Asteroid Population Research Articles

Thermal emission measurements of ordinary chondrite mineral analogs in a simulated asteroid environment: 2. Representative mineral mixtures

We investigate a suite of synthetic mineral mixtures designed to act as bulk mineralogical analogs to H, L, and LL ordinary chondrite meteorites in order to probe how the thermal emission characteristics of such materials change between ambient and simulated asteroid environmental conditions. Due to the parent body link with certain S-type asteroids, studying these analog mixtures in an environment that is relevant to actual asteroid surfaces advances our understanding of the thermal emission properties of one of the most common regolith types among the main-belt and near-Earth asteroid populations. The observed changes in spectral emissivity features due to cold, vacuum conditions are not as large as previously observed for single mineral (silicate) samples. We interpret this difference to be the result of metallic and opaque components weakening near-surface thermal gradients in the mixtures. As such, we predict that near-surface thermal gradients on ordinary chondrite parent bodies (e.g., S-type asteroids) are likely much weaker than would be inferred from cold, vacuum measurements of individual mineral components. We tested whether the increased spectral contrast observed in fine-grained (<25 μm) samples measured in a cold, vacuum environment increases the efficacy of least squares linear unmixing methods. It is found that the accuracy of such models does not improve relative to measurements made under ambient conditions, thus linear unmixing models are not expected to yield accurate estimates of the modal mineralogy of airless planetary surfaces if they are dominated by fine-grained regolith. Mixtures with coarse particle sizes (125–250 μm) that were modeled using the coarse particle size endmembers yielded results that were largely independent of the environmental conditions, but with larger errors in spectral fits for samples measured in a simulated asteroid environment. At simulated asteroid environmental conditions, the bulk silicate composition and metal content play a more important role in determining the thermal state and brightness temperature of the sample than at ambient conditions. Modest changes in metal content (10–25 wt%) lead to large differences in the brightness temperature of a sample. Under simulated asteroid conditions, an ~10 K increase in maximum brightness temperature that tracks with increased iron content is observed at fine particle sizes (<25 μm) between each of the analog ordinary chondrite groups. Based on these results, it may be difficult to distinguish H, L, and LL compositions of suspected ordinary chondrite parent bodies using only Earth- or space-based thermal emission spectra. This is inferred from the spectral similarity of the analog mixtures and the absence of significant variation in spectral emissivity associated with reported differences in bulk metal content for ordinary chondrites.

Observational Completion Limit of Minor Planets from the Asteroid Belt to Jupiter Trojans

Abstract With the growing numbers of asteroids being discovered, identifying an observationally complete sample is essential for statistical analyses and for informing theoretical models of the dynamical evolution of the solar system. We present an easily implemented method of estimating the empirical observational completeness in absolute magnitude, , as a function of semimajor axis. Our method requires fewer assumptions and decisions to be made in its application, making results more transportable and reproducible among studies that implement it, as well as scalable to much larger data sets of asteroids expected in the next decade with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time. Using the values of (a) determined at high resolution in semimajor axis, a, we demonstrate that the observationally complete sample size of the main belt asteroids is larger by more than a factor of 2 compared with using a conservative single value of , an approach often adopted in previous studies. Additionally, by fitting a simple, physically motivated model of (a) to ∼7 × 105 objects in the Minor Planet Database, our model reveals statistically significant deviations between the main belt and the asteroid populations beyond the main belt (Hungarias, Hildas, and Trojans), suggesting potential demographic differences, such as in their size, eccentricity, or inclination distributions.

Near-infrared spectroscopy of the Sulamitis asteroid family: Surprising similarities in the inner belt primitive asteroid population

We present NIR spectra of 19 asteroids in the Sulamitis family as part of our survey of primitive inner belt asteroid families. The spectra were obtained with NASA's Infrared Telescope Facility and the Telescopio Nazionale Galileo between January 2017 and February 2020. We find spectral homogeneity in our sample despite the diversity within the family observed at visible wavelengths. The average Sulamitis spectrum is flat with a spectral slope of 0.89 ± 0.26%/1000 Å between 0.95 and 2.3 μm. We show that the Sulamitis family is spectrally similar to other inner belt families in the NIR, despite differences between families seen in the visible wavelength range. We also compare our obtained spectra with asteroids (101955) Bennu and (162173) Ryugu to show that the Sulamitis family is a plausible source of Ryugu.

