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Probing the history of Solar System through the cratering records on Vesta and Ceres
Abstract
Through its connection with HED meteorites, Vesta is known as one of the
first bodies to have accreted and differentiated in the Solar Nebula, predating
the formation of Jupiter and surviving the violent evolution of the early Solar
System. The formation time of Ceres instead is unknown, but it should not
postdate that of Jupiter by far. In this work we modelled the collisional
histories of Vesta and Ceres at the time of the formation of Jupiter, assumed
to be the first giant planet to form. In this first investigation of the
evolution of the early Solar System, we did not include the presence of
planetary embryos in the disk of planetesimals but we concentrated on the role
of the forming Jupiter and the effects of its possible inward migration due to
disk-planet interactions. Our results clearly indicate that the formation of
the giant planet caused an intense early bombardment in the orbital region of
the Main Asteroid Belt. According to our results, Vesta and Ceres would not
have survived the Jovian early bombardment if the disk was populated mainly by
large planetesimals like those predicted to form in turbulent circumstellar
disks. Disks dominated by small bodies, like those predicted to form in
quiescent circumstellar disks, or with a varying fraction of the mass in the
form of larger (D \geq 100 km) planetesimals represent more favourable
environments for the survival of the two asteroids. In those scenarios where
they survive, both asteroids had their surfaces saturated by craters as big as
150 km and a few as big as 200 - 300 km. In the case of Vesta, the Jovian early
bombardment would have significantly eroded (locally or globally) the crust and
possibly caused effusive phenomena similar to the lunar maria, whose
crystallisation time would then be directly linked to the time of the formation
of Jupiter.
... We simulated a giant planet that forms and migrates on the disk midplane (i.e., on an orbit with inclination i = 0) over a timescale of 3 Myr, growing from a planetary embryo to a Jupiter-like giant planet. We adopted a two-phase approach to model both the mass growth and the evolution of the physical radius of the planet, following the growth tracks from Lissauer et al. (2009);Bitsch et al. (2015), and D' Angelo et al. (2021) using the parametric approach from Turrini et al. (2011Turrini et al. ( , 2019. In the first 2 Myr, the planet accretes its core and extended atmosphere and grows from an initial mass of M 0 = 0.1 M ⊕ (mass of the planetary embryo) to a critical mass of M c = 30 M ⊕ , equally shared between the core and the atmosphere. ...
... During the first phase of core growth, the planetary mass grows from a Mars-like planetary embryo (M 0 = 0.1 M ⊕ ) to a critical value of M c = 30 M ⊕ , following the growth curve (Turrini et al. 2011): ...
... During the second phase of runaway gas accretion, the planetary mass evolves as (Turrini et al. 2011): Shibata et al. 2020;Turrini et al. 2021 ...
Giant planets can interact with multiple and chemically diverse environments in protoplanetary disks while they form and migrate to their final orbits. The way this interaction affects the accretion of gas and solids shapes the chemical composition of the planets and of their atmospheres. Here we investigate the effects of different chemical structures of the host protoplanetary disk on the planetary composition. We consider both scenarios of molecular (inheritance from the prestellar cloud) and atomic (complete chemical reset) initial abundances in the disk. We focus on four elemental tracers of different volatility: C, O, N, and S. We explore the entire extension of possible formation regions suggested by observations by coupling the disk chemical scenarios with N -body simulations of forming and migrating giant planets. The planet formation process produces giant planets with chemical compositions significantly deviating from that of the host disk. We find that the C/N, N/O, and S/N ratios follow monotonic trends with the extent of migration. The C/O ratio shows a more complex behavior, dependent on the planet accretion history and on the chemical structure of the formation environment. The comparison between S/N* and C/N* (where * indicates normalization to the stellar value), constrains the relative contribution of gas and solids to the total metallicity. Giant planets whose metallicity is dominated by the contribution of the gas are characterized by N/O* > C/O* > C/N* and allow to constrain the disk chemical scenario. When the planetary metallicity is instead dominated by the contribution of the solids we find that C/N* > C/O* > N/O*.
... However, compositional studies of the asteroid belt (DeMeo and Carry, 2014;Michtchenko et al., 2016) disagree on whether an extensive migration of the giant planets is consistent with the current radial distribution of the different kinds of asteroids. On the other hand, the very mass growth of the giant planets was shown to also be capable of triggering phases of dynamical excitation and radial mixing of the planetesimals even in absence of migration (see Fig. 1 and Turrini et al. 2011Turrini et al. , 2012Turrini 2014;Turrini & Svetsov 2014;Turrini et al. 2015;Raymond & Izidoro 2017). This ambiguity in the early history of the giant planets severely hinders our understanding of the formation of the Solar System. ...
... In exploring the working of the astrochemical constraints provided by these three compositional characteristics, we will consider a proof-of-concept case study focusing on the collisional evolution of primordial Vesta across Jupiter's mass growth in different migration scenarios for the giant planet (the event also labelled as Jovian Early Bombardment or JEB, see Fig. 1 and Turrini et al. 2011Turrini et al. , 2012Turrini 2014;Turrini & Svetsov 2014;Turrini et al. 2015). This case study has been selected as it allows us to reuse previous simulations and results to explore the sensitivity of these astrochemical constraints to a number of physical parameters (namely flux, physical characteristics, size distribution and impact velocity distribution of the impactors and the mass of the primordial Vesta). ...
... In parallel, diogenites indicate that the underlying lower crust slowly solidified over tens of Myr (see McSween et al. 2011 and references therein). Because of the timing of Jupiter's formation mentioned above (i.e. the first ∼2 Myr after Vesta's differentiation) and of the duration of the bombardment it triggered (∼1 Myr, Turrini et al. 2011Turrini et al. , 2012, across the JEB both the eucritic and the diogenitic layers were in a partially molten state (see e.g. Formisano et al. 2013;Tkalcec et al. 2013 for the results of thermal and geophysical models and McSween et al. 2011;Greenwood et al. 2014;Steenstra et al. 2016;Roszjar et al. 2016 for the meteoritic evidences). ...
For decades the limited thickness of Vesta's basaltic crust, revealed by the link between the asteroid and the howardite-eucrite-diogenite family of meteorites, and its survival to collisional erosion offered an important constraint for the study of the early evolution of the Solar System. Some results of the Dawn mission, however, cast doubts on our understanding of Vesta's interior composition and of the characteristics of its basaltic crust, weakening this classical constraint. In this work we investigate the late accretion and erosion experienced by Vesta's crust after its differentiation and recorded in the composition of eucrites and diogenites and show that it offers an astrochemical window into the earliest evolution of the Solar System. In our proof-of-concept case study focusing on the late accretion and erosion of Vesta's crust during the growth and migration of Jupiter, the water enrichment of eucrites appears to be a sensitive function of Jupiter's migration while the enrichment in highly-siderophile elements of diogenites appears to be particularly sensitive to the size-frequency distribution of the planetesimals. The picture depicted by the enrichments created by late accretion in eucrites and diogenites is not qualitatively affected by the uncertainty on the primordial mass of Vesta. Crustal erosion, instead, is more significantly affected by said uncertainty and Vesta's crust survival appears to be mainly useful to study violent collisional scenarios where highly energetic impacts can strip significant amounts of vestan material while limitedly contributing to Vesta's late accretion. Our results suggest that the astrochemical record of the late accretion and erosion of Vesta's crust provided by eucrites and diogenites can be used as a tool to investigate any process or scenario associated to the evolution of primordial Vesta and of the early Solar System.
... In order to assess the collisional probability of the pre-LHB asteroids on Vesta, we applied the method developed by Turrini et al. (2011), i.e. we used a statistical approach based on solving the ray-torus intersection problem between the torus built on the osculating orbit of Vesta and the linearised path of a massless particle across a timestep. The basic idea is that the probability that both the impacting body and Vesta will occupy the same spatial region at the same time is the collision probability that we search. ...
... This probability can be evaluated as the ratio between the effective collisional time (T ef f ) and the orbital period of the asteroid 4 Vesta (T V ). Following Turrini et al. (2011), the effective collisional time is the amount of time available for the collision and it is evaluated as the minimum between the time spent by Vesta (τ V ) and the time spent by impacting particle (τ P ) into the crossed region of the torus. So the impact probability is given by: ...
... In particular, the survival of the relatively thin basaltic crust put strong constraints to the collisional evolution of the asteroid during the life of the Solar System. Using the collisional model developed by Turrini et al. (2011Turrini et al. ( , 2012 to study the pre-LHB evolution of Vesta we simulated the impact history of the asteroid during the Late Heavy Bombardment. ...
Vesta is the only currently identified asteroid for which we possess samples, which revealed us that the asteroid is differentiated and possesses a relatively thin basaltic crust that survived to the evolution of the asteroid belt and the Solar System. However, little is know about the effects of past events like the Late Heavy Bombardment on this crust. We address this gap in our knowledge by simulating the LHB in the different dynamical scenarios proposed for the migration of the giant planets in the broad framework of the Nice Model. The results of simulations generate information about produced crater population, surface saturation, mass loss and mass gain of Vesta and number of energetic or catastrophic impacts during LHB. Our results reveal that planet-planet scattering is a dynamically favourable migration mechanism for the survival of Vesta and its crust. The number of impacts on Vesta estimated as due to the LHB is $31\pm5$, i.e. about 5 times larger than the number of impacts that would have occurred in an unperturbed main belt in the same time interval. The contribution of a possible extended belt, instead, is quite limited and can be quantified in $2\pm1$ impacts. The chance of energetic and catastrophic impacts is less than 10\% and is compatible with the absence of giant craters dated back to 4 Ga ago and with the survival of the asteroid during the LHB. The mass loss translates in the erosion of $3-5$ meters of the crust, consistently with the global survival of the basaltic crust of Vesta confirmed by the Dawn mission. Our analysis revealed that the contribution of the LHB to the cratering of Vesta' surface is not significant and is actually erased by the crater population produced by the following 4 Ga of collisional evolution of the asteroid, in agreement with the data provided by the Dawn mission.
... Therefore, the basaltic (eucritic) crust should have completely solidified somewhere between the first 3-10 Ma. In order to reach the molten layer toward the end of this temporal interval (e.g., when the solid crust was about 15 km thick) an impact would cause the formation of a ∼200 km wide crater, with major implications for the survival of the basaltic crust (e.g., the effusion of diogenite-rich magma and the formation of diogenitic terrains units, [17,18]) that have not been observed by the Dawn mission [1,2]. As a consequence, whatever mechanism delivered the volatile elements incorporated in the eucritic samples studied by [4], it should have acted during the first few Ma of the life of Vesta and it should have preserved the basaltic crust of the asteroid, which we know survived until present time [1,2]. ...
... The giant planets of the Solar System should have formed across this timespan (see [21] for an overview on the processes and the timescales governing the formation of giant planets) and, in particular, theoretical [22,23] and observational [24] arguments suggest that Jupiter formed 3-5 Ma after the condensation of CAIs. The formation of Jupiter has been shown by different authors [17,18,[25][26][27][28] to trigger a sudden spike in the flux of impactors in the early history of the Solar System. This event, named the Jovian Early Bombardment ( [17,28], JEB in the following), is caused by the scattering of ice-rich planetesimals from the outer Solar System due to the gravitational perturbation of the giant planet [17,[25][26][27][28] and by the appearance of the Jovian mean motion resonances in the asteroid belt, in particular the 3:1 and 2:1 resonances [17,18,27,28]. ...
... The formation of Jupiter has been shown by different authors [17,18,[25][26][27][28] to trigger a sudden spike in the flux of impactors in the early history of the Solar System. This event, named the Jovian Early Bombardment ( [17,28], JEB in the following), is caused by the scattering of ice-rich planetesimals from the outer Solar System due to the gravitational perturbation of the giant planet [17,[25][26][27][28] and by the appearance of the Jovian mean motion resonances in the asteroid belt, in particular the 3:1 and 2:1 resonances [17,18,27,28]. The duration of the JEB is limited to about 1 Ma [17,26,28], with the bulk of the impacts taking place in the first (3-5) × 10 5 years [17]. ...
The asteroid (4) Vesta, parent body of the Howardite-Eucrite-Diogenite meteorites, is one of the first bodies that formed, mostly from volatile-depleted material, in the Solar System. The Dawn mission recently provided evidence that hydrated material was delivered to Vesta, possibly in a continuous way, over the last 4 Ga, while the study of the eucritic meteorites revealed a few samples that crystallized in presence of water and volatile elements. The formation of Jupiter and probably its migration occurred in the period when eucrites crystallized, and triggered a phase of bombardment that caused icy planetesimals to cross the asteroid belt. In this work, we study the flux of icy planetesimals on Vesta during the Jovian Early Bombardment and, using hydrodynamic simulations, the outcome of their collisions with the asteroid. We explore how the migration of the giant planet would affect the delivery of water and volatile materials to the asteroid and we discuss our results in the context of the geophysical and collisional evolution of Vesta. In particular, we argue that the observational data are best reproduced if the bulk of the impactors was represented by 1–2 km wide planetesimals and if Jupiter underwent a limited (a fraction of au) displacement.
