Ganymede is the largest of the Galilean moons and the second largest moon in the solar system. In size, it is located between Mercury and Mars. Its diameter was determined in the occultation of the star with an accuracy of 10 - 20 km, the average density, equal to 1.95 g/cm3, is known with an accuracy of several percent. The main uncertainty in this quantity arises from the fact that a significant atmosphere can exist on this satellite. If the pressure at the surface is greater than 10-3 mbar, then the diameter determined during coating will refer to some level in the atmosphere, and not to a solid surface, and the density given above will be the lower limit of the density of the solid. However, in theory the existence of such an atmosphere is doubtful. But we will have to consider the above figures as the diameter, but at the same time we will keep in mind that the true diameter may be somewhat smaller.
It, like Calisto, consists of water, but its bowels are completely differentiated. This may be due to its greater proximity to Jupiter and a slightly higher abundance of radioactive elements, which is possible due to its higher density. Models of the internal structure of this satellite assume a rocky core surrounded by a liquid water mantle with a thin (100–200 km) ice crust. It is believed that about 15% of the mass is contained in the core, which consists mainly of silicates and metal oxides, so we can assume that only a small amount of CH and NH3 is dissolved in water.
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