* Astronomy

Title: Are Large Trojan Asteroids Salty? An Observational, Theoretical, and Experimental Study
Authors: Bin Yang, Paul Lucey, Timothy Glotch

With a total mass similar to the main asteroid belt, the Jovian Trojan asteroids are a major feature in the solar system. Based upon the thermal infrared spectra of the largest Trojans obtained with the Spitzer space telescope, Emery et al. (2006) suggested that the surfaces of these Trojans may consist of fine-grained silicates suspended in a transparent matrix.
To explore the transparent matrix hypothesis, we adopted a modified radiative transfer model to fit the Trojan spectra simultaneously both in the near and the thermal infrared regions. Our model shows that the Trojan spectra over a wide wavelength range can be consistently explained by fine grained silicates (1-5 wt.%) and highly absorbing material (e.g. carbon or iron, 2-10 wt.%) suspended in a transparent matrix. The matrix is consistent with a deposit of salt on the surfaces of the large Trojans. However, this consistency is not an actual detection of salt and other alternatives may still be possible. We suggest that early in the Solar System history, short-lived radionuclides heated ice-rich Trojans and caused melting, internal circulation of water and dissolution of soluble materials. Briny water volcanism were facilitated by internal volatiles and a possibly global sill of frozen brine was formed beneath the cold primitive crust. The frozen brine layer was likely to be evacuated by impact erosions and evaporation of the exposed brines eventually left a lag deposit of salt. Over the Solar System's history, fine dust from comets or impacts contaminated and coloured these salty surfaces of the Trojans to produce the spectral properties observed today.

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Title: Eurybates - the only asteroid family among Trojans?
Authors: Miroslav Bro, Jakub Rozehnal

We study orbital and physical properties of Trojan asteroids of Jupiter. We try to discern all families previously discussed in literature, but we conclude there is only one significant family among Trojans, namely the cluster around asteroid (3548) Eurybates. It is the only cluster, which has all of the following characteristics: (i) it is clearly concentrated in the proper-element space; (ii) size-frequency distribution is different from background asteroids; (iii) we have a reasonable collisional/dynamical model of the family. Henceforth, we can consider it as a real collisional family.
We also report a discovery of a possible family around the asteroid (4709) Ennomos, composed mostly of small asteroids. The asteroid (4709) Ennomos is known to have a very high albedo p_V \simeq 0.15, which may be related to a hypothetical cratering event which exposed ice (Fernandez et al. 2003). The relation between the collisional family and the exposed surface of the parent body is a unique opportunity to study the physics of cratering events. However, more data are needed to confirm the existence of this family and its relationship with Ennomos.

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Title: The Capture of Trojan Asteroids by the Giant Planets During Planetary Migration
Authors: P. S. Lykawka, J. Horner

Of the four giant planets in the Solar system, only Jupiter and Neptune are currently known to possess swarms of Trojan asteroids - small objects that experience a 1:1 mean motion resonance with their host planet. In Lykawka et al. (2009), we performed extensive dynamical simulations, including planetary migration, to investigate the origin of the Neptunian Trojan population. Utilising the vast amount of simulation data obtained for that work, together with fresh results from new simulations, we here investigate the dynamical capture of Trojans by all four giant planets from a primordial trans-Neptunian disk. We find the likelihood of a given planetesimal from this region being captured onto an orbit within Jupiter's Trojan cloud lies between several times 10^-6 and 10^-5. For Saturn, the probability is found to be in the range <10^-6 to 10^-5, whilst for Uranus the probabilities range between 10^-5 and 10^-4. Finally, Neptune displays the greatest probability of Trojan capture, with values ranging between 10^-4 and 10^-3. Our results suggest that all four giant planets are able to capture and retain a significant population of Trojan objects from the disk by the end of planetary migration. As a result of encounters with the giant planets prior to Trojan capture, these objects tend to be captured on orbits that are spread over a wide range of orbital eccentricities and inclinations. The bulk of captured objects are to some extent dynamically unstable, and therefore the populations of these objects tend to decay over the age of the Solar System, providing an important ongoing source of new objects moving on dynamically unstable orbits among the giant planets. Given that a huge population of objects would be displaced by Neptune's outward migration (with a potential cumulative mass a number of times that of the Earth), we conclude that the surviving remnant of the Trojans captured during the migration of the outer planets might be sufficient to explain the currently known Trojan populations in the outer Solar system.

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Title: The resonant structure of Jupiter's trojan asteroids-II. What happens for different configurations of the planetary system
Authors: Philippe Robutel (IMCCE), Julien Bodossian (IMCCE)
(Version v2)

In a previous paper, we have found that the resonance structure of the present Jupiter Trojan swarms could be split up into four different families of resonances. Here, in a first step, we generalize these families in order to describe the resonances occurring in Trojan swarms embedded in a generic planetary system. The location of these families changes under a modification of the fundamental frequencies of the planets and we show how the resonant structure would evolve during a planetary migration. We present a general method, based on the knowledge of the fundamental frequencies of the planets and on those that can be reached by the Trojans, which makes it possible to predict and localize the main events arising in the swarms during migration. In particular, we show how the size and stability of the Trojan swarms are affected by the modification of the frequencies of the planets. Finally, we use this method to study the global dynamics of the Jovian Trojan swarms when Saturn migrates outwards. Besides the two resonances found by Morbidelli et al (2005) which could have led to the capture of the current population just after the crossing of the 2:1 orbital resonance, we also point out several sequences of chaotic events that can influence the Trojan population.

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