The icy shell of Europa

 

This week’s guess the planet image comes from Europa. It was acquired by NASA’s Gallileo spacecraft. Europa is one of Jupiter’s four large moons. It is an intriguing world, as it is entirely covered in a shell of ice. On a large scale this gives the moon a very smooth topography, with few high relief features. On the small scale the surface is crisscrossed with cracks and streaks, such as those shown in this image which is 163 km by 167 km. NASA produced the colour image by combining data from two different orbits of the Galileo spacecraft. Credit goes to NASA’s JPL

Beneath the ice there is believed to be a subsurface ocean of liquid water, kept warm by tidal heating from Jupiter. This ocean has long intrigued scientists and the public alike as it could be a good place to look for alien life in the outer solar system. There is more water on Europa than on Earth, and so it would be very interesting to break through the ice into the ocean below. This might be easier said than done however, since the ice layer is believed to be kilometres thick, shielding the water below it from space.

The brown streaks on Europa can be seen quite clearly from orbit, and are evidence that Europa is active. They are believed to consist of hydrated salts, which are extruded onto the surface of Europa when cracks open up in the ice. The description of this image over at the NASA Photojournal suggests that the bright regions of the image are pure, white water ice, while the brown streaks might consist of “magnesium sulfate or sulfuric acid”.

Experiments have shown that salts take on a yellow brown appearance when exposed to the unpleasant radiation environment around Jupiter. This makes them very distinctive, and helps to pick out some of the cracks and fissures which riddle the moon’s surface.

the most interesting regions of Europa are the “Chaos Terrains”, where the surface and subsurface appear to interact. These regions are riddled with fractures like those shown above. The ice is broken up into massive blocks, which look somewhat similar to terrestrial icebergs. It has been suggested that areas of chaos terrain are indicative of pockets of water in the subsurface. These buried lakes aren’t part of the massive ocean itself, but are rather areas of melt further up in the icy crust. Unlike the ocean, they are close enough to the surface to interact with it, causing rifts and fissures to develop above them, through which salty water can flow onto the surface.

While the large ocean might be hard for a spacecraft to reach, it could be more feasible to drill into one of these underground lakes, and look for signs of life.

Image Credits

Europaterrain at NASA PhotoJournal: NASA/JPL-Caltech/SETI Institute.

Europa Globe: NASA/ JPL/ DLR