The extremely water-ice-rich composition of Tethys remains unexplained. The conditions in the Saturnian sub-nebula likely favored conversion of the molecular nitrogen and carbon monoxide into ammonia and methane, respectively. This can partially explain why Saturnian moons including Tethys contain more water ice than outer Solar System bodies like Pluto or Triton as the oxygen freed from carbon monoxide would react with the hydrogen forming water. One of the most interesting explanations proposed is that the rings and inner moons accreted from the tidally stripped ice-rich crust of a Titan-like moon before it was swallowed by Saturn.