Anticipated Geological Assessment of the (65803) Didymos–Dimorphos System, Target of the DART–LICIACube Mission

Impact craters are ubiquitous on asteroids (Figure 7) and provide a means for characterizing an asteroid's surface history and its near-surface and interior properties. As the population and flux of impactors in the near-Earth environment are well understood (Brown et al. 2002; Ivanov 2006), the surface age of Didymos and Dimorphos, as informed by the population of impact craters, can be obtained once the strength of the surface is measured by the DART experiment. The surface age of a rubble-pile asteroid places a constraint on the time of its formation through the catastrophic disruption of its parent body (Michel et al. 2009; Sugita et al. 2019; Walsh et al. 2019) or could be indicative of its last major resurfacing event (Asphaug 2009). Independent of a direct determination of the absolute surface age for both asteroids, a relative surface age between Didymos and Dimorphos can be obtained by comparing their respective crater densities (e.g., Melosh 1989), accounting for possible differences in crater scaling due to the size and possible structural difference between the two components. If an observable difference is found in crater density between the two components of the binary, then crater statistics could inform the origin of the binary system, or it may be indicative of an active crater erasure mechanism that is more efficient on one of these bodies (e.g., Richardson et al. 2005; Bottke et al. 2020). Once the strength of the surface is measured, the crater statistics may also then provide an absolute timescale since the formation of the binary system, providing a crucial component to understanding the evolution of NEA binaries.

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The surface age of an asteroid is typically constrained by its largest craters, which are hypothesized to vary depending on asteroid size and/or structural properties. For example, the main-belt asteroid (253) Mathilde (∼50 km diameter) hosts several large and deep craters of up to ∼60% of its diameter (Veverka et al. 1999). Housen et al. (1999) proposed that these craters are indicative of a compaction-cratering process that can occur on highly porous objects such as Mathilde, which has a low density of 1.3 g cm⁻³. Alternatively, Asphaug (2008) proposed the size of the largest undegraded crater on an asteroid scale with asteroid diameter. A measurement of the size of the largest crater on the asteroid provides a way to characterize the asteroid's interior's ability to attenuate the seismic wave that resurfaced the asteroid. For Didymos and Dimorphos, the model of Asphaug (2008) would suggest that the largest craters on their surface are expected to be ∼0.1× their size. If larger craters existed, this could be indicative of an interior with strong attenuation properties that could rapidly weaken a seismic wave before it is able to traverse the entire length of the asteroid.

Expectations from DART and LICIACube. Critical to understanding the surface properties and evolution of Didymos and Dimorphos through their crater population will be an assessment of crater morphology. Crater morphometrics help elucidate the structural properties of an asteroid and its surface evolution (e.g., Prockter et al. 2002; Daly et al. 2020b; Noguchi et al. 2021). For example, some large craters (>500 m in diameter) on Eros exhibit polygonal square shapes indicative of structural control (Prockter et al. 2002). On Bennu, some of the larger craters (>10 m in diameter) exhibit central mounding, indicating the possibility of subsurface strength (Daly et al. 2020a). In some special circumstances, large depth-to-diameter ratios for craters can indicate the presence of a highly porous interior (Housen & Holsapple 2003; Schultz et al. 2007; Nakamura 2017). Finally, the shallowing of craters can be indicative of mass wasting. Some characteristics, such as the presence of square craters, can be more directly assessed or measured than others, such as the depth of the craters. The expected limited stereo range from DRACO and LEIA images (Daly et al. 2022b) will make it difficult to construct detailed local terrain models of craters as has been done on Eros, Bennu, and Ryugu, for example (Ernst et al. 2012; Daly et al. 2020b; Noguchi et al. 2021). However, we expect to measure the depths of craters ≥10 m in diameter using stereophotoclinometry (Gaskell et al. 2008; Barnouin et al. 2020; Palmer et al. 2022). We will also measure crater depths using shadow measurements, as was successfully done for Eros, if the surface is not rugged, which does not rely on generating topography. Indeed, Robinson et al. (2002) measured average depth-to-diameters of 0.12 from shadows, which is similar to what has been found via shape modeling (Ernst et al. 2012) and the use of Near Laser Rangefinder data (Marchi et al. 2015). Using shadows, we expect to be able to measure the depths of craters up to a few meters in size. Thus, we expect that an intra-asteroid comparison of crater depth-to-diameter ratios (d/D) could provide insight into either structural or surface age differences between Didymos and Dimorphos. Moreover, an intra-asteroid comparison of d/D would probe region-specific variations of the near-surface (e.g., boulder population and possible impact armoring, Bierhaus et al. 2022), the subsurface (e.g., presence of mascons), or surface evolution.

