Saturn's moon Enceladus is one of the Solar System's
prime extraterrestrial locations for life to thrive. It harbors a global salty
ocean that internal heating theoretically keeps at temperatures hospitable to
an alien marine ecosystem.
Detecting that life, however, is not such an easy
matter. The moon is enclosed by a shell of ice that's estimated to be 5
kilometers thick (3.1 miles) at its thinnest point, and the ocean below it is
10 kilometers deep. This would pose a huge enough challenge here on Earth,
never mind a moon half a Solar System away.
But we may not need to go to all the effort of
drilling through Enceladus's shell after all. A new study finds that we ought
to be able to detect life on the icy moon in the plumes of salty water that
erupt from its surface – even if there's not all that much life there.
"Clearly, sending a robot crawling through ice cracks
and deep-diving down to the seafloor would not be easy," says evolutionary
biologist Regis Ferrière of the University of Arizona.
"By simulating the data that a more prepared
and advanced orbiting spacecraft would gather from just the plumes alone, our
team has now shown that this approach would be enough to confidently determine
whether or not there is life within Enceladus' ocean without actually having to
probe the depths of the moon. This is a thrilling perspective."
Enceladus is very different from Earth; it's hardly
likely to be crawling with cows and butterflies. But deep under Earth's ocean,
far from the life-giving light of the Sun, a different kind of ecosystem
emerged. Clustered around vents in the ocean floor that spew forth heat and chemicals,
life relies not on photosynthesis but on harnessing the energy of chemical
reactions.
What we know of Enceladus suggests that similar
ecosystems might be lurking on its seafloor. It completes an orbit of Saturn
every 32.9 hours, traveling on an elliptical path that flexes the moon's
interior, generating enough heat to keep the water closest to the core liquid.
This isn't just theory: At the south pole, where the
ice shell is thinnest, giant plumes of water hundreds of kilometers high have
been seen erupting out from below the ice, spewing forth water that scientists
think helps contribute to the ice in Saturn's rings.
When Saturn probe Cassini flew through these plumes
over a decade ago, it detected several curious molecules – including high concentrations
of a collection associated with Earth's hydrothermal vents: methane and lesser
amounts of dihydrogen and carbon dioxide. These can be linked to
methane-producing archaea here on Earth.
"On our planet, hydrothermal vents teem with
life, big and small, in spite of darkness and insane pressure," Ferrière
said. "The simplest living creatures there are microbes called methanogens
that power themselves even in the absence of sunlight."
Methanogens metabolize dihydrogen and carbon
dioxide, releasing methane as a by-product. Ferrière and his colleagues modeled
the methanogen biomass we might expect to find on Enceladus if the biomass
existed around hydrothermal vents like those found on Earth.
They then modeled the likelihood that cells and
other biological molecules would be ejected through the vents and how much of
these materials we would be likely to find.
"We were surprised to find that the
hypothetical abundance of cells would only amount to the biomass of one single
whale in Enceladus' global ocean," says evolutionary biologist Antonin
Affholder, now of the University of Arizona, but who was at Paris Sciences et
Lettres University in France at the time of the research.
"Enceladus' biosphere may be very sparse. And
yet our models indicate that it would be productive enough to feed the plumes
with just enough organic molecules or cells to be picked up by instruments
onboard a future spacecraft."
Equipped with the expected abundances of these
compounds, an orbiting spacecraft might be able to detect them – if it could
make multiple plume flythroughs to collect sufficient material.
Even then, there might not be enough biological
material, and the chance that a cell might survive the trip through the ice and
being spewed out into space is probably pretty slim.
In the absence of such a smoking gun, the team
suggests that amino acids such as glycine would serve as an alternative,
indirect signature if abundances exceed a certain threshold.
"Considering that according to the
calculations, any life present on Enceladus would be extremely sparse, there
still is a good chance that we'll never find enough organic molecules in the
plumes to unambiguously conclude that it is there," Ferrière says.
"So, rather than focusing on the question of
how much is enough to prove that life is there, we asked, 'What is the maximum
amount of organic material that could be present in the absence of life?'"
These figures, the researchers say, could help
design future missions in the years to come. Meanwhile, we'll just be here on
Earth, wondering what an ecosystem deep under the ocean on a moon orbiting
Saturn might look like.
Reference: The Planetary Science Journal.
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