The Empty Primordial Asteroid Belt

Red (S > 10%/0.1 μm) spectral slopes are common among Centaurs and trans-Neptunian objects (TNOs) in the outer solar system. Interior to and co-orbital with Jupiter, the red (S ∼ 10%/0.1 μm) slopes of D-type main-belt and Jupiter Trojan asteroids are thought to reflect their hypothesized shared origin with TNOs beyond the orbit of Jupiter. In order to quantify the abundance of red-sloped asteroids within the main belt, we conducted a survey using the NASA Infrared Telescope Facility and the Lowell Discovery Telescope. We followed up on 32 candidate red objects identified via spectrophotometry from the Sloan Digital Sky Survey’s Moving Object Catalog to confirm their steep spectral slopes and determine their taxonomic classifications. We find that our criteria for identifying candidate red objects from the Moving Object Catalog result in a ∼50% confirmation rate for steeply red-sloped asteroids. We also compare our observations of main-belt asteroids to existing literature spectra of the Jupiter Trojans and steeply red-sloped main-belt asteroids. We show that some red-sloped asteroids have linearly increasing reflectance with increasing wavelength, while other red-sloped asteroids show a flattening in slope at longer near-infrared wavelengths, indicating a diversity among the population of spectrally red main-belt asteroids suggestive of a variety of origins among the population of steep-sloped asteroids.

Ice in main belt asteroids and near‐Earth objects is of scientific and resource exploration interest, but small airless bodies gradually lose their ice to space by outward diffusion. Here we quantitatively estimate the time it takes a porous airless body to lose all of its interior ice, based on an analytic solution for the interior temperature field of bodies in stable orbits. Without latent heat, the average surface temperature, which is lower than the classic effective temperature, is representative of the body interior and hence an appropriate temperature to evaluate desiccation timescales. In a spherically averaged model, an explicit analytic solution is obtained for the depth to ice as a function of time and the time to complete desiccation. Half of the ice volume is lost after 11% of this time. A bilobate structure emerges from the strong latitude dependence of desiccation rates. Cold polar regions can harbor subsurface ice, even when the body center does not. Latent heat retards ice loss, and we obtain a succinct expression for the temperature difference between the surface and the ice. In the outer main belt, nearly all bodies 10 km in size or larger should have been able to retain ice in their interiors over the age of the solar system. Each of the following factors favors the presence of ice inside near‐Earth objects: a semimajor axis in the outer belt or beyond, a mantle of very low thermal inertia, a young age, or a small and stable axis tilt.

The majority of basaltic objects in the main belt are dynamically connected to Vesta, the largest differentiated asteroid known. Others, due to their current orbital parameters, cannot be easily dynamically linked to Vesta. This is particularly true for all the basaltic asteroids located beyond 2.5 au, where lies the 3:1 mean motion resonance with Jupiter. In order to investigate the presence of other V-type asteroids in the middle and outer main belt (MOVs) we started an observational campaign to spectroscopically characterize in the visible range MOV candidates. We observed 18 basaltic candidates from TNG and ESO - NTT between 2015 and 2016. We derived spectral parameters using the same approach adopted in our recent statistical analysis and we compared our data with orbital parameters to look for possible clusters of MOVs in the main belt, symptomatic for a new basaltic family. Our analysis seemed to point out that MOVs show different spectral parameters respect to other basaltic bodies in the main belt, which could account for a diverse mineralogy than Vesta; moreover, some of them belong to the Eos family, suggesting the possibility of another basaltic progenitor. This could have strong repercussions on the temperature gradient present in the early Solar System, and on our current understanding of differentiation processes.

Preservation of polar ice on near-Earth asteroids originating in the outer main belt: A model study with dynamical trajectories

