Science Writing

Life Without Light

The case for life under the global ocean on Saturn's moon, Enceladus.

In the depths of the Atlantic Ocean there is a city. It teems with life of all shapes and sizes, it has existed for over thirty thousand years.

And it is completely devoid of light.

This place is known as The Lost City and it is a group of hydrothermal stacks that could be a clue as to the origins of life on Earth.

But it isn’t life on Earth that is the focus here.

Orbiting Saturn is a moon known as Enceladus. It’s small, only 500kms wide, and covered with a thick layer of ice; its surface makes it one of the most reflective bodies in the solar system.

This ice is roughly 23km thick around the majority of Enceladus’ surface, however at its south pole the thickness is dramatically reduced to less than 5kms. And it is the south pole that, in 2005 when Cassini flew by Enceladus for the first time, became of particular interests to scientists.

The plume of Enceladus in a false colour image as captured by Cassini Credit: NASA/JPL

Fissures and cracks in the ice at the south pole were discovered to be ejecting huge amounts of vapor and water ice particles into the surrounding space. In fact, it is believed that the majority, if not all, of Saturn’s E-ring is made up of the ejecta from Enceladus.

In a recent study published in The Planetary Science Journal, a group of scientist revisit and review the case for returning to Enceladus with the aim of sampling the plume for direct evidence of life.

“The plume of Enceladus is unique…providing direct access to fresh material from an extraterrestrial subsurface ocean.”

And it has been confirmed using the data from Cassini that the plume contains organic molecules such as carbon, oxygen, hydrogen, and nitrogen, as well as high mass organic molecules that are most likely fragments of even larger molecules.

Study of the plume revealed that the subsurface ocean is rich with methane, which either can be a rudimentary source of fuel for primitive cells, or a byproduct of life. Either way, it is a tantalising addition to the evidence that life could exist on Enceladus.

The process of retrieivng energy without light would be the same on Enceladus as it is on Earth at the Lost City and other places forever trapped in the dark; chemosynthesis.

In the Lost City energy is produced by redox disequilibrium wherein an electron released by a electron-donating reaction is taken up by an electron-accepting reaction. This change in energy is all that small microbes need to survive and thrive.

This redox reaction is one of the causes of a high level H2 found within the Lost City as it is released when reduced iron within rock is oxidised by water.

It so happens that H2 was found in abundance within the plume of Enceladus which suggests that redox reactions are occurring as the ocean interreacts with the rocky core of the moon.

A cross-section of the layers of Enceladus’ southern pole. The plume, and the ocean beneath, has been proven to contain the building blocks of life. Credit: NASA/JPL

Which means that the Lost City may have a counterpart orbiting Saturn. Whether or not this means that life surrounds those hydrothermal vents as well is still up for debate but as the authors of The Science Case for a Return to Enceladus put it:

“Enceladus is the only confirmed and most well-studied habitable environment beyond Earth with strong evidence supporting the presence of the ingredients considered necessary for life as we know it: liquid water, chemical building blocks including organics, and energy sources.”

Perhaps we will send a probe to Enceladus one day in the near future with updated sensors and the directive to search for life.

And maybe we will finally get an answer to the question, are we alone?

References:

The Science Case for a Return to Enceladus (iop.org)

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