In October 2016, astronomers announced that Saturn’s moon Dione most likely has an ocean of liquid water beneath the surface. This makes it the sixth such moon we know of in our solar system. Two other moons of Saturn, Titan and Enceladus, are also thought to have subsurface oceans, and the same is true for the three big Jovian moons Europa, Callisto and Ganymede.
We are not talking about small amounts of water. Even though the moons are far smaller than the planet Earth, each of their oceans are expected to hold more water than all the Earth’s seas combined. Unlike our planet’s oceans, the water in the moon’s oceans isn’t kept liquid by the Sun’s heat, but by tidal forces that heat the core and create internal volcanic activity. This is interesting, since the best current theories for how life originated on Earth say that it happened precisely around undersea volcanic vents, which even today nourish vital ecosystems. Life here isn’t fuelled by photosynthesis, but by chemosynthesis, where the conversion of sulphur and sulphites provides the necessary energy for life to thrive, either directly or indirectly through animals that eat chemosynthetic plants. For this reason, many scientists believe that there is a good chance that primitive lifeforms exist in the oceans of one or more of these moons.
When astronomers look for planets around other stars that may harbour life, they look in the so-called ‘habitable zones’ around the stars, at a distance where the water on a planet’s surface could exist in liquid form without either evaporating or being bound in permafrost. Our own solar system has three planets in the habitable zone – Venus, the Earth, and Mars – and of these, only the Earth has liquid surface water. Watery moons like Dione, Europa and the rest need not be at any particular distance from their sun, just within a suitable distance from their planet, and the range is wider here – Titan, for instance, lies more than five times as far from Saturn as Enceladus does, while Mars only is slightly more than twice as far from the Sun as Venus is. It is in fact not even necessary to have a moon orbiting a large planet to have subsurface oceans. Pluto, which is also thought to have a subsurface ocean, is not orbiting a larger planet, but instead has a relatively large and close moon. Planets orbiting small, cold stars such as red or brown dwarfs – which are far more common than Sun-type stars – could also hide subsurface oceans heated by tidal forces.
Overall, it does look like liquid water, and hence the opportunity for life to arise, is far more common beneath the surface of moons and similar celestial bodies than on the surface of Earth-like planets. Could we imagine that some such moons harbour advanced or even intelligent lifeforms? If so, how are we going to discover them?
From here on, this article will necessarily be far more speculative than the above. Consider yourself warned.
As mentioned, biologists think that life on Earth originated around undersea volcanic vents, and this would likely also be the case for watery moons, since photosynthesis would be impossible in a dark world where the only light comes from volcanic rifts with liquid magma. Life originating in such a world will be very unlike even what we find in the oceans of Earth. Sight, as we know it, will likely not be of much use, and sonar could very well be the primary sense, as in dolphins and bats on the Earth. Even so, many undersea organisms on Earth exhibit bioluminescence, and we can imagine animal species that bring their own light to see by.
The pressure will be enormous. The Jovian moon of Europa is thought to have a sea up to 170 km deep, covered by an icy layer 10 to 20 km thick. Even though Europa’s gravity is only 13 percent of the Earth’s, the weight of the ice sheet and the water creates a pressure at sea bottom that is more than twice the pressure in the deepest trenches in Earth’s oceans – we are talking thousands of atmospheres. This, however, does not rule out life, and in fact, there is a complex biotope at the bottom of the Mariana Trench, the deepest undersea canyon on Earth. The pressure does set some limits – it is e.g. not possible to have air-filled hollows, and calcium shells can’t handle the pressure, either. Life down there mainly consists of microbes, algae, molluscs, and animals with glass shells. Many of the lifeforms that have evolved to live under such pressure, can’t survive in conditions of lower pressure. If we were to imagine that somewhere in the universe, intelligent life may have arisen under such conditions, they would need one hell of a pressure suit to explore the upper layers of their buried oceans – not to mention the emptiness of space.
Life, however, has a habit of finding niches everywhere. Life on Earth originated in the sea, but walked onto land and in time evolved into human beings. In a similar manner, life originating at the bottom of the sea on Europa-like moons could move up through the layers of the ocean and for instance find a habitat in the ice canyons on the underside of the kilometres-thick ice sheet. Radiation from the sun, or the planet which the moon orbits, will break down minerals on the outer surface, and oxygen, among other things, could drift down through the ice and fuel chemical reactions that support life. The watery moons we know shoot up enormous geysers of water far above the surface, and this water falls back to the surface, creating new layers of ice. This creates a cycle where mineral-rich water from the subsurface ocean is thrown up on the surface, where the minerals are broken down by radiation and slowly drift down until they are released on the underside of the ice and can take part in biochemical reactions. Astronomers even think that life could arise on this basis in the absence of volcanic activity at the bottom of the sea.
It is one thing for life to arise in the subsurface oceans of a moon, another for such life to develop intelligence, civilisation, and technology. We do find quite intelligent aquatic lifeforms on Earth – including dolphins and octopi – but none of them have developed anything we would call a civilisation (though some octopi use tools). However, the same thing can be said about intelligent apes like chimpanzees, orangutans and bonobos, so this is not in itself an argument that aquatic intelligent species will be unable to develop advanced civilisations that build cities and practice aquaculture.
There will, however, be limits on the development of technology. Fire has meant a lot for mankind’s technological development, but it is difficult to make things burn underwater. It is also far more difficult to control electricity underwater, though some underwater species, such as the electric eel, use electricity for sensing and as a weapon. Alternatives could instead be found in the shape of e.g. chemical heat and light-based ‘electronics’, and we can also imagine purely biotechnological civilisations that through thousands of years of breeding and genetic engineering develop lifeforms that serve the same purposes as our fire-based and electronic technology.
An undersea civilisation could very well choose to explore the geysers that suck water out of the icy canyons and hence discover the surface of their moon. Unless the denizens of the moon wholly lack a sense of vision, technological or biological, they are unlikely to escape noticing the planet they are orbiting. After this, the discovery of other moons, planets, and stars will follow naturally.
It could perhaps seem as though aquatic creatures would find it hard to build space rockets, but we must remember that gravity and hence the escape velocity (the minimum speed a vessel needs to escape the gravity well of its celestial body; ed.) will be far less than on Earth. It will also be easier for these creatures to find worlds resembling their own – there may be several others orbiting the same planet – and start colonising them. With the discovery of fusion power, comets can be heated from within and turned into habitats, and in time, they could colonise the billions of ice planetoids and proto-comets that orbit their star, far more easily than we could do something similar.
Overall, it does seem more likely that we will find intelligent alien life beneath the surface of watery moons than on the surface of watery planets. If they have begun colonising space, it is likely that they will communicate by means that we could pick up, like radio or lasers. So perhaps, when we someday get in touch with intelligent beings from outer space, they may resemble jellyfish or squids more than they will resemble human beings.
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