The formation of terrestrial planets from a ring of planetesimals was initially proposed by Hansen (2009) as a solution to the planet’s mass distribution pattern, namely with two small planets at the edges of the semi major axis distribution and two major ones at the center, rather than a system of planets with uniform masses.An early attempt to explain why planetesimals should have been concentrated into a ring invoked a specific inward-then-outward migration pattern of Jupiter, dubbed the Grand Tack (Walsh et al., 2011). However, modern models of planetesimal formation predict that the first planetesimals naturally form in rings (Drazkowska et al., 2016; Morbidelli et al., 2022; Izidoro et al., 2022; Marchall and Morbidelli, 2024). These works support the fundaments of the ring formation model.Whereas early simulations of terrestrial planet formations assumed the existence of a narrow ring of planetesimals and planetary embryos after gas removal (Hansen et al., 2009; Nesvorny et al., 2021), more recent simulations start from a ring of sole planetesimals (super-particles) and simulate the growth of planetary embryos during the gas-disk stage (Woo et al. 2023, 2024). This phase leads to the spreading of the ring and the formation of numerous mini-planets, unless the radial profile of the gas surface density distribution peaks itself near 1 au (Broz et al., 2021). In these conditions, it is not necessary that the ring of planetesimals is located at 1 au.Although the terrestrial planets formed from almost exclusively non-carbonaceous material from the inner Solar system, the chemistry of the Earth’s mantle, in particular the moderately siderophile element depletion pattern and FeO and SiO2 abundances, reveal that our planet should have started its accretion by incorporating reduced material, akin (in terms of oxygen fugacity) to Aubrites and enstatite chondrites, and incorporated later more oxidized material, akin to the known non-carbonaceous iron meteorite parent bodies or of ordinary chondrites (Rubie et al., 2011). This cannot be explained if the terrestrial planets formed from a narrow ring of planetesimals (Dale et al., 2025 and at this conference).New, still unpublished models will be presented in this review, where the terrestrial planets form from two rings. The first ring, located near 0.5 au, is due to the silicate sublimation line and comprises reduced planetesimals, where the second one, located near 1.5 au, is made of more oxidized planetesimals. The growth of planetary embryos from each of these rings, and their merging near 1 au, gives the best reconstruction of the orbital and mass distribution of the terrestrial planets. The existence of the second ring is justified in Goldberg et al., this conference).In conclusions, the model of terrestrial planet formation from a ring of planetesimals is definitely more complicated than originally envisioned by Hansen (2009) but is now converging towards a more complex model that can explain both the physical, chemical and isotopic properties of the planets. Moreover, the new model is consistent with the structure of the asteroid belt and with our understanding of the evolution of the giant planets.Acknowledgment: This work is supported by the ERC grant N. 101019380 HolyEarthReferences.Hansen, B.M.S., 2009. Formation of the Terrestrial Planets from a Narrow Annulus. 703, 1131–1140. doi:10.1088/0004-637X/703/1/1131,Walsh, K.J., Morbidelli, A., Raymond, S.N., O’Brien, D.P., Mandell, A.M., 2011. A low mass for Mars from Jupiter’s early gas-driven migration. 475, 206–209. doi:10.1038/nature10201Drążkowska, J.,Alibert,Y.,Moore, B., 2016. Close-in planetesimal formation by pile-up of drifting pebbles. 594,A105. doi:10.1051/0004-6361/201628983,Morbidelli, A., Baillié, K., Batygin, K., Charnoz, S., Guillot, T., Rubie, D.C., Kleine, T., 2022. Contemporary formation of early Solar System planetesimals at two distinct radial locations. Nature Astronomy 6, 72–79. doi:10.1038/s41550-021-01517-7,Izidoro, A., Dasgupta, R., Raymond, S.N., Deienno, R., Bitsch, B., Isella, A. 2022. Planetesimal rings as the cause of the Solar System's planetary architecture. Nature Astronomy 6, 357–366. doi:10.1038/s41550-021-01557-zMarschall, R., Morbidelli, A. 2023. An inflationary disk phase to explain extended protoplanetary dust disks. Astronomy and Astrophysics 677. doi:10.1051/0004-6361/202346616Nesvorný, D., Roig, F.V., Deienno, R., 2021. The Role of Early Giant-planet Instability in Terrestrial Planet Formation. 161, 50. doi:10.3847/1538-3881/abc8efWoo, J.M.Y., Morbidelli, A., Grimm, S.L., Stadel, J., Brasser, R., 2023. Terrestrial planet formation from a ring. 396, 115497. doi:10.1016/j. icarus.2023.115497Woo, J.M.Y., Nesvorný, D., Scora, J.,Morbidelli, A., 2024. Terrestrial planet formation from a ring: Long-term simulations accounting for the giant planet instability. 417, 116109. doi:10.1016/j.icarus.2024.116109Broz, M., Chrenko, O., Nesvorny, D., Dauphas, N. 2021. Early terrestrial planet formation by torque-driven convergent migration of planetary embryos. Nature Astronomy 5, 898–902. doi:10.1038/s41550-021-01383-3Rubie, D.C., Frost, D.J., Mann, U., Asahara, Y., Nimmo, F., Tsuno, K., Kegler, P., Holzheid, A., Palme, H., 2011. Heterogeneous accretion, composition and core-mantle differentiation of the Earth. Earth and Planetary Science Letters 301, 31–42. doi:10.1016/j.epsl.2010.11.030.Dale, K.I., Morbidelli, A., Nathan, G., Woo, J., Nesvorný, D. and Rubie, D.C. Oxidation Constraints on Terrestrial Planet Formation from a Ring. Icarus, submitted.

