Jupiter, the largest planet, has a diameter of 88,846 miles and can easily be considered a jumbo planet with a volume capable of holding all of the other planets with room to spare. Everything about Jupiter is jumbo-size including storms large enough to swallow Earth, lightning that can span a continent, towering thunderclouds hundreds of miles high, and storms that rage for centuries.

Jupiter is completely covered with clouds, but unlike Venus there is no solid surface. Instead, the atmosphere compresses further down making the hydrogen gas behave like a liquid and eventually behave like metal. In this metallic state, along with the rapid rotation of 9 hours 50 minutes, it generates a powerful magnetic field that can instantly kill anyone and disable spacecraft. The rapid rotation also stretches the clouds into dark belts and bright bands. Jupiter’s colorful clouds are due to the interactions of hydrogen, helium, ammonia, methane, sulfur, phosphorus, arsenic, and other compounds making up the atmosphere. The tops of the highest clouds are around -250ºF and frigid because Jupiter averages 500 million miles from the Sun. About 60 miles below the cloud tops it is around 105ºF, and at 600 miles down it is already 4000ºF. The increasing heat and pressure further down generates storms and can fuel them for centuries since there is no solid surface to create friction and tear the storms apart. The white clouds are the highest and coldest, and composed of fine ammonia ice crystals. The yellow and brown clouds are further down and warmer, and composed of ammonium hydrosulfide. The bluish clouds are the lowest and composed mainly of water. The brighter belts, more correctly called zones, are areas of rising air that cools and condenses to form ammonia ice clouds. As the air cools, it eventually sinks and dries out to clear the air and reveals the region of lower, warmer, and darker clouds below that appear as the brownish belts. Jet stream winds over 400 miles per hour blow in opposite directions causing swirls and eddies in the clouds. Jupiter has a rocky core probably no larger than Earth and the pressure is so great that the core is still gradually collapsing, giving off huge amounts of heat which rises and powers the storms above.

The Great Red Spot is the largest storm in the Solar System, a little larger than Earth and has a hurricane-like circulation rising from deep within Jupiter that revolves in six days. Radiation from the Sun reacts with the phosphorus dredged up from within the Great Red Spot to create phosphine, a reddish compound. The Great Red Spot is like a huge bump in the atmosphere and its clouds are the highest and coldest on Jupiter. The strong jet streams keep it spinning and the heat from below feeds it along with several smaller storms and spots that venture close to it. This, and the lack of a solid surface to disrupt it, has kept the Great Red Spot going for centuries and probably long before the invention of the telescope.

Jupiter has a dusty sheet of rings and 79 moons, most of which are probably captured asteroids, but four of them are large enough to be respectable worlds if they orbited the Sun on their own. They are Io, Europa, Ganymede, and Callisto, also known as the Galilean moons after their discoverer, Galileo Galilei. Io is the most bizarre moon of all which, at 2250 miles in diameter, is slightly larger than Earth’s Moon. It is the most volcanically active world in the Solar System. Flying by it in March 1979, Voyager 1 discovered at least eight active volcanoes that stain it in a riot of colors including red, orange, yellow, white, brown, and black, making it look like a poorly made pizza. The volcanoes are huge with plumes up to 300 miles high and searing hot lava flows of molten sulfur at 3000ºF, twice as hot as Earth’s lava! It is the sulfur in various states of solidifying that gives Io its colors. The powerful tidal flexing of Io by Jupiter and the other three Galilean moons create surface tides as high as 300 feet and heats the interior to incredibly high temperatures. Volcanic activity is so intense that Io may have turned itself inside out several times in its lifetime! Io is a beautiful but deadly world with towering volcanic plumes, fiery geysers, glowing lava flows, fire pits and lava lakes, fissures, mesas, and mountains all laced in a deadly radiation belt, which it helps to power for Jupiter.

