Personal Science & Physics

Water worlds are oceans apart

In many ways, I’m glad to live in this time. Being an avid reader of popular science and accessible physics, I thrill over what is and what might be. But equally, I sometimes feel like I was born too early. It’s my grandchildren that will be the fortunate ones. They’re the ones that will walk upon foreign soils, or even swim in waters on alien worlds, far from our own Earth in some distant corner of our galaxy.

I mention swimming specifically, because that might be a very likely outcome of our continuing exploration of space and the planets that we will one day visit and most likely inhabit:

“Imagine a world with no land at all, merely the impenetrable depths of a seething ocean. Models of planet formation predict the existence of such worlds, even though our own solar system has none. Indeed, their formation should actually be rather common – and new satellites may soon detect them around other stars.”

It’s an amazing idea, that of all of the notions we have of what alien planets might be like, we very rarely consider the possibility of there being worlds entirely oceanic.

“Planets that form far from their parent star are expected to have a composition similar to comets (50% rock, 50% water by weight). Once a planet exceeds about ten Earth masses it has enough gravity to attract any hydrogen and helium near its orbit, and will rapidly transform into a gas giant. But what happens to planets in this region that never exceed the threshold?

It becomes an ‘ocean planet’, a term coined by Alain Léger (Université Paris-Sud, France) when he first proposed the existence of such worlds in 2003. An ocean planet that stays in the outer disk will probably be captured by the gas giants forming there, perhaps to become a moon like Europa. Such worlds will be composed mostly of rock and ice, and depending on their environment and formation history may harbor liquid oceans below their surface.”

I am fascinated with Europa and the possibility that beneath that thin veneer of ice lays a water world, one of shrouded, dark secrets. One possibility populated with deep sea hydrothermal vents, those in turn playing host to a multitude of organisms existing entirely not on photosynthetic, but of chemosynthetic energy.

What’s even more astonishing are the results of simulations of these worlds and what they may be like:

“The water content of the Earth is only about one part in 4400, yet water covers over two-thirds of its surface. Not only would water cover the entire surface of an ocean planet, but its average depth would on the order of 100 kilometers!

All the rocky matter would sink to the center of the planet, forming a dense core not unlike the Earth’s. Where the Earth has a thick mantle of magma, however, ocean planets would have a mantle of exotic ice. The pressure at the bottom of the ocean would be 10 million atmospheres or more; under such a crushing weight, water has no choice but to solidify.”

Exotic ice? What does that mean and what would exotic ice look like? I just want to know more.

Just from recollection (and in the absence of a good Google) I understand that there are 9 basic types of ice, all forming at different temperature gradients, only a few of which form naturally here on Earth.

And as for oceans hundreds of kilometers deep .. the mind is just ripped from one imagining to another. What, if anything, could live at even a fraction of those depths? And what might they look like?

Life here on Earth is hugely diverse, and I doubt that any life we might find elsewhere would be more strange or more unique than life here on Earth.

After all, there are only a finite number of ways of doing the same thing. So locomotion is limited to motion across land, through the air as well as above and below water. Many of these methods have been explored throughout the billions of years.

We know that here on Earth, some strange creatures can persist and live at the astonishing depths we know of, such as the various deep sea trenches like the Tonga and the Aleutian trenches.

But what might life be like on worlds where oceans are faceless and unremarkable for hundreds of kilometers both deep and wide, with no land to break the monotony?

As we move inexorably through the years, what we call science fiction often becomes science fact. And the prospect of such water worlds perfectly whets the appetites of not just this generation, but for those yet to come…

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By Wayne Smallman

Wayne is the man behind the Blah, Blah! Technology website, and the creator of the Under Cloud, a digital research assistant for journalists and academics.

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