Jupiter's Ice Moon Europa: Part 2

We discussed Europa's icy surface and possible sub-surface ocean last week. Some images that Galileo sent back showed bumpy features on Europa's surface, almost like blisters. These features (shown below)  look similar to places on Earth where glaciers sit on top of undersea volcanoes. Basically, these underwater vents heat the water above them which melts the overlaying ice. Since these volcanoes are not active all the time, there are periods where the water can re-freeze on the surface. This re-freezing does not leave a smooth sheet, but rather a chaotic frozen pattern, much like the pattern seen on Europa. So what does this all mean? It suggests that there is some sort of internal heating mechanism within the moon, and therefore a subsurface ocean! The depth of this ocean, and the thickness of the ice sheet is still unknown, but further observations and hopefully future missions will enlighten us on Europa's mysterious structure.

 

Thera Macula (false color) is a region of likely active chaos production above a large liquid water lake in the icy shell of Europa. Color indicates topographic heights relative to background terrain. Purples and reds indicate the highest terrain. Image Credit: Paul Schenk/NASA

Jupiter's Ice Moon Europa: Part 1

 Image of Europa taken by the Galileo spacecraft showing surface ice and cracks

The Galileo spacecraft was launched in 1989, and in the 1990's gave astronomers their first up close and personal interview with Jupiter and its moons. One of many big discoveries was that Jupiter's 2nd closest moon, Europa, is a giant ball of ice! And the best part… it's water ice! Astronomers know the surface is water ice because of two main features. One is that albedo (reflectivity) of the planet is what we would expect for a planet covered in ice. Second, spectroscopy of Europa done in the infrared from the earth shows strong water absorption lines. Now that we know what the surface of Europa is made of, the question becomes "is there liquid water underneath that icy surface?" Astronomers speculate that the answer is yes. Based on Galileo images (above), Europa has a mostly smooth surface but exhibits large surface cracks as well (brown lines in the image). This is indicative of something like plate tectonics, where the ice plates sit on top of a liquid ocean and move around due to the motion of the sub-surface ocean. The only way to know for sure if there is a liquid ocean is to send a probe there to drill through the ice. This sort of probe may be in NASA's future, but recent discoveries based on the old Galileo observations suggest that liquid water has already been discovered! Tune in next time to learn more about this amazing discovery!

Image Credit: NASA/JPL