet collisions have had a profoundly constructive role in the formation and evolution of planetary bodies, beginning with their bulk composition and extending through to the timing and duration of differentiation and exogenous delivery of chemical ingredients essential to life (e.g., Osinski et al. 2020). The scars of collisional events—impact craters—litter the surfaces of planetary bodies across the solar system and chronicle the role collisional events had in shaping large-scale geology, volcanism and magmatism, atmospheres, and core dynamos and magnetic fields.

As we improve our understanding of the formation and evolution of planetary bodies, we continue to discover that collisions were often—although not always—responsible for major planetary-scale events. One of the most poignant examples is the formation of the Earth–Moon system (see Question 3, Chapter 6), canonically considered to be the result of a large-scale collision event early in the solar system history. Additionally, several planetary bodies—such as the Moon, Mars, and Pluto—possess ancient hemispheric asymmetries often hypothesized to have originated with large-scale impact events (e.g., Andrews-Hanna et al. 2008). These hemispheric asymmetries profoundly shape the evolution of these planetary bodies and often plausibly give rise to subsequent hemispheric asymmetries in tectonic and magmatic activity, as is evident for the Moon and Mars. Additionally, collisions can alter the orbital parameters and rotation state of planetary bodies, leave lasting consequences on the surfaces and near-surfaces of planetary bodies (Question 5, Chapter 8), are capable of generating circumplanetary systems (Question 8, Chapter 11), and may even potentially affect core dynamo activity within a planetary body (Question 5).

Q4.3a How Did the Earliest and/or Largest Impact Events Influence the Physical Evolution of Solar System Worlds?

The initial conditions of planets and small bodies are determined by accretional processes, while their subsequent evolution is often shaped by impacts. Among the most famous examples are Earth’s Moon (see Question 3) and Pluto’s moon Charon (see Question 2, Chapter 5), thought to have formed by large impacts. In the case of the Moon, this event would have provided enough energy to melt a large portion and form a deep magma ocean (see Question 3). Subsequently, the Moon crystallized, allowing less dense minerals (anorthosites) to separate from the residual melt and float to the surface and forming the lunar highlands.

The surface topography of many worlds has also been shaped by numerous impacts at the largest scales, but uncertainties remain. For example, the farside of the Moon has a thicker crust (50–60 km) than the nearside (30–40 km; Wieczorek et al. 2013); on Mars, the crust on the northern hemisphere, where the 10,000 km Borealis basin is located, is much thinner than that on the southern hemisphere (32 versus 58 km, respectively; Andrews-Hanna et al. 2008; Goossens et al. 2017) (see Question 3); and Enceladus possesses a thinner crust in the south pole, associated with the tectonic fractures that source its enormous plume (Hemingway et al. 2018) (see Question 8, Chapter 11). The origin of these dichotomies is still debated and might be explained by impact-induced mantle convection, a large impact transporting the crustal material from one hemisphere to another, or alternative nonimpact hypotheses. One way to constrain this issue may be to explore the martian moons Phobos and Deimos that were plausibly formed by this impact event (see Question 3).

Mercury may represent another world that was substantially affected by a giant impact. Mercury’s high density may be explained by a large impact that removed its original crust and much of its mantle (see Question 3). Others suggest that Mercury’s building blocks were simply iron-rich. Given that high density planets have been found in extrasolar systems, it is now even more important to determine the origin of Mercury’s distinctively high density.

In the asteroid belt and TNO populations, many bodies have been shattered, disrupted, and scrambled by large impacts (Bottke et al. 2015; Nesvorný et al. 2018). In the process, interior materials normally hidden away at depth are now potentially accessible, some on small bodies that may eventually approach Earth. Accordingly, by interpreting the jigsaw puzzles created by impacts, and placing their samples into geologic context, we can probe the origin and evolution of planetesimals to a much greater extent than would be possible with intact bodies.

Large impact events may also have disrupted mid-size icy satellites in the saturnian and uranian systems (e.g., Movshovitz et al. 2015). Given that basins on Iapetus can exceed 500 km diameter, and comparable events would be expected on the inner satellites, it seems plausible that some events completely shattered protosatellites like Mimas or Enceladus, which could reassemble into the current satellites. Whether such events have occurred, or did so multiple times, is not known but could be revealed by detailed mapping of internal mass distributions or by other means. The reader is referred to Question 8 for additional discussion about circumplanetary systems.

