Tales of a Death Star

One of the most promising areas of study for astrobiologists -- scientists who are interested in the possibility of life elsewhere in the universe -- is the potential for life on the moons of Jupiter, Saturn, Uranus, and Neptune.  We're beginning to develop the technology to detect biosignatures -- chemical traces of living things in the atmospheres of moons or exoplanets -- but it's a hell of a lot easier to find those in our own Solar System than it is around the barely-visible specks of light that are all we can see of most exoplanetary systems.

Despite their distance from the Sun, due to tidal heating there are several of these moons that are thought to have liquid water beneath a frozen crust.  Four commonly-discussed possibilities are Europa (Jupiter), Enceladus and Titan (Saturn) and Triton (Neptune); the case is nearly certain for Europa and Enceladus, where fly-bys have detected liquid water geysers erupting from surface cracks in the ice sheet.

What could be down there, I wonder?  Single-celled life is the most likely, but with no further information... well, anything's possible.  We only have a sample size of one regarding how life forms and evolves, so trying to predict what it would look like somewhere else is going to be speculation at best.

The conventional wisdom has been that the smaller moons are unlikely places to look for life; being smaller, they lose heat faster, so any heat gains they get from the Sun and from tidal compression are far offset by heat loss from their small thermal mass. 

That assessment will have to be revised, apparently.  A new study -- out this week in Nature -- found that Saturn's moon Mimas, best known for having a huge crater that makes it look like the Death Star from Star Wars, has an ocean of liquid water underneath a crust of ice and frozen methane.  It's only four hundred kilometers in diameter, over eight times smaller than our own Moon.

A photograph of Mimas from the 2010 pass by the probe Cassini [Image is in the Public Domain courtesy of NASA/JPL]

The frozen crust of Mimas is thought to be so thick (something on the order of twenty to thirty kilometers) that it precludes the cracks that cause the geysers on Enceladus and Europa.  So the liquid water inside is trapped -- but the effects of tidal heating from the enormous planet it orbits are apparently enough to keep it well above freezing, and therefore very likely to enable the convection currents which overturn nutrients in our own oceans and are essential for the maintenance of ecosystems.  

Based on what we know about the formation of moons and their stability in orbit around their host planet, Mimas is estimated to be quite young, something on the order of between five and fifteen million years old.  This seems like a very short time even to evolve simple single-celled organisms, but as I said before -- it's not like we have a bunch of test cases from which to draw inferences.

"Mimas was probably the most unlikely place to look for a global ocean — and liquid water more generally," said study co-author Valéry Lainey, of the Paris Observatory.  "So that looks like a potential habitable world.  But nobody knows how much time is needed for life to arise."

I'm always fascinated when we find this sort of thing, because it seems like every time we get new information affecting the terms of the Drake Equation, the estimates are revised upward.  At first, we didn't know if planet formation was at all likely, or if the Solar System was a fluke; now it seems like exoplanets are kind of everywhere we look, and most stars have planetary systems.  Most stars that have been studied have at least one planet in the habitable zone, and the size of the habitable zone is way bigger than we used to think.  Forming the biochemistry of life turns out to be simple; like exoplanets, complex organic molecules turn out to be all over the place.  And so on.

So could Mimas host life?  Entirely possible.  "Not life as we know it, Jim" -- but life nonetheless.  I still think that Europa and Enceladus are more likely (remember the end of the movie 2010?  "All of these worlds are yours except Europa, attempt no landing there") but life could well be common, not just out in the galaxy but right here in our own Solar System.

And maybe I'll live to see confirmation of it.  What a monumental overturning of our self-importance that would be.  It'd be a total game changer.  Proving once and for all that life is abundant in the cosmos... and that we are not alone.

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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!]

A titanic undertaking

While I first ran into the idea of life on other worlds when I was a kid watching shows like Lost in Space and Star Trek, it wasn't until I was in college and read Arthur C. Clarke's followup to his novel 2001: A Space Odyssey, called 2010: Odyssey Two, that I first considered life around moons in our own Solar System.

