Earth’s oceans are impressive, but humanity’s history of space exploration has revealed many more seas beyond our own.
The moon was leaking.
Its planet’s gravity had strained and cracked its icy outer shell. From that slender crevice, a jet of saltwater, dozens of kilometres long, streamed out into the emptiness of space, where the cold froze liquid droplets into crystals almost instantly. But the moon wasn’t leaking simple H2O; the stream of water was laced with silicates, carbon compounds, and other chemicals needed for the evolution of life.
The moon in question wasn’t the familiar Earth-orbiting one, but one of Saturn’s moons. The leaking moon, which goes by the name Enceladus, was first discovered by British astronomer William Herschel in 1789, but no one suspected that it might harbour liquid oceans until the Cassini probe flew past in 2005. Cassini spotted the vast plumes of water, sampled the chemical composition, and measured the amount of heat energy coming from Enceladus’s surface. Enceladus’s subsurface ocean proved to be far warmer and more chemically active than anyone expected, leading some scientists to speculate that Enceladus’ watery layer might be one of the best locations for possible alien life in the solar system.
We know the liquid water oceans on Earth’s surface teem with life, but the majority of watery oceans in the solar system may be subterranean. Many moons of gas giants in the outer solar system have outer layers made of ice and rock with molten water oceans underneath. Other bodies — most famously, Saturn’s moon Titan — have liquid surface oceans, made out of chemicals that exist as explosive gases here on Earth. The chemistries of these alien worlds’ vast liquid expanses are complex, but our knowledge of these outer solar system oceans comes only in bits and pieces.
Few space agencies have the funding and expertise to send probes into the outer solar system. Deciding on a flight plan for such rare journeys requires an awful lot of negotiation between teams of scientists, designing equipment that can survive rocket launches is difficult — and those are just the obstacles planet-scouting probes face before they leave Earth. Projects like the Enceladus-sampling Cassini are few and far between.
Only nine human-built probes have ever visited the outer solar system, and only three have been orbiters, capable of making several passes around a planet and investigating multiple moons. (Those counts include NASA’s Juno, which arrived at Jupiter just last month.)
Although the European Space Agency (ESA) has plans to send spacecraft to Jupiter and Saturn in the coming 2020s — the Jupiter Icy Moon Explorer (aka “JUICE”) in 2022 and the more tentative Titan Saturn System Mission in 2029 — so far, outer solar system exploration has been NASA’s turf since the beginning. In the late 1960s, NASA administrators were looking for a follow-up act to landing a man on the Moon. One man, an engineer named Gary Flandro, proposed a “Planetary Grand Tour” with unmanned missions to every planet in the solar system. That proposal morphed into several different space exploration programs, including Pioneer 10 and Pioneer 11, as well as the famous Voyager missions.
At the time of its launch in March 1972, Pioneer 10 was the fastest spacecraft ever built. It rocketed out of Earth’s atmosphere, propelled by NASA’s famous Centaur rockets. Pioneer 10 flew past Earth’s moon — a three-day trip for the Apollo missions — just 11 hours after its launch. Within three months, it had crossed Mars’s orbital path and headed for the Asteroid Belt. Luckily, both Pioneer 10 and its sibling probe Pioneer 11 survived traversing the radioactive band of whirling boulders. Pioneer 10 made a close flyby of Jupiter in December 1973 before hurtling away toward interstellar space. Pioneer 11 stopped by Jupiter in November 1974 before becoming the first spacecraft to reach Saturn. The two Pioneer missions took up-close photos of Jovian and Saturnine landmarks that Earth-based astronomers had only seen hazily through telescopes before. However, the Pioneers’ focus was very much on the gas giants, not their moons.
Luckily, the planets’ orbits gave NASA an incentive to speed up their exploration of the gas giants. NASA’s leaders realised that in 1977, all of the gas giants — Jupiter, Saturn, Uranus, and Neptune — would align in and create a once-in-every-196-years opportunity. It might, NASA realised, be possible to slingshot a spacecraft past all four planets. That project yielded the twin probes — Voyager 1 and Voyager 2. While the Pioneers were built for speed, the Voyagers were built for data and durability.
When Voyager 1 flew past Jupiter and the innermost of its large moons, its team immediately saw something unexpected: the innermost moon, Io, was erupting. Unlike Enceladus, Io was spewing rocky magma, but Io’s volcanic hyperactivity showed that Jupiter’s gravity could pull on its moons enough to create geothermic activity and heat. At 778 million kilometres from the Sun, undisturbed water on Jupiter’s moons would be frozen stiffer than most terrestrial rocks. However, if Jupiter’s pull could cause the molten rock inside Io to churn and erupt as volcanoes, geothermal activity — and therefore heated water — might exist on Jupiter’s other moons, too. But the Voyager team would have to wait for Voyager 2 to reach Jupiter and collect data from Jupiter’s outer, icier moons.
When Voyager 2 flew by and took photos of Jupiter’s moon Europa, the photos revealed a world crisscrossed by mysterious red stripes. Voyager’s 2 also revealed that Europa was a bit ‘squishy.’ Jupiter’s gravity tugs on every layer of the moon, not just the outer crust, so when Europa passes close to Jupiter, the planet exerts a powerful tidal pull on the liquid beneath the outer crust. The subterranean tides of Europa are so strong that some surface features shift as much as 30 metres with every tidal cycle. Measuring the amount of tidal stretching gives scientists a way to estimate how deep the moon’s oceans may be.
