by Izzy Baker
figures by Sean Wilson
In the quiet darkness of night, a ship gently rocks back and forth on calm waters. An exhausted first mate peers out over the expansive seascape, seeing the ocean twinkle as it reflects the stars shining down from up above. The vastness of our oceans, she ponders, can only be outmatched by the very cosmos that they reflect; yet, in mirroring these stellar giants, the oceans also mirror their origins in these stars. Come morning, the sunlight dancing on the water’s surface will be glaring, and just as it blinds the first mate and the rest of her crew, the Sun has, in another sense, blinded scientists for decades as well.
It has been a long-held and quite reasonable belief that a sun’s radiation is necessary for liquid water. Recently, however, when scientists donned their metaphorical sunglasses, they found that planetary bodies in the darkest corners of our Solar System, hidden away from sunlight, can hold liquid water, and with it, the potential for life.
From space dust to oceans: where does Earth’s water come from?
Hydrogen and oxygen—the only ingredients of water—are some of the most common elements in the universe. It makes sense, then, that scientists have found massive reservoirs of gaseous water throughout our galaxy. Water particles are forged in ejections from dying stars; they have been found forming in both the Helix Nebula and the Orion Nebula, which are more than 500 light-years away.
In fact, the daily amount of water formed in the Orion Nebula is so immense that it could fill Earth’s oceans 60 times over every day. Clearly, supply is not the issue—so how did water find its way to Earth? The answer lies within asteroids (literally). Recent observations have hinted that ice, and possibly liquid water, exist in the interiors of asteroids and comets. Over Earth’s 4.5 billion-year lifetime, countless comets and asteroids have crashed into the planet, enriching it with water. Indeed, studies of the chemical markers in our ocean’s waters indicate their cosmic origins. This fact suggests that any planet in the path of an asteroid hurtling through space could receive water in the same way. Looking to our nearest neighbors—Venus and Mars—it would appear that this is exactly the case. So, where are their oceans?
Cosmic oceans past and present
Billions of years ago, when Earth’s oceans were still new, Venus and Mars were just as blue with abundant water. After about a billion years, though, as the Sun’s brightness increased, Venus encountered a runaway greenhouse effect through the increasing evaporation of its water. More and more heat became trapped under an increasingly thick blanket of atmospheric water vapor, a powerful greenhouse gas (Figure 1). The rest of Venus’s water was boiled away into intensely sweltering conditions, and with no water left on the planet’s rapidly frying surface, melting rocks released potent carbon dioxide into the atmosphere, further exacerbating the greenhouse effect. Meanwhile, any remaining water vapor continued to rise past the atmosphere and was eventually lost to space.
Liquid water on ancient Mars, on the other hand, was likely sparse due to frigid temperatures locking any hopeful seas in ice. Scientists attribute the disappearance of the rest of Mars’ liquid water to the loss of its protective magnetic field. This left it vulnerable to the unrelenting effects of solar winds that literally blew any water off of the planet’s surface, resulting in the depletion of approximately 87% of the water it had billions of years ago. Nowadays, Mars only maintains reservoirs of water frozen in its ice caps and trapped beneath the soil.
Given the harsh effects of the Sun on the oceans of both Venus and Mars, it may seem like Earth is the only planet in our Solar System that is perfectly positioned to host a large liquid body of water. Earth is the only planet in our Solar System’s located in the aptly named “Goldilocks Zone”. Earth is neither too hot (i.e., too close to the Sun) that its water evaporates nor too cold (i.e., too far) that all of the water is locked as ice, in other words: just right. However, contrary to common intuition, being located in the Goldilocks Zone is not the only way to maintain a liquid ocean (Figure 2).
Radioactive decay from a planetary body’s core can generate enough heat to sustain liquid water, as can tidal heating. The latter occurs when a planet’s orbit causes constant stretching and warping of the planetary body, heating it up from the inside out, just like squeezing a tennis ball to warm it up before a match. Now that scientists have recognized that a sun’s heat is not necessarily required for there to be liquid water, the search for ocean worlds has expanded beyond those planets within the Sun’s immediate reach—and the results thus far have been plentiful.
