Alien Oceans: The Search For Life in the Depths of Space (Kevin Hand, 2020) 

We were stuck on the bottom. Batteries were running low. Our air was running out. We had no way to communicate to the other submersible or to the team on the boat some 10,000 feet above us. We were nestled in the metal sphere of our tiny submersible, perched on some rocks at the bottom of the Atlantic Ocean.
This was my first trip to the ocean floor, and it had the makings to be my last.

Yet somehow it seemed peaceful. With what little light we had left I was able to look out through the porthole of three-inch glass and see a long, red creature exploring the surface of a rock, perhaps looking for its next meal. There it was, going about its business, with no concern or awareness of our precarious situation. It’s easy for things to turn surreal and serene in a tiny submarine. Our brains have no way of processing the reality of the situation: several thou- sand pounds of pressure per square inch, a landscape revealed only by the limited lights on the sub, odd sounds and whirls of the machine that’s protecting you from a gruesome, watery death. Unlike experiencing a fear of heights or a bone-dry desert, there’s nothing in our Homo sapiens soft- ware that knows what to do when cooped up in a metal ball at the bot- tom of the ocean. We had no connection to a mouthpiece and breathing regulator like scuba divers and had no need for decompression on the way up. In a submersible you simply breathe the air around you. Sure, there’s the potential for claustrophobia with three people crammed into a sphere two meters in diameter and the darkness of a world cut off from sunlight; but if you can get beyond that, it’s really quite nice. That is, if everything is working as it should. I actually had no business being down there. First, as a human I was clearly out of my biological comfort zone; I needed technology to make this trip possible. But second, and perhaps more important, my profes- sional realm had long been that of the stars, planets, and moons. A child- hood obsession with aliens had led me down a path to studying Europa, an ice-covered moon of Jupiter that had recently been revealed to har- bor a vast subsurface ocean. I was in the midst of my PhD studying Eu- ropa’s physics and chemistry when fellow space nerd and longtime friend George Whitesides rang me to gauge my interest in an exciting project: James Cameron, filmmaker of Titanic, Terminator, and many other successful movies, was looking for a young scientist to talk about Europa while exploring the depths of our ocean. Would I be interested in potentially joining the expedition? It was not your everyday phone call.

The year was 2003 and Cameron wanted to make a film about the deep sea and the prospect of searching for life in Europa’s ocean. The team would explore the seafloors of the Atlantic and Pacific Oceans, studying how life survives in the dark depths—conditions that might be compa- rable to those found on Europa. My role would be to help connect ocean exploration with the search for life beyond Earth. The deep-sea hydro- thermal vents that we would explore serve as chemical oases for life in the ocean’s depths and provide some guidance in our search for habit- able environments beyond Earth.
And so, a month after that phone call from George, I found myself on the Russian research vessel Keldysh, floating above the middle of the At- lantic Ocean, preparing to explore the darkness below instead of the stars above.

The ocean had long been magical to me—not just because of its vast- ness and great depths but because it was a place with which I was largely unfamiliar. I had grown up in the landlocked state of Vermont. Set me down in the mountains or in a cave and I’d do fine, but the ocean was a foreign environment to me. With a combined sense of uncertainty and anticipation, I turned my attention to the machines that would take us into this extreme environment—the Russian Mir submersibles (in Russian, mir means peace or world). I spent time in the machine shop on the Keldysh, com- municating with hand gestures and the occasional Russian word. The basic goal of the physics of a submersible is pretty straightforward: (1) don’t get crushed, and (2) make sure you can float when you need to. Unlike space exploration, gravity is your friend when exploring the ocean floor. To get back and forth from space requires any number of variations on rocket engines, heat shields, parachutes, and wings. To get back and forth from the seafloor, the general idea is to carry a weight on the way down and then drop it when you want to come up. Although the change in pressure in the ocean is fairly extreme, there’s not a huge tem- perature difference, and you’re never traveling that fast. As long as your sub can withstand the pressure and is buoyant once you drop the weights, you’ll rise to the top of the ocean like a cork.

