  Are Saturn's regular satellites young or old? And how old are Enceladus'
cratered plains? To answer these questions we computed model surface ages of
the most heavily cratered terrains on Saturn's regular icy satellites using new
high-resolution outer Solar System evolution simulations, and coupled with
improved estimates of the trans-Neptunian objects populations. The output of
the simulations allowed us to construct a model impact chronology onto Saturn
which automatically applies to its regular satellites. We used crater densities
and our impact chronology onto Saturn to construct model impact-crater
isochrons, i.e., the scaling of the satellite crater production function with
time. The surface ages derived for the cratered plains on Mimas, Enceladus,
Tethys, Dione and Rhea range from 4.1 Ga to 4.4 Ga, with the surfaces of Mimas
and Enceladus roughly 200 Myr younger than those of the outer three satellites.
Uncertainties in these ages are less than 300 Myr. The calculated model surface
ages of these satellites are consistent over as much as two orders of magnitude
in the observed crater diameter. The similarity of the crater production
function amongst all satellites suggests that they were bombarded by a single
impactor source. This work supports the idea that Saturn's regular satellites
are ancient, and has implications for their formation and their tidal
evolution.
