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Methane on Enceladus: a possible sign of life?

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View of the moon Enceladus from Saturn via the Cassini probe. A new study by a group of biologists shows that something produces a lot of methane in the moon’s underground ocean. Could this be life? Image via NASA / ESA / JPL / SSI / Cassini Imaging Team / Carnegie Science.

With its global ocean of groundwater, Saturn’s moon Enceladus is considered one of the best places to seek life. Now, new research from a team of biologists suggests that life on Enceladus may indeed be entirely possible … and that we strength already have evidence for this. Something is producing a lot of methane on Enceladus, or rather in its underground ocean. And methane can be a sign of life. Charlie wood discussed these curious results on June 8, 2021, Popular science. Oddly, according to Wood’s article, the initial analysis suggests that methane is produced in a manner similar to methanogen microbes in Earth’s oceans.

It is not yet proof of life on, or rather in, Enceladus, but it is certainly interesting.

the Peer reviewed the conclusions were published June 8, 2021 at Nature astronomy.

Methane in Enceladus: hydrothermal vents and methanogens

The original detection of methane comes from the analysis of data obtained by the Cassini spatialship. The probe made several close flights of the moon. At one point, it passed through plumes of water vapor bursting through cracks at the south pole of Enceladus. The water vapor in the plumes originates from the ocean deep below the icy outer surface.

Additionally, Cassini found particles of ice, salts, hydrogen, and organic molecules in the plumes, provisional clues to an ocean similar in composition to Earth’s oceans. There is also evidence of hydrothermal vents on the Enceladus seabed, similar to those that support methanogens in Earth’s oceans. As stated in the article summary:

Observations from NASA’s Cassini spacecraft have established that Saturn’s moon Enceladus has an internal liquid ocean. Analysis of a plume of oceanic material ejected into space suggests that alkaline hydrothermal vents are present on the seabed of Enceladus. On Earth, such underwater vents harbor microbial ecosystems rich in methanogenic archaea.

Planet with a large slice revealing a gray core surrounded by a thick blue layer with many small geysers at the bottom.
Cutaway view of the interior of Enceladus, showing the underground ocean and jets of water vapor erupting through cracks in the icy surface of the South Pole. Image via Popular science.

So what is it that produces methane on Enceladus?

Scientists already knew methane existed, but the new study shows that its surprisingly high amount is difficult to explain. Basically the most likely chemical process that could produce methane on Enceladus, serpentinization, would not be able to create as much as what is observed. So what could? As Antonin Affholder, at ENS Paris and main author, confided Popular science:

Methanogens are able to explain the amount of methane.

Curved edge of a cracked, cratered moon with very large geysers erupting in space.
The geysers of the moon Enceladus of Saturn. These enormous plumes of water vapor erupt through the cracks of the South Pole of Enceladus. The Cassini spacecraft analyzed the plumes, finding water vapor, ice particles, salts, methane, and a variety of complex organic molecules. Scientists believe the plumes originate from a global ocean below the moon’s icy surface. Image via Nasa.

Methanogens are microbial life forms that eat hydrogen and carbon dioxide and produce methane as a by-product. On Earth, they are found in deep-water hydrothermal vents like those thought to exist on the seabed of Enceladus.

So, could this really be the explanation for the methane on Enceladus?

Organic or non-organic?

With this in mind, the researchers considered all possible scenarios where methane could be produced in the Enceladus Ocean. In serpentization, the interaction of hot water and minerals in rocks can create hydrogen. Subsequently, chemical reactions can then combine hydrogen and carbon dioxide to produce methane.

It seemed plausible, so the researchers took a closer look at the possibility. They wanted to see how much methane could realistically be produced on Enceladus. They then compared these results, using Bayesian analysis, to a scenario where methanogens were involved. In fact, the researchers used real organisms on Earth for the comparison. Overall, it was important to keep their expectations realistic, as Affholder explains:

We cannot just imagine what we want to imagine. We need to base our assumptions on what we know.

Young smiling man with glasses, short beard and messy hair.
Antonin Affolder at the ENS in Paris led new research into the mystery of Enceladus methane. Image via ENS.

“Too much” methane

The results were surprising: the amount of methane detected by Cassini was significantly higher than what could be easily explained by serpentization alone. Affholder said:

The first hypothesis is totally disqualified, score of zero.

As stated in the summary, there was apparently “too much” of methane to be easily explained by inorganic chemistry alone. Extract from the summary:

Here, we use a Bayesian statistical approach to quantify the probability that methanogenesis (production of biotic methane) can explain the escape rates of molecular hydrogen and methane in the Enceladus plume, as measured by Cassini instruments.

We note that the observed escape rates (1) cannot be explained solely by the abiotic alteration of the rock core by serpentinization; (2) are compatible with the assumption of habitable conditions for methanogens; and (3) mark the highest probability under the methanogenesis hypothesis, assuming that the probability of emergence of life is high enough.

Too much hydrogen?

In addition, Cassini also found an abundance of hydrogen in the ocean, which could be used as a food source for microbes. But, it seemed that there was more of it left than there should be if it was consumed as food. Ultimately, there could also be an answer to this mystery. It is believed that most of the hydrogen molecules are very close to the vents, where it would be too hot for the methanogens. Instead, organisms could feast on hydrogen molecules farther away from the vents. In this scenario, there would be little impact on the overall abundance of hydrogen.

Ocean floor with insets showing hydrothermal features that could release methane over Enceladus.
Sectional view of the underground ocean of Enceladus. There is evidence of hydrothermal vents on the ocean floor, just like on Earth. Could methane come from methanogenic microbes? Image via Nasa/ JPL-Caltech / SwRI.

More data needed

Serpentization, at least on its own, seems like an unlikely solution to the methane mystery. It is always possible that there is an excess of primordial methane, left over from the formation of Enceladus, bubbling from its core. Or another still unknown process at work. As Affholder points out, we still don’t know enough about Enceladus to be sure:

We do not know the origin of Enceladus. We do not know the age of Enceladus. We do not know the precise nature of methane. To find out more, we might need a mission to examine methane.

In short, answering these questions will require more data. Unfortunately, no return mission is planned at this time. Cassini analyzed the plumes in detail, but it goes without saying that a probe that could specifically look for evidence of life itself in them would be even better. Here’s hoping!

Conclusion: something produces a lot of methane in the ocean of Enceladus. Could this be life? Although not yet proven, a new study by a team of biologists appears to support this exciting possibility.

Source: Bayesian analysis of data from the Enceladus plume to assess methanogenesis

Via popular science


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