Unfathomable Abodes of Life? –Water Worlds of the Milky Way

Alien Ocean


Before life appeared on land some 400 million years ago, all life on Earth including the mind evolved in the sea. Astronomers have recently conjectured that blue exoplanets with endless oceans may be orbiting many of the Milky Way’s one trillion stars. In 2016, for example, Kepler astronomers discovered planets that are unlike anything in our solar system –a “water world” planetary system orbiting the star Kepler-62. This five-planet system has two worlds in the habitable zone — their surfaces completely covered by an endless global ocean with no land or mountains in sight.

Three Criteria for Life

In essence there are three major things needed for life to thrive in ocean worlds– energy, nutrients and a source of organic matter that can be assembled into life-forms, geochemist Chris German at Woods Hole Oceanographic Institution detailed in an email to The Daily Galaxy:

When you dive into the sea, you are diving into the origin of us all –Peter Godfrey-Smith

“Organic matter seems to be abundant throughout the universe, based on spectroscopy so that brings us back to the first two ingredients (we’re assuming an ocean world already had water). 

“Carbon, Hydrogen Oxygen, Phosphorus and Sulfur. For life, the core astrobiology maxim is that one also needs these essential elements: C, H, N, O, P, S.  If you have a source of organic matter then you already solved having enough Carbon and if you have an ocean then you have H and O as well = halfway there. In deciding on which are the most promising ocean worlds for life, therefore, one key issue is whether there is also enough N (nitrogen), P phosphorous) and S (sulfur).  For N and S, the problem is well on the way to being solved for our solar system’s ocean worlds – there are multiple sightings/determinations of ammonia and related compounds in bodies of the outer solar system and as for sulfur (S) there is the case of Io which has loads of it in its volcanic exhalations, some of which has already been transferred to Europa.  

“There has been debate about whether any given ocean world has enough phosphorous to sustain life but the amounts needed (based on life as we know it here on Earth) are very small compared to what anybody has yet designed a spacecraft to look for.  At both Europa and Enceladus, for example, the past missions (Galileo, Cassini) didn’t have an instrument sensitive enough to detect at the low levels required to establish habitability.  Conversely, most rocks we know on Earth (including those that fit well to predictions for other ocean worlds) have more than enough P that even a modest amount of weathering should be able to release reasonable amounts into the oceans.

You could add one mile of water to Earth’s moon tomorrow and subseafloor fluid flow would start up even though it has been geologically “dead” for aeons.

“Energy —The biggest issue for whether an ocean world is habitable is whether there is any energy.  A ball of pure water wouldn’t have what you need.  But an ocean with a rocky interior has the potential to host water-rock interactions if any of the water can pass through the rock.  Assuming the whole planet hasn’t reacted to equilibrium then pumping water through the rock and back out again will lead to chemical reactions when those ground-waters flow back out into the base of the ocean. There will be chemical energy released when those reactions take place and that, in Earth’s deep oceans, is what underpins habitable “chemosynthetic” ecosystems that are independent of sunlight for photosynthesis.  Imagining the same processes on any other body with water and rocks underneath that water (i.e densest stuff in the middle, less dense stuff round the outside) has capacity to be habitable.  Note that the rocky interior doesn’t HAVE to be geologically active (volcanoes, tectonics) for this to be true. If one has tidal forcing (e.g. multiple moons round a common planet) then that can drive pumping of water in and out of pore-spaces across the seafloor.  Also, just pre-existing topography will generate a pressure differential from one place to another across that planet’s seafloor so that water will flow into the rocks in one location and out in another – so you could add one mile of water to Earth’s moon tomorrow and subseafloor fluid flow would start up even though it has been geologically ‘dead’ for eons.”

The rocky bottom of Europa’s vast ocean is like a miniature Earth

Within our own solar system, Jupiter’s moon. Europa, harbors a massive salty ocean beneath its icy surface that scientists believe reaches a depth of 100 kilometers –a depth 10 times greater than Earth’s Marianas Trench.

The rocky bottom of Europa’s vast ocean, suggests Caltech’s Mike Brown, may be almost like a miniature Earth, with plate tectonics, continents, deep trenches, and active spreading centers. “Think about mid-ocean ridges on Earth,’ Brown writes on his blog, “with their black smokers belching scalding nutrient-rich waters into a sea floor teeaming with life that is surviving on these chemicals. It doesn’t take much of an imagination to picture the same sort of rich chemical soup in Europa’s ocean leading to the evolution of some sort of life, living off of the internal energy generated inside of Europa’s core. If you’re looking for Europa’s whales – which many of my friends and I often joke that we are – this is the world you want to look for them on.”

The chances that water worlds are a common feature of the Milky Way was heightened by research published in Proceedings of the National Academy of Sciences (PNAS). Using computer simulations, Harvard University astronomer Li Zeng and his colleagues presented data showing that sub-Neptune-sized planets, that is, planets featuring radii about two to four times that of Earth, are likely to be water worlds, and not gas dwarfs surrounded by thick atmospheres as conventionally believed.

Utterly different worlds 

Some of these sub-Neptunian planets, Zeng said, have oceans deep enough to exert pressures equivalent to a million times our atmospheric surface pressure. Under those conditions, fluid water gets compressed into high-pressure phases of ice, such as Ice Seven or superionic ices. “These high-pressure ices are essentially like silicate-rocks within Earth’s deep mantle—they’re hot and hard,” he said. “These are utterly different worlds compared to our own Earth.”

