How to find life on Enceladus? Look in the plumes

Saturn’s moon Enceladus is one of the most promising places in the solar system to search for alien life. A global salty ocean, thought to be much like Earth’s oceans, lies beneath its outer crust of ice. Looking for evidence of that possible life is challenging, though. Sending a probe into the ocean would require drilling through a few miles of solid ice.

That’s why researchers at the University of Arizona pointed out late last year (December 2022) that there’s a much simpler way to find signs of life, if it exists, on Enceladus. How? By having an orbiting spacecraft sample the moon’s water vapor plumes instead. Those plumes – at least 100 of them – erupt from the ocean below and into space through massive cracks called Tiger Stripes in the surface at Enceladus’ south pole.

Michelle Starr wrote about the intriguing proposal for ScienceAlert on January 4, 2023.

And researchers published their new peer-reviewed paper in The Planetary Science Journal on December 13, 2022.

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Enceladus’ magnificent plumes

Since Enceladus’ ocean is hidden below a crust of ice – much like Europa’s – it seems logical that searching for life in that ocean requires being able to drill through the ice to reach the salty water. That would certainly be difficult, however. Senior author Regis Ferrière at the University of Arizona said:

Clearly, sending a robot crawling through ice cracks and deep-diving down to the seafloor would not be easy.

But Enceladus is special, as it provides scientists a unique glimpse into its interior, with no drilling required! It does this through its plumes. These are huge geyser-like sprays of water vapor that erupt from the moon’s South Pole through huge cracks in the surface ice.

NASA’s Cassini spacecraft first found them as it flew past Enceladus numerous times while orbiting Saturn. Data from Cassini indicated that they likely originated from the subsurface ocean (now pretty much agreed on by most scientists). Moreover, Cassini actually flew directly through the plumes multiple times, sampling the water vapor and analyzing it. Cassini found a rich assortment of ingredients, including water vapor, ice particles, ammonia, methane, various organic molecules, dihydrogen and carbon dioxide.

Cassini, however, wasn’t equipped to look for life itself at Enceladus. What it did find in the plumes, though, is tantalizing, providing clues as to the habitability of the ocean below. Indeed, scientists now say that Enceladus’ ocean is likely quite habitable by earthly standards. There is even evidence now for active hydrothermal vents on the ocean floor.

A simpler way to look for life on Enceladus

The researchers behind the new study, led by Antonin Affholder at the University of Arizona (previously at Paris Sciences et Lettres University in France at the time the research was conducted), say that the plumes provide a much simpler way to look for evidence of life. Why drill through miles of ice if you don’t need to?

Even if only microbes, finding life on Enceladus would still be an historic discovery. An alien ecosystem completely separate from ones on Earth.

The new proposal builds on Cassini’s findings. Another spacecraft could sample Enceladus’ plumes again, but this time with specific instruments for detecting evidence of active biology. ScienceAlert quoted Ferrière:

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.

Affholder added:

Our research shows that if a biosphere is present in Enceladus’ ocean, signs of its existence could be picked up in plume material without the need to land or drill. But such a mission would require an orbiter to fly through the plume multiple times to collect lots of oceanic material.

Methanogenic life on Enceladus?

Scientists say that any life in Enceladus’ ocean is likely fairly simple. There is no sunlight and little heat in this deep abyss. But on Earth, many types of organisms live in the deep oceans. And hydrothermal vents on Enceladus’ ocean floor – if indeed they do exist – could provide localized heat and nutrients just as they do in Earth’s oceans.

Microbes called methanogens are common in those environments. Methanogens metabolize dihydrogen and carbon dioxide, and then release methane as a by-product. Since all three of those have been found in Enceladus’ plumes, that could indicate a similar kind of ecosystem occurring in the moon’s ocean. As Ferrière said:

The simplest living creatures there are microbes called methanogens that power themselves even in the absence of sunlight.

Calculating the biomass in Enceladus’ ocean

If there are methanogenic-type microorganisms in Enceladus’ ocean, especially around the vents, how many might there be? Ferrière and his colleagues modeled the methanogenic biomass that could be expected to exist. Biomass is the overall amount of organic material in living organisms. They also addressed how much of that biomass might eject into space in the plumes. Affholder explained that it might not be a lot, at least in comparison to Earth:

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. 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 for instruments to pick up onboard a future spacecraft.

The paper states:

Here we extend this approach to quantify the ecosystem’s expected biomass stock and production and evaluate its detectability from the collection of plume material. We find that although a hypothetical biosphere in Enceladus’s ocean could be small (<10 tons of carbon), measurable amounts of cells and organics might enter the plume.

Even if there are only trace amounts of such organic molecules or cells, a future mission back to Enceladus would still have a good chance of finding them. And if it didn’t, it could still find amino acids such as glycine. Notably, if those amino acids exceeded a certain amount, that would also be a good – albeit more indirect – signature of life.

The paper also notes that, if necessary, a lander could also collect plume particles that land on Enceladus’ surface. Or perhaps do so in conjunction with the flybys of the spacecraft through the plumes.

Future missions

In 2018, NASA announced support for studying a privately funded return mission to Enceladus. The European Space Agency (ESA) is also proposing a return m mission called Moonraker. Instruments would study and analyze both the surface and the plumes. Moonraker would conduct multiple flybys of Enceladus over a nominal period of 13.5 years.

So is there life on Enceladus? There’s only one way to find out, as Ferrière said:

To know if that is the case, we must go back to Enceladus and look.

Affholder also cautions that the results may still not be definitive:

The possibility that actual cells could be found might be slim, because they would have to survive the outgassing process carrying them through the plumes from the deep ocean to the vacuum of space; quite a journey for a tiny cell. The definitive evidence of living cells caught on an alien world may remain elusive for generations. Until then, the fact that we can’t rule out life’s existence on Enceladus is probably the best we can do.

Bottom line: Do we need to drill through miles of ice to find life on Enceladus? A new study says a simpler strategy is to just re-analyze the moon’s water vapor plumes.

Source: Putative Methanogenic Biosphere in Enceladus’s Deep Ocean: Biomass, Productivity, and Implications for Detection

Via Science Alert

Via University of Arizona