Q4.3b How Do Impacts Affect Surface and Near-Surface Properties of Solar System Worlds?

Impacts modify surface morphologies on planetary bodies (see Q5.5b) (Melosh 1989). These modification processes mainly result from crater excavation, ejecta blanketing, and topographic diffusion. They occur on many scales ranging from a microscale, including small boulder erosion, up to surface modification on heavily bombarded planets and moons. While all of these surface modification processes directly connect the history of each planetary body, their stories are incomplete.

Small bodies have low surface gravitational acceleration, so their surface conditions are affected by impacts, dynamic processes, or both. As one example, outgassing and mass ejection events on comet 67P/Churyumov-Gerasimenko have led to the collapse of cliffs and surface changes from the reaccumulation of ejected dust (Pajola et al. 2017). A second example is impact-induced seismic shaking, likely responsible for substantial depletions in small crater populations on asteroids tens of km or smaller across (e.g., Marchi et al. 2015). A third example comes from the asteroids (101955) Bennu and (162173) Ryugu, both of which exhibit surface material flows that follow trends expected from YORP-driven spin-up (e.g., Jawin et al. 2020). A fourth example is Vesta’s Rheasilvia basin, whose unique central peak and escarpment were created by a combination of the impact itself and the asteroid’s spin. More work is required to understand how impact- and dynamical-related processes trigger mass wasting and crater erasure events on small bodies.

Impact-induced melt sheets play essential roles in altering the target structure, age determination, and inducing hydrothermal activities, and can affect craters formed on top of them. Melt sheets are commonly found in impact basins (Melosh 1989). If a deep melt sheet cools slowly, it can differentiate and form a compositionally stratified structure, altering the original target structure. This behavior is observed in the Sudbury impact basin, and it likely also occurred within the SPA basin on the Moon. Such melt sheets may also affect the later formation of superposed craters, making it more challenging to use crater spatial densities to estimate the ages of their source craters. With that said, melt sheets are ideal locations for sample return missions because the rocks provide accurate and unique impact ages. Melt sheets can also offer a long-term heat source, which contributes to forming hydrothermal systems if subsurface volatiles are present (Q3.4d).

Last, there are cases where planetary atmospheres are sufficiently dense to prevent asteroids and comets from reaching the surface prior to disruption (e.g., Venus, Earth, and Titan). For the case of Venus, radar-dark splotches observed by Magellan could be byproducts of bolide airbursts, with the shock wave from the blast producing fine-grained material on the surface. Modestly larger projectiles that can penetrate more deeply in the atmosphere may produce a shotgun blast of small craters. Insights into how small bolides break apart in atmospheric disruption events can also be gleaned from observations of comet impacts into giant planet atmospheres (e.g., fragments of comet Shoemaker-Levy 9 hitting Jupiter in 1994; small comet impacts into Jupiter whose effects can be observed by both amateur and professional astronomers; Huseo et al. 2018). Ultimately, the effects of bolide disruptions in all atmospheres require further research, in that they can tell us about the strength/nature of asteroids/comets and how atmospheres/surfaces react to energetic blast events.

Q4.3c How Do Impacts Affect the Deep Interior of Solar System Worlds, and Can They Expose Interior Compositions?

Large basin-forming impacts have taken place on many planetary bodies in the solar system (Figure 7-2) (see also Q4.3a). Our direct knowledge of their effects on the interiors of worlds is limited, but models provide several intriguing results. For example, models suggest that major impacts can generate substantial thermal anomalies in the interiors of certain planetary bodies. This can potentially reorganize convection patterns and may even induce the formation of plumes that stretch from the core-mantle boundary to the surface (O’Neill et al. 2020). Large thermal anomalies may even reduce the efficacy of core dynamos (e.g., Roberts and Arkani-Hamed 2012, 2017). For example, it has been argued that the large basin forming impact on Mars heated the outer part of the core and made it thermally stratified, temporarily shutting off martian core convection and its dynamo (see also Question 5).