... Vesta Turrini, 2013) and, more generally, the asteroid belt (Turrini et al., 2012) underwent a phase of enhanced collisional evolution at the time of the formation of Jupiter. Turrini et al. (2011) and Turrini (2013) showed that the flux of impactors on Vesta could amount, in mass, to up to ∼10% of the present mass of the asteroid. However, as discussed by Turrini (2013), at that time Vesta still possessed a limited solid crust overlying a mostly molten interior (see e.g. ...
... As the behavior of cometary impactors and asteroidal impactors is not necessarily comparable and for Vesta it is unlikely that the re-condensation of vapors played a significant role in the contamination of the surface of the asteroid (see e.g. Turrini et al. 2011), we focused only on the single value of the average retention efficiency reported for asteroidal impactors. To estimate the difference between the results of Ong et al. (2010) and those of Svetsov (2011), we computed the average retained mass fraction for the Moon using the scaling laws reported by Svetsov (2011). ...
... The assessment of the mass loss of Vesta from the formation of its basaltic crust to now is important to understand the global picture of the erosion of the surface of the asteroid (see e.g. McSween et al. 2013 for a discussion concerning Rheasilvia and Turrini 2013 for a more general one) and can play a significant role in unveiling the history of the asteroid belt and the Solar System Turrini et al., 2011Turrini et al., , 2012Turrini, 2013). ...
The Dawn spacecraft observed the presence of dark material, which in turn
proved to be associated with OH and H-rich material, on the surface of Vesta.
The source of this dark material has been identified with the low albedo
asteroids, but it is still a matter of debate whether the delivery of the dark
material is associated with a few large impact events, to micrometeorites or to
the continuous, secular flux of impactors on Vesta. The continuous flux
scenario predicts that a significant fraction of the exogenous material
accreted by Vesta should be due to non-dark impactors likely analogous to
ordinary chondrites, which instead represent only a minor contaminant in the
HED meteorites. We explored the continuous flux scenario and its implications
for the composition of the vestan regolith, taking advantage of the data from
the Dawn mission and the HED meteorites. We used our model to show that the
stochastic events scenario and the micrometeoritic flux scenario are natural
consequences of the continuous flux scenario. We then used the model to
estimate the amounts of dark and hydroxylate materials delivered on Vesta since
the LHB and we showed how our results match well with the values estimated by
the Dawn mission. We used our model to assess the amount of Fe and siderophile
elements that the continuous flux of impactors would mix in the vestan
regolith: concerning the siderophile elements, we focused our attention on the
role of Ni. The results are in agreement with the data available on the Fe and
Ni content of the HED meteorites and can be used as a reference frame in future
studies of the data from the Dawn mission and of the HED meteorites. Our model
cannot yet provide an answer to the fate of the missing non-carbonaceous
contaminants, but we discuss possible reasons for this discrepancy.
... A temporal interval not covered by the studies of Bottke et al. (2005a,b) is the one going from the beginning of the formation of Jupiter to the end of that of Saturn. The formation of Jupiter has been shown by different authors (Safronov, 1972;Weidenschilling, 1975;Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012 to trigger a sudden spike in the flux of impactors in the early history of the Solar System. This event, named the Jovian Early Bombardment (Turrini et al. , 2012, JEB in the following), is caused by the scattering of icerich planetesimals from the outer Solar System due to the gravitational perturbation of the giant planet (Safronov, 1972;Weidenschilling, 1975;Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012 and by the appearance of the Jovian mean motion resonances in the asteroid belt, in particular the 3:1 and 2:1 resonances (Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012. ...
... The formation of Jupiter has been shown by different authors (Safronov, 1972;Weidenschilling, 1975;Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012 to trigger a sudden spike in the flux of impactors in the early history of the Solar System. This event, named the Jovian Early Bombardment (Turrini et al. , 2012, JEB in the following), is caused by the scattering of icerich planetesimals from the outer Solar System due to the gravitational perturbation of the giant planet (Safronov, 1972;Weidenschilling, 1975;Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012 and by the appearance of the Jovian mean motion resonances in the asteroid belt, in particular the 3:1 and 2:1 resonances (Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012. The duration of the JEB is limited to about 1 Ma (Weidenschilling, 1975;Turrini et al., 2011Turrini et al., , 2012, with the bulk of the impacts taking place in the first 3 − 5 × 10 5 years . ...
... The formation of Jupiter has been shown by different authors (Safronov, 1972;Weidenschilling, 1975;Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012 to trigger a sudden spike in the flux of impactors in the early history of the Solar System. This event, named the Jovian Early Bombardment (Turrini et al. , 2012, JEB in the following), is caused by the scattering of icerich planetesimals from the outer Solar System due to the gravitational perturbation of the giant planet (Safronov, 1972;Weidenschilling, 1975;Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012 and by the appearance of the Jovian mean motion resonances in the asteroid belt, in particular the 3:1 and 2:1 resonances (Weidenschilling et al., 2001;Turrini et al., 2011Turrini et al., , 2012. The duration of the JEB is limited to about 1 Ma (Weidenschilling, 1975;Turrini et al., 2011Turrini et al., , 2012, with the bulk of the impacts taking place in the first 3 − 5 × 10 5 years . ...
This work explores the implications of the Jovian Early Bombardment (JEB) for
the evolution of the primordial Vesta, in particular in terms of crater
saturation, crustal excavation and surface erosion. Both scenarios assuming the
planetesimals having formed in a quiescent or a turbulent nebula were explored
and both primordial and collisionally evolved size-frequency distributions were
considered. The results obtained indicate that, if the basaltic surface of
Vesta were already formed, the JEB would saturate it with craters and could
erode it to depths that vary from hundreds of meters to tens of kilometres. In
the latter cases, the surface erosion caused by the JEB would be comparable
with the thickness of the eucritic and diogenitic layers of Vesta. In the cases
where the global surface erosion is limited, however, large impactors, if too
abundant, can excavate the whole crust and extract significant quantities of
material from the vestan mantle, incompatible with the present understanding of
HED meteorites. This appears to be the case if the impacting planetesimals
formed in a turbulent nebula and Jupiter migrated by 0.5 AU or more. Globally,
the results obtained suggest that the scenarios where the planetesimal formed
in a quiescent nebula and Jupiter underwent a modest migration (i.e. up to 0.5
AU) are the most consistent with our understanding of Vesta, even if the cases
of planetesimals formed in a turbulent nebula with Jupiter undergoing limited
(i.e. about 0.25 AU) or no migration cannot be ruled out. Recent results on the
differentiation of the asteroid, however, raised the possibility that Vesta
originally possessed a now-lost undifferentiated crust. In this case, the
favoured scenarios would be those where the planetesimals formed in a quiescent
nebula and Jupiter underwent a more significant migration (i.e. between 0.5 AU
and 1 AU).
... The growth of the giant planet in each n-body simulation follows the growth tracks from Lissauer et al. (2009), Bitsch et al. (2015, and D' Angelo et al. (2021) using the parametric approach from Turrini et al. (2011Turrini et al. ( , 2019. For consistency with these works, we adopted a common formation time of 3 Myr for all giant planets in our simulations. ...
... Following the approach described in Turrini et al. (2011Turrini et al. ( , 2012, Turrini (2014), and Turrini & Svetsov (2014), from the knowledge of their respective formation regions, we can associate each dynamical tracer impacting on the giant planet with a mass flux of planetesimals. Specifically, integrating the solids surface density profile from Equation (17) over a given orbital annular region of the disk allows for the computation of the total mass of solids, and hence planetesimals, it contains. ...
The composition of giant planets is imprinted by their migration history and the compositional structure of their hosting disks. Studies in recent literature have investigated how the abundances of C and O can constrain the formation pathways of giant planets forming within few tens of au from a star. New ALMA observations, however, suggest planet-forming regions possibly extending to hundreds of au. We explore the implications of these wider formation environments through n-body simulations of growing and migrating giant planets embedded in planetesimal disks, coupled with a compositional model of the protoplanetary disk where volatiles are inherited from the molecular cloud and refractories are calibrated against extrasolar and Solar System data. We find that the C/O ratio provides limited insight on the formation pathways of giant planets that undergo large-scale migration. This limitation can be overcome, however, thanks to nitrogen and sulfur. Jointly using the C/N, N/O, and C/O ratios breaks any degeneracy in the formation and migration tracks of giant planets. The use of elemental ratios normalized to the respective stellar ratios supplies additional information on the nature of giant planets, thanks to the relative volatility of O, C, and N in disks. When the planetary metallicity is dominated by the accretion of solids C/N* > C/O* > N/O* (* denoting this normalized scale), otherwise N/O* > C/O* > C/N*. The S/N ratio provides an additional independent probe into the metallicity of giant planets and their accretion of solids.
... Across their formation, however, giant planets drastically alter the dynamical equilibrium of the surrounding planetesimals by exciting their orbits, a process that acts in response to the mass growth of the giant planets independently of whether they migrate or not (Turrini et al. 2011(Turrini et al. , 2012(Turrini et al. , 2015(Turrini et al. , 2018Turrini 2014;Turrini & Svetsov 2014;Raymond & Izidoro 2017). This phase of dynamical excitation was shown to greatly enhance the collisional activity among the planetesimals (Turrini et al. 2012). ...
... The formation of the giant planets was assumed to occur on a relatively short timescale (Lambrechts & Johansen 2012;Bitsch et al. 2015), and their mass growth was modeled using the numerical approach from Turrini et al. (2011). During the first τ c =10 6 yr of the simulations, the giant planets accreted their cores, whose masses grew from an initial value of After the critical mass value M c was reached, the mass growth of each giant planet during the subsequent gas accretion phase was modeled as ...
The amount of dust present in circumstellar disks is expected to steadily decrease with age due to the growth from μ m-sized particles to planetesimals and planets. Mature circumstellar disks, however, can be observed to contain significant amounts of dust and possess high dust-to-gas ratios. Using HD 163296 as our case study, we explore how the formation of giant planets in disks can create the conditions for collisionally rejuvenating the dust population, halting or reversing the expected trend. We combine N -body simulations with statistical methods and impact scaling laws to estimate the dynamical and collisional excitation of the planetesimals due to the formation of HD 163296's giant planets. We show that this process creates a violent collisional environment across the disk that can inject collisionally produced second-generation dust into it, significantly contributing to the observed dust-to-gas ratio. The spatial distribution of the dust production can explain the observed local enrichments in HD 163296's inner regions. The results obtained for HD 163296 can be extended to any disk with embedded forming giant planets and may indicate a common evolutionary stage in the life of such circumstellar disks. Furthermore, the dynamical excitation of the planetesimals could result in the release of transient, nonequilibrium gas species like H 2 O, CO 2 , NH 3 , and CO in the disk due to ice sublimation during impacts and, due to the excited planetesimals being supersonic with respect to the gas, could produce bow shocks in the latter that could heat it and cause a broadening of its emission lines.
... The rapid changes in the gravitational potential across the circumsolar disk due to Jupiter's accretion and migration should have strongly affected the mass and velocity distribution of all the bodies present at that time in the Solar Nebula (see e.g. Coradini, Magni & Turrini 2010; Turrini, Magni & Coradini 2011). Quantitative information on the age and the duration of the Solar Nebula phase is supplied respectively by meteoritic studies and astronomical observations. ...
... Alongside the planetesimals, as we previously mentioned, the giant planets were forming across the Solar Nebula phase. It has been suggested (Turrini, Magni & Coradini 2011) Across the Jovian Early Bombardment and, by extension, the Primordial Heavy Bombardment, Vesta and Ceres would have undergone impacts from both rocky bodies formed in the inner Solar System and volatile-rich bodies from the outer Solar System (ibid). According to Turrini, depends both on the extent of Jupiter's radial migration due to disk-planet interactions and on the size-frequency distribution of the planetesimals populating the Solar Nebula. ...
The evolution of the Solar System can be schematically divided into three
different phases: the Solar Nebula, the Primordial Solar System and the Modern
Solar System. These three periods were characterized by very different
conditions, both from the point of view of the physical conditions and from
that of the processes there were acting through them. Across the Solar Nebula
phase, planetesimals and planetary embryos were forming and differentiating due
to the decay of short-lived radionuclides. At the same time, giant planets
formed their cores and accreted the nebular gas to reach their present masses.
