Jupiter Trojans Research Articles

Reprint of "Evidence for color dichotomy in the primordial Neptunian Trojan population"

In the current model of early Solar System evolution, the stable members of the Jovian and Neptunian Trojan populations were captured into resonance from the leftover reservoir of planetesimals during the outward migration of the giant planets. As a result, both Jovian and Neptunian Trojans share a common origin with the primordial disk population, whose other surviving members constitute today’s trans-Neptunian object (TNO) populations. The cold (low inclination and small eccentricity) classical TNOs are ultra-red, while the dynamically excited “hot” (high inclination and larger eccentricity) population of TNOs contains a mixture of ultra-red and blue objects. In contrast, Jovian and Neptunian Trojans are observed to be blue. While the absence of ultra-red Jovian Trojans can be readily explained by the sublimation of volatile material from their surfaces due to the high flux of solar radiation at 5 AU, the lack of ultra-red Neptunian Trojans presents both a puzzle and a challenge to formation models. In this work we report the discovery by the Dark Energy Survey (DES) of two new dynamically stable L4 Neptunian Trojans, 2013 VX30 and 2014 UU240, both with inclinations i > 30°, making them the highest-inclination known stable Neptunian Trojans. We have measured the colors of these and three other dynamically stable Neptunian Trojans previously observed by DES, and find that 2013 VX30 is ultra-red, the first such Neptunian Trojan in its class. As such, 2013 VX30 may be a “missing link” between the Trojan and TNO populations. Using a simulation of the DES TNO detection efficiency, we find that there are 162 ± 73 Trojans with Hr < 10 at the L4 Lagrange point of Neptune. Moreover, the blue-to-red Neptunian Trojan population ratio should be higher than 17:1. Based on this result, we discuss the possible origin of the ultra-red Neptunian Trojan population and its implications for the formation history of Neptunian Trojans.

OSSOS. XIX. Testing Early Solar System Dynamical Models Using OSSOS Centaur Detections

Abstract We use published models of the early solar system evolution with a slow, long-range and grainy migration of Neptune to predict the orbital element distributions and the number of modern-day Centaurs. The model distributions are biased by the Outer Solar System Origins Survey (OSSOS) simulator and compared with the OSSOS Centaur detections. We find an excellent match to the observed orbital distribution, including the wide range of orbital inclinations which was the most troublesome characteristic to fit in previous models. A dynamical model, in which the original population of outer disk planetesimals was calibrated from Jupiter trojans, is used to predict that OSSOS should detect 11 ± 4 Centaurs with semimajor axes of a &lt; 30 au, perihelion distances of q &gt; 7.5 au, and diameter of D &gt; 10 km (absolute magnitude H r &lt; 13.7 for a 6% albedo). This is consistent with 15 actual OSSOS Centaur detections with H r &lt; 13.7. The population of Centaurs is estimated to be 21,000 ± 8000 for D &gt; 10 km. The inner scattered disk at 50 &lt; a &lt; 200 au should contain (2.0 ± 0.8) × 107 D &gt; 10 km bodies and the Oort cloud should contain (5.0 ± 1.9) × 108 D &gt; 10 km comets. Population estimates for different diameter cutoffs can be obtained from the size distribution of Jupiter trojans (N(&gt;D) ∝ D −2.1 for 5 &lt; D &lt; 100 km). We discuss model predictions for the Large Synoptic Survey Telescope observations of Centaurs.

The Case for a Large-scale Occultation Network

Abstract We discuss the feasibility of and present initial designs and approximate cost estimates for a large (N ∼ 2000) network of small photometric telescopes that is purpose-built to monitor V ≲ 15 Gaia mission program stars for occultations by minor solar system bodies. The implementation of this network would permit measurement of the solar system’s tidal gravity field to high precision, thereby revealing the existence of distant trans-Neptunian objects such as the proposed “Planet Nine.” As a detailed example of the network capabilities, we investigate how occultations by Jovian Trojans can be monitored to track the accumulation of gravitational perturbations, thereby constraining the presence of undetected massive solar system bodies. We also show that the tidal influence of Planet Nine can be discerned from that of smaller, nearer objects in the Kuiper Belt. Moreover, ephemerides for all small solar system bodies observed in occultation could be significantly improved using this network, thereby improving spacecraft navigation and refining solar system modeling. Finally, occultation monitoring would generate direct measurements of size distributions for asteroid populations, permitting a better understanding of their origins.

