IAPETUS (MOON)

'Iapetus' (''eye-ap'-ə-təs'', , Greek ''Ιαπετός''), also 'Japetus' ()^[4], is the third-largest moon of Saturn, discovered by Giovanni Domenico Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the ''Cassini'' mission have revealed several other unusual physical characteristics. These mysteries are currently under investigation by scientists, and new information about Iapetus is accumulating continuously.

Contents

Name

Physical characteristics

Two-tone coloration

Sources from space

Internal sources

Overall shape

The equatorial ridge

Temperatures

Orbit

See also

Iapetus in fiction

References

External links

Name

Iapetus is named after the mythological Iapetus. It is also designated 'Saturn VIII'.
Giovanni Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) ''Sidera Lodoicea'' ("the stars of Louis") to honour King Louis XIV. However, astronomers fell into the habit of referring to them and Titan as ''Saturn I'' through ''Saturn V''. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended to ''Saturn VII''.
The names of all seven satellites of Saturn then known come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope'',^[5] wherein he suggested the names of the Titans, sisters and brothers of the Titan Kronos (whom the Romans equated with their god Saturn), be used. The name was first Latinized as ''Japetus'', a spelling still in occasional use.

Physical characteristics

The low density of Iapetus indicates that it is primarily composed of ice, with only a small (~10%) amount of rocky materials.

Furthermore, the overall shape of Iapetus is neither spherical nor ellipsoid, as most moons are, but has a bulging waistline and squashed poles^[6]; moreover, its unique equatorial ridge (see below) is so high that it visibly distorts the moon's shape even when viewed from a distance.
Iapetus is a heavily cratered body, and ''Cassini'' images have revealed large impact basins in the dark region, at least three of which are over 350 km wide. The largest has a diameter over 500 km; its rim is extremely steep and includes a scarp over 15 km high.

Two-tone coloration

In the 17th century, Giovanni Cassini observed that he could see Iapetus only on one side of Saturn and not on the other. He drew the conclusion that one side of Iapetus was darker than the other, a conclusion confirmed by images from the ''Voyager'' and ''Cassini'' spacecraft.
The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere is dark (albedo .03–.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo .5-.6, almost as bright as Europa). Thus, the magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9 - beyond the capacity of the best telescopes in the 17th century. The pattern of coloration is analogous to a spherical yin-yang symbol. The dark region is named Cassini Regio, and the bright region Roncevaux Terra.
The origin of this dark material is not currently known, though several theories have been proposed (see below). Its thickness is also unknown; there are no bright craters present on the dark hemisphere, so if the dark material is thin it must either be extremely recent or constantly renewed, as otherwise a meteor impact would have punched through the layer to reveal brighter underlying material.
When NASA's ''Voyager 2'' flew past Iapetus on August 22 1981 at a relatively distant 966,000 km (600,000 mi), the spacecraft's cameras could make out few details in the area of dark material, but revealed the bright side to be icy and heavily cratered. On December 31 2004, the ''Cassini'' spacecraft passed within 123,000 km (77,000 mi) of Iapetus and photographed Cassini Regio at far a higher resolution than ''Voyager'' was able, but the mystery surrounding its origin has only deepened.
''Cassini'' is scheduled for a much closer approach on September 10 2007 — 1,200 km (800 mi).

Sources from space

The dark material might be formed of organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets; Earth-based observations have shown it to be carbonaceous, and it probably includes cyano-compounds such as frozen hydrogen cyanide polymers.
There have also been suggestions that the dark material originated from other Saturnian moons. For example, it was long suspected that the material may have spiralled in from Phoebe, having been knocked free from the smaller moon's surface by micrometeoroid impacts and then swept up by Iapetus's leading hemisphere. However, despite being widely cited, this theory is no longer tenable: observations have shown Phoebe's surface to have a different color from that of the dark material of Iapetus (indeed in 2005 it was announced that Phoebe's composition is closer to the bright Iapetian hemisphere than the dark one).^[7]
Another suggested source is Titan,^[8] based on the fact that the observed infrared spectra fit with a mixture of water ice, amorphous carbon and tholins.
Yet another possible explanation involves coating by dark material created in the aftermath of the destruction of the object which went on to become Hyperion, whose irregular shape is consistent with its formation in a violent impact.^[9] However, there remain doubts as to whether or not such an event can produce a stream of debris able to produce the distribution of dark material seen on Iapetus.

Internal sources

It is possible that the dark material may have originated from some internal source, perhaps brought to the surface by a combination of meteor impact and cryovolcanism. This theory is supported by the apparent concentration of the material on crater floors. It has been suggested that since Iapetus is far from Saturn and would have avoided much of the heating its other moons received during the formation of the Solar System, Iapetus may have retained methane or ammonia ice in its interior that later erupted to the surface as cryovolcanic lava and was then blackened by solar radiation, charged particles, and cosmic rays. A dark ring of material about 100 km in diameter straddling the border between the leading and trailing hemispheres of Iapetus is suggestive of such volcanism, resembling structures that have formed on the Moon and on Mars as a result of volcanic material flowing into impact craters with a central peak.