Heating produced by large impact events has also been proposed to explain the putative differences in the interior structures of Ganymede and Callisto (see Questions 5 and 8; Chapters 8 and 11, respectively). Ganymede, being closer to Jupiter, should have been hit by more impactors (and with higher impact speeds) than Callisto owing to the gravitational focusing of Jupiter. This may have triggered planetary differentiation in Ganymede while leaving Callisto in a partially differentiated state (Barr and Canup 2010).

Additionally, impact events are capable of excavating material from substantial depths (e.g., Melosh et al. 2017). For example, SPA may have excavated the early lunar mantle, which would provide an unprecedented view into the interior structure and thermochemical evolution of the Moon. More extreme cases of excavation have been suggested for worlds like Mercury and asteroid (16) Psyche, proposed to be remnants of much larger planetary bodies that were stripped in hit-and-run collisions (Asphaug and Reufer 2014). Understanding the viability of such scenarios would provide insights into the magnitude and type of collisional events that occurred early in solar system history.

Q4.3d How Frequently Are Impact-Driven Hydrothermal Systems Formed and Which Processes and Target Properties Control Their Evolution?

Impacts have many ways of changing a planetary surface. Not only do they create craters and ejecta, but they also fragment the surface, changing its porosity, and produce heat that can mobilize volatiles (like water and CO₂). This residual impact heat can lead to the formation of hydrothermal systems beneath impact craters. Through permeable channels, such systems are thought to be possible abodes for life for Earth, Mars, and possibly other worlds such as Ceres. Thus, impact-driven hydrothermal systems may strongly influence geological, geochemical, and biological evolution, especially on those worlds whose internal heat flows and volcanic activity may be modest (see also Q5.3d and Question 10, Chapter 13).

Hydrothermal systems have been observed in ~80 craters out of 180 craters on Earth (see Osinski et al. 2020), where the crater size ranges from 1.8 km to 250 km. They can exist anywhere in the crater, but most commonly are found in the central uplift, crater-fill, ejecta, and rim, and have been identified in drilled core samples from the Chicxulub impact crater (Kring et al. 2021).

Impact heating can be limited when projectile velocities are less than 10 km/s (e.g., Marchi et al. 2013), potentially explaining why little impact melt is seen on Vesta or within asteroidal meteorites, because asteroids in the main belt tend to have impact speeds near 5 km/s. For higher impact velocities, common on Earth because of its 11.2 km/s escape velocity, heating correlates with kinetic energy, and this means large craters on Earth can have long-lived hydrothermal systems. For example, it could take ~1 million years for a Sudbury-sized impact crater to cool.

In addition to endogenic hydrothermal activity, Sudbury-like hydrothermal systems may have been ubiquitous on many planetary surfaces hosting water. For example, given the likely existence of ice in the martian subsurface (e.g., Bramson et al. 2017; Dundas et al. 2018), it is expected that large martian craters hosted numerous hydrothermal systems. Indeed, associations between hydrated rock phases and martian craters have already been identified using spectral data. Another example may be the bright spots on Ceres, which are likely salt and carbonate deposits extruded through channels formed by an impact (Castillo-Rogez et al. 2019). Future sample return missions may provide key constraints on this issue.

The frequency of impacts large enough to sustain hydrothermal processes and environments has potential importance for the origin of life on several water-rich worlds. Large impact events were common in the first billion years, decreasing afterward. During these early times, it is likely that links can be found between hydrothermal processes and the alteration of planetary interiors, heating of planetary crusts, delivery of volatiles, formation of atmospheres, and other processes. But as seen in Q4.2 and Q4.3, there remain significant uncertainties in the flux of impactors during the early epochs of solar system evolution. To understand the role of larger impacts in the development of viable biospheres, we need to understand how early bombardment affected the history of the solar system. Determining how impact parameters and surface geology controlled such environments necessitates a deeper understanding of the mineralogy and chemistry (Figure 7-3) of such deposits on different planets.

Q4.3e What Exogenic Volatile and Nonvolatile Materials Are Delivered to Planetary Bodies?

