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Rickman H.,Pas Space Research Center | Rickman H.,Uppsala University | Wisniowski T.,Pas Space Research Center | Gabryszewski R.,Pas Space Research Center | And 6 more authors.
Astronomy and Astrophysics | Year: 2017

Context. The Nice model predicts that the trans-planetary planetesimal disk made a large or even dominant contribution to the cratering in the inner solar system during the late heavy bombardment (LHB). In the presence of evidence that lunar craters and mare basins may be mainly of asteroidal origin, there is a dilemma of the missing comets that is not yet resolved. Aims. We aim to revisit the problem of cometary impact rates on the Moon and the terrestrial planets during the LHB with a flexible model, allowing us to study the influences of physical destruction of comets, the mass of the primordial disk, and the distribution of this mass over the entire size range. Methods. We performed a Monte Carlo study of the dynamics of the cometary LHB projectiles and derive the impact rates by calculating individual collision probabilities for a huge sample of projectile orbits. We used Minimum Orbit Intersection Distances (MOIDs) according to a new scheme introduced here. Different calculations were performed using different models for the physical evolution of comet nuclei and for the properties of the primordial, trans-planetary disk. Results. Based on the capture probability of Jupiter Trojans, we find a best fit radius of the largest LHB comet impacting the Moon for a low-mass primordial disk. For this disk mass, the LHB cratering of the Moon, Mercury and Mars were dominated by asteroids. However, some smaller lunar maria were likely preceded by comet impacts. The volatile delivery to the Earth and Mars by LHB comets was much less than their water inventories. Conclusions. There is no excessive cometary cratering, if the LHB was caused by a late planetary instability in the Nice Model. The Earth and Mars obtained their water very early in their histories. The Noachian water flows on Mars cannot be attributed to the arrival of LHB-related H2O or CO2. © 2017 ESO.

Rickman H.,Pas Space Research Center | Rickman H.,Uppsala University | Marchi S.,Southwest Research Institute | A'Hearn M.F.,University of Maryland University College | And 40 more authors.
Astronomy and Astrophysics | Year: 2015

Context. One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range. Aims. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models. Methods. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system. Results. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded. Conclusions. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution. © ESO, 2015.

Fouchard M.,Lille University of Science and Technology | Fouchard M.,Institute Of Macanique Caleste Et Calcul Daaphamarides | Rickman H.,Pas Space Research Center | Rickman H.,Uppsala University | And 2 more authors.
Astronomy and Astrophysics | Year: 2011

Context. This work is a follow-up of a previous study, where we simulated the dynamical evolution of the Oort Cloud over 5 Gyr with special attention to the injection of comets into observable orbits. Aims. We wish to clarify how comet injection operates with two types of perturbers: Galactic tides and passing stars. We illustrate why attempts to identify the stars that might have played an important role in injecting the observed new Oort Cloud comets are as yet unlikely to succeed, and investigate how large an improvement can be expected from the Gaia mission. Methods. We simulate a 5 Gyr time span, concentrating on the injections found during the last 3 Gyr by extracting detailed information about the last revolution of the injected comets. We analyse the contributions of both the Galactic tides and the stars separately, and assess their importance as a function of the semi-major axis of the comets. We also compute the distances and motions of the perturbing stars at the time the comets reach their perihelia and thus estimate their observability. Results. By studying more than 20  000 injected comets, we determine how the likelihood of tidal and stellar injections varies with the semi-major axis. We establish the range of semi-major axis for which a real-time synergy between stellar and tidal perturbations is important. We find how many perturbing stars could be identified using Hipparcos and Gaia data, and how the dynamics of injections would change, if only the observable stars were acting. Conclusions. The number of injected comets peaks at a semi-major axis (a) of about 33  000 AU but the comets spread over a wide range around this value. The tides are unable to inject any comets at a < 23000 AU but would be able to inject almost all of them at a > 50000 AU. The real-time synergy is found to extend between a ∼ 15000 AU and a ∼ 45000 AU and to be the main contributor at a ∼ 25000 AU. Stellar perturbations make important contributions at all semi-major axes. On the basis of Hipparcos data, only a minority of the stars that may contribute to comet injections are detectable, since most stars have escaped to distances beyond the Hipparcos detection limit. For Gaia, on the other hand, a large majority of the perturbing stars will be both identifiable and measurable. © 2011 ESO.