The basic underlying simplicity of how a submersible moves up and down through the water was central to my getting comfortable with the Mir subs. Genya, Viktor, and Anatoli—three of the pilots and engineers who endured my questioning—explained the various backup systems and redundancies the subs boasted. For the most part, the subs followed the prime KISS rule of engineering design: Keep It Simple, Stupid. They had relatively few moving parts, and the electronics seemed like hardy relics from the Cold War. Nevertheless, my stream of “What ifs?” even- tually led to the worst-case scenario: What if you’re many kilometers down at the bottom of the ocean and your power fails, your thrusters fail, your communications link fails, and you start running out of air? You’re just sitting in a fancy hunk of metal trapped at the bottom of the ocean. What then?

Not surprisingly, there was a plan. In that scenario, you lift up one of the seats in the sub and find a big wrench. That wrench is used to loosen a large nut on a bolt that is connected to a weight. Once that weight has dropped, the sub becomes positively buoyant. It should start to rise off the bottom of the ocean and gradually accelerate as it rises upward. Ac- cording to the engineers, by the time the sub reaches the surface, it will have amassed so much momentum that it would likely pop out of the ocean and into the air, rising a few meters above the ocean surface. It’s not pretty or high-tech, but at least you’re not dead on the bottom.

Being in a submersible at the bottom of the ocean feels like a hot air balloon ride, scuba dive, and space mission rolled into one. (Mind you, of the three, I’ve only ever scuba dived.) Motions are, for the most part, slow and smooth. The Russian Mir submersibles offer a small (approxi- mately eight-inch) porthole from which to peer out into the depths: one porthole for each occupant, three in total.
On the way down to the bottom, when the machine is dropping like a stone, you pass rapidly through the photic zone—the uppermost layer of water, approximately 300 meters deep, through which sunlight shines and in which life thrives off of photosynthetic organisms like phytoplankton.

As you descend through this region, light begins to fade. Blue goes to black. The sub starts to cool. You can’t feel that you’re falling, but the sounds from the acoustic communication system serve as a metronome reminding you of the distance between you and the rest of the world. Loud pings bounce between the sub and the Keldysh every few seconds—exactly the clichéd pinging you would expect to hear while sitting in a submersible but a little higher in pitch and shorter in dura- tion. Every so often, words trickle through the speaker: a sentence or two of Russian, significantly broken up by its journey through the water. For me, with my limited Russian vocabulary, the foreign patter and pings made the environment feel even more alien.

Here, however, I was scouring the depths for a glimpse of biolumines- cence from the host of bizarre creatures that populate our ocean’s depths and that give off pulses of light as they, or things around them, move. The plummeting submersible created a shock wave of bioluminescence radiating away from us. Creatures large and small, from jellyfish to microbes, flashed. It was a sight I’ll never forget, and it’s one that I trea- sured on each of my nine dives. I got into the habit of donning earphones, pairing the biological fireworks with the music of Radiohead and Pink Floyd. I half-expected to find those bands playing on the ocean floor.

Finding this spot on the bottom of the ocean may not seem like a big achievement, but it really was. All of those mapping luxuries that we em- ploy on the surface of the Earth are useless in the ocean. The wave- lengths we use for GPS navigation barely penetrate beyond a few milli- meters into the ocean’s surface. In fact, the ocean is a bad place for transmitting just about any wavelength in the electromagnetic spectrum. Water—the key ingredient to life as we know it—turns out to be very good at hiding much of our own planet from us. Liquid water readily absorbs light, from short to long wavelengths, and thus we can’t “see” or communicate from the bottom of the ocean. This simple fact has con- founded engineers for decades: no electronic mode of navigation or communication works underwater—no cell phones, no Wi-Fi, no GPS, no AM, no FM, no ham (amateur) radio, nothing.

Nothing makes it through the ocean over very long distances, except sound. This is partly why whales and dolphins use sound to communi- cate. And it’s why those acoustic pings kept coming through into our sub from the team on the Keldysh, checking to see whether we were still there and whether everything was okay. The scene through that porthole could be what “home” looks like to most of life in our Universe. Deep, dark, seemingly desolate ocean floors may be some of the best real estate for biology. Recent explorations of our solar system have taught us that—while planets like Earth may be comparatively rare (one per solar system, if you’re lucky)—worlds with deep oceans, covered with ice and cut off from any sky or atmosphere above, could be ubiquitous.

