Category: Astronomy

Three LIGOs make a Banana Slicer

On By rolcottIn Astronomy: Techniques, Gravitational astronomy, UncategorizedLeave a comment

Ponder for a moment what Space throws at you.  Photons of all sizes, of course —  infra-red ones that warm your skin, visible ones that show you the beach, ultra-violet ones that give you tan and sunburn.  Neutrinos and maybe dark matter particles that pass right through you without even pausing.  All of those act upon you in little bits at little places — gravity pervades you.  You can put up a parasol or step into a cave, but there’s no shielding yourself from gravity.

Gravity’s special character has implications for LIGOs.  A word first about words.  LIGO as a generic noun unwinds to Laser Interferometer Gravitational-Wave Observatory, a class of astronomical instruments. LIGO as a proper noun denotes a project that culminated in the construction of a specific pair of devices that went live in 2002.

That hardware wasn’t sensitive enough to detect the gravitational waves it was created to seek.  To improve the initial LIGO’s power and sensitivity, the LIGO infrastructure and organization morphed into the Advanced LIGO (aLIGO) project.  Concurrently, the LIGO instrument was upgraded and renamed.  No surprise, the instrument’s new name is aLIGO.  An early phase of aLIGO bore uncannily fortunate fruit with the Sept 14 gravitational wave detection.

Four other LIGOs are proposed, under construction or in operation around the world — KARGA in Japan, INDIGO in India, GEO600 in Germany and VIRGO in Italy.  Why so many, and why even consider space-borne LIGOs like LISA Pathfinder and eLISA?

Astronomers ask a series of questions of the Universe:

  • What objects are out there?
  • Where are they?
  • What are they doing?
  • Why are they doing that?

September’s aLIGO incident gave us a gratifyingly unexpected answer to the first question.  To the surprise of theoreticians, the detected event was the collision of two black holes, each of which was in a size range that current theory says shouldn’t be populated.  Even more surprising, such objects are apparently common enough to meet up, form binary pairs and eventually merge.

1 LIGO localizationThe second question is harder.  The best the aLIGO team could do was point to a “banana-shaped region” (their words, not mine) that covers about 1% of the sky.  The team marshaled a world-wide collaboration of observatories to scan that area (a huge search field by astronomical standards), looking for electromagnetic activities concurrent with  the event they’d seen.  Nobody saw any.  That was part of the evidence that this collision involved two black holes.  (If one or both of the objects had been something other than a black hole, the collision would have given off all kinds of photons.)

Why such poor localization?  Blame gravity’s pervasive character and Geometry.  With a telescope, any kind of telescope, you know which direction you’re looking.  Telescopes work only with photons that enter through the front; photons aimed at the back of the instrument stop there.

2 LIGO localizationIn contrast, a LIGO facility is (roughly speaking) omni-directional.  When a LIGO installation senses a gravitational pulse, it could be coming down from the visible sky or up through the Earth from the other hemisphere — one signal doesn’t carry the “which way?” information.  The diagram above shows that situation.  (The “chevron” is an image of the LIGO in Hanford WA.)  Models based on the signal from that pair of 4-km arms can narrow the source field to a “banana-shaped region,” but there’s still that 180o ambiguity.

The good news is that the LIGO project built not one but two installations, 2500 miles apart.  With two LIGOs (the second diagram) there’s enough information to resolve the ambiguity.  The two also serve as checks on each other — if one sees a signal that doesn’t show up at the other that’s probably a red herring that can be discarded.

3 LIGO localizationThe great “if only” is that the VIRGO installation in Italy was not recording data when the Hanford WA and Livingston LA saw that September signal.  With three recordings to reconcile, the aLIGO+VIRGO combination would have had enough information to slice that banana and localize the event precisely.

When the European Space Agency puts Evolved LISA (eLISA) in orbit (watch the animation, it’s cool) in 2034, there’ll be a million-kilometer triangle of spacecraft up there, slicing bananas all over the sky.

