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The Near-Earth binary asteroid Didymos was the target of a planetary defense demonstration mission DART in September 2022. The smaller binary component, Dimorphos, was impacted by the spacecraft in order to measure momentum transfer in kinetic impacts into rubble piles. DART and associated Earth-based observation campaigns have provided a wealth of scientific data on the Didymos-Dimorphos binary. DART revealed a largely oblate and ellipsoidal shape of Dimorphos before the impact, while the post-impact observations suggest that Dimorphos now has a prolate shape. Here we add those data points to the known properties of small binary asteroids and propose new paradigms of the radiative binary YORP (BYORP) effect as well as tidal dissipation in small binaries. We find that relatively spheroidal bodies like Dimorphos made of small debris may experience a weaker and more size-dependent BYORP effect than previously thought. This could explain the observed values of period drift in several well-characterized binaries. We also propose that energy dissipation in small binaries is dominated by relatively brief episodes of large-scale movement of (likely surface) materials, rather than long-term steady-state tidal dissipation. We propose that one such episode was triggered on Dimorphos by the DART impact. Depending on the longevity of this high-dissipation regime, it is possible that Dimorphos will be more dynamically relaxed in time for the Hera mission than it was in the weeks following the impact., Comment: Accepted for PSJ

In our recent work, we combined dynamical simulations, meteoritic data and thermal models as well as asteroid observations to argue that the current parent body of the EL meteorites was implanted into the asteroid belt not earlier than 60 Myr after the beginning of the Solar System and that the most likely capture mechanism was the giant planet orbital instability. In the study "The link between Athor and EL meteorites does not constrain the timing of the giant planet instability" that appeared in arXiv, Izidoro and collaborators argue that the implantation of Athor into the asteroid belt does not necessarily require that the giant planet orbital instability occurred at the implantation time. Here we provide further arguments that, in the end, the giant planet instability is still the most likely dynamical process to implant asteroid Athor into the asteroid main belt between 60 and 100 Myr after the beginning of the Solar System., Comment: 4 pages, 1 figure; comment on arXiv:2404.10828

The main asteroid belt (MAB) is known to be primarily composed of objects from two distinct taxonomic classes, generically defined here as S- and C-complex. The former probably originated from the inner solar system (interior to Jupiter's orbit), while the latter probably from the outer solar system. Following this definition, (4) Vesta, a V-type residing in the inner MAB (a < 2.5 au), is the sole D > 500 km object akin to S-complex that potentially formed in-situ. This provides a useful constraint on the number of D > 500 km bodies that could have formed, or grown, within the primordial MAB. In this work we numerically simulate the accretion of objects in the MAB region during the time when gas in the protoplanetary disk still existed, while assuming different MAB primordial masses. We then accounted for the depletion of that population happening after gas disk dispersal. In our analysis, we subdivided the MAB into five sub-regions and showed that the depletion factor varies throughout the MAB. This results in uneven radial- and size-dependent depletion of the MAB. We show that the MAB primordial mass has to be $\lesssim$ 2.14$\times$10$^{-3}$ Earth masses. Larger primordial masses would lead to the accretion of tens-to-thousands of S-complex objects with D > 500 km in the MAB. Such large objects would survive depletion even in the outer sub-regions (a > 2.5 au), thus being inconsistent with observations. Our results also indicate that S-complex objects with D > 200-300 km, including (4) Vesta, are likely to be terrestrial planetesimals implanted into the MAB rather than formed in-situ., Comment: Accepted for publication in PSJ, 23 pages, 7 figures, 4 tables

We explore binary asteroid formation by spin-up and rotational disruption considering the NASA DART mission's encounter with the Didymos-Dimorphos binary, which was the first small binary visited by a spacecraft. Using a suite of $N$-body simulations, we follow the gravitational accumulation of a satellite from meter-sized particles following a mass-shedding event from a rapidly rotating primary. The satellite's formation is chaotic, as it undergoes a series of collisions, mergers, and close gravitational encounters with other moonlets, leading to a wide range of outcomes in terms of the satellite's mass, shape, orbit, and rotation state. We find that a Dimorphos-like satellite can form rapidly, in a matter of days, following a realistic mass-shedding event in which only ${\sim}2-3\%$ of the primary's mass is shed. Satellites can form in synchronous rotation due to their formation near the Roche limit. There is a strong preference for forming prolate (elongated) satellites, although some simulations result in oblate spheroids like Dimorphos. The distribution of simulated secondary shapes is broadly consistent with other binary systems, measured through radar or lightcurves. Unless Dimorphos's shape is an outlier, and considering the observational bias against lightcurve-based determination of secondary elongations for oblate bodies, we suggest there could be a significant population of oblate secondaries. If these satellites initially form with elongated shapes, a yet-unidentified pathway is needed to explain how they become oblate. Finally, we show that this chaotic formation pathway occasionally forms asteroid pairs and stable triples, including co-orbital satellites and satellites in mean motion resonances., Comment: 41 pages, 23 figures, accepted to PSJ, movies available at https://zenodo.org/doi/10.5281/zenodo.8387043