In our solar system, these worlds are actually moons of the giant plan- ets, with names like Europa, Ganymede, Callisto, Titan, Enceladus, and Triton. These are worlds that likely harbor oceans of liquid water today, right now, and their oceans have likely been in existence for much of the history of the solar system. A few of these worlds—Europa, Ence- ladus, and Titan—could even be hospitable to life as we know it.

These ice-covered oceans have no beaches or sandy shores, but they are potentially wonderful, and plentiful, places to call home. The dark depths of those distant oceans may look similar to the deepest regions of our own ocean. Microbes and sea creatures that inhabit our ocean depths might do fine under the physical and chemical conditions thought to exist within the oceans of Europa, Enceladus, and Titan.

The trip back to the surface has a bizarre elegance. Unlike spaceflight, where the return to Earth involves an intense and fiery trip through the atmosphere, followed by aggressive thrusters to fight gravity’s attempt to turn you into ash, the journey back from the bottom of the ocean feels like a slow elevator ride. The laws of physics are a gentle friend, not a foe, as the sub’s buoyancy guides you back home. Gravity does all the work— no thrusters needed, no engines fired, no fear of a parachute not deploying.

If you happen to ascend during daytime, the dark abyss eventually gives way to subtle hints of a star above. Black fades to blue as the most intrepid of the Sun’s rays pierce through the water. With each moment, blue pushes black farther down, and an ocean fed by sunlight emerges. In the final moment, the sub rises and then falls as it bobs up onto the surface of the ocean, once more touching the atmosphere. Sunlight blasts through the portholes, reminding your brain of where it really belongs.

Now safely returned to the surface, we sat bobbing in a tiny speck of orange and white on the vast blue of the ocean. A tiny but considerable speck, returned from an otherworldly experience in the depths, waiting to be picked up by the Keldysh and brought on deck by its massive crane.

NOT PALE BLUE DOT / PALE BLUE DYAD 

If we have learned anything from life on Earth, it is that where you find liquid water, you generally find life. Water is essential to all life as we know it. It is the solvent, the watery broth that makes possible all the chemistry in our cells. Water dissolves many of the compounds that life, large and small, needs to grow and metabolize. Every living cell is a tiny bag of water in which the complex operations of life take place. Thus, as we search for life elsewhere in the solar system, we are primarily searching for places where liquid water can be found today or where it might have existed in the past.
The story of the search for life beyond Earth is, in part, the story of our planet, the pale blue dot,1 reaching out into space, seeking signs of life on other worlds. Like a plant stretching vines out into its environment, our little planet has been sending its robotic emissaries out in spiral ten- drils that circle other planets, probing for answers and sending back information.

We humans have been exploring our solar system with robotic vehi- cles for over 55 years. The first robotic mission to another planet was the flyby of Venus by the Mariner 2 spacecraft on December 14, 1962. Since then, we have sent an armada of spacecraft to study the Sun and a variety of planets, moons, asteroids, and comets, most of which are in the inner reaches of our solar system. Over that same period, we have sent only eight spacecraft beyond the asteroid belt to study the many worlds in the outer reaches of the solar system.

Spacecraft that have gone beyond the asteroid belt—Pioneer, Voyager, Galileo, Cassini, New Horizons, and Juno—have revealed something pro- found about what it means for a world to be habitable. The data returned from those missions have served to revolutionize our understanding of where liquid water exists in our solar system, and by extension, where life might find a home.
We now have good reason to predict that at least six moons of the outer solar system likely harbor liquid water oceans beneath their icy crusts. These are oceans that exist today, and in several cases we have good rea- son to predict that they have been in existence for much of the history of the solar system. Three of these ocean worlds—Europa, Ganymede, and Callisto—orbit Jupiter. They are three of the four large moons first discovered in 1610 by Galileo. The fourth moon, Io, is the most volcani- cally active body in the solar system and does not have water. At least two more ocean worlds, Titan and Enceladus, orbit Saturn. Neptune’s curious moon Triton, with an orbit opposite to the direction it rotates, also shows hints of an ocean below. 

Throughout the history of the solar system, ocean worlds may have come and gone; for example, the large asteroid Ceres likely had a liquid water ocean for much of its early history. Mars and Venus may also have had oceans previously. Early in our solar system’s history, oceans might have been commonplace, be they on the surface of worlds like Venus, Earth, and Mars, or deep beneath icy crusts of worlds in the as- teroid belt and beyond. Today, however, it is the outer solar system that harbors the most liquid water.

