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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.03.033 Byline: S. Fornasier (a)(b), E. Dotto (c), O. Hainaut (d), F. Marzari (e), H. Boehnhardt (f), F. De Luise (c), M.A. Barucci (b) Keywords: Trojan asteroids; Asteroids; composition; Photometry; Spectroscopy Abstract: We present the results of a visible spectroscopic and photometric survey of Jupiter Trojans belonging to different dynamical families. The survey was carried out at the 3.5 m New Technology Telescope (NTT) of the European Southern Observatory (La Silla, Chile) in April 2003, May 2004 and January 2005. We obtained data on 47 objects, 23 belonging to the L5 swarm and 24 to the L4 one. These data together with those already published by Fornasier et al. [Fornasier, S., Dotto, E., Marzari, F., Barucci, M.A., Boehnhardt, H., Hainaut, O., de Bergh, C., 2004a. Icarus 172, 221-232] and Dotto et al. [Dotto, E., Fornasier, S., Barucci, M.A., Licandro, J., Boehnhardt, H., Hainaut, O., Marzari, F., de Bergh, C., De Luise, F., 2006. Icarus 183, 420-434], acquired since November 2002, constitute a total sample of visible spectra for 80 objects. The survey allows us to investigate six families (Aneas, Anchises, Misenus, Phereclos, Sarpedon, Panthoos) in the L5 cloud and four L4 families (Eurybates, Menelaus, 1986 WD and 1986 TS6). The sample that we measured is dominated by D-type asteroids, with the exception of the Eurybates family in the L4 swarm, where there is a dominance of C- and P-type asteroids. All the spectra that we obtained are featureless with the exception of some Eurybates members, where a drop-off of the reflectance is detected shortward of 5200 A. Similar features are seen in main belt C-type asteroids and commonly attributed to the intervalence charge transfer transition in oxidized iron. Our sample comprises fainter and smaller Trojans as compared to the literature's data and allows us to investigate the properties of objects with estimated diameter smaller than 40-50 km. The analysis of the spectral slopes and colors versus the estimated diameters shows that the blue and red objects have indistinguishable size distribution, so any relationship between size and spectral slopes has been found. To fully investigate the Trojans population, we include in our analysis 62 spectra of Trojans available in literature, resulting in a total sample of 142 objects. Although the mean spectral behavior of L4 and L5 Trojans is indistinguishable within the uncertainties, we find that the L4 population is more heterogeneous and that it has a higher abundance of bluish objects as compared to the L5 swarm. Finally, we perform a statistical investigation of the Trojans's spectra property distributions as a function of their orbital and physical parameters, and in comparison with other classes of minor bodies in the outer Solar System. Trojans at lower inclination appear significantly bluer than those at higher inclination, but this effect is strongly driven by the Eurybates family. The mean colors of the Trojans are similar to those of short period comets and neutral Centaurs, but their color distributions are different. Author Affiliation: (a) University of Paris 7 'Denis Diderot,' 10 rue Alice Domon et Leonie Duquet, 75013 Paris, France (b) LESIA, Paris Observatory, Batiment 17, 5 Place Jules Janssen, 92195 Meudon Cedex, France (c) INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone (Roma), Italy (d) European Southern Observatory, Casilla 19001, Santiago, Chile (e) Dipartimento di Fisica, Universita di Padova, Via Marzolo 8, 35131 Padova, Italy (f) Max-Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany Article History: Received 2 December 2006; Revised 20 March 2007 Article Note: (footnote) [star] Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, ESO proposals 71.C-0650, 73.C-0622, 74.C-0577.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.02.012 Byline: E. Dotto (a), S. Fornasier (b), M.A. Barucci (c), J. Licandro (d)(e), H. Boehnhardt (f), O. Hainaut (g), F. Marzari (h), C. de Bergh (c), F. De Luise (a) Keywords: Asteroids; Photometry; Spectroscopy Abstract: Asteroid dynamical families are supposed to be formed from the collisional disruption of parent bodies. As a consequence, the investigation of the surface properties of small and large family members may give some hints on the nature of the dynamical group, the internal composition of the parent body, and the role played by space weathering processes in modifying the spectral behavior of the members' surfaces. In this work we present visible-near-infrared observations of 24 Jupiter Trojans belonging to seven dynamical families of both the L4 and L5 swarms. The most important characteristics we found is the uniformity of the Trojans population. All the investigated Trojans have featureless spectra and a spectral behavior typical of the primitive P and D taxonomic classes. In particular, no signatures of water ice have been found on the spectra of these primordial bodies. From our investigation, the L4 and L5 clouds appear to be compositionally indistinguishable. Tentative models of the surface composition, based on the Hapke theory, are presented and discussed. Author Affiliation: (a) INAF, Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monteporzio Catone (Roma), Italy (b) Department of Astronomy, University of Padova, Vicolo dell'Osservatorio 2, I-35122 Padova, Italy (c) LESIA, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92195 Meudon, France (d) Isaac Newton Group of Telescopes, P.O. Box 321, E-38700 Santa Cruz de la Palma, Tenerife, Spain (e) Instituto de Astrofisica de Canarias, c/Via Lactea s/n, E-38200 La Laguna, Tenerife, Spain (f) Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, D-37191 Katlenburg-Lindau, Germany (g) European Southern Observatory, Casilla 19001, Santiago, Chile (h) Department of Physics, University of Padova, Via Marzolo 8, I-35131 Padova, Italy Article History: Received 27 June 2005; Revised 14 February 2006 Article Note: (footnote) [star] Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, ESO proposals 69.C-0524 and 71.C-0650, and at the Telescopio Nazionale Galileo, La Palma, Canary Island, proposals TAC06 (AOT7) and TAC705 (AOT6).

