150 research outputs found
The Librations, Tides, and Interior Structure of Io
No full textsponsorship: We thank William B. Moore and an anonymous reviewer for their detailed review and insightful comments. This work was financially supported by the Belgian PRODEX program managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office. Data about the interior structure models used and the libration results are available on Zenodo (Van Hoolst et al., 2020). (Belgian PRODEX program, Belgian Federal Science Policy Office)status: Publishe
Survey of capabilities and applications of accurate clocks: directions for planetary science
No full textFor planetary science, accurate clocks are mainly used as part of an onboard radioscience
transponder. In the case of two-way radio data, the dominating data type for planetary
radioscience, an accurate spacecraft clock is not necessary since the measurements can
be calibrated using high-precision clocks on Earth. In the case of one-way radio data, however,
an accurate clock can make the precision of one-way radio data be comparable to the
two-way data, and possibly better since only one leg of radio path would be affected by the
media. This article addresses several ways to improve observations for planetary science, either
by improving the onboard clock or by using further variants of the classical radioscience
methods, e.g., Same Beam Interferometry (SBI). For a clock to be useful for planetary science,
we conclude that it must have at least a short-time stability (< 1,000 s) better than
10^−13 and its size be substantially miniaturized. A special case of using laser ranging to the
Moon and the implication of having an accurate clock is shown as an example
Influence of the internal structure of Europa on the Doppler signal of an orbiter
No full textEuropa, the second Galilean satellite starting from Jupiter, probably has a liquid ocean beneath the icy shell but the thickness of these layers is poorly known. From the values of the gravitational coefficients C20 and C22 of Europa determined by the Galileo mission, the to- tal thickness of the ice and water layer is evaluated to 80 - 170 km [1]. However, the thickness of the indi- vidual ice and water layers could not be determined, since their densities are similar. An important goal of the Europa Jupiter System Mission under study by NASA and ESA is to better constrain the ice shell and subsurface ocean of Eu- ropa. Important information on the interior structure is thought to result from observation of the tides, li- brations and obliquity of Europa. Here we assess the possibility to measure those quantities with a Radio Science instrument, which is part of the scientific core payload of the Europa orbiter of EJSM Together with the static gravity field of Europa, tides and rotation determine the orbital motion of a spacecraft around Europa. The quantities we take into account are C20, C22, the libration amplitude, the obliq- uity, the Love number k2 and its quality factor Q. For their dependence on the internal structure, see e.g. [2], [3], [4], [5], [6], [7] and [8]. ll these quantities induce perturbations (secular, long term and short period) of the orbital elements of the orbiter, thereby changing the spacecraft position and the relative radial velocity between the orbiter and a terrestrial observer (or Doppler signal). We calculate these perturbations by integration of Lagrange’s equa- tions in order to obtain an analytical expression for the Doppler signal (see [9] for a similar study for Mars). The effect of each parameter on the Doppler signal is determined for different initial orbital elements of the orbiter. To test the measurability, the effects are compared with the expected accuracy of the Doppler signal. [1] Anderson, J. D. et al. (1999) Science , 281, 2019– 2022. [2] Van Hoolst, T. et al. (2008) Icarus , 195, 386–399. [3] Van Hoolst, T. et al. (2009) Icarus , 200, 256–264. [4] Peale, S. J. et al. (1988) Mercury, University of Arizona Press , 461–493. [5] Bills, B. G. and Nimmo F. (2008) Icarus , 196, 293–297. [6] Wu, X. et al. (2001) Geophysical Research Let- ters , 28, 2245–2248. [7] Wahr, J. M. et al. (2006) JGR , 111, 12005–. [8] Lainey, V. (2005) PhD Thesis, Observatoire de Paris [9] Yseboodt, M. (2003) PhD Thesis, UC
Updated Europa gravity field and interior structure from a reanalysis of Galileo tracking data
No full textThe Galileo radio tracking data were reanalysed exploiting the new knowledge of Jupiter obtained by the Juno mission, together with modern orbit determination techniques developed for the Cassini data analysis. Using Doppler data acquired during six encounters of Europa an updated gravity field of the moon was obtained, resulting in a value of C22 statistically different from the available literature. The new value suggests a thinner ice-water shell and a less dense interior
Librations of the Galilean satellites: The influence of global internal liquid layers
