page_title: Iapetus (moon)
section_title: Two-tone coloration
hierarchical_section_title: Iapetus (moon) / Physical characteristics / Two-tone coloration
caption_attribution_description: English: Saturn's two-faced moon tilts and rotates for Cassini in this mesmerizing movie sequence of images acquired during the spacecraft's close encounter with Iapetus on Nov. 12, 2005. The encounter begins with Cassini about 850,000 kilometers (530,000 miles) from Iapetus. Cassini approached over the moon's northern hemisphere, allowing for excellent full views of a 575-kilometer-wide (360-mile) impact basin in northeastern Cassini Regio. Astronomer Giovanni Cassini discovered the light and dark faces of Iapetus' two hemispheres (among his other Saturn discoveries), and the dark region. The spacecraft also bears his name. Also prominent in these images is a 380-kilometer-wide (235-mile) basin to the northwest of the larger basin, in the transition zone between Cassini Regio and a brighter region called Roncevaux Terra, with its 150-kilometer-wide (95-mile) crater Roland (at the top, with a prominent central peak). The movie takes Cassini to its closest approach, at about 415,000 kilometers (260,000 miles) from Iapetus, and then looks back at the moon's receding crescent. The sequence ends with Cassini at a distance of about 460,000 kilometers (285,000 miles) from the moon. Iapetus is 1,468 kilometers (912 miles) across. Images taken using ultraviolet, green and infrared spectral filters with the Cassini spacecraft narrow-angle camera were combined to create false-color frames for this movie. The color seen here is similar to that produced in (red, green and blue) natural color views. Resolution in the original images taken at closest approach to Iapetus was about 3 kilometers (2 miles) per pixel. The color frames were resized to create the movie. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
context_page_description: Iapetus 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. Discoveries by the Cassini mission in 2007 revealed several unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon.
context_section_description: In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and never on the east. He correctly deduced that Iapetus is locked in synchronous rotation about Saturn and that one side of Iapetus is darker than the other, conclusions later confirmed by larger telescopes.
  The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5–0.6, almost as bright as Europa). Thus, the apparent 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 or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it.  Before optical observations could be made by deep space probes, theories about the reason for this dichotomy included an asteroid shearing off part of the moon's crust. The original dark material is believed to have come from outside Iapetus, but now it consists principally of lag from the sublimation of ice from the warmer areas of Iapetus's surface. 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.
  On September 10, 2007 the Cassini orbiter passed within 1,227 km (762 miles) of Iapetus and returned images showing that both hemispheres are heavily cratered. The color dichotomy of scattered patches of light and dark material in the transition zone between Cassini Regio and the bright areas exists at very small scales, down to the imaging resolution of 30 metres (98 ft). There is dark material filling in low-lying regions, and light material on the weakly illuminated pole-facing slopes of craters, but no shades of grey. The dark material is a very thin layer, only a few tens of centimeters (approx. one foot) thick at least in some areas, according to Cassini radar imaging and the fact that very small meteor impacts have punched through to the ice underneath.
  A better hypothesis is that the dark material is lag (residue) from the sublimation (evaporation) of water ice on the surface of Iapetus, possibly darkened further upon exposure to sunlight. 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. 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. 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. 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
caption: View of Cassini Regio. Large craters visible include Falsaron (upper left), Turgis (above and right of center) and Ganelon (lower right)
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