Impactors deliver materials from one world to another, although the extent to which exogenic materials survive an impact event is not well known. Hypervelocity impacts are thought to result in the vaporization of most of the impactor, but this may vary depending on the impactor’s velocity, composition, structure, the nature of the target material, and whether the target body has an atmosphere (see Q4.4). For example, bolides with modest mechanical strengths often manage to land on Earth and Mars in the form of meteorites. For larger asteroid or comet strikes on Earth, however, the recovered fragments tend to be tiny.

Fragments of exogenic materials have been observed in lunar samples. The most common type is carbonaceous chondrite-like materials delivered by meteoroids. This steady rain of tiny impactors explains why the lunar regolith is composed of ~2 percent exogenic materials (e.g., Heiken et al. 1991). An investigation of early Earth samples retained as meteorites on the Moon, such as the putative terrestrial meteorite fragment in an Apollo 14 breccia (Bellucci et al. 2019), may provide unique insights into the evolution of our own planet during a time for which scant evidence remains. Evidence for larger-scale impactor debris is also found on the Moon; examples would include the traces of highly siderophile elements found within returned samples (see Question 3, Chapter 6).

Impact velocities are lower among main belt asteroids than on the terrestrial planets, such that projectiles are more likely to survive as small fragments during cratering and catastrophic disruption events. This means the bombardment of a large main belt asteroid by other asteroids will lead to increasing concentrations of exogenic materials near the surface of that body. The mixing of these materials in the regolith, and the subsequent formation of breccias, could explain why exogenic clasts are found in many kinds of meteorites. More broadly, much of the northern hemisphere of Vesta is littered with exogenic carbonaceous material (e.g., Marchi et al. 2015). Apparently impact heating was low enough to preserve much of the volatile content of this debris.

The disruption of a main belt parent body with exogenic materials will produce smaller asteroids with varying concentrations of foreign fragments. The exogenous materials found on the surfaces of Bennu and Ryugu (DellaGiustina et al. 2020; Tatsumi et al. 2021), as well as the plethora of meteorite types associated with the progenitor of the Almahata Sitta bolide (e.g., Goodrich et al. 2015), are telltale signs of impact mixing among main belt asteroids. Similar mixing is likely in the outer solar system, modulated by the lower impact rates.

Comets, primitive asteroids, or meteoroids have also delivered volatiles and organics to the inner solar system over many billions of years (see Question 5, Chapter 8). It is possible that at least some ices preserved at cryogenic temperatures within the permanently shadowed regions near the poles of the Moon and Mercury are from these sources, although volcanic outgassing and/or solar wind implantation are potential competitors. The composition of these ices could provide us with important clues to deduce their origin. The polar volatile inventories of the Moon are also much smaller than Mercury, and it is not known whether this is owing to differences in the volatile sources and stability, the timing of volatile delivery, or local processing.

Strategic Research for Q4.3

• Constrain the origin of Mercury’s high density by obtaining in situ compositional information from the surface.
• Determine the composition and depth of the materials, possibly lunar mantle, excavated by the SPA basin formation by returning samples from near or within the basin with the characteristics of the deep interior.
• Determine the origin of polar volatiles by obtaining and analyzing the properties of ices found within the permanently shadowed craters located near the lunar and hermean poles.
• Determine the nature and global distribution of hydrothermal deposits in large martian or cerean craters that may have been habitable zones by mapping at high resolution the mineralogy, composition, and distribution of these deposits or returning samples.
• Identify physical interactions between impacts and Venus’s atmosphere by conducting remote sensing observations of fireball events and by characterizing unique chemical anomalies in the atmosphere owing to such events.

• Characterize uplifted deeper icy crustal materials and projectile contaminants on icy bodies by obtaining high-resolution spectroscopic identification of mineralogy, crystallinity, and chemistry of impact crater floors, peaks, and ejecta.
• Determine the distribution of exogenic materials in comets to identify impactor material conditions by performing high-resolution spectroscopic and imaging observations, and by identifying exogenic materials in sampled materials.

Q4.4 HOW DO THE PHYSICS AND MECHANICS OF IMPACTS PRODUCE DISRUPTION OF AND CRATERING ON PLANETARY BODIES?

