Across ages, epochs, and eras, meanwhile, a paltry puff in the cloud that would become our own oozed fluidly and with a sonorous turbulence. From its nascency, gravity really worked on our little parcel of nebula, drawing it ever inward on itself, slowly but inexorably, until at last, nearly five billion years ago, it buckled and collapsed. As it did, its erstwhile inner turbulence, those fluid motions, caused our contracting cloud to exhibit a net sense of rotation. Our tiny wisp began to spin. The more material it drew in, the faster it spun, and though the sun today could hold more than one million Earths, it began like this, as a swelling union of dust and hydrogen, and it grew and grew and grew, its rapacious core inhaling everything available, growing ever denser, more massive and molten, one mote of dust at a time until nearly all of the cloud was consumed. It was becoming a protostar.
The spared, swirling fraction of a fraction of gas and dust on the protostar’s fringes flattened all the while into a thin disc of vast diameter, the way pizza dough stretches and flattens as it is tossed. These particles, they aspired to such sizes as grains of sand, and perhaps one day grains of rice, but for now, they were but specks circling some crazy ball bursting from within. Over time they clustered by chance into particle pelotons behind which other specks could hide, drag diminished, energy saved, the groups growing bigger and bigger still. The clusters’ comfortable wakes tempted more things yet, and in due course these growing balls of material reached sizes sufficient to start self-gravitating and attracting yet more stuff, until they themselves collapsed into solid celestial objects: the first asteroids. Onward they went, colliding with one another and growing larger and larger as they accumulated debris and other solid material. The farther from the swirl’s center you were, the lower the temperature, and thus the more solids these planetesimals had handy, because a new building block was introduced to the material available: ice. This swarming supplementary matter allowed planetesimals to grow ten times larger than Earth, itself now forming from rock and metal nearer to the disc’s interior. Planetesimals soon showed signs of becoming protoplanets with attendant superhot centers and such distinct, differentiated layers as crust, mantle, and core. All of this was happening at once: the protoplanetary disc, the protostar, and the whole thing still submerged in the thinning local nebula.
Then, the awakening. The heat and pressure at the disc’s core, ever increasing and increasing, could increase no more in its present state, until fantastically, this enormous, round, burning-hot thing—almost the entire mass of what was once merely a haze of atoms—reached eighteen million degrees Fahrenheit, and its pressure and heat were now so great that the nuclei of its constituent hydrogen atoms commenced fusing together, creating helium and releasing astounding amounts of energy. As the newborn star flickered to life, it blasted a wave of heat and plasma outward, concentrating the cloud at the orbits of the protoplanets Jupiter, Saturn, Uranus, and Neptune, each of which immediately went to work acquiring this sudden influx of hydrogen and helium. Jupiter, the hungriest of the four, was largest and best positioned to dominate, and within a million years, its atmosphere was as massive as its core: ten Earth masses of solid stuff surrounded by ten Earth masses of hydrogen and helium. Feeling now like a real master of the universe, it then dove headlong into an unstable, runaway phase of accretion, and in ten thousand years—on cosmic timescales, the firing of a brain’s neuron—Jupiter inhaled three hundred Earth masses’ worth of gas.
When a planet forms that fast and that heedlessly, weird things happen around and inside it. Jupiter migrated, first inward, truncating the disc of debris available to the protoplanets nearer to the sun, leaving Mars material enough only to grow twice the size of Earth’s newborn moon. Saturn saved the solar system by pulling Jupiter again outward and away from the sun. The planetesimals now settled into place between Mars and Jupiter, and were so agitated by events and the giant world’s gravity that they were unable to organize into proper planets. Thus was born the asteroid belt.
At the severe pressures of Jupiter’s interior, hydrogen acts like a metal. Hydrogen is simple: it’s basically just a proton with an electron going around it. Take a ball of hydrogen atoms and squeeze it tightly enough—say, a million times the pressure of Earth’s atmosphere—and the atoms get so uncomfortably close to one another that the electrons stop caring which protons they’re orbiting. As long as there’s a proton nearby—any proton at all—the electrons are happy, and they will just start hopping around from atom to atom. The interior has, at this point, become a highly conductive metal fluid: liquid metallic hydrogen. The planet’s core heats the liquid metallic hydrogen, causing it to rise, and once it reaches the outer layer, it cools and sinks, again and again and endlessly, generating in the process a massive magnetic field.
