When it come to habitable environments in our solar system, there’s Earth, perhaps Mars billions of years ago and then a slew of ice-covered moons that are likely to have global oceans under their crusts. Many of you are familiar with Europa (a moon of Jupiter) and Enceladus (a moon of Saturn) — which have either been explored by NASA or will be in the years ahead.
But there quite a few others icy moons that scientists find intriguing and just possibly habitable. There is Ganymede, the largest moon of Jupiter and larger than Mercury but only 40 percent as dense, strongly suggesting a vast supply of water inside rather than rock.
There’s Saturn’s moon Titan, which is known for its methane lakes and seas on the surface but which has a subterranean ocean as well. There is Callisto, the second largest moon of Jupiter and an subsurface-ocean candidates and even Pluto and Ceres, now called dwarf planets that show signs of having interior oceans.
And of increasing interest are several of the icy moons of Uranus, particularly Ariel and Miranda. Each has features consistent with a subsurface ocean and even geological activity. Although Uranus is a distant planet, well past Jupiter and Saturn and would take more than a decade to just get there, the possibility of a future Uranus mission is becoming increasingly real.
The National Academy of Sciences (NAS) Decadal Survey for planetary science rated a Uranus mission as the highest priority in the field, and just today (Aug. 18) NASA embraced the concept.
At a NASA Planetary Science Division town hall meeting, Director Lori Glaze said the agency was “very excited” about the Uranus mission recommendation from the National Academy and that she hoped and expected some studies could be funded and begun in fiscal 2024.
If a Uranus mission is fully embraced, it would be the first ever specifically to an ice giant system — exploring the planet and its moons. This heightened interest reflects the fact that many in the exoplanet field now hold that ice giant systems are the most common in the galaxy and that icy moons may well be common as well.
If that Uranus proposal eventually becomes a scheduled mission, that will mean that NASA has approved two multi-billion-dollar Flagship missions to icy moons and their environs — the one in progress is the Europa Clipper mission scheduled to launch in 2024, and then the Uranus mission for years later.
Added to the icy-moon exploration is a major European Space Agency (ESA) mission to Ganymede, Europa and Callisto, scheduled to launch 2023. The Jupiter Icy Moons Explorer (or JUICE) mission will fly by the three icy moons in the Jupiter system discovered by Galileo in 1610 and then go into orbit around Ganymede. And the less elaborate but unprecedented Dragonfly terrain-hopping mission is also scheduled for a moon thought to have an inner ocean with a rock-ice surface with lakes of methane — Titan.
“Many outer solar systems moons are believed to have large subsurface oceans, and they have become of great interest as the community turns more and more to astrobiology and habitability,” said Francis Nimmo of the University of California, Santa Cruz. “The question of whether some are habitable, or might even support life now, is front and center.”
As explained by Nimmo, who is a member of the NASA Europa Clipper team, icy moon oceans are so important to planetary scientists and astrobiologists because they provide many of the key components needed for life: water, a source of energy (heat produced by tidal forces) and a chemical gradient that could provide food for microbial or larger creatures that might live in the oceans.
But while it may be possible to determine whether moons ranging from Ganymede to Callisto and Miranda are theoretically habitable, it will be impossible to determine if they are inhabited, he said. All have miles of ice crust separating life’s potential ocean homes from the punishing surface. The technology, know how and funds available reach those inner oceans is many decades off.
“We think we can answer questions such as how old the oceans are, how salty they may be and we can model simulations of how the oceans might circulate, but we won’t know about the composition of the water,” Nimmo said. “This explains why we’re so excited about Enceladus.”
That moon of Saturn is one of three where the inner ocean water might be accessible. The NASA Cassini mission to the Saturn system discovered that the small moon spewed out plumes of water vapor — an unexpected finding that excites astrobiologists enormously because it potentially allows them to sample the water. Several proposals for smaller NASA mission to bring life-detection instruments to Enceladus have been proposed and are in the hopper.
And some less prominent plumes may have (or may not have) been spotted by the Hubble Space Telescope on Europa. The finding is considered tentative and many efforts to find more plumes have failed.
But Europa is very far from the Hubble, while the Europan surface will be close to the Europa Clipper when it arrives at the moon, scheduled for the early 2030s. Clearly, one of the main objectives is to search for Enceladus-like plumes on Europa. The spacecraft will make almost 50 flybys of the moon, so it is likely to image a plume if they are indeed present.
The moon Triton of distant Neptune is also believed to have an ocean inside and some geysers. Neptune has 14 moons but Triton is by far the largest and the only one with sufficient gravity to make itself spherical, leaving the others to remain as objects akin to asteroids. Uranus was prioritized ahead of Neptune for a potential NASA Flagship because it was considered more feasible.
