Earth's atmosphere is a relatively minor feature of our planet. It hugs the surface and thins out to almost nothing at an altitude of about 100 kilometers. By contrast, Jupiter’s atmosphere is tens of thousands of kilometers thick, and there is no solid surface to stand on. You might say that Jupiter is all atmosphere.

What would you encounter if you descended into Jupiter? The air pressure near the cloud tops is roughly the same as at Earth’s surface. But it increases dramatically as you drop, and so does the temperature. Assuming you could survive the trip, you would fall through thickening air, crushing pressure, and searing heat. The mostly hydrogen gas would gradually become as thick as pudding. Deep inside the planet, the tremendous pressure creates a form of hydrogen that conducts electricity like a metal. Still lower, you reach a solid core larger than Earth.

When you put your eye to a telescope and gaze at Jupiter, you see it in visible light. Other wavelengths beyond visible reveal additional information about the planet. Astronomers use cameras with special detectors to record this information. They assign “false colors” to different wavelengths in these images to highlight different details.

How many Earths can fit inside Jupiter’s Great Red Spot?

Jupiter's many moons range in size from rocky bodies only a few kilometers across to one that is larger than Mercury. New moons are being found all the time. The four largest, in order of distance from Jupiter, are Io, Europa, Ganymede, and Callisto – known as the Galilean satellites because of their discovery by the astronomer Galileo in 1610. From 1995 to 2002, a spacecraft named Galileo made some of our most spectacular pictures of these worlds.

Why is Jupiter’s moon, Io, the most volcanically active world in the Solar System? Io’s core should have cooled off long ago. But this little moon is kept hot by a combination of gravitational influenced from Jupiter and two of Jupiter’s other large satellites.

Here's how it works: Over time, Io, Europa, and Ganymede have developed an orbital relationship called a resonance, which periodically pulls Io farther away from Jupiter. Then Jupiter pulls Io back. The result is that Jupiter’s powerful gravity causes tidal effects on Io that are sometimes weaker, sometimes stronger. Something similar happens when you flex and unflex a piece of plastic: very soon it heats up! On Io, the “flex” heating is enough to melt a portion of the interior and cause constant volcanic eruptions.

With the unaided eye, Jupiter looks like a bright star. The first person to study it with a telescope was Galileo in 1610. Half a century later, bigger telescopes showed cloud bands and the Great Red Spot. The first spacecraft to visit Jupiter flew by in 1973. Since then, more probes have inspected the planet up close. In recent years, telescopes based on Earth and in space have also viewed Jupiter in remarkably sharp detail.