* Astronomy

Cassini Finds Saturn Moon has Planet-Like Qualities

Data from NASA's Cassini mission reveal Saturn's moon Phoebe has more planet-like qualities than previously thought.
Scientists had their first close-up look at Phoebe when Cassini began exploring the Saturn system in 2004. Using data from multiple spacecraft instruments and a computer model of the moon's chemistry, geophysics and geology, scientists found Phoebe was a so-called planetesimal, or remnant planetary building block. The findings appear in the April issue of the Journal Icarus.
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Title: Origin of craters on Phoebe: comparison with Cassini's data
Authors: R. P. Di Sisto, A. Brunini

Phoebe is one of the irregular satellites of Saturn; the images taken by Cassini-Huygens spacecraft allowed us to analyse its surface and the craters on it. We study the craters on Phoebe produced by Centaur objects from the Scattered Disk (SD) and plutinos escaped from the 3:2 mean motion resonance with Neptune and compare our results with the observations by Cassini. We use previous simulations on trans-Neptunian Objects and a method that allows us to obtain the number of craters and the cratering rate on Phoebe. We obtain the number of craters and the greatest crater on Phoebe produced by Centaurs in the present configuration of the Solar System. Moreover, we obtain a present normalized rate of encounters of Centaurs with Saturn of \dot F = 7.1 x  10^{-11} per year, from which we can infer the current cratering rate on Phoebe for each crater diameter. Our study and the comparison with the observations suggest that the main crater features on Phoebe are unlikely to have been produced in the present configuration of the Solar System and that they must have been acquired when the SD were depleted in the early Solar System. If this is what happened and the craters were produced when Phoebe was a satellite of Saturn, then it had to be captured, very early in the evolution of the Solar System.

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Title: A precise modelling of Phoebe's rotation
Authors: L.Cottereau, E.Aleshkina, J.Souchay

Although the rotation of some Saturn's satellites in spin-orbit has already been studied by several authors, this is not the case of the rotation of Phoebe, which has the particularity of being non resonant. The purpose of the paper is to determine for the first time and with precision its precession-nutation motion. We adopt an Hamiltonian formalism of the motion of rotation of rigid celestial body set up by Kino****a (1977) based on Andoyer variables and canonical equations. First we calculate Phoebe's obliquity at J2000,0 from available astronomical data as well as the gravitational perturbation due to Saturn on Phoebe rotational motion. Then we carry out a numerical integration and we compare our results for the precession rate and the nutation coefficients with pure analytical model. Our results for Phoebe obliquity (23° 95) and Phoebe precession rate (5580".65/cy) are very close to the respective values for the Earth. Moreover the amplitudes of the nutations (26" peak to peak for the nutaton in longitude and 8" for the nutation in obliquity) are of the same order as the respective amplitudes for the Earth. We give complete tables of nutation, obtained from a FFT analysis starting from the numerical signals. We show that a pure analytical model of the nutation is not accurate due to the fact that Phoebe orbital elements e, M and Ls are far from having a simple linear behaviour. The precession and nutation of Phoebe have been calculated for the first time in this paper. We should keep on the study in the future by studying the additional gravitational effects of the Sun, of the large satellites as Titan, as well as Saturn dynamical ellipticity.

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Well beyond the main rings is the Phoebe ring which is tilted at an angle of 27 degrees to the other rings and like Phoebe has a retrograde orbit.
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