A comparative analysis of the outer-belt primitive families

Context.Asteroid families are witnesses to the intense collisional evolution that occurred on the asteroid belt. The study of the physical properties of family members can provide important information about the state of differentiation of the parent body and provide insights into how these objects were formed. Several of these asteroid families identified across the main belt are dominated by low-albedo, primitive asteroids. These objects are important for the study of Solar System formation because they were subject to weaker thermophysical processing and provide information about the early conditions of our planetary system.Aims.We aim to study the diversity of physical properties among the Themis, Hygiea, Ursula, Veritas, and Lixiaohua families.Methods.We present new spectroscopic data, combined with a comprehensive analysis using a variety of data available in the literature, such as albedo and rotational properties.Results.Our results show that Themis and Hygiea families, the largest families in the region, present similar levels of hydration. Ursula and Lixiaohua families are redder in comparison to the others and present no sign of hydrated members based on the analysis of visible spectra. Conversely, Veritas presents the highest fraction of hydrated members.Conclusions.This work demonstrates a diverse scenario in terms of the physical properties of primitive outer-belt families, which could be associated with dynamical mixing of asteroid populations and the level of differentiation of the parental body.

Bennu's near-Earth lifetime of 1.75 million years inferred from craters on its boulders.

An asteroid's history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75±0.75 million years.

Spin pairs in the Koronis asteroid family

Asteroid groups may either form because of collisions or the rotational failure of a parent body. Collisional asteroid families tend to be more spread in proper element domains, while young spin clusters are usually much more compact. Recently, it has been shown that young spin clusters may form at higher rates in extremely young asteroid families (families younger than ≃3 Myr) than in more evolved groups. Among asteroid families, the Koronis family is a natural laboratory to test this phenomenon. While the family itself is very evolved, with an estimated age of more than 2 Byr, two young sub-families are known to be located among Koronis members: the extremely young family of Karin, with an age of 5.75±0.05 Myr, and the relatively young sub-family of Koronis(2), with an age of 10±5 Myr. This fact allows studying the occurrence of young spin clusters in extremely young, young and evolved asteroid populations, all sharing the same physical properties. Using methods based on backward numerical integrations, we identify one possible pair candidate in the Karin family, and retrieved the pair 1741–258640 recently found in other works. No convincing spin pair candidate was found in the Koronis(2) sub-family. This suggests that the occurrence of young spin clusters in families older than ≃ 3 Myr may be a rarer event than what previously thought.

Evolution of an Asteroid Family under YORP, Yarkovsky, and Collisions

Abstract Any population of asteroids, like asteroid families, will disperse in semimajor axis due to the Yarkovsky effect. The amount of drift is modulated by the asteroid spin state evolution, which determines the balance between the diurnal and seasonal Yarkovsky forces. The asteroid’s spin state is, in turn, controlled in part by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. The otherwise smooth evolution of an asteroid can be abruptly altered by collisions, which can cause impulsive changes in the spin state and can move the asteroid onto a different YORP track. In addition, collisions may also alter the YORP parameters by changing the superficial features and overall shape of the asteroid. Thus, the coupling between YORP and Yarkovsky is also strongly affected by the impact history of each body. To investigate this coupling, we developed a statistical code modeling the time evolution of semimajor axis under YORP–Yarkovsky coupling. It includes the contributions of NYORP (normal YORP), TYORP (tangential YORP), and collisions whose effects are deterministically calculated and not added in a statistical way. We find that both collisions and TYORP increase the dispersion of a family in semimajor axis by making the spin axis evolution less smooth and regular. We show that the evolution of a family’s structure with time is complex and collisions randomize the YORP evolution. In our test families, we do not observe the formation of a “YORP-eye” in the semimajor axis versus diameter distribution, even after a long period of time. If present, the “YORP-eye” might be a relic of an initial ejection velocity pattern of the collisional fragments.