Various forms of mass wasting have been observed in the solar system, not only on Earth but also on multiple other bodies, such as the Moon (e.g., Bart 2007; Xiao et al. 2013), Mars (e.g., Crosta et al. 2018; Pajola et al. 2022), Mercury (e.g., Malin & Dzurisin 1978), Venus (e.g., Malin 1992), the Martian moon Phobos (e.g., Shi et al. 2016), Jovian moons (e.g., Schenk & Bulmer 1998; Moore et al. 1999), Saturnian moons (e.g., Singer et al. 2012), and Charon (Beddingfield et al. 2020). In addition, mass-wasting features have also been detected and investigated on the surfaces of minor bodies, such as on comets and asteroids (e.g., Figure 8(A), (B); Sullivan et al. 1996; Thomas et al. 2002; Miyamoto et al. 2007; Barnouin-Jha et al. 2008; Massironi et al. 2012; Pajola et al. 2017b; Schmidt et al. 2017; Barnouin et al. 2019; Lucchetti et al. 2019; Walsh et al. 2019; Daly et al. 2020a; Jawin et al. 2020; Barnouin et al. 2022). These investigations reveal that one can get a broad range of surface characteristics associated with mass-transport events and their related accumulation deposits. The mechanism responsible for mass movement is controlled by different factors, such as the slope failure or the mechanical properties of the material. On small bodies, the occurrence of mass movements driven by rotation can play a dominant role in shaping regional surfaces, and maybe even their overall shape (e.g., Walsh et al. 2008). Detailed analysis of such events can provide insight into the mechanical behavior of their constituting material. For instance, high-resolution images of comet 67P/Churyumov–Gerasimenko allowed the characterization of the deposits originating from mass-wasting events that reveal a cometary material more akin to Earth dry landslides than to mass movements on icy satellites, despite the cometary composition (Lucchetti et al. 2019). Such geomechanical analyses are also possible on NEAs. On Bennu (Barnouin et al. 2022), evidence for ongoing surface creep and formation of observed surface terraces indicates that Bennu's top 8–10 m of unconsolidated surface material is very weak and likely has very little to no cohesion (<0.6 Pa). On Eros, bright and dark streaks have been identified as the results of downslope mass movements/landslides of mature regolith (Figure 8(B), Veverka et al. 2001; Thomas et al. 2002). Deposits of granular material have been observed at the base of slopes (Cheng 2002) suggesting that the asteroid surface is characterized by a regolith layer overlying a substrate with some competence several tens to hundreds of meters below the surface (Robinson et al. 2002). In the case of Itokawa, smooth terrains of loose small-sized granular material coincide with regions of low gravitational potential or elevation (Miyamoto et al. 2007; Barnouin-Jha et al. 2008). Observations suggest transportation of these small-sized granular regolith particles by seismic shaking, thermal processes, and/or tidal effects that gradually cover up the boulder-rich terrain (Miyamoto et al. 2007; Barnouin-Jha et al. 2008). The evidence that the regolith migration is a gravitationally induced motion is supported by gradual changes in surface roughness from the roughest high-standing terrains to the smoother low-lying regions (Barnouin-Jha et al. 2008; Susorney et al. 2019). It is also evidenced by the long axes of boulders oriented transverse to the direction of gravel migration, which consistently matches the local slopes (Miyamoto et al. 2007).

Features indicating mass wasting on Ryugu are in the form of crater wall slumping, showing the existence of unconsolidated particles, asymmetric regolith deposition on imbricated boulders, and an indication of boulders burial (Michikami et al. 2019; Sugita et al. 2019). On Ryugu, the mass movement's direction is in agreement with the current topographic profile and the geopotential of the asteroid (Sugita et al. 2019).

