Iapetus (moon)

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"Japetus" redirects here. For the mythological figure, see Iapetus. For the insect genus, see Japetus (planthopper).
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Iapetus
Iapetus as imaged by the Cassini orbiter in false color, September 2007. Iapetus's unusual coloration can be seen, with the equatorial ridge on the right limb. The large craters Engelier and Gerin are near the bottom.
Discovery
Discovered byG. D. Cassini
Discovery dateOctober 25, 1671
Designations
Designation
Saturn VIII
Pronunciation/aɪˈæpətəs/[1]
Named after
Ἰαπετός Īapetus
AdjectivesIapetian /aɪəˈpiːʃən/[2]
Orbital characteristics
3560820 km
Eccentricity0.0276812 [3]
79.3215 d
3.26 km/s
Inclination
  • 17.28° (to the ecliptic)
  • 15.47° (to Saturn's equator)
  • 8.13° (to Laplace plane)[4]
synchronous)
zero
Albedo0.05–0.5[7]
Temperature90–130 K
10.2–11.9[8]

Iapetus (

Iapetus, the moon was discovered in 1671 by Giovanni Domenico Cassini
.

A relatively low-density body made up mostly of ice, Iapetus is home to several distinctive and unusual features, such as a striking difference in coloration between its leading hemisphere, which is dark, and its trailing hemisphere, which is bright, as well as a massive equatorial ridge running three-quarters of the way around the moon.

History

Discovery

Giovanni Domenico Cassini, discoverer of Iapetus

Iapetus was discovered by

Christaan Huygens spotted Titan
16 years prior in 1655; and the sixth extraterrestrial moon to be discovered in human history.

Cassini discovered Iapetus when the moon was on the western side of Saturn, but when he tried viewing it on the eastern side some months later, he 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.[10][11]

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.[12]

Name

Iapetus from Greek mythology. The name was suggested by John Herschel (son of William Herschel) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,[13] in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan Cronus (whom the Romans equated with their god Saturn); and Giants, the massive but lesser relatives of the Titans who sided with the Titans against Zeus and the Olympian Gods.[14]

The name has a largely obsolete variant, Japetus[13][15] /ˈdʒæpɪtəs/,[16] with an adjectival form Japetian.[15] These occurred because there was no distinction between the letters ⟨i⟩ and ⟨j⟩ in Latin, and authors rendered them differently.

When first discovered, Iapetus was among four the

The Song of Roland.[17]

Orbit

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

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 also has the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. 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. The cause of this highly inclined orbit is unknown; however, the moon is not likely to have been captured. One suggestion for the cause of Iapetus's orbital inclination is an encounter between Saturn and another planet in the distant past.[18]

Despite being, on average, 2.4 times further from Saturn than Hyperion, the next moon inward, Iapetus is tidally locked to Saturn while Hyperion is not.[19]

Formation

The moons of Saturn are typically thought to have formed through

giant impacts between pre-existing moons. Iapetus and Rhea are thought to have formed from part of the debris of these collisions.[20] More-recent studies, however, suggest that all of Saturn's moons inward of Titan are no more than 100 million years old; thus, Iapetus is unlikely to have formed in the same series of collisions as Rhea and all the other moons inward of Titan, and—along with Titan—may be a primordial satellite.[21]

Physical characteristics

Main article: List of geological features on Iapetus
Size comparison between Iapetus (lower left), the Moon (upper left) and Earth
A map of the surface of Iapetus with its surface features labeled with their official names. 0° longitude marks the side of the moon that always faces Saturn.

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.[22]

Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles.[23] Its unique equatorial ridge (see below) is so high that it visibly distorts Iapetus's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.

Iapetus is heavily cratered, and Cassini images have revealed large impact basins, at least five of which are over 350 km (220 mi) wide. The largest, Turgis, has a diameter of 580 km (360 mi);[24] its rim is extremely steep and includes a scarp about 15 km (9.3 mi) high.[25] Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding.[26]

Two-tone coloration

Natural-color image of Iapetus

The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (

sublimation (evaporation) of ice from the warmer areas of the moon's surface, further darkened by exposure to sunlight.[27][28][29] It contains 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
.

A colored map of the surface of Iapetus by the Lunar and Planetary Institute clearly showing the dichotomy between its light and dark hemisphere.

Images from the Cassini orbiter, which passed within 1,227 km (762 miles), show that both Cassini Regio and the Terra's are heavily cratered.

meteor impacts have punched through to the ice underneath.[29][33]

View of Cassini Regio. The large craters that are visible include Falsaron (upper left), Turgis (above and right of center) and Ganelon (lower right)

Because of its slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), Iapetus would have had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system even before the development of the color contrast; near the equator, heat absorption by the dark material results in a daytime temperatures of 129 K (−144 °C) in the dark Cassini Regio compared to 113 K (−160 °C) in the bright regions.[29][34] The difference in temperature means that ice preferentially sublimates from Cassini Regio, and deposits in the bright areas and especially at the even colder poles. Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback thermal runaway process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio.[29] It is estimated that over a period of one billion years at current temperatures, dark areas of Iapetus would lose about 20 meters (70 ft) of ice to sublimation, while the bright regions would lose only 10 cm (4 in), not considering the ice transferred from the dark regions.[34][35] This model explains the distribution of light and dark areas, the absence of shades of grey, and the thinness of the dark material covering Cassini Regio. The redistribution of ice is facilitated by Iapetus's weak gravity, which means that at ambient temperatures a water molecule can migrate from one hemisphere to the other in just a few hops.[29]