Numerical modeling has long suggested that gravitationally-bound (or so-called rubble-pile) near-Earth asteroids (NEAs) can be destroyed by tidal forces during close and slow encounters with terrestrial planets. However, tidal disruptions of NEAs have never been directly observed nor have they been directly attributed to any families of NEAs. Here we show population-level evidence for the tidal disruption of NEAs during close encounters with the Earth and Venus. Debiased model distributions of NEA orbits and absolute magnitudes based on observations by the Catalina Sky Survey during 2005--2012 underpredict the number of NEAs with perihelion distances coinciding with the semimajor axes of Venus and the Earth. A detailed analysis of the orbital distributions of the excess NEAs shows that their characteristics agree with the prediction for tidal disruptions, and they cannot be explained by observational selection effects or orbital dynamics. Accounting for tidal disruptions in evolutionary models of the NEA population partly bridges the gap between the predicted rate of impacts by asteroids with diameters of tens of meters and observed statistics of fireballs in the same size range., Comment: 8 pages, 5 figures, accepted for publication in ApJL

We present an approach for the inclusion of non-spherical constituents in high-resolution N-body discrete element method (DEM) simulations. We use aggregates composed of bonded spheres to model non-spherical components. Though the method may be applied more generally, we detail our implementation in the existing N-body code pkdgrav. It has long been acknowledged that non-spherical grains confer additional shear strength and resistance to flow when compared with spheres. As a result, we expect that rubble-pile asteroids will also exhibit these properties and may behave differently than comparable rubble piles composed of idealized spheres. Since spherical particles avoid some significant technical challenges, most DEM gravity codes have used only spherical particles, or have been confined to relatively low resolutions. We also discuss the work that has gone into improving performance with non-spherical grains, building on pkdgrav's existing leading-edge computational efficiency among DEM gravity codes. This allows for the addition of non-spherical shapes while maintaining the efficiencies afforded by pkdgrav's tree implementation and parallelization. As a test, we simulated the gravitational collapse of 25,000 non-spherical bodies in parallel. In this case, the efficiency improvements allowed for an increase in speed by nearly a factor of three when compared with the naive implementation. Without these enhancements, large runs with non-spherical components would remain prohibitively expensive. Finally, we present the results of several small-scale tests: spinup due to the YORP effect, tidal encounters, and the Brazil-nut Effect. In all cases, we find that the inclusion of non-spherical constituents has a measurable impact on simulation outcomes., Comment: 35 pages, 8 figures

Near-Earth asteroids Ryugu and Bennu, were visited, characterised, and sampled by the Hayabusa2 and OSIRIS-REx missions: remote sensing data and sample return analysis showed that both asteroids have primitive, hydrated and organic-rich compositions. The dark families of the inner main belt (IMB) that belong to the spectroscopic C-complex have been claimed to be the sources of both Ryugu and Bennu. Hence, there has been large effort to characterise them. Here we used the Gaia Data Release 3 (DR3) asteroid reflectance spectra to investigate the 11 known IMB C-complex families (Chaldaea, Chimaera, Clarissa, Erigone, Eulalia, Klio, Polana, Primordial, Sulamitis, Svea, Tamara). For each family, we extracted the family members that have known geometric visible albedo values and Gaia DR3 data and we created an average reflectance spectrum per family between 370 and 950 nm. The average DR3 reflectance spectra of each family were compared with the previous literature data and to Bennu's and Ryugu's spectra. We found that DR3 reflectance spectra of the IMB C-complex families are in general consistent with previous findings with the only exception of the Svea family. We also showed that the Polana and the Eulalia families can be distinguished in the wavelength region 370 - 500 nm. Among all the IMB C-complex families, we determined that the average reflectance spectra of the Eulalia and Polana families are the most similar to those of Bennu and Ryugu, respectively. In particular, Eulalia family's average spectrum is a good match to Bennu's in the wavelength range 450 - 800 nm, while beyond 800 nm the spectrum of Bennu is bluer than that of Eulalia. Moreover, the spectrum of the Polana family has the smallest discrepancy against the spectrum of Ryugu, although this match is formally unsatisfactory (reduced chi^2 ~ 1.9)., Comment: Accepted for publication in Astronomy and Astrophysics

