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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Completing the recipe

Last week, I wrote a piece on the peculiarities of Jupiter's moon Io -- surely one of the most inhospitable places in the Solar System, with hundreds of active volcanoes, lakes of liquid sulfur, and next to no atmosphere.  But there's a place even farther out from the warmth of the Sun that is one of our best candidates for an inhabited world -- and that's Saturn's icy moon Enceladus.

It's the sixth largest of Saturn's eighty-some-odd moons, and was discovered back in 1789 by astronomer William Herschel.  Little was known about it -- it appeared to be a single point of light in telescopes -- until the flybys of Voyager 1 and Voyager 2 in 1980 and 1981, respectively, and even more was learned by the close pass in 2005 by the Cassini spacecraft.  

One of Cassini's spectacular photographs of Enceladus [Image is in the Public Domain courtesy of NASA/JPL]

Enceladus, like Io, is an active world.  It has a thick crust mostly made of water ice, but there are "cryovolcanoes" -- basically enormous geysers -- that jet an estimated two hundred kilograms of water upward per second.  Some of it falls back to the surface as snow, but the rest is the primary contributor to Saturn's E ring, 

Where it gets even more interesting is that beneath the icy crust, there is an ocean of liquid water estimated to be ten kilometers deep (just a little shy of the depth of the Marianas Trench, the deepest spot in Earth's oceans).  Like Io's wild tectonic activity, the geysers of Enceladus are maintained primarily by tidal forces exerted by its host planet and the other moons.  But that's where any resemblance to Io ends.  Chemically, it could hardly be more different.  Analysis of the snow ejected by the cryovolcanoes of Enceladus found that dissolved in the water was ordinary salt (sodium chloride), with smaller amounts of ammonia, carbon dioxide, methane, sulfur dioxide, formaldehyde, and benzene.

What jumped out at scientists about this list is that these compounds contain just about everything you need to build the complex organic chemistry of a cell -- carbon, nitrogen, oxygen, hydrogen, and sulfur.  I say "just about" because one was missing, and it's an important one: phosphorus.  In life on Earth, phosphorus has two critical functions -- it forms the "linkers" that hold together the backbones of DNA and RNA, and it is part of the carrier group for energy transfer in the ubiquitous compound ATP.  (In vertebrates, it's also a vital part of our endoskeletons, but that's a more restricted function in a small subgroup of species.)

But just last month, a paper was presented at the annual meeting of the American Geophysical Union describing the research that finally found the missing ingredient.  There is phosphorus in Enceladus's ocean -- in fact, it seems to have a concentration thousands of times higher than in the oceans of Earth.

This is eye-opening because phosphorus is a nutrient that is rather hard to move around, as vegetable gardeners know.  If you buy commercial fertilizer, you'll find three numbers on the package separated by hyphens, the "N-P-K number" representing the percentage by mass of nitrogen, phosphorus, and potassium, respectively.  These three are often the "limiting nutrients" for plant growth -- the three necessary macronutrients that many soils lack in sufficient quantities to grow healthy crops.  And while the nitrogen and potassium components usually (depending on the formulation) "water in" when it rains and spread around to the roots of your vegetable plants, phosphorus is poorly soluble and tends to stay pretty much where you put it.

The fact that the snow on Enceladus has amounts of phosphorus a thousand times higher than the oceans of Earth must mean there is lots down there underneath the ice sheets.

This strongly boosts the likelihood that there's life down there as well.  Primitive life, undoubtedly; it's unlikely there are Enceladian whales swimming around under the ice.  But given how quickly microbial life evolved on Earth after its surface cooled and the oceans formed, I feel in my bones that there must be living things on Enceladus, given the fact that all the ingredients are there.  (The oceans on Earth formed on the order of 4.5 billion years ago, and the earliest life is likely to have begun on the order of four billion years ago; given a complete recipe of materials and an energy source, complex biochemistry seems to self-assemble with the greatest of ease.)

Maybe I'm being overly optimistic, but the discovery of phosphorus in the snows of Enceladus makes me even more certain that extraterrestrial life exists, and must be common in the universe.  If we can show that there are living things down there, on a mostly frozen moon 1.4 billion kilometers from the Sun, then it will show that life can occur almost anywhere -- as long as you have all the ingredients for the recipe.

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The water world

Coming hard on the heels of an encouraging paper about the possibilities of near-light-speed travel, at which we might potentially have probe data from the nearest star to the Sun in ten years or so, we have an even more encouraging study of a place right here in the Solar System that might be worth looking at as a home of extraterrestrial life.

The place is Enceladus, the sixth largest moon of the planet Saturn.  It's a pretty decent-sized object, about one-seventh the diameter of the Earth.  Flyby data from the spacecraft Cassini in 2014 showed that it's a curious place, with a liquid water ocean capped by a shell of solid ice.  There are geysers coming up through cracks in the surface, and Cassini was able to sample the spray and confirm that it is, indeed, water.

Enceladus [Image is in the Public Domain courtesy of NASA/JPL and the Cassini probe]

But it's kind of a topsy-turvy world even so.  Here on the Earth, oceans are warmest at the top and coolest at the bottom; the deep parts of the ocean are the most stable ecosystems on Earth, always completely dark, under crushing pressure, and about four degrees Celsius (the temperature at which water is densest).  On Enceladus, it's the other way around; coldest on top, where it's in contact with the undersurface of the ice cap, and warmest at the bottom, where it's in contact with the core of the moon.  There's no land surface; the oceans on Enceladus are estimated at thirty kilometers deep (contrast that to an average three kilometers for Earth's oceans).

The upside-down temperature structure on Enceladus is what makes it an excellent place to look for extraterrestrial life, but to see why, we'll need to take a brief digression for a physics lesson.