When Galileo first turned his telescope toward the night sky and began charting the faint points of light he saw around Jupiter, he set in motion a revolution in physics. Night after night he drew Jupiter and the arrange- ment of these points of light. At first, he concluded that they must be stars that he could not see with the naked eye. He even named them the “stars of Medici” in honor of the Medici family since they were funding his research (Galileo was no idiot).

But through his diligent charting of these points of light, Galileo soon realized that they were not stars; they were moons orbiting Jupiter. His discovery got him into deep trouble with the Spanish Inquisition, and he ended up under house arrest. The idea that a celestial body would orbit anything other than the Earth was heretical. The world view at the time was framed around Aristotelian cosmology—the Earth is at the center of the universe and everything revolves around the Earth. Galileo’s discovery put him at odds with this world view and provided strong evidence for the growing Copernican Revolution, the idea that the planets orbit the Sun and that the stars we see could well be suns with planets of their own.

In the decades that followed Galileo, advances in math and physics would lead to an appreciation that the laws of physics work beyond Earth. Gravity, energy, and momentum govern objects here on Earth as well as on worlds and wonders beyond. Is biology an incredibly rare phenomenon, or does life arise wherever the conditions are right? Do we live in a biological universe?
We don’t yet know. But for the first time in the history of humanity, we can do this great experiment. We have the tools and technology to explore and see whether life has taken hold within the distant oceans of our solar system.

The real leap in deep ocean exploration came in the late 1920s and early 1930s, when the engineering and science team of Otis Barton and William Beebe created and deployed their bathysphere—a hollow, steel sphere only 4 feet, 9 inches (1.5 meters) in diameter, with 3-inch-thick quartz windows. This sphere was connected to a cable on a ship’s winch that could lower it down and haul it up. Electrical cables also enabled communication to the surface and provided power for lights. Beebe’s description of a dive to 2,500 feet in early August 1934 captures his surreal experience: “There are certain nodes of emotion in a descent such as this, the first of which is the initial flash. This came at 670 feet, and it seemed to close a door upon the upper world. Green, the world-wide color of plants, had long since disappeared from our new cosmos, just as the last plants of the sea themselves had been left behind far overhead.”4

On numerous occasions Beebe’s writings and radio broadcasts linked the dark sea, peppered with bioluminescent creatures, to the twinkling stars of the night sky. After his successful dive with Barton to 3028 feet, Beebe wrote: “The only other place comparable to these marvelous nether regions, must surely be naked space itself, out far beyond atmo- sphere, between the stars, where sunlight has no grip upon the dust and rubbish of planetary air, where the blackness of space, the shining plan- ets, comets, suns, and stars must really be closely akin to the world of life as it appears to the eyes of an awed human being, in the open ocean, one half mile down.”5

The connection between sea and space appears time and again in ex- ploration. Indeed, when NASA launched the first planetary spacecraft toward Venus in 1962, it was not given a name of astronomical significance but one that was connected to our ocean: Mariner. And just two years before Mariner flew by Venus, humans themselves would make the plunge to the deepest part of the ocean for the first time, seven miles down in the Challenger Deep region of the Mariana Trench. In 1960 the Trieste, a 100-ton vehicle consisting of a sphere that fit two people ( Jacques Piccard and Don Walsh) and a giant, buoyant carafe of gasoline, dropped to the deepest point in our ocean.

The dive of the Trieste bathyscaphe6 marked what some hoped would be the beginning of an ambitious program to explore the deepest regions of our ocean. Designed by a Swiss inventor (Auguste Piccard, father of Jacques), built in its namesake region in Italy, and purchased by the United States Navy, it was the culmination of centuries of ocean explo- ration that sought to answer the question of what lies below not what lies above and beyond.

On that historic dive little was actually seen, as sediment that stirred up from the seafloor clouded much of the view, and Piccard and Walsh ould not stay on the bottom for long. The deep ocean remained largely unseen. But seventeen years after the Trieste landed in the Mariana Trench, in the spring of 1977, the abyss would give way to new insights into how life works in some of the most extreme environments on planet Earth. The veritable aliens within our own ocean would finally be revealed.

