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Jupiter's Galilean Satellites: Europa

by Dani Johnson

June 7, 2013

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Above image: These images show the trailing hemisphere of Jupiter's moon Europa taken by the Galileo spacecraft at a distance of about 677,000 km. The left image shows Europa in approximately true color and the right image shows Europa in enhanced color to bring out details.

Image Credit: NASA

Name & Discovery

Europa was discovered in 1610 by Galileo Galilei along with Io, Ganymede and Callisto. He called them "The Medicean Stars" after the wealthy Medici family. Fellow astronomers Simon Marius and Johannes Kepler discussed the idea of naming the moons after the mythical loves of Jupiter, but the idea didn't catch on for a couple hundred years. Most astronomers called them I, II, III and IV in the order of distance from Jupiter. In the 17th century we were discovering so many moons of Jupiter and Saturn that astronomers finally started calling them the familiar mythological names we know now.

In the myth, Jupiter (Zeus) is enamored by Europa and turns himself into a white bull and mixes in with her father's herds. Europa sees how gentle this white bull is and winds up climbing onto his back. Jupiter takes advantage of her trust and runs into the sea, with Europa still on his back, and swims all the way to the island of Crete. He then reveals himself to her, and she becomes the first queen of Crete.

Quick Facts:

  • Jupiter's four largest satellites are Io, Ganymede, Callisto, and Europa.

  • Europa is thought to be about the same age as Jupiter, 4.5 billion years old.

  • Europa is an average of 780 million kilometers (485 million miles) away from the sun.

  • Europa is the sixth satellite from Jupiter.

  • On average, Europa is about 671 thousand kilometers (414 thousand miles) away from Jupiter.

  • It takes about three and a half Earth-days for Europa to orbit Jupiter.

  • Europa is tidally locked to Jupiter, meaning that one side faces the planet at all times.

  • Europa is just smaller than Earth's moon at a little over three thousand kilometers in diameter.

  • Europa is the 15th largest body in the entire Solar System.

  • It is thought that Galileo actually saw Europa and Io on January 7th, 1610, but they were so close together in their orbit that he couldn't tell them apart until the next day.

  • The discovery of the Galilean moons orbiting Jupiter helped scientists realize that the planets in our solar system, including Earth, revolved around the sun and not the Earth.

  • Europa's icy crust gives it an albedo—light reflectivity—of 0.64, one of the highest of all of the moons in the entire solar system.

  • It is thought that Europa's young surface is about 20 million to 180 million years old.

  • Europa's surface temperature at the equator never rises above minus 260 degrees Fahrenheit (minus 160 degrees Celsius). At the poles of the moon, the temperature never rises above minus 370 degrees Fahrenheit (minus 220 degrees Celsius).


Pioneer 10 was the first mission to return pictures of the Jovian moon. Voyager 1 and Voyager 2 were the first to deliver spectacular, high-res pictures of the satellite, but Galileo has gathered the most data in its nine flybys past Europa. On its way to Pluto, New Horizons has returned images of circles on the ice that suggest wandering poles which supports the idea that an ocean is present under the icy surface. Right now, Juno is en route to Jupiter and will arrive in 2016. Much like Cassini, the orbiter's primary objective is to learn about the parent planet but the close proximity will surely provide countless opportunities to observe and gather data from the Jovian satellites. The European Space Agency is also planning to launch an orbiter, JUICE, in 2022 that will spend at least three years making detailed observations and gathering data about Jupiter and the four Galilean Moons.

There are many things that we hope to learn about Europa from Juno and JUICE, specifically including:

  • What are the characteristics of its ocean?

  • How thick is the icy shell?

  • Is there near-surface water within the ice shell?

  • What is the global distribution of geological features?

  • Is liquid water involved in surface feature formation?

  • Is the icy shell warm and convecting?

  • What does the red material on the surface tell us about ocean composition?

  • How active is Europa today?

  • What is the plasma and radiation environment at Europa?

  • What is the specific nature of organics in salts at Europa?

  • The Galileo spacecraft did detect a weak magnetic field around Europa, nonetheless-one created within Europa as Jupiter's own powerful magnetic field sweeps past the moon.

We may not have those answers yet, but we can make certain observations and develop theories based on the data gathered by past missions.

Surface Characteristics:

The smooth, bright surface of Europa is unlike any other planet or moon in the entire Solar System. Most of the planet's surface features don't exceed a few hundred meters tall. The smoothness suggests that the surface of the moon is young and still active, much like its sister planet Io. If the surface weren't still active we would expect to see much larger land features and many more impact structures like on Callisto.

