Linking planetary embryo formation to planetesimal formation I: The impact of the planetesimal surface density in the terrestrial planet zone

The growth time scales of planetary embryos and their formation process are imperative for our understanding on how planetary systems form and develop. They determine the subsequent growth mechanisms during the life stages of a circumstellar disk. We quantify the timescales and spatial distribution of planetary embryos via collisional growth and fragmentation of dynamically forming 100km sized planetesimals. In our study, the formation timescales of viscous disk evolution and planetesimal formation are linked to the formation of planetary embryos in the terrestrial planet zone. We connect a one dimensional model for viscous gas evolution, dust and pebble dynamics and pebble flux regulated planetesimal formation to the N-body code LIPAD. Our framework enables us to study the formation, growth, fragmentation and evolution of planetesimals with an initial size of 100km in diameter for the first million years of a viscous disk. Our study shows the effect of the planetesimal surface density evolution on the preferential location and timescales of planetary embryo formation. A one dimensional analytically derived model for embryo formation based on the local planetesimal surface density evolution is presented. This model manages to reproduce the spatial distribution, formation rate and total number of planetary embryos at a fraction of the computational cost of the N-body simulations. The formation of planetary embryos in the terrestrial planet zone occurs simultaneously to the formation of planetesimals. The local planetesimal surface density evolution and the orbital spacing of planetary embryos in the oligarchic regime serve well as constraints to model planetary embryo formation analytically. Our embryo formation model will be a valuable asset in future studies regarding planet formation

Linking planetary embryo formation to planetesimal formation II: The impact of pebble accretion in the terrestrial planet zone

The accretion of pebbles on planetary cores has been widely studied in recent years and is found to be a highly effective mechanism for planetary growth. While most studies assume planetary cores as an initial condition in their simulation, the question how, where and when these cores form is often neglected. We study the impact of pebble accretion during the formation phase and subsequent evolution of planetary embryos in the early stages of circumstellar disk evolution. In doing so we aim to quantify the timescales and local dependency of planetary embryo formation, based on the solid evolution of the disk. We connect a one dimensional two population model for solid evolution and pebble flux regulated planetesimal formation to the N-body code LIPAD. In our study we focus on the growth of planetesimals with an initial size of 100 km in diameter by planetesimal collisions and pebble accretion for the first 1 million years of a viscously evolving disk. We compare 18 different N-body simulations in which we vary the total planetesimal mass after 1 million years, the surface density profile of the planetesimal disk, the radial pebble flux and the possibility of pebble accretion. Pebble accretion leads to the formation of fewer, but substantially more massive embryos. The area of possible embryo formation is weakly influenced by the accretion of pebbles and the innermost embryos tend to form slightly earlier compared to the simulations in which pebble accretion is neglected. Pebble accretion strongly enhances the formation of super earths in the terrestrial planet region, but it does not enhance the formation of embryos at larger distances

Isotopic Trichotomy of Main Belt Asteroids from Implantation of Outer Solar System Planetesimals

Recent analyses of samples from asteroid (162173) Ryugu returned by JAXA's Hayabusa2 mission suggest that Ryugu and CI chondrites formed in the same region of the protoplanetary disk, in a reservoir that was isolated from the source regions of other carbonaceous (C-type) asteroids. Here we conduct $N$-body simulations in which CI planetesimals are assumed to have formed in the Uranus/Neptune zone at $\sim15$--25 au from the Sun. We show that CI planetesimals are scattered by giant planets toward the asteroid belt where their orbits can be circularized by aerodynamic gas drag. We find that the dynamical implantation of CI asteroids from $\sim15$--25 au is very efficient with $\sim 5$\% of $\sim 100$-km planetesimals reaching stable orbits in the asteroid belt by the end of the protoplanetary gas disk lifetime. The efficiency is reduced when planetesimal ablation is accounted for. The implanted population subsequently evolved by collisions and was depleted by dynamical instabilities. The model can explain why CIs are isotopically distinct from other C-type asteroids which presumably formed at $\sim5$--10 au.Comment: EPSL, in pres

Implications of Jupiter Inward Gas-Driven Migration for the Inner Solar System

The migration history of Jupiter in the sun's natal disk remains poorly constrained. Here we consider how Jupiter's migration affects small-body reservoirs and how this constrains its original orbital distance from the Sun. We study the implications of large-scale and inward radial migration of Jupiter for the inner solar system while considering the effects of collisional evolution of planetesimals. We use analytical prescriptions to simulate the growth and migration of Jupiter in the gas disk. We assume the existence of a planetesimal disk inside Jupiter's initial orbit. This planetesimal disk received an initial total mass and size-frequency distribution (SFD). Planetesimals feel the effects of aerodynamic gas drag and collide with one another, mostly while shepherded by the migrating Jupiter. Our main goal is to measure the amount of mass in planetesimals implanted into the main asteroid belt (MAB) and the SFD of the implanted population. We also monitor the amount of dust produced during planetesimal collisions. We find that the SFD of the planetesimal population implanted into the MAB tends to resemble that of the original planetesimal population interior to Jupiter. We also find that unless very little or no mass existed between 5 au and Jupiter's original orbit, it would be difficult to reconcile the current low mass of the MAB with the possibility that Jupiter migrated from distances beyond 15 au. This is because the fraction of the original disk mass that gets implanted into the MAB is very large. Finally, we discuss the implications of our results in terms of dust production to the so-called NC-CC isotopic dichotomy.Comment: Accepted for publication in The Astrophysical Journal Letters; In pres

