A lighter core for Phobos?

¹ State Key Laboratory of Information Engineering in Surveying, Mapping and Remote sensing, Wuhan, PR China
e-mail: jgyan@whu.edu.cn
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

Received: 4 July 2020
Accepted: 6 May 2021

Abstract

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.