57 Galileo's Moons-Mid

Galileo's Moons FOCUS Question What makes each of Jupiter's four largest moons unique?

Many Moons When you look up at the sky, you may see the Moon. It is Earth's only moon. But Jupiter has more than sixty moons, and counting! The four largest are known as the Galilean moons. They are named for Galileo Galilei, who discovered them in 1610. Why so many moons? Jupiter is the largest planet in the solar system. More than one thousand planets the size of Earth would fit inside Jupiter! Because of its massive size, Jupiter has very strong gravity. Dust, gases, asteroids, and space debris got pulled into orbit around Jupiter by this strong gravity. This material clumped together to form Jupiter's many moons. Do You Know? Galileo spotted four of Jupiter's moons using a telescope he made himself. His scope could see objects three times farther away than other telescopes of the time.

Meet the Moons The Galilean moons are Europa, Io, Callisto, and Ganymede. Europa (yuh-ROW-puh) is the smallest of the four moons. Io (EYE-oh) is a little larger than Europa. Callisto (kuh-LIH-stow) is about the same size as the planet Mercury. The largest is Ganymede (GA-nuh-meed), which is even larger than Mercury. Their orbits around Jupiter are very short compared to the orbit of Jupiter around the Sun. Use the chart to compare these four moons. Galilean Moons Europa Io Callisto Ganymede Diameter (km) 3,138 3,630 4,800 5,262 Distance from Jupiter (millions of km) 0.671 0.422 1.883 1.070 Orbital Period (days) 3.6 1.8 16.7 7.2

Europa Scientists want to know more about Europa because it could have life! Europa's surface is covered in ice. But beneath the ice may be a liquid ocean. In 1998, the Galileo spacecraft found signs of liquid water on Europa. The spacecraft took pictures of cracks in the ice. Scientists think that the cracks were caused by moving water below the ice. Most of Earth's life is found in the ocean. The same may be true of Europa. The Hubble Space Telescope recorded water vapor spewing from Europa's surface in 2013. This gives scientists more hope of finding liquid water on that moon. Wowser! Scientists crashed the Galileo spacecraft into Jupiter on purpose Why? They didn't want it to crash on Europa and possibly harm any life in its ocean.

EUROPA: A RECIPE FOR LIFE? If we could find living things—even very simple life forms—beyond Earth, it would be very exciting! Scientists think several "ingredients" are needed for life to exist. Does Europa have them? 1. Is there water? Yes! Scientists believe Europa has a vast saltwater ocean beneath its icy surface. 2. Is the temperature just right? Yes! The climate needs to be warm enough for liquid water to flow but not too hot. As with Io, friction caused by the gravitational tug of Jupiter and other moons may keep water below the surface liquid. But Europa is not fiery like Io. 3. Does it contain the building blocks of life? Yes! Probes have found signs of organic, or living, compounds.

Io Closest to Jupiter, Io is covered with active volcanoes. In 1979, the Voyager spacecraft took pictures of volcanoes erupting. Lava from the volcanoes flows over Io's surface. Some places have hot spots, which may be lakes of lava. The atmosphere is a poisonous mix of gases given off by the eruptions. Io may be such a fiery place because of the way Jupiter "tugs" on it. The mighty planet's strong pull on Io causes materials inside the moon to rub together. All that friction heats Io up from the inside. Try This Rub your hands together as fast as you can. The rubbing causes friction. Are your hands getting warm? Friction gives off heat.