An impressive candidate mass movement deposit was first identified inside a large crater on Bennu's equator (Figure 8(C); Barnouin et al. 2019; Walsh et al. 2019). Smaller and asteroid-wide features were mapped by Jawin et al. (2020), identifying rock fragments of various sizes sitting on other boulders, boulder imbrication, and partly buried boulders. Such features have been observed at different scales ranging from meter scale, as individual boulders, to the aforementioned single ∼100 m-long debris flow. In all cases, their movement is consistent with the local downslope direction (which is generally oriented N–S toward the equator, Jawin et al. 2020). Daly et al. (2020a) show that the existence of ancient but pervasive mass movements on the northern hemisphere of Bennu and their absence in the south may in part explain the hemispherical shape difference seen at Bennu. Due to the continued spin-up of the asteroid and the resulting steepening of its surface slopes, mass movements are expected to continue occurring on the surface of the NEA (Jawin et al. 2020). Currently, surface creep is ongoing, resulting in the formation of terraces, as well as the toppling of boulders (Barnouin et al. 2022). This includes reorientation of the long axis of boulders both parallel (for boulders that slide into position) and perpendicular (for boulders that roll) to the slope direction (Barnouin et al. 2022).

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Expectations from DART and LICIACube. The DART and LICIACube images will provide an opportunity to enhance our understanding of mass movements affecting the surface of NEAs and, in particular, those on the surfaces of objects in a binary system. Indeed, we expect to detect and investigate several features related to mass movements, such as the presence of variable boulder concentration, organized boulders, and possibly imbricated boulders. We may even see topographic evidence of creep-driven terracing and surface flows. In some cases, the downslope direction of mass movements and the resulting surface expression can change depending on the asteroid rotation velocity, which changes over time, while in other cases impacts on the surface of an asteroid can induce seismic shaking that results in downslope motion of loose, unconsolidated material. Roughness changes with surface elevation not only provide clues to surface displacements by mass movements, but they can also provide evidence for how asteroid landscapes evolve over time due to, for example, changes in spin rates (Susorney et al. 2021). Determining the triggering mechanism for mass movements from DART and LICIACube observations is key to providing information about the surface properties of the regolith that DART may encounter. As discussed in Section 3, evidence for mass movements on Didymos feeds directly into our understanding of the evolution of binary asteroids and similar fast-rotating bodies.

The tidal interaction between Didymos and Dimorphos may also contribute to mass movements on either object (Zhang et al. 2021) as well as mass exchange between the two. Evidence for longitudinal or latitudinal variations of surficial mass concentrations will be critical to assess, to aid in understanding how any of the tidal interactions may obscure insight into the origins of Dimorphos. We do predict that variations will exist in part as a consequence of Dimorphos' likely spin-locked state.

In 1965, the first unearthly boulders were revealed on the lunar surface thanks to a wealth of photographs obtained by a large number of spacecraft (Kuiper 1965). In 1977, the Viking landers photographed Martian boulders (Mutch et al. 1977), suggesting that they might be ubiquitous on solid planetary surfaces. From that moment on, the increasing resolutions of the images of planetary surfaces led to the possibility of acquiring the SFD of boulders present on the solid surfaces of all explored solar system bodies.

Today, the boulder SFD analysis is a widely accepted and used tool to test and investigate some of the geomorphological processes that shaped a planetary surface (e.g., Grant et al. 2006; Yingst et al. 2007; Golombek et al. 2008; Yingst et al. 2010; Pajola et al. 2017c, 2022). This approach is not only used for planets, indeed there is increasing literature documenting the SFD of boulders on minor bodies and satellites such as the Martian Phobos and Deimos (e.g., Lee et al. 1986); on asteroids (243) Ida (Geissler et al. 1996), (433) Eros (Thomas et al. 2001), (25143) Itokawa (Michikami et al. 2008; Mazrouei et al. 2014), (21) Lutetia (Küppers et al. 2012), (4) Vesta (Schröder et al. 2020), (101955) Bennu (Daly et al. 2020a; Burke et al. 2021), and (162173) Ryugu (Michikami et al. 2019); on the dwarf planet Ceres (Schröder et al. 2021); on the Moon (Bart & Melosh 2010); on Enceladus (Pajola et al. 2021); and, more recently, on cometary nuclei, such as 67P/Churyumov–Gerasimenko and 103P/Hartley 2 (Pajola et al. 2015, 2016a, 2016b).