After the gas dispersal, the Primordial Solar System began its evolution. In
the inner Solar System, planetary embryos formed the terrestrial planets and,
in combination with the gravitational perturbations of the giant planets,
depleted the residual population of planetesimals. In the outer Solar System,
giant planets underwent a violent, chaotic phase of orbital rearrangement which
caused the Late Heavy Bombardment. Then the rapid and fierce evolution of the
young Solar System left place to the more regular secular evolution of the
Modern Solar System. Vesta, through its connection with HED meteorites, and
plausibly Ceres too were between the first bodies to form in the history of the
Solar System. Here we discuss the timescale of their formation and evolution
and how they would have been affected by their passage through the different
phases of the history of the Solar System, in order to draw a reference
framework to interpret the data that Dawn mission will supply on them.
... (Lissauer et al. 2009;D'Angelo et al. 2010;Bitsch et al. 2015;Johansen & Lambrechts 2017;Johansen et al. 2019;D'Angelo et al. 2021) using the parametric approach fromTurrini et al. (2011Turrini et al. ( , 2019Turrini et al. ( , 2021a. The first phase is common to all planets, from super-Earths to gas giants, and accounts for their growth bypebble and planetesimal accretion (Bitsch et al. 2015; Johansen & Lambrechts 2017; Johansen et al. 2019). ...
Observational data on the dust content of circumstellar disks show that the median dust content in disks around pre-main-sequence stars in nearby star-forming regions seems to increase from ~1 to ~2 Myr and then decline with time. This behavior challenges the models where the small dust grains steadily decline by accumulating into larger bodies and drifting inwards on a short timescale (≤1 Myr). In this Letter we explore the possibility to reconcile this discrepancy in the framework of a model where the early formation of planets dynamically stirs the nearby planetesimals and causes high-energy impacts between them, resulting in the production of second-generation dust. We show that the observed dust evolution can be naturally explained by this process within a suite of representative disk-planet architectures.
... We simulated a giant planet that forms and migrates on the disc midplane (i.e., on an orbit with inclination i = 0) over a timescale of 3 Myr, growing from a planetary embryo to a Jupiter-like giant planet. We adopted a two-phase approach to model both the mass growth and the evolution of the physical radius of the planet, following the growth tracks from Lissauer et al. (2009), Bitsch et al. (2015, and D' Angelo et al. (2021) using the parametric approach from Turrini et al. (2011Turrini et al. ( , 2019. In the first 2 Myr, the planet accretes its core and extended atmosphere and grows from an initial mass of M 0 = 0.1 M ⊕ (mass of the planetary embryo) to a critical mass of M c = 30 M ⊕ , equally shared between the core and the atmosphere. ...
Giant planets can interact with multiple and chemically diverse environments in protoplanetary discs while they form and migrate to their final orbits. The way this interaction affects the accretion of gas and solids shapes the chemical composition of the planets and of their atmospheres. Here we investigate the effects of different chemical structures of the host protoplanetary disc on the planetary composition. We consider both scenarios of molecular (inheritance from the pre-stellar cloud) and atomic (complete chemical reset) initial abundances in the disc. We focus on four elemental tracers of different volatility: C, O, N, and S. We explore the entire extension of possible formation regions suggested by observations by coupling the disc chemical scenarios with N-body simulations of forming and migrating giant planets. The planet formation process produces giant planets with chemical compositions significantly deviating from that of the host disc. We find that the C/N, N/O, and S/N ratios follow monotonic trends with the extent of migration. The C/O ratio shows a more complex behaviour, dependent on the planet accretion history and on the chemical structure of the formation environment. The comparison between S/N* and C/N* (where * indicates normalisation to the stellar value), constrains the relative contribution of gas and solids to the total metallicity. Giant planets whose metallicity is dominated by the contribution of the gas are characterised by N/O* > C/O* > C/N* and allow to constrain the disc chemical scenario. When the planetary metallicity is instead dominated by the contribution of the solids we find that C/N* > C/O* > N/O*.
... Myr (refs. 37,39,40 ), whereas the energetic state of the inner Solar System recorded by the Pd-Ag system persisted for at least 3.9 +2.3 / −3.1 Myr after the closure of the IVA core (Table 3). This longevity problem persists if using the lower SSI 107 Pd/ 108 Pd ratio of ~3.5 × 10 −5 , which yields impacts at ~2.9-6.8 ...
Rapid cooling of planetesimal cores has been inferred for several iron meteorite parent bodies on the basis of metallographic cooling rates, and linked to the loss of their insulating mantles during impacts. However, the timing of these disruptive events is poorly constrained. Here, we used the short-lived 107Pd–107Ag decay system to date rapid core cooling by determining Pd–Ag ages for iron meteorites. We show that closure times for the iron meteorites equate to cooling in the time frame ~7.8–11.7 Myr after calcium–aluminium-rich inclusion formation, and that they indicate that an energetic inner Solar System persisted at this time. This probably results from the dissipation of gas in the protoplanetary disk, after which the damping effect of gas drag ceases. An early giant planet instability between 5 and 14 Myr after calcium–aluminium-rich inclusion formation could have reinforced this effect. This correlates well with the timing of impacts recorded by the Pd–Ag system for iron meteorites. Impact-related cooling of asteroids happened between 7.8 and 11.7 million years after Solar System formation due to dynamical forcing following the gas disk dissipation, an early giant planet instability, or a combination of the two.
... (Lissauer et al. 2009;D'Angelo et al. 2010;Bitsch et al. 2015;Johansen & Lambrechts 2017;Johansen et al. 2019;D'Angelo et al. 2021) using the parametric approach fromTurrini et al. (2011Turrini et al. ( , 2019Turrini et al. ( , 2021a. The first phase is common to all planets, from super-Earths to gas giants, and accounts for their growth bypebble and planetesimal accretion (Bitsch et al. 2015; Johansen & Lambrechts 2017; Johansen et al. 2019). ...
Observational data on the dust content of circumstellar disks show that the median dust content in disks around pre-main-sequence stars in nearby star-forming regions seems to increase from ∼1 to ∼2 Myr and then decline with time. This behavior challenges the models where the small dust grains steadily decline by accumulating into larger bodies and drifting inwards on a short timescale (≤1 Myr). In this Letter we explore the possibility to reconcile this discrepancy in the framework of a model where the early formation of planets dynamically stirs the nearby planetesimals and causes high-energy impacts between them, resulting in the production of second-generation dust. We show that the observed dust evolution can be naturally explained by this process within a suite of representative disk-planet architectures.
... Improved knowledge of the composition and interior structure of Uranus will also provide deeper insight into the processes that remixed material in the protoplanetary disk, caused for example by the formation of Jupiter (Safronov, 1972;Turrini et al., 2011) or due to extensive primordial migration of the giant planets (Walsh et al., 2011 thermal energy during formation and is steadily radiated away through their tenuous atmospheres as they age. Voyager measurements suggest that Uranus' evolution produced a planet with negligible self-luminosity, smaller than any other planet in our Solar System (Pearl et al., 1990). ...
Giant planets helped to shape the conditions we see in the Solar System today and they account for more than 99% of the mass of the Sun's planetary system. They can be subdivided into the Ice Giants (Uranus and Neptune) and the Gas Giants (Jupiter and Saturn), which differ from each other in a number of fundamental ways. Uranus, in particular is the most challenging to our understanding of planetary formation and evolution, with its large obliquity, low self-luminosity, highly asymmetrical internal field, and puzzling internal structure. Uranus also has a rich planetary system consisting of a system of inner natural satellites and complex ring system, five major natural icy satellites, a system of irregular moons with varied dynamical histories, and a highly asymmetrical magnetosphere. Voyager 2 is the only spacecraft to have explored Uranus, with a flyby in 1986, and no mission is currently planned to this enigmatic system. However, a mission to the uranian system would open a new window on the origin and evolution of the Solar System and would provide crucial information on a wide variety of physicochemical processes in our Solar System. These have clear implications for understanding exoplanetary systems. In this paper we describe the science case for an orbital mission to Uranus with an atmospheric entry probe to sample the composition and atmospheric physics in Uranus' atmosphere. The characteristics of such an orbiter and a strawman scientific payload are described and we discuss the technical challenges for such a mission. This paper is based on a white paper submitted to the European Space Agency's call for science themes for its large-class mission programme in 2013.
... A related issue is that the primordial main belt has likely been struck by sizable but transient populations on planetcrossing orbits, such as leftover planetesimals , ejecta from giant impacts in the terrestrial planet region (Bottke et al., 2015b), comet-like planetesimals dispersed from the primordial disk during giant planet migration (Brož et al., 2013), and Jupiter-Saturn-zone planetesimals pushed into the inner solar system via giant planet migration and/or evolution (Walsh et al., 2011;Turrini et al., 2011Turrini et al., , 2012. Most of these dramatic events are thought to take place during the first 500 m.y. of solar system history. ...
Collisional and dynamical models of the main asteroid belt allow us to glean insights into planetesimal- and planet-formation scenarios as well as how the main belt reached its current state. Here we discuss many of the processes affecting asteroidal evolution and the constraints that can be used to test collisional model results. We argue the main belt’s wavy size-frequency distribution for diameter D < 100-km asteroids is increasingly a byproduct of comminution as one goes to smaller sizes, with its shape a fossil-like remnant of a violent early epoch. Most D > 100-km asteroids, however, are primordial, with their physical properties set by planetesimal formation and accretion processes. The main-belt size distribution as a whole has evolved into a collisional steady state, and it has possibly been in that state for billions of years. Asteroid families provide a critical historical record of main-belt collisions. The heavily depleted and largely dispersed “ghost families," however, may hold the key to understanding what happened in the primordial days of the main belt. New asteroidal fragments are steadily created by both collisions and mass shedding events via YORP spinup processes. A fraction of this population, in the form of D < 30 km fragments, go on to escape the main belt via the Yarkovsky/YORP effects and gravitational resonances, thereby creating a quasi-steady-state population of planetcrossing and near-Earth asteroids. These populations go on to bombard all inner solar system worlds. By carefully interpreting the cratering records they produce, it is possible to constrain how portions of the main-belt population have evolved with time.
... Like other objects in the solar system, Ceres has been heavily bombarded by impactors [de Elía and di Sisto, 2011;Turrini et al., 2011;O'Brien and Sykes, 2011, and references therein]. These impacts have undoubtedly sculpted the surface of Ceres. ...
New experiments predict that Ceres should be extensively contaminated with meteoritic debris derived from the asteroid belt. All types of impactors likely contribute to the contamination. Ceres may accrete debris more efficiently if it is ice-rich because of enhanced projectile survival and retention in porous ice targets. Experiments indicate that if a silicate regolith lag protects subsurface ice, then some of the projectile should be injected into the regolith during high-angle impacts, thereby hiding part of the projectile component from view. If impacts excavate ice, sublimation will gradually concentrate projectile relics into a surficial lag. In contrast, if the near-surface lacks ice, then accreted meteoritic debris should be distributed throughout a vertically mixed regolith. High-resolution images may reveal pristine projectile relics lining some crater floors. Moreover, we predict that the surface of Ceres is not exclusively endogenic and may be dominated by delivered exogenic debris.
... Improved knowledge of the composition and interior structure of Uranus will also provide deeper insight into the processes that remixed material in the protoplanetary disk, caused for example by the formation of Jupiter (Safronov, 1972;Turrini et al., 2011) or due to extensive primordial migration of the giant planets (Walsh et al., 2011 thermal energy during formation and is steadily radiated away through their tenuous atmospheres as they age. Voyager measurements suggest that Uranus' evolution produced a planet with negligible self-luminosity, smaller than any other planet in our Solar System (Pearl et al., 1990). ...
Giant planets helped to shape the conditions we see in the Solar System today and they account for more than 99% of the mass of the Sun’s planetary system. They can be subdivided into the Ice Giants (Uranus and Neptune) and the Gas Giants (Jupiter and Saturn), which differ from each other in a number of fundamental ways. Uranus, in particular is the most challenging to our understanding of planetary formation and evolution, with its large obliquity, low self-luminosity, highly asymmetrical internal field, and puzzling internal structure. Uranus also has a rich planetary system consisting of a system of inner natural satellites and complex ring system, five major natural icy satellites, a system of irregular moons with varied dynamical histories, and a highly asymmetrical magnetosphere. Voyager 2 is the only spacecraft to have explored Uranus, with a flyby in 1986, and no mission is currently planned to this enigmatic system. However, a mission to the uranian system would open a new window on the origin and evolution of the Solar System and would provide crucial information on a wide variety of physicochemical processes in our Solar System. These have clear implications for understanding exoplanetary systems. In this paper we describe the science case for an orbital mission to Uranus with an atmospheric entry probe to sample the composition and atmospheric physics in Uranus’ atmosphere. The characteristics of such an orbiter and a strawman scientific payload are described and we discuss the technical challenges for such a mission. This paper is based on a white paper submitted to the European Space Agency’s call for science themes for its large-class mission programme in 2013.