A spacecraft has visited the Uranus system but once, and it was in 1986 with Voyager 2. That spacecraft was not designed to study the Uranus system per se and so it did its flyby, produced the only close images the world has of the planet and some of its moons. After it’s brief flyby, Voyager 2 then sped further into the solar system and ultimately into interstellar space.
Uranus has 27 moons, most named after characters from Shakespeare and a few from Alexander Pope. The five largest ones are Ariel, Miranda, Umbriel, Titania and Oberon and all are considered candidate ocean worlds with internal saltwater oceans.
The two of greatest interest are Ariel and Miranda.
Large canyons dot the surface of Ariel and there is reason to believe that it has a geologically active surface. That surface and atmosphere are rich in carbon dioxide and may feature ammonia and carbonates.
All these conditions suggest to scientists that Ariel has, or has had, a subsurface ocean that reaches the surface through cryovolanic ice flows (volcanoes that erupts with ice, water, and other materials, such as methane and ammonia, instead of molten rock and ash.) The deep canyons have fissures that suggest volcanic activity as well.
Miranda is one of the odder-looking objects in the solar system. Only about 300 miles in diameter, it has canyons 12 times deeper than our Grand Canyon and a 14-mile-high cliff that has is the tallest in the solar system.
Water is known to exist on Miranda’s surface and it’s suspected that it also hosts methane, ammonia, carbon monoxide, or nitrogen. It has those three mysterious polygonally shaped coronae, a feature uncommon in the solar system. It’s thought they’re caused by warm ice rising to cause tectonic faults that deform the moon’s surface, and there are possibly many more.
While all Uranian large moons have significant amounts of water ice, Miranda is on the high side with an estimated 60 percent water and ice, as determined from afar by its low density.
What produces a subsurface ocean inside an icy moon?
Being located far from the Sun is an initial requirement. Planets and moons from Mars inward are too close to the Sun to have maintained the original water that they were all formed with. A body’s location in relation to the solar system “snow line” defines whether that primordial water remains, in a frozen form. (The oceans on Earth are generally described as having been delivered via comets and asteroids later in the planet’s evolution, after the original water had been burned off.)
So subsurface ocean worlds are an outer solar system phenomenon. But the icy moons would remain just icy were it not for tidal heating — the continual flexing and stretching of a moon caused by the shape of its orbit and the gravitational pulls from its planet and other nearby orbiting moons. This gravitation shape-shifting causes friction, which in terms produces the heat needed to warm a subsurface ocean.
But not all outer solar system moons have oceans; indeed many do not, or have water ice that does not melt.
An iconic example of the peculiarities of these moons is the difference between the Saturnian moons Mimas and Enceladus.
Mimas is closer to Saturn than Enceladus and so should be subject to more tidal heating and melting. Yet it appears to be frozen solid and heavily cratered, suggesting that it has persisted that way for a long time. Yet Enceladus — the closest major moon to Mimas — has a smooth surface and has those famous plumes of water vapor.
This produces a paradox which has led astronomers to use the “Mimas test” for any theory regarding the plumes of Enceladus. Any theory to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.
Nimmo is an advocate for all the icy moon missions; there are just so many open questions, he said. As an indication of his informed enthusiasm, he was selected to be one of two “Science Champions” to advocate for the Uranus mission during the National Academy Decadal Review. He is also a member of one of the teams putting together proposals to send a small spacecraft back to Enceladus to test for possible biology in the plume.
So the future looks bright for icy moon scientists. But there’s a problem: today’s excitement will require years of patience before the data rolls in.
Jupiter, Saturn and Uranus are distant, taking some six, eight and twelve years of travel to respectively. The Europa Clipper does have tentative a launch date of 2024 and the ESA’s JUICE mission is tentatively scheduled for 2023, which means they won’t reach their destinations until the 2030s.
For Uranus, there is some celestial timing to take into account when planning for a mission. Uranus orbits the Sun every 84 years, and arriving at the Uranian system in the mid-2040s would allow for an optimal approach to the moons’ southern hemispheres. The timing would also give planetary scientists a chance to watch seasonal changes as the Uranus system approaches winter.
So despite the great scientific interest and effort, icy moon scientists will focus on the important work of modeling possible subsurface ocean scenarios — a well as developing missions and their instruments — for years to come before spacecraft arrive near the moons and start taking long-awaited measurements.
(For an earlier Many Worlds story on how oceans are formed on icy moons, click here.)