As previously mentioned, small asteroids (<10 km) are commonly considered to be reaccumulated remnants from disrupted parent bodies (Ryan & Melosh 1997; Durda et al. 1997; Benz & Asphaug 1999; Walsh 2018; Delbo et al. 2019). Surface boulders, therefore, represent (i) directly the fragments of those parent-body disruptions, (ii) secondarily, the collisional evolution of those fragments subject to cratering and thermal breakdown on the surface of the small asteroid, and (iii) the size-sorting and migration of granular materials in the mobilized regolith of a small asteroid (e.g., Miyamoto et al. 2007). Hence, deriving the boulder SFD on the surface of an NEA and the corresponding power-law indices is a powerful tool for understanding their initial formation and subsequent evolution.

The two S-type NEAs Eros and Itokawa and the two carbonaceous/organic-rich C-type Ryugu and B-type Bennu have been imaged from meter to submeter resolutions, allowing the most detailed boulder SFD analyses ever performed on small bodies. Thanks to the NASA/NEAR mission to Eros, the collected images allowed Thomas et al. (2001) to detect ∼7000 rocks ≥15 m over the entire surface, deriving a power-law index of the best SFD fitting curve of −3.2, while a later analysis allowed investigating ∼34,000 boulders ≥1 m, deriving a comparable power-law index of −3.1 (Dombard et al. 2010). By using images of Itokawa, Michikami et al. (2008) identified nearly 5000 boulders larger than 1.6 m in diameter and a total of ∼490 boulders larger than 5 m (Figure 9(A)), getting a power-law index of −3.1 ± 0.1. A later analysis performed by Mazrouei et al. (2014) identified a power-law index of −3.5 ± 0.1 for all boulders ≥6 m (Figure 9(B)).

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In the mentioned works, the power-law index range of −3.1 to −3.5 has been thoroughly discussed as representative of an impact-related size distribution as validated by impact simulations on low-gravity bodies (Thomas et al. 2001; Michikami et al. 2008; Küppers et al. 2012; Mazrouei et al. 2014) and on the Moon (Shoemaker & Morris 1970; Bart & Melosh 2010).

On Ryugu, Michikami et al. (2019) identified nearly 4400 boulders ≥5 m that appear to be uniformly distributed across the full surface, with some latitudinal and longitudinal differences. In particular, a lower number density in the equatorial region has been identified, while the resulting SFD of the entire surface is characterized by a power-law index of −2.65 ± 0.05. For Bennu, DellaGiustina et al. (2019) and Burke et al. (2021) counted a total number of 3136 boulders with diameters spanning between 1.3 and 58.4 m. The estimated completeness limit used to compute the global SFD is 8 m and the resulting power-law index is −2.9 ± 0.3. Nevertheless, multiple centimeter-scale images (Figure 9(C)) returned the possibility of counting more than 55,000 pebbles/boulders on the two sampling site finalists, deriving power-law indices ranging from −2.2 ± 0.1 to −2.7 ± 0.6 (Burke et al. 2021).

The boulders on Ryugu and Bennu are suggested to be the result of their larger parent bodies' disruption (Sugita et al. 2019; DellaGiustina et al. 2019); nevertheless, their SFDs are shallower when compared to the S-type NEAs. Watanabe et al. (2019) proposed that the past migration of small particles toward the equatorial ridge of such asteroids might have buried the smallest boulders, hence decreasing the power-law index value. This is supported by boulder counts performed on smaller areas and with higher-spatial-scale images, where the SFD gradually decreases with boulder size. This trend might also be present on Bennu (Burke et al. 2021); nevertheless, it is worth mentioning that the coupled thermal stresses (Molaro et al. 2020b) and micrometeorite bombardment (Ballouz et al. 2020) might affect more the smaller rather than the larger sizes, hence fully disintegrating them and shallowing the resulting global SFD.