... It has also been proposed that the formation of Jupiter may have scattered planetesimals through the asteroid belt region and caused a 'Jovian Early Bombardment' (Turrini et al., 2011;Turrini, 2014). The actual bom-bardment rate in that model can be quite large, potentially intense enough to erode Vesta's surface, but varies significantly based on chosen initial conditions. ...
Vesta has a complex cratering history, with ancient terrains as well as
recent large impacts that have led to regional resurfacing. Crater counts can
help constrain the relative ages of different units on Vesta's surface, but
converting those crater counts to absolute ages requires a chronology function.
We present a cratering chronology based on the best current models for the
dynamical evolution of asteroid belt, and calibrate it to Vesta using the
record of large craters on its surface. While uncertainties remain, our
chronology function is broadly consistent with an ancient surface of Vesta as
well as other constraints such as the bombardment history of the rest of the
inner Solar System and the Ar-Ar age distribution of howardite, eucrite and
diogenite (HED) meteorites from Vesta.
... From a dynamical standpoint, such a time-limited event may have occurred during the primordial solar system (∼10 Myr after the formation of the first solids) when it is believed that the water was accreted on the Earth (Morbidelli et al. 2000;Turrini et al. 2011) or much later, during the Late Heavy Bombardment, when primitive outer solar system objects were implanted in the outer belt (Levison et al. 2009). For instance, implanted primitive bodies in the outer main belt would have produced a significant amount of dust due to collisional grinding for several 100 Myr after the Late Heavy Bombardment (Levison et al. 2009) that may have undergone low speed accretion with Vesta (as well as on other bodies). ...
Water plays a key role in the evolution of the terrestrial planets, and notably the occurrence of Earth’s oceans. However, the mechanism by which water has been incorporated into these bodies -including the Earth- is still extensively debated. Here we report the detection of widespread 2.8-µm OH absorption bands on the surface of asteroid Vesta by the VIR imaging spectrometer on board Dawn. These observations are surprising as Vesta is fully differentiated with a basaltic surface. The 2.8-µm OH absorption is distributed across Vesta’s surface and shows areas enriched and depleted in hydrated materials. The uneven distribution of hydrated mineral phases is unexpected and indicates ancient processes that differ from those believed to be responsible for OH on other airless bodies, like the Moon. The origin of Vestan OH provides new insight on the delivery of hydrous materials in the main belt, and may offer new scenarios on the delivery of hydrous minerals in the inner solar system, suggesting processes that may have played a role in the formation of terrestrial planets.
... This is surprising evidence that Vesta once had a molten interior, like the Earth does. After the crust formation, Vesta underwent an intense and extended resurfacing due to collisional evolution Turrini et al. 2011), as can also be inferred by the presence of a large crater on the south pole (Schenk et al. 2012). ...
In this work we study the link between the evolution of the internal
structure of Vesta and thermal heating due to 26Al and 60Fe and long-lived
radionuclides, taking into account the chemical differentiation of the body and
the affinity of 26Al with silicates. Differentiation takes place in all
scenarios in which Vesta completes its accretion in less than 1.4 Ma after the
injection of 26Al into the Solar Nebula. In all those scenarios where Vesta
completes its formation in less than 1 Ma from the injection of 26Al, the
degree of silicate melting reaches 100 vol. % throughout the whole asteroid. If
Vesta completed its formation between 1 and 1.4 Ma after 26Al injection, the
degree of silicate melting exceeds 50 vol. % over the whole asteroid but
reaches 100 vol. % only in the hottest, outermost part of the mantle in all
scenarios where the porosity is lower than 5 vol. %. If the formation of Vesta
occurred later than 1.5 Ma after the injection of 26Al, the degree of silicate
melting is always lower than 50 vol. % and is limited only to a small region of
the asteroid. The radiation at the surface dominates the evolution of the crust
which ranges in thickness from 8 to about 30 km after 5 Ma: a layer about 3-20
km thick is composed of primitive unmelted chondritic material while a layer of
about 5-10 km is eucritic.
... In the case of Ceres, instead, the most energetic collisions reaches only 1/100 of the self-gravitation energy of the aster- oid. For a detailed description of our results we refer the readers to [2]. ...
... The collisional histories of Vesta and Ceres have been reproduced statistically by evaluating the impact probabilities of the massless particles which crossed the orbits of the two asteroids. For further details on the collisional treatment we refer the reader to our manuscript in preparation [10]. EPSC Abstracts Vol. 5, EPSC2010-541, 2010 European Planetary Science Congress 2010 c Author(s) 2010 ...
Vesta and Ceres are among the oldest objects that formed in the Solar
System, likely predating the formation of the giant planets. Through
their histories, the surfaces of the two targets of the Dawn mission
would have suffered several periods of intense bombardment which shaped
their present morphologies. Here we report the results of our
investigation of the collisional histories of Vesta and Ceres at the
time of the formation of Jupiter. The formation of the giant planet
caused in fact an intense early bombardment in the asteroid belt. In
those scenarios where they survived, both asteroids had their surfaces
saturated by craters as big as 150 km and a few as big as 200 - 300 km.
In the case of Vesta, such Jovian early bombardment would have
significantly eroded the crust, likely exposing the upper mantle or
causing effusive phenomena similar to lunar maria.
Rapid cooling of planetesimal cores has been inferred for several iron meteorite parent bodies based on metallographic cooling rates, and linked to the loss of their insulating mantles during impacts. However, the timing of these disruptive events is poorly constrained. Here, we used the short-lived 107Pd / 107Ag decay system to date rapid core cooling by determining Pd-Ag ages for iron meteorites. We show closure times for the iron meteorites equate to cooling in the timeframe ~7.8 to 11.7 Myr after CAI, and indicate that an energetic inner Solar System persisted at this time. This likely results from the dissipation of gas in the protoplanetary disk, after which the damping effect of gas drag ceases. An early giant planet instability between 5 and 14 Myr after CAI could have reinforced this effect. This correlates well with the timing of impacts recorded by the Pd Ag system for iron meteorites.
Small bodies - comets, asteroids, TNOs are relics of the planetesimals that formed during the early stages of the Solar System and thus, studying these objects is crucial for understanding the formation and evolution of the Solar System. We are currently living in the golden age for the exploration of small bodies that includes many recent and ongoing space missions, of which the Rosetta mission has conducted the most detailed study of a comet so far and gathered an enormous amount of data on comet 67P/Churyumov-Gerasimenko, some of which deserve a closer investigation.My thesis applies a multidisciplinary approach to the investigation of the primitive surface of comet 67P/CG, com-bining analysis of observational data and laboratory experiments. During the first half of my thesis, I studied the morphology, spectrophotometry and activity of comet 67P through the analysis of over one thousand Rosetta/OSIRIS images taken between 2014 and 2016, some of which are high resolution images that had not yet been published in literature. On the other hand, the second half of the thesis was dedicated to producing and measuring the spectra of different types of analogs of icy cometary surfaces. Our research focused on a number of regions of interest on the surface: the Wosret region on the small lobe of the comet, including the two final landing sites of the Philae lander; and the bright spots enriched in water ice exposed to the comet surface.Our study of the Wosret region indicates that the surface layer of the small lobe of comet 67P may consist of more consolidated materials and have lower volatile content than the big lobe, which supports the hypothesis that comet 67P is the result of a merge between two independently formed bodies. We also confirmed that water ice had been exposed at Philae’s second touchdown point in Wosret, implying that comet 67P’s surface is made of low strength materials that covers highly ice-rich subsurface layers, which helps provide constraints for future space missions to comets. In addition to the ice exposed due to Philae, we also identified approximately 700 bright ice-rich spots that were seen from 2014 to 2016, some of which are correlated with cometary activity and surface changes. About 1/10 of the spots have unusually blue spectral slope compared to most other spots, whose high reflectance factor mostly matched pure water ice/frost samples, indicating that these “blue” spots formed by water recondensation into frost instead of outcrops of underlying dirty ice. On the other hand, measurements of other types of icy surface analog show that the reflectance of a mixture between a dark component and a bright component is controlled by three parameters: volume fraction of the dark particles, grain size of the dark and bright particles, therefore determining the composition of a “dirty” icy surface is a complicated process that requires constraining the grain size.6
Meteorites are excavated fragments from asteroid surfaces and planets, and determining their thermophysical properties is important since they contain valuable information about internal structures of their parent bodies. We investigated thermophysical properties of the Sariçiçek meteorite by differential scanning calorimetry (DSC), measuring phase transition temperatures, enthalpy changes and specific heat capacities of samples and thermogravimetric analysis (TGA), investigating weight change in a sample as a function of temperature or time. DSC results indicate that troilite α/β and β/γ phase transition temperatures of the interior part of the meteorite were at 421.98 ± 0.02 and 581.74 ± 1.71 K, and troilite content of interior and crust parts of the meteorite were 0.28 and 0.02 wt%, respectively. Relict temperatures were calculated as 453 ± 10, 465 ± 17, and 588 ± 55 K; specific heat capacities were measured as 779, 745, and 663 J kg–1 K–1 at 300 K; and predicted as 568, 537, and 480 J kg–1 K–1 at 200 K, for interior, edge, and crust, respectively. TGA results revealed that Sariçiçek's weight loss was 0.98% at 1170 °C, and water content and hydrogen abundance at 200–800 °C were 0.34% and 0.04%, respectively. Obtained results shed light on a thermal history of Sariçiçek’s parent body and provide further knowledge on thermal alteration of 4 Vesta.
This project details my PhD work on Cerean impact craters. The work includes information of the Cerean crater database and additional analysis preformed using this database
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 L4 and L5 Lagrangian 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.
Processes governing the evolution of planetesimals are critical to understanding how rocky planets are formed, how water is delivered to them, the origin of planetary atmospheres, how cores and magnetic dynamos develop, and ultimately, which planets have the potential to be habitable. Theoretical advances and new data from asteroid and meteorite observations, coupled with spacecraft missions such as Rosetta and Dawn, have led to major advances in this field over the last decade. This transdisciplinary volume presents an authoritative overview of the latest in our understanding of the processes of planet formation. Combining meteorite, asteroid and icy body observations with theory and modelling of accretion and orbital dynamics, this text also provides insights into the exoplanetary system and the search for habitable worlds. This is an essential reference for those interested in planetary formation, solar system dynamics, exoplanets and planetary habitability.
In this paper it has been studied the possibility that Ceres has or had in the past a crust heavier than a pure or muddy ice mantle, in principle gravitationally unstable. Such a structure is not unusual in the solar system: Callisto is an example. In this work, we test how the composition (i.e. the volumetric quantity of ice) and the size of the crust can affect its survival during the thermo-physical evolution after the differentiation. We have considered two different configurations: the first, characterized by a dehydrated silicate core and a mantle made of pure ice; the second one, with a hydrated silicate core and a muddy mantle (ice with silicate mpurities). In both cases, the crust is composed by a mixture of ice and silicates. These structures are constrained by recent measurement of the mean density by Park et al.. The Rayleigh-Taylor instability, which operates in such an unstable structure, could reverse the overall or part of the crust. The whole unstable crust (or a part of it) can chemically interact with the underlying mantle and what currently observe could be a partially/totally new crust. Our results suggest that, in case of a pure ice mantle, the primordial crust has not survived until today, with a stability timespan always less than 3 Gyr. Conversely, in case of a muddy mantle, with some “favorable” conditions (low volumetric ice percentage in the crust and small crustal thickness), the primordial crust could be characterized by a stability timespan compatible with the lifetime of the solar system.
Ceres appears likely to be differentiated and to have experienced planetary evolution processes. This conclusion is based on current geophysical observations and thermodynamic modeling of Ceres' evolution. This makes Ceres similar to a small planet, and in fact it is thought to represent a class of objects from which the inner planets formed. Verification of Ceres' state and understanding of the many steps in achieving it remains a major goal. The Dawn spacecraft and its instrument package are on a mission to observe Ceres from orbit. Observations and potential results are suggested here, based on number of science questions. © 2012 Springer Science+Business Media New York. All rights are reserved.
It is difficult to find a Vesta model of iron core, pyroxene and olivine-rich mantle, and HED crust that can match the joint constraints of (a) Vesta’s density and core size as reported by the Dawn spacecraft team; (b) the chemical trends of the HED meteorites, including the depletion of sodium, the FeO abundance, and the trace element enrichments; and (c) the absence of exposed mantle material on Vesta’s surface, among Vestoid asteroids, or in our collection of basaltic meteorites. These conclusions are based entirely on mass-balance and density arguments, independent of any particular formation scenario for the HED meteorites themselves. We suggest that Vesta either formed from source material with non-chondritic composition or underwent after its formation a radical physical alteration, possibly caused by collisional processes, that affected its global composition and interior structure.
