What is Consuming Hydrogen and Acetylene on Titan?

Jun 03, 2010
This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Image credit: NASA/JPL

(PhysOrg.com) -- Two new papers based on data from NASA's Cassini spacecraft scrutinize the complex chemical activity on the surface of Saturn's moon Titan. While non-biological chemistry offers one possible explanation, some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan's surface. According to one theory put forth by astrobiologists, the signatures fulfill two important conditions necessary for a hypothesized "methane-based life."

One key finding comes from a paper online now in the journal Icarus that shows molecules flowing down through Titan's atmosphere and disappearing at the surface. Another paper online now in the Journal of Geophysical Research maps hydrocarbons on the Titan surface and finds a lack of .

This lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Center, Moffett Field, Calif., who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005. One interpretation of the acetylene data is that the hydrocarbon is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of their proposed mechanisms involved the consumption of hydrogen.

"We suggested hydrogen consumption because it's the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth," McKay said. "If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth."

To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on or produce it as a waste product. On Titan, where temperatures are around 90 Kelvin (minus 290 degrees Fahrenheit), a methane-based organism would have to use a substance that is liquid as its medium for living processes, but not water itself. Water is frozen solid on Titan's surface and much too cold to support life as we know it.

The list of liquid candidates is very short: liquid methane and related molecules like ethane. While liquid water is widely regarded as necessary for life, there has been extensive speculation published in the scientific literature that this is not a strict requirement.

The new hydrogen findings are consistent with conditions that could produce an exotic, methane-based life form, but do not definitively prove its existence, said Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md., who authored the paper on hydrogen.

Strobel, who studies the upper atmospheres of Saturn and Titan, analyzed data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer in his new paper. The paper describes densities of hydrogen in different parts of the atmosphere and the surface. Previous models had predicted that hydrogen molecules, a byproduct of ultraviolet sunlight breaking apart acetylene and methane molecules in the upper atmosphere, should be distributed fairly evenly throughout the atmospheric layers.

Strobel found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escape out of the upper atmosphere.

"It's as if you have a hose and you're squirting hydrogen onto the ground, but it's disappearing," Strobel said. "I didn't expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should 'float' to the top of the atmosphere and escape."

Strobel said it is not likely that hydrogen is being stored in a cave or underground space on Titan. The Titan surface is also so cold that a chemical process that involved a catalyst would be needed to convert hydrogen molecules and acetylene back to methane, even though overall there would be a net release of energy. The energy barrier could be overcome if there were an unknown mineral acting as the catalyst on Titan's surface.

The hydrocarbon mapping research, led by Roger Clark, a Cassini team scientist based at the U.S. Geological Survey in Denver, examines data from Cassini's visual and infrared mapping spectrometer. Scientists had expected the sun's interactions with chemicals in the atmosphere to produce acetylene that falls down to coat the Titan surface. But Cassini detected no acetylene on the surface.

In addition Cassini's spectrometer detected an absence of water ice on the Titan surface, but loads of benzene and another material, which appears to be an organic compound that scientists have not yet been able to identify. The findings lead scientists to believe that the organic compounds are shellacking over the water ice that makes up Titan's bedrock with a film of hydrocarbons at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as does on Earth.

"Titan's atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again," Clark said. "All that implies Titan is a dynamic place where organic chemistry is happening now."

The absence of detectable acetylene on the Titan surface can very well have a non-biological explanation, said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team. Allen is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Allen said one possibility is that sunlight or cosmic rays are transforming the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.

"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," Allen said. "We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results - for example, reactions involving mineral catalysts."

"These new results are surprising and exciting," said Linda Spilker, Cassini project scientist at JPL. "Cassini has many more flybys of Titan that might help us sort out just what is happening at the surface."

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User comments : 19

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Caliban
2.2 / 5 (5) Jun 03, 2010
The comment by NASA's Allen, that cosmic rays or sunlight might be chemically altering acetylene in Titan's atmosphere is ludicrous. Cosmic rays are extremely weakly-interacting, and solar radiation incident to Titan's atmosphere is quite weak as well- or so I would assume- correct me if I'm wrong.

From the description, my guess would be that the shellack that forms over the frozen water is the likeliest candidate for life form.

The definition of life should be: possessing an energy metabolism, self-organizing, self replicating. But even that might ultimately prove too restrictive a definition. There doesn't have to be carbon or H2O involved. That is just hydrocarbon chauvinism speaking.
Quantum_Conundrum
1.7 / 5 (3) Jun 03, 2010
Considering we've only had space probes directly monitoring Titan for a few years, I find it ridiculous that anyone could even claim to have a grasp on the overall composition or norms for this moon.
zevkirsh
2.5 / 5 (2) Jun 03, 2010
the answer here is obvious, it's the hydrogen and acetylene monsters. that's what they eat.
plastikman
2 / 5 (4) Jun 03, 2010
I wonder .. If someone lit a match on Titan would the moon blow up?
Shootist
2.7 / 5 (7) Jun 03, 2010
I wonder .. If someone lit a match on Titan would the moon blow up?


only if you supply the oxidizer.
ubavontuba
2.6 / 5 (11) Jun 03, 2010
I wonder .. If someone lit a match on Titan would the moon blow up?


only if you supply the oxidizer.
Oxygen is the third most common element in the solar system. I'm guessing that Titan's early history was replete with episodic combustion, which bound up all the free oxygen into water (hence all the ice).