Expectations from DART and LICIACube. If we take into consideration both the ∼60° phase angle images of DRACO, which is good for identifying surface features thanks to the presence of elongated shadows, as well as the three-pixel sampling rule (e.g., Michikami et al. 2008; Pajola et al. 2015), the DRACO image data set will be enough to count all boulders ≥18 m visible in the illuminated hemisphere of Didymos, reaching dimensions ≥10 m in some of its areas. For Dimorphos, we will be able to identify all boulders ≥1.5 m on its visible surface, reaching minimum sizes ≥0.2–0.3 m surrounding the DART impact location. This data set will be complemented by the LEIA images that will allow the identification of boulders ≥4.5 m on the illuminated sides of Dimorphos and Didymos (6–7 m on the nonimpacted Dimorphos hemisphere, as well as on the Didymos side not observed by DRACO). Such identifications will be crucial to determine the boulder's SFD and compare it with the other NEA values, hence deriving their formation processes and morphologies. It is indeed not yet clear whether, despite similar disruptions and rubble-pile formations, as well as impact degradations, S-type objects commonly present steeper SFDs than carbonaceous asteroids or not. For the case of Didymos, which is an S-type NEA, one might expect a similar power-law index range to that of Eros and Itokawa if impacts modified its surface-generating boulders in a similar way. Nevertheless, because Didymos is top-shaped like Ryugu and Bennu, it might have experienced a similar equatorial pebble migration as explained by Watanabe et al. (2019), hence it could show a shallower SFD. In addition, Didymos is a binary asteroid, therefore any tidal interaction or particle exchange between the two bodies might have modified the original boulder SFD. This, in particular, is extremely important to be considered when the first results will be obtained. For Dimorphos, we do not know yet if it is a single monolithic body or a rubble pile itself that formed from Didymos: this means that until arrival, we can only speculate on the boulder SFDs we are going to derive and the corresponding boulder surface densities. Nevertheless, it is clear that the boulder SFD analysis will provide hints to answer these questions, hence returning crucial information on the system origin and surface modifications.

Lastly, throughout the DRACO and LEIA data sets, we might detect impact degradation that has happened to the largest boulders. This aspect is of great importance because the size of the largest boulder compared to the size of the largest crater is an important indicator of the formation of the system (catastrophic disruption) and its rubble-pile nature (DellaGiustina et al. 2019). Moreover, the size of the largest boulder (as the 160 m-wide Otohime Saxum on Ryugu, Figure 9(D)) will be a necessary component for constraining the strength of monolithic asteroids (Ballouz et al. 2020), hence providing important implications for the targets' interior properties.

Lineaments are common features that have been found on the surface of many asteroids and provide insights into the internal structure and history of the body (Veverka et al. 1994; Prockter et al. 2002; Sullivan et al. 2002; Besse et al. 2014; Marchi et al. 2015; Simioni et al. 2015). Linear features are usually identified in variable forms, such as ridges, troughs, grooves, and pit chains (Thomas & Prockter 2010). In many instances, primarily for larger asteroids (>30 km), lineament and groove orientation are associated with impact craters (Figures 10(A) and (B)) providing information about internal stiffness (Buczkowski et al. 2008; Marchi et al. 2015). Lineaments can also be the response to stress readjustment, while long linear grooves are usually considered evidence of structural coherence (Barnouin et al. 2014; Marchi et al. 2015).

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In order to understand the lineaments' formation process, it is important to know their 3D orientation and distribution, especially for small bodies. For instance, on Eros, lineaments were found to be ubiquitous at different scales and their formation of some lineament sets is clearly related to impact craters (Buczkowski et al. 2008; Figure 10(B)). However, there are some lineated sets that are not associated with craters. These may have formed during the disruption of Eros' parent body and provide hints that a broad regional boundary exists within the asteroid (Buczkowski et al. 2008; Buczkowski & Wyrick 2015).

In the case of the rubble-pile asteroid Itokawa, which does not have many impact craters on its surface, linear structures have also been identified (Sasaki et al. 2006; Cheng et al. 2007). Some lineaments may be the result of boulders movements whose presence does not require a coherent interior (Sasaki et al. 2006), while others have been observed to be confined to the body of the asteroid and could infer some localized coherence or at least an ability to transmit stress waves over a portion of the asteroid (Barnouin et al. 2014).

Despite its small size, Bennu possesses some lineaments (Barnouin et al. 2019) that may appear to be similar in density to the lineaments present on Eros, but with more concentration in the northern hemisphere (Perry et al. 2019). Shallow troughs are the most common linear features present on the surface of the asteroid and a preliminary analysis agrees with the interpretation that Bennu is a rubble-pile asteroid that may contain large subsurface blocks or some other form of interior rigidity (Barnouin et al. 2019; Perry et al. 2019).

