Does life exist elsewhere besides Earth, or are we alone in the universe? This central question is one of the primary reasons behind solar system and planetary exploration today. As humankind develops faster and more technologically savvy ways to delve deeper into space, the desire to discover life-nurturing worlds becomes ever more intense. And one of the most fascinating possibilities remains in Jupiter's ice-rich moon, Europa.
Europa [yur-ROH-pah] has intrigued scientists and astronomers for centuries. Its relatively smooth, icy surface reflects sunlight, and because of this, it is one of the brightest celestial bodies in the solar system.
In 1610, Galileo Galilei discovered four satellites of Jupiter: Io, Callisto, Ganymede, and Europa. Then, in the mid-twentieth century, astronomer Gerard Kuiper showed that Europa's crust was composed of water and ice.
In the 1970s, Kuiper's analysis of Europa's surface sparked an ambitious plan to explore Jupiter's satellite system, beginning with the Pioneer and Voyager missions. These missions verified Kuiper's analysis and also discovered many other features and characteristics of the Jovian moons. In 1995, the Galileo spacecraft began gathering more detailed images of Europa, bridging the gap between past and present understanding of this intriguing moon.
Europa is roughly the size of Earth's moon, but is otherwise markedly different. To begin with, at 790 million kilometers (490 million miles) from the sun, Europa's surface is a bone-chilling minus 230 degrees Fahrenheit (145 degrees Celsius). That's much too cold to support life as we know it.
Currently, there is good evidence to suggest that a layer of liquid water could exist beneath Europa's surface ice -- a sub-surface ocean as much as 50 kilometers (31 miles) deep. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities. On Earth, wherever we have found water, we have found evidence of life. Scientists believe the interior of Europa is heated by tidal flexing, a process that results from the gravitational tug-of-war among Jupiter and its moons. The heating may be sufficient to keep the inner layers liquid.
The Galileo spacecraft beamed close-up images of Europa's surface to Earth throughout the late 1990's, until the end of its mission in 2003. The images revealed patterns of ridges and cracks in the crust that are suggestive of an icy shell moving over a liquid ocean.
Even though scientists are uncertain whether Europa contains organic compounds, the Galileo spacecraft DID detect carbon dioxide, sulfur dioxide, cyanogens (colorless, poisonous, flammable gases), and hydrocarbons (hydrogen-carbon compounds often found in natural gas and petroleum) on neighboring moons Callisto and Ganymede. In addition, Europa's closest Jovian satellite Io, has active volcanoes, which could contain organic materials that might spew from within its surface. This shows that the likelihood of these compounds existing on Europa is very high, given the proximity to its neighboring satellites.
One interesting debate now centers on the thickness of Europa's icy shell. An ocean could melt through a thin ice shell only a few kilometers thick exposing water and anything swimming in it to sunlight (and radiation). A thin ice shell could melt through, exposing the ocean to the surface, and granting easy access of photosynthetic organisms to sunlight. A thick ice shell tens of kilometers thick would be very unlikely to melt through.1
The thickness will also ultimately determine how we can explore Europa's ocean and search for evidence of any life or organic chemistry there. Presently, we have not fully developed the means in which to drill or sample the ocean directly through such a thick crust and must resort to searching for ocean material that may have been exposed on the surface.
Most likely, measurable ocean material will have to be embedded as small bubbles or pockets or as layers within ice that has been brought to the surface by other geologic means. Three geologic processes could cause this to occur:
1. Impact craters excavate crustal material from deep within Europa and eject it onto the surface, where its compounds could be discovered and analyzed. Unfortunately, the largest known crater on Europa, Tyre, excavated material from only 3 kilometers deep, not deep enough to get near the ocean. Due to geometry and mechanics, craters excavate from the upper part of the crater, not the lower. So in this case, the mystery remains.
2. There is strong evidence that Europa's icy shell is somewhat unstable and is convecting. This means that samples of deep crustal material rise upward toward the surface where they are sometimes exposed as domes several kilometers wide. Any ocean material embedded within the lower crust could then be exposed to the surface.
3. Resurfacing of wide areas has occurred on Europa's surface where the icy shell has literally torn through and split apart. These areas are not empty but have been filled with new crustal material from below. It is possible that oceanic material could be found within this new crustal material.
Our understanding of Europa's surface and history is still very limited. Unknown processes could occur that bring organic materials from Europa's ocean depths to the surface, but only a definitive return to this icy moon will tell. 1
1 Source: Paul Schenk, Lunar and Planetary Institute, Houston, TX
Last Updated: 22 February 2011