Multiband Photometry of a Patroclus–Menoetius Mutual Event: Constraints on Surface Heterogeneity

Abstract We present the first complete multiband observations of a binary asteroid mutual event. We obtained high-cadence, high-signal-to-noise photometry of the UT 2018 April 9 inferior shadowing event in the Jupiter Trojan binary system Patroclus–Menoetius in four Sloan bands—g′, r′, i′, and z′. We use an eclipse light-curve model to fit for a precise mideclipse time and estimate the minimum separation of the two eclipsing components during the event. Our best-fit mideclipse time of is 19 minutes later than the prediction of Grundy et al. The minimum separation between the center of Menoetius’s shadow and the center of Patroclus is 72.5 ± 0.7 km—slightly larger than the predicted 69.5 km. Using the derived light curves, we find no evidence for significant albedo variations or large-scale topographic features on the Earth-facing hemisphere and limb of Patroclus. We also apply the technique of eclipse mapping to place an upper bound of ∼0.15 mag on wide-scale surface color variability across Patroclus.

IDP-like Asteroids Formed Later than 5 Myr After Ca–Al-rich Inclusions

Abstract The parent bodies of ordinary chondrites, carbonaceous CM chondrites, and interplanetary dust particles (IDPs) represent most of the mass of the solar system’s small (D ≤ 250 km) bodies. The times of formation of the ordinary and carbonaceous CM chondrite parent bodies have previously been pinpointed, respectively, to ≈2 and 3–4 million years after calcium–aluminum-rich inclusions (CAIs). However, the timing of the formation of IDP parent bodies such as P- and D-type main-belt asteroids and Jupiter Trojans has not been tightly constrained. Here, we show that they formed later than 5–6 million years after CAIs. We use models of their thermal and structural evolution to show that their anhydrous surface composition would otherwise have been lost due to melting and ice-rock differentiation driven by heating from the short-lived radionuclide 26Al. This suggests that IDP-like volatile-rich small bodies may have formed after the gas of the protoplanetary disk dissipated and thus later than the massive cores of the giant planets. It also confirms an intuitive increase in formation times with increased heliocentric distance, and suggests that there may have been a gap in time between the formation of carbonaceous chondrite (chondrule-rich) and IDP (chondrule-poor) parent bodies.

Hubble Ultraviolet Spectroscopy of Jupiter Trojans

Abstract We present the first ultraviolet spectra of Jupiter Trojans. These observations were carried out using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and cover the wavelength range 200–550 nm at low resolution. The targets include objects from both of the Trojan color subpopulations (less-red and red). We do not observe any discernible absorption features in these spectra. Comparisons of the averaged UV spectra of less-red and red targets show that the subpopulations are spectrally distinct in the UV. Less-red objects display a steep UV slope and a rollover at around 450 nm to a shallower visible slope, whereas red objects show the opposite trend. Laboratory spectra of irradiated ices with and without H2S exhibit distinct UV absorption features; consequently, the featureless spectra observed here suggest H2S alone is not responsible for the observed color bimodality of Trojans, as has been previously hypothesized. We propose some possible explanations for the observed UV-visible spectra, including complex organics, space weathering of iron-bearing silicates, and masked features due to previous cometary activity.

