# Astronomy college course/Jupiter

Material taken from Jupiter and Atmosphere of Jupiter
Jupiter is the fifth planet from the Sun and the largest planet in the Solar System (but not the largest known planet if exoplanets are included). Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian or outer planets. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, although helium only comprises about a tenth of the number of molecules. It may also have a rocky core of heavier elements, but like the other gas giants, Jupiter lacks a well-defined solid surface. Because of its rapid rotation, the planet's shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around its equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a powerful magnetosphere. There are also at least 67 moons, including the four large moons called the Galilean moons.
Jupiter has been explored on several occasions by robotic spacecraft, first by Pioneer in 1973, and most recently by New Horizons in 2007.

## Composition

Jupiter's upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume or fraction of gas molecules. Since a helium atom has about four times as much mass as a hydrogen atom, the composition changes when described as the proportion of mass contributed by different atoms. Thus, the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. The outermost layer of the atmosphere contains crystals of frozen ammonia.
The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, which is about one tenth as abundant as in the Sun. Helium is also depleted, although only to about 80% of the Sun's helium composition. This depletion may be a result of precipitation of these elements into the interior of the planet.

## Mass

Jupiter's mass is 2.5 times that of all other planets in the Solar System combined—and so massive that its barycenter with the Sun lies above the Sun's surface. Although Jupiter dwarfs Earth with a diameter 11 times as great, it is considerably less dense. Jupiter's volume is that of about 1,321 Earth's, yet the planet is only 318 times it's mass.
Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter. Jupiter does radiate more heat than it receives from the Sun. This additional heat radiation is generated by the Kelvin–Helmholtz mechanism through contraction. This process results in the planet shrinking by about 2 cm each year. When it was first formed, Jupiter was much hotter and was about twice its current diameter.

## Internal Structure

Wikipedia claims that there is "considerable uncertainty" regarding the internal structure of Jupiter. From the center outward, we have:
* A dense core with a mixture of elements that has been described as "rocky". It is not certain that this core exists, but if it does, calculations indicate that it might be dozens of times more massive than Earth.
* A surrounding layer of (mostly) hydrogen so compressed that it conducts electricity, and is for that reason called liquid metallic hydrogen. This layer is believed to extends outward to about 78 percent of 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. The friction associated with this "rain" creates so much heat that Jupiter emits more energy than it receives from the Sun. It also creates a situation where the center of Jupiter is hot at the center.
* An outer layer that is not really a gas or a liquid because at these pressures, liquid becomes compressible and there is no phase transition between liquid and solid states. While there is no specific radius at which the matter becomes a gas, it definitely is a gas at the outer layers, and for that reason Jupiter is appropriately called a gas planet.

## Atmosphere

Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. The clouds are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence.
The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic that is approximately 2.5 times larger than Earth. It rotates with a period of about six days, and is known to have existed since at least the 17th century when it was first seen by telescope. Mathematical models suggest that the storm is stable and may be a permanent feature of the planet.
Neptune has a similar such storm. Such storms are common within the turbulent atmospheres of gas giants. Smaller versions of these storms can last as little as a few hours. The storms are driven by the fact that two counter-prevailing streams of a fluid are unstable. You see this if you paddle a canoe in calm water and note the vortexes coming off the edges of the paddle.
   * See computer simulation of this "Kelvin Helmholtz" instability