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Why is there a single giant satellite on Saturn?


Saturn's number of satellites has been found to be 82. However, the rest of the satellites are much smaller than the largest one, the Titan. The reason why Saturn has only one giant satellite has remained a question so far.

On the 9th, the Japanese astronomy research team published the results of a simulation of the process of satellite formation on a gas planet with a new model in the journal Astronomy & Astrophysics. This study corrects the errors of several models developed so far, explains why giant satellites such as Titan have formed on Saturn, and evaluates that progress has been made in understanding the principles of the creation of solar system satellites.

Gas planets have many satellites. Of the 79 known satellites of Jupiter, Ganymede (diameter 5268 km) and Callisto (diameter 4820 km) are slightly larger or similar to Mercury (diameter 4880 km), while Io (diameter 3643 km) and Europa (diameter 3122 km) are the Earth's moon (diameter 3474 km). It is a giant satellite of similar size. These four satellites were discovered by Galileo Galilei in the early 17th century and named "Galileo's Satellite."

On the other hand, Saturn's satellites are relatively small except for Titan (5149 km in diameter), followed by Lea (1527 km in diameter) and Iapetus (1470 km in diameter). It is not enough to explain this just by the fact that Jupiter is three times heavier than Saturn.

Previous studies of the formation process of gas planets have shown the possibility of multiple giant satellites, such as Jupiter's Galileo satellite, appearing in the gas and dust disks surrounding the gas planet just before birth, but how a single giant satellite like Titan It was not known whether it was born.


Development of a new model that compensates for the defects of the existing model

Prof. Yuri Fujii and Masahiro Ogihara of Nagoya University, in collaboration with the National Observatory of Japan (NAOJ), created a new satellite system model that incorporates realistic temperature distributions based on the mass and density of dust and ice on various sizes of primitive planetary disks. Developed. The researchers then conducted a series of simulations with this model, taking into account the gas pressure of the disk and the gravitational effects of several satellites.

Simulations of the evolution of satellite systems over 100,000 years have confirmed that gas planets like Saturn can actually absorb satellites, but only within certain distances. Thus, the Titans, orbiting relatively far from Saturn, may have been absorbed by the planet and destined to be destroyed.


The researchers discovered that as the disk gas disk evolved, a "safe zone" could form that would prevent large satellites from falling into the planet. The scenario in which this occurs consists of four stages, the third of which only takes place in simulations of Professor Fujii and Professor Ogihara.

In stage 1, a gas disk of ice and dust condenses and rotates around the planet. In the second stage, the solid components of the disk gradually aggregate and grow as much as the size of the satellite. In the third stage, the orbits of these satellites gradually change due to the influence of the gas in the disc.

It is from this point that many moons orbit closer to the planet and eventually fall. Directly outside the danger zone is a thick buffer zone where the temperature drops dramatically. The warm high-pressure gas around the planet pushes all the satellites out there without pulling it toward the planet. That's why giant satellites orbiting the safe zone can keep their distance from the planet.

In the final stage, the gas on the disk is lost and the satellites that survive the safety zone remain in stable orbit.

"It is the first time that a satellite system with only one giant satellite can be formed around a giant planet," said Professor Fujii. "This is an important milestone in understanding the origin of Titan."

Difficult to prove with solar system satellites alone

The Japanese team's model of satellite system formation is limited to explaining both the Titan and other satellite systems in the solar system. This is because the solar system's satellites formed billions of years ago with the planet. However, it can be very useful for astronomers who are studying the extraterrestrial planetary system.

Professor Ogihara said, “It is difficult to investigate or prove that Titan has actually gone through a simulation-like process. Our scenario could be verified by satellite studies around other planetary systems. "If we find many satellites in the extraterrestrial planetary system, discussions about the results of this study will be active."

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