We do not have "concrete" evidence for a ocean presently existing below Europa's ice crust. However, the Galileo data have considerably strengthened the case for this. First, the images show clear evidence for near-surface melting and movements of large blocks of icy crust in ways that are similar to icebergs or ice rafts on the Earth. Second, there are very few impact craters on the surface, suggesting that this activity took place recently, geologically speaking. The problem is that we have no precise way to measure the exact age of the surface, and it is possible that we are looking at an ancient "frozen" ocean, not a current one. We feel the evidence favors an ocean now but that it is not conclusive.
What do most scientists believe about the possibility of life in Europa's oceans?
This of course depends on when there was an ocean and how long it lasted (including up to the present). When the Viking went to Mars in 1976, most life scientists felt that to have a chance for extraterrestrial life you had to have light (for photosynthesis), liquid water, and oxygen. Since then we have discovered places on the Earth (ocean floor hydrothermal vents, geothermal hotsprings, etc.) where life is currently sustained in the dark, without oxygen, using the heat and chemical energy from volcanic fluids and water. Some of these life forms such as thermophillic (or heat loving) bacteria are among the most ancient types of life on Earth. Many scientists now speculate that life may actually have arisen under such conditions here on Earth. So, of course, we are now more interested in places like Mars and Europa where there may have been liquid water and volcanic activity as a place to look for primitive life. If there is an ocean on Europa, there is no easy way to estimate the chances that life could exist there but that's exactly the question we're trying to answer by continued exploration! In other words, we have to go look. Developing experiments and missions to this is obviously challenging but we're trying.
What future missions are being considered to explore Europa?
Galileo will continue to observe Europa and we hope in the next year to have eight more close encounters. Beyond that, NASA is studying a possible Europa Orbiter mission and perhaps landers in the early part of the next century. A Europa Orbiter might be able to definitively answer the ocean question by combining a number of techniques. One approach is to use radar to penetrate the ice and perhaps measure its thickness. Another is to use very precise gravity and altimetry measurements to observe the tides raised by Jupiter on Europa (these will be much larger - as great as 40 meters or so - if there is a liquid layer than if the water is all frozen).
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