Looking for a signature

Finding unequivocal evidence of extraterrestrial life is not as easy as science fiction makes it sound.

The problem is, we're biased toward detecting terrestrial life -- living things that have a similar chemistry to the familiar life forms here on Earth.  It's understandable; I mean, why would we not be?  While there is a great diversity of species here on our planet (something on the order of nine million extant, at an estimate), they all share the same basic biochemistry, including:

• ATP as an energy driver
• some form of sugar-fueled cellular respiration to produce that ATP
• phospholipid bilayers as cell membranes, and (for eukaryotes) for the internal membranes that compartmentalize the cell
• proteins to facilitate structure, movement, and catalysis (the latter are called enzymes)
• nucleic acids such as DNA and RNA for information storage and retrieval
• lipids for long-term energy storage

While there are obviously different twists on how exactly these things work, those features are common to just about all life on Earth.  (Interestingly, a 2010 paper claiming a microbe had been discovered in California's Mono Lake that incorporates arsenic into the backbone of its DNA instead of phosphorus was just retracted by the journal Nature -- although the retraction is controversial, and the authors are still defending their work as valid.)

The question remains unsolved, therefore, of the extent to which the genesis and evolution of life are constrained -- by which we mean that the pathways taken by biology might be expected to repeat on other Earth-like worlds.  (Or, to use Stephen Jay Gould's pithy phrase, we might find that evolution would produce similar forms again here on Earth if we were somehow able to "replay the tape of life.")  Would living things, down to the biochemical level, be at least somewhat like terrestrial life, or would they be entirely different?

To be fair to the science fiction writers, there have been instances where they've made a significant effort to consider what life "not as we know it" might look like.  Star Trek's Horta ("The Devil in the Dark"), Crystalline Entity ("Silicon Avatar"), and Tholians ("The Tholian Web") come to mind, as well as Doctor Who's Vashta Nerada ("Silence in the Library"), Boneless ("Flatline"), Not-Things ("Wild Blue Yonder"), and Midnight Entity ("Midnight").  

Even Lost in Space gave it a creditable try with the Bubble Creatures in "The Derelict."

The problem is more complex than that, though.  Since we can't actually go to other planets and search for living things -- even going to planets and moons in our own Solar System is crazy expensive and fraught with difficulties -- we're stuck with looking for biosignatures, traces (either structural or chemical) that show unequivocal evidence of being created by living things.

The sticking point is that word unequivocal.  Two good examples of this came to light in the last couple of weeks, one of them (from the standpoint of people like me who would love nothing better than to find out we're not alone in the universe) bad news, and the other one -- at least tentatively -- good news.

Let's start with the bad news first.

Back in 2005, NASA's Cassini mission spotted something exciting -- the presence of organic molecules in water-rich plumes erupted from Saturn's icy moon Enceladus.  The surmise was that these plumes were created by pressure in the moon's interior, where water might be kept liquid by tidal deformation from the huge gas giant's gravitational pull.  Now, organic doesn't mean produced by life; it's a chemistry term meaning "containing carbon and hydrogen."  (Formaldehyde, for example, is an organic compound, and has been found by its spectral signature in interstellar gas, and no one's claiming that it was made by aliens.  At least, no one I'd want to have a conversation with.)  On the other hand, lots of organic compounds are made by living things, so their presence in Enceladus's geysers certainly seemed to raise at least the possibility that underneath the ice, there might be a watery ocean that harbored life.

Well, a study led by scientists from Italy's Istituto Nazionale di Astrofisica e Planetologia Spaziale has found another possible explanation.  The organic molecules could be formed right there on the surface by radiation funneled in by Saturn's magnetic field, then blasted off the surface -- i.e., they didn't come from an interior ocean at all.  "While the identification of complex organic molecules in Enceladus's environment remains an important clue in assessing the moon's habitability, the results demonstrate that radiation-driven chemistry on the surface and in the plumes could also create these molecules," said Grace Richards, who led the study.  "Molecules considered prebiotic could plausibly form in situ through radiation processing, rather than necessarily originating from the subsurface ocean.  Although this doesn't rule out the possibility that Enceladus's ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes."

The second paper had more hopeful news.  It comes out of Stony Brook University, and is an analysis of some mudstones found in Jezero Crater on Mars by the Perseverance rover.  The analysis -- done long-distance, obviously -- found traces of organic material, along with ferrous sulfate and ferric sulfide.  Most interestingly, the organic material seems to have undergone post-deposition redox reactions; redox reactions are the mechanism by which all terrestrial life forms harvest energy for their life processes (cellular respiration is, basically, one long string of redox reactions).  The minerals, the researchers say, "challenge some aspects of a purely abiotic explanation" -- cautious science-speak for, "Life?  Yeah, could be."

