Europa Facts for Kids
1. Europa is the smallest of Jupiter's four Galilean moons.
Jupiter's four biggest moons are known as the Galilean moons after Galileo (1564-1642) who discovered them in 1610. Europa is 3100 km (1900 miles) in diameter, only slightly smaller than our own Moon.
2. Europa is over 670,000 km (420,000 mi) from Jupiter, and its orbit takes just over three and a half days.
Europa gets around much faster than the Moon does. It orbits Jupiter almost eight times while the Moon is getting around the Earth once.
3. Most of our detailed knowledge of faraway Europa comes from space probes.
NASA's Galileo mission is the main source of information. In the 1990s it spent eight years studying Jupiter and its moons. In addition, the Pioneer and Voyager missions, as well as New Horizons, photographed Europa during fly-bys.
4. Europa is large enough to be round and layered like a planet.
Europa is made mostly of silicate rock. This is the main type of rock in the Earth's crust. Silicate rocks are a feature of the inner planets, asteroids and rocky moons. Europa probably has an iron-nickel core, and the crust is definitely ice. At the low temperatures found on Europa, ice is as hard as granite.
5. Europa's surface is very bright and amazingly smooth.
Europa is bright because it reflects 64% of the light that hits it. Our Moon reflects, on average, only 12% of incoming sunlight. Europa is also the smoothest object that we know of in the Solar System. It should be covered in craters from impacts like those that cratered the Moon and Mercury, but it isn't. Yet we know that Solar System objects were hit by meteorites in large numbers. Europa's smooth surface must be new – somehow the craters got filled in and the surface smoothed over.
6. Europa is cold – extremely cold.
Europa is nearly 500 million miles from the Sun, about five times as far away as Earth is. It doesn't have a thick atmosphere to hold in heat, and its shiny surface reflects most of the Sun's energy. Even its temperature at the equator is only -160 °C (-260 °F). The poles are much colder, -220 °C (-370 °F). The coldest temperature ever recorded on Earth was about half as cold as Europa's equatorial temperature.
7. Europa has a salty ocean under the ice crust.
There is scientific evidence that points to a salty ocean. And two different teams of astronomers have found evidence of water plumes. (The Cassini spacecraft has observed such plumes close up on Saturn's moon Enceladus.) A plume is a stream of material that looks like a long feather. Although the surface of Europa is smooth, it's crossed by lines that are cracks. Water from inside the moon seems to be occasionally shooting up through these cracks.
It looks as though Europa's outer layer may be about 100 km (60 mi) thick. The outer part is the frozen crust, but underneath that it's liquid. There could be as much as three times the amount of water on Europa as there is in Earth's oceans.
8. Europa is geologically active.
A body like Earth is geologically active. The heat inside our planet produces volcanoes, earthquakes and many other changes. But our Moon is not active – slow surface changes are mostly from meteorite impacts. For a body to be active it needs an internal heat source to support a fluid layer such as Earth's magma. Europa's plumes are evidence of activity, and it must have internal heat and a liquid layer in order to renew its surface.
9. Although Europa is extremely cold, something keeps the ocean liquid.
Astronomers don't have enough information to agree completely about the heat source. They do generally agree that much of the heat – if not all – is supplied by gravitational forces from Jupiter and Europa's companion moons. They stretch and squeeze Europa, and that releases heat. This action may also explain Europa's cracks.
10. Europa would be a good place to search for life.
The life we know on Earth mostly relies on food chains that begin with plants turning the Sun's energy into food. But we also know that deep in the ocean there are living things well beyond the reach of sunlight. They exist on the ocean floor where long narrow cracks allow sea water to be heated by magma, and chemicals to be released from the rocks. Their food chains begin with bacteria that use minerals for energy. There could be something similar happening in Europa's ocean.
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