A bit more on this, from the abstract at
http://www.sciencemag.org/content/344/6179/78:
"We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°.
And a little more on methodology, from the full article:
"The design of the Cassini spacecraft does not allow radio tracking from Earth during remote-sensing observations. Therefore, only 3 of the 19 flybys of Enceladus completed so far have been used for gravity measurements. In these close encounters, the spacecraft was continuously tracked from ground antennas while flying within 100 km of the moon’s surface, twice above the southern hemisphere (in the flybys labeled E9 and E19) and once over the northern hemisphere (E12). We determined Enceladus’ quadrupole gravity field and degree-3 zonal harmonic coefficient J3 from measurements of spacecraft range-rate.
And further detail on the effects modeled:
"Microwave links between the onboard transponder and ground stations of NASA’s Deep Space Network enabled precise measurements of the spacecraft range-rate. In addition to gravitational forces, our analysis accounts for the main nongravitational accelerations, most notably neutral particle drag exerted by the substantial gas plume formed by the jets of the south-polar region. Flying by the moon at latitudes below –70°, the spacecraft interacts with the plume at distances of up to 500 km from Enceladus’ surface..."
Amazing results. They're flying ~100 km from the moon's surface, while at a distance of 1.3 billion km. And getting very accurate velocities due to the mighty Doppler effect.