Deep in our solar system, a new era of space exploration
is unfolding. Beneath the thick ice of Europa, in the vapor plumes on Enceladus, and within the methane lakes of Titan, astrobiologists are on the hunt
for extraterrestrial life. We’ve honed in on these three moons
because each is an ‘ocean world,’ an environment that contains
a liquid ocean– and liquid can support
the formation of life. Living organisms have to be able to grow,
reproduce, and feed themselves, among other things. All of those functions require the
formation of complex molecules from more basic components. Liquids such as water allow chemical
compounds to remain in suspension instead of sinking under
the force of gravity. This enables them to interact frequently
in a 3-dimensional space and, in the right conditions, go through chemical reactions that
lead to the formation of living matter. That alone isn’t enough; the small but complex biomolecules
that we’re familiar with are sensitive to temperature— too hot or cold, and they won’t mix. Liquid water has an additional advantage in that it’s relatively
temperature-stable, meaning it can insulate molecules against
large shifts in heat. On Earth, these and other conditions in
aquatic environments may have supported the emergence
of life billions of years ago. Tantalizingly, the same could be true
in other parts of our solar system, like these three icy moons. Europa, which is a moon of Jupiter, is probably the most
intriguing ocean world. Beneath a surface layer of ice thicker
than Mount Everest, there exists a liquid ocean as much as
100 kilometers deep. Astrobiologists think this hidden
ocean could harbor life. Thanks to the Galileo probe, we can deduce that its
potential salt content is similar to that of some lakes on Earth. But most of its characteristics will be a
mystery until we can explore it further. Like Jupiter, Saturn also has moons that
might have the right conditions for life. For instance– Enceladus is a tiny ball of
ice that’s small enough to nestle within the surface area
of the Gulf of Mexico. Similarly to Europa, it likely contains an
ocean deep under the ice. But Enceladus also has geysers that frequently vent water vapor and
tiny ice grains into space. Astrobiologists are curious about whether
these geysers are connected to the ocean below. They hope to send a probe to test whether
the geysers’ plumes of vapor contain life-enabling material
from that hidden sea. Although it’s the best known substance
for nurturing life, water isn’t necessarily the only medium
that can support living things. Take Titan, Saturn’s largest moon, which has a thick nitrogen atmosphere containing methane and many other
organic molecules. Its clouds condense and
rain onto Titan’s surface, sustaining lakes and seas
full of liquid methane. This compound’s particular chemistry means
it’s not as supportive a medium as water. But, paired with the high quantities
of organic material that also rain down from the sky, these bodies of liquid methane could
possibly support unfamiliar life forms. So what might indicate that life exists on
these or other worlds? If it is out there, astrobiologists
speculate that it would be microscopic, comparable to the bacteria
we have on earth. This would make it difficult to directly
observe from a great distance, so astrobiologists seek clues
called biosignatures. Those may be cells, fossils, or mineral
traces left behind by living things. And finding any biosignatures will be
challenging for many reasons. One of the biggest concerns is to make sure we sterilize our
probes extremely thoroughly. Otherwise we could accidentally
contaminate ocean worlds with Earth’s own bacteria, which could destroy alien life. Titan, Enceladus, and Europa are just three of possibly many
ocean worlds that we could explore. We already know of several other
candidates in our solar system, including Jupiter’s moons Callisto and
Ganymede, Neptune’s Triton, and even Pluto. If there’s this much potential for life
to exist in our own tiny solar system, what unimagined secrets might the
rest of the universe contain?
