Quick—How many ocean worlds exist in our solar system?
You’re probably familiar with at least one; that lovely blue marble we call home. But the truth is that nobody is quite sure exactly how many other briny deeps could be hiding relatively close to Earth. Water ice is a common building block in the outer solar system and even a small amount of heating can melt that ice into a vast liquid sea. From the asteroid belt to the moons of the giant planets to Pluto and its Kuiper Belt friends, there could be as many as a dozen other nearby oceans. This fact has taken planetary scientists decades to fully digest and is only now getting widespread attention. But it could soon shape our exploration efforts and force us to rethink the prospects of life around both our local star and others.
How did all these other oceans spring up?
To answer that we have to return to the early days of the solar system when the young sun blazed at the center of a thick disk containing lots of dust and gas. The parts of this protoplanetary disk closest to the newly-formed star were hot, meaning that light molecules like water and ammonia evaporated away. But out beyond what’s known as the frost line—located at roughly the same place as the present-day asteroid belt—temperatures remained cold enough that many volatiles stuck around as ices.
This disk eventually gave rise to the planets, moon, and asteroids of our solar system. The inner worlds like Earth and Mars started out dry and rocky, with comets probably delivering their subsequent water reserves. But the giant planets beyond the frost line formed with lots of water ice, as did their many moons. In fact, “water ice is the dominant solid-forming material in the outer solar system,” according to a 2015 review, Interiors and Evolution of Icy Satellites. That means places like Saturn’s moon, Titan, and the dwarf planet, Pluto, have entire mountain ranges built out of ice.
Still, it’s cold when you’re far away from the sun. And ice, even water ice, does not a global ocean make. But you have to remember the icy crusts of frozen worlds act similar to the rocky crust of a planet like Earth. Rock may be solid on our planet’s surface but if you go deeper, things begin to heat up. The mantle is an interior layer of flowing magma sustained by the decay of radioactive materials and the Earth’s primordial heat. Now think what would happen if you replaced the rock of our crust with ice.
“If Earth were a lot farther from the sun and had a layer of ice covering it, its internal heat would produce prodigious amounts of volcanism and probably a liquid ocean,” said geophysicist Steven Vance, who heads the Icy Worlds Astrobiology group at NASA’s Jet Propulsion Laboratory.
As far back as 1971, some scientists had created models showing that the icy moons of our solar system could theoretically sustain large bodies of liquid water. The Voyager probes proved these simulations right when they toured the outer solar system in the late 70s and early 80s and discovered lots of geological activity. Several of Jupiter’s large moons were covered in ice shells, beneath which seemed to be evidence of global oceans. Neptune’s natural satellite Triton spewed water and ammonia from volcano-like structures. More recently, NASA’s Cassini mission to Saturn discovered geysers of water shooting out of the frozen moon Enceladus—once thought too small to be capable of such pyrotechnic abilities. The more we explore the outer solar system, the more potentially large deposits of liquid water we seem to encounter.
Water, water, everywhere…
Let’s take a quick expedition through the most likely places to find oceans in the solar system. Traveling out from Earth, our first stop is the dwarf planet Ceres, the largest object in the asteroid belt. When NASA’s Dawn spacecraft reached this rocky world, it noticed odd bright patches all over its surface. Mounting evidence suggests that these bright patches are salt, coming from the sublimation of ice buried in a layer just below the rocky surface. Scientists currently suspect that asteroids punching holes in the rocky exterior expose this buried ice layer, beneath which could be a subsurface ocean.
Next up we reach Jupiter and its mini-solar-system of moons. The largest four are roughly the same size as the planet Mercury. Europa is the most famous hypothetically water-bearing world, with possible watery plumes spraying from its surface. Planetary scientists have long wanted to send a mission to further explore its secrets. Ganymede, the largest moon in our solar system, has a thick icy crust and a global magnetic field that suggests its interior is hot. “Callisto has even better evidence for an ocean than Ganymede,” said Vance. “And I’m utterly confident there’s one on Ganymede.”
At Saturn, we find Titan, another of our solar system’s planet-sized moons. Beneath its hazy atmosphere are lakes and seas filled with liquid hydrocarbons like methane and ethane. And underneath those are an icy crust, which may contain a layer of liquid water deeper in the interior. Whether or not the liquid hydrocarbons and the liquid water interact—a potentially life-giving recipe—remains an open question. Enceladus and its jets are another spectacular example of a possible ocean, and tiny Mimas also has hints of potential liquid water. Future observations of the Saturn system could yield further surprises.
Beyond the gas giants are the ice giants, Uranus and Neptune. Though little explored, Uranus’ Ariel, Umbriel, Oberon, Miranda, and Titania all contain significant amounts of water ice in their crusts. Neptune’s Titan has its active cryovolcanoes. Pluto—it is becoming abundantly clear—could have a buried ocean at the heart of its Sputnik Planum. And if the king of the dwarf planets has an ocean, then perhaps other large objects in the Kuiper belt also harbor liquid water beneath their surfaces.
Given this wealth of water, many researchers are starting to realize we need to reconfigure our explorations. Thus far we’ve mainly focused on our neighbor Mars, which lost most of its water early on, for habitability and ancient organisms. But the possibility of oceans in the outer solar system suggests we could discover microbes living and thriving on ice-covered moons. NASA and Congress are already leaning in this direction. The latest budget proposal from the House of Representatives directs the agency to set up an Ocean Worlds Exploration Program. Vance said that scientists have begun meeting periodically to outline an exploratory roadmap, with the first targets being those moons with the best evidence for liquid water: Europa, Titan, and Enceladus. NASA is already working on the Europa Clipper mission, which could launch in the 2020s and send a small lander to the moon’s surface. The European Space Agency is also gearing up for the Jupiter Icy Moon Explorer (JUICE) mission, which will tour Europa as well as Ganymede and Callisto.
Even if none of the ocean worlds of our solar system contain life, perhaps icy planets in other systems will prove to be hospitable. Astronomers are currently focused on finding a second Earth—a small, rocky planet with an atmosphere that orbits at just the right distance from its parent star to sustain liquid water. But maybe we shouldn’t just scope out places like home. The alien environments of an Europa- or Titan-like world could be just as good for life as our own world.
NASA’s Jet Propulsion Laboratory made this gorgeous infographic about the best-known candidates for finding water in our solar system, and maybe even life.