Iapetus (moon)

The Saturnian satellite Iapetus is one of the most unusual satellites in the solar system. It has a dark side, a bright side, and a ridge along its equator that sits atop an equatorial bulge.

Overview

Iapetus was first noticed on one side of Saturn by Giovanni Domenico Cassini in October 1671. Searching for it later on the other side of the planet was futile with the telescope that he was using. He tracked it many times over several years, but only when he had a better telescope in 1705 did Cassini finally see Iapetus on the other side of the planet. He concluded that Iapetus had a dark side and a bright side. It was named for the Titan god Iapetus, a brother of Cronus (the Greek name for the Roman god Saturn) in Greek mythology. Iapetus was originally called Saturn V because the Saturnian satellites were originally numbered, and many scientists continued to use the numbers. With the discovery of other Saturnian satellites, Iapetus’s scientific referent changed to Saturn VII and eventually Saturn VIII. Except for its dark region, Iapetus's geological features are named after characters and places from the epic French poem The Song of Roland. The dark region is called Cassini Regio, in honor of Iapetus’s discoverer, Cassini.

Cassini deduced that Iapetus always presents the same face as Saturn, which is synchronous with Saturn. Therefore, the time of revolution and the time to circle the planet are identical, 79.32 days. Iapetus is prograde, meaning it turns in the same direction as Saturn. Iapetus’s orbit is about 15.5° out of the plane of Saturn’s equator; its inclination is 15.5°. The inclination is large enough to cause questions about whether Iapetus was “captured” by Saturn’s gravitational field or was generated by the same process that produced Saturn and the other satellites. Iapetus’s elliptical orbit (its eccentricity is 0.029) and a synchronous satellite 3,561,000 kilometers from the planet also add to the doubt about whether Iapetus was formed by the same process that formed Saturn.

Iapetus is an oblate spheroid with a 1,494-kilometer diameter along the axis pointed at Uranus. The equatorial axis is 1,498 kilometers, and the pole-to-pole axis is 1,426 kilometers. With its equatorial bulge and squashed poles, Iapetus looks like a walnut with a twenty-kilometer ridge that girds most of the satellite along its equator. No other known satellite has such a ridge. It is triangular, with a 200-kilometer wide base. The ridge is also cratered, proving it has existed for a long time, and some theorize that it was created because the crust formed while the interior was still flexible enough for the weight of the shell to crush the interior. The interior material forced its way to the surface, fracturing the shell at the equator and forming a ridge. The material cooled relatively quickly, locking it in the shape of modern Iapetus. This theory requires that Iapetus spin much faster during its formation and cooling periods. The slowing of its spin, despinning, had to occur near a large object such as a planet. Lapetus’s composition is similar to that of the other Saturnian satellites, indicating that Iapetus was formed in the process that formed Saturn and the other large satellites.

The first feature of Iapetus that astronomers noted was its albedo, which is entirely different on the two sides of this satellite. The pictures from the Cassini spacecraft (2007) show a tar-black leading hemisphere with a bright backside hemisphere. The albedo is about 0.05 for the leading side and 0.6 for the trailing hemisphere. This variation in albedo is noted not only in the visible range but also in the ultraviolet and radio ranges. The Cassini images suggested that the dark material is not solid but is in streaks large enough to appear solid from a distance. Near-infrared spectra indicate that the bright material is ice. The moon’s 1.1 grams per centimeter³ cubed density indicates the rock fraction can be no more than 22 percent. The dark side's composition appears to be ice contaminated with materials like ammonia, amorphous carbon, poly-hydrogen cyanide (poly-HCN), and hematite (Fe₂ O₃).