Consequences of planetary migration on the minor bodies of the early solar system

Pebble accretion is an efficient mechanism that is able to build up the core of the giant planets within the lifetime of the protoplanetary disc gas-phase. The core grows via this process until the protoplanet reaches its pebble isolation mass and starts to accrete gas. During the growth, the protoplanet undergoes a rapid, large-scale, inward migration due to the interactions with the gaseous protoplanetary disc. In this work, we have investigated how this early migration would have affected the minor body populations in our solar system. In particular, we focus on the Jupiter Trojan asteroids (bodies in the coorbital resonance 1:1 with Jupiter, librating around the L4and L5Lagrangian points called, respectively, the leading and the trailing swarm) and the Hilda asteroids. We characterised their orbital parameter distributions after the disc dispersal and their formation location and compare them to the same populations produced in a classical in situ growth model. We find that a massive and eccentric Hilda group is captured during the migration from a region between 5 and 8 au and subsequently depleted during the late instability of the giant planets. Our simulations also show that inward migration of the giant planets always produces a Jupiter Trojans’ leading swarm more populated than the trailing one, with a ratio comparable to the current observed Trojan asymmetry ratio. The in situ formation of Jupiter, on the other hand, produces symmetric swarms. The reason for the asymmetry is the relative drift between the migrating planet and the particles in the coorbital resonance. The capture happens during the growth of Jupiter’s core and Trojan asteroids are afterwards carried along during the giant planet’s migration to their final orbits. The asymmetry and eccentricity of the captured Trojans correspond well to observations, but their inclinations are near zero and their total mass is three to four orders of magnitude higher than the current population. Future modelling will be needed to understand whether the dynamical evolution of the Trojans over billions of years will raise the inclinations and deplete the masses to observed values.

A comparative study of size frequency distributions of Jupiter Trojans, Hildas and main belt asteroids: A clue to planet migration history

Since 2002, we have obtained size frequency distributions (SFDs) of main belt asteroids (MBAs), Hildas, and Jupiter Trojans (JTs) by using the 8.2-m Subaru Telescope equipped with the wide-field CCD cameras: Suprime-Cam (SC) or Hyper Suprime-Cam (HSC). After combining these SFDs with SFDs obtained from other surveys, we performed a comparative study of SFDs for each group of small bodies in an attempt to obtain clues about planet migration that affected those populations. The large aperture of the Subaru Telescope and the wide field of view of SC or HSC allowed us to detect small moving objects up to apparent magnitudes 24.4–24.5 mag (Rc-band), which corresponds to sub–km in diameter (D) for MBAs and about 1 km for Hildas and JTs. We combined the SFDs obtained from our surveys with those derived from published data to obtain the individual representative SFD for MBAs, Hildas and JTs in the size range of sub–km to 1000 km. We found that the SFDs of JTs and Hildas are roughly flat in the R-plot while that of MBAs has a wavy structure. We also investigated the SFDs of MBAs in the inner, middle, and outer regions of the main belt. We found that the shape of the SFDs changes gradually with increasing heliocentric distance across these regions. This trend continues beyond the outer region, where the SFD becomes flatter as shown by the SFDs of JTs and Hildas. Recent planet migration models suggest that the current JTs originated in the trans-Neptunian region and were captured as Trojans during planet migration. The finding of a gradual change of the SFDs from the inner MBAs to JTs is in line with the idea that trans-Neptunian objects (TNOs) were implanted not only into the JT region, but also into the main belt outer region (including the Hildas) at the early solar system.In order to investigate this implantation hypothesis, we considered a synthetic population of TNOs assuming with a SFD represented by a power-law distribution of N>D∝D-3, (estimated from crater record on Pluto and Charon). We then added this synthetic population to the MBA populations in various proportions. We found that the higher the proportion, the flatter the wavy SFD of MBAs becomes. This simple model yields a rough explanation for the gradual change of SFDs found from the inner main belt to the JT region. However, the shape of the modelled SFDs does not match observations for all sizes. In particular, because important discrepancies are seen in the small size range, we need to consider the removal of small objects by collisional evolution and/or Yarkovsky effect in the future.

