The oceans of Europa

On Monday, we looked at some new techniques for detecting exoplanets, and how exciting that is for people like me who are obsessed with extraterrestrial life.  The difficulty, of course, is that even if you find a thousand Earth-like planets in the habitable zone, how could you tell if they're inhabited?  Consider how long the Earth has hosted life (on the order of three billion years) and how long it's been that we've had living things that would be detectible to distant aliens via such signals as radio and television transmissions (on the order of a hundred years).

Life has been present on Earth thirty million times longer than it would be easily detectible to a technological species in another stellar system.

So life could be more or less everywhere out there.  But how could we find it?

The possibilities are limited.  There's been a suggestion that one way is to analyze the atmosphere and look for chemicals like oxygen that are highly reactive and probably wouldn't persist unless there was something (i.e. something living) pumping it in at a constant rate.  But finding planets at a distance is one thing, and analyzing the atmosphere of those planets from something like the faint absorption spectrum lines of the light reflected from the surface, is another thing entirely.

So unless we stumble across a technological society whose signals (deliberate or otherwise) we detect, we could be looking at stupendous biodiversity and never know it.

A different approach is to look for life closer to home.  One of the goals of the Mars rover expeditions was to analyze the soil for the presence of bacterial metabolism, the results of which were equivocal at best.  Venus is out, unless there are life forms that enjoy being in an acidic pressure cooker, which seems unlikely.

But what about the outer planets?

If there is life on the gas giants themselves, it'd have to be unlike anything we have here on Earth.  Carl Sagan hypothesized giant "floaters," creatures like enormous parachutes, that would ride the stormy updrafts of the thick atmosphere and metabolize the methane that makes up a good percentage of it.  But what about the moons?

The moons of the gas giants are possible candidates, as they're small and rocky, and at least some of the larger ones have a stable atmosphere.  The problem is, they're cold.  Ganymede, for example, Jupiter's largest satellite, has a mean surface temperature of about -160 C.

Not exactly your next tropical vacation spot.

But it's not as hopeless as all that, because there's more going on here than meets the eye.  Which brings me to Jupiter's moon Europa.

Europa, showing the cracks in the ice sheet colored by what may be a mix of minerals and organic compounds from the subsurface liquid water ocean [Image is in the Public Domain courtesy of NASA/JPL]

Europa is the smallest of the four "Galilean" moons, so called because they were first observed by Galileo Galilei, which led him afoul of the powers-that-be when he claimed they were orbiting Jupiter (Earth, remember, was thought to be the center of the universe, with everything orbiting around it).  But eppur si muove, as Galileo muttered when he was found guilty of heresy ("and yet it moves"), and the total number of Jovian satellites now stands at 79.

Europa, however, is especially fascinating.  Pioneer 10 and 11, the (aptly-named) Galileo orbiter, and most recently the New Horizons probe, have all brought us back a picture of Europa that's curious to say the least.  It's got a surface made mostly of water ice, but its overall density is consistent with being mostly composed of silicate minerals (so it's a rocky ball underneath its icy covering).  Most interesting, though, is that magnetometer readings support the conclusion that between the two is a layer of ion-rich liquid water on the order of thirty kilometers in depth -- a subsurface sea rich in magnesium, sodium, and calcium, traces of which are found on the surface when the cracking of shifting of the ice sheet allows the seawater to bubble up and freeze onto the surface.

This makes Europa an excellent candidate for hosting life, at least the microbial type.  But what's keeping the water liquid, so far out there in the cold reaches of the Solar System?

Apparently, its host planet is.  Jupiter is enormous, and has Europa tidally locked (the same side faces Jupiter all the time).  Its orbit isn't perfectly circular, though, and as it zips around, tracing out an ellipse, the changes in gravitational pull generate tidal flexing.  The giant planet pulls and distorts the core of the moon, and the friction that creates generates enough heat to keep the subsurface ocean liquid.

So we have something a little analogous to the Earth's hydrothermal vents, albeit powered by a different process.  It means that (though it saddens me to admit) it's probably better to put our money into sending a surface probe to Europa, to look for traces of life frozen onto the surface, than scanning the skies looking for life outside of the Solar System.

We could do both, of course.  That'd make me happy.  But given the budget cuts to NASA in the last few years, they've got to put their time, effort, and money into whatever is going to have the greatest likelihood of working.  At the moment, that looks like the search for extraterrestrial life should focus on our own neighborhood -- starting with a moon that initially looked like a lifeless ball of ice.

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This week's Skeptophilia book recommendation of the week is brand new -- only published three weeks ago.  Neil Shubin, who became famous for his wonderful book on human evolution Your Inner Fish, has a fantastic new book out -- Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA.

Shubin's lucid prose makes for fascinating reading, as he takes you down the four-billion-year path from the first simple cells to the biodiversity of the modern Earth, wrapping in not only what we've discovered from the fossil record but the most recent innovations in DNA analysis that demonstrate our common ancestry with every other life form on the planet.  It's a wonderful survey of our current state of knowledge of evolutionary science, and will engage both scientist and layperson alike.  Get Shubin's latest -- and fasten your seatbelts for a wild ride through time.

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!]