Asteroid Belt Region Research Articles

Asteroids and Life: How Special Is the Solar System?

Abstract Asteroid impacts with Earth may have played an essential role in the emergence of life on Earth through their creation of favorable niches for life, changes to the atmosphere, and delivery of water. Consequently, we suggest two potential requirements for life in an exoplanetary system: first, that the system has an asteroid belt, and second, that there is a mechanism to drive asteroids to impact the terrestrial habitable planet. Since in the solar system the ν 6 secular resonance has been shown to have been important in driving these impacts, we explore how the masses and locations of two giant planets determine the location and strength of this secular resonance. Examining observed exoplanetary systems with two giant planets, we find that a secular resonance within the asteroid belt region may not be uncommon. Hence, the solar system is somewhat special, but the degree of fine-tuning that may be necessary for the emergence of life is not excessive. Finally, with n-body simulations, we show that when the two giant planets are close to the 2:1 mean motion resonance, the asteroid belt is unstable, but this does not lead to increased asteroid delivery.

Realistic collisional water transport during terrestrial planet formation

Context. According to the latest theoretical and isotopic evidence, Earth’s water content originates mainly from today’s asteroid belt region, or at least from the same precursor material. This suggests that water was transported inwards to Earth, and to similar planets in their habitable zone, via (giant) collisions of planetary embryos and planetesimals during the chaotic final phase of planet formation. Aims. In current dynamical simulations water delivery to terrestrial planets is still studied almost exclusively by assuming oversimplified perfect merging, even though water and other volatiles are particularly prone to collisional transfer and loss. To close this gap we have developed a computational framework to model collisional water transport by direct combination of long-term N-body computations with dedicated 3D smooth particle hydrodynamics (SPH) collision simulations of differentiated, self-gravitating bodies for each event. Methods. Post-collision water inventories are traced self-consistently in the further dynamical evolution, in accretionary or erosive as well as hit-and-run encounters with two large surviving bodies, where besides collisional losses, water transfer between the encountering bodies has to be considered. This hybrid approach enables us for the first time to trace the full dynamical and collisional evolution of a system of approximately 200 bodies throughout the whole late-stage accretion phase (several hundred Myr). As a first application we choose a Solar System-like architecture with already formed giant planets on either circular or eccentric orbits and a debris disk spanning the whole terrestrial planet region (0.5–4 au). Results. Including realistic collision treatment leads to considerably different results than simple perfect merging, with lower mass planets and water inventories reduced regularly by a factor of two or more. Due to a combination of collisional losses and a considerably lengthened accretion phase, final water content, especially with giant planets on circular orbits, is strongly reduced to more Earth-like values, and closer to results with eccentric giant planets. Water delivery to potentially habitable planets is dominated by very few decisive collisions, mostly with embryo-sized or larger objects and only rarely with smaller bodies, at least if embryos have formed throughout the whole disk initially. The high frequency of hit-and-run collisions and the differences to predominantly accretionary encounters, such as generally low water (and mass) transfer efficiencies, are a crucial part of water delivery, and of system-wide evolution in general.

Chondrule Formation by the Jovian Sweeping Secular Resonance

Abstract Chondrules are silicate spheroids found in meteorites, and they serve as important fossil records of the early solar system. In order to form chondrules, chondrule precursors must be heated to temperatures much higher than the typical conditions in the current asteroid belt. One proposed mechanism for chondrule heating is the passage through bow shocks of highly eccentric planetesimals in the protoplanetary disk in the early solar system. However, it is difficult for planetesimals to gain and maintain such high eccentricities. In this paper, we present a new scenario in which planetesimals in the asteroid belt region are excited to high eccentricities by the Jovian sweeping secular resonance in a depleting disk, leading to efficient formation of chondrules. We study the orbital evolution of planetesimals in the disk using semi-analytic models and numerical simulations. We investigate the dependence of eccentricity excitation on the planetesimal’s size, as well as the physical environment and the probability for chondrule formation. We find that 50–2000 km planetesimals can obtain eccentricities larger than 0.6 and cause effective chondrule heating. Most chondrules form in high-velocity shocks, in low-density gas, and in the inner disk. The fraction of chondrule precursors that become chondrules is about 4%–9% between 1.5 and 3 au. Our model implies that the disk depletion timescale is τ dep ≈ 1 Myr, comparable to the age spread of chondrules, and that Jupiter formed before chondrules, no more than 0.7 Myr after the formation of calcium aluminum inclusions.

The Delivery of Water During Terrestrial Planet Formation

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.