Besides Earth, who else has an ocean?
Water is not hard to find here on Earth—about 71% of Earth’s surface is covered by water, with more than 96% contained in the oceans. That said, an alien from our outer Solar System would consider the Mariana Trench as shallow as a puddle.
Europa, one of Jupiter’s moons and a top target in the search for extraterrestrial life, is estimated to have an ocean that is over 100 miles deep in some parts—that’s 16 times deeper than the Challenger Deep, the deepest place on Earth (Figure 3). While Europa’s outer shell is a roughly 15-mile-thick sheet of ice, tidal heating from its parent, Jupiter, is thought to maintain its ocean’s liquid state below. In 2014 and again in 2016, the Hubble telescope spotted water plumes erupting off of Europa, further indicating a vast ocean lying just below the surface. Capturing material from these plumes is now a goal of NASA’s upcoming Clipper mission, set to launch in 2025. Europa is not the only ocean world orbiting Jupiter, as fellow moons Ganymede and Callisto are also thought to be laden with cavernous oceans beneath extremely icy crusts extending more than 120 miles deep.
Looking even further from the Sun, Saturn’s moons Titan and Enceladus also have vast subsurface oceans beneath their thick, icy shells. Enceladus, like Jupiter’s Europa, is also a prime candidate in the search for places capable of sustaining life. Its 6-mile-deep ocean lies beneath a shell of ice more than 20 miles thick (Figure 3). Its underground ocean feeds dramatic jets which spray from deep crevices in Enceladus’s surface. When the recently retired Cassini space probe flew through these jets in 2015, the scientific community was surprised to find hydrogen, a key ingredient for life on Earth.
Given that these thrilling features have been found in some of our nearest neighbors, it is not hard to imagine what other ocean worlds lay waiting to be discovered beyond our Solar System. The more we look for water, the more we seem to find it; but how does this set the stage in our search for life beyond Earth? Without sunlight, how could life even exist on these faraway planets? One might reasonably assume that in the absence of sunlight, there would be no plants or algae, and without them, so goes the rest of the food chain.
It’s always sunny in the cosmos…or is it?
Returning to Earth for a moment, and traveling back in time to the year 1976, it was indeed a well-established fact that life required access to sunlight. However, the following year, scientists in the submersible Alvin diving beyond the reach of sunlight made a discovery that turned this notion on its head. Massive, rocky, chimney-like structures, bedazzled with deposits of fool’s gold and other iron-sulfur minerals, were found spewing hot chemical soup, that, when mixed with the surrounding cold, oxygenated seawater, created a dazzling plume resembling huge clouds of black smoke. The discovery that stunned the scientific community, however, was not the appearance of these structures, but rather the unbelievable consortium of life that they hosted.
Teeming with thick mats of microbes, alien-looking tube worms, and ghostly white crabs, these hydrothermal vents, as they are now known, showed us an entirely new kind of ecosystem—one based on microbes converting chemicals into food that can be used by other organisms like animals, and all without the help of the Sun. The discovery that life can exist in the darkness was revolutionary.
While the oceans of worlds like Enceladus and Europa may be too distant from the Sun and too thickly covered in ice to accommodate photosynthesis, they do contain water in direct contact with potentially hydrothermally active sea floors, making vent-hosted life a possibility. As the hunt for watery worlds progresses at a promising pace, our own oceans still remain largely unexplored. Thus, the search for alien oceans must be just as inward-looking as it is outward.
Izzy Baker is a fourth-year PhD candidate in Organismic and Evolutionary Biology at Harvard University
Sean Wilson is a sixth-year graduate student in the Department of Molecular and Cellular Biology at Harvard University
For More Information:
- To learn more about ocean worlds, check out this NASA guide
- Find out more about ongoing efforts to combine deep sea research on Earth with astronomical surveys to better understand the potential for life on other planets
- Check out this article to learn more about the science of maintaining liquid water
- Follow up on the exciting potential for life’s ingredients on Enceladus