Of course, this is not proof, and Sagan's principle that I mentioned in a post a few days ago -- "Extraordinary claims require extraordinary evidence" -- certainly applies here.  But it is fascinating, and if further inquiries support the biotic explanation for the odd chemistry of the Jezero mudstones, it'll be somewhere beyond exciting.  I've always thought it likely that life is common in the universe, but having a real, honest-to-Gallifrey extraterrestrial example would be amazing.

In any case, keep your eyes on the science news.  Despite the ridiculous budget cuts NASA is facing, they're still doing some wonderful science.  And hopefully, at some point, we'll actually find proof of aliens.  Wouldn't that be cool?

I hope it's not the Vashta Nerada, though.  Those mofos are scary.

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The signature

As much as I love the movie Contact, trying to find extraterrestrial life isn't just a matter of tuning in to the right radio frequency.

There's no guarantee that even intelligent life would use radio waves to communicate, and if they did, that they'd do it in such a way that we could decipher the message.  I must admit, though, that the whole "sequence of prime numbers" thing as a beacon was a pretty cool idea; it's hard to imagine a natural phenomenon that would result in blips in a pattern of prime numbers.

Even besides the issue with how exactly a technological species would choose to communicate, there's the problem that this method would miss the vast majority of life that's potentially out there.  Consider the fact that there's been life on Earth for 3.8 billion years, give or take a day or two, and until about a hundred years ago, we weren't producing any radio waves ourselves.  To a civilization two hundred light years away -- so, seeing us as we were two hundred years ago -- Earth would be, to borrow C. S. Lewis's pithy phrase, a completely silent planet, even though there was a thriving biosphere that included at least one intelligent, soon-to-be-technological species.

So except for those presumably few planets that host intelligent beings who communicate kind of like we do, detecting extraterrestrial life is a tricky question.  The most promising approach has been to look for biosignatures -- chemical traces that (as far as we know) can only be produced by living things.  One example on Earth is the fact that our atmosphere contains both oxygen and methane.  Both are highly reactive (especially with each other); to keep stable levels of these gases in the atmosphere requires that something is continuously producing them, because they're constantly being removed by oxidation/reduction reactions.  In this case, photosynthesis and bacterial methanogenesis, respectively, pump them into the atmosphere as fast as they're being destroyed, so the levels remain relatively stable over time.

Two other chemicals that, on the Earth at least, are entirely biological in origin are dimethyl sulfide and dimethyl disulfide.  You've undoubtedly encountered these before; they're partly responsible for the unpleasant smell when you cook cabbage.  They're produced by a variety of living things, including bacteria, plants, and fungi -- dimethyl sulfide is what truffle-hunting pigs are homing in on when they're after truffles. 

Well, data from the James Webb Space Telescope showed that an exoplanet called K2-18b has measurable quantities of both dimethyl sulfide and dimethyl disulfide -- to the point that even the astronomers, who ordinarily have zero patience with the "It's aliens!" crowd, are saying "this is the strongest hint yet of biological life on another planet."

So far, the spectroscopic data that found the chemicals is at a significance level of "3-sigma" -- meaning there's a 0.3% chance that the signal was a statistical fluke (or, put another way, a 99.7% chance that it's the real deal).  It's exciting, but we've seen 3-sigma data do a faceplant before, so I'm trying to restrain myself.  Generally 5-sigma -- a 0.00006% chance of it being a fluke -- is the standard for busting out the champagne.  But even so, this is pretty amazing.

K2-18b is 124 light years away, and is thought to be a "Hycean world" -- an ocean-covered world with a thick, hydrogen-rich atmosphere.  So whatever life is there is very likely to be marine.  But even if we're not talking about your typical Star Trek-style planet with lots of rocks and an orange sky and aliens that look like humans but with rubber facial appendages, the levels of DMS and DMDS suggest a thriving biosphere.

"Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds," said Nikku Madhusudhan of Cambridge University, who co-authored the study, which appeared this week in Astrophysical Journal Letters.  "And now we've observed it, in line with what was predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have."

The issue, of course, is not just the statistical significance; 99.7% seems pretty good to me, even if it doesn't satisfy the scientists.  The problem is that sneaky little phrase that was in my description of biosignatures earlier; "as far as we know."  We don't know of a way to produce DMS and DMDS in significant quantities except by biological processes, but that doesn't mean one doesn't exist.  It could be that in the weird chemical soup on an planet in another star system, there's an abiotic way to produce a stable amount of these two compounds, and we just haven't figured it out yet.

Be that as it may, it's still pretty damn exciting.  It's certainly the closest we've gotten to "there's life out there."  And being only 124 light years away -- in our stellar neighborhood, really -- it's right there for us to study more intensively.  Which the astronomers will definitely be doing.

