Europa, moon of Jupiter

Jupiter's moon Europa has a large ocean under its icy crust. And never mind Mars or Venus! Many scientists think that Europa's ocean would be a good place to look for life.

1. Galileo (1564-1642) discovered Jupiter's four biggest moons in 1610. They're known as the Galilean moons.
Europa is the smallest of the four. It's 3100 km (1900 miles) in diameter, so only a bit smaller than our Moon.

2. Europa is over 670,000 km (420,000 mi) from Jupiter. It orbits the planet in just over three and a half days.
While our Moon is taking a leisurely 27 days to orbit Earth once, Europa can whiz eight times around Jupiter.

3. We knew almost nothing about the distant moons of the giant planets until spacecraft went visiting.
In the 1990s NASA's Galileo mission spent eight years studying Jupiter and its moons. Other space missions have also photographed Europa on the way to their destinations.

4. Europa's surface is very bright, but it's extremely cold.
Europa is five times farther from the Sun than Earth is. In addition, its shiny surface reflects most of the Sun's energy and it has no atmosphere to hold in any heat. At the equator, Europa's temperature is -160 °C (-260 °F). The poles are even colder, -220 °C (-370 °F).

5. Europa is rounded and layered like a planet.
If Europa orbited the Sun instead of orbiting Jupiter, it could be a dwarf planet. It's big enough that gravity has pulled it into a round shape. It also has layers. The crust is ice, which is as hard as granite at Europa's low temperatures. Under the crust there is a liquid layer. Europa's mantle is mostly of silicate rock like Earth's mantle and that of other inner planets. It probably also has an iron core.

6. Europa has a salty ocean under the ice crust.
Europa's outer layer of ice and liquid may be about 100 km (60 mi) thick. There could be as much water on Europa as there is on Earth. The surface of Europa is smooth, but you can see lines in the image at the top of the page. These are cracks. Plumes of water from inside the moon sometimes shoot up through these cracks. Space missions have photographed this on Europa and on Saturn's moon Enceladus. (A plume is a stream of material that looks like a long feather.)

7. Europa is the smoothest object that we know of in the Solar System.
Meteors hitting Europa should have covered it in craters like they did on other moons. That means that Europa's surface must be new – somehow the craters got filled in and smoothed over.

8. Europa is geologically active.
A body like Earth is geologically active. There's a hot mantle inside the planet feeds volcanoes, and causes earthquakes and other dramatic changes. Our Moon isn't active in this way. However, the plumes and the renewed surface on Europa show there's activity there.

9. Although Europa is extremely cold, something keeps the ocean liquid.
Much of the heat comes from the gravitational pull of Jupiter and Europa's companion moons. They stretch and squeeze Europa, and that releases heat. This may also explain the moon's cracks.

10. Europa would be a good place to search for life.
We used to think that all life on Earth needed plants to turn sunlight energy into food. But we now know that life also exists deep in the ocean where sun never shines. Instead of plants and light energy, there are microbes using chemical energy from rocks. Europa seems to have the conditions for this kind of life to exist, but is there anything there? NASA is planning a mission to study Jupiter's moons, so maybe we'll find the answer in a few years.