Illuminating the dark side of the asteroid population: Visible near-infrared (0.7–2.45 μm) surface mineralogy modeling of D-type asteroids using Shkuratov theory

D-type asteroids are a prime example of the many dark, low-albedo asteroids which do not reflect sufficient light to reveal detectable mineral absorptions. While D-type asteroids are relatively rare in the inner solar system and the main asteroid belt, they are dominant among the Jovian Trojans. In this study, we have applied Shkuratov radiative transfer modeling to laboratory spectra of meteorites for which mineral abundances have been measured using X-ray diffraction (XRD) and Rietveld refinement. The general agreement of radiative transfer and XRD estimates of mineral abundances demonstrates the applicability of the radiative transfer approach to featureless, low-albedo spectra. Shkuratov modeling was then applied to new spectral observations of D-type asteroids, along with numerous previously published spectra. The surface mineral abundances of 81 D-type objects, including NASA's Lucy Mission target (21900) Orus, were modeled using assemblages that are plausible based on meteorite analogues. Modeling results reveal D-types are composed of: low-iron olivine; magnesium saponite-dominant phyllosilicates; opaques such as pyrrhotite and tholin; as well as traces of water-ice and other constituents. Subtle compositional differences in model mineralogies exist between Trojan and non-Trojan D-types as well as between L4 and L5 Trojans suggesting differing formational as well as evolutional conditions have affected these bodies.

Aspects of nanospacecraft design for main-belt sailing voyage

We present a detailed mechanical and thermal analysis of a stand-alone nanospacecraft that performs asteroid flybys in the main asteroid belt (2.75 AU) and one Earth flyby at the end of the mission to return the gathered data. A fleet of such nanospacecraft (<10 kg) has been proposed as part of the Multi-Asteroid Touring mission concept, a nearly propellantless mission where the electric solar wind sail (E-sail) is used for primary propulsion. The fleet makes flybys of thus far poorly characterised asteroid populations in the main belt and downlinks scientific data during the returning Earth flyby. The spacecraft size is close to a three-unit cubesat with a mass of less than 6 kg. The spacecraft is designed for a 3.2-year round trip. A 20-km-long E-sail tether is used. A remote unit is attached to the tether’s tip and stowed inside the spacecraft before the E-sail commissioning. The remote unit is slightly smaller than a one-unit cubesat with a mass of approximately 750 g. With an electrospray thruster, it provides angular momentum during tether deployment and spin-rate management while operating the E-sail. The selection of materials and configurations is optimised for thermal environment as well as to minimise the mass budget. This paper analyses the main spacecraft and remote-unit architectures along with deployment and operation strategies from a structural point of view, and thermal analysis for both bodies.

A Global Fireball Observatory

The world’s meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroids) together is a major challenge for planetary science. Directly probing asteroids achieves this at a high cost. Observing meteorite falls and calculating their pre-atmospheric orbit on the other hand, is a cheaper way to approach the problem. The Global Fireball Observatory (GFO) collaboration was established in 2017 and brings together multiple institutions (from Australia, USA, Canada, Morocco, Saudi Arabia, the UK, and Argentina) to maximise the area for fireball observation time and therefore meteorite recoveries. The members have a choice to operate independently, but they can also choose to work in a fully collaborative manner with other GFO partners. This efficient approach leverages the experience gained from the Desert Fireball Network (DFN) pathfinder project in Australia. The state-of-the art technology (DFN camera systems and data reduction) and experience of the support teams is shared between all partners, freeing up time for science investigations and meteorite searching. With all networks combined together, the GFO collaboration already covers 0.6% of the Earth’s surface for meteorite recovery as of mid-2019, and aims to reach 2% in the early 2020s. We estimate that after 5 years of operation, the GFO will have observed a fireball from virtually every meteorite type. This combined effort will bring new, fresh, extra-terrestrial material to the labs, yielding new insights about the formation of the Solar System.

