Jupiter's Galilean Satellites: Europa
by Dani Johnson
June 7, 2013
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.
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:
We may not have those answers yet, but we can make certain observations and develop theories based on the data gathered by past missions.
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
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.
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.
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.
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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