The strangest moon in the solar system.  The mystery has been solved after more than 300 years

When the topic of Saturn’s moons appears, Titan or Dione usually comes to mind. The name Iapetus appears less frequently, despite the fact that it is one of the greatest satellites on the planet. Moreover, it is also a very special object that has no counterpart in the solar system. Iapetus was discovered by Giovanni Cassini in 1671 Already as the second satellite of Saturn. He immediately caught the astronomer’s attention due to his very strange behavior.

When Cassini spotted Iapetus, it was the only one visible for fifty percent of its orbit on Saturn’s western side. Another 50 percent. (on the eastern side of the planet) It was nowhere to be found. At first, the scientist did not understand what the result of this might be. Only by upgrading his telescope was he able to see Iapetus also on the eastern side of Saturn. Here, however, again surprised, because The brightness of the object was two times less than when it was observed on the western side of the planet. Cassini quickly concluded that the moon must be bicolored. He also stated that the Moon must be tidal with Saturn with the same side always facing it. The question was why was that so. Cassini was unable to test his theories due to technical limitations.

Photo: NASA/JPL-Caltech/Space Science Institute

The dark and light side of Iapetus

To find out, one has to take a broader look at Iapetus. It is the farthest moon of Saturn, and is located farthest from all the other moons. What’s more, it’s also at exactly the exact location of the satellite. If we look at the rings of Saturn, we can see that they rotate in the same plane. The same is true of seven of the eight moons, each orbiting at a distance of 1.6 degrees from the plane, the minor exception to this rule being Mimas, which has an inclination of more than 0.5 degrees.

Iapetus is a real sensation in this case, because surprisingly it leans so strongly in relation to the rest of the system. Its inclination with respect to the plane is 15.5 degrees and so far it is not known what it produces. There are many different theories about this, some pointing to gravitational interception, others pointing to a cosmic collision. However, it’s hard to say exactly what made Iapet’s orbit look like this. It should be noted, however, that no other moon formed with a planet has such characteristics, which may be a hint.

Another curiosity about this moon is its distinctive hills. Iapetus, of course, is not a perfect sphere, nor the Earth, nor the Moon. Our planet is slightly compressed from the poles, which makes the equator a little bulging. This is due to the balance between gravity and momentum, also known as hydrostatic balance. As you probably guessed, the whole thing in Japan looks very different. The diameter of the equator is 1,492 km, and the diameter between the poles is 1,424 km. Knowing these data, it can be calculated that the already mentioned equilibrium would be reached if Iapetus made a full revolution every 16 hours. The problem is every 79 days. This only confirms his close tidal association with Saturn. The hills themselves are 20 km wide and 13 km high. It also appears to run almost perfectly along the equator. No known object has a similar property.

Characteristic hills on Iapetus

Photo: NASA/JPL-Caltech/Space Science Institute/Moon and Planetary Institute

Characteristic hills on Iapetus

Let’s finally move on to the most interesting characteristics of Iapetus. Cassini was right when he said that The moon is bicolorBut figuring out why was not easy. Iapetus is an ice giant, so such color differences don’t seem to make sense. For 300 years, scientists have not been able to determine the reason for such a peculiarity of the appearance of the moon. In the end, it turns out that the dark side of Iapetus turns dark in exactly the same way a car window gets dirty after being run over by a large group of insects. In this case, only the windshield is dirty, while the rear windshield, which does not come into contact with mosquitoes and midges, looks exactly the same as before driving into the swarm.

The question is, what is the cosmic equivalent of the swarm of insects polluting the dark side of Iapetus. There were many hypotheses, but in the end it was proved that it was a stream of particles from the asteroid Phoebe. One of the largest asteroids in our system orbits in the opposite direction from Saturn’s moon. The cloud he leaves, which is actually only visible with infrared rays, is present thanks to the influence of solar radiation and micro-effects. The resulting ring is very rare – only 6-7 dust grains per cubic kilometer.

Iapetus in reference to Phoebe and his dust ring

Photo: NASA/JPL-Caltech

Iapetus in reference to Phoebe and his dust ring

But most importantly, this ring is also huge and Iapetus pierces its orbit. The deposition of dust on a particular side is facilitated by the already mentioned tidal connection with Saturn. This, however, does not completely solve the issue. An explanation was needed as to why the ice did not cover the adsorbents from the ring. Here comes the issue of heat absorption, which darker materials are known to absorb better.

The bright side of Iapetus when exposed to sunlight retains much less heat than the side with particles, so the ice on the latter side sublimates a lot and accumulates on the “cooler” white side. In the “hottest hemisphere” only non-volatile heat-absorbing particles remain from Phoebe. Scientists say that over billions of years, Iapetus accumulated on the surface Only 30 cm of black dust. It is not much, but it is enough to “color” the entire hemisphere.

ibts surface map

Photo: NASA/JPL/Space Science Institute

ibts surface map

Although answers to many questions about this moon have been found, there are still many unresolved problems. The data collected by Cassini did not help – more information will be needed in future spaceflights. Astronomers are eagerly awaiting confirmation of the origin of Iapetus, for example. Although his intercept theory makes perfect sense, and it makes sense when looking at Triton – Neptune’s moon, no one is 100 percent capable. Confirm that it was.

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