OCEAN WORLDS

If physics and chemistry are the same throughout the universe, is biology too?

Earth isn’t the only world which accommodates water. There are moons in the Solar System half the size of Earth that hold much more of it. These are known as ocean worlds. Astronomers continue to scrutinise these places in great detail, as they have the potential to change how biology is viewed throughout the universe. “An ocean world is any planet, or sometimes moon, that has at least ten times the amount of water that Earth does. For reference, Earth is about 0.1 per cent water, so an ocean world has a water content of one per cent or greater,” explains Dr Lynnae Quick, an ocean worlds planetary scientist at NASA’s Goddard Space Flight Center in Maryland.

Water is a necessity for life as we know it. Every living thing on Earth consumes water to stay hydrated and survive, from the smallest single-celled organisms to giant mammals. Water also provides opportunities for exploration, whether it’s taking to the oceans to discover a new bit of land or diving into the depths below, where many mysteries remain unanswered. Astrobiologists now want to up the ante and explore the oceans of other worlds millions of miles – and possibly even light years – away in order to find signs of alien life.

How water worlds end up having global oceans, either visible on the surface or buried underneath an icy exterior, is a story in itself. Water does not float through space as liquid droplets waiting to splash down on a body’s surface. As space is so cold, water travels through space as grains of ice. As new planetary systems begin to form around a young star, water can only stay in the form of these icy grains beyond a certain radius, known as the ‘ice line’.

“There is a natural ‘ice line’ in the Solar System where volatile species migrated away from the centre, akin to a tree line on tall mountains. The outer Solar System is replete with water, among other such species,” says Dr Chris German, a senior scientist at the Woods Hole Oceanographic Institution in Falmouth, Massachusetts. “Planets tend to have geothermal gradients, whether it’s from the cooling of magma, the radioactive decay of longlived isotopes or tidal heating. As you go deeper, things warm up and the potential for melting arises.

”Of course, if the oceans get too deep, then as you dive further and further in, higher pressures arise than what we see on Earth, and compaction can lead to new and different forms of solid-phase minerals with the same water composition to form,” says German. He also describes this in a way that’s simpler to understand: “Some ocean worlds can be ice sandwiches.”

Tidal heating is a term that’s frequently mentioned when talking about ocean worlds, but more specifically the ocean moons that orbit larger planets. Known ocean moons in the Solar System beyond the asteroid belt include three of Jupiter’s four Galilean moons – Ganymede, Callisto and Europa – as well as Saturn’s moons Enceladus, Titan and Mimas and Neptune’s moon Triton. The fact that they are all gravitationally bound to enormous planetary bodies means that there’s consequently a gravitational sloshing effect in the moons’ cores.

This sloshing effect creates heat in the form of friction within the core, which in turn heats up the outer layers. Tidal heating, when combined with radiogenic heating from radioactive decay, melts the water ice accumulated from the moons’ formation beyond the ice line and creates interior oceans. When you see one of these moons through your telescope and acknowledge its icy surface, remember that there is an enormous quantity of subsurface liquid water lying beneath, which astrobiologists are working incredibly hard to try and investigate.

“Ocean worlds are important to study because we can test an important hypothesis in science: biology,” explains Dr Morgan Cable, a research scientist and supervisor of the astrobiology and ocean worlds group at NASA’s Jet Propulsion Laboratory in California. “We already know that the rules of physics – the speed of light, the theory of general relativity and so on – seem to apply everywhere in the observable universe. The same is true for the rules of chemistry; those also seem to apply everywhere. But we haven’t tested this for biology yet. If you have an environment with habitable conditions and you wait long enough, will life emerge?”

On 15 September 2017, NASA’s Cassini mission came to an end. During its 13 years at Saturn it returned breathtaking images and invaluable data – not just about the ringed planet, but its moons, too. The data returned about the icy moon Enceladus was key in revitalising public attention towards ocean world research, which had been growing over the last few decades. A notable event which strengthened this shift was when the spacecraft flew through a plume of material erupting from the icy surface of Enceladus on 28 October 2015. With its instruments turned on and ready to go, Cassini dove head first into the plume, and the data taken from this encounter led to remarkable discoveries about Enceladus’ interior. The most important finding was that within this world, which is 25 times smaller than Earth, lies the three main ingredients for life: liquid water, an energy source for metabolism and a tantalising selection of chemical ingredients such as carbon, nitrogen, oxygen, phosphorus and sulphur.

With this incredible discovery, astrobiologists made an extremely compelling case that future exploration missions should look to visit more ocean worlds in the outer Solar System. This was once again strengthened when astrobiologists found something very interesting in the data collected by NASA’s Dawn mission, which was operational at the dwarf planet Ceres between 2015 and 2018. “Measurements from the Dawn mission indicate salts on the surface of Ceres, which can only be there if they are being constantly replenished from underneath. This means Ceres has activity due to brines [salty liquids], making it an ‘evolved’ or ‘mature’ ocean world,” says Cable. “Based on our current understanding, only a few per cent of Ceres’ subsurface ocean is left, so Ceres may serve as an excellent example of what an ocean world without tidal heating looks like at the end stages of its life when the energy runs out.”

