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.”

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.

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.

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

Long ago in a far-away career, I taught a short-course about then-current theories on the origin of life.  The lab portion of the course centered on the 1952 Miller-Urey experiment the first demonstration that amino acids could be produced abiotically.

Imagine my surprise when I learned that Miller’s original lab apparatus is on exhibit at the Denver Museum of Nature and Science, where I volunteer now.

The diagram’s notes describe the basic experiment.  Load up the system with whatever gases you think might have been in the primeval atmosphere.  Start cooling water running through the condenser (double-wall tubing below the upper sphere) and gently heat the water sample you’ve put in the bottom sphere.  Vapors travel up the tube into the top sphere where there’s a spark arcing between two electrodes (the black lines at 45º).  Water vapor passing by sweeps any gas-spark reaction products back down to the bottom sphere.

Let the whole thing stew for a while (Miller ran his for a week, we let ours go for two), then draw off and analyze a sample of the solution in the bottom sphere.  In Stanley’s day his analytical techniques found 5 amino acids.  In 1971 (I think) we found (I think) 8 or 9.  More recent work-ups of Miller’s sealed original samples found 25, including all 20 considered essential to life.  So yeah, if you supply enough energy to a methane-ammonia-water system (Miller added hydrogen to that; we didn’t, for safety reasons) you can make the building blocks for proteins.

The experiment has been repeated probably thousands of times by different researchers in the last half-century.  Some replications were duplicates of Miller’s, some started with recipes derived from other theories about what Earth’s early atmosphere looked like.

And there’s the problem.  In Miller’s day we thought that Earth’s atmosphere was basically comet-tail concentrate.  That’d be mostly water vapor along with a couple of volatiles like methane and ammonia.  Later on we realized that much of our atmosphere is volcano belch — a hodgepodge of carbon dioxide, carbon monoxide, methane, hydrogen, sulfur gases, nitrogen, argon, helium, various acids, multiple kinds of rock dust….  Some of that is left-overs from Earth’s initial stages; some has been generated by subsequent geological processes like serpentinization, which can generate both methane and hydrogen.

If you’re running a Miller experiment you’re free to load the apparatus with whatever mixture you think other scientists might think is reasonable for an Earth on the verge of Biology.  No oxygen, though — the chemistry of ancient rocks rules out significant atmospheric O₂ before about 3½ billion years ago.

Now the researchers are playing variations on the theme, asking whether conditions on (or within) Titan could also generate complex compounds that given a billion years could self-organize into anything we’d recognize as life.  So what did Titan’s atmosphere look like a few billion years ago?

That’s a toughie, because we don’t have on-the-ground (or out-of-the-ground) data like we have for Earth.  We’ll have to make do with theory, which starts with this chart.

At any given temperature you can calculate the average energy per gas molecule (any kind of gas).  Combine that with the known mass of a specific kind of molecule and you can compute its speed.

On any given world you can calculate the minimum speed (the escape velocity) that an object (rocket, rock or gas molecule) needs to have in order to overcome the world’s gravitational pull.

The chart combines both calculations for some important molecules for worlds in the Solar System that have atmospheres.  For instance, Earth’s average temperature (give or take a few dozen degrees) is  300ºK=27ºC≈80ºF.  From the chart, hydrogen and helium should be able to (and do) leave our atmosphere quickly.  However, Earth’s gravity is sufficient to hold onto its original dowry of the heavier species.  By contrast, the four massive planets would have to warm up by hundreds of degrees before they lose even the light gases.

Sure enough, Titan’s atmosphere is mostly nitrogen.  The astronomers measure its methane and hydrogen content in parts per thousand but those concentrations aren’t the same going from top to bottom of Titan’s atmosphere.  Therein lies an intriguing tale, but it’ll have to wait for the next post.

~~ Rich Olcott