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.01.022 Byline: P. Thebault (a)(b), F. Marzari (c), H. Scholl (d) Keywords: Planetary formation; Planetary dynamics; Accretion Abstract: We investigate classical planetesimal accretion in a binary star system of separation a.sub.ba[c]1/250AU by numerical simulations, with particular focus on the region at a distance of 1 AU from the primary. The planetesimals orbit the primary, are perturbed by the companion and are in addition subjected to a gas drag force. We concentrate on the problem of relative velocities [DELTA]v among planetesimals of different sizes. For various stellar mass ratios and binary orbital parameters we determine regions where [DELTA]v exceed planetesimal escape velocities v.sub.esc (thus preventing runaway accretion) or even the threshold velocity v.sub.ero for which erosion dominates accretion. Gaseous friction has two crucial effects on the velocity distribution: it damps secular perturbations by forcing periastron alignment of orbits, but at the same time the size-dependence of this orbital alignment induces a significant [DELTA]v increase between bodies of different sizes. This differential phasing effect proves very efficient and almost always increases [DELTA]v to values preventing runaway accretion, except in a narrow e.sub.ba0 domain. The erosion threshold [DELTA]vv.sub.ero is reached in a wide (a.sub.b,e.sub.b) space for small Author Affiliation: (a) Stockholm Observatory, Albanova Universitetcentrum, SE-10691 Stockholm, Sweden (b) Observatoire de Paris, Section de Meudon, F-92195 Meudon Principal Cedex, France (c) Dipartimento di Fisica, Universita di Padova, Via Marzolo 8, I-35131 Padova, Italy (d) Observatoire de la CA[acute accent]te d'Azur, Dept. Cassiopee, B.P. 4229, F-06304 Nice, France Article History: Received 25 July 2005; Revised 27 January 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.01.018 Byline: H. Scholl (a), F. Marzari (b), P. Tricarico (c) Abstract: In this paper we explore the dynamical stability of the Mars Trojan region applying mainly Laskar's Frequency Map Analysis. This method yields the chaotic diffusion rate of orbits and allows to determine the most stable regions. It also gives the frequencies which are responsible for the instability of orbits. The most stable regions are found for inclinations between about 15[degrees] and 30[degrees]. For inclinations smaller than 15[degrees], we confirm, by applying a synthetic secular theory, that the secular resonances [nu].sub.3, [nu].sub.4, [nu].sub.13, [nu].sub.14 rapidly excite asteroid orbits within a few Myrs, or even faster. The asteroids are removed from the Trojan region after a close encounter with Mars. For large inclinations, the secular resonance [nu].sub.5 clears a small region around 30[degrees] while the Kozai resonance rapidly removes bodies for inclinations larger than 35[degrees]. The dynamical lifetimes of the three L5 Trojans, (5261) Eureka, 1998 VF31, 2001 DH47, and the only L4 Trojan 1999 UJ7 are determined by numerically integrating clouds of corresponding clones over the age of the Solar System. All four Trojans reside in the most stable region with smallest diffusion coefficients. Their dynamical half-lifetime is of the order of the age of the Solar System. The Yarkovsky force has little effect on the known Trojans but for bodies smaller than about 1-5 m the drag is strong enough to destabilize Trojans on a timescale shorter than 4.5 Gyr. Author Affiliation: (a) Observatoire de la CA[acute accent]te d'Azur, Nice, France (b) Dipartimento di Fisica, Universita di Padova, 35131 Padova, Italy (c) Department of Physics, Washington State University, Pullman, WA 99164, USA