So that's our cool news for today.  I don't know about you, but now I'm daydreaming about what kind of life there might be on a world entirely covered by water.  I'm sure that whatever they are, they'll be "forms most beautiful and most wonderful" beyond Charles Darwin's wildest dreams.

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The fingerprint of life

Springboarding off yesterday's post, which suggested that -- from a biochemical standpoint, at least -- extraterrestrial life might be way more common than we'd thought, today we look at how we might find out where it lives.

This is a thornier problem than it might seem at first.  Despite hopeful movies like Contact, picking up an alien radio signal makes looking for a needle in a haystack seem like child's play.  Consider the difficulties; you have to have your radio telescope pointed at exactly the right place in the sky, at exactly the right time, and tuned to exactly the right frequency, to pick it up as it sweeps by the Earth at the speed of light.  Even if you posit an extremely simple message, which repeats indefinitely (like Ellie Arroway's string of prime number blips), there's the problem that any kind of electromagnetic signaling follows the inverse-square law, meaning if you double the distance between the sender and the receiver, the intensity of the received signal goes down by a factor of four.  Triple it, and it goes down by a factor of nine, and so forth.

And the fact is, the distances we're talking about here are...

... astronomical.  (*rimshot*)

So the possibility of detecting some sort of radio signal (whether or not deliberately sent to attract our attention) is not zero, but pretty damn small.  And the other downside is that if that's all we're looking for, we're going to miss a huge slice of the living creatures that could be out there -- we'd only see the ones that have a technological civilization that uses radio waves to communicate.  From that approach, Earth itself would have appeared to be barren and lifeless until the use of radio became widespread, back in the 1930s.

Is there another way?

An alternate approach -- one that avoids at least some of these pitfalls -- is to look for biosignatures, chemical traces that might indicate the presence of life on a planet even if it hasn't reached the point of being technological.  The studies done on Mars that attempted to find Martian microbes took this approach; take a sample of soil, add some likely nutrients, and look for a sign of metabolism.  But this, too, has its inherent difficulties.  How do you tell the difference between Martian microbes chowing down on the food you gave them, and some exotic but abiotic chemical reaction?

A team of astronomers and biologists from the University of Birmingham and MIT have come up with a possible answer.  According to a paper in Nature Astronomy last week, there is a pair of dead giveaways; an atmosphere depleted in carbon dioxide but enriched in ozone.

Carbon dioxide is a highly stable compound, and on lifeless, dry planets like Venus and Mars, it makes up a significant percentage of the atmosphere.  (96.5% on Venus, 95.3% on Mars.)  The fact that despite the amount of carbon on the Earth, the quantity in the air is only 0.04%, is due mostly to the fact that the water in the oceans acts as a huge carbon sink, first dissolving the carbon dioxide, then reacting it with dissolved metal ions like calcium and magnesium to form minerals like the calcite and magnesite in limestone.  Without the oceans, all of that carbon would stay in the atmosphere -- and we'd be a lot more like the inferno that is Venus than the temperate world where we reside.

As far as ozone, the real tipoff for the presence of life would be gaseous oxygen, which is a highly reactive substance that, in the absence of something producing it pretty much continuously, would all be bound up chemically.  Ozone -- a chemical relative of oxygen, O3 instead of O2 -- is expected to be present in small amounts in any atmosphere with free oxygen, but is the astronomers' choice because its spectral signature is much easier to detect than oxygen's.

Likewise, carbon dioxide's spectral fingerprint is obvious because of its strong absorption in the infrared (a property that is directly related to the greenhouse effect and carbon dioxide's warming effect on atmospheres).

So it should be possible to analyze the light reflected from the surface of exoplanets that seem to be in the right temperature range, and look for two things -- low carbon dioxide (indicating liquid water on the surface) and high ozone (indicating something, possibly life, keeping molecular oxygen in the atmosphere).  See both of those things, the team said, and you're very likely looking at a planet that is inhabited.

Like I said yesterday, of course, "inhabited" doesn't mean "inhabited by bipedal humanoids with spaceships and laser guns."  But even so, the technique is intriguing in its simplicity.  The team suggested starting with relatively nearby planetary systems like TRAPPIST-1, which has seven known exoplanets and is only a little over forty light years away from Earth.

TRAPPIST-1 and its lineup of seven planets [Image is in the Public Domain courtesy of NASA/JPL]

So this is all tremendously exciting -- that astronomers are now taking the possibility of extraterrestrial life seriously enough to start proposing methods for searching for it other than just scanning the skies and hoping for the best.  After all, to go back to the movie Contact -- "if we're all alone in the universe, it seems like an awful waste of space."

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Assembling aliens

There are a lot of hurdles in detecting extraterrestrial life, and that's not even counting the possibility that it might not exist.