Besides decameter-long lineaments located on the surface of the NEAs, the formation and propagation of fractures is a common process seen on rocks and boulders (Delbo et al. 2014; Molaro et al. 2020a, 2020b). Cracking can be driven by different processes, such as stresses from impact and thermal cycling (Figures 10(C)–(E)), variation in tectonic stresses, volatile loss, and dehydration and can eventually lead to rock exfoliation, breakdown, and rockfalls. The morphology, arrangement, orientation, and spatial density of the fractures provide hints about the process(es) generating them. In the case of impact origin, fractures are expected to usually propagate along all azimuthal directions (although hoop stress can also be created circumferentially around an impact), while fractures presenting preferred direction orientation can be the result of cyclic Sun-induced thermal stresses, as in the case of rocks in Earth's midlatitude desert and on Mars (McFadden et al. 2005; Eppes et al. 2015). It has been shown that the temperature variations due to cycles between day and night can damage the material on airless bodies of our solar system, allowing the nucleation and growth of microfractures due to the mechanical stresses induced by diurnal temperature cycles. Hence, thermal fatigue has been invoked for explaining the disintegration process of surface rocks and the production of regolith over spatial and temporal scales (Dombard et al. 2010; Delbo et al. 2014). In the case of Bennu, the evidence of fractured boulders (Figures 10(C) and (D)) coupled with the morphology of the rocks are consistent with models of fatigue-driven exfoliation (although impacts can also produce such morphologies) and demonstrate the possibility that crack propagation through thermal stress can lead to their development (Molaro et al. 2020b, 2020a). Better evidence for the effect of thermal fatigue is provided by an analysis of the orientation of boulder cracks on Ryugu, which reveal the existence of a preferred orientation in the meridional direction (Figure 10(E); Sasaki et al. 2021). On the other hand, on Itokawa, several cracked boulders have been observed and compared with cracked fragments generated by impact experiments, suggesting that impacts remain an important factor in generating rock cracks (Nakamura et al. 2008).

Expectations from DART and LICIACube. In the meter-scale DRACO images of Dimorphos, it may be possible to detect and analyze (if present) both global or local linear features located on the asteroid's surface. Recognition of longer lineaments on very rocky and rough surfaces could be complicated by the small handful of viewing geometries available. A nondetection may not mean that these features are not present. But if large and small lineaments are indeed seen, their presence will constrain the internal structure of Dimorphos when taking into account lessons from other NEAs. Such analyses will allow testing of Didymos and Dimorphos formation and evolution and provide insights into the stresses affecting the binary system. From the centimeter-scale DRACO images of the final DART impact site, it will be possible to detect the presence (if any) of cracks on Dimorphos boulders, therefore advancing the knowledge of fractures' formation processes, such as thermal fatigue, in the context of an S-type body.

Active asteroids, small bodies that display activity but are not necessarily volatile rich or possess cometary origins (Nuth et al. 2020), exhibit mass ejection from mechanisms due to rotational instabilities and impacts and other nongravitational effects (Jewitt 2012; Jewitt et al. 2015). Among such activities, rotational-driven activities depend strongly on the geological (presence of loose unconsolidated rubble) surface and geophysical (spin-state) conditions (Hirabayashi et al. 2014; Nakano & Hirabayashi 2020; Barnouin et al. 2022; Jackson et al. 2022). Data collected at asteroid Bennu (Lauretta et al. 2019b; Chesley et al. 2020; Pelgrift et al. 2020; Hergenrother et al. 2020) highlight that natural particle ejection on asteroids (Jewitt 2012; Jewitt et al. 2015) is an important process and is probably an indicator of the geologic and geophysical state of small planetary bodies. The recent proximity observations of asteroid Bennu have suggested that besides rotational causes for particle ejections, additional mechanisms might be operating. An important contender remains mass ejection from micrometeoroid impacts (Bottke et al. 2020), and thermal fatigue (Molaro et al. 2020b; Rozitis et al. 2020). The presence of volatiles could also be a factor (Lauretta et al. 2019b). The ejected particle trajectories strongly depend on the complex force field surrounding Bennu, giving a relatively short particle lifetime (McMahon et al. 2020; Bierhaus et al. 2021), while the particles' influence on Bennu's dynamics is minimal (Scheeres et al. 2020). In addition, some ejecta particles escaping from Bennu finally may reach Earth (Kováčová et al. 2020).

