On July 13, 2015, the sharpest-yet images of Pluto from NASA’s New Horizons satellite arrived at the Johns Hopkins University Applied Physics Laboratory in Maryland. The satellite had traveled three billion miles and was now one day away from its closest encounter with the dwarf planet. As it approached Pluto, New Horizons began sending intriguing photographs of Pluto’s never-before-seen surface, including a range of ice mountains surrounding a vast heart-shaped basin of frozen nitrogen. Scientists saw strange shapes within the nitrogen basin; they looked like convection cells – pockets of heat generated by decaying radioactive elements. But scientists didn’t know why these convection cells only appeared in the basin.

Francis Nimmo, a planetary scientist at the University of California, Santa Cruz and a member of the New Horizons team, was the first to explain why. He knew that Pluto’s moon Charon orbits directly opposite the nitrogen basin and wondered why Charon was pulled toward that particular point on Pluto. Perhaps, he thought, some large mass in the basin was generating a strong gravitational pull that attracted Charon towards that specific region of Pluto. After considering multiple options, Nimmo concluded that only one thing could generate such a strong pull: water from an ocean hidden just beneath Pluto’s surface.

Scientists and public alike gushed with excitement at the news. After all, water is the key ingredient for life, and despite all odds, Pluto now had everything scientists look for when searching for worlds that can support life: a source of heat (from decaying radioactive elements), a liquid (in this case, water) and a layer of cream-colored carbon-rich chemicals coating its surface. But even if Pluto indeed has a subsurface ocean — an idea that quite recently would have been preposterous — as well as all the right ingredients for life, it’s unlikely scientists would return there: many other places in the solar system have similar characteristics, are much easier to get to and are just as intriguing as targets for exploration.

Over the past two decades, scientists have realized that nearly a dozen “ocean worlds” such as Pluto exist in our solar system. These ocean worlds are ice planets and moons that have a layer of liquid inside, and include Jupiter’s moon Europa, which holds twice as much water as Earth, and Saturn’s moon Enceladus.

The discovery of so many ocean worlds has forced a deep re-thinking of how and where life might arise. Scientists have traditionally looked for life on Earth-like planets sitting comfortably within a star’s habitable zone — the narrow, just-right “Goldilocks” region around a star where water, the key to life, forms on a planet’s surface as a liquid. But if Pluto and nearly another dozen worlds so far from the sun hold all the ingredients for life, perhaps even more unfathomable places may as well.

Europa: A shifting tide

NASA’s Voyagers 1 and 2 snapped images of Europa for the first time in the fall of 1979. Before that, she was only a dot in the sky that, like Pluto, scientists expected was little more than a frigid rock covered in craters and water ice. But the pictures showed much more. Europa was bright white and smooth in most locations. The surface was covered in criss-crossing rust-colored lines and a few regions of terrain where it seemed like broken icebergs had melted and refrozen haphazardly.

“It was like a white billiard ball faintly crossed by lines applied with a pen,” David Morrison, a senior scientist at NASA Ames in California, wrote of the Voyager team’s initial reactions. Shortly after, Voyager 1 looked to Europa’s sister moon Io. The team saw something that helped make sense of Europa’s faint lines and chaotic terrain.

From just under 300,000 miles from the surface, Voyager 1 imaged lava erupting 100 miles from Io’s surface into space. Nobody had seen such volatility before, but scientists thought they knew why Io, Jupiter’s fifth closest moon, was so explosive. They blamed tides.

On Earth, the Moon’s relatively small gravitational pull tugs on the oceans to create its daily ebbs and flows, which we call tides. Jupiter, on the other hand, is much larger than the Moon, and it pulls with a gravitational force 300 times stronger than that of the Moon. So, as Io orbits around Jupiter, Io’s surface stretches and deforms a bit like Play-Doh. Rocks slide back and forth, producing tremendous heat from friction, and the heat slowly builds until it causes volcanic eruptions.

Scientists thought Europa might experience tidal effects similar to those seen on Io. But because Europa orbited farther from Jupiter, these tidal impacts might be less dramatic. Heat from friction wouldn’t produce volcanic explosions, but it would melt Europa’s icy insides. This melting would create an ocean lying miles under Europa’s surface. The criss-crossing cracks on the surface might be stretch marks where the moon deformed to accommodate the swelling ocean within, which sometimes might even seep onto the surface. The picture seemed compelling and, for some, even prime for life.

But many scientists were skeptical. “Sure, several people said, ‘Gee, there could be an ocean on Europa, and maybe even Europa’s habitable,’” says Richard Greenberg, a planetary scientist at the University of Arizona who has studied Europa for roughly 30 years. But these ideas were mostly hypothetical..

