April 18, 2017

expert reaction to hydrothermal processes found on one of saturn’s moons, enceladus

Publishing in Science, researchers have reported that the Cassini spacecraft has detected molecular hydrogen on Enceladus, one of Saturn’s moons.

Prof Jeffrey Kargel, Senior Research Scientist at the University of Arizona, said:

“The new research by Hunter Waite and colleagues is a breakthrough in the molecular analysis of Enceladus’ plumes.  The other components mentioned in the article (water, methane, ammonia, and carbon dioxide) had been detected before, and molecular hydrogen had been suspected to exist, but now there is a detection of it.  Not only that, but hydrogen is among the most abundant components of the plume emissions.  In fact, it is probably one of the volatiles most responsible for the rapid jetting of water and gases from the interior.  It seems likely that the rate of hydrogen generation in the interior partly controls the rate of jetting of the plumes.

“As the authors model and describe, the hydrogen gas does indeed appear to be a telltale signature of hydrothermal activity occurring on the seafloor.

“This finding does not mean that life exists there, but it makes life more plausible and potentially quite abundant if a fraction of the hydrogen is used to drive biology. The combination of volatiles is extremely interesting with regard to potential biology.

“It’s a very exciting finding, and for me it is very special in that Enceladus was the first extraterrestrial object that I studied scientifically when I was a graduate student. My first work was to show how a fracture and crater chain on Enceladus, as imaged by Voyager 2, appears to be the site of explosive venting of gases and water, and now we see in more detail than before that this process is occurring and what is driving it.”

Prof Andrew Coates, Professor of Physics at UCL, said:

“This is an exciting and remarkable result which shows that Enceladus may actually be habitable. This follows a number of key discoveries on Enceladus from the Cassini mission, which has shown the existence of the water-rich plume jetting into space through cracks in the moon’s icy surface, a sub-surface ocean, sodium indicating the ocean may be salty, and silicates indicating possible hydrothermal vent activity on the ocean floor. Now, Cassini has shown the existence of molecular hydrogen in the plume, which indicates a chemical imbalance within Enceladus which would be a source of energy.

“We know that the four requirements for life as we know it are liquid water, the right chemistry, a source of energy and enough time for life to develop. But now, we know that 3 of the 4 conditions are there on Enceladus – and this distant moon now joins Mars and Europa as the best potential locations for life beyond Earth in our solar system.”

Dr David Clements, Astrophysicist at Imperial College London, said:

“This is very interesting, though there are often alternate explanations. We have, however, long suspected that hydrothermal processes are behind the Enceladus plumes and the liquid ocean that fuels them, so this result is fully consistent with that picture.

“This discovery does not mean that life exists on Enceladus, but it is a step on the way to that result. We need to know much more about the molecular species coming out of Enceladus and, ideally, that are inside it before we can make such claims. Cassini is a great resource for some of this work, but observations from the Earth can also help with this.

“Since we already suspected the presence of hydrothermal vents this discovery isn’t a shock, but it’s great to see that idea being supported by new data.

“It doesn’t really tell us anything about how life started on Earth. The recent paper in Nature about signs of early fossilised micro-organisms around at the site of an ancient hydrothermal vent, up to 4.28 billion years old, says rather more about that:

http://www.nature.com/nature/journal/v543/n7643/full/nature21377.html

“But it is great to see confirmation that similar hydrothermal processes are at work elsewhere in the Solar System.”

Prof David Rothery, Professor of Planetary Geosciences, The Open University, said:

“Life has NOT been discovered on Enceladus, but we do now have the last piece of evidence needed to demonstrate that life is possible there.

“A paper published in Science, and a NASA press conference, report the detection of molecules of hydrogen in the plume erupting through the cracks in Enceladus’s ice. This hydrogen is a necessary by-product of the chemical reactions between heated water and rock at the floor of Enceladus’s internal ocean, but it was not possible for the Cassini probe (orbiting Saturn) to detect this until its final and closest dive through the plumes in October 2015.

“Now we know that hydrogen is present, this shows that a chemical reaction between dissolved carbon dioxide and dissolved hydrogen can take place, to make methane and water:

CO2 + 4H2 = CH4 + H2O

“This reaction gives out energy, and it can be used by ‘methanogenic’ microbes to fuel their metabolisms. For example, such methanogens are abundant around hot vents (hydrothermal vents) on the floor of the Earth’s oceans. This is life that needs neither oxygen nor sunlight, and may be the form in which life on Earth began, before some of it adapted to other conditions.

“‘Methanogens’ are microorganisms that produce methane as a metabolic by product in anoxic conditions (i.e. where there is no free oxygen). On Earth the only known methanogens belong to the domain known as ‘archaea’ (thus they are not ‘bacteria’). Archaea and Bacteria are equally-ranked taxonomic divisions of life on Earth, sufficiently different genetically to be classified separately, but are both simple, single-celled organisms (i.e. ‘microbes’ or ‘microorganisms’).

“Now we know that ALL the necessary conditions to support microbial life exist inside Enceladus, it is important to find out whether or not life DOES exist there. Why? Because if there is life in Enceladus’s internal ocean, then it probably began there. At present, we know of only ONE genesis of life (the one that led to us), so it is still possible that life could be an incredibly rare fluke. If we knew that life had started independently in two places in our Solar System, then we could be pretty confident that life also got started on some of the tens of billions of planets and moons around other stars in our galaxy that also have the right conditions (are ‘habitable’).

“Key facts:

Enceladus is a 502 km diameter moon of Saturn

Icy surface, rocky interior, with an ocean sandwiched between the two. Tidal heating of the rock prevents the ocean from freezing.

Discovered 28 Aug 1789 by British astronomer William Herschel

Named 1847 by his son, John Herschel (also an astronomer0

Cassini orbiter (NASA in partnership with ESA) went into orbit about Saturn 2005, and quickly discovered plumes of water vapour venting to space from cracks near Enceladus’s south pole

Cassini’s multiple fly-pasts of Enceladus were adjusted to let it observe and sample these plumes.

Plume contents found to be mainly water (tiny ice particles) with traces of methane, ammonia, carbon monoxide, carbon dioxide and various salts and simple organic molecules (meaning carbon compounds, this does NOT imply life).

Later ‘silica nanoparticles’ were found in the plume, which is what we would expect if the hot rocky interior was reacting chemically with alkaline water.

This should release hydrogen, but that had not been detected (was not detectable during normal operations).

During Cassini’s final dive through the plumes on 28 Oct 2015, its mass spectrometer was operated in a special mode allowing it to detect hydrogen. The results of that study are finally published today, and show that hydrogen IS present. This is what makes methanogenic life possible.”

* ‘Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes’ by Waite et al. will be published in Science at 19:00 UK time on Thursday 13th April, which is also when the embargo will lift. 

Declared interests

None declared