Experimental constraints on the ordinary chondrite shock darkening caused by asteroid collisions

Context.Shock-induced changes in ordinary chondrite meteorites related to impacts or planetary collisions are known to be capable of altering their optical properties. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed dark C/X asteroid population.Aims.The exact pressure-temperature conditions of the shock-induced darkening are not well constrained. Thus, we experimentally investigate the gradual changes in the chondrite material optical properties as a function of the shock pressure.Methods.A spherical shock experiment with Chelyabinsk LL5 was performed in order to study the changes in its optical properties. The spherical shock experiment geometry allows for a gradual increase of shock pressure from ~15 GPa at a rim toward hundreds of gigapascals in the center.Results.Four distinct zones were observed with an increasing shock load. The optical changes are minimal up to ~50 GPa. In the region of ~50–60 GPa, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa due to an onset of silicate melting. At pressures higher than ~150 GPa, recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains. The shock darkening affects the ultraviolet, visible, and near-infrared region while changes to the MIR spectrum are minimal.Conclusions.Shock darkening is caused by two distinct mechanisms with characteristic pressure regions, which are separated by an interval where the darkening ceases. This implies a reduced amount of shock-darkened material produced during the asteroid collisions.

Modeling the Dielectric Properties of Minerals From Crystals to Bulk Powders for Improved Interpretation of Asteroid Radar Observations

AbstractPlanetary radar has provided a growing number of data sets on the inner planets and near‐Earth and main belt asteroid populations in the solar system. Physical interpretation of radar data for inference of surface properties requires constraints on the constitutive parameters of the material making up a given surface. In this study, the complex permittivity of seven minerals as a function of frequency and porosity is measured using the coaxial transmission line method to determine the mixing equation that best describes the relationship between the real part of the complex permittivity of single mineral crystals and granular mineral powders. We find the Looyenga‐Landau‐Lifshitz and Bruggeman symmetric mixing equations to describe our experimental results with the highest accuracy. The variation in the real part of the permittivity of solid mineral crystals between different minerals is shown to depend on the grain density and the chemical composition of the minerals. These mixing relationships are incorporated into an asteroid radar model and used to calculate the porosity in the near‐surface of seven asteroids visited by robotic spacecraft using Earth‐based radar observations. The results of the asteroid radar model support the presence of significant porosity in the boulders on the surface of asteroid 101955 Bennu. This research highlights the ability of radar to measure the porosity on asteroid surfaces and provides theoretical and experimental justification for the inversion of permittivity to bulk density assumed by the asteroid radar model.

Interpreting the Cratering Histories of Bennu, Ryugu, and Other Spacecraft-explored Asteroids

Abstract Asteroid crater retention ages have unknown accuracy because projectile–crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size–frequency distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. For D &gt; 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. The same scaling law applied to 0.3 &lt; D &lt; 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for Bennu, Ryugu, and Itokawa and ∼2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is that f ≫ 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way between D &gt; 10 km asteroids, where f ∼ 10, and 0.3 &lt; D &lt; 2.5 km asteroids, where f ≫ 10.

Solar System Objects Observed with TESS—First Data Release: Bright Main-belt and Trojan Asteroids from the Southern Survey

Compared with previous space-borne surveys, the Transiting Exoplanet Survey Satellite (TESS) provides a unique and new approach to observe Solar System objects. While its primary mission avoids the vicinity of the ecliptic plane by approximately six degrees, the scale height of the Solar System debris disk is large enough to place various small body populations in the field-of-view. In this paper we present the first data release of photometric analysis of TESS observations of small Solar System Bodies, focusing on the bright end of the observed main-belt asteroid and Jovian Trojan populations. This data release, named TSSYS-DR1, contains 9912 light curves obtained and extracted in a homogeneous manner, and triples the number of bodies with unambiguous fundamental rotation characteristics, namely where accurate periods and amplitudes are both reported. Our catalogue clearly shows that the number of bodies with long rotation periods are definitely underestimated by all previous ground-based surveys, by at least an order of magnitude.

Asteroid Diameters and Albedos from NEOWISE Reactivation Mission Years 4 and 5

Abstract The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft has been conducting a two-band thermal infrared survey to detect and characterize asteroids and comets since its reactivation in 2013 December. Using the observations collected during the fourth and fifth years of the survey, our automated pipeline detected candidate moving objects that were verified and reported to the Minor Planet Center. Using these detections, we perform thermal modeling of each object from the near-Earth object (NEO) and Main Belt asteroid (MBA) populations to constrain their sizes. We present thermal model fits of asteroid diameters for 189 NEOs and 5831 MBAs detected during the fourth year of the survey, and 185 NEOs and 5776 MBAs from the fifth year. To date, the NEOWISE Reactivation survey has provided thermal model characterization for 957 unique NEOs. Including all phases of the original Wide-field Infrared Survey Explorer survey brings the total to 1473 unique NEOs that have been characterized between 2010 and the present.

