A better idea for Life on Titan

Oh, I do love it when a theoretician comes in from the dugout and turns the game around.  Billy Beane did that for the Oakland baseball team (see Moneyball, the book or the movie).  Now a team led by James Stevenson in Dr Paulette Clancy’s lab at Cornell has done it for the scientific game of “What’s Life?”

Yeah, we’ve all seen the functional criteria: metabolism, self-regulation, growth, responsiveness, reproduction.  But structurally, everyone since Robert Hooke has known that living organisms are made of cells.  For the past 60 years we’ve known that those cells wear a cell membrane, a flexibly fluid two-layer construction of partly-oily molecules designed to separate watery cell contents from watery outside.

The standard membrane model is on the left in these sketches.  The zigzags represent long (15-20 carbons) hydrocarbon chains (the oily part); the red circles they’re bonded to stand for phosphate-containing groups that prefer a watery environment.  The polka dots are salts and other molecules floating in water.  The whole thing depends on  “oil and water don’t mix.”

ambipolar
Alternative membrane structures
(diagram on the right adapted from the Stevenson paper)

But Titan’s liquid environment is hydrocarbons, non-polar and therefor inhospitable to dissolved salts.  In a year-ago post I followed other people in proposing that cells living in Titan’s lakes might use an inverted membrane structure (the middle sketch).  It’d separate oily inside from oily outside by interposing a thin layer of watery.

That might work on worlds whose temperature range matches ours.  Stevenson and his team pointed out that it’s seriously unworkable on cold worlds like Titan (surface temp -290ºF).  The watery parts would be frozen granite-hard and the oily parts would be stiff as high-grade candle wax.  If there are living cells on Titan, they can’t use either two-layer membrane design as they move, grow and do those other life-ish things.

conga-line
Acrylonitrile molecules
in ambipolar array
(adapted from Stevenson)

Stevenson’s team asked the next question, “What else might work?”  They decided to investigate single-layer structures with no salty component at all.  Such membranes could be held together by electric forces between charges that are slightly separated within the same ambipolar molecule (right-most sketch of the three above). For instance, a nitrogen atom holds onto its electrons more tightly than a carbon atom does, so a bonded C≡N pair will be slightly negative on the N side, slightly positive on the C side.

And to avoid the candle-wax problem the tails on those molecules would have to be short.

Titan’s atmosphere is 98% nitrogen and most of the rest is methane and hydrogen, so the group looked at nine ambipolar nitrogen-carbon-hydrogen molecules with short tails.  For each compound they asked, “Would a membrane made of this stuff be stable on Titan?  If so, would that membrane be flexible?”

Those questions are hard to answer experimentally (-290ºF is cold) so the team resorted to advanced molecular dynamics simulation programs that they just happened to have lying around because that’s what the authors do in their day-jobs.

Basically, what the programs do is arrange some virtual molecules (including solvent) in a starting configuration, then let them move around step-by-step under the influence of their mutual attractions and repulsions.  Meanwhile, the programs keep track of the total energy (and a few other things) for the entire assemblage.  Run the simulation until things settle down (if they do); see what virtually happens.

acrylonitrile-azotosome
Virtual acrylonitrile vesicle
(also from Stevenson)

In many (not all) of the computer runs, the molecules under test did indeed form a more-or-less regular membrane floating in the “solvent.”  Only some were “stable,” with calculated energies indicating they’d hold together for days or longer.  Some would even be able to form hollow spheres (vesicles) at least as large as a small virus.  Significantly, “flexibility” values for the stable membranes are in the same range as Earthly cell membranes.

It’s an exciting paper if you’re interested in alien life forms.  Among other things, it suggests that astronomers can’t limit their surveys to planets that exhibit signs of atmospheric O2.

But Life also depends on information storage and transfer.  Earth uses DNA, a huge polar molecule unsuited to Life on Titan.  How might Titan’s Life handle that problem?

~~ Rich Olcott

Bilayer membranes - Earth-standard and reversed

The basis for life on Titan — maybe

When the Huygens probe flashed us those images of lakes on Saturn’s moon Titan, people chattered that maybe there’s life in those hydrocarbon “waters.”

Huygens descending on Titan (image courtesy NASA/JPL-Caltech)
Huygens descending on Titan (image courtesy NASA/JPL-Caltech)

If there is life there, we might not even recognize it as such. Not just because of the frigid temperatures and other reasons laid out in en.wikipedia.org/wiki/Life_on_Titan, but for a couple of reasons having to do with the physical chemistry of solvents.

