258 research outputs found
Librational response of a deformed 3-layer Titan perturbed by non-keplerian orbit and atmospheric couplings
The analyses of Titan's gravity field obtained by Cassini space mission
suggest the presence of an internal ocean beneath its icy surface. The
characterization of the geophysical parameters of the icy shell and the ocean
is important to constrain the evolution models of Titan. The knowledge of the
librations, that are periodic oscillations around a uniform rotational motion,
can bring piece of information on the interior parameters. The objective of
this paper is to study the librational response in longitude from an analytical
approach for Titan composed of a deep atmosphere, an elastic icy shell, an
internal ocean, and an elastic rocky core perturbed by the gravitational
interactions with Saturn. We start from the librational equations developed for
a rigid satellite in synchronous spin-orbit resonance. We introduce explicitly
the atmospheric torque acting on the surface computed from the Titan IPSL GCM
(Institut Pierre Simon Laplace General Circulation Model) and the periodic
deformations of elastic solid layers due to the tides. We investigate the
librational response for various interior models in order to compare and to
identify the influence of the geophysical parameters and the impact of the
elasticity. The main librations arise at two well-separated forcing frequency
ranges: low forcing frequencies dominated by the Saturnian annual and
semi-annual frequencies, and a high forcing frequency regime dominated by
Titan's orbital frequency around Saturn. We find that internal structure models
including an internal ocean with elastic solid layers lead to the same order of
libration amplitude than the oceanless models, which makes more challenging to
differentiate them by the interpretation of librational motion.Comment: 38 pages, 4 figures. Accepted for publication in Planetary and Space
Scienc
Estimating the Lunar Core Equatorial Ellipticity using Lunar Laser Ranging
No abstract availabl
The rotation of Mimas
The Cassini mission in the Saturnian system is an outstanding opportunity to
improve our knowledge of the satellites of Saturn. The data obtained thanks to
this mission must be confronted to theoretical models. This paper aims at
modeling the rotation of Mimas, with respect to its possible internal
structure. For that, we first build different interior models, in considering
Mimas as composed of 2 rigid layers with different porosity. Then we simulate
the rotational behavior of these models in a 3-degree of freedom numerical
code, in considering complete ephemerides of a Mimas whose rotation is
disturbed by Saturn. We also estimate the deviation of its longitudinal
orientation due to tides. We expect a signature of the internal structure up to
0.53{\deg} in the longitudinal librations and an obliquity between 2 and 3
arcmin, the exact values depending on the interior. The longitudinal librations
should be detectable, inverting them to get clues on the internal structure of
Mimas is challenging
Analytical description of physical librations of Saturnian coorbital satellites Janus and Epimetheus
Janus and Epimetheus are famously known for their distinctive
horseshoe-shaped orbits resulting from a 1:1 orbital resonance. Every four
years these two satellites swap their orbits by a few tens of kilometers as a
result of their close encounter. Recently Tiscareno et al. (2009) have proposed
a model of rotation based on images from the Cassini orbiter. These authors
inferred the amplitude of rotational librational motion in longitude at the
orbital period by fitting a shape model to the recent Cassini ISS images. By a
quasiperiodic approximation of the orbital motion, we describe how the orbital
swap impacts the rotation of the satellites. To that purpose, we have developed
a formalism based on quasi-periodic series with long and short-period
librations. In this framework, the amplitude of the libration at the orbital
period is found proportional to a term accounting for the orbital swap. We
checked the analytical quasi-periodic development by performing a numerical
simulation and find both results in good agreement. To complete this study, the
results regarding the short-period librations are studied with the help of an
adiabatic-like approach
Conditions of Dynamical Stability for the HD 160691 Planetary System
The orbits in the HD 160691 planetary system at first appeared highly
unstable, but using the MEGNO and FLI techniques of global dynamics analysis in
the orbital parameter space we have found a stabilizing mechanism that could be
the key to its existence. In order to be dynamically stable, the HD 160691
planetary system has to satisfy the following conditions: (1) a 2:1 mean motion
resonance, combined with (2) an apsidal secular resonance in (3) a
configuration where the two apsidal lines are
anti-aligned, and (4) specific conditions on the respective sizes of the
eccentricities (high eccentricity for the outer orbit is in particular the most
probable necessary condition). More generally, in this original orbital
topology, where the resonance variables and librate
about while librates about , the HD
160691 system and its mechanism have revealed aspects of the 2:1 orbital
resonances that have not been observed nor analyzed before. The present
topology combined with the 2:1 resonance is indeed more wide-ranging than the
particular case of the HD 160691 planetary system. It is a new theoretical
possibility suitable for a stable regime despite relatively small semi-major
axes with respect to the important masses in interactions.Comment: 21 pages, 8 figures, 1 table, accepted version to ApJ (31 Jul 2003
Global dynamics and stability limits for planetary systems around HD 12661, HD 38529, HD 37124 and HD 160691
In order to distinguish between regular and chaotic planetary orbits we apply
a new technique called MEGNO in a wide neighbourhood of orbital parameters
determined using standard two-body Keplerian fits for HD 12661, HD 38529, HD
37124 and HD 160691 planetary systems. We show that the currently announced
orbital parameters place these systems in very different situations from the
point of view of dynamical stability. While HD 38529 and HD 37124 are located
within large stability zones in the phase space around their determined orbits,
the preliminary orbits in HD 160691 are highly unstable. The orbital parameters
of the HD 12661 planets are located in a border region between stable and
unstable dynamical regimes, so while its currently determined orbital
parameters produce stable regular orbits, a minor change within the margin of
error of just one parameter may result in a chaotic dynamical system.Comment: 12 pages, 3 figures, accepted ApJ, revised version following the
referee's repor
Influence of the coorbital resonance on the rotation of the Trojan satellites of Saturn
The Cassini spacecraft collects high resolution images of the saturnian
satellites and reveals the surface of these new worlds. The shape and rotation
of the satellites can be determined from the Cassini Imaging Science Subsystem
data, employing limb coordinates and stereogrammetric control points. This is
the case for Epimetheus (Tiscareno et al. 2009) that opens elaboration of new
rotational models (Tiscareno et al. 2009; Noyelles 2010; Robutel et al. 2011).
Especially, Epimetheus is characterized by its horseshoe shape orbit and the
presence of the swap is essential to introduce explicitly into rotational
models. During its journey in the saturnian system, Cassini spacecraft
accumulates the observational data of the other satellites and it will be
possible to determine the rotational parameters of several of them. To prepare
these future observations, we built rotational models of the coorbital (also
called Trojan) satellites Telesto, Calypso, Helene, and Polydeuces, in addition
to Janus and Epimetheus. Indeed, Telesto and Calypso orbit around the L_4 and
L_5 Lagrange points of Saturn-Tethys while Helene and Polydeuces are coorbital
of Dione. The goal of this study is to understand how the departure from the
Keplerian motion induced by the perturbations of the coorbital body, influences
the rotation of these satellites. To this aim, we introduce explicitly the
perturbation in the rotational equations by using the formalism developed by
Erdi (1977) to represent the coorbital motions, and so we describe the
rotational motion of the coorbitals, Janus and Epimetheus included, in compact
form
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