One of the main drivers of ocean currents -- the movement of water not only horizontally, but vertically -- is convection, which is fluid flow because of differences in density.  One of the best-studied examples, which I described more fully in a post a few weeks ago, is the Atlantic Conveyor (known to scientists as the Atlantic Meridional Overturning Circulation), in which evaporation from the warm Gulf Stream as it flows north cools the water and makes it more saline, both of which have the effect of increasing its density.  Eventually, the blob of water becomes cool and saline enough that it exceeds the density of the water surrounding it, and it sinks.  This usually occurs in the North Atlantic southwest of Iceland, and that draw-down is what pulls more warm water north, keeping the whole system moving.

This has multiple effects, two of which concern us here.  The first is that it acts as a heat transfer mechanism, warming the air (and the land near it) and giving the American Northeast, the Maritimes of Canada, Iceland, and northwestern Europe the temperate climate they have, which otherwise would be a lot more like Siberia.  Second, the water carries with it nutrients of various sorts, and redistribution of those nutrients forms the basis of phytoplankton growth and the food chain.  (The most obvious example of this latter effect is the El Niño Southern Oscillation, in which upwelling of nutrient-laden water off the coast of Peru supports a huge population of fish -- until an El Niño year, when warm water flowing east blocks the upwelling, and the entire food chain collapses.  The four-year lots-of-fish to no-fish cycle was observed as far back at the seventeenth century, when the Spanish rulers of Peru noted that the collapse often started in midwinter, and gave it the name El Niño, which refers to the baby Jesus.)

So as long as you have alterations in density, a fluid will move.  It's what drives all weather, in fact; ground heating raises the temperature of air, lowering its density and making it rise, generating a low-pressure system that draws in more air to replace what's moving aloft.  This causes wind, and if the air has moisture, it'll condense out as it rises and cools, causing rain and/or snow.

Of course, the water drawn down by the sinking of the Gulf Stream near Iceland (or the air moving upward because of warming) is only half the picture.  It's got to come back somehow, and both the atmosphere and ocean are filled with convection cells, swirling, more-or-less circular currents following the motion both vertically and horizontally.  And once again -- to return to why the topic comes up -- these redistribute not only heat, but (in the case of water), nutrients.

On Enceladus, the pattern is upside down as compared to Earth's oceans.  Water in contact with the underside of the ice shell cools and eventually sinks, drawing warmer water up from near the center of the moon.  This mixing stirs the pot, and any potential nutrient chemicals don't just settle out on the bottom.  Thus, Enceladus is a prime candidate for extraterrestrial life of some sort.

To be sure, it'd be different from what we have here on Earth.  A lot different.  Despite the cracks and geysers, the ice shell on Enceladus is thick and pretty much solid, so any living things under there would never come into contract with direct rays of the Sun (as dim as they'd be out there).  The only energy source would be the warmth of the core, so there'd be no photosynthesis, only chemosynthesis, perhaps similar to the weird organisms near Earth's hydrothermal vents in the deep oceans.  

Even so, it's a prime spot to look for signs of life.  And unlike Proxima Centauri, the nearest star, which in a best-case scenario would require ten years for an outward-bound near-light-speed probe and returned signal back on Earth, the same round-trip to Enceladus would take on the order of three hours.  

Once again highlighting that the universe is freakin' huge.

If we can develop near-light-speed travel, maybe the first thing to do is to send some probes to explore our own Solar System more thoroughly.  Not only Enceladus, but a similar water-world moon of Jupiter, Europa, which is even closer.  I'd say the likelihood of finding intelligent life on either one is slim to none, so I wouldn't be looking for anything like the super-tech civilization on a planet orbiting Vega in the movie Contact, but I think there's an excellent chance that there's something living down there, even if it turns out to be only as complex as bacteria.

But even so.  How cool would that be?  A life form completely unrelated to anything we have down here.  And if we did find life on Europa or Enceladus, it would really bolster the hunch I've had for years, which is that life is common in the universe.

And I, for one, would settle for that in a heartbeat.

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The sad truth of our history is that science and scientific research has until very recently been considered the exclusive province of men.  The exclusion of women committed the double injury of preventing curious, talented, brilliant women from pursuing their deepest interests, and robbing society of half of the gains of knowledge we might otherwise have seen.

To be sure, a small number of women made it past the obstacles men set in their way, and braved the scorn generated by their infiltration into what was then a masculine world.  A rare few -- Marie Curie, Barbara McClintock, Mary Anning, and Jocelyn Bell Burnell come to mind -- actually succeeded so well that they became widely known even outside of their fields.  But hundreds of others remained in obscurity, or were so discouraged by the difficulties that they gave up entirely.

It's both heartening and profoundly infuriating to read about the women scientists who worked against the bigoted, white-male-only mentality; heartening because it's always cheering to see someone achieve well-deserved success, and infuriating because the reason their accomplishments stand out is because of impediments put in their way by pure chauvinistic bigotry.  So if you want to experience both of these, and read a story of a group of women who in the early twentieth century revolutionized the field of astronomy despite having to fight for every opportunity they got, read Dava Sobel's amazing book The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars.

In it, we get to know such brilliant scientists as Willamina Fleming -- a Scottish woman originally hired as a maid, but who after watching the male astronomers at work commented that she could do what they did better and faster, and so... she did.  Cecilia Payne, the first ever female professor of astronomy at Harvard University.  Annie Jump Cannon, who not only had her gender as an unfair obstacle to her dreams, but had to overcome the difficulties of being profoundly deaf.

Their success story is a tribute to their perseverance, brainpower, and -- most importantly -- their loving support of each other in fighting a monolithic male edifice that back then was even more firmly entrenched than it is now.  Their names should be more widely known, as should their stories.  In Sobel's able hands, their characters leap off the page -- and tell you a tale you'll never forget.

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