JIRAM is an imager/spectrometer on board the Juno spacecraft bound for a polar orbit around Jupiter. JIRAM is composed of IR imager and spectrometer channels. Its scientific goals are to explore the Jovian aurorae and the planet's atmospheric structure, dynamics and composition. This paper explains the characteristics and functionalities of the instrument and reports on the results of ground calibrations. It discusses the main subsystems to the extent needed to understand how the instrument is sequenced and used, the purpose of the calibrations necessary to determine instrument performance, the process for generating the commanding sequences, the main elements of the observational strategy, and the format of the scientific data that JIRAM will produce. Keywords Juno · Jupiter · Image spectrometer · Jovian atmosphere · Jovian aurorae
In the course of the selection of the scientific themes for the second and third L-class missions of the Cosmic Vision 2015–2025 program of the European Space Agency, the exploration of the ice giant planets Uranus and Neptune was defined “a timely milestone, fully appropriate for an L class mission”. Among the proposed scientific themes, we presented the scientific case of exploring both planets and their satellites in the framework of a single L-class mission and proposed a mission scenario that could allow to achieve this result. In this work we present an updated and more complete discussion of the scientific rationale and of the mission concept for a comparative exploration of the ice giant planets Uranus and Neptune and of their satellite systems with twin spacecraft. The first goal of comparatively studying these two similar yet extremely different systems is to shed new light on the ancient past of the Solar System and on the processes that shaped its formation and evolution. This, in turn, would reveal whether the Solar System and the very diverse extrasolar systems discovered so far all share a common origin or if different environments and mechanisms were responsible for their formation. A space mission to the ice giants would also open up the possibility to use Uranus and Neptune as templates in the study of one of the most abundant type of extrasolar planets in the galaxy. Finally, such a mission would allow a detailed study of the interplanetary and gravitational environments at a range of distances from the Sun poorly covered by direct exploration, improving the constraints on the fundamental theories of gravitation and on the behaviour of the solar wind and the interplanetary magnetic field.
In the course of the selection of the scientific themes of the second and
third L-class missions of the Cosmic Vision 2015-2025 program of the European
Space Agency, the exploration of the ice giant planets Uranus and Neptune was
defined "a timely milestone, fully appropriate for an L class mission". Among
the proposed scientific themes, in the white paper "The ODINUS Mission Concept"
we discussed the scientific case of exploring both planets and their satellites
in the framework of a single L-class mission and proposed a mission scenario
that could allow to achieve this result. In this work we present an updated and
more complete discussion of the scientific rationale for a comparative
exploration of the ice giant planets Uranus and Neptune and of their satellite
systems. The first goal of comparatively studying these two similar yet
extremely different systems is to shed new light on the ancient past of the
Solar System and on the processes that shaped its formation and evolution.
This, in turn, would reveal whether the Solar System and the very diverse
extrasolar systems discovered so far all share a common origin or if different
environments and mechanisms were responsible for their formation. A space
mission to the ice giants would also open up the possibility to use Uranus and
Neptune as templates in the study of one of the most abundant type of
extrasolar planets in the galaxy. Finally, such a mission would allow a
detailed study of the interplanetary and gravitational environments at a range
of distances from the Sun poorly covered by direct exploration, improving the
constraints on the fundamental theories of gravitation and on the behaviour of
the solar wind and the interplanetary magnetic field.
Over the last twenty years, the search for extrasolar planets revealed us the
rich diversity of the outcomes of the formation and evolution of planetary
systems. In order to fully understand how these extrasolar planets came to be,
however, the orbital and physical data we possess are not enough, and they need
to be complemented with information on the composition of the exoplanets.
Ground-based and space-based observations provided the first data on the
atmospheric composition of a few extrasolar planets, but a larger and more
detailed sample is required before we can fully take advantage of it. The
primary goal of the Exoplanet Characterization Observatory (EChO) is to fill
this gap, expanding the limited data we possess by performing a systematic
survey of hundreds of extrasolar planets. The full exploitation of the data
that EChO and other space-based and ground-based facilities will provide in the
near future, however, requires the knowledge of what are the sources and sinks
of the chemical species and molecules that will be observed. Luckily, the study
of the past history of the Solar System provides several indications on the
effects of processes like migration, late accretion and secular impacts, and on
the time they occur in the life of planetary systems. In this work we will
review what is already known about the factors influencing the composition of
planetary atmospheres, focusing on the case of gaseous giant planets, and what
instead still need to be investigated.
With crater counts from Dawn imaging and topography data, along with
insights provided by dynamical models and constraints from the HED
meteorites, we are working towards developing an absolute cratering
chronology for Vesta's surface.
Using recent constraints on the shape and density of (2) Pallas, we model the thermal evolution of the body as a function of possible formation scenarios that differ in the time of formation and composition assumed for the protoplanet. We develop possible evolution scenarios for Pallas and compare these to available observations. Our models imply two distinct types of end states: those with a hydrosphere and silicate core, and those where the body is dominated by hydrated silicates. We show that for an initial ice-rock mixture with density 2400 kg/m3, Pallas is likely to differentiate and form a rocky core and icy shell. If Pallas accreted from material with lower initial ice content, our models indicate that Pallas’s interior is dominated by hydrated silicates, possibly with a core of anhydrous silicates.
The Martian satellite Phobos has been observed on 2007 February 24 and
25, during the pre- and post-Mars closest approach (CA) of the ESA
Rosetta spacecraft Mars swing-by. The goal of the observations was the
determination of the surface composition of different areas of Phobos,
in order to obtain new clues regarding its nature and origin.
Near-ultraviolet, visible and near-infrared (263.5-992.0 nm) images of
Phobos's surface were acquired using the Narrow Angle Camera of the
OSIRIS instrument onboard Rosetta. The six multi-wavelength sets of
observations allowed a spectrophotometric characterization of different
areas of the satellite, belonging respectively to the leading and
trailing hemisphere of the anti-Mars hemisphere, and also of a section
of its sub-Mars hemisphere. The pre-CA spectrophotometric data obtained
with a phase angle of 19° have a spectral trend consistent within
the error bars with those of unresolved/disc-integrated measurements
present in the literature. In addition, we detect an absorption band
centred at 950 nm, which is consistent with the presence of pyroxene.
The post-CA observations cover from NUV to NIR a portion of the surface
(0° to 43°E of longitude) never studied before. The reflectance
measured on our data does not fit with the previous spectrophotometry
above 650 nm. This difference can be due to two reasons. First, the
OSIRIS observed area in this observation phase is completely different
with respect to the other local specific spectra and hence the spectrum
may be different. Secondly, due to the totally different observation
geometry (the phase angle ranges from 137° to 140°), the
differences of spectral slope can be due to phase reddening. The
comparison of our reflectance spectra, both pre- and post-CA, with those
of D-type asteroids shows that the spectra of Phobos are all redder than
the mean D-type spectrum, but within the spectral dispersion of other
D-types. To complement this result, we performed an investigation of the
conditions needed to collisionally capture Phobos in a way similar to
that proposed for the irregular satellites of the giant planets. Once
put in the context of the current understanding of the evolution of the
early Solar system, the coupled observational and dynamical results we
obtained strongly argue for an early capture of Phobos, likely
immediately after the formation of Mars.
We report the first results of a joint study of the differentiation of
Vesta and the bombardment triggered by the formation of Jupiter to
assess the possible implications of the interpretation of the data
supplied by the Dawn mission.
Vesta and Ceres are the largest members of the asteroid belt, surviving from the earliest phases of Solar System history.
They formed at a time when the asteroid belt was much more massive than it is today and were witness to its dramatic evolution,
where planetary embryos were formed and lost, where the collisional environment shifted from accretional to destructive, and
where the current size distribution of asteroids was sculpted by mutual collisions and most of the asteroids originally present
were lost by dynamical processes. Since these early times, the environment of the asteroid belt has become relatively quiescent,
though over the long history of the Solar System the surfaces of Vesta and Ceres continue to record and be influenced by impacts,
most notably the south polar cratering event on Vesta. As a consequence of such impacts, Vesta has contributed a significant
family of asteroids to the main belt, which is the likely source of the HED meteorites on Earth. No similar contribution to
the main belt (or meteorites) is evident for Ceres. Through studies of craters, the surfaces of these asteroids will offer
an opportunity for Dawn to probe the modern population of small asteroids in a size regime not directly observable from Earth.
KeywordsAsteroids–Asteroid dynamics–Asteroid collisional evolution–Asteroid Vesta–Asteroid Ceres–Asteroid families–Planet formation–Impacts
Ceres appears likely to be differentiated and to have experienced planetary evolution processes. This conclusion is based
on current geophysical observations and thermodynamic modeling of Ceres’ evolution. This makes Ceres similar to a small planet,
and in fact it is thought to represent a class of objects from which the inner planets formed. Verification of Ceres’ state
and understanding of the many steps in achieving it remains a major goal. The Dawn spacecraft and its instrument package are
on a mission to observe Ceres from orbit. Observations and potential results are suggested here, based on number of science
questions.
KeywordsDawn–Ceres–Evolution of ice-silicate bodies
A New Dawn
Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684 ) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687 ) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690 ) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694 ) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697 ) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700 ) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.
The asteroid belt is an open window on the history of the Solar System, as it
preserves records of both its formation process and its secular evolution. The
progenitors of the present-day asteroids formed in the Solar Nebula almost
contemporary to the giant planets. The actual process producing the first
generation of asteroids is uncertain, strongly depending on the physical
characteristics of the Solar Nebula, and the different scenarios produce very
diverse initial size-frequency distributions. In this work we investigate the
implications of the formation of Jupiter, plausibly the first giant planet to
form, on the evolution of the primordial asteroid belt. The formation of
Jupiter triggered a short but intense period of primordial bombardment,
previously unaccounted for, which caused an early phase of enhanced collisional
evolution in the asteroid belt. Our results indicate that this Jovian Early
Bombardment caused the erosion or the disruption of bodies smaller than a
threshold size, which strongly depends on the size-frequency distribution of
the primordial planetesimals. If the asteroid belt was dominated by
planetesimals less than 100 km in diameter, the primordial bombardment would
have caused the erosion of bodies smaller than 200 km in diameter. If the
asteroid belt was instead dominated by larger planetesimals, the bombardment
would have resulted in the destruction of bodies as big as 500 km.
The asteroid 4 Vesta is the only known differentiated asteroid with an
intact internal structure, probably consisting of a metal core, an
ultramafic mantle, and a basaltic crust. Considerable evidence suggests
that the HED meteorites are impact ejecta from Vesta, and detailed
studies of these meteorites in terrestrial laboratories, combined with
ever more sophisticated remote sensing studies of the asteroid, have
resulted in a good understanding of the geological evolution of this
fascinating object. Extensive mineralogical, petrological, geochemical,
isotopic, and chronological data suggest that heating, melting, and
formation of a metal core, a mantle, and a basaltic crust took place in
the first few million years of solar system history. It is likely that
many more Vesta-like asteroids formed at the dawn of the solar system
but were destroyed by impact, with the iron meteorites being remnants of
their cores. Such differentiated objects may have played an important
role in the accretion and formation of the terrestrial planets, and it
is therefore highly desirable to explore by spacecraft this world that
can be viewed as the smallest of the terrestrial planets.
We review the theory of terrestrial planet formation as it currently stands. In anticipation of forthcoming observational capabilities, the central theoretical issues to be addressed are (1) what is the frequency of terrestrial planets around nearby stars; (2) what mechanisms determine the mass distribution, dynamical structure, and stability of terrestrial-planet systems; and (3) what processes regulated the chronological sequence of gas and terrestrial planet formation in the solar system? In the context of solar system formation, the last stage of terrestrial planet formation will be discussed along with cosmochemical constraints and different dynamical architectures together with important processes such as runaway and oligarchic growth. Observations of dust around other stars, combined with models of dust production during accretion, give us a window on exoterrestrial planet formation. We discuss the latest results from such models, including predictions that will be tested by next-generation instruments such as GMT and ALMA.
Gravitational instabilities and formation of planetesimals in the
sedimentating dust component of the protoplanetary disk are studied,
taking into account the interaction between dust grains and gas. The
timescales, and the influence on the various processes of the mass of
the grains and of their dynamical evolution inside the disk, are
discussed. The physical characteristics of the planetesimals in
different regions of the disk are also obtained.
The masses of the three largest asteroids: (1) Ceres, (2) Pallas and (4)
Vesta were determined from gravitational perturbations exerted on
respectively 25, 2, and 26 selected asteroids. These masses were
calculated by means of the least-squares method as weighted means of the
values obtained separately from the perturbations on single asteroids.