After performing a reassessment of the known dynamical asteroid families in the inner main belt, we report a newly discovered ancient asteroid family with an estimated age of $4.3\pm1.7$ billion years. Additionally, we report the most comprehensive list of planetesimals, which are asteroids that survived since the planet forming days of the solar system., Comment: 21 pages, 13 figures

Polluted white dwarfs offer a unique way to study the bulk compositions of exoplanetary material, but it is not always clear if this material originates from comets, asteroids, moons, or planets. We combine N-body simulations with an analytical model to assess the prevalence of extrasolar moons as white dwarf (WD) polluters. Using a sample of observed polluted white dwarfs we find that the extrapolated parent body masses of the polluters are often more consistent with those of many solar system moons, rather than solar-like asteroids. We provide a framework for estimating the fraction of white dwarfs currently undergoing observable moon accretion based on results from simulated white dwarf planetary and moon systems. Focusing on a three-planet white dwarf system of Super-Earth to Neptune-mass bodies, we find that we could expect about one percent of such systems to be currently undergoing moon accretions as opposed to asteroid accretion., Comment: Submitted to AAS journals, comments welcome

Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus and Sulfur (CHNOPS) play key roles in the origin and proliferation of life on Earth. Given the universality of physics and chemistry, not least the ubiquity of water as a solvent and carbon as a backbone of complex molecules, CHNOPS are likely crucial to most habitable worlds. To help guide and inform the search for potentially habitable and ultimately inhabited environments, we begin by summarizing the CHNOPS budget of various reservoirs on Earth, their role in shaping our biosphere, and their origins in the Solar Nebula. We then synthesize our current understanding of how these elements behave and are distributed in diverse astrophysical settings, tracing their journeys from synthesis in dying stars to molecular clouds, protoplanetary settings, and ultimately temperate rocky planets around main sequence stars. We end by identifying key branching points during this journey, highlighting instances where a forming planets' distribution of CHNOPS can be altered dramatically, and speculating about the consequences for the chemical habitability of these worlds., Comment: 39 pages, 7 figures; under review for publication as a chapter in Protostars and Planets VII (Editors: Shu-ichiro Inutsuka, Yuri Aikawa, Takayuki Muto, Kengo Tomida, and Motohide Tamura)

Most close-in planetary satellites are in synchronous rotation, which is usually the stable end-point of tidal despinning. Saturn's moon Hyperion is a notable exception by having a chaotic rotation. Hyperion's dynamical state is a consequence of its high eccentricity and its highly prolate shape (Wisdom et al. 1984). As many binary asteroids also have elongated secondaries, chaotic rotation is expected for moons in eccentric binaries (\'Cuk and Nesvorn\'y, 2010), and a minority of asteroidal secondaries may be in that state (Pravec et al. 2016). The question of secondary rotation is also important for the action of the BYORP effect, which can quickly evolve orbits of synchronous (but not non-synchronous) secondaries (\'Cuk and Burns, 2005). Here we report results of a large set of short numerical simulations which indicate that, apart from synchronous and classic chaotic rotation, close-in irregularly-shaped asteroidal secondaries can occupy an additional, intermediate rotational state. In this ``barrel instability'' the secondary slowly rolls along its long axis, while the longest axis is staying largely aligned with the primary-secondary line. This behavior may be more difficult to detect through lightcurves than a fully chaotic rotation, but would likewise shut down BYORP. We show that the binary's eccentricity, separation measured in secondary's radii and the secondary's shape are all important for determining whether the system settles in synchronous rotation, chaotic tumbling, or barrel instability. We compare our results for synthetic asteroids with known binary pairs to determine which of these behaviors may be present in the Near-Earth Asteroid binary population., Comment: Accepted for PSJ

