Iapetus: the strangest moon of the Solar System
In 1671, Giovanni Cassini was looking through a telescope at Saturn and found a number of incredible miracles: the famous gap in its rings, cloud bands in the atmosphere and a few satellites. The second discovered Saturn's moon - Iapetus - immediately proved its uniqueness: it was seen only half of its orbit. The other 50% of the time Iapetus was completely invisible, it can not be detected in any way, even though the rest of it was subject to the ordinary laws of gravity. Thirty years later, improvements Telescope Cassini was finally able to find the moon in the west and east side, but it was six times fainter than on the eastern side.
Cassini has developed a theory about the moon, now known as the Iapetus. He argued that it is primarily Iapetus must be a two-tone, with one side more vivid, and the other - less bright. Second, it must be tidally locked to Saturn, to meet him is always one and the same party. By combining these factors, the "cutting edge" will lapetus dimmer and darken its rear. The idea was interesting, but check it out, there was no opportunity.
This difference of color is not the only thing that makes Iapetus remarkable or even unique among all the satellite. All the major satellites of Saturn are orbiting in the same plane as his ring. All but Iapetus, which is significantly tilted. No one knows why. No other large moon in the solar system, which formed together with the parent planet, has no such inclination as Iapetus. Still at Iapetus has a giant ridge along the equator: about 10 kilometers higher than the rest of the solid ice of the world. But the satellite is rotating fast enough to explain the origin of the ridge and the surface of Iapetus many billions of years, so it's definitely not accumulated debris. None of the theories explaining the appearance of the ridge has no advantages and superiority. Iapetus is very unusual for a satellite of the solar system, and we figured out it is not so much of his mysteries.
Yet one of the mysteries of Iapetus we unraveled 300 years after its inception. Thanks to the "Cassini" - NASA's mission, rather than to the Italian scientist - we got to Iapetus and photographed it. It turned out that one of its sides looks as though it plowed into a muddy mess. Iapetus was also true two-color; one hemisphere represents 10-20 times more light than the other. The situation was even more severe than imagined himself Cassini because the distinction between light and dark hemispheres do not perfectly match the orbit of Iapetus.
What led to an even greater enigma: why Iapetus looks like this?
You see, the Iapetus is in orbit two times more than any other major satellite of Saturn. For dark debris that has accumulated on the front side, "bugs on the windshield," would be very strange explanation, since the satellite is far from the other major players in the Saturn system, including the planet's rings. No other moon of the planet does not have such features.
A little further on Iapetus is Phoebus, a small moon, which is likely a planet captured from the Kuiper Belt. In contrast to all other outside of Saturn's moons, Phoebe rotates in the opposite direction, there is a very far and very, very dark moon that is important. She darker all the other large moons orbiting Saturn, and is comparable to the dark part of Iapetus on this parameter. In addition Phœbus radiated constant stream of particles for a long time, since the sun radiation and tiny clashes were strong enough to blow off the small dust mote of the outer surface of the satellite.
Through infrared observatories like the Spitzer Space Telescope, we discovered something interesting about Phoebe: moon formed around Saturn's rings. It is more diffuse and less dense than the other rings are not currently open. The ring is so scattered - seven dust particles per cubic kilometer - and so extended that even a distant Iapetus passes through it on its orbit. Phoebe and her ring rotate clockwise around Saturn, Iapetus but is counter-clockwise, so we get the same effect of "bugs on the windshield."
Over time these dark particles accumulated on one side of Iapetus, but not on the other. But this is only the beginning of history. If only this happened, "bright stuff" on Iapetus (ice) would simply covered with dark material Phoebe in the short term. Cumulatively, the dark material would be provided under the layer of ice, and Iapetus would have become entirely white.
But the same physics that leads to the fact that a black car heats up on sun faster and stronger than the white car in the same conditions, and plays on Iapetus. When water is trying to condense, freeze and settle on the light areas of Iapetus, it does not interfere. But if the ice lands on the dark side, heat the surface sufficiently to sublimate it (i.e., from the solid phase to boil) and send to the other side.
So our system has a two-tone world, like a yin-yang symbol. 300 years later, the mystery was solved. Unusual moon painted in two colors, thanks to a comet that Saturn has captured a long time ago. Hundreds of millions of years of debris and dust hid the moon another, changing its color. It remains to solve the problems of the ridge and the slope, but who knows how many more mysteries await us beyond Earth.