Back in 1997 I read a very thought-provoking article in The Atlantic Monthly magazine by physicist Freeman Dyson in which he discussed some novel ideas for thinking about space exploration. Among other things, he note that a
place where life might now be flourishing is in a deep ocean on Jupiter's satellite Europa. Jupiter has four large satellites, discovered almost 400 years ago by Galileo: Io, Europa, Ganymede, and Callisto, in order of their increasing distance from Jupiter. The Galileo spacecraft now orbiting Jupiter is sending back splendid pictures of the satellites. The new pictures of Europa show a smooth, icy surface with many large cracks but very few craters. It looks as if the ice is floating on a liquid ocean and being fractured from time to time by movements of the water underneath. The pictures are strikingly similar to some pictures of the ice that floats on the Arctic Ocean; it would not be surprising if Europa had a warm ocean under the ice. Io is blazing hot, with active volcanoes on its surface; Ganymede's surface is icy like Europa's but not so smooth; and Callisto looks like a solid ball of ice covered with ancient craters. All four satellites are heated internally by the tidal effects of the huge mass of Jupiter, but the internal heating falls off rapidly with distance from Jupiter. We should expect that below the surface Europa is much cooler than Io and much warmer than Ganymede and Callisto. Since Io is hot enough to boil away all its water, and Callisto is cold enough to freeze solid, Europa might well have a warm liquid ocean. Ganymede might also have a liquid ocean, but it would be covered by a much thicker layer of ice. Of all the worlds that we have explored beyond Earth, Mars and Europa are the most promising places to look for life.
To land a spacecraft on Europa, with the heavy equipment needed to penetrate the ice and explore the ocean directly, would be a formidable undertaking. A direct search for life in Europa's ocean would today be prohibitively expensive. But just as asteroid and comet impacts on Mars have given us an easier way to look for evidence of life on that planet, impacts on Europa give us an easier way to look for evidence of life there. Every time a major impact occurs on Europa, a vast quantity of water is splashed from the ocean into the space around Jupiter. Some of the water evaporates, and some condenses into snow. Creatures living in the water far enough from the impact have a chance of being splashed intact into space and quickly freeze-dried. Therefore, an easy way to look for evidence of life in Europa's ocean is to look for freeze-dried fish in the ring of space debris orbiting Jupiter. Sending a spacecraft to visit and survey Jupiter's ring would be far less expensive than sending a submarine to visit and survey Europa's ocean. Even if we did not find freeze-dried fish in Jupiter's ring, we might find other surprises -- freeze-dried seaweed, or a freeze-dried sea monster.
Freeze-dried fish orbiting Jupiter is a fanciful notion, but nature in the biological realm has a tendency to be fanciful. Nature is usually more imaginative than we are. Nobody in Europe ever imagined a bird of paradise or a duck-billed platypus before it was discovered by explorers. Even after the platypus was discovered and a specimen brought to London, several learned experts declared it to be a fake. Many of nature's most beautiful creations might be dismissed as wildly improbable if they were not known to exist. When we are exploring the universe and looking for evidence of life, either we may look for things that are probable but hard to detect or we may look for things that are improbable but easy to detect. In deciding what to look for, detectability is at least as useful a criterion as probability. Primitive organisms such as bacteria and algae hidden underground may be more probable, but freeze-dried fish in orbit are more detectable. To have the best chance of success, we should keep our eyes open for all possibilities.
When Dyson wrote that he had no way of knowing that in 2005 the Cassini spacecraft would make close observations of Saturn's moon Enceladus (en-SELL-a-duss) that make it an even better candidate. As its Wikipedia article says,
Enceladus seems to have liquid water under its icy surface. Cryovolcanoes at the south pole shoot large jets of water ice particles into space. Some of this water falls back onto the moon as "snow", some of it adds to Saturn's rings, and some of it reaches Saturn. Because of this apparent water at or near the surface, Enceladus may be one of the best places for humans to look for extraterrestrial life. By contrast, the water thought to be on Jupiter's moon Europa is locked under a very thick layer of surface ice.
Those cryovolcanoes (also referred to as geysers) mean that it's not necessary to look for debris knocked off of the moon by the occasional meteorite. The moon itself is blasting out samples at a pretty reliable rate. Moreover, Cassini's instruments detected evidence of large organic molecules, meaning carbon compounds that are not necessarily biological in origin, but consistent with life.
On December 5 Peter Tsou et al made a presentation at the fall meeting of the American Geophysical Union proposing a relatively inexpensive sample-return mission to Enceladus. The abstract says,
Cassini’s discovery of geysers on Enceladus and organic materials, indicate that there is an exceptional opportunity and science rational to do a low-cost flyby sample return mission, similar to what was done by the Stardust. [...] Enceladus and comets are the only known solar system bodies that have jets enabling sample collection without landing or surface contact. [...] Four prime criteria for habitability are liquid water, a heat source, organics and nitrogen. [...] Out of all the NASA designated habitability targets, Enceladus is the single body that presents evidence for all four criteria.
(Stardust was a relatively low-cost sample-return mission to Comet Wild 2 launched in 1999. It was also a pretty decent 2007 movie based on a Neil Gaiman novel, but I digress.)