Asteroid dynamics inside Venus' orbit

Asteroids Most asteroids reside in the main-belt region between the orbits of Mars and Jupiter. A population of asteroids within the orbit of Venus has been hypothesized, but searching that region is difficult because it can only be observed just before sunrise or just after sunset. The first asteroid with an orbit entirely inside Venus' orbit, designated 2020 AV2, was discovered on 4 January 2020. Greenstreet used simulations to assess its orbital dynamics and found that the orbit is unstable: Within a million years, the asteroid will either collide with Venus or be scattered onto an Earth-crossing orbit. A nearby 3:2 resonance with Venus could extend the lifetime to a few million years. Mon. Not. R Astron. Soc. Lett. 2020 , slaa025 (2020).

Linking asteroids and meteorites to the primordial planetesimal population

Meteorites provide a unique insight into early Solar System processes. However, to fully interpret this record requires that these meteorites are related back to their source asteroids and ultimately to the original planetesimal population that formed early in Solar System history. As a first step in this process an assessment has been undertaken of the likely number of distinct source asteroids sampled by meteorites and related extraterrestrial materials. The results of this survey indicate that there are between 95 and 148 parent bodies represented in our sample collections. This number has been steadily increasing as new “anomalous” meteorites are characterized. Attempts to link these parent bodies to identified asteroidal sources has so far been of limited success, due to the non-unique reflectance spectra of almost all known asteroids. Asteroid (4) Vesta and the HED (howardites, eucrite, diogenite) meteorites are the best example of a relatively non-disputed asteroid-meteorite linkage.As part of this study the “parent body” concept has been examined and was found to be a widely, but loosely, used term in the literature to designate “a body that supplies meteorites to Earth.” This concept could be rendered more meaningful by discriminating between primary and secondary parent bodies. A primary parent body is the source asteroid from which the meteorite is ultimately derived, and a secondary parent body is an asteroid derived through impact or break-up of the primary body. A clear example of this usage is provided by (4) Vesta, with the main asteroid being the primary parent body and the Vestoids representing secondary parent bodies. The concept of primary vs. secondary parent bodies may have important implications for early Solar System evolution. Chondritic parent bodies are known to have accreted between 1 and 4 Myr after CAIs. This timing difference may reflect the fact that their source asteroids, particularly those of the carbonaceous chondrites, are secondary bodies, with the original CAI-bearing primary bodies destroyed during early collisional processing.The number of primary parent bodies represented by meteorites (95–148) appears low when compared to the estimated number of asteroids in the main belt (>100,000 with diameters exceeding ∼2 km). A range of potential reasons may explain this apparent mismatch: (i) meteorites provide an unrepresentative sampling of the main belt, (ii) the belt may only contain a limited number of primary parent bodies, (iii) meteorites may be preferentially derived from the ∼120 identified asteroid families, (iv) loosely consolidated types are filtered by Earth’s atmosphere, (v) multiple, near-identical, “clone” parent bodies may be present in the belt. At present, it is not possible to determine which of these potential mechanisms are dominant and all may be operating to a greater or lesser extent.Based on classical accretion models the meteorite record appears to be highly unrepresentative of the primordial asteroid population. In contrast, pebble accretion models suggest that these first-generation bodies may have been relatively large, in which case meteorites may provide a more unbiased record of early Solar System processes.