In Earth’s night sky, Jupiter is not all that special: a flickerless pinhole of light in a dome of darkness. If its magnetic field were visible, however, it would be the size of three full moons in our sky.30 When charged particles in space travel through that massive magnetic field, they get trapped and start zipping around at the speed of light. Space is a deep, dark, deadly domain, infinite in its dangers, but it is a boundless Switzerland compared with the wilderness of this, the Jovian radiation belt.
As for the rest of the solar system, what nebular material was not consumed by Jupiter, Saturn, Uranus, and Neptune was at last blown away by the stellar winds. It took five hundred million years for all of this to happen, from collapsed cloud to solar system, planets circling and a star to steer them by.
After four and a half billion years, the solar system more or less settled, with a thin layer of life having taken hold on Earth. There, an artist named Giusto Sustermans painted what would become the definitive portrait of physicist-philosopher Galileo Galilei.31 The somber, sober subject was seventy by then, round, wrinkled, bearded lavishly, and posed touching a telescope. Draped in black, a white collar wraps around his neck, and higher up, silver brows furrow below a hairline struggling valiantly to hold on a little longer. The man in the painting seems wise, but more than anything else, he just looks tired.
But when Galileo discovered what he called the “stars of the Medici” orbiting Jupiter, he was preening, dynamic, forty-six, and famous. Already, he was a caustic seventeenth-century version of Carl Sagan. He wrote poetry. He loved wine and women. He sold science to the masses (his books were published in Italian rather than Latin), and in salons from Pisa to Padua, he took his ideas on tour, throwing down against fellow philosophers.32 A graceful winner he was not (and win he did; he was Galileo, after all). He seemed to find a satisfaction only in the scholarly equivalent of Mortal Kombat finishing moves, extracting twitching spines and still-beating hearts from his debate opponents.
Galileo certainly had the CV for such high self-regard. At twenty, he had discovered the law of the pendulum: that the period of its swing is independent of its amplitude. (This had real ramifications for timekeeping, though he would be dead before they could be leveraged.) At twenty-five, he took a swing at Aristotle, asserting that two objects dropped from a great height would land simultaneously regardless of weight; density, he declared, was the deciding factor. He was right and, naturally, was not modest about it.
Cosimo II de’ Medici, the grand duke of Tuscany, eventually made Galileo court mathematician. (Only the pope would have been a better or more powerful patron, and Galileo was friends with him, too.) In 1610 the acerbic Italian fixed a modified spyglass on planet Jupiter and discovered three tiny stars in its vicinity. Observations over several evenings found a fourth, and movement, and it didn’t take long for Galileo to work out what he was looking at: objects in space circling another object. There was no end of implications to this, and no better scientist to make the discovery. Heliocentrism had been around for a while—the Greek astronomer Aristarchus of Samos advanced the idea in the third century BC, though his work was lost in the centuries to follow.33 More recently, just before his death in 1543, the Polish astronomer Nicolaus Copernicu
s had published On the Revolutions of the Heavenly Spheres, positing that there might be multiple centers of motion in the universe: that the planets circle the sun, and the moon orbits Earth. Absent proof, however, everyone went right on thinking that Earth was center of all things because it was safer, made more sense, and had zero incompatibilities with Scripture and its armed enforcers. Galileo had no such qualms, however, and beat his drum—hard—upending cosmology itself and humanity’s Very Special Place Indeed in the universe. He probably thought a lot about that while spending his last days under house arrest, having annoyed the Inquisition with this heliocentrism business.
