The Biggest Oceans in the Solar System

...red sky thinking for an open and diverse left

The Biggest Oceans in the Solar System

Steve Arnott looks at the icy gas giant moons Europa and Enceladus and investigates how unmanned robotic missions have overturned previous scientific thinking on the possibility of life elsewhere in the solar system.

Europa Rising

Reader, let me ask you a couple of simple questions. They’re not trick questions – you either know the answer or you don’t. I’m hoping your answer, right or wrong, can provide an instructive beginning for this short article.

How many planetary bodies in our solar system do we know have free flowing salt water oceans?

What is the biggest free flowing salt water ocean we know of?

If your answer to 1. was ‘one – the Earth’ and to 2. ‘the Pacific Ocean’, then you haven’t been watching enough Discovery Channel, I’m afraid, and have at least a few years of catching up to do in your science knowledge. But I’m not surprised if you didn’t know the answer. There’s a current advert for an all singing all dancing household android device on telly where it’s asked the latest news headlines and replies that geysers of water have been observed erupting from Jupiter’s moon Europa. In fact, these geysers were first observed years ago. Science and space stuff rarely features as headline news these days, even when the implications of the discoveries our remote machines are making out in the dark reaches of our solar system have paradigm shifting implications.

The correct answer to question 1, according to the best scientific data and observations we currently have, is that there are three planetary bodies in our solar system with free flowing salt water oceans: our own Earth, Jupiter’s moon Europa, and Saturn’s small moon Enceladus. The correct answer to question 2 is not the Pacific, but the sixty to 200 kilometres deep ocean of Europa, locked under its frozen surface ice, and which scientists believe has twice the volume of salt water than all the surface water on Earth put together.

If you did know the right answers then go to the top of the class – but I’m really writing this for those on the left who some of this stuff has passed by, hoping to share some of the wonder and importance of the cresting wave of scientific knowledge on free water elsewhere in our galactic backyard, and thus the realistic possibility of extra-terrestrial life existing and being discovered within the life time of this generation; depending on how old you are, perhaps your lifetime.

So how do we know this stuff? How do we know that free flowing salt water exists under water ice crusts on moons so far away that using current propulsion technologies it takes our robotic explorers 7 – 10 years to get there?

Let’s deal with each of the two moons – and the two - oceans separately, before bringing our narrative to a conclusion.

Europa

Jupiter is the largest planet in our solar system, a gas giant with no less than sixty four moons. Most of these are relatively small, but there are four moons considered to be the major moons, Io, Europa, Ganymede and Callisto. These are the Galilean moons, named after their discoverer, the renaissance giant Galileo Galilea . All of these moons are of major scientific interest. Europa is the second moon out from Jupiter and until relatively recently was, yes, thought to be icy, because of its high albedo (light reflectivity), but basically a dead ice ball in space. And, of course, ice needn’t imply the presence of water. Other substances, much more inimical to life can freeze to an icy mass at the correct temperature.

The Galileo space probe mission to the Jovian system in 1996 changed all that. Much closer and more detailed examination of Europa than had been possible with previous Jupiter missions, Pioneer and Voyager, showed a water ice surface with darker regions that were mineral rich, a changing active surface, and long, strange lines of various lengths and breadth criss-crossing the surface. Crucially, Galileo magnetic field data also

“…showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. The layer is likely a salty liquid water ocean. The crust is estimated to have undergone a shift of 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle.^[27] Europa probably contains a metallic iron core.^[28]’”

^{(source: Wikipedia)}

Planetary geologists recognised the line features as similar to a phenomena often seen at Earth’s own icy poles, where cracks in the ice allow subsurface open water at higher temperatures to rise above the ice mantle and then rapidly freeze, causing parallel lines or ridges to develop along the crack. Further examination of Europa’s lines (or linae) showed that nearly all were parallel ridged in this way, that many were relatively recent geological features, and the spectroscopy confirmed the composition as mainly water ice.