Evidence for color dichotomy in the primordial Neptunian Trojan population

In the current model of early Solar System evolution, the stable members of the Jovian and Neptunian Trojan populations were captured into resonance from the leftover reservoir of planetesimals during the outward migration of the giant planets. As a result, both Jovian and Neptunian Trojans share a common origin with the primordial disk population, whose other surviving members constitute today’s trans-Neptunian object (TNO) populations. The cold (low inclination and small eccentricity) classical TNOs are ultra-red, while the dynamically excited “hot” (high inclination and larger eccentricity) population of TNOs contains a mixture of ultra-red and blue objects. In contrast, Jovian and Neptunian Trojans are observed to be blue. While the absence of ultra-red Jovian Trojans can be readily explained by the sublimation of volatile material from their surfaces due to the high flux of solar radiation at 5 AU, the lack of ultra-red Neptunian Trojans presents both a puzzle and a challenge to formation models. In this work we report the discovery by the Dark Energy Survey (DES) of two new dynamically stable L4 Neptunian Trojans, 2013 VX30 and 2014 UU240, both with inclinations i > 30°, making them the highest-inclination known stable Neptunian Trojans. We have measured the colors of these and three other dynamically stable Neptunian Trojans previously observed by DES, and find that 2013 VX30 is ultra-red, the first such Neptunian Trojan in its class. As such, 2013 VX30 may be a “missing link” between the Trojan and TNO populations. Using a simulation of the DES TNO detection efficiency, we find that there are 162 ± 73 Trojans with Hr < 10 at the L4 Lagrange point of Neptune. Moreover, the blue-to-red Neptunian Trojan population ratio should be higher than 17:1. Based on this result, we discuss the possible origin of the ultra-red Neptunian Trojan population and its implications for the formation history of Neptunian Trojans.

When did the planets migrate?

The existence of a unique binary object captured in the Jovian Trojan population requires an early migration of the giant planets.

The dynamical evolution of escaped Jupiter Trojan asteroids, link to other minor body populations

The Jupiter Trojans constitute an important asteroidal population both in number and also in relation to their dynamical and physical properties. They are asteroids located around L4 and L5 Lagrangian points on relatively stable orbits, in 1: 1 mean motion resonance with Jupiter. However, not all of them lie in orbits that remain stable over the age of the Solar System. Unstable zones allow some Trojans to escape in time scales shorter than the Solar System age. This may contribute to populate other small body populations. In this paper, we study this process by performing long-term numerical simulations of the observed Trojans, focusing on the trajectories of those that leave the resonance. The orbits of current Trojan asteroids are taken as initial conditions and their evolution is followed under the gravitational action of the Sun and the planets. We built “occupancy maps” that represent the zones in the Solar System where escaped Trojans should be found. We find the rate of escape of Trojans from L5, ∼ 1.1 times greater than from L4. The majority of escaped Trojans have encounters with Jupiter although they have encounters with the other planets too. The median lifetime of escaped Trojans in the Solar System is ∼ 264, 000 years for L4 and ∼ 249, 000 years for L5. Almost all escaped Trojans reach the comet zone, ∼ 90% cross the Centaur zone and only L4 Trojans reach the transneptunian zone. Considering the real asymmetry between L4 and L5, we show that 18 L4 Trojans and 14 L5 Trojans with diameter D > 1 km are ejected from the resonance every Myr. The contribution of the escaped Trojans to other minor body populations would be negligible, being the contribution from L4 and L5 to Jupiter-family comets (JFCs) and no-JFCs almost the same, and the L4 contribution to Centaurs and TNOs, orders of magnitude greater than that of L5. Considering the collisional removal, besides the dynamical one, and assuming that Trojans that escape due to collisions follow the same dynamical behavior that the ones removed by dynamics, we would have a minor contribution of Trojans to comets and Centaurs. However, there would be some specific regions were escaped Trojans could be important such as Asteroids in Cometary Orbits (ACOs), Encke-type comets, Shoemaker-Levy 9-type impacts on Jupiter and Near-Earth objects (NEOs).