However, a separate process of color segregation would be required to get the thermal feedback started. The initial dark material is thought to have been debris blasted by meteors off small outer moons in

retrograde orbits and swept up by the leading hemisphere of Iapetus. The core of this model is some 30 years old, and was revived by the September 2007 flyby.[27][28]

The bright regions of Iapetus. Roncevaux Terra is at the top (north); while Saragossa Terra with its prominent basin Engelier, Iapetus's second largest, is at the bottom.

Light debris outside of Iapetus's orbit, either knocked free from the surface of a moon by micrometeoroid impacts or created in a collision, would spiral in as its orbit decays. It would have been darkened by exposure to sunlight. A portion of any such material that crossed Iapetus's orbit would have been swept up by its leading hemisphere, coating it; once this process created a modest contrast in albedo, and so a contrast in temperature, the thermal feedback described above would have come into play and exaggerated the contrast.[28][29] In support of the hypothesis, simple numerical models of the exogenic deposition and thermal water redistribution processes can closely predict the two-toned appearance of Iapetus.[29] A subtle color dichotomy between Iapetus's leading and trailing hemispheres, with the former being more reddish, can in fact be observed in comparisons between both bright and dark areas of the two hemispheres.[28] In contrast to the elliptical shape of Cassini Regio, the color contrast closely follows the hemisphere boundaries; the gradation between the differently colored regions is gradual, on a scale of hundreds of kilometers.[28] The next moon inward from Iapetus, chaotically rotating Hyperion, also has an unusual reddish color.

The largest reservoir of such infalling material is

tenuous disk of material in the plane of and just inside Phoebe's orbit was announced on 6 October 2009,[37] supporting the model.[38] The disk extends from 128 to 207 times the radius of Saturn, while Phoebe orbits at an average distance of 215 Saturn radii. It was detected with the Spitzer Space Telescope
.

Overall shape

Current triaxial measurements of Iapetus give it radial dimensions of 746 km × 746 km × 712 km (464 mi × 464 mi × 442 mi), with a mean radius of 734.5 ± 2.8 km (456.4 ± 1.7 mi).[5] However, these measurements may be inaccurate on the kilometer scale as Iapetus's entire surface has not yet been imaged in high enough resolution. The observed oblateness would be consistent with hydrostatic equilibrium if Iapetus had a rotational period of approximately 16 hours, but it does not; its current rotation period is 79 days.[39] 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.[23]

Equatorial ridge

Main article: Equatorial ridge on Iapetus
Closeup of the equatorial ridge

A further mystery of Iapetus is the

tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than 200 km (120 mi) and sections with three near parallel ridges.[41] Within the bright regions there is no ridge, but there are a series of isolated 10 km (6.2 mi) peaks along the equator.[42] The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives Iapetus a walnut
-like appearance.

A series of images of Iapetus's north pole taken by Voyager 2 as it flew by. The white dots at the lower part of the images (Iapetus's equator) hinted at the presence of high mountains there which will later turn out to be the equatorial bulge and duly named Voyager Mountains.

It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are many hypotheses, but none explain why the ridge is confined to Cassini Regio. Theories include that the ridge is a remnant of Iapetus's oblate shape during its early life, that it was created by the collapse of a ring system, that it was formed by icy material welling from Iapetus's interior, or that it is a result of convective overturn.[43]

Exploration

One of the first-ever images that clearly shows Iapetus's light and dark areas, taken by Voyager 1 on Nov. 12, 1980, from a distance of 3.2 million kilometers (1.9 million miles).[44]

The first spacecraft to visit Saturn, Pioneer 11, did not provide any images of Iapetus and it came no closer than 1,030,000 km (640,000 miles) from the moon.[45] Nonetheless, Pioneer 11 was humanity's first attempt to obtain direct measurements from the objects within the Saturnian system.

Voyager 1 arrived at Saturn on November 12, 1980, and it became the first probe to return pictures of Iapetus that clearly show the moon's two-tone appearance from a distance of 2,480,000 km (1,540,000 mi).[46] as it was exiting the Saturnian system.[47]

Voyager 2's image of the north pole of Iapetus taken on August 22, 1981, from a distance of 910,000 km (570,000 mi). The crater on the lower part along the terminator is Roland[48]

Voyager 2 became the next probe to visit Saturn on August 22, 1981, and made its closest approach to Iapetus at a distance of 909,000 km (565,000 mi). It took photos of Iapetus's north pole as it entered the Saturnian system - opposite the approach direction of Voyager 1.

The latest probe to visit Iapetus was the Cassini orbiter which went into orbit around Saturn starting on July 1, 2004.[49] Iapetus has been imaged many times from moderate distances by Cassini but its great distance from Saturn makes close observation difficult.

