1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, MA 02139 , USA
2. School of Physics and Astronomy, University of Birmingham , Edgbaston, Birmingham B15 2TT , UK
3. Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange , Bd de l’Observatoire, CS 34229, F-06304 Nice Cedex 4 , France
4. European Space Agency (ESA), European Space Research and Technology Centre (ESTEC) , Keplerlaan 1, NL-2201 AZ Noordwijk , The Netherlands