Europa is slightly smaller than Earth’s Moon at 1950 miles in diameter and a world so fascinating that plans are in the works by NASA to explore it during the next decade and possibly land on it. It is unusually smooth with the highest hill hardly 300 feet high and bright with cracks everywhere. This led to the speculation that a vast global ocean may lie beneath a thin, icy crust. There is evidence of upheaval in the crust in areas where there may be an upwelling of warmer water. The darker patches on the surface may be evaporate deposits that percolated through the cracks in the crust, erupted onto the surface, and evaporated leaving a salty residue. There may also be weak geysers erupting as the Hubble Space Telescope detected possible faint, briefly erupting plumes of water ice in the southern hemisphere. Europa also undergoes tidal flexing that heats and maintains the underground ocean.

Ganymede is the largest moon in the Solar System with a diameter of 3267 miles; it is larger than Mercury. Being a rock and ice mixture, it looks like a large marble of light and dark contrasting features. Denser, dark rock covers the surface in large, straight-edged blocks broken by brighter icy flows that squeezed up beneath the crust through long fissures. The darker surface is the oldest, while most of the brighter ice may have been vaporized by countless meteorite collisions. Ganymede also has whitish polar ice caps of thin water frost deposits. These may have formed when fissures released water vapor that was then carried by Ganymede’s weak magnetic field towards the poles where it condensed. The fissures, plus radiation from the Sun interacting with the surface ice, may also be responsible for the thin oxygen atmosphere discovered around Ganymede. The craters have dark and light rays that indicate the crust is layered like sedimentary rock. The Galileo spacecraft detected the possible existence of a shallow ocean deep beneath the crust, deeper than Europa’s. Ganymede is so fascinating that it is the destination of an ambitious mission next decade by the European Space Agency (ESA).

Callisto is 2980 miles in diameter and it is completely covered with craters of various sizes and textures. There are so many craters that new ones can only form by destroying old ones. A giant impact basin, Valhalla, is nearly 1000 miles across and is multi-ringed with concentric rings of bright ridges. It has no relief which indicates the ice softened from the heat generated by the impact that then flowed and filled in the crater. All large craters have evidence of softening and flow fills while smaller craters are distinct with no fill, indicating that insufficient heat was generated to melt the subsurface ice. There should be a vast area of chaotic terrain on the opposite side of the Valhalla basin where the shock waves would converge, however nothing unusual was seen. This could only mean that something inside Callisto absorbed the shock waves and the only material that could do this would be a liquid, most likely a vast subsurface ocean. This shallow ocean is theorized to lie very deep within Callisto, even deeper than Ganymede’s.

Jupiter and its moons are dynamic and fascinating, plus they are close enough to Earth that spacecraft can reach them in reasonable time, usually from two to six years. The next decade will be exciting with two new missions planned. Juno will continue to orbit Jupiter around its poles, studying its deep atmosphere and magnetic field, until at least 2021 when radiation will eventually degrade it and cause it to take a fiery plunge into Jupiter. NASA is planning to launch Europa Clipper in 2022 or 2023, arriving at Jupiter by 2030 and orbiting it at least 45 times, flying as close as 15 miles above Europa to study its surface, underground ocean, and any possible geysers using a suite of highly sophisticated instruments. A separate lander mission may follow a few years later and could land in a suspected warm area of possible geyser activity near the bright crater Pwyll. ESA is planning to launch JUICE in June 2022 with the spacecraft arriving at Jupiter in 2030 to orbit it several times and study Europa, Ganymede, and Callisto. The main goal is for JUICE to go into orbit around Ganymede in 2032 and study it at extremely close range until at least 2034. These are exciting times for exploring Jupiter and its moons, and who knows what new discoveries lie ahead. A new era in the exploration of Jupiter in the late 2020’s is a fitting golden jubilee for the beginning of its intensive exploration with the Voyagers in 1979. The more we explore, the more we discover, the more we are surprised. These are truly going to be great times ahead to celebrate Jupiter!