No full textThe four Galilean satellites are thought to harbor one or even two global internal liquid layers beneath their surface layer. The iron core of lo and Ganymede is most likely (partially) liquid and also the core of Europa may be liquid. Furthermore, there are strong indications for the existence of a subsurface ocean in Europa, Ganymede, and Callisto. Here, we investigate whether libration observations can be used to prove the existence of these liquid layers and to constrain the thickness of the overlying solid layers. For lo, the presence of a small liquid core increases the libration of the mantle by a few percent with respect to an entirely solid lo and mantle libration observations could be used to determine the mantle thickness with a precision of several tens of kilometers given that the libration amplitude can be measured with a precision of 1 m. For Europa, Ganymede, and Callisto, the presence of a water ocean close to the surface increases by at least an order of magnitude the ice shell libration amplitude with respect to an entirely solid satellite. The shell libration depends essentially on the shell thickness and to a minor extent on the density difference between the ocean and the ice shell. The possible presence of a liquid core inside Europa and Ganymede has no noticeable influence on their shell libration. For a precision of several meters on the libration measurements, in agreement with the expected accuracy with the NASA/ESA EJSM orbiter mission to Europa and Ganymede, an error on the shell thickness of a few tens kilometers is expected. Therefore, libration measurements can be used to detect liquid layers such as lo's core or water subsurface oceans in Europa, Ganymede, and Callisto and to constrain the thickness of the overlying solid surface layers. (C) 2010 Elsevier Inc. All rights reserved
Librations and tides of icy satellites: model comparison for Enceladus
No full textThe latest measurements of the librations of Enceladus suggest that it could have a global subsurface ocean or a non-hydrostatic core (Thomas et al. 2014). Further observations should constrain the properties of the ice shell, and similar insights are expected from future investigation of Europa and Ganymede. Detailed models of the librations and tides are therefore required to properly interpret these measurements in terms of interior structure. Here we compare the `classical', separate tide and libration models (where spherical symmetry is assumed to compute the tides, Van Hoolst et al. 2013) with our combined tide+libration model (Trinh et al. 2013), both extended to account for non-hydrostatic structure. Even with a global ocean, different mechanisms act to prevent Enceladus's shell from moving independently from the rest. Among those, pressure coupling across the flattened boundaries of the ocean requires special care if the shape is not fully relaxed. We discuss how it should be modelled in the classical approach to be consistent with the combined model
Linear Isentropic Oscillations of Stars: Theoretical Foundations
No full textThe study of stellar oscillations is the preeminent way to investigate the stability of stars and to interpret their variability. The theory of the linear, isentropic oscillations of isolated gaseous stars, and thus of compressible spherically symmetric equilibrium configurations, has largely been developed from the viewpoint of the hypothesis of the physical radial pulsations of stars. Written for doctoral students and researchers, this monograph aims to provide a systematic and consistent survey of the fundamentals of the theory of free, linear, isentropic oscillations in spherically symmetric, gaseous equilibrium stars. The first part of the book presents basic concepts and equations, the distinction between spheroidal and toroidal normal modes, the solution of Poisson’s differential equation for the perturbation of the gravitational potential, and Hamilton’s variational principle. The second part is devoted to the possible existence of waves propagating in the radial direction, the origin and classification of normal modes, the completeness of the normal modes, and the relation between the local stability with respect to convection and the global stability of a star. The third part deals with asymptotic representations of normal modes and with slow period changes in rapidly evolving pulsating stars
Slichter modes of large icy satellites
No full textBecause of the presence of an ocean below the ice shell of icy satellites such as Europa, Callisto, Ganymede and Titan the solid interior of these satellites can be displaced with respect to the ice shell, similarly to the translational oscillation of the inner core of the Earth called the Slichter modes of the Earth. We construct a set of interior structure models of Europa, Callisto, Ganymede and Titan satisfying the observed mass, radius and moment of inertia and study the properties of the Slichter mode for these models. The periods obtained range from a few hours to a few tens of hours depending mainly on the ocean thickness. Ganymede has two Slichter modes since it is thought to have a liquid outer core besides a global subsurface ocean. The second Slichter mode describes essentially the oscillation of the solid inner core inside the liquid outer core and its period is determined principally by the thickness of the outer core. We study the possible observation of these modes with a lander on the surface or a spacecraft in orbit about Europa, Callisto, Ganymede or Titan. We show that an impactor with a radius of at least a few kilometers to a few tens of kilometers could excite the Slichter modes to a level observable by a lander. Such impacts occur on average once in >30. My for Europa, once in >70. My for Callisto, once in >40. My for Ganymede and once in >0.4. Gy for Titan. Observation of the Slichter mode would allow constraining the thickness of the ocean. © 2013 Elsevier Inc
Jovian tidal dissipation from inner satellite dynamics
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