At that time, it was hard to imagine that there were still entire ecosystems on our planet yet to be discovered: the continents had been mapped; the poles had been reached; humans had touched down in the deepest point within Earth’s ocean; the footprints of 12 humans even dotted the landscape of the Moon. What game-changing discoveries were left to be made?
Plenty, it turns out.

In that spring of 1977, a team of scientists set off to explore the Galápagos Rift, a region of the seafloor near the Galápagos Islands. They wanted to find out what was causing temperature anomalies in the region. Previous expeditions had measured these anomalies with instruments dropped down on cables and dragged around the ocean. The thinking at the time was that the plate tectonics of the spreading Galápagos Rift was creating a lot of localized heat; hot rocks were creating hot water, simple enough.

As part of the expedition, the team used Alvin, a US submersible, expecting to make important observations and discoveries about how geology works. But what they saw instead called into question how biol- ogy works. At a depth of over 6,500 feet (2,000 meters), the lights on Alvin re- vealed structures that resembled tall and tortuous chimneys, billowing out “smoke” like active smelting plants from the Industrial Revolution. This was not smoke but clouds of fluids jetting out into the ocean at tem- peratures well beyond boiling—nearly 750 °F (400 °C). These fluids do not boil because they can’t: the pressure is too high at those depths. These “superheated” fluids contain gases like hydrogen, methane, and hydro- gen sulfide, as well as minerals that dissolve in the high-temperature and high-pressure fluids. The Alvin team had come across what we now call a hydrothermal vent—essentially a powerful, gushing hot spring at the bottom of the ocean. The surprise was not so much the vents, but rather the bizarre and beautiful ecosystem surrounding the vent chimneys. Like a deep ocean version of animals congregating at a watering hole in the African savanna, the chimneys were host to never-before-seen creatures—red tube worms, stark white eel-like fish, and golden mounds of mussels—that were thriv- ing in this extreme environment where conventional wisdom had said no animals should exist. And yet there they were.

How were these creatures surviving? What was sustaining this aston- ishing ecosystem?
On the surface of the Earth, the base of the food chain is driven by photosynthesis. Algae and plants harness the Sun’s energy, breathing in carbon dioxide, extracting the carbon to build the structures of life, and then exhaling oxygen. Small organisms and animals eat the photosyn- thetic organisms, and then larger organisms eat those, and so on.
At the bottom of the ocean, however, the Sun is nowhere to be seen, and the food chain as we know it breaks down. Light from the Sun pen- etrates about 1,000 feet (300 meters) down, but beyond that, photosyn- thesis is not an option.

What was the base of the food chain at these hydrothermal vents? This is where the chemistry of the vents come in to play, offering essential nu- trients and forming oases for life on the seafloor. The vents erupt hydro- gen, methane, hydrogen sulfide, and a host of metals, many of which turn out to be tasty treats for microbes. The microbes utilize chemosynthesis instead of photosynthesis. Here the prefix “chemo” denotes that the mi- crobes are synthesizing what they need for life with chemicals from the chimneys instead of photons from the Sun.

Only two years later, in March and July 1979, twin Voyager spacecraft would fly past Jupiter, capturing the first close-up images of Europa and Jupiter’s other large moons. Those images would lay the foundation for thinking there might exist oceans of liquid water in a region where most would have said it was not possible.

In those brief years of the late 1970s, two seemingly disparate but phe- nomenal discoveries helped pave the way for a new approach to the search for life beyond Earth. The prospect of a liquid water ocean within Europa was all the more exciting once it became clear, through the dis- covery of the hydrothermal vents, that life could thrive in the dark regions of our ocean, cut off from the Sun, in a manner perhaps similar to that of an ice-covered ocean.

Our own alien ocean, hidden in the abyss, provided a glimmer of hope that distant oceans beyond Earth might also harbor life. In the chapters ahead, we dive deep into how we think we know these oceans beyond Earth exist and why we think they could be habitable

Only two years later, in March and July 1979, twin Voyager spacecraft would fly past Jupiter, capturing the first close-up images of Europa and Jupiter’s other large moons. Those images would lay the foundation for thinking there might exist oceans of liquid water in a region where most would have said it was not possible. In those brief years of the late 1970s, two seemingly disparate but phe- nomenal discoveries helped pave the way for a new approach to the search for life beyond Earth. The prospect of a liquid water ocean within Europa was all the more exciting once it became clear, through the dis- covery of the hydrothermal vents, that life could thrive in the dark regions of our ocean, cut off from the Sun, in a manner perhaps similar to that of an ice-covered ocean.
Our own alien ocean, hidden in the abyss, provided a glimmer of hope that distant oceans beyond Earth might also harbor life. In the chapters ahead, we dive deep into how we think we know these oceans beyond Earth exist and why we think they could be habitable

