In 1671, Giovanni Cassini looked through a telescope at Saturn and discovered a number of incredible wonders: the famous gap in its rings, cloud stripes in the atmosphere and several satellites. The second discovered moon of Saturn - Iapetus - immediately demonstrated its uniqueness: only half of its orbit was visible. The other 50% of the time Iapetus was completely invisible, it could not be detected in any way, although otherwise it obeyed the usual laws of gravity. After thirty years of improvements to the telescope, Cassini was finally able to find this moon on the west and east sides, but it was six times fainter on the east side.
Cassini developed a theory about this moon, now known as Iapetus. He argued that first of all Iapetus should be two-color, with one side brighter and the other less bright. Second, it must be tidally blocked with Saturn in order to always meet it on the same side. Due to the combination of these factors, Iapetus's "front line" will be dimmer and darker than his rear. The idea was interesting, but there was no way to test it.
This color difference is not the only thing that makes Iapetus a remarkable or even unique companion among all. All of Saturn's major moons orbit in the same plane as its rings. Everyone except Iapetus, who is significantly inclined. Nobody knows why. No other large moon in the solar system that formed with its parent planet has the same tilt as Iapetus.
Iapetus also has a giant ridge along the equator: about 10 kilometers higher than the rest of the solid ice world. But the moon is not rotating fast enough to explain the origin of the ridge, and the surface of Iapetus is many billions of years old, so this is definitely not accumulated debris. None of the theories explaining the emergence of this ridge has advantages and advantages. Iapetus is a very unusual satellite for the solar system, and we have solved not so many of its mysteries.
And yet we have solved one of the mysteries of Iapetus 300 years after its appearance. Thanks to Cassini - a NASA mission, not an Italian scientist - we got to Iapetus and photographed it. It turned out that one of its sides looks like it was plowed into a dirty mess. Iapetus turned out to be really two-colored; one hemisphere reflects 10-20 times more light than the other. The situation turned out to be even more severe than Cassini himself imagined, since the demarcation between the light and dark hemispheres did not perfectly match the orbit of Iapetus.
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Which led to an even bigger mystery: why does Iapetus look like this?
You see, Iapetus is in orbit twice as far as any other large satellite of Saturn. For the dark debris accumulating on the front, "bugs on the windshield" would be a very strange explanation, since the moon is far away from other major players in the Saturn system, including the planet's rings. No other moon on the planet has this feature.
A little further than Iapetus is Phoebus, a small moon most likely captured by the planet from the Kuiper belt. Unlike all the other outer satellites of Saturn, Phoebus rotates in the opposite direction, is very far away and a very, very dark moon, which is important. It is darker than all other large moons orbiting Saturn, and is comparable to the dark part of Iapetus in this parameter. In addition to this, Phoebus emitted a constant stream of particles for a long time, as the radiation from the Sun and tiny collisions were strong enough to blow off the outer surface of the satellite, speck by speck.
Thanks to infrared observatories like the Spitzer Space Telescope, we discovered something interesting about Phoebe: the moon formed its own ring around Saturn. It is larger, diffuse, and less dense than other rings that are not currently open. The ring is so scattered - seven grains of dust per cubic kilometer - and so long that even distant Iapetus passes through it in its orbit. Phoebus and her ring rotate clockwise around Saturn, but Iapetus goes counterclockwise, which means we get the same effect of "beetles 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 the story. If only this were happening, the "bright matter" on Iapetus (ice) would simply cover the dark material of Phoebus in a short time. Accumulating, the dark material would be under a layer of ice, and Iapetus would become entirely white.
But the same physics that leads to the fact that the black car heats up on the Sun faster and stronger than the white car under the same conditions plays on Iapetus. When water tries to condense, freeze and settle on the bright areas of Iapetus, nothing prevents it. But if the ice lands on the dark side, there is enough surface heat to sublimate it (i.e., boil it out of the solid) and send it to the other side.
Therefore, a two-color world appeared in our system, like the Yin-Yang symbol. After 300 years, this mystery was solved. The unusual moon was painted in two colors thanks to a comet that Saturn captured a long time ago. For hundreds of millions of years, its debris and dust covered another moon, changing its color. It remains to solve the issues of the ridge and slope, but who knows how many more mysteries await us outside the Earth.
ILYA KHEL