~~ Rich Olcott

aLIGO and eLISA: Tuning The Instrument

On By rolcottIn Astronomy: Techniques, Gravitational astronomy, UncategorizedLeave a comment

Oh, it’s good to see Big News in hard science get big attention in Big Media.  The LIGO story and Columbia’s Dr Brian Greene even made it to the Stephen Colbert Late Show.  Everyone chuckled at the final “boowee-POP” audio recording (simulation at 7:30 into this clip; get for-real traces and audio from this one).

There’s some serious science in those chirps, not to mention serious trouble for any alien civilization that happened to be too close to the astronomical event giving rise to them.

LIGO trace 3
Adapted from the announcement paper by Abbot et al

The peaks and valleys in the top LIGO traces represent successive spatial compression cycles generated by two massive bodies orbiting each other.  There’s one trace for each of the two LIGO installations.  The spectrograms beneath show relative intensity at each frequency.  Peaks arrived more rapidly in the last 100 milliseconds and the simulated sound rose in pitch because the orbits grew smaller and faster.  The audio’s final POP is what you get from a brief but big disturbance, like the one you hear when you plug a speaker into a live sound system.  This POP announced two black holes merging into one, converting the mass-energy of three suns into a gravitational jolt to the Universe.

Scientists have mentioned in interviews that LIGO has given us “an ear to the Universe.”  That’s true in several different <ahem> senses.  First, we’ve seen in earlier posts that gravitational physics is completely different from the electromagnetism that illuminates every kind of telescope that astronomers have ever used.  Second, black hole collisions generate signals in frequencies that are within our auditory range.  Finally, LIGO was purposely constructed to have peak sensitivity in just that frequency range.

Virtually every kind of phenomenon that physicists study has a characteristic size range and a characteristic frequency/duration range.  Sound waves, for instance, are in the audiophile’s beloved “20 to 20,000” cycles per second (Hz).  Put another way, one cycle of a sound wave will last something between 1/20 and 1/20,000 second (0.05-0.000 05 second).  The speed of sound is roughly 340 meters per second which puts sound’s characteristic wavelength range between 17 meters and 17 millimeters.

No physicist would be surprised to learn that humans evolved to be sensitive to sound-making things in that size range.  We can locate an oncoming predator by its roar or by the snapping twig it stepped on but we have to look around to spot a pesky but tiny mosquito.

So the greenish box in the chart below is all about sound waves.  The yellowish box gathers together the classes of phenomena scientists study using the electromagnetic spectrum.  For instance, we use infra-red light (characteristic time range 10-15-10-12 second) to look at (or cause) molecular vibrations.

RegimesWe can investigate things that take longer than an instrument’s characteristic time by making repeated measurements, but we can’t use the instrument to resolve successive events that happen more quickly than that.  We also can’t resolve events that take place much closer together than the instrument’s characteristic length.

The electromagnetic spectrum serves us well, but it has its limitations.  The most important is that there are classes of objects out there that neither emit nor absorb light in any of its forms.  Black holes, for one.  They’re potentially crucial to the birth and development of galaxies.  However, we have little hard data on them against which to test the plethora of ideas the theoreticians have come up with.

Dark matter is another.  We know it’s subject to gravity, but to our knowledge the only way it interacts with light is by gravitational lensing.  Most scientists working on dark matter wield Occam’s Razor to conclude it’s pretty simple stuff.  Harvard cosmologist Dr Lisa Randall has suggested that there may be two kinds, one of which collects in disks that clothe themselves in galaxies.

That’s where LIGO and its successors in the gray box will help.  Their sensitivity to gravitational effects will be crucial to our understanding of dark objects.  Characteristic times in tens and thousands of seconds are no problem nor are event sizes measured in kilometers, because astronomical bodies are big.

GrWave Detectors
Gravitational instrumentation, from Christopher Berry’s blog and Web page

This is only the beginning, folks, we ain’t seen nothin’ yet.