Honestly, I don't think that last stumbling block is all that likely, and it's not just because proving we're not alone in the universe has been one of my dearest wishes since I was six years old and watching the original Lost in Space.  Since astronomer Frank Drake came up with his famous Drake equation in 1961, which breaks down the likelihood of extraterrestrial intelligence into seven individual parameters (each with its own, independent probability), the estimates of the values of those parameters have done nothing but increase.  As only one example, one of the parameters is f(p) -- the fraction of stars that have planetary systems.  When Drake first laid out his equation, astronomers had no certainty at all about f(p).  They were working off a sample size of one; we know the Solar System exists because we live in it.  But was its formation a fluke?  Were stars with planets extremely uncommon?

No one knew.

Now, exoplanet discovery has become so routine that it barely even makes the news any more.  The first exoplanet around a main-sequence star -- 51 Pegasi b -- was discovered in 1995.  Since then, astronomers have found 5,297 exoplanets, with new ones being announced literally every week.  It seems like damn near a hundred percent of stable main-sequence stars have planetary systems, and most of them have at least one planet in the "Goldilocks zone," where the temperatures are conducive to the presence of liquid water.

Even setting aside my hopes regarding aliens, the sheer probability of their existence has, from a purely mathematical standpoint based upon the current state of our knowledge of the universe, improved significantly.

But this still leaves us with a problem: how do we find it?  The distances even to the nearest stars are insurmountable unless someone comes up with warp drive.  (Where are you, Zefrem Cochrane?)  So we're left with remote sensing -- looking for biosignatures.  The most obvious biosignature would be a radio transmission that's clearly from intelligent life, such as the one Ellie Arroway found in Contact; but it bears keeping in mind that through almost all of the Earth's 3.7-billion-odd years it's been inhabited by living creatures, it would have been entirely silent.  Alien astronomers looking from their home worlds toward the Earth would not have heard so much as a whisper.  It's only since we started using radio waves to transmit signals, a century ago, that we'd be detectable that way; and given how much transmission is now done via narrow-beam satellite and fiber optics cables rather than simple wide-range broadcast, it's entirely possible that once the technology improves Earth will go silent once again.  There may only be a short period during which a technological civilization is producing signals that are potentially detectable from a long way away.

So the question remains: how could we determine if an exoplanet had life?

I'm guessing that whatever the aliens look like, it's not this.  Unfortunately.

The tentative answer is to look for other kinds of biosignatures, and the most obvious one is chemicals that "shouldn't be there" -- in other words, that would not form naturally unless there were life there producing them through its metabolic processes.  This, too, is not a simple task.  Not only is there the technological challenge of detecting what's in a distant exoplanet's atmosphere (something we're getting a lot better at, as spectroscopy improves), there's the deeper question of how we know what should be there.  If we find an odd chemical in a planet's atmosphere, how do we know if it was made by life, or by some exotic (but abiotic) chemistry based on the planet's composition and conditions?

We've gotten caught this way before; three years ago, scientists discovered traces of a chemical called phosphine in the atmosphere of Venus, and a lot of us -- myself included -- got our hopes up that it might be a biosignature of something alive in the clouds of our hostile sister planet.  The consensus now is that it isn't -- the amounts are vanishingly small, and any phosphine on Venus is a product of its wild convection and bizarre atmospheric makeup.  So once we detect a chemical on an exoplanet, is there a way to do a Drake-equation-style estimate of its likelihood of forming abiotically?

Astrobiologist Leroy Cronin, of the University of Glasgow, has proposed an answer, based on something he calls "assembly theory."  Assembly theory, significantly, doesn't rely on any kind of analogy to terrestrial life.  Cronin and others are now trying to figure out strategies to find life as we don't know it -- living creatures that might be based upon extremely different chemistry.

What he's done is given us a purely mathematical way to index chemicals according to how many independent steps it takes to create them from simple, pre-existing building blocks.  This molecular assembly number, Cronin says, is directly proportional to its likelihood of being created by a living thing.  As a simple analogy, he shows how you would find the molecular assembly number for the word abracadabra:

1. add a + b;
2. add ab + r;
3. add abr + a;
4. add abra + c;
5. add abrac + a;
6. add abraca + d;
7. add abracad + abra (we'd already created abra in step three).

Seven steps from the primordial building blocks, so the molecular assembly number for abracadabra is seven.

Replace putting letters and letter groups together with steps in a chemical reaction chain, and you have an idea how assembly theory works.