In this paper we present results obtained in the framework of a visible spectroscopic and photometric survey of Trojan asteroids. We concentrated on bodies orbiting at the L5 Lagrangian point of Jupiter that are also members of dynamical families. Spectroscopy is a crucial tool that allows us to characterize the mineralogical composition of families and their parent bodies, gives evidence of ongoing space weathering, and confirms family membership. We have observed 18 objects belonging to the Aneas, Astyanax, Sarpedon, and Phereclos families as defined by Beauge and Roig (2001, Icarus 53, 391). In addition, we have determined the spectroscopic properties of 8 background Jupiter Trojans. The observed spectra are reddish with a dominance of D-type asteroids. As expected, the spectra of the non-family members are more heterogeneous compared to the spectra of family members, with the exception of the members of the Aneas family. We also confirm the lack of absorption features in the visible region, as already reported by other authors. Keywords: Asteroids; Photometry; Spectroscopy: Jupiter Trojans; Families

In this paper we study the statistical effect of planetary flybys on the rotation rates and states of Near Earth Objects (NEOs). Our approach combines numerical and analytical methods within a Monte Carlo model that simulates the evolution of the NEO spin rates. We take as input for the simulation a source distribution of spin states and evolve it to find their steady state distribution. In performing this evolution we track the changes in the spin rate and state distribution for the different components of the NEO population. We show that the cumulative effect of planetary encounters is to spin up the overall population of NEOs. This spin up effect holds on average only, and particular members of the population may experience an overall decrease in rotation rate. This effect is clearly seen across all components of the NEO population and is significant both statistically and physically. For initially slow rotators the spin up effect is strong, lowering the mean rotation period by 32%. For faster rotating populations the effect is less, lowering the spin period by 15% for the intermediate case, 6% for fast rotating rubble piles, and 8% for fast rotating monoliths. Physically, the spin up effect pushes 1% of the fast rotating rubble-pile NEOs over the disruption limit, while 6% of these bodies experience a sub-disruption event that could modify their physical structure. For monolithic NEOs, the spin up effect is self-limiting, reaching a minimum spin period of 1.1 hr, with a strong cut-off between 2-3 hr. This has two implications. First, it may not be necessary to invoke the rubble-pile hypothesis to recover a cut-off in spin period. Second, it shows that planetary flybys cannot account for the extremely rapid rotation rates of some small NEOs. We also tested a different balance between the effects of Earth and Venus by treating the Aten sub-class of asteroids separately. Due to increased interactions with the planets, the spin up effect is more pronounced (10%) and disruptions increase by a factor of three. The slow rotation tails of the spin distributions are increased to longer periods, in general, with rotation periods of over 100 hr occurring for a few tenths of a percent for some component populations. Thus, this mechanism may account for some of the noted excess in slow rotators among the NEOs. Planetary flybys also cause NEOs to enter a tumbling state, with approximately 0.5% of the population being placed into a long-axis rotation mode. Finally, based on the evolution of spin states of different components of the NEO population, we compared the evolved states with the measured distribution of NEOs to estimate the relative populations of these components that comprise the NEOs. Keywords: Asteroids; Rotation; Rotational dynamics; Comets; Dynamics