Special attention was paid to the selection of the observations of the
asteroids. For this purpose, a criterion based on the requirement that
the post-selection distribution of the (O - C) residuals should be
Gaussian was implemented. The derived masses are: (4.70 ∓ 0.04)
× 10-10 Msun, (1.21 ∓ 0.26) ×
10-10 Msun, and (1.36 ∓ 0.05) ×
10-10 Msun for (1) Ceres, (2) Pallas and (4)
Vesta, respectively. We also show how the fact that a statistically
substantial number of perturbed asteroids is used in the determination
of the mass of (1) Ceres and (4) Vesta increases the reliability of
their mass determination because effects like the flaws of the dynamical
model and/or the observational biases cancel out. In case of Ceres and
Vesta, these effects have a very small influence on the final result.
The number of acceptable mass determinations of Pallas is much smaller,
but can be increased in the future when the dynamical model is improved.
We indicate some promising encounters with Pallas.
Constraining the timescales for the assembly and differentiation of planetary bodies in our young solar system is essential for a complete understanding of planet-forming processes. This is best achieved through the study of the daughter products of extinct radionuclides with short half-lives, as they provide unsurpassed time resolution as compared to long-lived chronometers. Here we report high-precision Mg isotope measurements of bulk samples of basalt, gabbro, and pyroxenite meteorites obtained by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). All samples from the eucrite and mesosiderite parent bodies (EPB and MPB) with suprachondritic Al/Mg ratios have resolvable 26Mg excesses compared to matrix-matched samples from the Earth, the Moon, Mars, and chondrites. Basaltic magmatism on the EPB and MPB thus occurred during the life span of the now-extinct 26Al nuclide. Initial 26Al/27Al values range from (1.26 ± 0.37) × 10-6 to (5.12 ± 0.81) × 10-6 at the time of magmatism on the EPB and MPB, and are among the highest 26Al abundances reported for igneous meteorites. These results indicate that widespread silicate melting and differentiation of rocky bodies occurred within 3 million years of solar system formation, when 26Al and 60Fe were extant enough to induce planetesimal melting. Finally, thermal modeling constrains the accretion of these differentiated asteroids to within 1 million years of solar system formation, that is, prior to the accretion of chondrite parent bodies.
In this paper, we use N-body integrations to study the effect that planetary embryos spread between ∼0.5 and 4 AU would have on primordial asteroids. The most promising model for the formation of the terrestrial planets assumes the presence of such embryos at the time of formation of Jupiter. At the end of their runaway growth phase, the embryos are on quasi-circular orbits, with masses comparable to that of the Moon or Mars. Due to gravitational interactions among them, and with the growing Jupiter, their orbits begin to cross each other, and they collide, forming bigger bodies. A general outcome of this model is that a few planets form in a stable configuration in the terrestrial planet region, while the asteroid belt is cleared of embryos. Due to combined gravitational perturbations from Jupiter and the embryos, the primordial asteroids are dynamically excited. Most of the asteroids are ejected from the system in a very short time, the dynamical lifetime being on the order of 1 My. A few asteroids (less than 1%) survive, mostly in the region 2.8–3.3 AU, and their eccentricity and inclination distribution qualitatively resembles the observed one. The surviving asteroids have undergone changes in semimajor axis of several tenths of an AU, which could explain the observed radial mixing of asteroid taxonomic types. When the distribution of massive embryos is truncated at 3 AU, we obtain too many asteroids in the outer part of the belt, especially too many Hildas. This suggests that the formation of Jupiter did not prohibit the formation of large embryos in the outer belt and Jupiter did not accrete them while it was still growing.
HST images obtained over an interval of 71 hr at a scale of 52 km/pixel have been used to determine the spin pole, size, and shape of Vesta. Several bright and dark surface features allow control point stereogrammetric measurement of spin pole orientation. An independent solution of spin pole and shape can be made from the limb and terminator coordinates. A combined control point and limb solution for Vesta's rotation pole is at α0= 308° ± 10°, δ0= 48° ± 10°, J2000). Its shape can be fit by an ellipsoid of radii 280, 272, 227 (±12) km. The mean density of Vesta from the mass reported by Schubart and Matson (1979) is 3.8 ± 0.6 gm/cm3. For this density Vesta's shape is close to that of a Maclaurin spheroid with superposed variations of ∼15 km.
Determining the origins of our solar system and, by proxy, other planetary systems, depends on knowing accurately and precisely the timing and tempo of the transformation of the disk of gas and dust to the solids that formed the planets. Relative ages based on the short-lived nuclide 26Al indicate that high-temperature calcium-aluminum inclusions (CAIs) formed before lower temperature chondrules but these ages are heavily dependant on a model of homogeneous distribution of 26Al within the protoplanetary disk. The competing X-wind model argues for heterogeneous distribution of 26Al due to its formation by intra-solar system irradiation such that this system would have no chronological significance. We report a207Pb-206Pb isochron age of 4565.45 ± 0.45 Myr for chondrules from the CV chondrite Allende, an age that is 1.66 ± 0.48 Myr younger than the accepted Pb-Pb age for CAIs from this chondrite group. This age offset is in excellent agreement with the relative ages determined using the 26Al-26Mg system, an observation that supports a supernova origin for 26Al and, importantly, the chronological significance of the 26Al-26Mg system in general. This is consistent with an early and brief CAI-forming event followed by recurrent chondrule formation throughout the life span of the protoplanetary disk. The paucity of old chondrules in chondrite meteorites may reflect their early incorporation into the parent bodies of differentiated meteorites after CAIs were effectively removed from the innermost regions of the protoplanetary disk. Lastly, the agreement between the absolute and relative chronology of CAIs and chondrules requires a solar system age younger than ~4567.5 Myr
The subject of satellite formation is strictly linked to the one of planetary formation. Giant planets strongly shape the evolution of the circum-planetary disks during their formation and thus, indirectly, influence the initial conditions for the processes governing satellite formation. In order to fully understand the present features of the satellite systems of the giant planets, we need to take into account their formation environments and histories and the role of the different physical parameters. In particular, the pressure and temperature profiles in the circum-planetary nebulae shaped their chemical gradients by allowing the condensation of ices and noble gases. These chemical gradients, in turn, set the composition of the satellitesimals, which represent the building blocks of the present regular satellites. Comment: 23 pages, 6 figures, accepted for publication, to appear on Space Science Reviews and on Europlanet vol. on Icy Satellites
We investigate the evolution of protoplanets with different masses embedded in an accretion disk, via global fully three-dimensional hydrodynamical simulations. We consider a range of planetary masses extending from 1.5 M**oplus; up to 1 MJ, and we take into account physically realistic gravitational potentials of forming planets. In order to calculate accurately the gravitational torques exerted by disk material and to investigate the accretion process onto the planet, the flow dynamics has to be thoroughly resolved on long as well as short length scales. We achieve this strict resolution requirement by applying a nested-grid refinement technique that allows us to greatly enhance the local resolution. Our results from altogether 51 simulations show that for large planetary masses, approximately above 0.1 MJ, migration rates are relatively constant, as expected in a type II migration regime and in good agreement with previous two-dimensional calculations. In a range between 7 and 15 M**oplus;, we find a dependency of the migration speed on the planetary mass that yields timescales considerably longer than those predicted by linear analytical theories. This property may be important in determining the overall orbital evolution of protoplanets. The growth timescale is minimum around 20 M**oplus;, but it rapidly increases for both smaller and larger mass values. Significant differences between two- and three-dimensional calculations are found in particular for objects with masses smaller than 10 M**oplus;. We also derive an analytical approximation for the numerically computed mass growth rates.
Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters' rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.
The lead-lead isochron age of chondrules in the CR chondrite Acfer 059 is 4564.7 +/- 0.6 million years ago (Ma), whereas the lead isotopic age of calcium-aluminum-rich inclusions (CAIs) in the CV chondrite Efremovka is 4567.2 +/- 0.6 Ma. This gives an interval of 2.5 +/- 1.2 million years (My) between formation of the CV CAIs and the CR chondrules and indicates that CAI- and chondrule-forming events lasted for at least 1.3 My. This time interval is consistent with a 2- to 3-My age difference between CR CAIs and chondrules inferred from the differences in their initial 26Al/27Al ratios and supports the chronological significance of the 26Al-26Mg systematics.
The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures. The second most massive asteroid--4 Vesta--has differentiated to a crust, mantle and core. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.
In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.1-1 Myr, and these collided more energetically to form planets. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5-200 km in diameter. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr(-1), increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed approximately 4,500 Myr ago in a 'hit-and-run' collision between Moon-to-Mars-sized protoplanets. This demonstrates that protoplanets of approximately 10(3) km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.
During the initial stages of planet formation in circumstellar gas disks, dust grains collide and build up larger and larger bodies. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem: boulders are expected to stick together poorly, and to spiral into the protostar in a few hundred orbits owing to a 'headwind' from the slower rotating gas. Gravitational collapse of the solid component has been suggested to overcome this barrier. But even low levels of turbulence will inhibit sedimentation of solids to a sufficiently dense midplane layer, and turbulence must be present to explain observed gas accretion in protostellar disks. Here we report that boulders can undergo efficient gravitational collapse in locally overdense regions in the midplane of the disk. The boulders concentrate initially in transient high pressure regions in the turbulent gas, and these concentrations are augmented a further order of magnitude by a streaming instability driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar disks.
The discovery of close orbiting extrasolar giant planets led to extensive studies of disk planet interactions and the forms of migration that can result as a means of accounting for their location. Early work established the type I and type II migration regimes for low mass embedded planets and high mass gap forming planets respectively. While providing an attractive means of accounting for close orbiting planets intially formed at several AU, inward migration times for objects in the earth mass range were found to be disturbingly short, making the survival of giant planet cores an issue. Recent progress in this area has come from the application of modern numerical techniques which make use of up to date supercomputer resources. These have enabled higher resolution studies of the regions close to the planet and the initiation of studies of planets interacting with disks undergoing MHD turbulence. This work has led to indications of how the inward migration of low to intermediate mass planets could be slowed down or reversed. In addition, the possibility of a new very fast type III migration regime, that can be directed inwards or outwards, that is relevant to partial gap forming planets in massive disks has been investigated.
Major advances in deciphering the record of nebula processes in chondrites can be attributed to analytical improvements that allow coordinated isotopic and mineralogical studies of components in chondrites and to a wealth of new meteorites from hot and cold deserts. These studies have identified a few rare pristine chondrites that largely escaped heating and alteration in asteroids, which have matrices composed of submicrometer-sized grains of enstatite and forsterite and amorphous silicates, as found in comets. Isotopic analyses of components in pristine chondrites using short-lived nuclide chronometers, Pb-Pb dating, and oxygen isotopes aided by laboratory and theoretical studies of chondrites and differentiated meteorites have provided key constraints on the processes that shaped the early solar system. These processes were once thought to have followed one another sequentially over a period of several million years: chondrule formation; planetesimal accretion; alteration, metamorphism, and melting in planetesimals; and finally, high-velocity collisions between asteroids. Radiometric dating shows, however, that these processes overlapped so that chondrules were still forming in the nebula several million years after early-formed planetesimals had melted and collided. Chondrites are extraordinary mixtures of presolar and solar nebula materials and asteroidal debris.