There is a possibility that natural particle ejections may also occur in the Didymos system. Unlike the small bodies explored by spacecraft missions in the past, Didymos is a binary asteroid, which for most origin scenarios implies that particle ejection and the resulting dynamic behavior directly contributed to its formation and evolution. There are multiple processes generating mass ejection such as rotational instability, impacts, and nongravitation processes. Rotational instability may be one of the key processes causing mass ejection from Didymos because of its 2.26 hr spin period (Hirabayashi et al. 2017; Zhang et al. 2017, 2021; Naidu et al. 2020). A high centrifugal acceleration causes steep surface slopes (Zhang et al. 2017), possibly giving the entire body some sensitivity to structural failure (Naidu et al. 2020). Under this condition, failed regions may induce mass movements followed by particle ejection. Impacts and other nongravitational effects also play significant roles in particle ejection. Meteoroid impacts can expel grains with a wide range of ejection speeds (Gault et al. 1968; Richardson et al. 2007; Raducan et al. 2020, 2022). Large impacts can induce this process on a large scale (Stickle et al. 2022). Micrometeoroid impacts (Ballouz et al. 2020) may have limited influence on particle ejection but pulverize surface materials (Cambioni et al. 2021). The combination of this process with other nongravitational effects such as electrostatic lofting (Hartzell 2019), solar radiation pressure (McMahon et al. 2020), and thermal fracturing (Molaro et al. 2020b; Rozitis et al. 2020) can also lead to mass ejection.

Expectations from DART and LICIACube. Whether or not DART will be able to detect ejected particles remains a question. Interestingly, the system's L4 and L5 Lagrange points are linearly stable, meaning that these regions satisfy a condition that a small number of small particles can stay there. Though this does not mean that ejecta particles should exist there, these locations may scientifically be interesting to observe. However, DART's approach to observing particle ejection in the Didymos system is analogous to EPOXI's, which conducted observations of large particles in the coma of the hyperactive comet 103P/Hartley 2 during a flyby (Hermalyn et al. 2013). OSIRIS-REx used repeated long-duration observations to identify ejecta particles and track their trajectories to understand their origin and ultimate fate. Hayabusa and Hayabusa2 hovered over their respective target asteroids at near-uniform lighting conditions and never detected ejected particles, possibly because of unsuitable viewing geometries. Like EPOXI, the DART approach and the LICIACube flyby will have a short window of a few minutes to track particles around Didymos before these measurements are confounded by ejecta from the DART impact. With this type of data set, the particles' positions may be determined if observed for long enough by LICIACube to obtain stereo parallax, but measuring their velocities will be limited to those particles that exhibit detectable motions during the short window of observations. Thus, it may not be possible to characterize their trajectories in detail even if detected.

Among all visited NEAs, the largest one, called Eros (34 × 11 × 11 km size), is a low macroporous (∼20%; Wilkinson et al. 2002), cratered world, covered in unconsolidated surface regolith. The asteroid possesses several grooves and ridges (Veverka et al. 2000) that indicate the presence of broad planes of weakness that could have been produced by multiple impacts that shattered the asteroid and/or its parent body (e.g., Buczkowski et al. 2008; Tonge et al. 2016); alternatively, these structures may be fissures in fine-grained granular material (Asphaug et al. 2002). In either case, the extent of the lineaments provides some evidence for coherence across the asteroid (Marchi et al. 2015). Moreover, large boulders (>30 m in diameter) have been linked with the formation of the large 8 km size Shoemaker crater (IAU Charlois Regio) on the asteroid (Thomas et al. 2001), while a scarcity of small craters (0.177 to 1 km in diameter) near Shoemaker provides evidence for seismic shaking (Thomas & Robinson 2005). Some of the large boulders on Eros (which equal the size of Dimorphos) show aprons at their base, evidence of their slow breakdown.

Itokawa was the first unquestionable rubble-pile asteroid (Fujiwara et al. 2006) ever visited, with a low bulk density, high (∼40%) porosity, and boulder-rich appearance (Abe et al. 2006) as well as its elongated shape (0.5 × 2.9 × 2.1 km). In particular, its geology is dominated by two regions: a smooth low-lying region and a rougher boulder-rich highland (Fujiwara et al. 2006; Barnouin-Jha et al. 2008). Moreover, Itokawa looks to be composed of two parts that might have experienced collisional disruption followed by reaccumulation (Nakamura et al. 2008; Michel & Richardson 2013; Mazrouei et al. 2014).

Also elongated (4.8 × 2.4 × 2.0 km), Toutatis has a large crater at its observed limb, and like Itokawa, is characterized by a clear bilobate shape suggesting a contact binary origin. This asteroid possesses widespread evidence for boulders and regolith on its surface (Huang et al. 2013), and their identification has been possible despite a much coarser imagery data set than the other NEAs visited.