Moons of Jupiter: Io

Jupiter has at least 16 moons. Io, Europa, Ganymede, and Callisto are the largest. They are called the Galilean moons after Galileo Galilei, who discovered them in 1610 using one of the first telescopes.

Io is the most famous moon of Jupiter because active volcanoes dot its surface. They erupt frequently, spewing yellow and red sulfur dust up to 180 miles (300 km) high. When the Galileo spacecraft first scanned Io in early 1996, it found that much of the moon÷s surface had been repaved by volcanic eruptions since the Voyager probes visited in the late 1970s.

Gravity drives Io's volcanoes. Io moves in an elliptical orbit around Jupiter. It moves fastest when it's closest to Jupiter, and slowest when its far away. But it spins on its axis at a constant speed, so rotation sometimes gains on orbital speed and sometimes drops behind. So as seen from Jupiter, Io shakes its head "no." Jupiter tries to stop the shaking with its mighty gravitational grip.

That makes rocks inside Io rub together and heat up, melting sulfur and other elements. Sulfur compounds jet from the volcanoes. Sometimes material blasted from the volcanoes escapes Io entirely. For example, sulfur from Io colors its neighboring moon Amalthea bright red.

Sulfur on Io assumes different colors depending on the specific chemical compound and on its temperature. Black, white, and even blue sulfur stains the surface. But mostly Io is orange and yellow.

The Voyager space probes observed up to nine major eruptions on Io at one time. But the eruptions might be cyclic. The moons of Jupiter tug at each other, slowly shifting their orbits. Sometimes Io might be quiet. At other times it might be even more violent than it is today.

Io also has a thin atmosphere of volcanic gases. The atmosphere is thicker above active volcanoes and patches of evaporating ice. The outer region of the atmosphere is gradually escaping into space. meteor.

Moons of Jupiter: Europa

Of all the icy bodies in our solar system, Europa may be the most intriguing. Its surface looks like a cracked egg, with jagged grooves radiating across a smooth, white skin.

Europa is covered by a thick layer of ice. Surface temperatures are around 230 degrees below zero Fahrenheit. But the dark cracks suggest that Europa's surface ice might float atop liquid water -- one of the main ingredients for life.

One hemisphere of Europa always faces Jupiter, just as the same side of the Moon always faces Earth. But the gravity of Jupiter's other large moons tries to turn Europa around, so it gets pulled and twisted. This gravitational battle generates heat in Europa's core -- perhaps enough to melt some of its ice.

The Galileo spacecraft found that vast seas or oceans of liquid water may lie below the ice, warmed by the moon's internal heat. Since water is an essential ingredient for life -- at least life as we know it -- there's speculation that life might have evolved in Europa's ocean.

Some scientists and fiction writers have speculated that exotic organisms might live in these oceans, but no one will know for sure until probes can dive into the frigid waters. Someday, a robotic submarine may glide through that ocean, looking for simple bacteria, or even odd sea creatures.

Scientists don't know how deep Europa's seas might be, or how warm -- or even if there really ARE seas. And even if seas do exist, there's no guarantee that anything lives in them. But considering how pervasive life is on Earth, few scientists are willing to rule out the possibility.

Not everyone agrees that there's an ocean beneath Europa's ice, though. That's because scientists have some very different ideas on the age of Europa's surface.

Europa itself is about four and a half billion years old. But its surface is younger. Some scientists say it's as much as two or three billion years old. If that's the case, then there's no liquid water on Europa -- just solid ice.

But other scientists think Europa's surface is just a few MILLION years old. Europa has relatively few impact craters on its surface, indicating that the surface changes quickly. That's a good sign that the ice is floating atop liquid water.

Moons of Jupiter: Ganymede

Ganymede is the solar system's largest moon ? about half again the size of our own Moon. In fact, it's larger than the planets Mercury and Pluto. Ganymede grew so large because ice was plentiful around newborn Jupiter. Ganymede consists of about half ice and half rock and metal. A thin icecap covers one of its poles.

Grooves and ridges that crisscross its surface indicate much change over the eons. The Galileo spacecraft already has seen great fault lines on Ganymede, signs of a strong internal heat source in the past. Surface ice slumped and flowed, filling in craters blasted by mountain-sized boulders. And parallel grooves called Sulci cover more than half of Ganymede's surface. Scientists aren't sure yet how the Sulci formed; maybe pieces of Ganymede's crust pulled apart or bumped together, creating wrinkles. Ganymede has far fewer craters than its neighbor Callisto.

During its first pass, Galileo photographed two small regions of Ganymede's rugged surface. It saw mountains of ice and great sheets of ice that were spewed forth by volcanoes. It also saw deep canyons and broad, smooth plains created by the motions of Ganymede's crust ? like the motions of the continents on Earth. And it detected hints of a thin, cold atmosphere.

Galileo discovered that Ganymede has its own magnetic field. The field indicates that Galileo's core might consist of molten iron ? like Earth's core. As Ganymede rotates on its axis, the iron core would act like an electric motor, generating a magnetic field around the moon.

Moons of Jupiter: Callisto

Callisto, the Galilean satellite farthest from Jupiter, is an icy, rocky moon covered with craters. Callisto is almost as big as the planet Mercury.

Callisto's interior probably consists of a rocky core surrounded by ice mixed with rock and dirt. But Callisto's most interesting feature is its surface. It's more heavily cratered than any other planet or moon in the solar system. That means that not much has happened to Callisto over the last four billion years.

The other large solid bodies in the solar system underwent the same pounding ? including Earth. But on the other worlds, many of the craters were erased by wind, rain, ice, volcanoes, or other forces. Giant lava flows even paved over some of the craters on our own Moon. But Callisto has preserved its ancient craters with little or no change.

The Galileo spacecraft focused on one of the largest craters, known as Valhalla. It consists of a bright central region almost 400 miles across, surrounded by rings that span almost 2,000 miles. The crater probably formed when a large asteroid or comet slammed into Callisto several billion years ago. If the impact had been much more powerful, it might have shattered the big moon.