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Modeling Life On Titan

Modeling Life On Titan

Lifeforms that live off methane instead of water are possible on Titan’s surface.

Is extraterrestrial life on Titan possible? A research team lead by James Stevenson at Cornell University have recently published a paper that claims to show that a cell wall, essential for carbon-based life as we know it, could also form in the methane-and-nitrogen-rich, ungodly-cold conditions found on the surface of Titan.

Cells in water

Cell walls are essential to life. Here on Earth, cell walls are formed as a result of a spontaneous action performed by lipid molecules (aka, fat/soap molecules) when in contact with water – since lipids contain a polar ‘head’ and a long, non-polar ‘tail’, the polar end faces outwards, towards the water, while the non-polar end faces inwards, away from the water (non-polar molecules hate polar ones! This is why oil and water do not mix). This is how soap bubbles form here on Earth – you’re literally, mostly, made of soap and water!

However, on Titan, while there is definitely no shortage of non-polar molecules (all hydrocarbons are non-polar), the oxygen component of lipids, which on Earth comes from the water, is frozen more than solid on Titan.

Cells in nitrogen

Fortunately, though, there is also an abundance of nitrogen on Titan, in its atmosphere, and nitrogen also forms polar compounds.

And the Cassini probe was able to detect some of these compounds:

Titan, extraterrestrial, cells, nitrogen, carbon, cyanide, lipids, bubble, molecule

Don’t get it? The paragraphs in italic at the bottom of the article will help you.

The challenge now was to figure out which of the above molecules would spontaneously form ‘bubbles’ in a sea of liquid methane at temperatures only 100 K or so above absolute zero.

Surprisingly, the researchers were able to render ‘bubbles’ from a handful of the given molecules.

By far the best candidate was the acrylonitrile molecule (third molecule down from the top). It was most stable in the Titanic conditions, and it formed ‘bubbles’ most spontaneously in said conditions.

Perfect!

Cyanide’s the key

Let’s see how this is possible. On Earth, the polar end of the lipids used to form the cell walls come from the oxygen in water. On Titan, it seems, the polar end of the acrylonitriles that could possibly form in the methane seas comes from the nitrogen in the cyanide (cyanide = ‘CN’) that floats around as an aerosol in the atmosphere.

On Earth, the stability of the lipid ‘bubble’ derives from the non-polar part of the molecules. On Titan, that stability appears to be derived from the polar cyanide end of the much shorter acrylonitride molecules.

Oh, and these ‘bubbles’, called ‘liposomes’ for Earth, are called ‘azotosomes’ for Titan.

Titan, extraterrestrial, cells, nitrogen, carbon, cyanide, lipids, bubble, molecule

Now, of course, the next step here is to model a way in which Titan could possibly form nucleotides, proteins, and enzymes using only carbon, hydrogen, and nitrogen.

And then it’d be spectacular if we could actually find such lifeforms on Titan.

Explain a little more please?
For those that didn’t take Chemistry in school: ‘C’ = carbon, ‘H’ = hydrogen, ‘N’ = nitrogen; ‘HCN’ = hydrogen cyanide (‘CN’ = cyanide – the ‘poison’ in rat poison; and smells like almonds); ‘ppm’ = ‘parts per million’ (which means, if you collect a million molecules from a given system, beit air, a liquid, or solid – in this case, Titan’s atmosphere – then, in the case of ‘HCN’, you will expect to find 200 of those million molecules to be of ‘HCN’). The lines denote chemical bonds (you’ll notice that only the bonds for carbon are shown – that’s how it’s done in Organic Chemistry; the chemistry of the carbon atom (yes, carbon has its very own branch of Chemistry set aside just for itself)); one ‘line’ denotes a single electron from each atom being shared in the bond, two denotes two electrons being shared, and three denotes three electrons from each of the two atoms in the bond.

Other things of note
Anywhere you see ‘C≡N’ in the middle column of the chart above, think ‘cyanide’, because that’s what it is. Thus, in names like ‘cyanoacetylene’, the ‘C≡N’ part is the ‘cyanide’, so the rest of the molecule is the ‘acetylene’ part (yes, acetylene is the same that’s in the name ‘acetylene torch’). In other cases, the word ‘nitrile’ is used; that’s just the ‘N’ inhabiting the very left end of the molecule – thus, in the case of ‘acrylonitrile’, the ‘acrylo’ part (the same ‘acrylo’ that’s the root for ‘acrylic paints’) is all the atoms and their bonds except that one nitrogen atom. Why organic chemists name these molecules the way they do is largely to do with how the atoms bond to form the molecule (and minimally to do with no good reason at all…). Lastly, the last two molecules have a <CH2>x part to them. The ‘x’ means that the number of <CH2> in that part of the molecule varies in number, because there is more than one molecule that belongs to that group and each is distinguished by the number of <CH2>’s it contains. In the case of the ‘primary amine’, this is because we’re talking about amino acids – those that only contain C, H, and N, that is (and yes, those be the same ‘amino acids’ that life here on Earth uses to form proteins and enzymes).

Reference
Stevenson, J., Lunine, J., & Clancy, P. (2015). Membrane alternatives in worlds without oxygen: Creation of an azotosome Science Advances, 1 (1) DOI: 10.1126/sciadv.1400067

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Jeffrey Daniels
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