Soiled white ball with a belt at the equator
Where in space to find dumplings, a bagel and a walnut, why the two-faced satellite was named Iapetus, and not Janus, and how mountains can fall on an astronomical object, read the material Indicator. Ru.
The very history of the discovery of Iapetus suggests that this very large satellite (almost 1500 kilometers across, the 11th largest satellite in the solar system, the third largest satellite of Saturn) is very unusual.
In October 1671, he was seen through a telescope by the famous astronomer Giovanni Domenico Cassini, the same one after whom the NASA probe was named, which had worked in the orbit of Saturn for more than a dozen years. Cassini spotted a new satellite from the west of the giant planet, however, when he tried to see it from the east a few months later, nothing came of it. The same story happened the next year. Only a third of a century later, he was able to see Iapetus from the east side of Saturn using a much more powerful telescope. Even then, the astronomer made an assumption: firstly, the satellite is always turned to Saturn by the same side, and secondly, one hemisphere is dark, and the other is light. Therefore, when it moves towards an observer (from the east of Saturn), it is poorly visible through a telescope, but from an observer it is good.
Paris Observatory, in which Cassini discovered Iapetus / Wikimedia Commons
This is probably why Iapetus somehow became one of the favorite celestial bodies of Soviet and foreign science fiction writers. Arthur Clarke had an alien stargate in his "Odyssey" on Iapetus, and the works of Sergei Pavlov and Vladimir Mikhailov took place there ("Moonlight Rainbow" and "Stream on Iapetus. However, there seem to be no streams there).
The first relatively normal and, more importantly, a close look at Iapetus was thrown by both Voyagers, confirming Cassini's assumption. Indeed, one - the "leading" - side of the satellite is very dark, reflects 3-5% of the light falling on it, and the other is one of the brightest in our solar system, returning 50-60% of the light to outer space. The riddle of such two-facedness (oh, the wrong moon of Saturn was named Janus) was to be solved by Cassini.
The bright side of Iapetus / NASA / JPL
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Another "incorrectness" of Iapetus (by the way, the name itself is given in honor of the ancient Greek titan, the son of Uranus, the father of Prometheus, and, in general, the father of mankind - some researchers put an equal sign between the ancient Iapetus and the biblical Japheth) in comparison with all the large satellites of Saturn this is its remote and 150-tilted orbit.
Orbit of Iapetus (in red) in relation to other large moons of Saturn / NASA / JPL
Cassini's meeting with Iapetus happened on New Year's Eve - December 31, 2004. And instead of providing answers to old questions, this rendezvous asked planetary scientists new riddles. It turned out that Iapetus is not a ball, and is the largest body in the solar system that has not reached hydrodynamic equilibrium. If he were on his own, he would not have been chosen even in dwarf planets, like Ceres comparable to him.
Most of all Iapetus looks like … a walnut. A mountain belt runs along the equator of this satellite, which is higher than our Everest. With the width of the mountain range 20 kilometers in height, the Wall of Iapetus - as astronomers informally call it - reaches as much as 13 km.
Iapetus and his Wall / NASA / JPL
How did such features of the planet appear?
Most likely, Iapetus itself is a kind of snowball or ice - judging by its density (1.088 grams per cubic centimeter), it is almost entirely composed of water ice. This is probably why it is originally a very white object. The "dark side" of Iapetus appeared over hundreds of millions of years, that it revolves around Saturn with "one" side, "picking up" on the leading side dark particles from the Phoebe ring - a huge rarefied "donut" of dust particles stretching over a hundred radii of Saturn.
As for the Wall of Iapetus, there were three main hypotheses of its appearance. One of them - Iapetus was more rarefied, and then shrank, squeezing out rocks along the equator - it seems that it has already been thrown away. Astronomers are now suggesting an external origin for this belt. But there are also two versions, each of which has the right to exist. First, Iapetus once had its own rings, which gradually "crumbled" to the equator. The second - once Iapetus moved along a different trajectory, passed through the rings of Saturn and a mountain ridge "froze" on him. This version is supported by recent pictures of "space dumplings" - Atlas and Pan.
Wall of Iapetus, close look / NASA / JPL
Well, in conclusion, it is worth saying a little about the names of objects on the surface of Iapetus. The first 20 appeared during the flight of Voyager satellites, which captured 40% of the satellite's surface, and in 2008 the names were approved for the rest of the territory. It's funny that all the names - except one, the International Astronomical Union decided to take from … Song of Roland. So there are the mountains of Toledo, Seville and Cordoba, and the Land of Zaragoza, and the craters Roland, Oliver, Oton and many others. And only the "dark" side of Iapetus is called the Cassini Region. In honor of the astronomer, not the probe.
Map of Iapetus with the names of objects / NASA / JPL
Alexey Paevsky