Iapetus is Saturn's third satellite in size, with a diameter of about 1,500 km, after the larger satellites Titan and Rhea. It was discovered by Giovanni Cassini in 1671. It takes 79.33 days to complete a lap around Saturn, at an average distance of 3,561,300 km. One of the satellite's hemispheres is much darker than the other, a peculiar characteristic that could be due to a different composition of the surface material, originating from inside the moon itself or from matter from other satellites or rings. The real reason is not known with certainty, although the second hypothesis is increasingly supported by observational evidence.
