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Jupiter is the fifth planet in the Sol System and the first gas giant from Sol. It is also the largest planet in the system by both mass and radius, with both being much higher than the next largest planet and fellow gas giant Saturn. The planet bares the name of the Roman god of the sky and king of the gods. Jupiter is one of the five planets visible with the naked eye and the third brightest object in the night sky behind Luna and Venus. Because of this, Jupiter has been known to Humans for thousands of years. While there is no known first record of the observation of the planet, it is known that Jupiter was known of by the Babylonians during the 2nd millennium BCE.
Jupiter is a gas giant, and as such has no hard surface. Jupiter's upper atmosphere consists mainly of gaseous hydrogen and helium (89% H2 and 10% He), with other gases such as methane, ammonia, and water vapor making up the bulk of the remaining 1%. Ice particles of solid ammonia and water are also present. The hydrogen and helium are thought to be remnants of the solar nebula in which Jupiter was formed, while the other trace gases are thought to be seeded from supernovas early in the planet's formation. Deeper within the atmosphere, the gasses get denser and pressure begins to increase, with pressure reaching into the hundreds of atm.
Core & Surrounding Parts
Jupiter's dense core primarily consists of various ices and rock, with various other elements like Iron, nickel, and platinum. With Jupiter being the largest of the planets in the Sol System, it's core is also massive, being around 24 times the size of Earth (roughly 8% of the planet's mass).
Surrounding the core is an extremely thick layer of metallic hydrogen, which extends to about 73% the radius of the planet. Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere. Rainfalls of diamonds have been suggested to occur on Jupiter, as well as on the other outer planets. Above the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above hydrogen's critical pressure of 12.85 atm and critical temperature of -240 C. In this state, there are no distinct liquid and gas phases, as hydrogen is said to be in a supercritical fluid state. It is convenient to treat hydrogen as gas in the upper layer extending downward from the cloud layer to a depth of about 1,000 km, and as liquid in deeper layers. Physically, there is no clear boundary, the gas smoothly becomes hotter and denser as one descends.