The four stars he found were, in fact, moons—Io, Europa, Ganymede, and Callisto, now collectively called the Galilean moons. (They are named for the lovers of Zeus, king of the gods on Mount Olympus, though Galileo did not name them. A German astronomer named Simon Marius did the christening, having had the misfortune of discovering the moons one day after Galileo.)34 So dominant was Jupiter in the whirling, collapsed cloud of cosmic dust and gas that formed the solar system, that it attracted a disc of its very own called the Jovian subnebula. It was a microcosm of the wider solar system, with Jupiter playing the role of the sun and its moons the planets.35 The stuff closest to Jupiter formed a world of rock and metal—Io—and moving outward, as temperatures dropped, ice increased as a formative factor. Europa is made of a lot of rock and some ice. Farther out, Ganymede is icier still, and Callisto—the most distant of the lot—is the iciest of them all.
Europa is the smallest of the quartet. It is a little smaller than Earth’s moon, with a little less gravity. “Small” in celestial objects is relative; one standing on Europa’s surface would notice no curvature on its horizon. Its atmospheric pressure is about one-trillionth that of Earth, meaning, in effect, that to stand on Europa is to stand in space. Above, two major moons hang in the darkness: Ganymede, giant, its pale-bronze hammered surface sometimes smooth, sometimes sharp, scratched, and scaly, splotches of snow seemingly splattered at random; and Io, an unsettling, Gigeresque orb, yellow with stains of orange and brass. Farther out, Callisto, brown and speckled like amphibian skin. There are more than a hundred other moons that could be witnessed on the dome above if you brought a good pair of binoculars, but nothing unsettles the soul like Jupiter, twenty-four times larger than a full moon in Earth’s sky, this looming leviathan, this aptly named and veritable god of planets, robed in bands of tans and reds—a spherical windstorm in space—its clouds of hydrogen and helium ever a slow churn driven by some unknown force from deep within its interior.
Looking down and to the horizon, an astronaut on Europa is casting her eyes across a postapocalyptic Antarctica: an endless tundra of gashed ice. In places, it is snowman white—the stuff of pure water. Elsewhere, it is sepia, seared and poisoned by the radiation belt into which Europa is submerged. Those gashes: in shadows they are cinnamon, scarlet, sienna, and they break up the landscape as though the whole world had been smashed on a marble floor and then reassembled haphazardly. There are steep cliffs and deep troughs and Grand Canyons of ice the color of prison cells. In places, the lacerations curl and meander like spaghetti. Some icebergs tower, some stoop in subjugation, and they meet chaotically across the expanse. It is three hundred degrees below zero Fahrenheit, and there is no weather, no wind, no rain, but there is ferocious radiation: Io-borne ions beating endlessly into the ice for billions of years, making some of its surface something almost like snow, depths unknown.36
Beneath the ice, the ocean, the seafloor, is Europa’s thousand-mile-diameter core, which is made of iron. (Just like Earth’s.) The mantle surrounding it is four hundred miles thick and made of silicate rock. (Just like Earth’s.) A sixty-mile layer of liquid water covers the mantle—not some green alien goo that technically fits some Poindexter’s definition of “water,” or water with an asterisk, or water for extremely large values of x, but liquid water. A saltwater ocean. And the whole world is wrapped in an ice shell fifteen miles or so thick. (Unlike Earth.)
Here is why there is a liquid saltwater ocean on a small moon so far from the sun that its surface is three hundred below zero: 1. gravity, 2. mighty Jove, and 3. the odd interplay of cohort moons. In the time it takes Ganymede to make a single lap around Jupiter, Europa has circled it twice, and Io four times. As they orbit, they pull each other toward and away from Jupiter. Because of the clean numbers—4:2:1—their “orbital resonances” are stable. They’ll go right on doing this forever. (Were they unstable, the forces at play would eventually rip the moons apart. This happened once at Saturn, and its breathtaking rings are a cemetery made of the moons’ mortal remains.) An effect of these contra dancing orbs is that their orbits are not perfectly circular. Sometimes they are nearer Jupiter in an orbit; sometimes they are farther away. When a moon is close to Jupiter, the giant planet’s astounding gravity, in effect, stretches it. When a moon is farther out, it gets respite. Closer, clenching. Farther, solace. It is like squeezing a tennis ball repeatedly. The more you do it, the warmer the ball gets.