The identification of meteorite parent bodies provides the context for understanding planetesimal formation and evolution as well as the key solar system dynamical events they have witnessed. We identified that the family of asteroid fragments whose largest member is asteroid (161) Athor is the unique source of the rare EL enstatite chondrite meteorites (Avdellidou et al. 2022), the closest meteorites to Earth in terms of their isotopic ratios. The Athor family was created by the collisional fragmentation of a parent body 3 Gyr ago in the inner main belt (Delbo et al. 2019), however the diameter of the Athor family progenitor was much smaller than the putative size of the EL original planetesimal (Triellof et a. 2022). Therefore, we deduced that the EL planetesimal that accreted in the terrestrial planet region underwent a first catastrophic collision in that region, and one of its fragments suffered a more recent catastrophic collision in the main belt, generating the current source of the EL meteorites.  We investigated the possible ways that could have brought the Athor family progenitor in its current position in the inner main belt. To do so, we used an interdisciplinary methodology where we combined laboratory meteorite thermochronometric data, thermal modelling, and dynamical simulations.  We showed that planetesimal fragments from the terrestrial zone must have been implanted into the main asteroid belt at least 60 Myr after the beginning of the solar system. We concluded that the giant planet instability is the only dynamical process that can enable such implantation so late in the solar system timeline.  Acknowledgements. We acknowledge support from the ANR ORIGINS (ANR- 18-CE31-0014). This work is based on data provided by the Minor Planet Physical Properties Catalogue (MP3C) of the Observatoire de la Côte d’Azur (mp3c.oca.eu).

We use the Fokker-Planck equation and model the dispersive dynamics of solid particles in annular protoplanetary disks whose gas component is more massive than the particle phase. We model particle--gas interactions as hard sphere collisions, determine the functional form of diffusion coefficients, and show the existence of two global unstable modes in the particle phase. These modes have spiral patterns with the azimuthal wavenumber $m=1$ and rotate slowly. We show that in ring-shaped disks, the phase space density of solid particles increases linearly in time towards an accumulation point near the location of pressure maximum, while instabilities grow exponentially. Therefore, planetesimals and planetary cores can be efficiently produced near the peaks of unstable density waves. In this mechanism, particles migrating towards the accumulation point will not participate in the formation of planets, and should eventually form a debris ring like the main asteroid belt or classical Kuiper belt objects. We present the implications of global instabilities to the formation of ice giants and terrestrial planets in the solar system.

Phase curves of nine Trojan asteroids over a wide range of phase angles

Basaltic asteroids (V-types) are believed to be fragments of large differentiated bodies. The majority of them are found in the inner part of the asteroid belt, and are current or past members of the Vesta family. Recently, some V-type asteroids have been discovered far from the Vesta family supporting the hypothesis of the presence of multiple basaltic asteroids in the early solar system. The discovery of basaltic asteroids in the outer belt challenged the models of the radial extent and the variability of the temperature distribution in the early solar system. We aim to identify new basaltic V-type asteroids using near-infrared colors of ~40000 asteroids observed by the VHS-VISTA survey and compiled in the MOVIS-C catalogue. We also want to study their near-infrared colors and to study the near-infrared color distribution of the Vesta dynamical family. We performed a search in the MOVIS-C catalogue of all the asteroids with (Y-J) and (J-Ks) in the range (Y-J) > 0.5 and (J-Ks) < 0.3, associated with V-type asteroids, and studied their color distribution. We have also analyzed the near-infrared color distribution of 273 asteroid members of the Vesta family and compared them with the albedo and visible colors from WISE and SDSS data. We determined the fraction of V-type asteroids in the family. We found 477 V-type candidates in MOVIS-C, 244 of them outside the Vesta dynamical family. We identified 19 V-type asteroids beyond the 3:1 mean motion resonance, 6 of them in the outer main belt, and 16 V-types in the inner main belt with proper inclination i_p < 3.0deg, well below the inclination of the Vesta family. We computed that ~85% of the members of the Vesta dynamical family are V-type asteroids, and only 1-2% are primitive class asteroids and unlikely members of the family. This work almost doubles the sample of basaltic asteroid candidates in regions outside the Vesta family.

An Upper Limit of Mass for a Stable Minor Planet in the Main Asteroid Belt

The planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on Earth. We review the various mechanisms proposed for the origin of water on the terrestrial planets. Various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. Comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single Earth ocean of water this way. The most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. Two main scenarios are discussed in detail. First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. Second is the Grand Tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. Sufficient water is delivered to the terrestrial planets in both scenarios. While the Grand Tack scenario provides a better fit to most constraints, namely the small mass of Mars, planets may form too fast in the nominal case discussed here. This discrepancy may be reduced as a wider range of initial conditions is explored. Finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets.