Impact events have been ubiquitous across the solar system. Resulting craters are strongly controlled by target and projectile properties as well as the physical nature of the impact. Target body disruption events, and possible target reaccumulation, are also controlled by these factors but introduce dynamical processes as well. While investigations have extensively explored the physics and mechanics of impacts, there are still numerous questions to be resolved.

Because projectile and target properties can be strongly altered by their formation and subsequent evolution (e.g., Wiggins et al. 2019), resulting impact signatures can be unique and can differ from planet to planet (Zahnle et al. 2003; Robbins et al. 2018) and even from place to place on one planet (e.g., van der Bogert et al. 2017). Impact processes in the inner solar system, where most projectile and target types are rocks, may differ significantly from those in the outer solar system, where most objects are ice-rich bodies. Material and impact variations cause different thermal, chemical, and mechanical processes, including the generation of ejecta. How ejected debris contributes to the reaccumulation of new bodies and ejecta deposition on planetary surfaces is an outstanding question in impact cratering. While high-speed ejecta escapes immediately, low-speed ejecta returns to the target surface after crater formation or is reaccumulated after a catastrophic disruption, depending on the target size. Collectively, impacts are a rich topic for scientific investigations.

Q4.4a How Does the Impact Process Vary with Projectile/Target Body Properties and Impact Parameters?

Understanding how impact craters form or disruption events occur is fundamental to interpreting the observed crater and small body record, whether it is modeling regolith generation by small impacts, converting crater sizes to projectile sizes, modeling excavation of a world’s mantle, or quantifying how an asteroid family formed. Projectile and target body properties, including material strength, porosity, and projectile sizes, speeds, rotations, and orientations, are fundamental parameters controlling the resulting impact craters or disruption events (Melosh 1989). Very low kinetic energy impacts cause particles to bounce against each other or stick together without disruption (Brisset et al. 2018). Higher-energy impacts induce plastic flows, generating craters and ejecta. Impacts catastrophically disrupting both projectiles and targets generate a multitude of new objects (for planet-forming impacts, see Q4.3a) or debris disks around target worlds. The existence of an ocean and an atmosphere can also change impact processes. These widely ranging properties affect the outcomes of impact events (also see Q5.5b), but the physics of this process remain incompletely understood.

Uniquely determining projectile properties and velocities from impact site constraints is challenging, but in some cases, it is possible to identify remnant traces of the projectile. It is easier to find such traces when the impact kinetic energy is relatively low. This allows the projectile materials to be more easily mixed with target materials. An example would be the carbonaceous chondrite-like material observed on Vesta; its context suggests that it was delivered by a large carbonaceous chondrite-like projectile.

Characterizing how target properties control impacts is another key issue. Thermal, chemical, and mechanical conditions can change the amount of melt created and how the material flows (Grieve and Cintala 1992). Porosity can control the compaction of target materials. Mechanical strength and structure can control fragmentation by modifying how shock and rarefaction waves propagate through the target. Overall, the wide range of properties in planets and small bodies helps explain the variations in impact processes observed from world to world across the solar system.

Last, there is still limited knowledge about how the entire impact process plays out to its end. Impacts consist of multiple physical processes (i.e., contact and compression, excavation, and modification, as well as ejecta formation and deposition, and subsequent relaxation). Because each component has a different timescale (ranging from milliseconds to millions of years, depending on the process in question and the size of the impact), investigations of short-term processes do not characterize all cratering mechanics, and state-of-the-art numerical models still have limited capabilities to integrate all physical effects. Alternatively, experimental approaches can yield useful insights, but by necessity they focus on small-scale impacts, and uncertain scaling relationships are needed to compare them to larger impacts. Technological innovations for all these approaches are urgently needed. Furthermore, collaborations in geological studies and numerical/experimental approaches are also recommended to further constrain full-timescale impact processes.

Q4.4b What Materials Ejected from Impact Craters Are Deposited on Planetary Surfaces?

