Volume 651, July 2021
|Number of page(s)||7|
|Section||Planets and planetary systems|
|Published online||27 July 2021|
A lighter core for Phobos?
State Key Laboratory of Information Engineering in Surveying, Mapping and Remote sensing,
2 Institute of Space Technology and Space Applications, Universität der Bundeswehr München, Neubiberg, Germany
3 Rheinisches Institut für Umweltforschung, Abteilung Planetenforschung, an der Universität zu Köln, Köln, Germany
4 Geodesy Observatory of Tahiti, University of French Polynesia, BP 6570, 98702 Faaa, Tahiti, French Polynesia, France
Accepted: 6 May 2021
Context. The origin of the Martian moons Phobos and Deimos is still poorly understood, and is the focus of intense debate.
Aims. We demonstrate that a stratified internal structure of Phobos is compatible with the observed gravity coefficients.
Methods. We fit previously derived C20 and C22 Phobos gravity coefficients derived from the combined MEX Doppler-tracking data from the close flybys in +2010 and 2013 with respect to the corresponding coefficients of a core–mantle stratification model of Phobos, with two opposite cases: a core denser than the mantle, and a core lighter than the mantle.
Results. Only the case with a core lighter than the mantle fits at the 3σ level the previously reported observed second degree and order coefficient C20, but a homogeneous Phobos cannot be strictly ruled out at the 3σ level.
Conclusions. This possible loosening of the core density might be the result of a displacement of material toward the surface, may be caused by centrifugal forces acting on a loosely packed rubble-pile structure, and/or by a hot-then-cold in-orbit accretion process. These two hypotheses are by no means exhaustive.
Key words: gravitation / planets and satellites: formation
© ESO 2021
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