Asteroid triple-system 2001 SN263: surface characteristics and dynamical environment

ABSTRACT The (153591) 2001 SN263 asteroid system, a target of the first Brazilian interplanetary space mission, is one of the known three triple systems within the population of near-Earth asteroids. One of the mission objectives is to collect data about the formation of this system. The analysis of these data will help in the investigation of the physical and dynamical structures of the components (Alpha, Beta, and Gamma) of this system, in order to find vestiges related to its origin. In this work, we assume the irregular shape of the 2001 SN263 system components as uniform-density polyhedra and computationally investigate the gravitational field generated by these bodies. The goal is to explore the dynamical characteristics of the surface and environment around each component. Then, taking into account the rotational speed, we analyse their topographic features through the quantities geometric altitude, tilt, geopotential, slope, and surface accelerations among others. Additionally, the investigation of the environment around the bodies made it possible to construct zero-velocity curves, which delimit the location of equilibrium points. The Alpha component has a peculiar number of 12 equilibrium points, all of them located very close to its surface. In the cases of Beta and Gamma, we found four equilibrium points not so close to their surfaces. Then, performing numerical experiments around their equilibrium points, we identified the location and size of just one stable region, which is associated with an equilibrium point around Beta. Finally, we integrated a spherical cloud of particles around Alpha and identified the location on the surface of Alpha where the particles have fallen.

Tidal disruption of planetary bodies by white dwarfs – II. Debris disc structure and ejected interstellar asteroids

ABSTRACT We make use of a new hybrid method to simulate the long-term, multiple-orbit disc formation through tidal disruptions of rocky bodies by white dwarfs, at high-resolution and realistic semimajor axis. We perform the largest yet suite of simulations for dwarf and terrestrial planets, spanning four orders of magnitude in mass, various pericentre distances, and semimajor axes between 3 and 150 au. This large phase space of tidal disruption conditions has not been accessible through the use of previous codes. We analyse the statistical and structural properties of the emerging debris discs, as well as the ejected unbound debris contributing to the population of interstellar asteroids. Unlike previous tidal disruption studies of small asteroids which form ring-like structures on the original orbit, we find that the tidal disruption of larger bodies usually forms dispersed structures of interlaced elliptic eccentric annuli on tighter orbits. We characterize the (typically power law) size distribution of the ejected interstellar bodies as well as their composition, rotation velocities, and ejection velocities. We find them to be sensitive to the depth (impact parameter) of the tidal disruption. Finally, we briefly discuss possible implications of our results in explaining the peculiar variability of Tabby’s star, the origin of the transit events of ZTF J0139+5245 and the formation of a planetary core around SDSS J1228+1040.

On the inclinations of the Jupiter Trojans

Jupiter Trojans are a resonant asteroidal population characterised by photometric colours that are compatible with trans-Neptunian objects, high inclinations, and an asymmetric distribution of the number of asteroids between the two swarms. Different models have been proposed to explain the high inclination of the Trojans and to interpret their relation with the Trans-Neptunian objects, but none of these models can also satisfactorily explain the asymmetry ratio between the number of asteroids in the two swarms. It has recently been found that the asymmetry ratio can arise if Jupiter has migrated inwards through the protoplanetary disc by at least a few astronomical units during its growth. The more numerous population of the leading swarm and the dark photometric colours of the Trojans are natural outcomes of this new model, but simulations with massless unperturbed disc particles led to a flat distribution of the Trojan inclinations and a final total mass of the Trojans that was 3–4 orders of magnitude larger than the current mass. We here investigate the possible origin of the peculiar inclination distribution of the Trojans in the scenario where Jupiter migrates inwards. We analyse different possibilities: (a) the secular evolution of an initially flat Trojan population, (b) the presence of planetary embryos among the Trojans, and (c) capture of the Trojans from a pre-stirred planetesimal population in which Jupiter grows and migrates. We find that the secular evolution of the Trojans and secular perturbations from Saturn do not affect the inclination distribution of the Trojans appreciably, nor is there any significant mass depletion over the age of the Solar System. Embryos embedded in the Trojan swarms, in contrast, can stir the Trojans to their current degree of excitation and can also deplete the swarms efficiently, but it is very difficult to remove all of the massive bodies in 4.5 Gyr of evolution. We propose that the disc where Jupiter’s core was forming was already stirred to high inclination values by other planetary embryos competing in the feeding zone of Jupiter’s core. We show that the trapped Trojans preserve their high inclination through the gas phase of the protoplanetary disc and that Saturn’s perturbations are more effective on highly inclined Trojans, leading to a lower capture efficiency and to a substantial depletion of the swarms during 4.5 Gyr of evolution.