In 1671 Giovanni Cassini stared through a telescope at Saturn and discovered a series of incredible miracles: the famous gap in its rings, the cloud bands in the atmosphere and more satellites. The second discovered moon of Saturn — yapet — immediately showed its uniqueness: it was visible only half of its orbit. The other 50% of the time yapet was completely invisible, could not be detected in any way, even if it obeyed the normal laws of gravity. Thirty years later, improvements to the telescope, Cassini was finally able to find this moon in the West and East side, but on the Eastern side it was six times dimmer.
Cassini developed a theory about this moon, now known as yapet. He argued that first of all yapet needs to be two tone, with one side brighter and the other less bright. Second, it needs to be tidal locked with Saturn to meet its always the same side. Thanks to the combination of these factors, the “front front” of yapet will be dimmer and darker in the rear. The idea was interesting, but check it out there was no way.
This difference in coloration is not the only thing that makes yapet remarkable or even unique companion among all. All the main moons of Saturn are orbiting in the same plane as its rings. All except yapet, which is significantly tilted. No one knows why. No other large moon in the Solar system that formed with the parent planet, has no such inclination, as of yapet.
Even the yapet is a giant ridge along the equator: about 10 km higher than the rest of the solid ice of the world. But the satellite rotates fast enough to explain the origin of the ridge and the surface of yapet many billions of years, so it’s definitely not the accumulated garbage. None of the theories explaining the appearance of this ridge, has advantages and advantage. Yapet is very unusual for Solar system satellite, and we figured not too many of his mysteries.
And yet one of the mysteries of yapet we solved 300 years after its occurrence. Thanks to the Cassini mission of NASA, and not the Italian scientist, we got to yapet and photographed. It turned out that one of its sides looks like it’s been plowed in a dirty mess. Yapet was really two-tone, one hemisphere reflects 10-20 times more light than the other. The situation was even more severe than Cassini did himself, because the distinction between the light and dark hemispheres are not perfectly consistent with the orbit of yapet.
That led to an even greater mystery: why yapet looks like this?
You see, yapet is in orbit twice as far, than any other major moon of Saturn. For dark debris that has accumulated on the front side, the “bugs on a windshield” would be a very strange explanation, since the satellite is far from the other major players in the system of Saturn, including the rings of the planet. No other moon of the planet does not have such features.
A little further of yapet is Phoebe, a small moon, which is likely the planet was captured from the Kuiper belt. Unlike all the other outer satellites of Saturn, Phoebe revolves in the opposite direction, away very far and very, very dark moon that’s important. She is darker than all the other large moons orbiting Saturn, and is comparable to the dark part of yapet on this parameter. In addition, the Phoebus radiated a steady stream of particles for a long time, since the radiation of the Sun and a tiny collision was strong enough to speck for speck of dust to blow off the outer surface of the satellite.
Thanks to infrared space observatories like the Spitzer telescope, we discovered something interesting about Phoebe: the satellite is formed around Saturn own ring. It is more diffusae and less dense than other rings, not open the moment. The ring is so scattered seven dust particles per cubic kilometer — and so extended that even a distant yapet passes through it on its orbit. Phoebus and her ring rotate clockwise around Saturn, but yapet counter-clockwise, and thus we get the same effect “bugs on windshield”.
Over time, these dark particles accumulated on one side of yapet, but not on the other. But this is only the beginning of the story. If it was only that, “the bright stuff” for the yapet (ice) simply would be covered with a dark material of Phoebus in a short time. The accumulating dark material was in a layer of ice, and yapet would be entirely white.
But the same physics that leads to that black car heats up in the Sun faster and stronger than a white car in the same conditions, plays and yapet. When the water is trying to condense, freeze and settle on bright areas of yapet, she did not interfere. But if the ice lands on the dark side, heat the surface enough to sublimate it (ie boil from the solid phase) and send it to the other side.
Therefore, in our two-tone peace, like the symbol of Yin and Yang. 300 years later the mystery was solved. Unusual moon painted in two colors, thanks to the comet which Saturn was captured a long time ago. Hundreds of millions of years of debris and dust hid the other the moon, changing its colour. It remains to solve the questions of the ridge and slope, but who knows how many more mysteries await us beyond Earth.