The distribution of heavy elements is anomalously low in the asteroid main belt region compared with elsewhere in the solar system. Observational surveys also indicate a deficit in the number of small (≲50 km size) asteroids, which is two orders of magnitude lower than what is expected from the single power-law distribution that results from a collisional coagulation and fragmentation equilibrium. Here, we consider the possibility that a major fraction of the original asteroid population may have been cleared out by Jupiter’s secular resonance, as it swept through the main asteroid belt during the depletion of the solar nebula. This effect leads to the excitation of the asteroids’ orbital eccentricities. Concurrently, hydrodynamic drag and planet–disk tidal interaction effectively damp the eccentricities of sub-100 km-size and of super-lunar-size planetesimals, respectively. These combined effects lead to the asteroids’ orbital decay and clearing from the present-day main belt region (∼2.1–3.3 au). Eccentricity damping for the intermediate-size (50 to several hundreds of kilometers) planetesimals is less efficient than for small or large planetesimals. These objects therefore preferentially remain as main belt asteroids near their birthplaces, with modest asymptotic eccentricities. The smaller asteroids are the fragments of subsequent disruptive collisions at later times as suggested by the present-day asteroid families. This scenario provides a natural explanation for both the observed low surface density and the size distribution of asteroids in the main belt, without the need to invoke special planetesimal formation mechanisms. It also offers an explanation for the confined spatial extent of the terrestrial planet building blocks without the requirement of extensive migration of Jupiter, which is required in the grand-tack scenario.

NASA’s Deep Space 1 mission demonstrated that a spacecraft can be navigated autonomously during deep-space cruise operations using only optical navigation measurements. A methodology is developed to evaluate the feasibility and accuracy of the Deep Space 1 orbit determination approach throughout the solar system as a function of a spacecraft’s imaging capabilities. Feasibility can be addressed by comparing the apparent magnitudes of the known population of asteroids against the imaging system capabilities. An upper limit on the accuracy of the spacecraft position estimate at a given location can be formulated by assuming observations of at least two asteroids simultaneously. Example results are presented for three camera implementations that span the range of capabilities flown in deep space to date using orbit and absolute magnitude data for 50,129 of the brightest known asteroids. Broadly speaking, achievable accuracies range from approximately 200 to 12,000 km () interior to the main asteroid belt and from 50 to 2000 km within the main belt, depending strongly on the chosen camera implementation. Between the main belt and Jupiter, only a current state-of-the-art imaging system is consistently capable of kinematic positioning using only asteroids. Beyond Jupiter, there are insufficient known asteroids to support this approach without including images of planets and moons. Although these levels of accuracy are far inferior to those achievable with radiometric navigation, they may yet be sufficient to satisfy cruise-phase requirements for many deep-space missions.

We measure the mid-plane of the main asteroid belt by using the observational data of a nearly complete and unbiased sample of asteroids and find that it has inclination and longitude of ascending node (in J2000 ecliptic-equinox coordinate system). This plane differs significantly from previously published measurements, and it is also distinctly different than the solar system’s invariable plane as well as Jupiter’s orbit plane. The mid-plane of the asteroid belt is theoretically expected to be a slightly warped sheet whose local normal is controlled by the gravity of the major planets. Specifically, its inclination and longitude of ascending node varies with semimajor axis and time (on secular timescales) and is defined by the forced solution of secular perturbation theory; the ν 16 nodal secular resonance is predicted to cause a significant warp of the mid-plane in the inner asteroid belt. We test the secular theory by measuring the current location of the asteroids’ mid-plane in finer semimajor axis bins. We find that the measured mid-plane in the middle and outer asteroid belt is consistent, within the 3σ confidence level, with the prediction of secular perturbation theory, but a notable discrepancy is present in the inner asteroid belt near ∼2 au.

Preferably located in the outer main belt, D-type asteroids experienced less heating and represent an important population for studies on the origin and evolution of the asteroid belt, as well as the relations between asteroidal and cometary bodies. Their surface mineralogy is currently related to a mixture of organics, anhydrous silicates, opaque material and ice. However, like other taxonomic classes, a large spectral diversity can be seen among D-type objects. We use the Visible spectra of 100 D-type objects available in the literature to search for minor absorptions in those objects. The presence of minor absorptions around 0.6 and 0.8 microns is reported for a large number of objects in the sample. The presence of such bands is not related to the heliocentric distance of the objects, since the absorptions can be seen in the whole main belt, up to the the Trojans region.