Like the Drake equation, Cronin's method isn't proof.  Finding some complex chemical in an exoplanet's atmosphere, the gas of a nebula, or a meteorite might be suggestive of life, but almost certainly wouldn't convince the doubters without a lot more in the way of evidence.  Still, just as Frank Drake did in 1961, it's nice to have a protocol for determining the likelihood of a biosignature that doesn't depend on our unavoidable Earth-centrism.  Like with the formation of the Solar System, we're familiar with only one kind of life -- the kind all around us, that we ourselves are examples of.  Shaking the bias that all life is Earth-like is not easy.

It's understandable that the creators of Lost in Space and Star Trek visualized almost all of the aliens as basically humans with odd facial excrescences, and that's granting the difficulty of finding a way to portray non-humanoid aliens convincingly using human actors.  When they did manage to get beyond humans with rubber noses, such as in the Lost in Space episode "The Derelict" and the Star Trek episodes "The Devil in the Dark" and "Obsession," the aliens were, respectively, giant mobile bubbles, a tunneling, acid-spewing rock, and a disembodied vampiric mist cloud, all of a which at least gave a shot at trying to visualize what truly non-Earthlike life might be.

I'm hopeful that the work of Cronin and others is moving the new field of astrobiology forward from simple "what ifs" to actual rigorous algorithms for analyzing the spectroscopic data we're gathering from exoplanet atmospheres.  And maybe... just maybe... within my lifetime we'll have enough data to feel confident we've identified for certain what I've been waiting for since I was six: life on another planet.

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A life-like glow

One of the problems faced by people who would dearly love to find unequivocal proof of extraterrestrial life is: space is big.

No, not big.  Really fucking huge.  Here's an analogy that may help.  Let's start out with saying the Earth has been shrunk to the size of the period at the end of this sentence.  The Sun would be the size of a pool ball, and would be located about six meters away.  The farthest decent-sized object in the Solar System we know of -- Pluto (yes, yes, I know it's not a planet, I don't want to discuss it) -- would be a dust speck 230 meters away, a bit more than twice the length of an American football field.  The nearest star to the Sun, Proxima Centauri, would be another pool ball 1,570 kilometers away, roughly the distance between where I sit now (in upstate New York) and Jacksonville, Florida.

And in between us and it is a whole lot of bugger-all.

Just seeing any kind of detail in objects that far away is tremendously difficult, and that's even considering the amazing strides we've taken in telescope design.  Not only is there the distance involved, but there's dust and debris in between us and everywhere else, blurring the image further.  There could be friendly aliens on one of the planets orbiting Proxima Centauri leaping about and waving their six arms and holding up signs saying, "HERE WE ARE!", and we wouldn't see them.

And that's the nearest star.

Things become even worse when you consider actually going there.  Voyager 1, currently the most distant human-made object from Earth, is traveling outward at a little over sixty thousand kilometers per hour.  A decent clip, right?  Well, even so, it would take ten thousand years to reach Proxima Centauri, if it were heading that way.

Which it's not.

To me, this is the strongest argument against UFOs having an extraterrestrial origin.  Every indication we have is that the laws of the Special and General Theories of Relativity, which prohibit faster-than-light travel, are enforced in every jurisdiction.  It's hard to imagine space-faring aliens crossing all this distance to come see us (only to abduct some cows and leave a crop circle in Farmer Bob's wheat field, then leaving).  We may well not be the only intelligent life in the cosmos, but the likelihood of having a face-to-face (or face-to-whatever-they've-got) visit is slim to none.

Even having a nice chat with them from a distance is gonna be tricky, not to mention boring.  Once again, using Proxima Centauri (at 4.2 light years distant) as an example, if we were to beam a focused radio wave signal toward it containing some kind of encoded message, the best-case scenario of what it'd be like in Earth's SETI Command Central would go something like this:

Us (into microphone): Hey, Proxima Centaurians, how are y'all doing?

[8.4 year silence]

PCs (voice from speaker): We're doing fine.  The weather's been nice, although we could use some rain.  How are you?

Us (into microphone): Same old, same old.  You know how it goes.

[8.4 year silence]

PCs (voice from speaker): Don't we ever.  It's the same everywhere in the universe, amirite?  LOL

So anything approaching scintillating repartee would be kind of out of the question.

Another complication is that intelligent life doesn't mean intelligent life we can communicate with.  Consider the fact that until the invention of the radio telescope (1937), there could have been extraterrestrials positively screaming at us, and we'd have had no way to know.  And it's no better with messages going the other way.  Prior to our own radio signals, the Earth itself would have appeared completely silent; there would have been little in the way of indication that there was anything alive down here, despite the fact that the Earth had already hosted life for three billion years.  

As an aside, it's an interesting question as to whether we're going silent again, given the increasing efficiency of signal transmission -- our "radio bubble" is getting weaker, not (heaven knows) because we've got less to say, but because less of the signal is leaking out into space.  This might not be a bad thing, although it's probably already too late.  Recall in the brilliant send-up of the original Star Trek, Galaxy Quest, that the aliens (the Thermians from the Klaaaaaatu Nebula) thought our early television signals were documentaries:

Lieutenant Madison: They're not all "historical documents."  Surely you don't think that Gilligan's Island...

