Could life thrive where the Sun never shines? The
answer to this unorthodox question bears directly on
the tantalizing possibility that life exists in the
hidden, perpetually dark oceans that are thought to
shroud some of Jupiter’s moons, most prominently
Europa.
Recent work by Christopher Chyba (SETI Institute) and
Kevin Hand (Stanford University) suggests that there
may be ways to nourish biology on watery environments
where the Sun’s rays don’t penetrate. The two
researchers have published their work in the June 15
issue of Science.
“Most surface life on Earth – on land or in the seas
– depends on photosynthesis,” notes Chyba. “The first
link in the food chain is chlorophyll’s conversion of
sunlight into chemically stored energy. But imagine
an ocean on Europa, a huge, bottled-up body of water
capped with miles of ice. Photosynthesis isn’t going
to work there. Nonetheless, there are other ways to
make a metabolic living in those dark seas.”
Recent results from NASA’s Galileo spacecraft have
strongly suggested the presence of subsurface oceans
not only on Europa, but also its sister moons,
Callisto and Ganymede. Since liquid water is usually
considered a prerequisite for the development of
life, these nearby worlds are intriguing locales to
search for extraterrestrial biology.
However, more than water is required. An energy
source is necessary to support life. Chyba and Hand
point out that this is usually obtained by
oxidation-reduction reactions in which two substances
(for example, carbon and oxygen) bond to share an
electron, releasing energy during the reaction. An
important oxidizing agent in Earth’s oceans is
molecular oxygen (O2), the product of photosynthesis.
But one would expect this to be in short supply in
the inky abysses of the Jovian moons.
However, Chyba and Hand note that Europa’s icy
exterior is routinely bombarded with high-speed
particles accelerated in Jupiter’s magnetosphere.
When they slam into the Europan ice, they form
oxidants such as H2O2 and O2. If, as could be the
case, this surface food supply eventually gets
churned into the ocean below, it could provide
sustenance to a substantial biomass.
“We can’t be certain at this point whether the
oxidants would actually make it into the water, even
over geological time scales,” says Chyba. “But if
not, there are other mechanisms that might be a
source for molecular oxygen in the oceans.”
One of these is the radioactive decay of a potassium
isotope 40K, which would be present in both the ice
crust and the liquid water. The decay splits water
molecules and produces O2. Although the quantity of
oxidant produced in this way is less than could be
supplied by the surface effects of charged particles,
it would still be enough to support a biosphere.
“Obviously, we don’t know if life exists on these
moons,” Chyba emphasizes, “but at least we can say
that if the oceans are there, the compounds that
could supply energy for life seem likely to be
present.”
For further information, contact:
Christopher Chyba, SETI Institute
(650) 961-6633
chyba@seti.org
Related links
“Recent planetary exploration has provided evidence for large subsurface oceans on several jovian moons. In their Perspective,
Chyba and Hand assess the chances for life in these oceans, which are deprived of sunlight. Photosynthesis is therefore ruled
out, but other sources may provide molecular oxygen in sufficiently high concentration to serve as an energy source for life. “
Background Information
