If all the planets were lined up, Jupiter would be easy to spot: it's the big one in the middle. With four planets on either side of it, Jupiter is indeed in the middle. But even without lining up the planets, Jupiter is easy to find because it is the largest of the planets.
In fact, Jupiter is huge. Its mass is more than double all the other planets' masses put together. If it was hollow, more than 1000 Earth's could fit inside. It is nearly 143,000 km around its equator.
Jupiter is also easy to spot because it actually gives off light. It is more like a Sun than a planet. Jupiter actually radiates more energy into space than it receives from the Sun. Its atmosphere is very deep and is composed mainly of hydrogen and helium. The hydrogen is under such great pressure that it becomes metallic. From Earth, Jupiter is only dimmer than the Sun, Moon and Venus (and sometimes Mars). It is in a great spot for observation for several months each year and the dark belts on its surface and its Great Red Spot are always visible.
But Jupiter's weather is anything but stable. The Great Red Spot is actually a complex storm moving in a counter-clockwise direction (like our storms) on its outside and more randomly closer to the planet.. Jupiter's colorful latitudinal bands also change with the weather.
This heat is the best explanation for what causes the complex motion that we observe in the clouds. Because of this heat, combined with the fact that Jupiter is so massive, some scientists speculate whether Jupiter was starting to develop into a star but didn't finish the process. If Jupiter had become a star, there would be too much heat pounding down on the Earth for life to survive because we would then have two Suns, one on each side of the Earth.
Jupiter has about the biggest diameter possible for a gas planet. Basically, it's a big ball of hydrogen: in the form of gas on the outside, liquid hydrogen in the middle, and at its core it is hydrogen that is under so much pressure that it has become metallic If there was more material there, it would be compressed by gravity and the diameter would barely increase. A star can be larger that this because of its internal heat source. For Jupiter to be a star, it would have to be at least eighty-ninety times more massive. It is interesting to compare Jupiter and Earth (right). Jupiter could be filled with 1,318 Earth's and still have room left over. Jupiter rotates about two and half times per day and the incline of its axis (left) is almost straight up and down (2-3° off the perpendicular). Thus, even though Jupiter has a long year (4333 days go travel around the sun), there are no seasons on Jupiter.
Jupiter's vivid colors change over time as its atmospheric clouds change. These clouds move in layers around the planet, mix together, and leave openings for us to see the inside layers. The layers closest to the planet are blue, with browns and whites above, and reds at the highest altitudes (as in the Great Red Spot). Some of the colors may be due to chemical reactions occurring within Jupiter's atmosphere. Jupiter's Great Red Spot is basically a high pressure storm that stays in one area.
It is not surprising, given Jupiter's size, that its magnetic field is very strong. This field not only keeps Jupiter's atmosphere and 16 moons in place but traps high levels of energetic particles. These radiation belts are much stronger than those around Earth. When the Galileo space probe investigated Jupiter, it found a new belt of radiation of high energy helium. So vast is Jupiter's oval-shaped magnetic field/atmosphere that it extends 2-3 million toward the Sun and 400 million miles out toward Saturn.
One very interesting feature on Jupiter is the Great Red Spot. This "spot" (called the GRS) is an oval storm system that is larger than Earth and at least 300 years old--that's a long storm! The GRS shifts up and down (longitude) but stays at about the same latitude. Jupiter has other, smaller storm systems but no one has called them LRS--little red spots. :)
In modern times, the Pioneer 10 became the first spacecraft to visit Jupiter (1973). Pioneer 10 was later followed by Pioneer 11, Voyager 1 (see left), Voyager 2, and Ulysses. Right now, the Galileo spacecraft is orbiting Jupiter and is sending back our most recent information.
Perhaps because it is made of gas, Jupiter's layers of clouds form fascinating patterns. In addition to the Great Red Spot, there is the Large Brown Oval (left) and lots of shifts in cloud patterns and colors. There is still much controversy about the amount of water in Jupiter's atmosphere, the cause of atmospheric storms and how many layers of clouds are present.
Not surprisingly, Jupiter is also a windy place. The winds are probably the result of heat generated by Jupiter itself and form layers and blow in opposite directions. These bands of winds give Jupiter a striped look, a collection of light bands (zones) and dark bands (belts) of clouds.
* Distance is from Jupiter's center to the ring's inner edge.
The non-Galilean outer satellites are found in 2 clusters of 4 each. Leda, Himalia, Lysithea, and Elara orbit Jupiter at about 11 million km but Ananke, Carme, Pasiphae, and Sinope orbit at more than 20 million km from Jupiter. Three more inner satellites were also found: Metis, Adrastea, and Thebe. More probably exist, but will have to be found by space probes.
*Values for the smaller moons are approximate.
Since the Galileans are the biggest, they get most of the attention. All four are easily seen with a telescope, and a few people can even spot some without any instruments. These four also deserve extra attention because they were the first found and because they are so different from the others. Two of the satellites (Io and Europa) are dense, rocky moons that orbit closest to Jupiter. The other two (Ganymede and Callisto) are mostly water (ice) and are farther away from Jupiter.
Io is bright-red, yellowish-brown and white with black dots. It has its own (transient) atmosphere, has active volcanoes (see right), and is coated with sulfur. In 1979, Voyager 1 & 2 detected ten erupting volcanoes, and the Galileo spacecraft (in 1997) confirmed the volcanic activity. Europa is the smallest of the Galileans, and is quite different from Io. First of all, it is very smooth. There are no volcanoes and almost no craters visible. Perhaps they were covered with ice because the surface is white and most likely made of ice. There may be large amounts liquid water below Europa's crust.
Ganymede is the largest known planetary satellite (bigger than Mercury). But its density is low, so it likely has an icy crust with a mantle of water below and a large silicate core below that. The surface of Ganymede has dark craters and younger regions with long grooves. Callisto is also made of ice, low in density and is heavily cratered. It's the farthest from Jupiter (of the Galilean satellites), is the least reflective of the four and is hard to see from Earth. The surface temperature on Callisto varies from a high of -118° F to a low of -193°.
As you can see, all four Galilean satellites are very different and very interesting.