The main belt is believed to have originally contained an Earth mass or more of material, enough to allow the asteroids to accrete on relatively short timescales. The present-day main belt, however, only contains ˜5×10 Earth masses. Numerical simulations suggest that this mass loss can be explained by the dynamical depletion of main belt material via gravitational perturbations from planetary embryos and a newly-formed Jupiter. To explore this scenario, we combined dynamical results from Petit et al. [Petit, J. Morbidelli, A., Chambers, J., 2001. The primordial excitation and clearing of the asteroid belt. Icarus 153, 338 347] with a collisional evolution code capable of tracking how the main belt undergoes comminution and dynamical depletion over 4.6 Gyr [Bottke, W.F., Durda, D., Nesvorny, D., Jedicke, R., Morbidelli, A., Vokrouhlický, D., Levison, H., 2005. The fossilized size distribution of the main asteroid belt. Icarus 175, 111 140]. Our results were constrained by the main belt's size frequency distribution, the number of asteroid families produced by disruption events from diameter D>100 km parent bodies over the last 3 4 Gyr, the presence of a single large impact crater on Vesta's intact basaltic crust, and the relatively constant lunar and terrestrial impactor flux over the last 3 Gyr. We used our model to set limits on the initial size of the main belt as well as Jupiter's formation time. We find the most likely formation time for Jupiter was 3.3±2.6 Myr after the onset of fragmentation in the main belt. These results are consistent with the estimated mean disk lifetime of 3 Myr predicted by Haisch et al. [Haisch, K.E., Lada, E.A., Lada, C.J., 2001. Disk frequencies and lifetimes in young clusters. Astrophys. J. 553, L153 L156]. The post-accretion main belt population, in the form of diameter D≲1000 km planetesimals, was likely to have been 160±40 times the current main belt's mass. This corresponds to 0.06 0.1 Earth masses, only a small fraction of the total mass thought to have existed in the main belt zone during planet formation. The remaining mass was most likely taken up by planetary embryos formed in the same region. Our results suggest that numerous D>200 km planetesimals disrupted early in Solar System history, but only a small fraction of their fragments survived the dynamical depletion event described above. We believe this may explain the limited presence of iron-rich M-type, olivine-rich A-type, and non-Vesta V-type asteroids in the main belt today. The collisional lifetimes determined for main belt asteroids agree with the cosmic ray exposure ages of stony meteorites and are consistent with the limited collisional evolution detected among large Koronis family members. Using the same model, we investigated the near-Earth object (NEO) population. We show the shape of the NEO size distribution is a reflection of the main belt population, with main belt asteroids driven to resonances by Yarkovsky thermal forces. We used our model of the NEO population over the last 3 Gyr, which is consistent with the current population determined by telescopic and satellite data, to explore whether the majority of small craters (D
Preface; 1. Introduction; 2. Dynamics; 3. Solar heating and energy
transport; 4. Planetary atmospheres; 5. Planetary surfaces; 6. Planetary
interiors; 7. Magnetic fields and plasmas; 8. Meteorites; 9. Minor
planets; 10. Comets; 11. Planetary rings; 12. Extrasolar planets; 13.
Planet formation; Appendices; References; Index.
Major advances in deciphering the record of nebula processes in chondrites can be attributed to analytical improvements that allow coordinated isotopic and mineralogical studies of components in chondrites and to a wealth of new meteorites from hot and cold deserts. These studies have identified a few rare pristine chondrites that largely escaped heating and alteration in asteroids, which have matrices composed of submicrometer-sized grains of enstatite and forsterite and amorphous silicates, as found in comets. Isotopic analyses of components in pristine chondrites using short-lived nuclide chronometers, Pb-Pb dating, and oxygen isotopes aided by laboratory and theoretical studies of chondrites and differentiated meteorites have provided key constraints on the processes that shaped the early solar system. These processes were once thought to have followed one another sequentially over a period of several million years: chondrule formation; planetesimal accretion; alteration, metamorphism, and melting in planetesimals; and finally, high-velocity collisions between asteroids. Radiometric dating shows, however, that these processes overlapped so that chondrules were still forming in the nebula several million years after early-formed planetesimals had melted and collided. Chondrites are extraordinary mixtures of presolar and solar nebula materials and asteroidal debris.
Water is ubiquitous in the Universe, and also in the Solar System. By setting the snow line at its condensation level in the protosolar disk, water was responsible for separating the planets into the terrestrial and the giant ones. Water ice is a major constituent of the comets and the small bodies of the outer Solar System, and water vapor is found in the giant planets, both in their interiors and in the stratospheres. Water is a trace element in the atmospheres of Venus and Mars today. It is very abundant on Earth, mostly in liquid form, but it was probably also abundant in the primitive atmospheres of Venus and Mars. Water is found in different states on the three planets, as vapor on Venus and ice (or permafrost) on Mars. Most likely, this difference has played a major role in the diverging destinies of the three planets.
Abstract— Many lines of evidence indicate that meteorites are derived from the asteroid belt but, in general, identifying any meteorite class with a particular asteroid has been problematical. One exception is asteroid 4 Vesta, where a strong case can be made that it is the ultimate source of the howardite-eucrite-diogenite (HED) family of basaltic achondrites. Visible and near-infrared reflectance spectra first suggested a connection between Vesta and the basaltic achondrites. Experimental petrology demonstrated that the eucrites (the relatively unaltered and unmixed basaltic achondrites) were the product of approximately a 10% melt. Studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal-scale magma ocean. Mass balance considerations point to a parent body that had its surface excavated, but remains intact. Modern telescopic spectroscopy has identified kilometer-scale “Vestoids” between Vesta and the 3:1 orbit-orbit resonance with Jupiter. Dynamical simulations of impact into Vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations. Hubble Space Telescope images show a 460 km diameter impact basin at the south pole of Vesta. It seems that nature has provided multiple free sample return missions to a unique asteroid. Major challenges are to establish the geologic context of the HED meteorites on the surface of Vesta and to connect the remaining meteorites to specific asteroids.
A model solar nebula is constructed by adding the solar complement of light elements to each planet, using recent models of planetary compositions. Uncertainties in this approach are estimated. The computed surface density varies approximately asr
–3/2. Mercury, Mars and the asteroid belt are anomalously low in mass, but processes exist which would preferentially remove matter from these regions. Planetary masses and compositions are generally consistent with a monotonic density distribution in the primordial solar nebula.
Recently it has been shown that long-lasting self-gravitating protoplanets can arise from a gravitationally unstable protoplanetary gaseous disks (Mayer et al., 2002). We are convinced that these calculations are extremely important to explain the existence of large extrasolar planets. Here we adopted a different approach, using as a reference scenario, the nucleated instability. This process takes place when the nebular gas is collected by an already formed solid core. This paper describes a long lasting project that simulates the formation of giant planets implementing the gas accretion onto the core via a 3-D hydro-dynamical model, recently developed in its present version (Magni and Coradini, 2003). Here we describe the structure of the central planet, while in a further paper we will treat the disk formation where possibly satellites were formed. To study the planet structure and the gas surrounding it we have considered three main regions: the central planet, a turbulent accretion disk which surrounds it and an extended region from which the gas is collected. The strong interaction between the growing planet and the surrounding gas warms the gas, creating a “barrier” that tends to reduce the accretion rate. The planet's thermodynamic structure depends upon the mass accretion rate onto it. The gas also exchanges angular momentum with the central body. Particular attention in the model has been paid to the analysis of angular momentum distribution between the growing planet and the disk. This work shows that we can match Jupiter present angular momentum.
The growth and orbital evolution of a swarm of ∼1026-g “planetary embryos,” originally distributed throughout both the terrestrial planet and the asteroidal regions has been simulated using a Monte Carlo technique previously used by the author to study the formation of the terrestrial planets alone. The effects of the giant planets, primarily Jupiter, are simply assumed to be those operative at present: chaotic acceleration in resonance regions and gravitational ejection of objects by encounters with Jupiter at aphelia ≳4.75 AU. It is found that the asteroidal embryos are very effective in scattering one another into resonance regions. The resulting orbital evolution clears the asteroid belt of embryos in about half of the simulations. Embryos accumulate in the terrestrial planet region to form a stochastically determined variety of planetary configurations that are similar in total mass, specific energy, specific angular momentum, planetary size, and orbital elements to our Solar System. The small quantity of material remaining in the asteroid region has a velocity distribution very much like that of the observed asteroids. It is concluded that this model, although certainly imperfect, represents a viable alternative to models in which the initial asteroidal population is limited to bodies the size of the present asteroids.
How big were the first planetesimals? We attempt to answer this question by conducting coagulation simulations in which the planetesimals grow by mutual collisions and form larger bodies and planetary embryos. The size frequency distribution (SFD) of the initial planetesimals is considered a free parameter in these simulations, and we search for the one that produces at the end objects with a SFD that is consistent with Asteroid belt constraints. We find that, if the initial planetesimals were small (e.g. km-sized), the final SFD fails to fulfill these constraints. In particular, reproducing the bump observed at diameter in the current SFD of the asteroids requires that the minimal size of the initial planetesimals was also ∼100 km. This supports the idea that planetesimals formed big, namely that the size of solids in the proto-planetary disk “jumped” from sub-meter scale to multi-kilometer scale, without passing through intermediate values. Moreover, we find evidence that the initial planetesimals had to have sizes ranging from 100 to several 100 km, probably even 1000 km, and that their SFD had to have a slope over this interval that was similar to the one characterizing the current asteroids in the same size range. This result sets a new constraint on planetesimal formation models and opens new perspectives for the investigation of the collisional evolution in the Asteroid and Kuiper belts as well as of the accretion of the cores of the giant planets.
We have modeled the growth of Jupiter incorporating both thermal and
hydrodynamical constraints on its accretion of gas from the circumsolar
disk. We have used a planetary formation code, based on a Henyey- type
stellar evolution code, to compute the planet's internal structure and a
three-dimensional hydrodynamics code to calculate the planet's
interactions with the protoplanetary disk. Our principal results are:
(1) Three dimensional hydrodynamics calculations show that the flow of
gas in the circumsolar disk limits the region occupied by the planet's
tenuous gaseous envelope to within about 0.25 Rh (Hill sphere radii) of
the planet's center, which is much smaller than the value of ~ 1 Rh that
was assumed in previous studies. (2) This smaller size of the planet's
envelope increases the planet's accretion time, but only by 5-- 10%. In
general, in agreement with previous results of Hubickyj et al.
[Hubickyj, O., Bodenheimer, P., Lissauer, J.J., 2005. Icarus, 179,
415-431], Jupiter formation times are in the range 2.5--3 Myr, assuming
a protoplanetary disk with solid surface density of 10 g/cm2 and dust
opacity in the protoplanet's envelope equal to 2% that of interstellar
material. Thermal pressure limits the rate at which a planet less than
a few dozen times as massive as Earth can accumulate gas from the
protoplanetary disk, whereas hydrodynamics regulates the growth rate for
more massive planets. (3) In a protoplanetary disk whose alpha-viscosity
parameter is ~ 0.004, giant planets will grow to several times the mass
of Jupiter unless the disk has a small local surface density when the
planet begins to accrete gas hydrodynamically, or the disk is dispersed
very soon thereafter. The large number of planets known with masses near
Jupiter's compared with the smaller number of substantially more massive
planets is more naturally explained by planetary growth within
circumstellar disks whose alpha-viscosity parameter is ~ 0.0004. (4)
Capture of Jupiter's irregular satellites within the planet's diffuse
and extended thermally-supported envelope is not consistent with the
Jupiter formation models presented in this study.
We have performed new simulations of two different scenarios for the excitation and depletion of the primordial asteroid belt, assuming Jupiter and Saturn on initially circular orbits as predicted by the Nice Model of the evolution of the outer Solar System [Gomes, R., Levison, H.F., Tsiganis, K., Morbidelli, A., 2005. Nature 435, 466–469; Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459–461; Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462–465]. First, we study the effects of sweeping secular resonances driven by the depletion of the solar nebula. We find that these sweeping secular resonances are incapable of giving sufficient dynamical excitation to the asteroids for nebula depletion timescales consistent with estimates for solar-type stars, and in addition cannot cause significant mass depletion in the asteroid belt or produce the observed radial mixing of different asteroid taxonomic types. Second, we study the effects of planetary embryos embedded in the primordial asteroid belt. These embedded planetary embryos, combined with the action of jovian and saturnian resonances, can lead to dynamical excitation and radial mixing comparable to the current asteroid belt. The mass depletion driven by embedded planetary embryos alone, even in the case of an eccentric Jupiter and Saturn, is roughly 10–20× less than necessary to explain the current mass of the main belt, and thus a secondary depletion event, such as that which occurs naturally in the Nice Model, is required. We discuss the implications of our new simulations for the dynamical and collisional evolution of the main belt.
We use a smooth particle hydrodynamics method to simulate colliding rocky and icy bodies from centimeter scale to hundreds of kilometers in diameter in an effort to define self-consistently the threshold for catastrophic disruption. Unlike previous efforts, this analysis incorporates the combined effects of material strength (using a brittle fragmentation model) and self-gravitation, thereby providing results in the “strength regime” and the “gravity regime,” and in between. In each case, the structural properties of the largest remnant are examined.Our main result is that gravity plays a dominant role in determining the outcome of collisions even involving relatively small targets. In the size range considered here, the enhanced role of gravity is not due to fracture prevention by gravitational compression, but rather to the difficulty of the fragments to escape their mutual gravitational attraction. Owing to the low efficiency of momentum transfer in collisions, the velocity of larger fragments tends to be small, and more energetic collisions are needed to disperse them.We find that the weakest bodies in the Solar System, as far as impact disruption is concerned, are about 300 m in diameter. Beyond this size, objects become more difficult to disperse even though they are still easily shattered. Thus, larger remnants of collisions involving targets larger than about 1 km in radius should essentially be self-gravitating aggregates of smaller fragments.