Some scientists have theorized that the dark material originates from another satellite, Phoebe. Phoebe has a retrograde orbit, meaning it is traveling in the direction opposite to Saturn’s rotation and, thus, in the direction opposite to Iapetus’s orbital motion. Material lost by Phoebe because of its retrograde motion can bombard the front face of Iapetus. However, data from the ultraviolet spectra collected by Cassini show that the composition of Phoebe is not the same as that of the dark material on Iapetus. There is the possibility that the dark material might be from Hyperion since the composition of Hyperion does match that of Iapetus. A second option is that the material from Phoebe changes before it bombards Iapetus. The dark material is thought to be a thin layer containing only a small amount (5 percent) of ice. A third opinion is that the dark material is consistent with an external impact. This would cause the poles of Iapetus to be bright, which they are. The bright poles may be bright because of frost. The peak temperature of the dark side of Iapetus was measured at 130 kelvins, warm enough to allow ice to sublime and refreeze at the poles, producing bright areas. The dark material has a reddish color that might indicate organic compounds. Organics would darken with radiation exposure. Indications are that the material is porous and in the form of fine particles.

The topography of Iapetus was not well known before the Cassini mission. Even pictures from the Voyager 1 mission did not show any feature on the dark side. The bright side was also largely unknown. Iapetus is cratered but is not saturated with craters. Around the craters were tall, steep, wall-like features called scarps. The craters on Iapetus may not have retained the full height of the crater rim, which would indicate the lithosphere is not thick or solid. The largest crater is 800 kilometers across, with a rim topography of ten kilometers (raised ten kilometers from the crater floor). Some scientists believe that Iapetus was formed and its shell hardened very early. The surface might be the oldest surface known.

Knowledge Gained

Iapetus has been studied with land-based telescopes since it was discovered. Its dichotomous albedo made it an unusual object and, therefore, an object of interest. Reflectance spectra taken using the instruments at the McDonald Observatory produced data that showed reddening of the dark-side material, as dark-side reflectance was compared to that of the bright side. Other land-based studies determined the size of Iapetus, the albedo range, and the longitudinal symmetry of the dark material. One of the most in-depth studies of Iapetus’s composition was done from the observatory at Hawaii’s Mauna Kea, using visible and near-infrared spectrometry.

The Voyager 1 mission revealed the diameter of Iapetus. Images of Iapetus were taken, revealing the great amount of cratering on the bright side and at the poles. Perturbations in the path of Voyager allowed scientists to calculate the satellite’s density.

In addition to the Image Science Substation (ISS), a version of which was on both of the Voyager spacecraft, the Cassini spacecraft (2004) had an ultraviolet imaging spectrograph (UVIS), which produces simultaneous spectral and spatial images. The visual and infrared mapping spectrometer (VIMS) was used to study the icy satellites by generating reflectance spectra, phase curves, and visual pictures. These data are especially important to compare with Earth-based data, providing an evaluation of the Earth-based data and possibly ideas of methods to correct the Earth-based data for two of the problems that occur using Earth-based instruments. Those two problems are the small phase angle seen from Earth and the extra light generated by reflection from the rings and Saturn. The composite infrared spectrometer (CIRS) on Cassini studied the thermal infrared spectrum for emissivity features.

Certain compounds emit a thermal signature in the infrared range that is noticeable in the background thermal spectra. The thermal spectra from Iapetus did not show any robust features. The lack of emission features and the data from near-infrared spectra caused scientists to believe that the surface must be covered with small particles with a high porosity. One bit of information gleaned from Cassini's images is that scarps and crater walls are bright on their north sides and covered with dark material on their south sides.

Context

Iapetus is undoubtedly an enigma. The black face on one side and the bright face on the other side remain unexplained. Scientists have ideas for explaining these phenomena, but none have definitive answers. If the dichotomy is due to material from other satellites, what is the transport mechanism, and what changes occur during transport? A second question about Iapetus concerns its formation. How did the equatorial ridge form, and why? What does the existence of the ridge tell scientists about the solar system's formation? Is the surface of Iapetus the oldest surface in the solar system? Can Iapetus help scientists determine how the solar system was formed?

Many scientists are surprised that Iapetus has such a large angle of inclination and is so far from Saturn, yet it is in synchronous relationship with the planet. Being synchronous requires a very slow revolution, which is unusual and can be explained only if one concludes that Saturn's gravitational force has caused despinning. The gravitational force on Iapetus increases and decreases as Iapetus gets closer and farther from the planet, which causes tidal flexing in the satellite. The heat from this tidal flexing may explain the heat needed to generate some of the satellite’s features, such as the ridge and the dichotomous albedo.

Bibliography

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