Impact events excavate planetary surfaces and eject materials from depth away from the impact point. They also redistribute target body materials across planetary bodies in the form of particulate and impact melt ejecta deposits, secondary craters, rays, and sesquinaries. Materials pulverized in this process are ejected ballistically as a “curtain” that moves away from the impact point and often results in the emplacement of an ejecta deposit around the crater. The complex interactions among ejected fragments while in motion are not fully understood. The radial extent and thickness of ejecta deposits vary for different planetary bodies and are influenced by the impact parameters (e.g., impact angle, target composition, and target gravity), with implications for the composition and age determinations of lunar samples.

Impact melt deposits are often formed during an impact event and occur both exterior and interior to the final crater. Different impact conditions, such as impact velocity, however, can affect the extent, distribution, and volume of impact melt generated. Given the desire to date impact craters from recrystallized impact melt by in situ methods or sample return missions, this topic needs further study.

Secondary craters form when large ejecta fragments strike the target surface and produce craters. Some ejecta can also escape into deep space before returning or hitting a different body, forming craters known as sesquinaries. Both secondary and sesquinary craters sometimes look like primary craters. This allows them to inflate the crater populations formed by primary impactors, and therefore can complicate the determination of surface ages via the spatial density of craters. Significant issues remain on this difficult problem, but progress can be made by better understanding the morphologies, populations, and spatial distributions of craters formed by ejecta (e.g., Bierhaus et al. 2018), and by improvements in modeling and experimental techniques.

The redeposition and orbital distribution of fragments and ejecta becomes more complex as a body’s size decreases, with an increasing portion of an ejecta field escaping or entering into a long-lived orbit. For some impacts, ejecta can become distributed globally across the body and can cause measurable geophysical effects owing to angular momentum transfer. The interaction between ejecta and a small body’s gravitational field can become strongly coupled, and through careful observation can provide a natural opportunity for remote determination of small body gravity fields (Chesley et al. 2020). The efficiency with which impact ejecta is lost is also a key question related to the efficiency of kinetic impactors proposed for planetary defense.

Strategic Research for Q4.4

• Determine how impact physics and mechanics changes at different impact sites by mapping those sites with high-resolution remote sensing images, identifying target material compositions using in situ and/or remote methods, and applying the information as constraints for numerical impact simulations.
• Determine the formation of the SPA basin’s asymmetric structure by characterizing the chemical compositions and geologic structures (e.g., shock fragmentation, etc.) of the surface and internal materials returned near or within SPA and by constraining the internal structure beneath SPA.
• Determine how impacts affect oceans and ices on Mars by characterizing martian surface conditions (i.e., geologic and chemical compositions, and the existence of water or ice) through time and then by simulating impacts into such target materials.

• Map structural deformation and characterize impact mechanics in icy target bodies by performing high-resolution imaging of impact crater morphology, with example high-value targets, including the uranian satellites, Europa, Ganymede, and trans-neptunian objects.
• Improve crater counts on icy bodies to characterize secondary crater mechanics and constrain smaller projectile populations (especially focused on gaps in the observational record on Europa, Ganymede, in the uranian system, and on additional >100 km trans-neptunian objects) by performing high resolution imaging of impact crater morphology on various outer solar system bodies
• Determine how impacts crush porous structures and materials on comets and asteroids in microgravity by characterizing the density variations beneath impact craters based on impact experiments, high-resolution gravity measurements, and remote sensing observations

SUPPORTIVE ACTIVITIES FOR QUESTION 4

• Establish a well-constrained chronology for events in the solar system through improved cataloging of impactor reservoirs using ground- and space-based assets, improved dynamical simulations of the formation and evolution of small bodies, improved mapping of new craters on all planetary surfaces, more complete observations of present-day small body impacts in different contexts, remote dating of planetary surfaces, dating of samples via in situ methods and/or returned samples from diverse bodies, and improved modeling of crater formation.
• Understand variations in impact mechanics owing to the target and projectile properties and impact conditions at all scales, with geophysical and geochemical constraints coming from geologic mapping efforts and through modeling of impact processes at higher spatial and temporal resolutions.
• Establish better equations of state for potential projectile/target materials as well as improved impact scaling relationships for cratering and disruption events through experimental and numerical work.

REFERENCES