We model with numerical algorithms the dynamical processes that possibly lead to the trapping of Jupiter's Trojans from a primordial population of planetesimals orbiting nearby a proto-Jupiter. The predictions of models based on mutual planetesimal collisions and on the mass growth of Jupiter are compared with observations. In particular, we concentrate on the distribution of the libration amplitude. The two mechanisms for trapping reproduce closely the libration amplitude distribution of the real Trojans only when the long-term dynamical diffusion described by Levison et al. (1997, Nature 385, 42-44) is taken into account. Keywords: Celestial mechanics; Asteroids

We have numerically explored the mechanisms that destabilize Jupiter's Trojan orbits outside the stability region defined by Levison et al. (1997, Nature 385, 42-44). Different models have been exploited to test various possible sources of instability on timescales on the order of [approximately equal to] [10.sup.8] years. In the restricted three-body model, only a few Trojan orbits become unstable within ~[10.sup.8] years. This intrinsic instability contributes only marginally to the overall instability found by Levison et al. In a model where the orbital parameters of both Jupiter and Saturn are fixed, we have investigated the role of Saturn and its gravitational influence. We find that a large fraction of Trojan orbits become unstable because of the direct nonresonant perturbations by Saturn. By shifting its semimajor axis at constant intervals around its present value we find that the near 5:2 mean motion resonance between the two giant planets (the Great Inequality) is not responsible for the gross instability of Jupiter's Trojans since short-term perturbations by Saturn destabilize Trojans, even when the two planets are far out of the resonance. Secular resonances are an additional source of instability. In the full six-body model with the four major planets included in the numerical integration, we have analyzed the effects of secular resonances with the node of the planets. Trojan asteroids have relevant inclinations, and nodal secular resonances play an important role. When a Trojan orbit becomes unstable, in most cases the libration amplitude of the critical argument of the 1:1 mean motion resonance grows until the asteroid encounters the planet. Libration amplitude, eccentricity, and nodal rate are linked for Trojan orbits by an algebraic relation so that when one of the three parameters is perturbed, the other two are affected as well. There are numerous secular resonances with the nodal rate of Jupiter that fall inside the region of instability and contribute to destabilize Trojans, in particular the [v.sub.16]. Indeed, in the full model the escape rate over 50 Myr is higher compared to the fixed model. Some secular resonances even cross the stability region delimited by Levison et al. and cause instability. This is the case of the 3:2 and 1:2 nodal resonances with Jupiter. In particular the 1:2 is responsible for the instability of some clones of the IA Trojan (3540) Protesilaos. Key Words: asteroids; dynamics.

Most extrasolar planets discovered to date are more massive than Jupiter, in surprisingly small orbits (semimajor axes less than 3 AU). Many of these have significant orbital eccentricities. Such orbits may be the product of dynamical interactions in multiplanet systems. We examine outcomes of such evolution in systems of three Jupiter-mass planets around a solar-mass star by integration of their orbits in three dimensions. Such systems are unstable for a broad range of initial conditions, with mutual perturbations leading to crossing orbits and close encounters. The time scale for instability to develop depends on the initial orbital spacing; some configurations become chaotic after delays exceeding [10.sup.8] y. The most common outcome of gravitational scattering by close encounters is hyperbolic ejection of one planet. Of the two survivors, one is moved closer to the star and the other is left in a distant orbit; for systems with equal-mass planets, there is no correlation between initial and final orbital positions. Both survivors may have significant eccentricities, and the mutual inclination of their orbits can be large. The inner survivor's semimajor axis is usually about half that of the innermost starting orbit. Gravitational scattering alone cannot produce the observed excess of 'hot Jupiters' in close circular orbits. However, those scattered planets with large eccentricities and small periastron distances may become circularized if tidal dissipation is effective. Most stars with a massive planet in an eccentric orbit should have at least one additional planet of comparable mass in a more distant orbit.