In Europa’s case, it’s hard to tell precisely where the icy crust stops and the saltwater liquid ocean starts, but the oceans are estimated to be as much as 100 kilometres deep. If that’s the case, Europa may contain two or three times as much liquid water as all of Earth’s oceans, despite only being the size of Earth’s moon. Beneath its alleged aquatic depths, Europa likely has a rocky mantle and core, similar to Earth’s, which could erupt into the ocean layer as hydrothermal vents. In the decades since Voyager 2’s flyby, evidence in favour of Europa’s liquid oceans has grown. However, figuring out exactly what lies inside Europa is still a game of speculation.
At least two more of Jupiter’s large moons — Ganymede and Callisto, which were discovered along with Io and Europa by Galileo Galilei in 1610 — show signs of having at least a bit of subsurface liquid, but Europa has captured the most imaginations as a possible location for alien life in the solar system.
In fact, in the early 2000s, NASA had plans to send an unusually large nuclear-powered probe to the Galilean moons to investigate their potential subsurface oceans. The project, Jupiter Icy Moons Orbiter (JIMO), would have included a robot that could land on Europa’s surface, drill down through several kilometres of ice, and then release yet another robot that could begin underwater explorations of the moon. JIMO and its ambitious goals captured the attention of none other than future Avatar director James Cameron, who highlighted the JIMO project in his 2005 documentary Aliens of the Deep.
Unfortunately, in 2005, NASA cut off funding to the JIMO project in favour of funding more manned missions to closer locations such as Earth’s moon and Mars. Competition between proposed space exploration projects is intense, and sadly for Europa researchers and aficionados, a moon from a different planet may be starting to challenge its status as most intriguing moon.
Titan, Saturn’s largest moon and Enceladus’s neighbour, has long been a mystery to astronomers because of its dense atmosphere. No telescope can see through it, and the atmosphere extends so far past Titan’s surface that planetary scientists used to think Titan was the largest moon in the solar system. (According to current estimates, it’s actually the second largest moon in our solar system with a 5,152km diameter to Ganymede’s 5,268km.Still, both Titan and Ganymede are larger than planet Mercury.) Despite being far too cold to host liquid water at its surface,Titan is a moon with clouds, rain, and storms of its own.
Much of what we know about Titan’s surface comes from data collected over just a couple of hours. In 2005, the Cassini/Huygens spacecraft — a joint NASA and ESA mission — arrived in Saturn’s orbit. While Cassini perused Saturn’s rings, the Huygens probe detached from Cassini and headed for Titan. The Huygens probe, named for the Dutch astronomer who discovered Titan via telescope in 1655, was the first (and so far only) human-made vehicle to land on a planetary body beyond the asteroid belt. The footage of Huygen’s descent onto Titan is famous in planetary science circles, because it revealed shockingly Earth-like terrain. Titan has dried methane river beds, lakes, ridges, and even smooth, rounded pebbles.
Unfortunately, Huygens didn’t have much time to rove and explore on Titan’s surface. The probe only continued transmitting for about 90 minutes after landing, but the information it sent back to Earth revealed a shockingly Earth-like world, albeit with liquid methane and ethane in place of liquid water.
Titan’s atmosphere exerts four times as much pressure at its sea level as Earth’s air does at our sea level. The moon’s air has traces of many different carbon-based compounds, including several that no one expected to see. Titan’s complex chemistry that makes some people wonder if some alternative form of life may be dwelling in the ethane-methane seas.
However, Cassini data also revealed that Titan likely has an underground liquid water ocean in addition to its surface-level ethane-methane seas. Like Europa, Titan is “squishy”; it too bends in response to its gas giant’s gravitational pull. Although Titan’s liquid water layer would likely be shallower and saltier — less likely to be habitable than Europa’s — it’s still the only body we know of with two chemically distinct ‘oceans.’ In terms of potential for housing alien life, Titan may be the solar system’s first double threat.
Yet, it’s so far away that we’re unlikely to hear of new probe visiting Titan or Enceladus’s oceans any time soon. Mars, with its potential for hosting human astronauts, still draws more funding and public interest than even the most promising moon oceans.
A few researchers have said that specially designed robot submarines may be able to scout Titan’s surface-level seas and lakes. However, missions like the Voyagers and Cassini/Huygens take years, if not decades, to plan and carry out. Additionally, such missions are at the mercy of planetary alignment. Planetary researchers will want to time launches such that probes can take advantage of the gas giants’ closest approaches to Earth and clear lines of communication between the moons of interest and Earth-based mission control. We will probably have to wait until at least the 2030s to gather more data on Titan and Enceladus.
Juno, NASA’s newly arrived Jupiter orbiter, is primarily focused on the planet, not its moons. The Galilean moons will receive their next round of close-up study in 2030, almost exactly half a century after Voyager 2’s discovery of Europa’s hidden oceans. The probe in question will be a European Space Agency project named JUICE (JUpiter ICy moon Explorer). JUICE will build off the data collected by NASA’s Galileo orbiter in the 1990s and early 2000s but will bring more sophisticated instruments to the table, including a radar system that may be able to detect structures kilometres beneath the moons’ outer shells. JUICE will spend the majority of its time out near Ganymede and Callisto in order to avoid Jupiter’s intense (and spacecraft-damaging) radiation but will make two flybys of highly irradiated Europa. By the close of the 2030s, we’ll likely know Jupiter’s large, ocean-carrying moons much better than we do now.
But until then, we’ll just have to wait and see.
Edited by Jack Scanlan