An alternative internal source may be the evaporation of water ice. Because of its slow rotation, Iapetus has the warmest surface in the Saturnian system (130 K in the dark region) allowing the sublimation of water ice on the surface. After sublimation, the water then freezes back to the surface and re-heats until it reaches a location where it is no longer able to sublimate. The dark areas may be the result of such a process, since the material there lacks water. However, this hypothesis fails to explain why only one hemisphere is dark.
Yet another explanation assumes a primordial, thick, dark sub-surface material covered originally on both hemispheres by a thin layer of water ice. The impacts would then erode mostly the leading hemisphere, exposing the dark material, accounting for the asymmetry that exists on Iapetus.^[10]

Overall shape

Current triaxial measurements of Iapetus give it dimensions of 747.1 × 749 × 712.6 km, with a mean radius of 736 ±2km. However, these measurements may be inaccurate on the kilometer scale as Iapetus' entire surface has not yet been imaged in high enough resolution. The observed flattening corresponds to a rotation period of 10 hours, and not to the 79 days observed. A possible explanation for this is that the shape of the moon was frozen shortly after its formation, while its rotation was slowed at a later time.

The equatorial ridge

A further mystery was discovered when the ''Cassini'' spacecraft imaged Iapetus on December 31 2004, and revealed an equatorial ridge about 20 km wide and 13 km high extending 1300 km through the center of Cassini Regio. Parts of the ridge rise more than 20 km over the surrounding plains. The ridge forms a complex system including isolated peaks, segments extending for more than 200 km and sections with three near parallel ridges.^[11] The prominent equatorial bulge gives the moon a "walnut-like" appearance.
The ridge follows the moon's equator almost perfectly and is not confined to Cassini Regio. Some bright mountains near the boundary of Cassini Regio that apparently belong to this ridge were seen in ''Voyager'' photos; however, the ''Voyagers'' were unable to make out any details in the dark region itself, so the extent of the ridge is only now apparent. The ridge system is heavily cratered, indicating that it is ancient.
The images are currently being analyzed by scientists and no firm conclusions have yet been announced about the ridge's origin. At least three hypotheses are in circulation:
One possibility is that the ridge is a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today.^[12] The height of the ridge suggests a maximum rotational period of 17 hours. In order for Iapetus to have cooled quickly enough to preserve the ridge, but remain plastic long enough for the tides raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus could only have been heated by the radioactive decay of aluminium-26. This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. The quantities of 'Al' needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the asteroids started to form.
Another possibility is that the ridge is icy material that has welled up from beneath the surface and then solidified. Given that Iapetus is locked in a synchronous spin, the bulge represented by the ridge would be driven to the equator even if formed elsewhere.
A third possibility has been suggested by Paulo C. C. Freire of Arecibo Observatory, who proposes that the ridge and Cassini Regio were created when Iapetus grazed the outer edges of Saturn's rings in the distant past.^[13] However, Freire's theory requires Iapetus to have been later ejected to its current, distant orbit around Saturn.^[14]

Temperatures

Temperatures on the dark region's surface can reach up to 130 K (-143.2°C or -226°F) at the equator, heating being made more effective by Iapetus's slow rotation. The brighter surfaces, on the other hand, absorb less sunlight so temperatures there only reach about 100 K (-173.2°C or -280°F).^[15]

Orbit

Polar view of Iapetus's orbit (red) compared to the other large moons of Saturn

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown.
Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see.

Side view of Iapetus's orbit (red) compared to the other large moons, showing its unusually high inclination

See also

★ List of geological features on Iapetus

★ Richard Hoagland's theory that Iapetus is artificial

Iapetus in fiction

''See Saturn's moons in fiction.''

References

1. http://home.gwi.net/~pluto/mpecs/ss08.htm#elements
2. Shapes of the Saturnian Icy Satellites, , P. C., Thomas, 37th Annual Lunar and Planetary Science Conference, 2006
3. The gravity field of the saturnian system from satellite observations and spacecraft tracking data, , R. A., Jacobson, The Astronomical Journal, 2006
4. Satellites of Saturn, , William, Lassell, Monthly Notices of the Royal Astronomical Society, 1848
5. /
6. Cowen, R. (2007). Idiosycratic Iapetus, ''Science News'' vol. 172, pp. 104-106. references)
7. Iapetus and Phoebe as Measured by the Cassini UVIS, , A. R., Hendrix, 36th Annual Lunar and Planetary Science Conference, 2005
8. Decoding the Domino: The Dark Side of Iapetus, , T. C., Owen, Icarus, 2001
9. Matthews, Robert A. J.; ''The Darkening of Iapetus and the Ongin of Hyperion'' Quarterly Journal of the Royal Astronomical Society, Vol. 33 (September 1992), p. 253
10. Spectrophotometry and Organic Matter on Iapetus. Models of Interhemispheric Asymmetry, , P. D., Wilson, Icarus, 1996
11. Cassini Imaging Science: Initial Results on Phoebe and Iapetus, , C. C., Porco, Science, 2005
12. Kerr, R. A.; ''Planetary Science: How Saturn's Icy Moons Get a (Geologic) Life'', Science, Vol. 311, No. 5757 (January 6 2006), p. 29 DOI: 10.1126/science.311.5757.29
13. Freire, P. C.; ''Solving the mystery of Iapetus'', submitted to the Journal of Geophysical Research - Planets
14. Did Iapetus Consume One of Saturn's Rings? T. Plotner
15. http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1281

External links

★ Cassini mission page - Iapetus

★ Iapetus Profile by NASA's Solar System Exploration

★ The Planetary Society: Iapetus

★ ''Astronomy Picture of the Day'' article on Iapetus

★ Mirror Objects in the Solar System? - refereed article discussing the speculative mirror matter, and Iapetus in this context

★ A Moon with a View - Richard C. Hoagland's highly creative discussion of Iapetus' oddities

★ New attempts to crack Saturn's 'walnut' moon - equatorial ridge formation theories
''... | Hyperion | 'Iapetus' | Kiviuq | ...''