~~ Rich Olcott

LIGO: Gravity Waves Ain’t Gravitational Waves

On By rolcottIn Astronomy: Techniques, Gravitational astronomy, Physics: Black Holes and Relativity, Physics: Foundations of Measurement, Physics: Newton and Gravity, UncategorizedLeave a comment

Sometimes the media get sloppy.  OK, a lot of times, especially when the reporters don’t know what they’re writing about.  Despite many headlines that “LIGO detected gravity waves,” that’s just not so.  In fact, the LIGO team went to a great deal of trouble to ensure that gravity waves didn’t muck up their search for gravitational waves.

Spring2A wave happens in a system when a driving force and a restoring force take turns overshooting an equilibrium point AND the away-from-equilibrium-ness gets communicated around the system.  The system could be a bunch of springs tied together in a squeaky old bedframe, or labor and capital in an economic system, or the network of water molecules forming the ocean surface, or the fibers in the fabric of space (whatever those turn out to be).

If you  were to build a mathematical model of some wavery system you’d have to include those two forces plus quantitative descriptions of the thingies that do the moving and communicating.  If you don’t add anything else, the model will predict motion that cycles forever.  In reality, of course, there’s always something else that lets the system relax into equilibrium.

The something else could be a third force, maybe someone sitting on the bed, or government regulation in an economy, or reactant depletion for a chemical process.  But usually it’s friction of one sort or another — friction drains away energy of motion and converts it to heat.  Inside a spring, for instance, adjacent crystallites of metal rub against each other.  There appears to be very little friction in space — we can see starlight waves that have traveled for billions of years.

Physicists pay attention to waves because there are some general properties that apply to all of them.  For instance, in 1743 Jean-Baptiste le Rond d’Alembert proved there’s a strict relationship between a wave’s peakiness and its time behavior.  Furthermore, Jean-Baptiste Joseph Fourier (pre-Revolutionary France must have been hip-deep in physicist-mathematicians) showed that a wide variety of more-or-less periodic phenomena could be modeled as the sum of waves of differing frequency and amplitude.

Monsieur Fourier’s insight has had an immeasurable impact on our daily lives.  You can thank him any time you hear the word “frequency.”  From broadcast radio and digitally recorded music to time-series-based business forecasting to the mode-locked lasers in a LIGO device — none would exist without Fourier’s reasoning.

Gravity waves happen when a fluid is disturbed and the restoring force is gravity.  We’re talking physicist fluid here, which could be sea water or the atmosphere or solar plasma, anything where the constituent particles aren’t locked in place. Winds or mountain slopes or nuclear explosions push the fluid upwards, gravity pulls it back, and things wobble until friction dissipates that energy.

Gravitational waves are wobbles in gravity itself, or rather, wobbles in the shape of space.  According to General Relativity, mass exerts a tension-like force that squeezes together the spacetime immediately around it.  The more mass, the greater the tension.

Binary BH with AENAn isolated black hole is surrounded by an intense gravitational field and a corresponding compression of spacetime.  A pair of black holes orbiting each other sends out an alternating series of tensions, first high, then extremely high, then high…

Along any given direction from the pair you’d feel a pulsing gravitational field that varied above and below the average force attracting you to the pair.  From a distance and looking down at the orbital plane, if you could see the shape of space you’d see it was distorted by four interlocking spirals of high and low compression, all steadily expanding at the speed of light.

The LIGO team was very aware that the signal of a gravitational wave could be covered up by interfering signals from gravity waves — ocean tides, Earth tides, atmospheric disturbances, janitorial footsteps, you name it.  The design team arrayed each LIGO site with hundreds of “seismometers, accelerometers, microphones, magnetometers, radio receivers, power monitors and a cosmic ray detector.”  As the team processed the LIGO trace they accounted for artifacts that could have come from those sources.

So no, the LIGO team didn’t discover gravity waves, we’ve known about them for a century.  But they did detect the really interesting other kind.

~~ Rich Olcott

Would the CIA want a LIGO?