Following the break up of a parent body, the Yarkovsky effect causes asteroid family members to spread in orbital semimajor axis with a rate often inversely proportional to their diameter. This size dependent semimajor axis drift causes family members to form structures in the semimajor axis vs inverse diameter plane that have the shape of the letter V. The V-shape method consists in finding the borders of such V-shapes of unknown center and opening. Although successfully employed to find some very old families in the inner main asteroid belt, the V-shape searching method is very sensitive to many parameters. In this work, we first created and evolved a synthetic asteroid family over billions of years. Then, by adding uncertainties to semimajor axis and diameter of the evolved synthetic family components, we randomly generated additional 99 similar, but not perfectly V-shaped, family clones. We chose a fairly low initial velocity dispersion of 20 m/s for our family. Thus, we can more easily relate the spreading in semimajor axis with the family's age (slope of the evolving V). A synthetic background with an initially randomly distributed components was also created and evolved for 100 Myr. Thus, by setting different levels of ratio of the synthetic family and background asteroids, we derived a detection efficiency map for the V-shape method and determined how sensitive the results can be based on signal-to-noise levels. We also determined optimal parameter values for the method's efficiency. We found that, families older than ~3 Gyr are likely undetectable, with a method efficiency of 50% or less, whereas younger families (0.5--2.5 Gyr) are more easily detected by the method, with an efficiency of >80%., Comment: 21 pages, 16 figures, Accepted for publication in Icarus

Recent spacecraft and radar observations have found that ~70 percent of short-period comet nuclei, mostly Jupiter-family comets (JFCs), have bilobate shapes (two masses connected by a narrow neck). This is in stark contrast to the shapes of asteroids of similar sizes, of which ~14% are bilobate. This suggests that a process or mechanism unique to comets is producing these shapes. Here we show that the bilobate shapes of JFC nuclei are a natural byproduct of sublimative activity during their dynamical migration from their trans-Neptunian reservoir, through the Centaur population, and into the Jupiter family. We model the torques resulting from volatile sublimation during this dynamical migration and find that they tend to spin up these nuclei to disruption. Once disrupted, the rubble pile-like material properties of comet nuclei (tensile strengths of ~1-10 Pa and internal friction angles of ~35$^\circ$) cause them to reform as bilobate objects. We find that JFCs likely experienced rotational disruption events prior to entering the Jupiter family, which could explain the prevalence of bilobate shapes. These results suggest that the bilobate shapes of observed comets developed recently in their history (within the past ~1-10 Myr), rather than during solar system formation or collisions during planet migration and residency in the trans-Neptunian population.

Proximity observations by OSIRIS-REx and Hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) Bennu and (162173) Ryugu. Their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. Different shape evolution scenarios were previously proposed; Bennu's top shape may have been driven by surface processing, while Ryugu's may have been developed due to large deformation. These two scenarios seem to be inconsistent. Here, we revisit the structural analyses in earlier works and fill a gap to connect these explanations. We also apply a semi-analytical technique for computing the cohesive strength distribution in a uniformly rotating triaxial ellipsoid to characterize the global failure of top-shaped bodies. Assuming that the structure is uniform, our semi-analytical approach describes the spatial variations in failed regions at different spin periods; surface regions are the most sensitive at longer spin periods, while interiors fail structurally at shorter spin periods. This finding suggests that the shape evolution of a top shape may vary due to rotation and internal structure, which can explain the different evolution scenarios of Bennu's and Ryugu's top shapes. We interpret our results as the indications of top shapes' various evolution processes., Comment: 27 pages, 1 table, 7 figures, In Press in Icarus (2020)

In 2015, the New Horizons spacecraft flew past Pluto and its moon Charon, providing the first clear look at the surface of Charon. New Horizons images revealed an ancient surface, a large, intricate canyon system, and many fractures, among other geologic features. Here, we assess whether tidal stresses played a significant role in the formation of tensile fractures on Charon. Although presently in a circular orbit, most scenarios for the orbital evolution of Charon include an eccentric orbit for some period of time and possibly an internal ocean. Past work has shown that these conditions could have generated stresses comparable in magnitude to other tidally fractured moons, such as Europa and Enceladus. However, we find no correlation between observed fracture orientations and those predicted to form due to eccentricity-driven tidal stress. It thus seems more likely that the orbit of Charon circularized before its ocean froze, and that either tidal stresses alone were insufficient to fracture the surface or subsequent resurfacing remove these ancient fractures., Comment: 12 pages, 5 figures, 2 tables