Asteroid impacts on terrestrial planets: the effects of super-Earths and the role of the ν6 resonance

With N-body simulations of a planetary system with an asteroid belt we investigate how the asteroid impact rate on the Earth is affected by the architecture of the planetary system. We find that the $\nu_6$ secular resonance plays an important role in the asteroid collision rate with the Earth. Compared to exoplanetary systems, the solar system is somewhat special in its lack of a super-Earth mass planet in the inner solar system. We therefore first consider the effects of the presence of a super-Earth in the terrestrial planet region. We find a significant effect for super-Earths with a mass of around $10\,\rm M_\oplus$ and a separation greater than about $0.7\,\rm AU$. For a super-Earth that is interior to the Earth's orbit, the number of asteroids colliding with Earth increases the closer the super-Earth is to the Earth's orbit. This is the result of multiple secular resonance locations causing more asteroids to be perturbed onto Earth-crossing orbits. When the super-Earth is placed exterior to Earth's orbit, the collision rate decreases substantially because the $\nu_6$ resonance no longer exists in the asteroid belt region. We also find that changing the semi-major axis of Saturn leads to a significant decrease in the asteroid collision rate, while increasing its mass increases the collision rate. These results may have implications for the habitability of exoplanetary systems.

Asteroid flux towards circumprimary habitable zones in binary star systems

Secular and mean motion resonances (MMR) are effective perturbations for shaping planetary systems. In binary star systems, they play a key role during the early and late phases of planetary formation, as well as for the dynamical stability of a planetary system. In this study, we aim to correlate the presence of orbital resonances with the rate of icy asteroids crossing the habitable zone (HZ), from a circumprimary disk of planetesimals in various binary star systems. We modelled a belt of small bodies in the inner and outer regions, interior and exterior to the orbit of a gas giant planet, respectively. The planetesimals are equally placed around a primary G-type star and move under the gravitational influence of the two stars and the gas giant. We numerically integrated the system for 50 Myr, considering various parameters for the secondary star. Its stellar type varies from a M- to F-type; its semimajor axis is either 50 au or 100 au, and its eccentricity is either 0.1 or 0.3. For comparison, we also varied the gas giant's orbital and physical parameters. Our simulations highlight that a disk of planetesimals will suffer from perturbations owing to a perturbed gas giant, mean motion, and secular resonances. We show that a secular resonance -- with location and width varying according to the secondary star's characteristics -- can exist in the icy asteroid belt region and overlap with MMRs which have an impact on the dynamical lifetime of the disk. In addition, we point out that in any case, the 2:1 MMR, the 5:3 MMR and the secular resonance are powerful perturbations for the flux of icy asteroids towards the HZ and the transport of water therein.

Binary formation in planetesimal disks. II. Planetesimals with a mass spectrum

Abstract Many massive objects have been found in the outer region of the solar system. How they were formed and evolved has not been well understood, although there have been intensive studies of the accretion process of terrestrial planets. One of the mysteries is the existence of binary planetesimals with near-equal mass components and highly eccentric orbits. These binary planetesimals are quite different from the satellites observed in the asteroid belt region. The ratio of the Hill radius to the physical radius of the planetesimals is much larger for the outer region of the disk, compared to the inner region of the disk. The Hill radius increases with the semimajor axis. Therefore, planetesimals in the outer region can form close and eccentric binaries, while those in the inner region would simply collide. In this paper, we carried out N-body simulations in different regions of the disk and studied whether binaries form in the outer region of the disk. We found that large planetesimals tend to form binaries. A significant fraction of large planetesimals are components of binaries. Planetesimals that become the components of binaries eventually collide with a third body, through three-body encounters. Thus, the existence of binaries can enhance the growth rate of planetesimals in the trans-Neptunian object region.

Populating the asteroid belt from two parent source regions due to the migration of giant planets—“The Grand Tack”