For decades, we judged a planet’s potential habitability according to this “Goldilocks” scenario of too hot, too cold, and just right, with Venus, Mars, and the Earth representing the little bowls of porridge that Goldi- locks tastes, before the bears come home. But recently, we’ve learned that there’s more to the story. On the ice- covered moons of the outer solar system, we’ve discovered a new Goldi- locks zone—a new way of determining if a world could be habitable. It turns out that there’s more than one way for a world to maintain a liquid water ocean. In this chapter, I will describe how the tidal tug of a moon as it orbits a planet can sustain liquid water oceans and how decay of radioactive heavy elements can contribute to the heat needed for main- taining liquid water.
Water is an amazingly elegant substance. When exposed to a cold en- vironment, it naturally forms a protective insulating barrier that, floating atop it, shields the liquid water from further exposure to the cold. It’s such a commonplace sight that it’s easy to take for granted, but this simple fact of physics may be the key to the largest volume of habitable real estate in our universe.
The properties of ice help explain how ocean worlds retain heat, but we haven’t yet answered the question of where the heat actually comes from. The source of the heat that makes these oceans possible is a game changer for habitable worlds. It is the truly new Goldilocks condition that moves us away from the constraints of the traditional habitable zone, de- fined by a star’s energy and a planet’s distance from the star. The new Goldilocks requirements move us into a realm of wide-ranging possibili- ties for creating and sustaining liquid water oceans. The source of energy
in this new Goldilocks scenario comes from tides.

If an ice-covered moon orbiting a giant planet contains an alien ocean,
then the heat for that world is likely generated by tidal energy. Tidal en- ergy is a consequence of a world’s gravitational interaction with another massive object, such as a planet or moon. As the two bodies move rela- tive to one another, the world’s solid mass actually stretches and relaxes because of the tug of its tides, like a rubber ball being squeezed again and again. If you squeeze and release a rubber ball dozens of times, it will start to heat up from all of the internal friction. Similarly, the tug of tides cre- ates mechanical energy and friction within the object—which, in turn, creates heat. Hereafter we’ll only consider the case of tidal heating in moons since that is most relevant to ocean worlds.

Two of the most important considerations for tidal heating are (1) the difference in the gravitational force across a moon, and (2) the change
in gravity as a moon moves around a planet in its elliptical orbit. Two other critical parameters are the mass of the planet around which the moon orbits, and the period, or time it takes a moon to complete its orbit (which, as Johannes Kepler taught us, is a function of its distance from the planet). In the analogy to squeezing a rubber ball, these parameters can all basically be summarized as how intense is the squeezing, and how often does the squeeze and relaxation process occur?

At this point, it may be useful to put our own Earth–Moon system in context. We see the tides rising and falling on the shores of our ocean. Are tides a significant source of heating for the Earth? In short, no. The Earth and Moon orbit each other in nearly circular orbits, and they are relatively small, low-gravity objects, at least by planetary standards. What this means is that, as they orbit each other, the distance between them doesn’t change much—thus, the gravitational field doesn’t change, and the tides don’t either. Fixed and constant, they generate almost no heat from their motion. Think of that rubber ball, squished in your hand, but you never let go. It’s deformed, but no heat is being generated. Without repeated squeezing and releasing, the ball doesn’t heat up from chang- ing shape. In order for a moon or planet to experience significant tidal heating it must experience a changing gravitational field, which is like your hand squeezing and letting go of that rubber ball.
The little tidal heating that does occur as the Earth and Moon orbit each other results from the solid part of Earth’s surface—continents and seafloors—rising and falling ever so slightly. The rocky part of the Earth rises and falls by a few centimeters to as many as 25 centimeters, depend- ing on the alignment of the Earth, Moon, and Sun. The amount of heat produced from tides is just a few milliwatts per square meter (mW/m2), which is negligible compared to the 1,388 W/m2 we get from the Sun.