Evidence for very early migration of the Solar System planets from the Patroclus–Menoetius binary Jupiter Trojan

The orbital distribution of trans-Neptunian objects provides strong evidence for the radial migration of Neptune1,2. The outer planets’ orbits are thought to have become unstable during the early stages3, with Jupiter having scattering encounters with a Neptune-class planet4. As a consequence, Jupiter jumped inwards by a fraction of an au, as required from inner Solar System constraints5,6, and obtained its current orbital eccentricity. The timing of these events is often linked to the lunar Late Heavy Bombardment that ended ~700 Myr after the dispersal of the protosolar nebula (t0)7,8. Here, we show instead that planetary migration started shortly after t0. Such early migration is inferred from the survival of the Patroclus–Menoetius binary Jupiter Trojan9. The binary formed at t ≲ t010,11 within a massive planetesimal disk once located beyond Neptune12,13. The longer the binary stayed in the disk, the greater the likelihood that collisions would strip its components from one another. The simulations of its survival indicate that the disk had to have been dispersed by migrating planets within ≲100 Myr of t0. This constraint implies that the planetary migration is unrelated to the formation of the youngest lunar basins. The Patroclus–Menoetius Jupiter Trojan binary could survive its travel from the Kuiper belt to Jupiter’s orbit only if the giant planets’ migrations happened within ~100 Myr after the birth of the Solar System and not after ~700 Myr, as suggested instead by the Late Heavy Bombardment.

Constructing a refined model of small bodies in the solar system

Aims. We construct a new arc model to represent the global perturbation induced by the Jupiter Trojans for the development of the modern planetary ephemerides. Methods. The Jupiter Trojans are divided into two groups: (1) the 226 biggest ones with absolute magnitudes H &lt; 11, have well determined masses and are treated as individual bodies; (2) the remaining small ones with H ≥ 11, are modeled by two discrete arcs centered at Jupiter’s L4 and L5 points, respectively. Then we derived the parameters characterizing the arcs, such as the total mass, the mass ratio, and the spatial distributions. Uncertainties on the fitted parameters have also been taken into account. Results. We find that the total mass of all the Jupiter Trojans, including the undiscovered ones, is most likely about 1.861 × 10−5 Earth mass. Then the global perturbation of the trojan population induced on the Earth–Mars distance is numerically estimated to be about 70 m during the 2014–2114 time interval. We also give a simple analytic expression. For the satellites of Mars and Jupiter, however, the change of the distance between the Earth and a satellite can be much more significant, reaching on tens of thousands of meters over one century, due to perturbations of the Jupiter Trojans.

Families among the Hildas and Trojans

AbstractA search for asteroid families among the Hildas and Jupiter Trojans was performed with the use of a new set of proper elements. The proper elements were calculated by the empirical method. Besides well known families, several new probable families were found in addition.

Evidences of asymmetry in properties of L4 &amp; L5 Jupiter Trojans

AbstractWe search for possible differences in rotational frequencies, diameters, albedos, and orbital parameters between Trojans belonging to the L4 and L5 swarms using our own observations and literature data. With increasing number of observational data it becomes evident that the L4 and L5 populations have very similar distributions of most parameters with an exception of orbital inclination distribution.

Dynamical Evolution of the Early Solar System

Several properties of the Solar System, including the wide radial spacing of the giant planets, can be explained if planets radially migrated by exchanging orbital energy and momentum with outer disk planetesimals. Neptune's planetesimal-driven migration, in particular, has a strong advocate in the dynamical structure of the Kuiper belt. A dynamical instability is thought to have occurred during the early stages with Jupiter having close encounters with a Neptune-class planet. As a result of the encounters, Jupiter acquired its current orbital eccentricity and jumped inward by a fraction of an astronomical unit, as required for the survival of the terrestrial planets and from asteroid belt constraints. Planetary encounters also contributed to capture of Jupiter Trojans and irregular satellites of the giant planets. Here we discuss the dynamical evolution of the early Solar System with an eye to determining how models of planetary migration/instability can be constrained from its present architecture. Specifically, we review arguments suggesting that the Solar System may have originally contained a third ice giant on a resonant orbit between Saturn and Uranus. This hypothesized planet was presumably ejected into interstellar space during the instability. The Kuiper belt kernel and other dynamical structures in the trans-Neptunian region may provide evidence for the ejected planet. We favor the early version of the instability where Neptune migrated into the outer planetesimal disk within a few tens of millions of years after the dispersal of the protosolar nebula. If so, the planetary migration/instability was not the cause of the Late Heavy Bombardment. Mercury's orbit may have been excited during the instability.