Captain Mathazar (sadly): Oh, those poor people.

So those of us who are kind of desperate to demonstrate that we're not alone in the universe have to figure out another way to do it other than the obvious ones.

Enter the Compact Color Biofinder.  This amazing device, developed at the University of Hawaii - Manoa, uses an interesting feature of many organic compounds -- fluorescence.  Fluorescence occurs when light at one frequency is absorbed by a molecule, resulting in the electrons in its atoms bouncing to higher energy levels; when those electrons fall back into the ground state, they emit light at certain characteristic frequencies.  (An example you may, unfortunately, know about; if you shine an ultraviolet light on cat piss, it fluoresces green, which will allow you to find where you need to clean up if Mr. Fluffums decides not to use his litter box.)

Because the fluorescence spectrums of different types of organic compounds are pretty well known, this allows you to analyze the light coming from an object that contains organic residues and determine what those residues are made of.  The concept, of course, is hardly new; it's the basic idea of spectroscopy, which has been around for two hundred years.  But the Compact Color Biofinder has refined the process to unbelievable levels.  It was able to detect and identify traces of the biological compounds in a fifty-million-year-old fish fossil from which you'd think every organic trace would have disappeared long ago.

"The Biofinder is the first system of its kind," said Anupam Misra, who led the team that developed the new device.  "At present, there is no other equipment that can detect minute amounts of bio-residue on a rock during the daytime.  Additional strengths of the Biofinder are that it works from a distance of several meters, takes video and can quickly scan a large area...  If the Biofinder were mounted on a rover on Mars or another planet, we would be able to rapidly scan large areas quickly to detect evidence of past life, even if the organism was small, not easy to see with our eyes, and dead for many millions of years.  We anticipate that fluorescence imaging will be critical in future NASA missions to detect organics and the existence of life on other planetary bodies."

So we may be fast approaching the point that we'll be able to analyze the faint light reflected from a distant exoplanet and say, "Yes, that's an organic biosignature."

As much fun as it'd be actually to meet aliens -- well, most aliens, I'll take a pass on Daleks, the Sycorax, and the Vashta Nerada -- at this point, I'll happily settle for evidence that they're out there.  The Compact Color Biofinder is looking like it may be our best tool yet for doing exactly that.  Until we can find a way around Relativity, we'll have to content ourselves with looking up into the night sky and saying, "They're out there, even if we can never get to have a conversation with them."

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Looking for a biosignature

From Gordon's Obsession #1 (paleontology) yesterday, today we're moving on to Gordon's Obsession #2: outer space and extraterrestrial life.

No, unfortunately, I'm not about to announce we've located the Andorian home world.  Maybe next time.

We're returning to this favorite topic of mine because of a paper this week in Astrophysical Journal Letters by a team led by Lisa Kaltenegger, of the Carl Sagan Institute of Cornell University, right in my part of the world in upstate New York.  Kaltenegger et al. describe models they've developed to refine how we look for Earth-like exoplanets -- by trying to figure out what our own world would have looked like from the depths of space during its entire geological history.

In "High-Resolution Transmission Spectra of Earth Through Geological Time," Kaltenegger's team recognizes the phenomenon we've discussed here before -- that seeing farther out into space means seeing further back into time.  An intelligent, technological alien species as little as 150 light years away wouldn't have any way of knowing that the Earth hosted a complex civilization with its own sophisticated scientific and technological capabilities, because they'd be seeing us as we were 150 years ago -- before the invention of long-distance radio-wave communication.  To them, the Earth would be a small rocky planet that was entirely silent, and apparently, devoid of life.

So are we making the same mistake with the exoplanets we're seeing?  And is there a way to get beyond that, and find "biosignatures" -- detectable traces of life on a far-distant world?

The key, says Kaltenegger, is in the world's atmosphere.  As the light from its host star passes through the thin envelope of gases surrounding the planet, the light is altered; each kind of gas has a specific set of frequencies it can absorb, and those are selectively removed from the stellar light, creating a dark-line or absorption spectrum.  This gives a fingerprint of what gases are there -- and, potentially, tells us what's going on down on the planet's surface, including whether or not there's anything alive.

The data they're using comes primarily from two sources -- the orbiting James Webb Space Telescope, and the Extremely Large Telescope out in the Atacama Desert of Chile.

