Jul 15 2021

Methane in Enceladus Plumes

Published by under Astronomy
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Enceladus is the 6th largest moon of Saturn, about 500 km in diameter. It is completely covered with mostly fresh ice, making it highly reflective (in fact, it is the object in the solar system with the highest albedo, reflecting almost 100% of the light that hits it). Given its small size, astronomers assumed it was likely frozen solid. This small chunk of ice, however, became significantly more interesting in 2005 when Cassini first observed plumes ejecting from its southern pole. This suggested that Enceladus has liquid water beneath that surface crust of ice – and any place with liquid water is a potential candidate location for life.

Over the next 10 years Cassini made many Enceladus flybies, collecting data that is still being analyzed. NASA now estimates that there is an ocean beneath the southern pole of the moon, below 30-40 km of surface ice, and 10 km deep. Further, analysis of the misty plumes finds that it is salty water with a higher level of organic material than predicted.

Now we have a new analysis of Cassini data looking at the methane content of the Enceladus plumes. Methane is of particular interest to exobiologists looking for telltale signs of life. This is for two good reasons. One is that methane is a highly reactive gas, and will not persist for long in an atmosphere or liquid water. So if it is present in significant amounts it must be being constantly replenished. It shares this feature with oxygen, which is why oxygen is also a significant sign of potential life. Further (and again like oxygen) methane is known to be a byproduct of metabolism of certain kinds of critters. On Earth deep sea vents contain methanogenic archaea, bacteria-like single-celled organisms that live by chemosynthesis.

Therefore, the fact that the plumes of Enceladus have methane is extremely intriguing. We would expect any life in the ocean beneath the surface of Enceladus to be chemosynthetic, because there is no light for photosynthesis. Further there is evidence of alkaline hydrothermal vents on the sea floor of that ocean, similar to hydrothermal vents on Earth that harbor the chemosynthetic archaea. A very compelling picture is emerging.

But good scientists have to consider all options, and there are known geological processes that can produce methane. This same issue came up with the detection of methane on Mars, and NASA is still trying to definitively solve the question of this methane’s origin. So the question is – is the methane we are detecting in the plumes of Enceladus more likely to result from geological processes or life? That is the focus of the new study, which takes a Bayesian approach to determine which source is more likely. They conclude:

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

That’s pretty exciting. They are saying that life is the most likely explanation for the methane in the plumes, however this does depend on how likely it is for life to form in the first place, and this is a huge unknown. In any case, however, known geological processes cannot account for the methane. So at the very least astronomers may discover a previously unknown geological process, making Enceladus very attractive for study by planetary astronomers (and not a bad consolation prize).

But of course, everyone is hoping to find methanogenic life on Enceladus. This would be a massive scientific discovery, the first detection of life off Earth. If we could get sufficient samples of this life to fully examine it we might find a number of things, all equally fascinating. First, Enceladus life might be completely separate from Earth life, indicating an independent origin and evolution. This would greatly inform our ability to estimate the probability of life forming in the universe, raising our data points from one to two. One instance can be an incredibly unlikely fluke, but two so close together means life is likely common.

It’s also possible we will find that Enceladus life is distantly related to Earth life. This too would have huge implications, indicating that life can be seeded from one planetary object to another, at least within the same solar system. We would then want to reverse engineer the path that life took. Perhaps it came to Earth from another body in our solar system.

This result supports Enceladus as a prime candidate for extraterrestrial life in our solar system, and certainly justifies a dedicated mission. Any such mission is likely decades away, however, so for now we are left pouring over all that Cassini data.

The other top candidates for life include Mars and Europa. Europa is also an icy moon, although of Jupiter, and is much larger than Enceladus, with a subsurface ocean about twice the volume of Earth’s combined oceans. Mars once had surface liquid water and a thicker atmosphere, and it’s possible that life evolved at that time and some remnant of it is clinging to the dry cold world still.

In 2020 Venus briefly became a potential candidate for life following the report of phosphine in the upper atmosphere. This is another gas that can result from living processes. However, the claim did not survive skeptical scrutiny of the scientific community. Reanalysis of the data showed that the original results were in error, and the phosphine signal was really sulfur dioxide, which is not associated with life. The phosphine signal was a false alarm. Dang.

So we are left for now with Mars, Europa and Enceladus, and of the three you can make a solid argument that Enceladus is the most encouraging. This new methane analysis just makes that case stronger.


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