The upshot of the book is that there is a developing intelligent species on Europa, one of the so-called "Galilean" moons of Jupiter.  It's not such a far-fetched idea; Europa has a water-ice crust and might well have liquid water underneath it, so it's entirely possible there's some life form or another living down there.  (In the book, there was, and the super-intelligent civilization that sent the famous monolith to Earth in the previous book starts broadcasting the message, "All these worlds are yours -- except Europa.  Attempt no landings there" in an attempt to keep humans from dropping in and fucking things up, which you have to admit we have a tendency to do.)

Europa is only one candidate for hosting life, however.  An even better bet is Titan, the largest moon of Saturn and the second largest (after Jupiter's moon Ganymede) moon in the Solar System.  It's larger than the planet Mercury, although less than half as massive, and its surface seems to be mostly composed of water and ammonia -- although in 2004 the Cassini-Huygens probe found liquid hydrocarbon geysers at its poles, which is certainly suggestive of some fancy organic chemistry going on underneath the surface.

A photograph of Titan taken by Cassini-Huygens.  Its featurelessness is because we're seeing the tops of the clouds -- thought to be, basically, photochemical smog.  [Image is in the Public Domain, courtesy of NASA/JPL]

In any case, it's a place ripe for some serious exploration.  And it's certainly looking better than even the nearest stars; our fastest spacecraft, Deep Space 1, would take about 81,000 years to reach the nearest star, Proxima Centauri, which is a little long to wait for results.  So I was thrilled to find out that NASA is talking about a mission to Titan -- that involves packs of "shapeshifting" robot drones.

One limitation of any probe we've sent out is that even if it's working optimally, it still can only survey a minuscule percentage of the target's surface.  What the planned Shapeshifter mission does is to send a spacecraft out there that's composed of hundreds (or more) smaller, self-propelled, robotic spacecrafts that can then roam around exploring the surface or dive down and puncture the crust and see what's down in the oceans that we believe exist below it.

"We have very limited information about the composition of the surface," said team leader Ali Agha, of NASA's Jet Propulsion Laboratory.  "Rocky terrain, methane lakes, cryovolcanoes – we potentially have all of these, but we don't know for certain.  So we thought about how to create a system that is versatile and capable of traversing different types of terrain but also compact enough to launch on a rocket."

The difficulty -- well, one of the many difficulties -- is whether we'll recognize life on Titan if we find it.  Besides an atmosphere that seems to be mostly made of ammonia and methane, Titan has an average surface temperature of around -180 C, which is a little chilly.  So any living thing there would have to be adapted to seriously different conditions than anything we've found on Earth.  There's no reason to believe that it would share characteristics with any terrestrial life form besides the most basic requirements for life -- reproduction, metabolism, and some kind of inheritable genetic code -- so we'll have to be pretty willing to expand our definition of "living thing" or we'll likely miss it entirely.  (Remember the Horta from the famous original Star Trek episode "The Devil in the Dark?"  It was a silicon-based life form that used hydrofluoric acid instead of water as its principal circulatory solvent -- and also as a defense mechanism, as various red-shirted unfortunates found out. The intrepid crew of the Enterprise at first thought the Horta was some bizarre geological formation -- which, of course, it sort of was.)

In any case, I hope Agha's project gets off the ground, both figuratively and literally.  If we can't develop faster-than-light travel, and unfortunately Einstein's ultimate universal speed limit seems to be strictly enforced in most jurisdictions, investigating other star systems is kind of impractical.  So we probably should focus on what's going on here at home -- and hope we're not told, "Attempt no landings on Titan."

Although if we were, that would be eye-opening in an entirely different way.

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This week's Skeptophilia book recommendation is especially for those of you who enjoy having their minds blown.  Niels Bohr famously said, "Anyone who is not shocked by quantum theory has not understood it."  Physicist Philip Ball does his best to explain the basics of quantum theory -- and to shock the reader thereby -- in layman's terms in Beyond Weird: Why Everything You Thought You Knew About Quantum Physics is Different, which was the winner of the 2018 Physics Book of the Year.

It's lucid, fun, and fascinating, and will turn your view of how things work upside down.  So if you'd like to know more about the behavior of the universe on the smallest scales -- and how this affects us, up here on the macro-scale -- pick up a copy of Beyond Weird and fasten your seatbelt.

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