An honourable mention also goes to NASA’s New Horizons mission, which returned the first close-up images of the dwarf planet Pluto from the darkest, most distant region of the Solar System, known as the Kuiper Belt. The images that were transmitted back to Earth from over 5 billion kilometres (3 billion miles) away were truly extraordinary. They revealed cracks on the surface, possible ice volcanoes and an extremely thin atmosphere made up mostly of molecular nitrogen. These are all lines of evidence that could point towards a subsurface ocean that is replenishing the dwarf planet’s atmosphere via the eruption of ice volcanoes or immediate evaporation after escaping through surface cracks.

When looking at these ocean worlds and the data collected from telescopes and exploration missions, it’s understandable that astrobiologists can’t wait for the next mission to launch and reveal a brand-new host of discoveries. Two missions due to launch within the next decade are NASA’s Europa Clipper mission, which is due to explore Jupiter’s moon Europa around 2030, and the European Space Agency’s (ESA) JUpiter ICy moons Explorer (JUICE), due to arrive at the Jovian system in 2031.

These two bespoke moon missions aim to provide a thorough reconnaissance of the three watery Galilean moons and will look to fully characterise their current states and compositions while also informing us about their evolution over the last 4.6 billion years. Europa is roughly 90 per cent the size of Earth’s Moon but holds twice as much water than all of Earth’s oceans combined. The astrobiological processes that may exist in this moon could answer the age- old question of whether there’s life elsewhere in the universe. And if Europa doesn’t host life, does it at least have a habitable and hospitable environment for it? “If we can find evidence in the outer Solar System that would imply a second origin of life here in our corner of the universe,” says German. “Occam’s razor then suggests that life should be pervasive in every other star system and we’re not that special at all.”

There are even efforts underway to test how we could explore these subsurface oceans directly. In 2019, a team of engineers from NASA’s Jet Propulsion Laboratory travelled to Antarctica to test the Buoyant Rover for Under-Ice Exploration (BRUIE), a roving vehicle built with the long-term goal of exploring the subsurface oceans of icy moons. BRUIE uses a unique combination of buoyancy and wheels with tiny spikes in them to move along the underside of ice sheets. This rover would be able to provide direct measurements of an ocean’s water salinity, dissolved oxygen levels, temperature and pressure. With these advancements, one day in the future there could be a documentary series talking about the oceans of Europa and what ocean life is swimming around in there!

“Ceres may serve as an excellent example of what an ocean world without tidal heating looks like at the end stages of its life”

Morgan Cable

Looking beyond our own Solar System, there are many exciting opportunities for discovering ocean worlds in other stellar systems. Unfortunately, these are a little bit harder to detect, but there are still techniques that can be used to infer their existence. “We can identify ocean worlds in extrasolar planetary systems based on their size and inferred density,” says Quick. “Low-density, low-mass planets are likely to be ocean worlds.”

A simple technique that’s commonly employed in astronomy when it comes to aiding research regarding an exoplanet’s potential for accommodating liquid water is calculating if a planet resides in its host star’s habitable zone. This is the radius range around a host star where the temperature is perfect for water to exist as a liquid. If the planet orbited any closer the water would evaporate, but if the planet was too far away the water would freeze into ice. This basic technique is used by astrobiologists when determining if an exoplanet could host liquid water as it’s known that Earth resides in the Sun’s habitable zone. It’s important to remember that this is an extreme simplification and does not take into account things such as a planet’s composition, atmosphere, evolution history and so many more factors.

To make a discovery of this calibre with great confidence and little uncertainty, the techniques that astrobiologists and astronomers use to detect exoplanets and analyse them will need to be improved in order to infer other aspects of a planet’s properties, and this includes its atmosphere. “Astronomers can further their techniques by improving spectral resolution and sensitivity,” says Cable. “This would help us identify possible biosignature molecules in the atmospheres or on the surfaces of exoplanets.”

Recent research suggests that the Milky Way could be littered with ocean worlds. A study led by Quick suggests that out of the 53 exoplanets surveyed, at least 30 of them could be ocean worlds, as they fit the assumed parameters. This is based on restricted knowledge, however, as current technologies and techniques only allow astronomers to definitively measure an exoplanet’s size, mass and distance from the host star, which can only imply a planet’s surface temperature. “If we see that a planet’s density is lower than Earth’s, that’s an indication that there might be more water there and not as much rock and iron,” Quick says. “But if a planet’s surface temperature is less than 32 degrees Fahrenheit [0 degrees Celsius], where water is frozen, then we have an icy ocean world, and the densities for those planets are even lower.”

With the number of known exoplanets currently over 5,600, Quick’s results could be extrapolated, suggesting that there are thousands of ocean worlds dotted across the Milky Way. Quick also makes the point that there could be other signatures of ocean worlds that are currently unknown, and that upcoming missions, for example the Europa Clipper, will identify these signals. These parameters could then be applied to exoplanet research in order to identify even more ocean worlds. This could provide greater reliability and accuracy about whether an exoplanet is an ocean world, since it provides something more meaningful than just inferring information based on the planet’s size, mass and density. Even the news regarding Ceres likely being a more mature ocean world could give astronomers new signs and biosignatures to look out for when investigating exoplanets.

The study of ocean worlds, both within our Solar System and elsewhere in the cosmos, is certainly one to keep a keen eye on. Through upcoming missions, astrobiologists will become even more equipped with highly precise data that could answer one of the most profound questions there is, finally revealing if life on Earth is special or not.