sentences:  A possible explanation for this is that the shape of Iapetus was frozen by formation of a thick crust shortly after its formation, while its rotation continued to slow afterwards due to tidal dissipation, until it became tidally locked.A further mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about long, wide, and high.It was discovered when the "Cassini" spacecraft imaged Iapetus on December 31, 2004.