On the sixth day, the huge rings of Saturn were already visible to the naked eye. The little girl in the group couldn't help jumping and shouted:

"Look! what a beautiful planet!"

Everyone gathered around to watch the real rings of Saturn that they had seen countless times in the mimic classroom. Hua Feng noticed that although Yun Meng and Bai Feng were a little reserved, there was also a fiery light flashing in their eyes, they looked at each other for a moment, smiled slightly, and nodded.

......

On the way, another astrophysicist, who accompanied the group, took Hua Feng in the bright learning pod as if he were on Earth, and they recalled Iapetus.

In October 1671, Giovanni Domenico Cassini discovered Titan on the west side of Saturn. In early 1672, Cassini tried to observe the moon from the east side of Saturn, but was unsuccessful.

This was the case again: Cassini observed Iapetus in December 1672 and February 1673 – both two weeks later – but in the interval between those two weeks, he was still unable to observe the moon on the eastern side of Saturn. Finally, in 1705, Cassini used a modified telescope to observe Titan on the east side of Saturn, and found that the apparent magnitude of the moon was reduced by two magnitudes.

Cassini correctly deduced that Iapetus has a brighter hemispheric surface and a darker hemispheric surface, and that the moon is tidally locked and always keeps the same side facing Saturn, so that the brighter side of Iapetus is always observed from the west side of Saturn, and the darker side is always observed on the other side. Later, the darker hemisphere of Iapetus was named "Cassini Zone".

Iapetus (Eapertus) is named after the titan giant Eapertus from Greek mythology.

Iapetus, along with three other Saturn moons (Titan, Titan, and Titan), was named "Side" by its discoverer, Cassini

a Lodoicea) in honor of King Louis XIV of France at the time. However, astronomers still followed the custom of naming it Titan, and in 1789 the discovery of Enceladus and Enceladus expanded the family of Saturn's moons, and Iapertus was renamed Iapetus, and after the discovery of Hyperion in 1848, it was renamed Titan.

Another name for Iapetus, which is still commonly used, was proposed by John Herschel in his 1847 book The Results of Astronomical Observations at the Cape of Good Hope. In the book, Herschel proposes that Saturn's moons be named after the Titans, the siblings of Cronus, who are the Roman equivalent of Saturnus, the god of saturn. The adjective case is Iapetia

or Japetia

。

The geological features on Titan are named after people and places from the French epic poem "The Song of Roland" (such as Charlemagne's crater and the bright area of Enceladus, the Lonsesvales district). The only exception is the dark area of the moon, the Cassini Zone, named after Giovanni Cassini, the discoverer of the area.

The low density of Iapetus suggests that it may be composed of ice and a small amount (about 20%) of the rock composition.

Unlike most of the moons, Titan is not spherical or ellipsoidal, with a convex equatorial part and a depression at the poles, and a unique ridge in the equatorial region that changes the shape of the moon even from a distance. These features make Titan look more walnut-shaped.

Iapetus has been subjected to heavy meteorite bombardment, and Cassini has discovered several massive craters on its dark side, at least five of which are more than 350 kilometers in diameter. The largest crater in Titan is the Tegis Crater (Tu

GIS), with a diameter of 580 km, has a steep crater rim, some of which are up to 15 km high.

In the 17th century, Cassini discovered that he could only observe Titan on the west side of Saturn, but never on the east side. He accurately deduced that Tuladus was a synchronous rotating moon orbiting Saturn, and that one side of it was much darker than the other. This hypothesis was later confirmed by larger telescopes.

The difference in brightness between the two hemispheres of Titan is enormous. The same orbital side is darker (albedo 0.3-0.5) with a reddish-brown tinge, while the other side is mostly brighter (albedo 0.5-0.6, close to Enceladus). So the magnitude on the reverse orbital side is 10.2, while the magnitude on the same orbital side is about 11.9 magnitude – beyond the discerning range of the best telescopes of the 17th century.

This chiaroscuro surface of Iapetus resembles the Tai Chi diagram in Taoism as well as the surface of a tennis ball. The dark side is named Cassini District, and the bright side is named the Lonsesvales District.

The original surface material that made up the dark surface was thought to have come from outside of Titan, but today it is made up of coarse detritus left over from the sublimation of ice in warmer regions, containing organic matter similar to those found on the surface of primitive meteorites and comets. Observations from Earth indicate that Titan is rich in carbon, and cyano-based compounds such as hydrogen cyanide polymers may be present in the intervening range.

On September 10, 2007, Cassini flew over Iapetus from a distance of 1,640 kilometers and found that the light and dark sides of the satellite had been heavily bombarded. It also found that the scattered patches of light and dark that make up the transition zone between Cassini and Longssvalles were small, even smaller than the maximum resolution of 30 meters for the photographs taken by Cassini.

The low-lying terrain on Iapetus is filled with dark material, and the crater's uplifted crater slopes are covered with bright material. Radar imagery of Cassini and the fact that a tiny meteor can form an impact crater in the ice beneath the cover, which is very thin, only tens of centimeters thick in some areas.

NASA scientists believe that the dark material is the coarse detritus left over from the sublimation of ice on the surface of Iapetus, which has further darkened due to exposure to sunlight.