On By rolcottIn Astronomy: Techniques, Gravitational astronomy, Physics: Black Holes and Relativity, Physics: Electromagnetism, Physics: Fundamentals, Physics: Light and Relativity, UncategorizedLeave a comment

So I was telling a friend about the LIGO announcement, going on about how this new “device” will lead to a whole new kind of astronomy.  He suddenly got a far-away look in his eyes and said, “I wonder how many of these the CIA has.”

The CIA has a forest of antennas, but none of them can do what LIGO does.  That’s because of the physics of how it works, and what it can and cannot detect.  (If you’re new to this topic, please read last week’s post so you’ll be up to speed on what follows.  Oh, and then come back here.)

There are remarkable parallels between electromagnetism and gravity.  The ancients knew about electrostatics — amber rubbed by a piece of cat fur will attract shreds of dry grass.  They certainly knew about gravity, too.  But it wasn’t until 100 years after Newton wrote his Principia that Priestly and then Coulomb found that the electrostatic force law, F = ke·q1·q2 / r2, has the same form as Newton’s Law of Gravity, F = G·m1·m2 / r2. (F is the force between two bodies whose centers are distance r apart, the q‘s are their charges and the m‘s are their masses.)

Jim and AlAlmost a century later, James Clerk Maxwell (the bearded fellow at left) wrote down his electromagnetism equations that explain how light works.  Half a century later, Einstein did the same for gravity.

But interesting as the parallels may be, there are some fundamental differences between the two forces — fundamental enough that not even Einstein was able to tie the two together.

One difference is in their magnitudes.  Consider, for instance, two protons.  Running the numbers, I found that the gravitational force pulling them together is a factor of 1036 smaller than the electrostatic force pushing them apart.  If a physicist wanted to add up all the forces affecting a particular proton, he’d have to get everything else (nuclear strong force, nuclear weak force, electromagnetic, etc.) nailed down to better than one part in 1036 before he could even detect gravity.

But it’s worse — electromagnetism and gravity don’t even have the same shape.

Electromagneticwave3D
Electric (red) and magnetic (blue) fields in a linearly polarized light wave
(graphic from WikiMedia Commons, posted by Lookang and Fu-Kwun Hwang)

A word first about words.  Electrostatics is about pure straight-line-between-centers (longitudinal) attraction and repulsion — that’s Coulomb’s Law.  Electrodynamics is about the cross-wise (transverse) forces exerted by one moving charged particle on the motion of another one.  Those forces are summarized by combining Maxwell’s Equations with the Lorenz Force Law.  A moving charge gives rise to two distinct forces, electric and magnetic, that operate at right angles to each other.  The combined effect is called electromagnetism.

The effect of the electric force is to vibrate a charge along one direction transverse to the wave.  The magnetic force only affects moving charges; it acts to twist their transverse motion to be perpendicular to the wave.  An EM antenna system works by sensing charge flow as electrons move back and forth under the influence of the electric field.

Gravitostatics uses Newton’s Law to calculate longitudinal gravitational interaction between masses.  That works despite gravity’s relative weakness because all the astronomical bodies we know of appear to be electrically neutral — no electrostatic forces get in the way.  A gravimeter senses the strength of the local gravitostatic field.

Maxwell and EinsteinGravitodynamics is completely unlike electrodynamics.  Gravity’s transverse “force” doesn’t act to move a whole mass up and down like Maxwell’s picture at left.  Instead, as shown by Einstein’s picture, gravitational waves stretch and compress while leaving the center of mass in place. I put “force” in quotes because what’s being stretched and compressed is space itself.  See this video for a helpful visualization of a gravitational wave.

LIGO is neither a telescope nor an electromagnetic antenna.  It operates by detecting sudden drastic changes in the disposition of matter within a “small” region.  In LIGO’s Sept 14 observation, 1031 kilograms of black hole suddenly ceased to exist, converted to gravitational waves that spread throughout the Universe.  By comparison, the Hiroshima explosion released the energy of 10-6 kilograms.

Seismometers do a fine job of detecting nuclear explosions.  Hey, CIA, they’re a lot cheaper than LIGO.

