Iapetus (moon)

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Iapetus (moon)
Iapetus dark ridge.jpg
Iapetus, showing Cassini Regio and the equatorial ridge, taken by Cassini
Date of discovery October 25, 1671[1]
Name of discoverer Giovanni Domenico Cassini[1][2]
Name origin A Titan, god of mortality and wounding
Orbital characteristics
Primary Saturn
Order from primary 22
Perikrone 3,460,515 km[3]
Apokrone 3,662,085 km[3]
Semi-major axis 3,561,300 km[4]
Orbital eccentricity 0.0283[4]
Sidereal month 79.330183 da[4]
Avg. orbital speed 3.27 km/s[2]
Inclination 14.72°[4] to Saturn's equator
Rotational characteristics
Sidereal day 79.330183 da[4]
Rotational speed 0.000658 km/s[3]
Axial tilt [4]
Physical characteristics
Mass 1.81 * 1021 kg[4]
Density 1090 kg/m³[4]
Mean radius 718 km[4]
Equatorial radius 748 km[5]
Polar radius 712.6 km[5]
Surface gravity 0.234 m/s²[3]
Escape speed 0.579 km/s[3]
Surface area 6,478,266 km²[3]
Minimum temperature 70 K[6]
Mean temperature 100 K[6]
Maximum temperature 130 K[6]
Composition Water ice and small amounts of rock
Color Light and dark gray
Albedo 0.5[4]
Iapetus or Japetus or Saturn VIII is the twenty-second moon of Saturn, the second moon of Saturn to be discovered, and one of the most remarkable objects in all the Solar system.

Discovery and naming

Giovanni Domenico Cassini discovered Iapetus in 1671. However, he noted that he could reliably observe Iapetus only on the western side of Saturn, and never on the eastern side. In 1705 he observed Iapetus on the eastern side, with an improved telescope, and noted that it was two visual magnitudes dimmer on that side.

After his discovery of Iapetus, Cassini discovered three other moons (Tethys, Dione, and Rhea) in the ensuing years, during the reign of "Sun-King" Louis XIV of France.[1] Cassini named these moons the "Sidera Lodoicea" in honor of the king. Later generations (especially after the French Revolution) would not retain such regal flattery.[7]

Sir John Herschel, son of the astronomer William Herschel, suggested the current names of the seven largest satellites of Saturn, including Iapetus. Titan received a generic name, and the other six received names of the Titans of mythology. Iapetus was one of four brothers who conspired with Kronos or Cronus to depose Uranus, father of all the gods.[8]

Orbital characteristics

Iapetus is in an eccentric orbit around Saturn, at an average distance of 3,561,300 km. This distance is nearly three times the average distance of Titan and 2.5 times the average distance of Hyperion. Its sidereal month is about 79.33 days.

The orbit of Iapetus is inclined to Saturn's equator by more than 14 degrees, more than any of the other large moons of Saturn. Only the much smaller irregular moons of Saturn have orbits more inclined than that of Iapetus.

Rotational characteristics

Iapetus is in tidal lock with Saturn.

Physical characteristics

Iapetus is slightly more dense than water. It therefore is likely composed of water ice with a small mineral component.

Two colors

Iapetus is best known for the surprisingly different colors of its leading and trailing hemispheres. Its leading hemisphere has an albedo of about 0.05, while the trailing hemisphere has an albedo as high as 0.6. The dark region is named Regio Cassini, in honor of the discoverer, and the bright region is named Roncevaux Terra. The boundary between these two regions is sharp, but not perfectly so. In the border between the two regions, the dark material often covers only one side of a crater or fills the floor of another.[9]

The origin of the dark material remains unsettled. Some astronomers speculate that the material came from Phoebe, a member of the Norse group of moons. Yet the colors of Phoebe and the dark region of Iapetus are not alike.[2][9]

The dark material is either several meters thick or else is constantly renewed. Cassini Regio has no bright-rimmed craters consistent with erosion of the dark material.[2]

Shape and surface features

Iapetus is an oblate spheroid.[5] The observed oblateness is far more pronounced than astronomers would expect of a body having a sidereal day of more than 79 Earth days. Moreover, Iapetus has a ridge of mountains, 20 km high, along its equator. To assume such a shape, Iapetus ought to have had a sidereal day of 10 hours, slowing later to 16 hours. Its crust would then have had to freeze and thicken over a period of 100 million to 900 million years, according to uniformitarian theories. But during this time, Iapetus would have required a source of internal heat sufficient to melt a vast quantity of water ice. Astronomers who have considered the problem believe that this internal heat source was radioactive decay, specifically of 26Al and 60Fe.[10]

The ridge system itself is an enigma. Astronomers have suggested at least two models for its formation:

  1. It formed during the process described above, as the crust buckled from the rapid rotation as the body cooled.[11]
  2. Iapetus initially had rings, which then collapsed onto the equator and persist today as the ridge.[12]

The ridge and all the rest of the surface is covered with multiple impact craters that show little or no erosion. These include a 400-kilometer-wide basin within Regio Cassini at about 30 degrees north latitude,[13] and a 450-kilometer-wide basin in Roncevaux Terra at about 50 degrees south latitude.[14] Iapetus has nine such very large impact basins in all.