Above Image: These images show a comparison of the surfaces of the three icy Galilean satellites, Europa, Ganymede, and Callisto, scaled to a common resolution of 150 meters per picture element (pixel). Despite the similar distance of 0.8 billion kilometers to the sun, their surfaces show dramatic differences. Callisto (with a diameter of 4817 kilometers) is "peppered" by impact craters, but is also covered by a dark material layer of so far unknown origin, as seen here in the region of the Asgard multi-ring basin. It appears that this layer erodes or covers small craters. Ganymede's landscape is also widely formed by impacts, but different from Callisto, much tectonic deformation can be observed in the Galileo images, such as these of Nicholson Regio. Ganymede, with a diameter of 5268 kilometers (one-and-a-half times larger than the Earth's moon), is the largest moon in the solar system. Contrary to Ganymede and Callisto, Europa (diameter 3121 kilometers) has a sparsely cratered surface, indicating that geologic activity took place more recently. Globally, ridged plains and the so-called "mottled terrain" are the main landforms. In the high-resolution image presented here showing the area around the Agave and Asterius dark lineaments, older ridges dominate the surface, while a small part of the younger mottled terrain is visible to the lower left of the image center.

Image Credit: NASA/JPL/DLR

Above Image: Europa's trailing hemisphere (the hemisphere that trails in the satellite's orbital motion about Jupiter) is seen in this color image. Much of the hemisphere is covered with reddish-brown material. The colors here are intended to approximate how the surface might be perceived by the human eye. The bright feature containing a central dark spot in the lower right portion of the image is a relatively young impact crater some 24 km (15 miles) in diameter. This crater is named Pwyll (pronounced "pwuk") from Celtic mythology.

Image Credit: NASA/JPL/Ted Stryk

Europa's bright surface is scarred with fissures of a reddish color, it looks almost as if an enormous ice-skater danced around the surface of the planet with really dirty skates. Some of these fissures are more than 14 kilometers (9 miles) wide, which indicates that something is causing these cracks and then "refilling" them. As far as we can tell, the cracks are caused by Jupiter's gravitational pull on Europa as it orbits. Europa has an eccentric orbit, which means that the orbital pattern is more like an oval than a circle. This eccentricity causes the gravitational pull from Jupiter to change depending on where the moon is in its orbit which causes the surface to deform and crack. We call this constant deformation "tides", and it is illustrated well in the following video by NASA,

The tides are thought to cause the surface to crack, and a briny material from below is thought to push its way into these cracks, causing the change in color. It is not known whether the material below is already reddish in color or if it changes once it is exposed to radiation on the surface of the moon. Europa's surface is a place with many ridges and bands, chaos regions and even some impact craters. Below are some jaw-dropping videos that NASA has put together using actual images from Galileo.!!

For the complete list of videos, visit NASA's Europa: Videos page.

Planet Composition:

Based on images from Galileo, it is thought that Europa consists of silicate rock and has an iron core with a rocky mantle, very similar to Earth, except Europa's crust is entirely ice. The core is probably not molten, though, because an internally generated magnetic field was not detected around Europa. The weak magnetic field that does exist around the satellite is thought to be caused by Jupiter's magnetic field sweeping across the icy surface, which suggests a salty composition. A combination of the strange surface features found on Europa and the idea that the tides that cause the cracks probably heat up the interior of the planet lead us to believe that there might be an ocean under all of the surface ice. We imagine that it might look something like a lava lamp, with denser and colder ice sinking to the bottom while less dense and warmer ice rises to the surface. This process is called convection, and it may also be partially responsible for some of the geological features of the Jovian satellite. It is thought that the chaos regions are caused by pockets of warmer ice pushing up into the frozen surface, causing the surface to "melt" over the warmer ice.


The atmosphere on Europa is thin and made up almost entirely of Oxygen. The atmosphere is caused by charged particles from Jupiter's radiation belt that hit Europa's icy surface and produce water vapor. The hydrogen in the water vapor escapes and leaves only Oxygen behind.

Extraterrestrial Life:

Europa is a very interesting satellite because it is one of the few places in the Solar System where it might actually be worth searching for the elusive extra terrestrial life form. It seems to have all the "ingredients" to make a life form. The atmosphere is made up of oxygen and it is very likely that the ocean contains quite a bit of oxygen, maybe even enough to sustain life. Even without oxygen, it's possible that Europa's core heats the ocean up enough for deep-sea vents to occur.


Extra Content:

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Featured Photo:

Above Image: This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera.

Image Credit: NASA/JPL

by Dani Johnson

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