Iapetus (moon)
Photomosaic of Cassini images taken on 31 December 2004. It shows the dark Cassini Regio and its border with the bright Roncevaux Terra, several large craters (Falsaron above center, Turgis, the largest, at right), and the equatorial ridge (the Toledo and Tortelosa Montes).
|
|
| Discovery | |
|---|---|
| Discovered by | G. D. Cassini |
| Discovery date | October 25, 1671 |
| Designations | |
| Saturn VIII | |
| Adjectives | Iapetian, Japetian |
| Orbital characteristics | |
| 3560820 km | |
| Eccentricity | 0.0286125 |
| 79.3215 d | |
|
Average orbital speed
|
3.26 km/s |
| Inclination |
|
| Satellite of | Saturn |
| Physical characteristics | |
| Dimensions | 1,492.0 × 1,492.0 × 1,424 km |
|
Mean radius
|
734.5±2.8 km |
| 6700000 km2 | |
| Mass | (1.805635±0.000375)×1021 kg |
|
Mean density
|
1.088±0.013 g/cm³ |
| 0.223 m/s2 | |
| 0.573 km/s | |
|
79.3215 d (synchronous) |
|
| zero | |
| Albedo | 0.05–0.5 |
| Temperature | 90–130 K |
| 10.2–11.9 | |
Iapetus (/aɪˈæpᵻtəs/; Greek: Ιαπετός), or occasionally Japetus /ˈdʒæpᵻtəs/, is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System, and the largest body in the Solar System known not to be in hydrostatic equilibrium. Iapetus is best known for its dramatic "two-tone" coloration. Discoveries by the Cassini mission in 2007 revealed several other unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon.
Iapetus was discovered by Giovanni Domenico Cassini, an Italian astronomer, in October 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.
Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honour.
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