If ever Io were an icy world, those days have long passed. No ice could survive the forces so near Jupiter, the heat, the insistent choking by a planet so immense. Io is the most violent body in the solar system; at any one time, there are more than a hundred volcanoes actively expelling hot rock into space. Under the force of Jupiter’s gravity, Io is literally turning itself inside out. Here on Earth, the moon we see today is the same one the cavemen saw, and the dinosaurs before them. Things just haven’t changed that much for eons. But in fifty years, the surface of Io—a moon just slightly larger than our own—will look totally different.
Europa has just enough space from Jupiter that it’s able to hold on to its ice, but not enough distance to be unaffected by the situation in which it finds itself. Those cracks on Europa’s surface are crushed ice under Jove’s flexing fist. It heaves, this body!—the Europan tides rising and falling by a hundred feet or more over its three-and-a-half day revolution of Jupiter.37 The friction caused by Europa’s eccentric orbit becomes heat welling up from the constricted mantle, and though Europa’s surface is six times colder than Antarctica in winter, twelve miles down, the ice becomes water in a flowing, relatively warm ocean.38 And there is a lot of water in that warm, swirling, gushing, meandering, swelling sea: three times as much as there is on Earth, with global currents more powerful than Earth’s as well.39
To create life as we know it, a world needs organic molecules—compounds with carbon, hydrogen, nitrogen, and oxygen, which most planets have. Such life requires water for processes such as ingestion, metabolism, and excretion, and while water is less common than organics, it is certainly found beyond the third planet. Life lastly craves chemical energy, which is the toughest of the three to come by. There is no sunlight in the Europan ocean, and thus no photosynthesis. It’s ink black down there. But on the ocean floor, water touches rock, which is conducive to interesting chemistry. And what Europa lacks in sunlight, it makes up for with unbridled chemical reactions powered by something else. Though its seafloor is a wondrous mystery, all the same, planetary scientists have a pretty good hypothesis for what it looks like: it looks like Io.40
It’s not quite as dramatic down there—Io is Mordor, and Europa’s distance helps stave off Sauron’s more malevolent designs. But the terrible forces causing Io to hemorrhage its entrails are at work on Europa as well, and, on its ocean floor, great geysers gush heat and chemical compounds from the mantle and into the water. This happens on Earth, too, but for different reasons: at the bottom of our ocean, hydrothermal vents ceaselessly billow brutal columns of scorching water, an endless supply of nutrients blasting from the bowels of Earth, and life teems there—life simple and complex—despite the total absence of sunlight.
Ganymede, meanwhile—the third of the three moons swinging about—is an icy world being squished by Jupiter. But because its shell is so thick, it manages to melt only in the middle, leaving its water sandwiched between ice layers. Its
water is never afforded the chance to touch rock, which is a bad thing indeed if you want the kind of chemistry that likely yields fish.
No one knows how you go from lifeless material to living material. It is the eternal mystery: Where did I come from? What scientists do know is that it takes a long time to happen. A celestial object can have all the right ingredients—organic material, water, chemical energy—but if the pie hasn’t had time to set, it doesn’t have life on it. The best guess for life’s baking time in the oven: five hundred million years.
Europa’s ocean has had more than four billion years for life to get started.
The possibility of a Jovian moon growing things that swim hasn’t escaped the notice of artists since Sustermans. Author Arthur C. Clarke and film director Stanley Kubrick developed the plot and themes of a story involving the Jovian system based loosely on a much older story by a poet named Homer. In 1964 Clarke went off and wrote his novel, 2001: A Space Odyssey, while Kubrick drafted a screenplay of the same name, and along the way, the two compared notes on each other’s works, incorporating the best stuff into each. The film premiered in 1968, two months before the book hit shelves, and though Clarke had written a masterpiece, Kubrick had created something superlative: a piece of artwork from which there was no turning back.
The pacing and storytelling of the two works track similarly. A magnetic anomaly pulsing from the lunar interior leads astronauts to drill a core sample of the area in order to work out what is happening there. Twenty feet down, when the drill is stopped cold by something unexpected and impenetrable, they begin to dig and soon call in a team with serious hardware to do a proper excavation.
The Mission Page 3