Cassini made its first targeted flyby of Iapetus on Dec. 31, 2004, at a distance of 123,400 km (76,700 mi) around the time when the spacecraft was settling in its orbit around Saturn.[50] Cassini did not crossed Iapetus's orbit when it flew by and remained inside the moon's orbit. Cassini's subsequent flybys of Titan would make the spacecraft's orbit smaller, preventing Cassini from flying close to Iapetus for months.

Cassini made a second flyby of Iapetus on November 12, 2005, at a distance of 415,000 km (258,000 mi),[51] also without crossing the moon's orbit.

Cassini then made a third and more distant flyby of Iapetus on January 22, 2006, at a distance of 1,300,000 km (810,000 mi).[52][53]

The fourth flyby happened on April 8, 2006, at a distance of approximately 866,000 km (538,000 mi), and this time, Cassini crossed Iapetus' orbit.[54] After this, Cassini's orbit was made smaller once again, preventing the probe from approaching Iapetus for more than a year this time.

Cassini's closest flyby of Iapetus happened on September 10, 2007, at a minimum range of 1,227 km (762 mi).[30] It approached Iapetus from its night side.[55]

After this encounter, Cassini made no further targeted flybys of Iapetus.

In popular culture

The monolith featured during the climax of Arthur C. Clarke's 1968 novel 2001: A Space Odyssey is located on Iapetus.

A team of scientists explore Iapetus in The Saturn Game, a science-fiction novella by Poul Anderson (1981).

The moon Iapetus is one of many galactic settings in the

2312
(2012).

Iapetus is also the setting for Czech writer Julie Nováková's story "The Long Iapetan Night", published in the November 2020 edition of Asimov's Science Fiction Magazine.[56]

Gallery

  • Iapetus seemingly in the shape of the famous Taijitu (also known as the Yin-Yang symbol). This was imaged by Voyager 1 on November 12, 1980.
    Iapetus seemingly in the shape of the famous Taijitu (also known as the Yin-Yang symbol). This was imaged by Voyager 1 on November 12, 1980.
  • Enhanced version of an image of Iapetus by Voyager 2. The left side of Iapetus in this image is its side that is always facing away from Saturn.[57]
    Enhanced version of an image of Iapetus by Voyager 2. The left side of Iapetus in this image is its side that is always facing away from Saturn.[57]
  • An image of a half-illuminated Iapetus, taken by Cassini on November 12, 2005 from a distance of 417,000 kilometers (259,000 miles) during its flyby of Saturn. The big crater at the center is Turgis, while the partly-shadowed crater with a central peak to the north is Roland.[58]
    An image of a half-illuminated Iapetus, taken by Cassini on November 12, 2005 from a distance of 417,000 kilometers (259,000 miles) during its flyby of Saturn. The big crater at the center is Turgis, while the partly-shadowed crater with a central peak to the north is Roland.[58]
  • An image of Iapetus from its nightside that was photographed by an approaching Cassini, showing it as a slender crescent with its massive equatorial ridge clearly visible at the middle. This image was taken on September 10, 2007 at a distance of 83,000 kilometers (51,600 miles).[59]
    An image of Iapetus from its nightside that was photographed by an approaching Cassini, showing it as a slender crescent with its massive equatorial ridge clearly visible at the middle. This image was taken on September 10, 2007 at a distance of 83,000 kilometers (51,600 miles).[59]
  • A super-close image of Iapetus's transition area between its light and dark side, showing white materials sharply spattering into the dark materials. Imaged by Cassini on September 10, 2007 at a distance of 9,240 kilometers (5740 miles) somewhere in the area of the Carassone Montes.[60]
    A super-close image of Iapetus's transition area between its light and dark side, showing white materials sharply spattering into the dark materials. Imaged by Cassini on September 10, 2007 at a distance of 9,240 kilometers (5740 miles) somewhere in the area of the Carassone Montes.[60]
  • A much clearer image of Iapetus's Taijitu (Yin-Yang) appearance, taken by Cassini on August 30, 2013 from a distance of 2.5 million kilometers (1.5 million miles). The crater near the terminator at the lower center is Ganelon.
    A much clearer image of Iapetus's Taijitu (Yin-Yang) appearance, taken by Cassini on August 30, 2013 from a distance of 2.5 million kilometers (1.5 million miles). The crater near the terminator at the lower center is Ganelon.
  • One of Cassini's final images of Iapetus taken on May 30, 2017, a few months before Cassini plunge into Saturn, at a distance of 2.5 million kilometers (1.5 million miles). The clearly-visible craters right of center on the white side are Naimon, Astor and Ivon (left to right).
    One of Cassini's final images of Iapetus taken on May 30, 2017, a few months before Cassini plunge into Saturn, at a distance of 2.5 million kilometers (1.5 million miles). The clearly-visible craters right of center on the white side are Naimon, Astor and Ivon (left to right).

See also

Notes

  1. ^ The moons more massive than Iapetus are: the Moon, the 4 Galilean moons (Ganymede, Callisto, Io, and Europa), Titan, Rhea, Titania, Oberon, and Triton.[9]

References

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  5. ^ a b c Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781.
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