I don't know about you, but the name of the latter always makes me laugh.  I'm picturing the scientists coming up with a name for the observatory after it was complete:

Scientist #1:  So, what are we gonna name our telescope?
Scientist #2:  How about naming it after Edwin Hubble?
Scientist #1:  No, that one's already taken.
Scientist #2:  Well, what's this thing's most outstanding feature?
Scientist #1:  It's extremely large.
*pause*
Scientist #1 and #2, together:  Heyyyyyy......!

But I digress.

Kaltenegger's team is looking for the presence of highly-reactive gases -- oxygen being the most obvious example -- that wouldn't be in an atmosphere unless something was continually pumping it out.  While there could be a non-biological way to inject large quantities of oxygen into an atmosphere, the better likelihood is some analogue to photosynthesis.

In other words, life.

The nice thing about this approach is that the presence of oxygen would have been detectable here on Earth over a billion years ago -- thus, potentially detectable by technological aliens from up to a billion light years away.  That's quite a window.  "Even though extrapolations from our findings suggest that one out of five stars hosts a planet which could be like Earth, it would be extremely surprising if all of them were at our Earth’s evolutionary stage," Kaltenegger said.  "So taking Earth’s history into account to me is critical to characterize other Earth-like planets."

What the team did is predict what the absorption spectra of the Sun's light would look like after passing through the Earth's atmosphere during the various periods of our prehistory -- the anoxic period (prior to the evolution of photosynthesis), the time during which aerobic life was present but uncommon, the transition to the land & evolution of plants, and so on, up through the Industrial Revolution, when (as James Burke put it in After the Warming) "instead of the atmosphere doing things to us, we started doing things to it."

The technique is not without its difficulties, however, most notably that the absorption spectrum of one of the biologically-produced reactive gases they studied -- methane -- is awfully close to that of water.  So teasing apart what's the signature of a ubiquitous compound, and what's the actual fingerprint of life, may not be simple.

What's certain is that we've only scratched the surface of what's out there.  At present there are a few more than 4,000 exoplanets identified, a lot of which are gas-rich Jovian planets that are likely not to have a solid surface.  (The reason for this is that the two main techniques for locating exoplanets, stellar occlusion and detection of a "wobble" in the star's position, work much better if the planet in question is large, biasing us against detecting small rocky worlds like our own.)  But if Kaltenegger is right that twenty percent of stars have Earth-sized planets, that's a lot of potential homes for alien life.

I don't know about you, but to me, that's tremendously exciting.  Even if we can't detect Vulcans and Klingons and Andorians yet, we might just be able to see if there's life at all out there.

And I'd be satisfied with that.  Just knowing we're not all alone in the cosmos would be reassuring, even if we don't know what that alien life is like, or whether they might be looking back at us through their own telescopes.

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This week's Skeptophilia book recommendation is an important read for any of you who, like me, (1) like running, cycling, and weight lifting, and (2) have had repeated injuries.

Christie Aschwanden's new book Good to Go: What the Athlete in All of Us Can Learn from the Strange Science of Recovery goes through all the recommendations -- good and bad, sensible and bizarre -- that world-class athletes have made to help us less-elite types recover from the injuries we incur.  As you might expect, some of them work, and some of them are worse than useless -- and Aschwanden will help you to sort the wheat from the chaff.

The fun part of this is that Aschwanden not only looked at the serious scientific research, she tried some of these "cures" on herself.  You'll find out the results, described in detail brought to life by her lucid writing, and maybe it'll help you find some good ways of handling your own aches and pains -- and avoid the ones that are worthless.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

The power of models

I get that scientific terminology can be daunting.  Scientists, and therefore scientific papers, have become so specialized that unless you are an expert, the vocabulary by itself can be an overwhelming barrier to understanding.  That only gets worse in disciplines like physics and chemistry, where complex mathematics throws another spanner into the works.  I have a B.S. in physics, enough credits for a second major in biology, and a minor in math, and am reasonably articulate, but just about every academic paper I've ever seen loses me within a couple of paragraphs, except for the two areas I know best -- population genetics and evolutionary biology.

So I'm not expecting laypeople to become experts in scientific jargon.  But there are two words I really wish everyone would familiarize themselves with -- theory and model.

Confusion over the first one is what gives rise to the "it's only a theory" *shrug* reaction a lot of people have when discussing the theory of evolution.  Theory, in scientific discussions, does not mean "a wild guess that could as easily be wrong as right."  In scientific parlance, a theory is an explanation of a natural phenomenon that has passed repeated tests and makes predictions that are in good accordance with the data.  This is why intelligent design creationism isn't a theory; it makes no predictions.  If things get complex, it defaults to "God did it," and the conversation ends.