Rickman H.,Pas Space Research Center | Rickman H.,Uppsala University | Winiowski T.,Pas Space Research Center | Wajer P.,Pas Space Research Center | And 3 more authors.
Astronomy and Astrophysics | Year: 2014

Context. Unraveling the events that took place in the solar system during the period known as the late heavy bombardment requires the interpretation of the cratered surfaces of the Moon and terrestrial planets. This, in turn, requires good estimates of the statistical impact probabilities for different source populations of projectiles, a subject that has received relatively little attention, since the works of Öpik (1951, Proc. R. Irish Acad. Sect. A, 54, 165) and Wetherill (1967, J. Geophys. Res., 72, 2429). Aims. We aim to work around the limitations of the Öpik and Wetherill formulae, which are caused by singularities due to zero denominators under special circumstances. Using modern computers, it is possible to make good estimates of impact probabilities by means of Monte Carlo simulations, and in this work, we explore the available options. Methods. We describe three basic methods to derive the average impact probability for a projectile with a given semi-major axis, eccentricity, and inclination with respect to a target planet on an elliptic orbit. One is a numerical averaging of the Wetherill formula; the next is a Monte Carlo super-sizing method using the target's Hill sphere. The third uses extensive minimum orbit intersection distance (MOID) calculations for a Monte Carlo sampling of potentially impacting orbits, along with calculations of the relevant interval for the timing of the encounter allowing collision. Numerical experiments are carried out for an intercomparison of the methods and to scrutinize their behavior near the singularities (zero relative inclination and equal perihelion distances). Results. We find an excellent agreement between all methods in the general case, while there appear large differences in the immediate vicinity of the singularities. With respect to the MOID method, which is the only one that does not involve simplifying assumptions and approximations, the Wetherill averaging impact probability departs by diverging toward infinity, while the Hill sphere method results in a severely underestimated probability. We provide a discussion of the reasons for these differences, and we finally present the results of the MOID method in the form of probability maps for the Earth and Mars on their current orbits. These maps show a relatively flat probability distribution, except for the occurrence of two ridges found at small inclinations and for coinciding projectile/target perihelion distances. Conclusions. Our results verify the standard formulae in the general case, away from the singularities. In fact, severe shortcomings are limited to the immediate vicinity of those extreme orbits. On the other hand, the new Monte Carlo methods can be used without excessive consumption of computer time, and the MOID method avoids the problems associated with the other methods. © 2014 ESO.

Morbidelli A.,University of Nice Sophia Antipolis | Rickman H.,Pas Space Research Center | Rickman H.,Uppsala University
Astronomy and Astrophysics | Year: 2015

Context. The Rosetta mission and its exquisite measurements have revived the debate on whether comets are pristine planetesimals or collisionally evolved objects. Aims. We investigate the collisional evolution experienced by the precursors of current comet nuclei during the early stages of the solar system in the context of the so-called Nice model. Methods. We considered two environments for the collisional evolution: (1) the transplanetary planetesimal disk, from the time of gas removal until the disk was dispersed by the migration of the ice giants; and (2) the dispersing disk during the time that the scattered disk was formed. We performed simulations using different methods in the two cases to determine the number of destructive collisions typically experienced by a comet nucleus of 2 km radius. Results. In the widely accepted scenario, where the dispersal of the planetesimal disk occurred at the time of the Late Heavy Bombardment about 4 Gy ago, comet-sized planetesimals have a very low probability of surviving destructive collisions in the disk. On the extreme assumption that the disk was dispersed directly upon gas removal, a significant fraction of the planetesimals might have remained intact. However, these survivors would still bear the marks of many nondestructive impacts. Conclusions. The Nice model of solar system evolution predicts that typical km-sized comet nuclei are predominantly fragments resulting from collisions experienced by larger parent bodies. An important goal for future research is to investigate whether the observed properties of comet nuclei are compatible with such a collisional origin. © ESO, 2015.

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