Europa, approximate natural colour, imaged by Galileo spacecraft

The startling conclusion was that under Europa’s frozen ice shell was a world girdling subsurface ocean, many, many kilometres deep. But how could this possibly be? So far away from the sun – many hundreds of millions of miles farther away than our own Earth in its comfortable ‘goldilocks’ zone – where was the heat coming from to keep such a vast quantity of water in a liquid state?

The answer emerged from the fundamental physics of gravity. Just as the moon’s orbit on the Earth produces tides on the surface of our homeworld, all planetary bodies exert tidal forces on one another to a varying degree – from the very, very small, to the large and significant. Mathematical modelling of the tidal effect that Jupiter and its other moons caused on Europa showed that the moon was constantly being stretched and flexed by these huge tidal forces. Here was a source of heat powerful enough to maintain water in a liquid state even out in the frozen depths of the outer solar system. The kinetic energy of the tidal stretching of Europa’s solid core would generate vast amounts of heat which would seek to escape outwards. Such heat would be enough to maintain water at a liquid state except near the surface where too much heat would be lost directly to space and the ice would freeze.

(If you are sceptical about this try getting a small but softish rubber ball, and squeeze it in your hand for a minute or so – you will feel the temperature rise as the kinetic energy you are transmitting to the ball turns to heat.)

This was a powerful explanatory model for all the data Galileo was sending, at it also excited astro-biologists tremendously. Not only was liquid water present under the surface of Europa in almost inconceivable oceanic quantities, but the heat generated at the interface between ocean and rocky mantle was such that tectonic, volcanic and ‘smoker’ activity would be likely. The discovery of ‘extremophiles’ – such as large tubeworm colonies - in the deepest parts of Earth’s oceans, where the pressure, dark and heat were immense had shown that life could not only exist in such conditions, without direct sunlight for photosynthesis, but actually flourish.

Europa is thought to have existed in its current state for some three billion years – a huge potential evolutionary timescale. And although it exists in a highly radioactive environment so deep inside Jupiter’s huge magnetic field, its icy crust probably shields much of its oceanic depths from the most harmful effects of that radiation. In any case, radiation that causes mutations at the right rate in a life rich environment where there are selectional forces at play is not necessarily a bad thing for biological evolution.

Could there be basic or even more complex life forms living in Europa’s ocean? As yet we just don’t know, of course. But over the space of a few years Europa has probably overtaken Mars as the place scientists think it most likely we might find extraterrestrial life in the solar system.

Enceladus

Enceladus is the sixth largest moon of the great ringed gas giant Saturn, and at about 1/10^th the size of Saturn’s largest moon Titan, has a surface area approximately the size of the UK. Little was known about this moon until the Voyager mission of the eighties other than a very high albedo indicating likely surface ice. Again, as was the case with Europa, it was a recent and significant robotic mission to the outer planets that has vastly increased our knowledge about Enceladus.

The Cassini probe of 2005, according to Wikipedia, discovered

“…a water-rich plume venting from the moon's southpolar region. This discovery, along with the presence of escapinginternal heatand very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped inorbital resonancesthat lead to forced librationororbital eccentricity; proximity to the planet can then lead totidal heatingof the satellite's interior, offering a possible explanation for the activity.”

Analysis of the out gassing and its chemical composition has suggested a substantial body of subsurface water. The discovery and analysis of the so-called ‘Tiger Stripes’ of Enceladus – four long fractures with ridges rising on either side located near the south pole – have solidified this picture and organic compounds discovered through spectrographic readings of these tiger stripes have again caught the interest of astro-biologists, pointing to the possibility of a subsurface ocean where at least some form of microbial life may have evolved.

Artists impression of surface activity on Enceladus

Sceptics point to the relatively small size of Enceladus and its much lower age compared to Europa – one billion years as compared to three billion – as reasons for caution in considering it a possible home for life, but other scientists have pointed to the organic compounds within the subsurface water as a reason for optimism. Indeed

NASA scientists at a conference on Enceladus in May 2011 were quoted as saying

“(Enceladus) is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it.”