Abstract– The asteroid belt is found today in a dramatically different state than that immediately following its formation. It is estimated that it has been depleted in total mass by a factor of at least 1000 since its formation, and that the asteroids’ orbits evolved from having near‐zero eccentricity and inclination to the complex distributions we find today. The asteroid belt also hosts a wide range of compositions, with the inner regions dominated by S‐type and other water‐poor asteroids and the outer regions dominated by C‐type and other primitive asteroids. We discuss a model of early inner solar system evolution whereby the gas‐driven migration of Jupiter and Saturn brings them inwards to 1.5 AU, truncating the disk of planetesimals in the terrestrial planet region, before migrating outwards toward their current locations. This model, informally titled “The Grand Tack,” examines the planetary dynamics of the solar system bodies during the final million years of the gaseous solar nebula lifetime—a few million years (Myr) after the formation of the first solids, but 20–80 Myr before the final accretion of Earth, and approximately 400–600 Myr before the Late Heavy Bombardment of the inner solar system. The Grand Tack attempts to solve some outstanding problems for terrestrial planet formation, by reproducing the size of Mars, but also has important implications for the asteroid population. The migration of Jupiter causes a very early depletion of the asteroid belt region, and this region is then repopulated from two distinct source regions, one inside the formation region of Jupiter and one between and beyond the giant planets. The scattered material reforms the asteroid belt, producing a population the appropriate mass, orbits, and with overlapping distributions of material from each parent source region.

Collisional process on Comet 9/P Tempel 1: Mass loss of its dust and ice by impacts of asteroidal objects and its collisional history

We have studied the collisional process on the nucleus of Comet 9/P Tempel 1 (T1) and estimated the mass loss of surface materials due to the impacts of asteroidal objects. The mass loss rate is around ∼2×105 kg yr-1, which is smaller than that of water sublimation of ∼6×109 kg yr-1. We then estimated the number density of craters formed by asteroid impacts to infer the collisional history of T1. We found that the time necessary to accumulate the crater population on T1 is as long as ∼ a few 104 years, suggesting that T1 crossed the asteroid belt region more than ∼ a few 104 years ago, though this estimated time may be reduced to several thousands of years by taking into account the influence of the sublimation process on the crater size. Alternatively, the total period of the recent orbit, in which the perihelion distance is small enough to cause significant sublimation by erasing a large number of small craters, may be less than ∼2×103 years.

Dynamical erosion of the asteroid belt and implications for large impacts in the inner Solar System

The cumulative effects of weak resonant and secular perturbations by the major planets produce chaotic behavior of asteroids on long timescales. Dynamical chaos is the dominant loss mechanism for asteroids with diameters D ≳ 10 km in the current asteroid belt. In a numerical analysis of the long-term evolution of test particles in the main asteroid belt region, we find that the dynamical loss history of test particles from this region is well described with a logarithmic decay law. In our simulations the loss rate function that is established at t ≈ 1 Myr persists with little deviation to at least t = 4 Gyr . Our study indicates that the asteroid belt region has experienced a significant amount of depletion due to this dynamical erosion—having lost as much as ∼50% of the large asteroids—since 1 Myr after the establishment of the current dynamical structure of the asteroid belt. Because the dynamical depletion of asteroids from the main belt is approximately logarithmic, an equal amount of depletion occurred in the time interval 10–200 Myr as in 0.2–4 Gyr, roughly ∼30% of the current number of large asteroids in the main belt over each interval. We find that asteroids escaping from the main belt due to dynamical chaos have an Earth-impact probability of ∼0.3%. Our model suggests that the rate of impacts from large asteroids has declined by a factor of 3 over the last 3 Gyr, and that the present-day impact flux of D > 10 km objects on the terrestrial planets is roughly an order of magnitude less than estimates currently in use in crater chronologies and impact hazard risk assessments.

Concentration and sorting of chondrules and CAIs in the late Solar Nebula

The high concentration and sorting of chondrules, sub-mm sized spherules found in undifferentiated meteorites, is one of the great unsolved mysteries in planetology. Here we present a unifying explanation for these phenomena based on the assumption that chondrules were present when the Solar Nebula was optically thin but had a significant amount of gas. An immediate consequence is that chondrules feel a force known as photophoresis. Photophoresis is based on a temperature gradient over the surface of a particle resulting from absorption of radiation and non-uniform interaction with its gaseous environment. In comparison to well-known forces originating from starlight, i.e. radiation pressure, Poynting–Robertson drag, or the Yarkovski effect, photophoresis can be stronger by many orders of magnitude in gaseous environments. In the application discussed here photophoresis concentrates chondrules and CAIs, which are both found in chondrites, in the region of the asteroid belt. Chondrules from any place in the Solar Nebula will be dragged to the asteroid belt region, while smaller dust particles and their aggregates will be removed from this region at the same time. This leads to a high relative concentration of chondrules, sorted with respect to their thermal conductivity, density, and size, for building chondrite parent bodies. Furthermore, photophoresis prevents CAIs from being lost to the Sun.

