This work was mainly funded by European Community’s
Seventh Framework Program (FP7/2007-2013) under grant agreement 263466
for the FP7-ESPaCE, and partially by UPMC-EMERGENCE (contract number
EME0911), for which R.T. and V.L. are grateful. R.T. was also supported
by the Cassini mission. In addition, this work was supported by the Science
and Technology Facilites Council (Grant No. ST/F007566/1) and C.D.M. and
N.J.C. are grateful to them for financial assistance. C.D.M. is also grateful to
the Leverhulme Trust for the award of a Research Fellowship