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| Titel |
An assessment of the determination of the tides and the rotation state of Ganymede with JUICE radio science experiment |
| VerfasserIn |
Rose-Marie Baland, Tim Van Hoolst, Gabriel Tobie, Veronique Dehant |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250110889
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| Publikation (Nr.) |
 EGU/EGU2015-10932.pdf |
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| Zusammenfassung |
| Besides being the largest natural satellite known in the Solar System, Ganymede most likely
also has the most differentiated internal structure of all satellites.Ganymede is thought to have
an external water/ice layer subdivided into three sublayers: an outer ice shell, a
global liquid water ocean, and a high pressure ice mantle. The presence of a water
layer is supported by the possible detection of an induced magnetic field with the
Galileo spacecraft. The metallic core is divided into a solid (inner core) and a liquid
(outer core) part. Between the water/ice and the metallic layers, a rock mantle is
expected.
The JUpiter ICy moons Explorer (JUICE) mission led by ESA is planned to be launched
in 2022. The spacecraft is expected to enter in orbit around Ganymede in september 2032.
The Ganymede Tour will alternate elliptic and circular phases at different altitudes.
The circular phases at altitudes of a few hundred kilometers are dedicated partly
to the study of the internal structure such as the determination of the extent and
composition of the ocean and of the surface ice shell. The payload of the spacecraft
comprises the radio science package 3GM (Gravity and Geophysics of Jupiter and the
Galilean Moons) that will be used to measure the Doppler effect on radio links
between the orbiter and the Earth which will be affected by the gravity field of
Ganymede.
The gravity field of Ganymede is the sum of the static hydrostatic field (related to the
secular Love number kf), of the periodically varying field due to tidal deformations (related
to the tidal Love number k2 and the tidal dissipation factor Q), of the periodically varying
field due to change in the rotation state (variations in the rotation rate and in the orientation of
the rotation axis), and of the non-hydrostatic field that may be due to mass anomalies. The
tidal and rotation parameters depend on the internal structure of the satellite (density, size,
rheological properties of the different layers) in a non-trivial way. Our aim is to assess for
which internal structure quantities of Ganymede information can be retrieved from Doppler
effect measurements.
The Doppler effect is modelled by the relative radial velocity between the orbiter and the
terrestrial observer, considering the tides and the rotation state of Ganymede, together with
the strong attraction exerted directly by Jupiter on the orbiter. The modelisation neglects
some effects such as a possible atmospheric drag and a possible non-hydrostatic part of the
gravity field. We aim to answer questions as “Is it possible to separate the tidal and rotational
signals?”, “What is the optimal orbital configuration?”. The inversion of the simulated noised
data is done by the least square method. An interesting configuration has to maximise the
effect of the tides and of the rotation on the Doppler signal, in order to maximise the
constraints inferred on the internal stucture of Ganymede. It also has to correspond to a quite
stable quasi-circular orbit as required for the circular phases of the Ganymede tour. |
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