Identification of a 4.3 billion year old asteroid family and planetesimal population in the Inner Main Belt

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

A race against the clock: Constraining the timing of cometary bombardment relative to Earth's growth

Comets are considered a potential source of inner solar system volatiles, but the timing of this delivery relative to that of Earth's accretion is still poorly understood. Measurements of xenon isotopes in comet 67P/Churyumov-Gerasimenko revealed that comets partly contributed to the Earth's atmosphere. However, there is no conclusive evidence of a significant cometary component in the Earth's mantle. These geochemical constraints would favour a contribution of comets mainly occurring after the last stages of Earth's formation. Here, we evaluate whether dynamical simulations satisfy these constraints in the context of an Early Instability model. We perform dynamical simulations of the solar system, calculate the probability of collision between comets and Earth analogs component embryos through time and estimate the total cometary mass accreted in Earth analogs as a function of time. While our results are in excellent agreement with geochemical constraints, we also demonstrate that the contribution of comets on Earth might have been delayed with respect to the timing of the instability, due to a stochastic component of the bombardment. More importantly, we show that it is possible that enough cometary mass has been brought to Earth after it had finished forming so that the xenon constraint is not necessarily in conflict with an Early Instability scenario. However, it appears very likely that a few comets were delivered to Earth early in its accretion history, thus contributing to the mantle's budget. Finally, we compare the delivery of cometary material on Earth to Venus and Mars. These results emphasize the stochastic nature of the cometary bombardment in the inner solar system.Comment: 26 pages, 12 figure

Early Bombardment of the Moon: Connecting the Lunar Crater Record to the Terrestrial Planet Formation

The lunar crater record features $\sim 50$ basins. The radiometric dating of Apollo samples indicates that the Imbrium basin formed relatively late -- from the planet formation perspective -- some $\simeq 3.9$ Ga. Here we develop a dynamical model for impactors in the inner solar system to provide context for the interpretation of the lunar crater record. The contribution of cometary impactors is found to be insignificant. Asteroids produced most large impacts on the terrestrial worlds in the last $\simeq 3$ Gyr. The great majority of early impactors were rocky planetesimals left behind at $\sim 0.5$--1.5 au after the terrestrial planet accretion. The population of terrestrial planetesimals was reduced by disruptive collisions in the first $t \sim 20$ Myr after the gas disk dispersal. We estimate that there were $\sim 4 \times 10^5$ diameter $d>10$ km bodies when the Moon formed (total planetesimal mass $\sim 0.015$ $M_{\rm Earth}$ at $t \sim 50$ Myr). The early bombardment of the Moon was intense. To accommodate $\sim 50$ known basins, the lunar basins that formed before $\simeq 4.35$--4.41 Ga must have been erased. The late formation of Imbrium occurs with a $\sim 15$--35\% probability in our model. About 20 $d>10$-km bodies were expected to hit the Earth between 2.5 and 3.5 Ga, which is comparable to the number of known spherule beds in the late Archean. We discuss implications of our model for the lunar/Martian crater chronologies, Late Veneer, and noble gases in the Earth atmosphere.Comment: Icarus, in pres

Born extra-eccentric: A broad spectrum of primordial configurations of the gas giants that match their present-day orbits

In a recent paper we proposed that the giant planets' primordial orbits may have been eccentric (~0.05), and used a suite of dynamical simulations to show outcomes of the giant planet instability that are consistent with their present-day orbits. In this follow-up investigation, we present more comprehensive simulations incorporating superior particle resolution, longer integration times, and eliminating our prior means of artificially forcing instabilities to occur at specified times by shifting a planets' position in its orbit. While we find that the residual phase of planetary migration only minimally alters the the planets' ultimate eccentricities, our work uncovers several intriguing outcomes in realizations where Jupiter and Saturn are born with extremely large eccentricities (~0.10 and ~0.25, respectively). In successful simulations, the planets' orbits damp through interactions with the planetesimal disk prior to the instability, thus loosely replicating the initial conditions considered in our previous work. Our results therefore suggest an even wider range of plausible evolutionary pathways are capable of replicating Jupiter and Saturn's modern orbital architecture.Comment: 12 pages, 3 figures, 2 tables, accepted for publication in Icaru