Detecting a Subsurface Ocean From Periodic Orbits at Enceladus

Current models about the interior structure of Enceladus entail a thin crust and a subcrustal water layer above an undifferentiated (rock+metal) interior. The determination of the existence of the putative subsurface ocean at Enceladus can be pursued with methods similar to those proposed for Europa, including the study of the gravitational perturbations of tidal origin on an Enceladus orbiter, and the use of altimeter measurements to the tidally deformed surface. The design of an Enceladus orbiter is difficult since the dynamical environment is made very unstable by the overwhelming presence of nearby Saturn. A nominal, high altitude, 64 degrees inclined reference orbit is proposed that enjoys long term stability and is favorable for long-term mapping and other scientific experiments. A brief excursion to lower altitude, slightly higher inclined, yet highly unstable orbits is proposed to improve gravity signatures and enable high resolution, nadir-pointing experiments on the geysers emanating from the tiger-stripes. Round-trip transfers between the two orbital phases is expected to conservatively cost between ~50 and ~100 m/s. In addition results are presented in terms of sensitivity of detection of the Love numbers to the different orbital geometries. Indications are provided for optimized orbit planning of future exploration missions aimed at investigating the internal structure of the satellite and the detection of its putative subcrustal ocean.