~~ Rich Olcott

LIGO, a new kind of astronomy

On By rolcottIn Astronomy: Techniques, Gravitational astronomy, Physics: Black Holes and Relativity, Physics: Fundamentals, UncategorizedLeave a comment

Like thousands of physics geeks around the world, I was glued to the tube Thursday morning for the big LIGO (Laser Interferometer Gravitational-Wave Observatory) announcement.  As I watched the for-the-public videos (this is a good one), I was puzzled by one aspect of the LIGO setup.  The de-puzzling explanation spotlit just how different gravitational astronomy will be from what we’re used to.

There are two LIGO installations, 2500 miles apart, one near New Orleans and the other near Seattle.  Each one looks like a big L with steel-pipe arms 4 kilometers long.  By the way, both arms are evacuated to eliminate some sources of interference and a modest theoretical consideration.

LIGO3The experiment consists of shooting laser beams out along both arms, then comparing the returned beams.

Some background: Einstein conquered an apparent relativity paradox.  If Ethel on vehicle A is speeding (like, just shy of light-speed speeding) past Fred on vehicle B, Fred sees that Ethel’s yardstick appears to be shorter than his own yardstick.  Meanwhile, Ethel is quite sure that Fred’s yardstick is the shorter one.

Einstein explained that both observations are valid.  Fred and Ethel can agree with each other but only after each takes proper account of their relative motion.  “Proper account” is a calculation called the Lorenz transformation.   What Fred (for instance) should do is divide what he thinks is the length of Ethel’s yardstick by √[1-(v/c)²] to get her “proper” length.  (Her relative velocity is v, and c is the speed of light.)

Suppose Fred’s standing in the lab and Ethel’s riding a laser beam.  Here’s the puzzle: wouldn’t the same Fred/Ethel logic apply to LIGO?  Wouldn’t the same yardstick distortion affect both the interferometer apparatus and the laser beams?

Well, no, for two reasons.  First, the Lorenz effect doesn’t even apply, because the back-and-forth reflected laser beams are standing waves.  That means nothing is actually traveling.  Put another way, if Ethel rode that light wave she’d be standing as still as Fred.

The other reason is that the experiment is less about distance traveled and more about time of flight.

Suppose you’re one of a pair of photons (no, entanglement doesn’t enter into the game) that simultaneously traverse the interferometer’s beam-splitter mirror.  Your buddy goes down one arm, strikes the far-end mirror and comes back to the detector.  You take the same trip, but use the other arm.

The beam lengths are carefully adjusted so that under normal circumstances, when the two of you reach the detector you’re out of step.   You peak when your buddy troughs and vice-versa.  The waves cancel and the detector sees no light.

Now a gravitational wave passes by (red arcs in the diagram).  In general, the wave will affect the two arms differently.  In the optimal case, the wave front hits one arm broadside but cuts across the perpendicular one.  Suppose the wave is in a space-compression phase when it hits.  The broadside arm, beam AND apparatus, is shortened relative to the other one which barely sees the wave at all.

The local speed of light (miles per second) in a vacuum is constant.  Where space is compressed, the miles per second don’t change but the miles get smaller.  The light wave slows down relative to the uncompressed laboratory reference frame.  As a result, your buddy in the compressed arm takes just a leetle longer than you do to complete his trip to the detector.  Now the two of you are in-step.  The detector sees light, there is great rejoicing and Kip Thorne gets his Nobel Prize.

But the other wonderful thing is, LIGO and neutrino astronomy are humanity’s first fundamentally new ways to investigate our off-planet Universe.  Ever since Galileo trained his crude telescope on Jupiter the astronomers have been using electromagnetic radiation for that purpose – first visible light, then infra-red and radio waves.  In 1964 we added microwave astronomy to the list.  Later on we put up satellites that gave us the UV and gamma-ray skies.

The astronomers have been incredibly ingenious in wringing information out of every photon, but when you look back it’s all photons.  Gravitational astronomy offers a whole new path to new phenomena.  Who knows what we’ll see.