I wonder if pockets of free oxygen might exist under the surface? Perhaps it might seep or break out occasionally? Might we find oxygen volcanoes and oxygen hot springs? That would be cool!

One day, BP will own it and accidentally blow open a huge oxygen reserve - and the whole thing will light up like a giant skyrocket! POOF!
Sean_W
2 / 5 (4) Jun 03, 2010
I wonder .. If someone lit a match on Titan would the moon blow up?


Not enough free oxygen and they don't sell matches at the gift shops.
Jim1965
1 / 5 (3) Jun 03, 2010
Hydrogen is great for fusion. To toss in one more possibility: our visiting e.t.'s harvest it to power their ships.
Titan's hydrogen consumption is most likely just basic chemistry of course, but nevertheless, e.t. is here, all you semi-clever skeptics.
Valentiinro
1 / 5 (3) Jun 03, 2010
I wonder .. If someone lit a match on Titan would the moon blow up?


Not enough free oxygen and they don't sell matches at the gift shops.


Oh, so that's why there was a tiny bit of the video that looked like a sign reading "Titanic Deals".
blengi
1 / 5 (2) Jun 04, 2010
My Hitchhiker's guide to the Metaverse entry (2008 ed.) -

Titan: A cryogaian uber world with methane soaked semi porous crust of cryonic ice corals quasi catalytically metabolising acethylene and other atmospheric products of photolysis into kelpy graphanelike biolaminates which pneumatically regulate this curious moon's subtle metabolism - 4.1 out of 10 on the GEI Index for transentropic cryptophenomena & sentience...
Gene_H
1 / 5 (1) Jun 04, 2010
What is Consuming Hydrogen and Acetylene on Titan?
I presume, physicists should learn chemistry first, as hydrogen and acetylene are well known to react mutually.

http://tinyurl.com/396fzhk
antialias
not rated yet Jun 04, 2010
hat cosmic rays or sunlight might be chemically altering acetylene in Titan's atmosphere is ludicrous.

The radiation could come from Saturn. Saturn emits 2.3 times the radiation that it gets from the sun. Like Jupiter it has intense radiation belts.

Oxygen is the third most common element in the solar system.

But it is not distributed evenly. During planet formation the heavier elements were (mostly) closer to the sun. The further out you go the less heavy elements you find (this is why you find so much hydrogen on Jupiter/Saturn/Neptune and Uranus)

So if you want to go mining for gold you should try Mercury ;-)
Skeptic_Heretic
1 / 5 (1) Jun 04, 2010
Hydrogen is great for fusion. To toss in one more possibility: our visiting e.t.'s harvest it to power their ships.
Titan's hydrogen consumption is most likely just basic chemistry of course, but nevertheless, e.t. is here, all you semi-clever skeptics.

The construction of life on Earth is "just basic chemistry".
I presume, physicists should learn chemistry first, as hydrogen and acetylene are well known to react mutually.

http://tinyurl.com/396fzhk

...when in the presence of oxygen. You need to read your source.
Gene_H
1 / 5 (2) Jun 04, 2010
...when in the presence of oxygen. You need to read your source.
How did you get it? Quote the sentence of my source, which supports your stance.
barakn
5 / 5 (4) Jun 04, 2010
I presume, physicists should learn chemistry first, as hydrogen and acetylene are well known to react mutually.
http://tinyurl.com/396fzhk

Maybe you should learn chemistry first, as you have provided a link to experiments performed at high partial pressures of both reactants at high temperature in the presence of metallic catalysts, i.e. very un-Titanlike conditions.
barakn
1 / 5 (1) Jun 04, 2010
I presume, physicists should learn chemistry first, as hydrogen and acetylene are well known to react mutually.

http://tinyurl.com/396fzhk

...when in the presence of oxygen. You need to read your source.

I read the source, and the reactions were not carried out in the presence of oxygen. Oxygen was used later to test for saturated hydrocarbons.
Skeptic_Heretic
5 / 5 (2) Jun 04, 2010
I read the source, and the reactions were not carried out in the presence of oxygen. Oxygen was used later to test for saturated hydrocarbons.

The reactions were carried out two ways. 1) in the presence of oxygen (for confirmation) 2) in the presence of a catalyzing agent.

The two do not hydrogenate without help. Second, the fact this paper is only relevant within a few degrees of 30 C. This research is not valid for the conditions in the above abstract. Which I think we're both in agreement on.
kevinrtrs
1.3 / 5 (3) Jun 09, 2010
Question:
Where does the acetylene and methane comes from, i.e. what is it's source on the moon?

How much hydrogen is lost to the surrounding space per year and how long can such a loss be sustained?

It would be very interesting to know the answers to these questions. For one it would certainly give an indication of the length of time Titan has been in existence.

Quantum_Conundrum
2 / 5 (4) Jun 10, 2010
antialias:

You offer that explanation as to why there is little oxygen in OSS, yet there seems to be plenty of methane and water ice in the comets; plus carbon is only 2 nucleons lighter than oxygen, so should not be significantly more likely to be in OSS than oxygen anyway.

Now if you take the mass of all of the comets and dwarf planets known and theorized to exist in the OSS, I would suspect that their mass may actually exceed that of all the terrestrial planets, asteroids, trojans, and rocky moons combined.

So what are comets and dwarf planets primarily composed of?

Dust(silica and metal), water ice(oxygen), and methane ice(carbon).

So in general, it doesn't add up. There's actually far more carbon and oxygen, and maybe even metals in the comets than there is in the gas giants....