We have investigated the creation and destruction of asteroid families using a collisional code which follows the evolution of the size distributions of both the main-belt background population and individual families which are produced by collisions within the population. Starting with a small-mass initial belt, the number of families produced by collisional breakup of parent bodies larger than 100 km and surviving to the present time is in good agreement with the observed number of families. Increasing the mass of the initial belt increases the number of families in the model; hence the number of families that we see today provides a significant constraint on the overall collisional history of asteroids. Most families formed by disruption of large (D > 200 km) parent bodies are still recognizable today. Families formed from smaller parent bodies early in Solar System history have been eroded away, and there is a general trend in our models to have younger ages associated with families formed from smaller parent bodies. Today about one-third of the overall population should be in families, consistent with the findings of V. Zappala et al. (1995, Icarus 116, 291-314).

We have refined a method to derive from Io's sodium cloud spectra precise estimates of the production rate and of the ejection velocity distribution at the exobase. This method is based on a detailed comparison between the observed spectra and synthetic spectra numerically simulated with a model derived from the Smyth and Combi (1988, Astrophys. J. Suppl. 66, 397-411) formalism. The innovative aspect of our approach consists in building up polienergetic spectra as linear combination of normalized single velocity spectra. In this way we simulate the dispersion of the sodium atom ejection velocities at the exobase and derive for each velocity component a production rate. Smyth and Combi (1988, Astrophys. J. 328, 888-918) proposed a similar approach to study the velocity distribution of sodium atoms at the exobase. However, they did not work with spectra but with bidimensional images of the Io's cloud. Moreover they did not try to derive from the model-observation comparison an estimate of the production rate. We have applied the polienergetic model spectra approach to analyze a sequence of 59 observations of the sodium cloud spanning a period of 4 years from February 1990 to April 1994. The echelle spectra have been reduced to separate the fast sodium components and then compared to polienergetic synthetic spectra. We report the behavior of the production rate in the timespan covered by our observations and the analysis of the velocity components at the exobase. We find a relationship between both the production rate and the velocity distribution and [[Lambda].sub.III].

We have studied the capture of planetesimals in Trojan orbits by a growing proto-Jupiter by integrating numerically the equations of motion of planetesimals in a four-body problem Sun-Jupiter-Saturn-planetesimal. The masses of Jupiter and Saturn increase exponentially with time. Nebular gas drag on small planetesimals is taken into account. We show that a fraction of the planetesimals near Jupiter's orbit and almost all planetesimals originally in horseshoe orbits are trapped into stable Trojan orbits. This mechanism was probably an important factor together with collisional diffusion, for delivering a large amount of planetesimals into Trojan orbits during the final growth of Jupiter by gas in-fall. We find also that the libration amplitude of trapped Trojans is damped by the increase of Jupiter's mass reinforcing the stability of the resonance lock. For planetesimals small enough to be perturbed by nebular gas drag, we find an asymmetry between the trapping rate in L5 compared to L4. Peale (1993, Icarus 106, 308-322) noted also that the stability of the L4 resonant region is reduced in presence of an intense frictional drag. We present here a simple semi-analytical explanation for the trapping asymmetry based on the evolution of the angular difference between the planetesimal's and proto-Jupiter's longitude of perihelion. The asymmetry between L4 and L5 could have led to the capture of a larger number of small planetesimals in L5. This may have favored a higher degree of collisional activity in L5 and might explain an alleged asymmetry in the size distribution of the tWO swarms.

