No one way to make an ocean world
Several decades of exploring the outer solar have revealed that alien oceans are common. Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan are just a few of the confirmed examples. Recent research, which I played a role in (shameless plug) also shows there may be subsurface oceans on the Uranian moons Miranda and Ariel.
What does it mean that tiny moons the size of smaller than Texas at the frigid edge of the solar system can have subsurface oceans of liquid water? For one, it implies that subsurface oceans are everywhere, and yet there still appear to be rules for where you can have an ocean.
Subsurface oceans, like Earth’s surface ocean, require a heat source. The heat source of Earth’s ocean, and likely the oceans which may have once existed on Mars and Venus, is heat directly from the sun. Heat from the sun is trapped by greenhouse gases in the atmosphere of Earth, which raises the temperature to allow for liquid water to exist on Earth’s surface. Because of the importance of liquid water in the presence of life, the zone around a star where a planet’s surface is warm enough for liquid water to exist, provided the right pressure and atmospheric parameters are also present, is referred to as the habitable zone.
The habitable zone not only requires sufficient heat from the parent star based on the intensity of sunlight at a given distance from the star (also called solar flux). There must also be an atmosphere thick enough to provide the necessary pressure for water to exist in its liquid form and not simply sublimate between ice and vapor the way it does on modern Mars.
Furthermore, the atmosphere must have the right heat trapping gases, such as CO2, in order to maintain enough heat. Otherwise it may still be too cold for water to be liquid for long periods of time. You also don’t want too high a concentration of greenhouse gases either…see Venus for details. The search for planets in the habitable zone of other stars is a very hot topic in exoplanet science right now.
While the habitable zone created by sunlight, greenhouse gases, and atmospheric pressure is well known and a frequent feature not just in science but also in science fiction, there is another habitable zone which is being revealed by the exploration of ocean moons. This habitable zone is based not on heat from the sun, but by tidal potential from the parent planet.
For example, the reason that Europa is hot enough to have a subsurface ocean is because of gravitational interactions between Jupiter and the other moons which cause Europa to stretch and contort generating heat the same way you would create heat from bending a paperclip back and forth. Just like a the solar habitable zone is illustrated by Venus (too hot), Mars (too cold) and Earth (just right?), a similar principle may apply to Europa, Io, and Callisto.
Jupiter’s moon Io, known for its volcanoes, may be too close to Jupiter so the tidal effects from Jupiter’s gravity produce too much tidal heating. Io long ago lost its ice layer and any chance of having a subsurface liquid water ocean. Jupiter’s moon Callisto, given its heavily cratered, and eroded surface, may be too far from Jupiter and the other moons for tidal heating to generate significant geologic activity. Even if it does have a subsurface ocean, there may not be volcanic avtivity on the ocean floor to provide energy for life. Europa appears to be the right distance so that it still has energy to heat its ocean, but not so much that it just becomes another Io.
Saturn’s moons are an interesting case in that Enceladus does not look like it is being actively heated by tidal effects from Saturn and the other moons. In fact, thermal models suggest that Enceladus should be geologically dead based on the tidal heat that would come from its tides due its tiny eccentricity.
Another interesting thing about Enceladus is that it is much smaller than Europa. Europa is roughly comparable to Earth’s moon in size (D = 3100 km or 1,926 miles), where as Enceladus is only 500 km (311 miles) in diameter. Europa also has evidence of active tectonics, where crustal fragments of ice are being moved along its surface analogous to tectonic plates on Earth. Enceladus has geyser-like plume activity coming from enormous fissure to be sure, but no evidence of anything analogous to a subduction zone or spreading ridge on Earth.
Thus there appear to be at least three types of ocean worlds in Earth’s solar system. There are Earth-type ocean worlds, which we will call Type 1. “Type 1” ocean worlds have liquid oceans on their surfaces and the lifespan of oceans is determined by the composition of the atmosphere and their distance from their parent star. Mars in this case could count as an extinct type 1 ocean world.
Type 2 ocean worlds are Europa-like worlds with subsurface oceans. The mechanism heating these oceans is tidal forcing due to gravitation interactions with their parent planet and other moons. The lifespan of their oceans has more to do with their distance from their parent planet, other planetary satellites, and the shape of their orbit (eccentricity) and their orientation in space (obliquity), since both eccentricity and obliquity will play a role in how much tidal forcing they receive. The tidal heating that type 2 ocean worlds can receive will also depend on the size of the planet they orbit.
Finally, Type 3 ocean worlds are similar to Type 2 but they are smaller, 500-1000 km in size, and lack evidence of active tectonics, though they may have geyser activity from fissures.
Which ocean world is more common? If subsurface oceans can form on Miranda and Ariel which orbit the much smaller Uranus (D ~50,000 km or ~30,000 miles) compared to Jupiter (D = ~86,000 km or ~53,000 miles), then icy moons orbiting any sufficiently large planet in the galaxy could be ocean worlds. Exoplanet studies show that Jupiter-sized planets and Uranus/Neptune-sized planets are among the most common planet types of the >6,000 discovered exoplanets. This could mean that there are thousands of potential ocean worlds just among the planetary systems we have discovered so far.
If oceans are common in the galaxy, what about a phenomenon that is known to take place in the oceans of Earth at least, life? Although there is a lot of interest in the habitability of subsurface oceans on icy worlds, it is not certain how hospitable to life such oceans actually are, romantic notions of alien squids living on Europa aside. For one thing, scientific studies suggest that subsurface oceans are nutrient-starved, which would limit the possibility of the emergence of complex life at least. It is possible that the only life we are likely to find on subsurface oceans across the outer solar system is a few microbes.
If there is a way that such nutrients could enter into the oceans, on the other hand, such as an analogue to subduction zones on Earth where material can be transfered from the surface into the ocean, that would mitigate this problem. On the other hand, critical ingredients for life, such as carbon, phosphorus and nitrogen, have all been shown or at least predicted to be present on many icy worlds, such as Enceladus and (1) Ceres.
Even if it is possible to get complex life on the icy moons, it is unclear that you would get technological civilizations which could communicate with us across interstellar distances since the basis of technological civilization on Earth, the control of fire, is obviously lacking in a subsurface ocean. At the same time, life finds a way.
My guess is that the first life we discover beyond Earth will come from a subsurface ocean of an icy moon, given the apparent prevalence of type 2 and 3 ocean worlds compared to type 1. The first extraterrestrial intelligence we discover is more likely to come from a type 1 ocean world given the difficulty in having a technologically advanced civilization without the possibility of combustion.
There are many unknowns about the many alien oceans that we have discovered in the solar system and may await interstellar probes across the galaxy. One thing that is certain though is that there are many exciting possibilities of what these oceans are like and whether anything living could be found in them. The only way to really answer these unknowns is of course sending spacecraft to explore them, starting with the ones in our own solar system.
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