In stark contrast, Earth has an obvious surface, with water compositions ranging between 25 to 50 percent of the planet’s total mass, these objects would be completely water-logged. They “may or may not have a well-defined surface,” said Li, and they “could be fluid all the way down—all the way down, to great depth.”

Could these ocean worlds support life? Perhaps even intelligent life? “There may be life there,” says Lisa Kaltenegger, Director of the Carl Sagan Institute at Cornell. “But could it be technology-based like ours? Life on these worlds would be under water with no easy access to metals, to electricity, or fire for metallurgy. Maybe life’s inventiveness to get to a technology stage will surprise us.”

Could alien waters worlds at some point evolve life as we know it on Earth? “Purely ocean worlds (without land on the surface),” Avi Loeb, former chairman of Harvard’s astronomy department told dailygalaxy.com, “are not likely to develop the diversity of life as we know it because they will be depleted of essential nutrients for life, such as phosphorus and molybdenum.”

“We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live,” explains astrophysicist Natalie Hinkel Senior Research Scientist at the Southwest Research Institute in San Antonio and a Co-Investigator for the Nexus for Exoplanet System Science (NExSS) research network at Arizona State University. “However, a planet that is a water world, or one that doesn’t have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.”

The chemistry of life is an aquatic chemistry. We can get by on land only by carrying a huge amount of salt water around with us. –Peter Godfrey-Smith

“I think it could be dangerous just thinking about everything in an Earth-mindset,” says Ramses Ramirez at the Tokyo Institute of Technology. “You might be missing out on other possibilities.”

The Alien Octopus Hypothesis

“I have a poster outside my office from a few years ago titled:  Exploring Ocean Worlds, Escaping Earthly Prejudice!”, Chris German at Woods Hole Oceanographic Institution wrote in an email to The Daily Galaxy. “But Ramses’ statement may already be revealing too much prejudice – he implies that we even know what lives in our own oceans and we demonstrably do not: less than 5% of Earth’s ocean floor has been explored sufficiently to even characterize what is living there. So even within one habitable planet we continue to surprise ourselves – and expand our known range of both habitable environments on Earth and the diversity of species that live there.  But what is exciting about thinking about the search for life on ocean worlds is that the robotic methods we will use – to operate at high pressures in cold, dark salt-water environments – are exactly the same that would be transformative for studying our own oceans, too.”

I have a poster outside my office from a few years ago titled:  Exploring Ocean Worlds, Escaping Earthly Prejudice!”

New research suggests that about 35% of all known exoplanets which are bigger than Earth should be water-rich. The recently-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb, will hopefully characterize their atmospheres with important implications for the search of life in the Milky Way.

Ocean’s Sentient Beings

The oceans of Earth itself became the conduit for evolution, says Peter Godfrey-Smith in Other Minds. Earth’s ocean-dwelling cephalopods – octopuses, squids and nautiluses, Godfrey-Smith writes – “are an island of mental complexity in the sea of invertebrate animals”, he writes, having developed on a different path from us, “an independent experiment in the evolution of large brains and complex behavior.”

Cephalopods are evolution’s only experiment in big brains outside of the vertebrates

“If we can make contact with cephalopods as sentient beings, it is not because of a shared history, not because of kinship, but because evolution built minds twice over,” says Godfrey-Smith. “This is probably the closest we will come to meeting an intelligent alien.”

The Last Word–Life on Water Worlds

“Whether life is possible on water worlds depends on the ability of essential chemicals to concentrate and initiate key reactions in the absence of plate tectonics,” Ramses Ramirez wrote in an email to The Daily Galaxy.  “Our idea,” he concluded, “was that rapidly-rotating planets with thick CO2 atmospheres could have effectively cycled CO2 through the interior and atmosphere and provide the necessary conditions for life to emerge.”

While the Earth’s oceans are teeming with life, it’s a much different situation when a planet is covered only in water.”

“While the Earth’s oceans are teeming with life,,” wrote Natalie Hinkle in an email to The Daily Galaxy, “it’s a much different situation when a planet is covered only in water. For one thing, it’s difficult to have active geologically-driven cycles on a planet — which are vital for moving around important minerals and materials necessary to keep a planet “alive” — when there is a massive, heavy layer of water on the planet’s surface. The Earth, for all intents and purposes, has a very thin coating of water on the surface. This means that life on an ocean world, should it develop, would have to survive with very limited resources. In addition, it’s likely that life would best exist on the ocean floor, near the minerals and nutrients, making any signatures of that life difficult to detect. I believe that the best hope for life on an ocean world is when the ocean is a small fraction of the overall planet, rather than a planet that is mostly made of water.”

Chris German told The Daily Galaxy: “In 30+ years of exploring Earth’s oceans, I have never failed to find new sites of seafloor fluid flow when searching systematically: even in what my peers may have considered the unlikeliest of places, based on what was previously known. All of them have been inhabited, frequently with food chains that are based on processes independent of sunlight.

“I continue to be humbled by the diversity of novel habitats that Earth’s oceans reveal, if we just explore them. Why should other Ocean Worlds be any less remarkable?”

Avi Shporer, Research Scientist, formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory, with MIT’s Kavli Institute for Astrophysics and Space Research via Natalie Hinkel, Ramses Ramirez, Chris German/Woods Hole Oceanographic Institution, New York Times, Caltech, Goldschmidt Conference, The Atlantic, and Scientific American

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