Science exploration and instrumentation of the OKEANOS mission to a Jupiter Trojan asteroid using the solar power sail

An engineering mission OKEANOS to explore a Jupiter Trojan asteroid, using a Solar Power Sail is currently under study. After a decade-long cruise, it will rendezvous with the target asteroid, conduct global mapping of the asteroid from the spacecraft, and in situ measurements on the surface, using a lander. Science goals and enabling instruments of the mission are introduced, as the results of the joint study between the scientists and engineers from Japan and Europe.

Size Distribution of Small Hilda Asteroids∗

Abstract We present the size distribution for the Hilda asteroid group using optical survey data obtained by the 8.2 m Subaru Telescope with the Hyper Suprime-Cam. Our unbiased sample consists of 91 Hilda asteroids (Hildas) down to 1 km in diameter. We found that the Hildas’ size distribution can be approximated by a single-slope power law in the ∼1−10 km diameter range with the best-fit power-law slope of α = 0.38 ± 0.02 in the differential absolute magnitude distribution. Directly comparing the size distribution of Hildas with that of the Jupiter Trojans measured from the same data set indicates that the two size distributions are similar to each other within a diameter of ∼10 km, while these shapes are distinguishable from that of main-belt asteroids. The results suggest that Hildas and Jupiter Trojans share a common origin and have a different formation environment from main-belt asteroids. The total number of the Hilda population larger than 2 km in diameter is estimated to be ∼1 × 104 based on the size distribution, which is less than that of the Jupiter Trojan population by a factor of about five.

Albedos of Centaurs, Jovian Trojans, and Hildas

Abstract We present optical V band albedo distributions for middle solar system minor bodies including Centaurs, Jovian Trojans, and Hildas. Diameters come mostly from the NEOWISE catalog. Optical photometry (H values) for about two-thirds of the ∼2700 objects studied are from Pan-STARRS, supplemented by H values from JPL Horizons (corrected to the Pan-STARRS photometric system). Optical data for Centaurs are from our previously published work. The albedos presented here should be superior to previous work because of the use of the Pan-STARRS optical data, which is a homogeneous data set that has been transformed to standard V magnitudes. We compare the albedo distributions of pairs of samples using the nonparametric Wilcoxon test. We strengthen our previous findings that gray Centaurs have lower albedos than red Centaurs. The gray Centaurs have albedos that are not significantly different from those of the Trojans, suggesting a common origin for Trojans and gray Centaurs. The Trojan L4 and L5 clouds have median albedos that differ by ∼10% at a very high level of statistical significance, but the modes of their albedo distributions differ by only ∼1%. We suggest the presence of a common “true background” in the two clouds, with an additional more reflective component in the L4 cloud. We find, in agreement with Grav et al. that the Hildas are darker than the Trojans by 15%–25%. Perhaps the Hildas are darker because of their passage near perihelion through zone III of the main asteroid belt, which might result in significant darkening by gardening.

Comparison of the orbital properties of Jupiter Trojan asteroids and Trojan dust

In a previous paper we simulated the orbital evolution of dust particles from the Jupiter Trojan asteroids ejected by the impacts of interplanetary particles, and evaluated their overall configuration in the form of dust arcs. Here we compare the orbital properties of these Trojan dust particles and the Trojan asteroids. Both Trojan asteroids and most of the dust particles are trapped in the Jupiter 1:1 resonance. However, for dust particles, this resonance is modified because of the presence of solar radiation pressure, which reduces the peak value of the semi-major axis distribution. We find also that some particles can be trapped in the Saturn 1:1 resonance and higher order resonances with Jupiter. The distributions of the eccentricity, the longitude of pericenter, and the inclination for Trojans and the dust are compared. For the Trojan asteroids, the peak in the longitude of pericenter distribution is about 60 degrees larger than the longitude of pericenter of Jupiter; in contrast, for Trojan dust this difference is smaller than 60 degrees, and it decreases with decreasing grain size. For the Trojan asteroids and most of the Trojan dust, the Tisserand parameter is distributed in the range of two to three.