~~ Rich Olcott

Squeezing past Newton’s infinity

On By rolcottIn Astronomy: Planetary Science, Physics: Fundamentals, Physics: Newton and GravityLeave a comment

One of the most powerful moments in musical theater — Philip Quast Quastin his Les Miz role of Inspector Javert, praising the stars for the steadfastness and reverence for law that they signify for him.  The performance is well worth a listen.

Javert’s certitude came from Newton’s sublimely reliable mechanics — the notion that every star’s and planet’s motion is controlled by a single law, F~(1/r2).  The law says that the attractive force between any pair of bodies is inversely proportional to the square of the distance between their centers.  But as Javert’s steel-clad resolve hid a fatal spark of mercy towards Jean Valjean, so Newton’s clockworks hold catastrophe at their axles.

Newton’s gravity law has a problem.  As the distance approaches zero, the predicted force approaches infinity.  The law demands that nearby objects accelerate relentlessly at each other to collide with infinite force, after which their combined mass attracts other objects.  In time, everything must collapse in a reverse of The Big Bang.

Victor Hugo wrote Les Misérables about 180 years after Newton published his Principia.  A decade before Hugo’s book, Professeur Édouard Roche (pronounced rōsh) solved at least part of Newton’s problem.

Roche realized that Newton had made an important but crucial simplification.  Early in the Principia, he’d proven that for many purposes you can treat an entire object as though all of its mass were concentrated at a single point (the “center of mass”).  But in real gravity problems every particle of one object exerts an attraction for every particle of the other.

That distinction makes no difference when the two objects are far apart.  However, when they’re close together there are actually two opposing forces in play:

  • gravity, which preferentially affects the closest particles, and
  • tension, which maintains the integrity of each structure.
contact_binary_1
Binary star pair demonstrating Roche lobes, image courtesy of Cronodon.com

Roche noted that the gravity fields of any pair of objects must overlap.  There will always be a point on the line between them where a particle will be tugged equally in either direction.  If two bodies are close and one or both are fluid (gases and plasmas are fluid in this sense), the tension force is a weak competitor.  The partner with the less intense gravity field will lose material across that bridge to the other partner. Binary star systems often evolve by draining rather than collision.

Now suppose both bodies are solid.  Tension’s game is much stronger.  Nonetheless, as they approach each other gravity will eventually start ripping chunks off of one or both objects.  The only question is the size of the chunks — friable materials like ices will probably yield small flakes, as opposed to larger lumps made from silicates and other rocky materials.  Roche described the final stage of the process, where the less-massive body shatters completely.  The famous rings of Saturn and the less famous rings of Neptune, Uranus and Jupiter all appear to have been formed by this mechanism.

Roche was even able to calculate how close the bodies need to be for that final stage to occur. The threshold, now called the Roche Limit, depends on the size and mass of each body. You can get more detail here.

Klingon3And then there’s spaghettification.  That’s a non-relativistic tidal phenomenon that occurs near an extremely dense body like a neutron star or a black hole.  Because these objects pack an enormous amount of mass into a very small volume, the force of gravity at a close-in point is significantly greater than the force just a little bit further out. Any object, say a Klingon Warbird that ignored peril markings on a space map (Klingons view warnings as personal challenges), would find itself stretched like a noodle between high gravity on the side near the black hole and lower gravity on the opposite side.  (In this cartoon, notice how the stretching doesn’t care which way the pin-wheeling ship is pointed.)

Nature abhors singularities.  Where a mathematical model like Newton’s gravity law predicts an infinity, Nature generally says, “You forgot something.”  Newton assumed that objects collide as coherent units.  Real bodies drain, crumble, or deform to slide together.  Look to the apparent singularities to find new physics.

~~ Rich Olcott

Bilayer membranes - Earth-standard and reversed

The basis for life on Titan — maybe

On By rolcottIn Astronomy: Planetary Science, Chemistry, Uncategorized1 Comment

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