With a rotation period of 79 Earth days (equivalent to its orbital period, the longest in Saturn's satellite system), Titan may have the highest solar surface temperature and the lowest dorsal surface temperature in Saturn's satellite system, while the near-equatorial region of the gloomy Cassini region will cause the dark matter to absorb heat to 128 degrees Kelvin during the day, compared to 113 degrees Kelvin in the bright Lonssvales region.

The difference in temperature means that the ice in Cassini is more likely to sublimate and eventually recondense in the Lonsesvales zone, especially in the polar regions where the temperatures are lowest. On geologic time scales, this effect will further darken the Cassini zone and brighten the Lonsesvalles zone and the polar regions.

The gradual loss of exposed ice in Cassini's zone drives the formation of a positive thermal feedback process that ultimately leads to greater contrast in the albedo of the light and dark surfaces. It is estimated that under current temperature conditions and without taking into account the shift of ice from the dark to the bright side, 20 meters of ice will be sublimated in 10 million years, while only 10 meters of ice will be lost in the Ronsesvalles area at the same time. This model explains the distribution of light and dark areas on Titan, the lack of gray ** domains, and the thinning of the dark matter covered by Cassini zones.

However, the premise for this thermal feedback mode to be activated is that there must be a difference between light and dark on the surface of Iapetus. It has been speculated that the first dark matter may have been debris raised by meteors bombarding small outer moons orbiting in retrograde orbit and adsorbed by Iapetus's co-orbital side. The core theory of this model has been established for more than 30 years, and it has been particularly important after Cassini's flyby in September.

As the orbit decays, the fine debris formed by the impact of micrometeoroids or meteorite impacts spirals into the inner orbit as a result of the detritus formed from the surface of the satellite. During this time, the detrition begins to darken due to exposure to sunlight. As the debris passes through Titan's orbit, it is likely to be adsorbed by Iapetus's orbital direction.

This layer of adsorbate covered by Titan causes a change in albedo, which in turn causes a change in temperature, which is exacerbated by the thermal feedback process that has also been initiated.

The largest donor of these debris is Titan, which is the largest outer moon. Although Titan's material composition is closer to the light side than the dark side of Titan, the detritus from Titan is only used to create the initial albedo difference, and it is likely that these debris have been masked by the sublimation residue that followed.

The triaxial length of Titan is 747.1×749×712.6 km, with an average radius of 736±2 km. However, because the overall surface of Titan has not been imaged with high resolution, there are still errors in the above data at the kilometer level. The observed flattening of Titan should correspond to a rotation period of 10 hours, rather than an actual rotation period of 79 days. A possible explanation is that in the early days of Iapetus's formation, it formed a thick shell that held the entire star shape in place. Later, due to gravitational tidal action, Titan's rotation period gradually lengthened until it finally formed a tidal locking state.

Another mystery of Iapetus is its equatorial ridge in the center of the Cassini district, which is about 1,300 kilometers long, 20 kilometers wide and 13 kilometers high. This terrain was spotted in photographs taken by the Cassini on December 31, 2004. Part of this equatorial ridge even rises up to 20 kilometers above the surrounding plain terrain.

The equatorial ridge is made up of a variety of complex terrains, including individual peaks, cliffs over 200 kilometers in length, and topographic units consisting of three parallel ridges that are closely apart. In the bright Lonsesvales Plateau, there is no equatorial ridge, but instead a series of independent peaks in the equatorial region with a height of up to 10 km. The equatorial ridge terrain has been heavily bombarded, which proves that its geological age is very old. This protruding terrain near the equator gives Titan a walnut-shaped shape.

It is still unclear how this terrain was formed. One of the inexplicable problems is why the equatorial ridges are so precisely located along the equatorial region. Three hypotheses exist, none of which explain why the equatorial ridge exists only in the Cassini zone.

A team of scientists involved in the Cassini project has proposed that the equatorial ridge is a remnant of the oblate-shaped star of the early formation of Iapetus, which rotated much faster than it does today.

The height of the equatorial ridge suggests that its shortest rotation period may have been 17 hours. If Titan had to cool fast enough for the equatorial ridge to be preserved, while remaining malleable enough for long enough to be able to maintain its plasticity – long enough for Saturn's tidal action to slow down Iapetus' rotation and eventually bring its rotation period to 79 Earth days – then Titan would need the isotope decay of Aluminum-26 to heat it up.

This isotope was found in the nebulae of the early solar system, but it is estimated that it was depleted in the early days of the formation of the solar system. To have the amount of aluminum-26 isotopes needed to heat Titan, Iapetus would have to form earlier than expected – 2 million years after the asteroid began to form.

The equatorial ridge may also be formed by the re-condensation of the ice that emerges from beneath the formation.

It has also been suggested that in the early days of formation, the Hull space on Titan (Hill Sphe

e) The area has formed a ring system, which later formed the present equatorial ridge due to the partial collapse of the ring system. However, the equatorial ridge, which appears to be very solid, does not appear to be caused by this collapse effect. In addition, recent observational images show a faulted structure that runs through the equatorial ridge, a phenomenon that seems to contradict the collapse ring hypothesis.

Finally, Carrivan said: It is still unclear how this terrain came to be. One of the inexplicable problems is why the equatorial ridges are so precisely located along the equatorial region. Three hypotheses exist, none of which explain why the equatorial ridge exists only in the Cassini zone.