To boldly go?

Water is essential to carbon based life ‘as we know it, Jim’. The discovery of such vast quantities of free flowing water on planetary bodies elsewhere in our solar system together with independent sources of heat energy and organic compounds means that there is certainly the possibility of life having emerged separately on these worlds. Of course, we cannot find out whether that possibility has been realised without going there and taking a closer and more detailed look.

Either way the answer to the question would be a hugely instructive one not just for science but for the whole of the human race. If life has evolved separately twice in just one stellar system then the bookies would probably stop taking bets – it would be a racing certainty that life was ubiquitous throughout our universe wherever it gets sufficient time to take hold. Failure to discover life where the conditions are ripe for it on the other hand might indicate that its much more difficult than we previously thought for life to get going. Such a discovery would make the little green blue ball we call home all the more precious.

Manned missions are probably ruled out for the foreseeable future due to the crisis of capitalism – and in any case, high levels of radiation make a manned trip to Europa with current technologies a death sentence. Closer and more detailed studies by robotic missions are certainly a possibility however, and one was already in the pipeline, only to be effectively shelved, at least for the time being, due to NASA budget constraints.

The Europa Jupiter System Mission was still in the planning stages, but it is not inconceivable a new Europa probe or probes could be launched in the next decade or so.

I’ll (almost) finish by extensively quoting an interview carried in Astrobiology magazine with Brad Dalton who was in charge of drafting up the proposal for the EJS mission.

“Scientists have dreamed of sending a surface lander or even asubmarineto investigate Europa's ocean, but Dalton says the current vision for EJSM does not include any such instrument.

"We tried very hard, within our cost and mass constraints, to come up with a realistic lander concept," says Dalton. "The truth is, for the amount of mass and cost it adds, it's very difficult to include sufficient scientific capability to make it worthwhile. Part of the problem is in understanding the surface well enough to constrain the design. Once you get there, of course you are going to want to dig -- and that is just outside of the current fiscal reality."

model of subsurface ocean, Europa

Dalton says they considered sending a probe that would slam into the ice at high velocity. Such an impactor could provide a lot of information about the composition of the icy shell – just like the impactor did for the Deep Impact mission to a comet. The orbiter itself even could act as the impactor at the end of the mission. However, says Dalton, "this brings up some nightmarish planetary protection issues."

"After a lot of discussion and head-scratching, it became clear that we really need to look before we leap," says Dalton. "There is a lot we can do from orbit, (work) that needs to be done before we can send the kind of lander everybody seems to want."

Still, Dalton says that a lander has not been absolutely ruled out. "There are some minimal instrument concepts still on the table, but nothing like a Viking or a Phoenix (lander)," he says.

One such minimal instrument could be a seismometer, in order to get a sense of how much and how frequently the ice shifts on Europa. The seismometer also could include a mass spectrometer to determine what sort of chemistry takes place within the ice.

This mission could answer the question of whether there is life on Europa by analyzing the ice shell. The underlying ocean on Europa occasionally wells up out of cracks in the ice shell and washes over the surface, erasing features like impact craters. If life is carried in these waters, then their remains could now be frozen in the ice and an orbiter could detect them.”

Whatever form forthcoming missions take and whenever they happen, what they find out could be epoch making, so a thought.

If the type of missions we want to launch to Enceladus or Europa (or even Mars) are currently beyond the fiscal means of any one nation – even a superpower like the USA – then why not a joint international mission funded by all of the worlds major space agencies and drawing together all the talents of scientists across the globe?

Finally, finally. If this article has sparked your imagination, seek out the movie Europa Rising, featured in our final still below. It's not 100% scientifically accurate, but it is food for thought, and a minor SF classic that slipped under most peoples' radar.

A bit like the fact that the biggest oceans in the solar system aren't on Earth, actually.