“People didn’t take it seriously,” Nimmo agrees. Surface features and hypothetical scenarios just weren’t hard evidence.
But that changed when NASA faced a crisis. Its budget was under threat in the post-Space Race world, and then the Challenger space shuttle exploded in 1986, killing all seven crew members aboard. NASA needed a new goal to justify space exploration—and fast.

Scientists’ interest in looking for life outside of Earth launched after the Galileo mission, which found compelling proof of a water ocean beneath Europa’s surface. Graphic: Jeremy Rehm

It was around then that teams of oceanographers discovered volcanic “black smoker” vents on the ocean floor around the Galápagos Islands. Defying the standard view that life depends on sunlight, life growing in these dark, frigid conditions disrupted everything scientists thought they knew about where life could thrive. Life in an ocean under Europa’s surface now didn’t seem as farfetched. Still, nobody knew if the ocean was there.

Enter Galileo. In October 1989, the Galileo satellite set-off for a six-year journey to Jupiter. Part of its mission was to confirm Europa’s ocean, looking for small changes in Jupiter’s magnetic field caused by salt in the water. If Europa had an ocean, its salt water would induce a temporary magnetic field around Europa as it passes through Jupiter’s magnetic field, causing a small blip. It would be the best evidence for an ocean, and it’s exactly what Galileo found.

“That really nailed the presence of an ocean,” says Nimmo.

But then there was the question of whether life was there. Perhaps hidden within the surface cracks were small microbes feeding on organics from the surface. Or maybe the ocean floor was replete with its own type of black smokers.  NASA now had a new mission—the search for life.

Oceans upon oceans

Researchers began racing to find life right in our solar system’s backyard, and Europa was the best place to start. But she wasn’t alone.

Space missions began finding “ocean moons” orbiting nearly every gas giant planet in the solar system. Voyager 2 found geysers spewing from Neptune’s moon Triton. Galileo found the first signs of oceans beneath Jupiter’s moons Ganymede and Callisto. Cassini coursed through water-ice jets rising from Saturn’s moon Enceladus. And scientists even supposed from surface shape and gravity measurements that Ceres, the solar system’s largest asteroid, conceals a now-frozen ocean below its surface.

Water was everywhere, and perhaps that meant life was, too. “A few decades ago, there was much skepticism that life could exist in the solar system,” wrote Dirk Schulze-Makuch, a German astrobiologist at the Technical University Berlin. “But more people are now optimistic to find other life on other moons and planets in our solar system.”

Among those moons, Titan, Saturn’s largest moon, became a prime candidate for potentially harboring life. Since the Voyager missions, scientists had learned little more about Titan other than it was the only moon with a thick, smog-orange atmosphere. What hid below remained mysterious until January 13, 2005, when the Huygens probe slowly parachuted through the haze. It touched Titan’s surface not with a bang, but a slight squish.

It was something like landing in “packed snow, wet clay, maybe rather stiff molasses,” wrote Ralph Lorenz, a planetary scientist at Johns Hopkins University and lead scientist on the Cassini-Huygens mission. One of his team members supplied an apt analogy: crème brûlée.

“I think with the discoveries we’ve had in our own solar system, we’re beginning to see there’s much more to it than just life on Earth.”

– Manasvi Lingam

Huygens had landed in what scientists believe is an old riverbed—a flat landscape dotted with smooth rocks. It took another year before Cassini—Huygens’ partner satellite orbiting from space—peered below Titan’s clouds and returned photos of a surface much like Earth’s, marked by snaking rivers, lakes as vast as the Great Lakes of North America and seas. Measurements of tidal forces even suggested a liquid water-ammonia ocean hiding below the surface. It was home away from home, almost, except that it was much colder than Earth. Day temperatures on Titan only climb to -290 degrees Fahrenheit, roughly twice as cold as the lowest recorded temperature on Earth. In place of rocks, there were chunks of ice; and instead of flowing water, there was liquid methane and ethane. But Titan’s similarities to Earth left many wondering if Titan could support life like Earth.

Titan stretches far beyond the imagination of a world that scientists ever considered could host life, says Schulze-Makuch. But he doesn’t believe life is impossible there. “It has liquids on the surface and tons of organic chemicals,” he says. “If we understand life correctly at all, you might have everything you need.”

Life might have emerged from the depths of Titan’s methane sea much in the way scientists first believed life emerged from Earth’s oceans: with lightning striking the methane seas to form the chemical building blocks of life – and then a bit of luck. Titan life would need to survive in punishingly cold conditions, and it would likely have evolved to use methane in place of water because methane is more abundant. So, life would likely look unlike anything we’ve ever seen on Earth.

