In many ways, Jupiter is like a kind of solar system, within the Solar System. That is to say, the natural satellites, around the massive planet, are worlds unto themselves. The Jovian system consists of Jupiter, orbited by dozens of other satellites, including Io, Europa, Ganymede, Callisto, Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and Sinope, to name but a few. Of course, the largest of these are the classic Galilean moons: Io, Europa, Ganymede, and Callisto. There is even a ring system around Jupiter, similar to Saturn, with four faint gossamer girdles: the halo ring, the main ring, the Amalthea ring, and the Thebe ring.
It all began almost 5 billion years ago when a nearby star in the Milky Way Galaxy exploded. This event caused a vast cloud of widely dispersed material to collapse, and then flatten into a spinning disk. Most of that debris went on to coalesce into a hot, dense core that became the Sun. After that, the remaining bits of loose matter came together to form the other astronomical objects in the Solar System. The majority of the gas in that overall bulk of the material was brought in on itself to produce Jupiter, long before that of Mercury, Venus, Earth, Mars, Saturn, Uranus, and Neptune were even seeded. That is to say that, hundreds of millions of miles from the center of the Solar System, the fifth planet from the Sun actually formed first.
Unlike the rocky planets, Mercury, Venus, Earth and Mars, Jupiter formed from the outside in. This is because the great gas giant is composed of 86% hydrogen and 13% helium, so it came together as a result of external pressure pushing in, not internal accumulation building up. Of course, Jupiter does contain a number of other heavier elements, like argon, nitrogen, and carbon, for instance. Many of these substances condensed at extremely low temperatures, far away from the Sun, much further out than that of Jupiter. Different chemical constituents were then brought to the great gas giant, on comets and asteroids. At the same time, the cumulative effects of the comets in the Oort cloud pulled on Neptune, Uranus, and Saturn, moving them away from the Sun. Meanwhile, Jupiter migrated inward, closer to the center of the Solar System.
At the very heart of the Jovian system is Jupiter’s molten metallic hydrogen core, which smoothly transitions out into a liquid and then a gaseous state. Since it is a gas planet, Jupiter does not rotate as a solid sphere. Moreover, the equator rotates a bit faster than its polar regions, at a speed in excess of 28,000 miles an hour. So, as the planet spins, this generates a magnetosphere, which creates the largest object in the Solar System. The churning hydrogen core creates an electromagnetic energy field around the planet, that stretches out 100 times further than the edge of the atmosphere, holding in charged particles as it does. The fact of the matter is that Jupiter is absolutely enormous. The great gas giant is about 11 times wider than the Earth, and 300 times more massive. As part of this, the planet’s atmosphere is hundreds of miles thick.
The weather on Jupiter is a maelstrom of lightning storms within swirling clouds, which can produce hurricane-type vortices. Since Jupiter is the fastest spinning planet in the solar system, moving 22 times faster than the Earth, it makes for very stable storms, leading to large-scale structures. Of these, the most famous is that of the Great Red Spot. This is a massive storm with 300 mile an hour winds, that have been raging on for centuries. The clouds in this bizarre phenomenon rise up more than 5 miles above the atmosphere, and they plunge deep into the bottomless depths of a surfaceless planet. In this way, the Great Red Spot serves as the largest storm is the Solar System. As part of this, furious jet streams blow alternating bands of clouds, in opposite directions, giving rise to the familiar stripes on the visible exterior. Hail storms, the size of your head, even fall from the endless sky. Plus, there are super bolts of lightning, a thousand times more powerful than those on Earth. These occur in immense thunderstorms that appear as white spots.
Moving out from Jupiter, Io is the closest moon to the planet. It is also the most volcanic world in the entire Solar System. On it, huge plumes of sulfur dioxide erupt, hundreds of miles, out into space. Meanwhile, the surface is covered in different colored chemicals, making it yellow, red, black and more. Lava lakes constantly boil and bubble. This continuously reshapes the surface of that world, so huge craters come and go. The heat, that drives all of it, is the result of stretching and squeezing, which happens from the pull of Jupiter and its other moons. As the surface rises and sinks, hundreds of feet at a time, the friction melts the rock within, thus producing volcanism. The surface of the Galilean moon also contains more than 100 mountains, which have been uplifted by extensive compression at the base of Io’s silicate crust. Plus, with over 400 active volcanoes, Io serves as the most geologically active object in the entire Solar System.
Further out, Europa is made, primarily, of silicate rock. As part of this, it has an iron-nickel core. There is even an oxygen atmosphere. This particular Galilean moon is an ice world, but it’s not frozen solid. Instead, there is a global ocean that covers Europa, to a depth of about 100 miles. This is the largest body of liquid water in the Solar System, but it’s covered by a thick ice crust. So, the surface temperature of the tundra is -135 degrees Fahrenheit. Europa also has the smoothest surface of any known solid object in the Solar System, although it is striated by streaks. These ridges crisscross the barren landscape because Europa is distorted by the gravity of the planet and other moons in the Jovian system, which cracks the surface. There are also geysers that spray out salt water, with various different minerals in it. Of course, the most interesting thing is that the internal heat drives hydrothermal vents, around which aquatic extraterrestrial organisms may live.
Next, out from this, Ganymede is the largest moon of Jupiter. It has its own magnetic field, as the result of a molten core. This is created by convection, within its liquid iron center. There is also a subsurface ocean, deep below the rocky crust of that world too. Ganymede is composed of roughly equal amounts of silicate rock and water ice. The surface of that distant world is composed of two main types of terrain. The darker regions are older and covered with craters, while the lighter regions are covered in grooves and ridges. This is the result of tectonic activity, due to tidal heating. Ganymede even has a thin oxygen atmosphere, with an ozone layer. The biggest Galilean moon also participates in orbital resonances with Europa and Io. So, for every orbit of Ganymede, Europa orbits twice and Io orbits four times.
Finally, Callisto is home to the oldest, most heavily cratered, terrain in the Solar System. The cold moon is covered with ancient meteorite impacts. The surface temperature is a chilling -274 degrees Fahrenheit. There is rock, ice, and metal throughout the distant Jovian satellite, which has the lowest density and surface gravity of the major moons. This is because, unlike most other moons and planets, Callisto never got hot. The strange satellite is not in an orbital resonance, like the other Galilean moons, therefore it is not appreciably tidally heated. The satellite’s rotation is even tidally locked to its orbit, so the same hemisphere always faces inward. Callisto is also less affected by Jupiter’s magnetosphere than the other inner moons. Its remote orbit is located just outside Jupiter’s main radiation belt. As a result, Callisto is the most suitable place for a base, in which the astronauts of the future can explore the Jovian system.
Over the centuries and millennia to come, Jupiter will continue to protect life on Earth, as well as other worlds, just as it has for eons. Every few decades, the great gas giant captures comets and asteroids that might have otherwise struck the Earth. The interception of near-Earth objects, that pose a potential hazard to this planet, is vital to maintaining life on our home world. This process was even seen in action with Shoemaker-Levy 9, back in 1994. Along with this, as the Sun grows larger, over millions and billions of years to come, life on the Jovian moons, particularly Europa, will evolve in strange new ways. After all, the frozen worldwide ocean, on that moon, will eventually liquefy, once the Sun grows big and hot enough. The thing is that only time will tell what the fate of the Jovian system actually holds in store.