With a mean diameter of 396.4 kilometres or 246.3 miles, Mimas is the smallest astronomical body known to be roughly rounded in shape due to its own gravity.

A notable feature of Mimas's surface is Herschel, one of the largest craters relative to the size of the parent body in the Solar System.

Herschel measures 139 kilometres (86 miles) across, about one-third of Mimas's mean diameter,[10] and formed from an extremely energetic impact event.

He recorded his discovery as follows: I continued my observations constantly, whenever the weather would permit; and the great light of the forty-feet speculum was now of so much use, that I also, on the 17th of September, detected the seventh satellite, when it was at its greatest preceding elongation.

The names of all seven then-known satellites of Saturn, including Mimas, were suggested by William Herschel's son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.

Italian Mimante, Russian Мимант for the mythological figure),[17] and so the English adjectival form is Mimantean[18] or Mimantian,[19] either spelling pronounced /maɪˈmæntiən/ ~ /mɪˈmæntiən/.

As a result of the tidal forces acting on it, Mimas is noticeably oblate; its longest axis is about 10% longer than the shortest.

By studying Mimas's movement, researchers have found that it has a water ocean beneath 20–30 km (12–19 mi) of surface ice.

The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap.

[25][26] In 2014, researchers noted that the librational motion of Mimas has a component that cannot be explained by its orbit alone, and concluded that it was due to either an interior that is not in hydrostatic equilibrium (an elongated core) or an internal ocean.

[27] However, in 2017 it was concluded that the presence of an ocean in Mimas's interior would have led to surface tidal stresses comparable to or greater than those on tectonically active Europa.

The presence of an internal ocean concealed by a stable icy shell between 24 and 31 km in thickness was found to match the visual and librational characteristics of Mimas as observed by Cassini.