Recent dynamical analyses suggest that some Jupiter family comets (JFCs) may originate in the main asteroid belt instead of the outer solar system. This possibility is particularly interesting given evidence that icy main-belt objects are known to be present in the Themis asteroid family. We report results from dynamical analyses specifically investigating the possibility that icy Themis family members could contribute to the observed population of JFCs. Numerical integrations show that such dynamical evolution is indeed possible via a combination of eccentricity excitation apparently driven by the nearby 2:1 mean-motion resonance with Jupiter, gravitational interactions with planets other than Jupiter, and the Yarkovsky effect. We estimate that, at any given time, there may be tens of objects from the Themis family on JFC-like orbits with the potential to mimic active JFCs from the outer solar system, although not all, or even any, may necessarily be observably active. We find that dynamically evolved Themis family objects on JFC-like orbits have semimajor axes between 3.15 au and 3.40 au for the vast majority of their time on such orbits, consistent with the strong role that the 2:1 mean-motion resonance with Jupiter likely plays in their dynamical evolution. We conclude that a contribution from the Themis family to the active JFC population is plausible, although further work is needed to better characterize this contribution., Comment: 12 pages, 6 figures; accepted for publication in AJ

We present numerical simulations of terrestrial planet formation that examine the growth continuously from planetesimals to planets in the inner Solar System. Previous studies show that the growth will be inside-out, but it is still common practice to assume that the entire inner disk will eventually reach a bi-modal distribution of embryos and planetesimals. For the combinations of disk mass, initial planetesimal radius and gas disk lifetime explored in this work the entire disk never reaches a simple bi-modal mass distribution. We find that the inside-out growth is amplified by the combined effects of collisional evolution of solid bodies and interactions with a dissipating gas disk. This leads to oligarchic growth never being achieved in different places of the disk at the same time, where in some cases the disk can simultaneoulsy support chaotic growth and giant impacts inside 1 au and runaway growth beyond 2 au. The planetesimal population is efficiently depleted in the inner disk where embryo growth primarily advances in the presence of a significant gas disk. Further out in the disk growth is slower relative to the gas disk dissipation, resulting in more excited planetesimals at the same stage of growth and less efficient accretion. This same effect drives mass loss due to collisional grinding strongly altering the surface density of the accreted planets relative to the initial mass distribution. This effect decreases the Mars-to-Earth mass ratios compared to previous works with no collisional grinding. Similar to some previous findings utilizing vastly different growth scenarios these simulations produce a first generation of planetary embryos that are stable for 10-20 Myr, or 5-10 e-folding times of the gas dissipation timescale, before having an instability and entering the chaotic growth stage., Comment: 17 figures

The solar system's dynamical state can be explained by an orbital instability among the giant planets. A recent model has proposed that the giant planet instability happened during terrestrial planet formation. This scenario has been shown to match the inner solar system by stunting Mars' growth and preventing planet formation in the asteroid belt. Here we present a large sample of new simulations of the "Early Instability" scenario. We use an N-body integration scheme that accounts for collisional fragmentation, and also perform a large set of control simulations that do not include an early giant planet instability. Since the total particle number decreases slower when collisional fragmentation is accounted for, the growing planets' orbits are damped more strongly via dynamical friction and encounters with small bodies that dissipate angular momentum (eg: hit-and-run impacts). Compared with simulations without collisional fragmentation, our fully evolved systems provide better matches to the solar system's terrestrial planets in terms of their compact mass distribution and dynamically cold orbits. Collisional processes also tend to lengthen the dynamical accretion timescales of Earth analogs, and shorten those of Mars analogs. This yields systems with relative growth timescales more consistent with those inferred from isotopic dating. Accounting for fragmentation is thus supremely important for any successful evolutionary model of the inner solar system., Comment: 16 pages, 7 figures, 4 tables, accepted for publication in Icarus

The moniker rubble pile is typically applied to all solar system bodies with Diameter between 200m and 10km - where in this size range there is an abundance of evidence that nearly every object is bound primarily by self-gravity with significant void space or bulk porosity between irregularly shaped constituent particles. The understanding of this population is derived from wide-ranging population studies of derived shape and spin, decades of observational studies in numerous wavelengths, evidence left behind from impacts on planets and moons and the in situ study of a few objects via spacecraft flyby or rendezvous. The internal structure, however, which is responsible for the name rubble pile, is never directly observed, but belies a violent history. Many or most of the asteroids on near-Earth orbits, and the ones most accessible for rendezvous and in situ study, are likely byproducts of the continued collisional evolution of the Main Asteroid Belt., Comment: 40 pages, 14 figures; Annual Review of Astronomy and Astrophysics, 2018, Vol. 56:593-624