In science, a model is a representation of a natural object, system, or phenomenon, often idealized or simplified, that can then be manipulated -- once again, to see if the results are consistent with observed data from the real world.  As an example, the computerized three-dimensional maps of the climate are models, breaking up the atmosphere into thousands of cubical regions and the land and ocean into square blocks of area, with specifications for atmospheric composition, heat absorption capacity for land and water, solar radiation input, and so on.  The software can take the known input parameters and then run a simulation to see if what comes out matches what we actually know of the real climate data (and they have, to a startling degree of accuracy, something that is simultaneously impressive and terrifying).

The problem with the idea of modeling is that to an outside observer, it may look like the scientists are just messing around -- playing Sims with the world, with no particular expectation that what they're doing has anything in common with reality.  This, of course, is the opposite of the truth -- if a model doesn't align very well with the natural world, it's rightly abandoned for one that works better.

Even models that seem to be a little bit out there are only retained because they describe a known part of the universe sufficiently well that their predictions can be useful for describing something not yet understood.  Take, for example, the paper last week in Proceedings of the National Academy of Sciences that used what's known about biochemistry to make a stab at the configuration and composition of the earliest proteins, molecules that were around 3.5 billion years ago -- produced abiotically before there were any living things on Earth.

In "Small Protein Folds at the Root of an Ancient Metabolic Network," Hagai Raanan, Saroj Poudel, Douglas Pike, Vikas Nanda, and Paul Falkowski, of Rutgers University, describe a sophisticated computer simulation that took what we know about the chemistry that is common to all living organisms (such as using oxidation/reduction reactions to power metabolism) and combined it with what is surmised about the conditions on the early Earth, and used it to infer what the earliest energy-transfer proteins looked like.  The authors write:

Life on Earth is driven by electron transfer reactions catalyzed by a suite of enzymes that comprise the superfamily of oxidoreductases (Enzyme Classification EC1).  Most modern oxidoreductases are complex in their structure and chemistry and must have evolved from a small set of ancient folds.  Ancient oxidoreductases from the Archean Eon between ca. 3.5 and 2.5 billion years ago have been long extinct, making it challenging to retrace evolution by sequence-based phylogeny or ancestral sequence reconstruction.  However, three-dimensional topologies of proteins change more slowly than sequences.  Using comparative structure and sequence profile-profile alignments, we quantify the similarity between proximal cofactor-binding folds and show that they are derived from a common ancestor.  We discovered that two recurring folds were central to the origin of metabolism: ferredoxin and Rossmann-like folds.  In turn, these two folds likely shared a common ancestor that, through duplication, recruitment, and diversification, evolved to facilitate electron transfer and catalysis at a very early stage in the origin of metabolism.

Here's one of the ancestral proteins the model generated:

Now, maybe you see this as a bunch of hand-waving in an intellectual vacuum.  After all, we have no way of going back 3.5 million years and checking to see if the model is correct.  But the key thing is that this was created within parameters of how we know proteins work, and what we see in the energy-transfer proteins of current organisms.  This model was very much constrained by reality -- meaning that its results have a really good chance of being accurate.

Further, like any good model (or theory, for that matter), it generates predictions -- in this case, what we might look for as a signature of emerging life on other planets.  "In the realm of deep-time evolutionary inference," the authors write, "we are necessarily limited to deducing what could have happened, rather than proving what did happen...  Ultimately, our goal is for the proposed effort to inform future NASA missions about detection of life on planetary bodies in habitable zones.  Our effort provides a unique window to potential planetary-scale chemical characteristics that might arise from abiotic chemistry, which must be understood if we are to recognize unique biosignatures on other worlds."

So models and theories aren't guesses, they're real-world descriptions, and the best ones give us deep insight into the workings of the universe.  As such, they are part of the scientist's stock-in-trade -- and essential to understand for laypeople who would like to know what's happening on the cutting edge of research.

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Any guesses as to what was the deadliest natural disaster in United States history?

I'd speculate that if a poll was taken on the street, the odds-on favorites would be Hurricane Katrina, Hurricane Camille, and the Great San Francisco Earthquake.  None of these are correct, though -- the answer is the 1900 Galveston hurricane, that killed an estimated nine thousand people and basically wiped the city of Galveston off the map.  (Galveston was on its way to becoming the busiest and fastest-growing city in Texas; the hurricane was instrumental in switching this hub to Houston, a move that was never undone.)

In the wonderful book Isaac's Storm, we read about Galveston Weather Bureau director Isaac Cline, who tried unsuccessfully to warn people about the approaching hurricane -- a failure which led to a massive overhaul of how weather information was distributed around the United States, and also spurred an effort toward more accurate forecasting.  But author Erik Larson doesn't make this simply about meteorology; it's a story about people, and brings into sharp focus how personalities can play a huge role in determining the outcome of natural events.

It's a gripping read, about a catastrophe that remarkably few people know about.  If you have any interest in weather, climate, or history, read Isaac's Storm -- you won't be able to put it down.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]