We use our multi-zone simulation code (D. Spaute, S. Weidenschilling, D. R. Davis, and F. Marzari, Icarus 92, 147-164, 1991) to model numerically the accretion of a swarm of planetesimals in the region of the terrestrial planets. The hybrid code allows interactions between a continuum distribution of small bodies in a series of orbital zones and a population of large, discrete planetary embryos in individual orbits. Orbital eccentricities and inclinations evolve independently, and collisional and gravitational interactions among the embryos are treated stochastically by a Monte Carlo approach. The spatial resolution of our code allows modeling of the intermediate stage when particle-in-a-box methods lose validity due to nonuniformity in the planetesimal swarm. The simulations presented here bridge the gap between such early-stage models and N-body calculations of the final stage of planetary accretion. The code has been tested for a variety of assumptions for stirring of eccentricities and inclinations by gravitational perturbations and the presence or absence of damping by gas drag. Viscous stirring, which acts to increase relative velocities of bodies in crossing orbits, produces so-called 'orderly' growth, with a power-law size distribution having most of the mass in the largest bodies. Addition of dynamical friction, which tends to equalize kinetic energies and damp the velocities of the more massive bodies, produces rapid 'runaway' growth of a small number of embryos. Their later evolution is affected by distant perturbations between bodies in non-crossing orbits. Distant perturbations increase eccentricities while allowing inclinations to remain low, promoting collisions between embryos and reducing their tendency to become dynamically isolated. Growth is aided by orbital decay of smaller bodies due to gas drag, which prevents them from being stranded between orbits of the embryos. We report results of a large-scale simulation of accretion in the region of terrestrial planets, employing 100 zones spanning the range 0.5 to .5 AU and spanning [10.sup.6] years of model time. The final masses of the largest bodies are several times larger than predicted by a simple analytic model of runaway growth, but a minimal-mass planetisimal swarm still yields smaller bodies, in more closely spaced orbits, than the actual terrestrial planets. Longer time scales, additional physical phenomena, and/or a more massive swarm may be needed to produce Earth-like planets.

We model the collisional evolution of Trojan asteroids using a numerical code which combines recent calculations of the intrinsic collision probabilities and impact speeds in the Trojan swarms (Marzari et al. 1996) with our current understanding of the outcomes of high-velocity collisions between asteroid-sized bodies. Using plausible collision parameters and energy scaling of impact strength with size, we obtain a good match to the present Trojan population, as inferred by Shoemaker et al. (1989). The steep slope of the current Trojan size distribution at diameters larger than about 50-100 km is essentially unaltered by the collisional process and must reflect the formative processes of these bodies. At smaller sizes, collisions have produced a power law size distribution having a slope characteristic of collisionally relaxed populations. Hence, we cannot distinguish whether the small-size end of the distribution was formed before or after the disruptive collisional regime characteristic of the present Trojan environment was established. The formation of prominent dynamical families in the Trojan swarms is a natural outcome of the collisional process; their number may allow us to constrain the degree of collisional evolution that has occurred in the Trojans. Finally, we find that Trojan collisional debris escaping from the libration regions and ending up into cometary orbits could supply [approximately equal to] 10% of the current population of short-period comets and Centaur asteroids. Whether this occurs depends on the dynamical lifetime of such bodies and whether they contain enough volatiles to become active comets.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.06.002 Byline: D.J. Scheeres (a), L.A.M. Benner (b), S.J. Ostro (b), A. Rossi (c), F. Marzari (d), P. Washabaugh (a) Keywords: Asteroids; Rotational dynamics; Rotation; Dynamics; Radar Abstract: We predict that when Asteroid 2004 MN4 passes 5.6[+ or -]1.4 Earth radii from Earth's center on April 13, 2029, terrestrial torques during the flyby will alter its spin state in a dramatic manner that will be observable using groundbased telescopes. Although the asteroid will most likely not undergo catastrophic disruption, it may be subject to localized failure across its surface and interior, providing a unique opportunity to measure otherwise inaccessible mechanical properties of an asteroid. Author Affiliation: (a) Department of Aerospace Engineering, University of Michigan, Ann Arbor, MI 48109-2140, USA (b) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA (c) ISTI-CNR, Via Moruzzi 1, 56124 Pisa, Italy (d) Department of Physics, University of Padova, Via Marzolo 8, I-35131 Padova, Italy Article History: Received 16 March 2005; Revised 2 June 2005