Temperature range

Iapetus temperature map
Temperatures on the leading (dark) hemisphere reach 130 K at local noon, because the dark material in the Regio Cassini can absorb more heat. Temperatures do not reach more than 100 K on the trailing hemisphere.[6]

Problems for uniformitarian theories posed by Iapetus

The marked inclination of the orbit of Iapetus from the equatorial plane of Saturn defies explanation. According to the nebula hypothesis, all the regular satellites of Saturn ought to remain in or near the same plane. But Iapetus is not likely to be a captured object. Its orbit, though eccentric, is still not consistent with such a model.

The markedly oblate shape of Iapetus, and its equatorial ridge, are inconsistent with its long sidereal day. Astronomers have modeled the possible evolutionary arrest of Iapetus shortly after its formation. But those same astronomers admit that the required sequence of events would have had to be exact, with virtually no tolerance for error.[10]

Observation and Exploration

Voyager 1 and Voyager 2 each made rendezvous with Iapetus and took photographs. Those photographs revealed the first details of the contrasting colors of Iapetus's leading and trailing hemispheres.

The Cassini-Huygens Mission Orbiter has made the closest rendezvous with Iapetus. Its camera first revealed the equatorial ridge, and its infrared sensors provided the first insights into the temperature on Iapetus.

No further rendezvous are currently planned.

In fiction

One of the most remarkable appearances of Iapetus (or Japetus as the author spelled it) was in Arthur C. Clarke's novel 2001: A Space Odyssey, based on the motion picture of the same name. This novel had as its theme the origins of human intelligence and a supposed history of contact with extraterrestrial scientists in deep time.[15] The author took especial note of Cassini's original astounding finding of radically different albedos on its different hemispheres. In this novel, either Iapetus was an artificial construct, or else extraterrestrial expeditionaries had seared the leading hemisphere and polished the trailing hemisphere to make it brilliant white. In the center of the white hemisphere the expeditionaries built a hyperspatial connecting "gateway" to their own dimension. They completed this project two million years ago and then waited with unfathomable patience for the first human astronaut to explore Iapetus and pilot a small vessel through their "gate."

The first edition of this novel appeared in the 1970s. The direct observation of Iapetus by Voyagers 1 and 2 rendered this speculation moot.


Video gallery


  1. 1.0 1.1 1.2 "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.
  2. 2.0 2.1 2.2 2.3 Hamilton, Calvin J. "Entry for Iapetus." The Views of the Solar System, 2004. Accessed June 16, 2008.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Calculated
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Williams, David R. "Saturnian Satellite Fact Sheet." National Space Science Data Center, NASA, November 23, 2007. Accessed June 4, 2008.
  5. 5.0 5.1 5.2 Thomas, P.C., Burns, J.A., Helfenstein, P., et al. "Shapes of the saturnian icy satellites and their significance." Icarus 190(2):573-584, October 2, 2007. doi:10.1016/j.icarus.2007.03.012 Accessed June 16, 2008.
  6. 6.0 6.1 6.2 6.3 "Catalog Page for PIA07004: Iapetus Thermal Radiation Image." JPL Photojournal, JPL, NASA, January 10, 2005. Accessed June 16, 2008.
  7. Boulay, J. C. "Saturne: les satellites." Astronomie-astronautique. Accessed June 5, 2008.
  8. Lassell, William. "Satellites of Saturn." Monthly Notices of the Royal Astronomical Society, 8(3):42-43, January 14, 1848. Accessed June 4, 2008.
  9. 9.0 9.1 Arnett, Bill. "Entry for Iapetus." The Nine 8 Planets, May 13, 2005. Accessed June 16, 2008.
  10. 10.0 10.1 Cowen, Ron. "Idiosyncratic Iapetus." Science News 172:104-106, August 14, 2007. Accessed June 16, 2008.
  11. Kerr, Richard A. "How Saturn's Icy Moons Get a (Geologic) Life." Science 311(5757):29, January 6, 2006. doi:10.1126/science.311.5757.29 Accessed June 16, 2008.
  12. Ip, Wing-Huen. "On a ring origin of the equatorial ridge of Iapetus." Geophys. Res. Lett. 33(L16203), 2006. doi:10.1029/2005GL025386 Accessed June 16, 2008.
  13. "PIA06166: Encountering Iapetus." JPL Photojournal, JPL, NASA, 2005. Accessed June 16, 2008.
  14. "PIA08384: The Other Side of Iapetus." JPL Photojournal, JPL, NASA, September 10, 2007. Accessed June 16, 2008.
  15. Clarke, Arthur C. 2001: A Space Odyssey, reprint. Penguin Group USA, 2005. ISBN 9780451452733. Paperback.