Earth’s microbes may offer at least a hint as to what Titan’s life could look like, though. Micro-organisms like bacteria and archaea have evolved to thrive in the freezing temperatures of Antarctic ice and acidic conditions of volcanic springs. Scientists have even discovered bacteria that munch on oil like candy. Microbial oddities like these give credence to the idea that life could exist on Titan, Schulze-Makuch says. “We don’t know whether it’s there,” he says, “but that is not really a reason that it couldn’t be.”

“There’s a lot of habitats that we haven’t really thought of before,” agrees Manasvi Lingam, an astrophysicist and astrobiologist at Harvard. “But I think with the discoveries we’ve had in our own solar system, we’re beginning to see there’s much more to it than just life on Earth.”

Where to find life

Lingam became an astrobiologist because he wondered how life might evolve on moons or planets with a subsurface ocean, like Europa and Enceladus. These moons are so distant from the sun that sunlight couldn’t supply energy for life. But tidal heating, as on Europa, or radioactive decay, as on Pluto, they calculated, could supply billions of years of heat to these worlds. And if the planet or moon has any organics leaking into its ocean, the conditions might be just right for life to take hold.

The constant stretching and contracting of a moon as it orbits a planet (left) and the breakdown of radioactive elements deep inside the moon (right) keep a subsurface ocean from freezing. Illustration: May Jernigan

“You have water and you have the right energy sources. There’s no reason why life wouldn’t take hold in those worlds,” he says.
If astronomers want to find life in the Cosmos, especially as hundreds of planets in other solar systems are discovered every year, Lingam believes astronomers should consider the entirety of a moon’s or planet’s circumstances rather than just its location close or near a star. “There could be planets or moons in very exotic locations where one would not have expected life, but under the right conditions, they might have the right temperature, the right amount of liquid water, the right energy sources,” he says.

In 2011, for instance, scientists proposed that life might exist on what are called “rogue planets,” worlds that have been jettisoned from their solar system to wander in the cold emptiness of interstellar space. To date, only a handful of these liberated vagabonds have been discovered, but astronomers estimate there could be billions. Even in their isolation and in subfreezing temperatures, heat from radioactive decay could prevent their subsurface liquid from freezing. And with serendipity, those conditions could still pave a path for life.

Lingam estimates icy ocean worlds are 100 to 1,000 times more common in the Milky Way Galaxy than are rocky earth-like planets close to stars. Earth, in that sense, may be rare. Scientists still hunt for its twin in the “Goldilocks” region around every new solar system discovered, searching for a rocky planet with liquid water on its surface. That’s partly because it’s what’s most familiar and easiest for us to look for, says Ravi Kopparapu, an exoplanet research scientist at NASA Goddard in Maryland. “It’s like going to a party,” he says. “When you go to a party, you first talk to the people you know, not the people you don’t know.”

But much of the problem, Lingam says, stems from our limited technology and the challenge of spotting life in distant solar systems.
“If we want to find life in our solar system say on Europa or Enceladus, we have to go there and look,” Lingam says.

NASA now plans to do just that. In March 2017, NASA greenlighted the Clipper mission to Europa, which will launch in 2022. It’s a reconnaissance mission to verify Europa has all the conditions necessary for life. Clipper will search for water that has seeped onto Europa’s surface, determine the subsurface ocean’s salinity by measuring Europa’s magnetic field, and use radar imaging to give scientist’s their first glimpse below the ice and possibly into an alien ocean.

But traveling to the planets of another solar system is currently infeasible, which means scientists would need to search for life remotely. And that’s challenging.

For perspective, the nearest planets outside our solar system lie in Proxima Centauri, a solar system sitting 4.25 light-years, or roughly 25 trillion miles, away. If one of those planets has an atmosphere, scientists can look for chemical clues of life, like oxygen, water vapor and methane gas. But if life existed below the surface as imagined on the icy ocean worlds on Europa, “it would be pretty much impossible to directly detect,” Lingam says.

That doesn’t deflate Lingam’s optimism for the future, though. More than 30 years’ worth of space exploration and discovery have widened the scope of worlds where scientists believe life might be found. We’ll eventually find life, Lingam says. “I don’t think ‘simple’ life, like microbes, is that rare.” With astronomers no longer stuck looking only for Earth’s twin, life could exist seemingly anywhere now. The possibilities are endless.

© 2017 Jeremy Rehm / UC Santa Cruz Science Communication Program