Tides do not create much heat for planet Earth or the Moon, but they obviously play a very significant role in the dynamics of our ocean. The motion of our ocean’s liquid water creates very little heat. Unlike rocks that resist deformation but eventually bend and stretch, and in so doing create friction and heat, liquid water just flows to accommodate the grav- ity of the tides. There is no significant heating because water moves freely and does not resist the motion of the tides.

The Moon’s gravity raises a tidal bulge of ocean water on the Earth directly beneath the Moon’s position, and there is a corresponding bulge of water on the opposite side of the Earth. It’s pretty intuitive that there should be a high tide bulge under the Moon, but why is there one on the other side of the Earth?

Recall that tides are caused by the difference in the gravitational force across a body. Since the force of gravity is inversely proportional to the square of the distance between two objects, the Moon tugs the Earth about 6% more strongly on the side that faces it than it does on the side farthest away. The Earth itself gets pulled closer to the Moon, leaving behind the water on the far side of the Earth, thus creating a high tide there.

The Earth rotates more quickly (one full rotation in 24 hours) than the Moon revolves around it (27.3 days), and thus the Moon “sees” a different region of the Earth all the time. The high tide bulges of ocean water stay aligned with the Moon, and the solid Earth rotates through them (Figure 2.3). Since there are two bulges, and the Earth completes its rotation in 24 hours, every place on Earth passes through two high tides and two low tides every day. This fixedness of the tides with respect to the Moon is critical. The tides on Earth are not really rising and falling, and so they do not, in fact, create a lot of mechanical energy.

While our Earth–Moon tidal dynamics do not create much internal heating, numerous moons of the outer solar system undergo very signifi- cant heating from tides. The key differences are as follows. First, these moons orbit giant planets that have very strong gravitational fields, within which the moons orbit. Second, if a moon has an elliptical orbit around a planet, then the distance between it and the planet is always changing and, therefore, so is the gravitational field. The tidal bulges will increase and decrease in size as the moon goes from its closest approach (periapse) to its most remote position (apoapse).

With each orbit, the rise and fall of these tidal bulges causes stretch- ing and relaxation, which creates friction and ultimately heat. Many of the ocean worlds of the outer solar system, including Europa, Ganymede, an Enceladus, have orbits that are elliptical and that cause this kind of tidal stretching and heating.

Jupiter’s four large moons provide a useful case study in this new Gold- ilocks framework for tidal heating. By studying the orbits of these moons, the potential for this new Goldilocks zone emerged. In 1979, just prior to the arrival of the Voyager spacecraft at Jupiter, Stanton Peale and Patrick Cassen (from the University of California Santa Barbara) and Ray Reynolds (from NASA Ames Research Center) published an article on the theory of tidal energy dissipation in the Jovian system.1 Remarkably, they concluded that tides could cause much of the interior of Io, Jupi- ter’s innermost moon, to be molten. Io could therefore be volcanically active. They even made the bold prediction that images from the Voyager 1 spacecraft might reveal such activity.

Peale, Cassen, and Reynolds made one of the most elegant and excit- ing predictions followed by discovery in the history of planetary sci- ence. Soon after their article was published, Voyager 1 flew by Jupiter and returned stunning images of volcanic plumes erupting from Io into space.

Not long after, the team published a similar article about the effect of tides within Europa. The title of their article, “Is there liquid water on Europa?,” was provocative.2 This was the first time that a scientifically rig- orous and robust mathematical argument had been made for an ocean existing within Europa. It was, in my opinion, the birth of the “New Gold- ilocks” conditions—a new way to determine a possible habitable zone.

With the Voyager 1 and 2 flybys, and the subsequent exploration of the Jovian moons with the Galileo spacecraft, we would come to understand the true power of tidal energy dissipation and this new Goldilocks re- quirement. For example, Io does not simply have volcanoes, it is the most volcanically active body in the solar system, even more volcanically ac- tive than the Earth. Volcanoes are erupting on Io right now.