Radial migration and dehydration of phyllosilicates in the solar nebula

While it is currently thought that Earth's water was delivered by hydrous minerals, the origin of these minerals is still debated. Nebular models suggest that the area where the Earth formed was too hot for these minerals to form, leading many to believe that they were delivered by large planetary embryos which formed in the outer asteroid belt region of the solar nebula. Others have argued that the hydrous minerals were present during the early accretion phase of the Earth in order to explain different aspects of its geochemistry and therefore, must have formed locally, implying that the nebula must have been cooler than the models predict. In this paper we explore a new possibility: that these hydrous minerals were formed in the outer asteroid belt region of the solar nebula and were then brought into the hotter regions of the nebula by gas drag where they were incorporated into the planetesimals which formed there. The hydrated minerals were able to survive for long periods of time in this hot region due to the sluggish dehydration kinetics. We find that this process need not have been efficient, requiring only a small amount (∼few percentages) of the material in the outer asteroid belt region of the nebula to be subject to this process. This delivery mechanism provides a way for hydrous minerals to be incorporated early on into the planetesimals which were accreted by the Earth without having to alter the generally accepted solar nebula models that are consistent with meteoritic and asteroidal observations.

Orbital Evolution of Asteroids during Depletion of the Solar Nebula

The pumping-up of inclinations and eccentricities caused by sweeping secular resonances in the asteroid belt is studied through numerical orbital integration and linear analysis. The present asteroids have large mean eccentricities and inclinations that cannot be explained by planetary perturbations alone. Sweeping of secular resonances associated with the depletion of the primitive solar nebula has been proposed for the origin of asteroids' orbits (e.g., by Ward, Colombo, and Franklin in 1976). We performed three-dimensional orbital integrations of asteroids under gravitational forces by Jupiter, Saturn, and the solar nebula. The asteroids' motions are also affected by hydrodynamic gas drag. We consider three types of nebula depletion models: (1) the uniform depletion model, in which the nebula density decreases exponentially with time and uniformly throughout the nebula; (2) the inside-out depletion model, in which nebula gas is depleted from the inside region; (3) the gap-opening model, in which a gap centered at Jupiter gradually expands. Previous studies have concentrated on the two-dimensional uniform depletion model. Our simulation shows that inclinations of asteroids are not pumped up enough in the first model to account for the observed magnitude.Moreover, most asteroids spiral into the Sun by gas drag if the depletion timescale is longer than 105 yr because pumped-up eccentricity induces strong gas drag. On the other hand, in the second and third models, inclinations are pumped up. Our linear analysis shows that the nonuniform depletion model is essential for the secular resonances pumping up inclination to sweep in the asteroid belt. In the case of the inside-out depletion model, both eccentricity and inclination are pumped up enough to be consistent with the observed magnitude in the entire asteroid belt, if the nebula depletion timescale (which is the time required for the nebula edge to migrate by the distance of 5 AU) is longer than 3 × 105 yr. Furthermore, since secular resonances pass after nebula gas has already been depleted in the passing region, gas drag does not damp the pumped-up eccentricities and inclinations and the semimajor axes of the asteroids. Therefore, the resultant eccentricity and inclination in the inside-out depletion model are consistent with observed ones for the nebula depletion time inferred from observed T Tauri stars (106-107 yr). Our gap-opening model shows similar results, although eccentricity and inclination are slightly smaller in the outer asteroid belt region. The essentially required condition to pump up both eccentricity and inclination highly enough in the asteroid region is that the nebula edge moves outward from 5 to 10 AU. The gap formation caused by Jupiter's tidal perturbations may be similar to our inside-out or gap-opening models, inside 10 AU. Therefore, it would be responsible for the high eccentricities and inclinations in the asteroid belt.

Panel discussion: does chemical evidence give diagnostic tests for the credibility of physical models of the origin of the Solar System?

M. M. Woolfson. Under the conditions of the capture theory, planets were originally formed in highly eccentric orbits which were close to, but not exactly coplanar. A resisting medium rounded off these orbits but, because it produced a non-central gravitational force on the planets, it also caused their orbits to precess. Differential precession gave intersecting orbits from time to time and it is possible to compute characteristic times for major interactions between pairs of planets. It turns out that these are similar to the rounding-off times and it can be concluded that some major event in the early Solar System was more likely than not. In 1977 Dormand and I postulated a planetary collision in the asteroid-belt region. Such a model readily explains the known characteristics of asteroids and meteorites, especially as a wide range of thermal regimes was present during the collision event. Other Solar System features that could be explained in a very straightforward way included the terrestrial planets, irregular satellites, e.g. the Moon and Triton, and the origin of cometary material.