Water is a polar solvent, good at dissolving salts and other substances in which centers of positive and negative charge are in different parts of the molecule. Conversely, water molecules interact so strongly with each other that interspersed hydrocarbons and other non-polar molecules are forced away and out of solution. Hence the existence of oil slicks … and cell membranes.

Every kind of life on Earth, or at least everything that a biologist would be willing to call life, is composed of units whose integrity depends upon hydrocarbon moieties (molecules that contain significant amounts of hydrocarbon structure) being forced together in escape from a polar environment.

For bacteria and multi-cellular life forms, the boundary between interior and exterior is the cell membrane (see diagram), two layers of two-tailed molecules laid tail-to-tail with their non polar (black) hydrocarbon chains sandwiched between negative (red) polar groups that face out of and into the polar (red and blue) cell. Bilayer membranes - Earth-standard and reversedFurthermore, our cellular life also depends upon a whole collection of two-layer membranes that isolate different metabolic functions within the cell — respiration over here, protein construction over there, and so forth.

By some definitions we have smaller kinds of life, too: viruses and phages. Many viruses (e.g., herpes) have a non-polar fatty coating. Others make do with proteins to hide their DNA.  However, biochemistry tells us that these structural proteins are almost certainly held together in large part by patches of non-polar regions with precisely matching shapes.

However, Titan’s surface is dominated by a hydrocarbon solvent, a liquid mix of methane and ethane, that behaves very differently from water. Critically, hydrocarbon solvents do not dissolve water and other polar materials. The amino acids from which we build our proteins, the nucleic acids from which we build our RNA and DNA, even the carbohydrate groups from which plants build sugars and cellulose — all are essentially insoluble in hydrocarbons.

If lightning or some other process were to generate some nucleic acids in a Titan lake (as in the Miller-Urey experiment, see en.wikipedia.org/wiki/Miller_experiment), those molecules would immediately aggregate and fall as a sludgy solid onto the lake floor. There’d be no opportunity for those small molecules to interact with each other, much less find some amino acids to tie together to produce a protein. Life as we know it could not begin.

Well, how about a non-polar kind of life? The properties of hydrocarbon solvents permit two possibilities, both of which are very strange from an Earth-life perspective.

The easier one to visualize turns that double-layered cell membrane inside out. A Titanic cell membrane could be a sandwich with a layer of polar stuff between two non-polar layers. Given that structure, the cell’s internal non-polar metabolic processes could operate in isolation from the non-polar outside, just as our cell membranes isolate our watery internal cellular metabolism from our watery outside. A reversed cell membrane on a Titanic cell would wrap around some very interesting biochemistry.

But things could be even stranger. All hydrocarbons can intermix in all proportions with all hydrocarbons. That’s why petroleum crude is such a complex mix, and why different crudes break out differently at the refinery.  Any non-polar molecule can slide in between any other hydrocarbon molecules with very little effort.  On Titan, then, non-polar materials can diffuse freely throughout those ethane seas.

Moreover, liquid hydrocarbons have very low surface tension compared to water.  At the surface of a pool or droplet of water, those H2O molecules cling to each other so tightly that another object must exert force to get between them and into the bulk liquid.  The threshold force, measured by the surface tension, is so high for water that pond skaters and similar bugs can live their lives on a pond rather than in it.  In contrast, surface tension for a liquid hydrocarbon is only one-third that of water.

What’s important here is that surface tension is the force that works to minimize the surface to-volume ratio of a blob of liquid.  The form with the smallest possible ratio is a sphere.  Sure enough, small droplets of water are spherical.  Hydrocarbon fluids, with their much lower surface tension, tend to accept looser forms.  Water on a tarry surface beads up; oil on a wet surface spreads out.  Scientists think that water’s powerful sphere forming propensity was crucial in creating proto-cells during Earth-life’s early stages.

Suppose that Titan’s hydrocarbon life doesn’t depend on nearly-spherical cells.  Maybe Titan life has no cell boundary as we know it. Titan’s “biology” could be one titanic (in both senses) “cell” with different metabolic processes isolated by geography rather than by membranes the way Earth life does it.

Maybe the lakes closest to Titan’s equator generate long-chain hydrocarbons.  Maybe another set of lakes links those molecules to form complex benzenes and graphenes that catalyze reactions in still other lakes.

Maybe Titan’s rivers and streams carry information the way our nerve cells do.

Maybe Titan thinks.

I wonder what Titan thought of the Huygens probe.

~~ Rich Olcott