Thermal models and radiometric ages for meteorites show that the peak temperatures inside their parent bodies were closely linked to their accretion times. Most iron meteorites come from bodies that accreted <0.5 Myr after CAIs formed and were melted by 26Al and 60Fe, probably inside 2 AU. Rare carbon-rich differentiated meteorites like ureilites probably also come from bodies that formed <1 Myr after CAIs, but in the outer part of the asteroid belt. Chondrite groups accreted intermittently from diverse batches of chondrules and other materials over a 4 Myr period starting 1 Myr after CAI formation when planetary embryos may already have formed at ∼1 AU. Meteorite evidence precludes accretion of late-forming chondrites on the surface of early-formed bodies; instead chondritic and non-chondritic meteorites probably formed in separate planetesimals. Maximum metamorphic temperatures in chondrite groups are correlated with mean chondrule age, as expected if 26Al and 60Fe were the predominant heat sources. Because late-forming bodies could not accrete close to large, early-formed bodies, planetesimal formation may have spread across the nebula from regions where the differentiated bodies formed. Dynamical models suggest that the asteroids could not have accreted in the main belt if Jupiter formed before the asteroids. Therefore Jupiter probably reached its current mass >3–5 Myr after CAIs formed. This precludes formation of Jupiter via a gravitational instability <1 Myr after the solar nebula formed, and strongly favors core accretion. Jupiter probably formed too late to make chondrules by generating shocks directly, or indirectly by scattering Ceres-sized bodies across the belt. Nevertheless, shocks formed by gravitational instabilities or Ceres-sized bodies scattered by planetary embryos may have produced some chondrules. The minimum lifetime for the solar nebula of 3–5 Myr inferred from the total spread of CAI and chondrule ages may exceed the median lifetime of 3 Myr for protoplanetary disks, but is well within the 1–10 Myr observed range. Shorter formation times for extrasolar planets may help to explain their unusual orbits compared to those of solar giant planets.
The target of the Deep Impact space mission (NASA), Comet 9P/Tempel 1, was observed from two nights before impact to eight nights after impact using the FORS spectrographs at the ESO VLT UT1 and UT2 telescopes. Low resolution optical long-slit spectra were obtained to study the evolution of the gas coma around the Deep Impact event. Following first results of this observing campaign on the CN and dust activity [Rauer, H., Weiler, M., Sterken, C., Jehin, E., Knollenberg, J., Hainaut, O., 2006. Astron. Astrophys. 459, 257–263], this work presents a study of the complete dataset on CN, C2, C3, and NH2 activity of Comet 9P/Tempel 1. An extended impact gas cloud was observed moving radially outwards. No compositional differences between this impact cloud and the undisturbed coma were found as far as the observed radicals are concerned. The gas production rates before and well after impact indicate no change in the cometary activity on an intermediate time scale. Over the observing period, the activity of Comet 9P/Tempel 1 was found to be related to the rotation of the cometary nucleus. The rotational lightcurve for different gaseous species provides indications for compositional differences among different parts of the nucleus surface.
Planet formation models suggest the primordial main belt experienced a short but intense period of collisional evolution shortly after the formation of planetary embryos. This period is believed to have lasted until Jupiter reached its full size, when dynamical processes (e.g., sweeping resonances, excitation via planetary embryos) ejected most planetesimals from the main belt zone. The few planetesimals left behind continued to undergo comminution at a reduced rate until the present day. We investigated how this scenario affects the main belt size distribution over Solar System history using a collisional evolution model (CoEM) that accounts for these events. CoEM does not explicitly include results from dynamical models, but instead treats the unknown size of the primordial main belt and the nature/timing of its dynamical depletion using innovative but approximate methods. Model constraints were provided by the observed size frequency distribution of the asteroid belt, the observed population of asteroid families, the cratered surface of differentiated Asteroid (4) Vesta, and the relatively constant crater production rate of the Earth and Moon over the last 3 Gyr. Using CoEM, we solved for both the shape of the initial main belt size distribution after accretion and the asteroid disruption scaling law . In contrast to previous efforts, we find our derived function is very similar to results produced by numerical hydrocode simulations of asteroid impacts. Our best fit results suggest the asteroid belt experienced as much comminution over its early history as it has since it reached its low-mass state approximately 3.9–4.5 Ga. These results suggest the main belt's wavy-shaped size-frequency distribution is a “fossil” from this violent early epoch. We find that most diameter D≳120 km asteroids are primordial, with their physical properties likely determined during the accretion epoch. Conversely, most smaller asteroids are byproducts of fragmentation events. The observed changes in the asteroid spin rate and lightcurve distributions near D∼100–120 km are likely to be a byproduct of this difference. Estimates based on our results imply the primordial main belt population (in the form of D<1000 km bodies) was 150–250 times larger than it is today, in agreement with recent dynamical simulations.
Observations of circumstellar disks around stars as a function of stellar properties such as mass, metallicity, multiplicity, and age, provide constraints on theories concerning the formation and evolution of planetary systems. Utilizing ground- and space-based data from the far-UV to the millimeter, astronomners can assess the amount, composition, and location of circumstellar gas and dust as a function of time. We review primarily results from the Spitzer Space Telescope, with reference to other ground- and space-based observations. Comparing these results with those from exoplanet search techniques, theoretical models, as well as the inferred history of our solar system, helps us to assess whether planetary systems like our own, and the potential for life that they represent, are common or rare in the Milky Way galaxy. Comment: To appear in IAU Symposium No. 258, Eds. E. Mamajek, D.R. Soderblom, and R.F.G. Wyse
Four stages in the accretion of planetesimals are described. The initial stage is the condensation of dust particles from the gaseous solar nebula as it cools. These dust particles settle into a thin disk which is gravitationally unstable. A first generation of planetesimals, whose radii range up to about 0.1 km, form from the dust disk by direct gravitational collapse to solid densities on a time scale of the order of 1 year. The resulting disk, composed of first-generation planetesimals, is still gravitationally unstable, and the planetesimals are grouped into clusters containing approximately 10,000 members. The contraction of these clusters is controlled by the rate at which gas drag damps their internal rotational and random kinetic energies. On a time scale of a few thousand years, the clusters contract to form a second generation of planetesimals having radii of the order of 5 km. Further coalescence of planetesimals proceeds by direct collisions which seem capable of producing growth at a rate of the order of 15 cm per year at 1 AU.
Several methods for estimating the outcomes of close planetary encounters are compared on the basis of the numerical integration of a range of encounter types. An attempt is made to lay the foundation for the development of predictive rules concerning the encounter outcomes applicable to the refinement of the statistical mechanics that apply to planet-formation and similar problems concerning planetary swarms. Attention is given to Oepik's (1976) formulation of the two-body approximation, whose predicted motion differs from the correct three-body behavior.
The petrology record on the Moon suggests that a cataclysmic spike in the cratering rate occurred approximately 700 million years after the planets formed; this event is known as the Late Heavy Bombardment (LHB). Planetary formation theories cannot naturally account for an intense period of planetesimal bombardment so late in Solar System history. Several models have been proposed to explain a late impact spike, but none of them has been set within a self-consistent framework of Solar System evolution. Here we propose that the LHB was triggered by the rapid migration of the giant planets, which occurred after a long quiescent period. During this burst of migration, the planetesimal disk outside the orbits of the planets was destabilized, causing a sudden massive delivery of planetesimals to the inner Solar System. The asteroid belt was also strongly perturbed, with these objects supplying a significant fraction of the LHB impactors in accordance with recent geochemical evidence. Our model not only naturally explains the LHB, but also reproduces the observational constraints of the outer Solar System.
Planetary formation theories suggest that the giant planets formed on circular and coplanar orbits. The eccentricities of Jupiter, Saturn and Uranus, however, reach values of 6 per cent, 9 per cent and 8 per cent, respectively. In addition, the inclinations of the orbital planes of Saturn, Uranus and Neptune take maximum values of approximately 2 degrees with respect to the mean orbital plane of Jupiter. Existing models for the excitation of the eccentricity of extrasolar giant planets have not been successfully applied to the Solar System. Here we show that a planetary system with initial quasi-circular, coplanar orbits would have evolved to the current orbital configuration, provided that Jupiter and Saturn crossed their 1:2 orbital resonance. We show that this resonance crossing could have occurred as the giant planets migrated owing to their interaction with a disk of planetesimals. Our model reproduces all the important characteristics of the giant planets' orbits, namely their final semimajor axes, eccentricities and mutual inclinations.
Long- and short-lived radioactive isotopes and their daughter products in meteorites are chronometers that can test models for Solar System formation. Differentiated meteorites come from parent bodies that were once molten and separated into metal cores and silicate mantles. Mineral ages for these meteorites, however, are typically younger than age constraints for planetesimal differentiation. Such young ages indicate that the energy required to melt their parent bodies could not have come from the most likely heat source-radioactive decay of short-lived nuclides ((26)Al and (60)Fe) injected from a nearby supernova-because these would have largely decayed by the time of melting. Here we report an age of 4.5662 +/- 0.0001 billion years (based on Pb-Pb dating) for basaltic angrites, which is only 1 Myr younger than the currently accepted minimum age of the Solar System and corresponds to a time when (26)Al and (60)Fe decay could have triggered planetesimal melting. Small (26)Mg excesses in bulk angrite samples confirm that (26)Al decay contributed to the melting of their parent body. These results indicate that the accretion of differentiated planetesimals pre-dated that of undifferentiated planetesimals, and reveals the minimum Solar System age to be 4.5695 +/- 0.0002 billion years.
The main asteroid belt has lost > 99:9% of its solid mass since the time at which the planets were forming, according to models for the protoplanetary disk. Here we show that the asteroid belt could have been cleared eciently if much of the original mass accreted to form planet-sized bodies, which were capable of perturbing one another into unstable orbits.. We provide results from 25 N-body integrations of up to 200 planets in the asteroid belt, with individual masses in the range 0.017-0.33 Earth masses. In the simulations, these bodies undergo repeated close encounters which scatter one another into unstable resonances with the giant planets, leading to collision with the Sun or ejection from the Solar System. In response, the giant planets' orbits migrate radially and become more circular. This reduces the size of the main-belt resonances and the speed with which they removed more material, although clearing continues. Typically, 90% of the solid mass in the belt is removed in 10-100 million years (Ma), with 1 or 2 asteroidal planets" surviving for up to several hundred Ma. These objects are ultimately removed by interactions with planets in the terrestrial region.
We report the results of the first sensitive L-band (3.5 micron) survey of the intermediate age (2.5 - 30 Myr) clusters NGC 2264, NGC 2362 and NGC 1960. We use JHKL colors to obtain a census of the circumstellar disk fractions in each cluster. We find disk fractions of 52% +/- 10%, 12% +/- 4% and 3% +/- 3% for the three clusters respectively. Together with our previously published JHKL investigations of the younger NGC 2024, Trapezium and IC 348 clusters, we have completed the first systematic and homogenous survey for circumstellar disks in a sample of young clusters that both span a significant range in age (0.3 - 30 Myr) and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high (> 80%) and rapidly decreases with increasing cluster age, such that half the stars within the clusters lose their disks in < ~3 Myr. Moreover, these observations yield an overall disk lifetime of ~ 6 Myr in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet building timescale in stellar clusters. The implications of these results for current theories of planet formation are briefly discussed. Comment: 12 pages, 1 figure, 1 table. To appear in ApJ Letters
We outline a scenario which traces a direct path from freely-floating nebula particles to the first 10-100km-sized bodies in the terrestrial planet region, producing planetesimals which have properties matching those of primitive meteorite parent bodies. We call this "primary accretion". The scenario draws on elements of previous work, and introduces a new critical threshold for planetesimal formation. We presume the nebula to be weakly turbulent, which leads to dense concentrations of aerodynamically size-sorted particles having properties like those observed in chondrites. The fractional volume of the nebula occupied by these dense zones or clumps obeys a probability distribution as a function of their density, and the densest concentrations have particle mass density 100 times that of the gas. However, even these densest clumps are prevented by gas pressure from undergoing gravitational instability in the traditional sense (on a dynamical timescale). While in this state of arrested development, they are susceptible to disruption by the ram pressure of the differentially orbiting nebula gas. However, self-gravity can preserve sufficiently large and dense clumps from ram pressure disruption, allowing their entrained particles to sediment gently but inexorably towards their centers, producing 10-100 km "sandpile" planetesimals. Localized radial pressure fluctuations in the nebula, and interactions between differentially moving dense clumps, will also play a role that must be allowed for in future studies. The scenario is readily extended from meteorite parent bodies to primary accretion throughout the solar system. Comment: Astrophys. J accepted April 11, 2008
- Weiler
- J. E. Chambers
- G. W. Wetherill
- J. N. Cuzzi
- R. C. Hogan
- K. Shariff
- M. Weiler
- H. Rauer
- J. Knollenberg
- C. Sterken
- M. J. Drake