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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Violent moon

If I had to vote for the single weirdest place in the Solar System, my choice would be Jupiter's moon Io.

Io is the innermost and third-largest of the "Galilean moons" of Jupiter, the ones that caused so much trouble for poor Galileo Galilei when he observed them in 1610 and informed the Catholic Church powers-that-be that we aren't the center of the universe.  It wasn't until the Voyager flybys in the late 1970s that we could see it as anything more than a fuzzy dot, even in the largest telescopes; the first close-up photographs invited comparisons to a moldy pizza,  Detailed photos from the Galileo probe in 1999 confirmed the original assessment: Io is one bizarre place.

1999 photograph of Io from the Galileo probe [Image is in the Public Domain courtesy of NASA/JPL]

The first weird thing about it is that it is the most tectonically active place in the Solar System.  Those pock-marks on the surface aren't impact craters, they're volcanoes.  In general, the smaller a body is, the less tectonically-active you might expect it to be.  Tectonic activity is (usually) triggered by convective fluid motion in a molten mantle or core, which requires a very hot interior to keep it going.  The heat comes from two sources; the energy released by its coalescence during its formation, and the decay of radioactive elements in its interior.  If that heat radiates away faster than it's being released, eventually the body cools off and freezes, and (most) tectonic activity stops.  Heat dissipates more rapidly from a small object, so they tend to shut down much sooner.  (That's what happened to the Moon, for example.)

But despite Io's small size, something is keeping it hot enough to create hundreds of active volcanoes.  But what?

It turns out it's the proximity to Jupiter.  The giant planet's gravitational pull creates significant tidal forces, and the stretching and compressing Io experiences generates enough friction in the moon's interior to keep the insides molten.  The result: violent volcanic activity that spews liquid sulfur jets into the sky, creating plumes as much as five hundred kilometers in height.  (It's the sulfur that's responsible for Io's bright colors.)

In fact, Io actually ejects so much material from its volcanoes that it has created a plasma torus around Jupiter in its wake -- a donut-shaped ring of charged particles tracing out its orbit.

Another cool thing about Io is that it's in orbital resonance with two of the other Galilean moons, Europa and Ganymede.  Io is the innermost, and has an orbital period exactly twice as fast as Europa and four times as fast as Ganymede -- a stable configuration that has since been found in other systems with multiple moons.  So every fourth revolution of Io, all three line up perfectly!

The reason this comes up is a new study out of Caltech that has found data suggesting an enormous underground magma ocean inside Io -- planetary scientists David Stevenson and Yoshinori Miyazaki believe the presence of a hundred-kilometer-thick liquid mantle explains the extremely active surface and its anomalous magnetic field, another feature Io shares with few other small bodies in the Solar System.

What lies deeper than the mantle is unknown.  Some astrophysicists believe it has a metallic core, but that question is far from settled.

What's certain is that Io is a peculiar place -- sulfur volcanoes, seething lava lakes on the surface, continuous "moonquakes" caused by the tidal forces exerted by the enormous planet Jupiter looming overhead.  And like anything odd and unexpected, it will continue to attract the attention of scientists, and we will continue to be astonished at what we learn about one of the weirdest places in our neighborhood.

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The dunes of Io

By anyone's standards, Jupiter's moon Io is a strange place.  It is by far the most geologically-active body in the Solar System, which is extremely unusual for an object its size.  Since tectonic forces are created by heat generated in the core, and smaller objects radiate away heat faster, it was thought that most planetary moons should be tectonically dead -- essentially, frozen in place.

What keeps the fires in Io going are the tidal forces between Jupiter and the other three "Galilean" moons (so called because they were first spotted by Galileo Galilei in January of 1610, and were instrumental in his championing of the heliocentric model of the Solar System).  But from earthbound telescopes all four just looked like points of light, despite the fact that as moons go, they're pretty big.  In fact, the largest of them -- Ganymede -- is bigger than Mercury, with a radius of 2,634 kilometers (as compared to Mercury's 2,440).  The four, the two aforementioned plus Europa and Callisto, were all named for various of Zeus's lovers, which meant astronomers had an extensive list of names to choose from, given that 95% of Greek mythology was driven by Zeus's inability to keep his toga on.

In any case, the push-and-pull of the gravitational forces from Jupiter and its moons stretches Io, and the friction thus created generates enough heat to keep its core (thought to be made mostly of iron, like Earth's) molten.  This thermal energy drives tectonic forces that dwarf the most violent volcanoes and earthquakes here on our planet.  Io has extensive lava flows, some over five hundred kilometers across.  Its volcanoes have ejected so much debris that there is a plasma ring surrounding Jupiter, sketching out Io's orbit.

We got our first good images of Io from Voyager 1 and Voyager 2 in 1979, and from its brightly-colored, pockmarked surface astronomers said it "looked like a moldy pizza" -- a vivid image that is certainly apt enough:

An image of Io taken, appropriately enough, by the spacecraft Galileo in 1995 [Image is in the Public Domain courtesy of NASA/JPL]

The bright yellows and oranges come from crystalline sulfur, which is abundant on the moon's surface.  Also common on its surface is sulfur dioxide, which at Earth's surface temperatures is a colorless gas that smells like rotten eggs; at Io's temperatures, averaging at 110 K (about -160 C), it's a crystalline solid.  The rest is mostly made up of silicate rock and sand.

There's still a lot we don't know about this peculiar place.  One of its odd features is that it has dunes, some of them over thirty meters high.  This should be impossible, as dunes are caused by fluid flow -- on Earth, either wind or water -- and Io has essentially no atmosphere and no liquid component of any kind on the surface.  But a recent paper published in Nature Communications explains a way that dunes can form without any wind; once again, it's caused by Io's extreme volcanism.  The study found that if there's at least a ten-centimeter thick layer of sulfur dioxide ice, and it is contacted by the subterranean (well, subionion) lava flows, the ice sublimates rapidly and explosively, blowing plumes of gas and debris at speeds of up to seventy kilometers and hour, reaching as much as two hundred kilometers high.

The force, though, isn't just exerted upwards, it's exerted outward.  This lateral blast moves enough of the sand and rock on the surface to generate Io's extensive dunes.  A combination of two things -- Io's low gravity and lack of an atmosphere -- means that the airborne debris can move a lot farther than a similar flow could do on Earth.  So while at first glance the processes seem similar to what we know of planetary geology, it's (as far as we know) unique in the Solar System.

"In some sense, these [other worlds] are looking more familiar," says George McDonald, a planetary scientist at Rutgers University, who co-authored the study, in an interview with Science News.  "But the more you think about it, they feel more and more exotic."

If you want to experience mystery and wonder, just look up.  The night sky is filled with a myriad places we are only just beginning to understand.  As French physicist and mathematician Jules Henri Poincaré put it, "Astronomy is useful because it raises us above ourselves; it is useful because it is grand; …  It shows us how small is man's body, how great his mind, since his intelligence can embrace the whole of this dazzling immensity, where his body is only an obscure point, and enjoy its silent harmony."

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