What NASA Discovered on Jupiter’s Icy Moon Europa

Jupiter, the largest planet in the solar system, has a total of 80 known moons, all of which are fascinating in their own way. But arguably the most interesting of them all is one of the Galilean moons - the mysterious icy world named Europa.

Crisscrossing Europa are vast cracks and discolored streaks that combine to create a complex yet beautiful world. And when looked at even closer, many more strange features can be seen such as Europa's mysterious Freckles. But what are these strange freckle-like features, and what is causing the chaotic fractures that weave across Europa's frozen landscape?

A total of six spacecraft have visited Europa up close, photographing the stunning Moon as they passed by. But most of what we know about it comes from several years of observations made by NASA's Galileo spacecraft. Galileo arrived at Jupiter in 1995 and successfully orbited the gas giant for almost eight years. It performed many close flybys of all of Jupiter's major moons, photographing them in all their glory, as these incredible images reveal. But it was the images and data collected about Europa that were the most tantalizing. In fact, the mission was extended to make a total of 12 close flybys of the icy Moon.

Despite its fractured and discolored appearance, scientists discovered that Europa is the smoothest solid object in the solar system. Its highest peaks, for example, are only a few hundred meters tall, and large craters are rare. This smoothness means that the surface of Europa is fairly young, probably only tens of millions of years old, rather than billions like most other moons. Somehow, the ice is being resurfaced on Europa and smoothed out faster than on many other worlds. The question of why that happens has not been definitively answered yet, but it probably has something to do with what's going on below Europa's mysterious surface.

Take this image captured by the Galileo spacecraft, for example. Several strange spots or freckle-like features can clearly be seen scattered across its frozen landscape. The spots visible in this image are about six miles across (which is about 10 kilometers across), and many are smooth domes. Some are jumbled pits, and some seem to have a reddish-brown hue. They are called lenticuli, which is the Latin term for freckles, and the similar size and shape of these spots suggest that warmer ice is rising up through the colder ice of the outer crust of Europa, much like magma chambers in Earth's crust. This process would be similar to that of a lava lamp, for example. The smooth dark spots may have formed when the warmer rising ice erupted through the surface, while the jumbled up rough lenticuli could be formed from small cracked fragments of the crust embedded in the dark ice from below, appearing like icebergs in a frozen sea. Whereas, if you look closely, the tops of the dome-like structures are very similar to the surface around them, suggesting that they were formed when the crust was pushed up but did not rupture like the darker spots in this area.

Recent studies have found that these mysterious features may sit above vast lakes of liquid water that are entirely encased within the icy outer shell of Europa. This cracked outer shell is estimated to be made mostly of water ice frozen as hard as granite and is approximately 15 miles thick (which is about 25 kilometers thick). But where is this liquid water coming from? Well, it turns out that these lakes may not be the only bodies of water Europa is concealing because below this icy shell, scientists have also found evidence of a global salty ocean.

Europa's most striking feature is its dark crisscrossing linear fractures and ridges that stretch around the Moon.