Because Io has no atmosphere, the eruptions spew plumes of gas and lava out into space, forming umbrella-like shapes. Io’s spectacular volca- nic activity is the result of the tidal stretching and deformation it experi- ences as it moves along its eccentric (that is, not concentric) orbit around Jupiter, which is 318 times as massive as the Earth.
Io is made of rock and has an iron-rich core, and its rocky mantle is perfectly conducive to tidal heating. Tidal heating generates 2,400 W/m2 on Io’s surface—over 1,000 watts more than the incoming solar energy flux received by the Earth, and nearly equal to the 2,600 W/m2 that Venus gets from the Sun! This is much too hot for a lot of water to exist on the surface. In this new Goldilocks scenario, Io is analogous to Venus: it has too much tidal energy and has lost almost all of its water. As a re- sult, Io has lots of heat but no ocean for life.

Jupiter has dozens of moons, and of the four largest ones, Callisto is the farthest out. Callisto does have an ocean, but it’s trapped beneath a very thick, old ice shell, likely sustained through the decay of radiogenic heavy elements in the moon’s interior. Callisto experiences very little tidal heating. Although its orbit is highly elliptical—more so even than Io’s—it is simply too far from Jupiter for this eccentricity to generate significant stretching and straining. Callisto’s higher eccentricity (0.0074 to Io’s 0.0041; where a value of 0 is a circular orbit) is largely offset by its greater distance from Jupiter.
In the new Goldilocks framework, Callisto is akin to Mars. It was, and perhaps still is, habitable, but the tidal energy dissipation is very small; as a result, Callisto is colder and less active than the large inner moons.

In between Io and Callisto lie Ganymede and Europa—occupying the sweet spot of the new Goldilocks zone. These two moons are stretched and squeezed by tides enough to generate tens to hundreds of milliwatts per square meter of internal heating. In the case of Europa, this is enough to maintain a liquid water ocean of approximately 100 km (60 miles) in depth, with a rocky seafloor—perhaps dotted with hydrothermal vents— all of which is overlaid by a relatively thin ice shell (a few kilometers thick to as much as 30 km thick). This ice shell is thin enough that its surface chemistry may provide a window into the chemistry—and pos- sibly the biology—of the ocean below.

Europa and Ganymede are also the beneficiaries of a curious property of the Jovian system that keeps their orbits elliptical and helps maintain their tidal heating. The three innermost large moons—Io, Europa, and Ganymede—are like three kids on a swing set whose back-and-forth mo- tion has gradually synchronized into a pattern. For every one orbit that Ganymede makes around Jupiter, Europa makes two; and for every one orbit that Europa makes, Io makes two. Ganymede, Europa, and Io’s or- bital periods are thus locked in a 1:2:4 ratio, known as the Laplace resonance (after the French mathematician Pierre-Simon Laplace, who discovered it in the early 1800s).

The Laplace resonance is important because it forces each moon to stay in an elliptical orbit. Typically, over time, orbits circularize and lose their eccentricity (i.e., they become less elliptical). But in the Jovian sys- tem, the three inner large moons regularly align in pairs: Io–Europa, Europa–Ganymede, and Io–Ganymede. When this happens, the aligned moons tug on each other, leading to a “forced eccentricity” and causing their orbits to each stay slightly stretched into an ellipse instead of be- coming perfectly circular. (Note that all three never line up together on the same side of Jupiter.)
Although the exact timing of the start of the Laplace resonance around Jupiter remains a topic of considerable study, at some point in the distant past (perhaps billions of years ago), Io began to retreat from Jupiter. Gradually, it got close enough to Europa to exert some gravita- tional influence on it. These two moons then engaged in a two-body resonance, systematically tugging on each other to create forced eccen- tricities and possibly settling into the 1:2 resonance they experience today.

Over time, both moons continued to lose energy and momentum to Jupiter, and their orbits grew larger. Eventually, they got close enough to Ganymede to begin to influence its orbit. The tug between the three moons then stabilized into the 1:2:4 resonance that we observe.

One day, Callisto will be part of this clockwork too. As the three in- nermost large moons continue to retreat outward, they will—perhaps hundreds of millions of years from now—expand far enough out to influence Callisto’s orbit. Will there then be a complete resonance of 1:2:4:8? It’s hard to predict since so many different factors influence en- ergy loss and momentum transfer, such as tidal energy dissipation or how Jupiter itself responds to the interaction.

NASA’s Juno spacecraft, which began orbiting Jupiter in 2016, is prob- ing some of these big picture questions about planetary dynamics and interiors. Once we have a clearer picture of how Jupiter works, we’ll be able to better understand its relationship to the ocean worlds trapped in its orbit.