Searching for Life Where the Sun Donít Shine (part 4): Explorations to the Seafloors of Earth and Europa
This is Part 4 of a 6-part series telling the story of humankind’s efforts to understand the origins of life, by looking for it in extreme environments where life thrives without relying on the Sun as an energy source. It follows an oceanographic expedition to the Mid-Cayman Rise led by Chris German of the Woods Hole Oceanographic Institution, and NASA’s efforts to plan a future mission to Jupiter’s moon, Europa. By understanding how life can live without the Sun we may discover how life began on our planet, and whether or not Earth is the only place in the universe capable of supporting a biosphere.
The largest of Jupiter’s moons are the four Galilean moons—Io, Ganymede, Callisto, and Europa—so named for their discoverer, Italian astronomer Galileo Galilee. Galileo made the discovery in 1610 after tweaking his telescope to reach a magnifying capability of 20x (20 times better than the naked eye). These four worlds were the first moons discovered orbiting a planet other than Earth (since our own moon is awfully hard to miss, we’ll probably never know which of the first humans looked up and said, “what’s that?” Thus, it’s probably safe to credit Galileo with discovering moons).
The Galilean moons proved to be full of surprises at close range. An active volcano was spotted on the innermost of the Galilean moons, Io, spouting ash and gases a whopping 174 miles in height—the first evidence of active volcanism beyond Earth. The team would find half a dozen more erupting volcanoes on Io over the course of the mission. Team leader Brad Smith said this made Io look like a pizza. Later, astronomers would identify more than 400 active volcanoes there, making Io the most geologically active world in our solar system.
You’d think such a world might be a hot one, but you’d be wrong. Most of Io’s surface is minus 150 degrees Celsius (minus 238 Fahrenheit). Colossal eruptions of gases condense and fall back to the surface as pastel yellow, orange and bluish white sulfur dioxide snow. This makes Io a surrealist’s dream: fiery hell plus winter wonderland with an everything-on-it sulfuric pizza surface (remember the egg smell?) and the churning clouds of Jupiter looming above hazy skies.
As previously mentioned, volcanic activity on Earth is produced by a heat source that comes from energy released from the decay of radioactive materials deep inside the planet and also leftover heat from its formation. But Io is too small for leftover heat and too active to explain its volcanoes by radioactive decay alone. Instead, the strong gravitational force from Jupiter pulls Io inward, while the weaker gravitational forces of its neighboring Galilean moons, Europa and Ganymede, pull Io outward. These three moons actually fall into an elegant, stabilizing dance coined orbital resonance, where Io travels around Jupiter exactly twice as fast as Europa, which travels twice as fast as Ganymede. This makes the distance between Io and Jupiter vary along its orbit as the moon fights with the competing forces (or tidal forces). These tremendous tidal forces alternate between squeezing and stretching the moon’s core, causing Io’s surface to rise and fall by the length of a football field every time it completes an orbit, which it does every 42.5 hours. This squeezing and stretching causes friction. Friction causes heat and pressure. Heat and pressure cause molten material and gases to explode from the surface. Hence, volcanoes.
Io’s smoking gun was pointing straight at its neighbor, the next closest Galilean moon to Jupiter: Europa. At the time of Voyager 1’s flyby of Io on March 5, 1979, scientists had just begun to re-think the internal makeup of the Galilean moons. If there was tidal heating on Io, partially caused by gravitational forces of Europa and Ganymede, wouldn’t there be similar forces acting on Europa and Ganymede, too?
By the time Voyager 2 reached Europa, the team’s hopes were soaring. They knew Europa was rich with water in its icy outer layer. If there was tidal heating on Europa, could it be enough to melt the bottom of the moon’s ice crust, perhaps enough—gasp—to sustain a liquid water ocean?
Some even imagined they’d see several miles-long geysers bursting from the icy shell. But when the first photos came in, the team found no geysers. Instead, they were confronted with a giant white eyeball staring back at the camera, striped with wide brown veins slicing across the entire surface.
If you were to walk across that surface, Europa’s stripes would appear almost painted on. There’s almost no elevation change across most of the moon. Some scientists think the stripes may be cracks in the ice that spread pieces of the crust apart—not unlike seafloor spreading on Earth—allowing for liquid water to well up from the ocean beneath and freeze again, forming a younger swath of icy surface.
Despite the absence of geysers or volcanoes on its surface, Europa does experience strong tidal forces. On Earth, the tidal force from our moon controls the ocean tides, which influence when ships leave for a day of fishing or whether a sand castle on the beach will meet its demise an hour after its construction. But on Europa, the effects of its host planet are a bit steeper. Jupiter is so much bigger than our moon—30,000 times more massive—and so close to Europa that its tide stretches the 2,000-mile wide moon by more than 100 feet. This also makes the moon’s orbit ever-so-slightly non-circular, giving the orbit a tiny eccentricity, but one with enormous implications. Huge tidal forces are the crucial factors that suggest Europa’s ocean is able to remain in its liquid water phase instead of freezing completely into the layer of ice above it. According to planetary scientists, everything interesting about Europa follows from this subtle eccentricity.
If an electrical conductor is placed in a varying magnetic field, electrical currents will be induced in it. Those currents will then induce a magnetic field oriented in the opposite direction to the original magnetic field that generated the current, partly canceling it out. Since Jupiter has a powerful magnetic field and its axis of rotation doesn’t exactly line up with its magnetic axis, Europa crosses from the north side to the south side of Jupiter’s magnetic field—or vice versa—every five and a half hours. Imagine you are Jupiter and Europa is orbiting around your belt. Now, imagine trying to learn to hula-hoop for the first time. The wobbly hoop is the orbit of Jupiter’s magnetic field—and every time it touches your belt, it’s wobbling in a different direction. Up, down, up, down, up, down. This causes the orientation of the magnetic field in Europa’s orbit to vary as Jupiter rotates.
In Europa’s case, a magnetic field oriented in the opposite direction to Jupiter’s would indicate the moon has a massive electrical conductor somewhere inside it. And it just so happens that a moon-swallowing subsurface saltwater ocean matches the prediction for this electrical conductor quite nicely.
Along with its two compatriots Io and Ganymede, Europa dances a violent, synchronized tango around its host, causing friction and inducing a magnetic field—enough, some scientists hope, to indicate the presence of a sloshing ocean somewhere beneath miles of occasionally shearing ice.
Below that potentially 100-mile deep ocean, there may be a seafloor that’s not so different from our own.