Distant Oceans

Nick Nielsen
Oct 14, 2017 · 22 min read

Introduction to Distant Oceans

Carl Sagan once spoke of the “cosmic ocean” in metaphorical terms, as an originary human home to which we long to return:

“The surface of the Earth is the shore of the cosmic ocean. From it we have learned most of what we know. Recently, we have waded a little out to sea, enough to dampen our toes or, at most, wet our ankles. The water seems inviting. The ocean calls. Some part of our being knows this is from where we came. We long to return. These aspirations are not, I think, irreverent, although they may trouble whatever gods may be.”

If we place Sagan’s reflection on the cosmic ocean in the context of JFK’s “sea speech,” we get a sense of the instinct to return to the cosmic ocean to which Sagan alluded:

“I really don’t know why it is that all of us are so committed to the sea, except I think it is because in addition to the fact that the sea changes and the light changes, and ships change, it is because we all came from the sea. And it is an interesting biological fact that all of us have, in our veins the exact same percentage of salt in our blood that exists in the ocean, and, therefore, we have salt in our blood, in our sweat, in our tears. We are tied to the ocean. And when we go back to the sea, whether it is to sail or to watch it we are going back from whence we came.”

But the cosmos is not only a metaphorical ocean to which we long to return; we know now that the universe has literal oceans waiting for us to explore them, and even within our own solar system.

Subsurface Ocean Worlds

It is only in recent years that the prevalence of subsurface ocean worlds has come to our attention, but we may need to eventually recognize a class of biospheres that consists of subsurface ocean worlds in isolation from a “terrestrial” biosphere. (I place “terrestrial” in scare quotes because this is a geocentric and anthropocentric concept specific to Earth, but its use here ought to be unproblematic as I assume that the reader can make the extrapolation from the particular case of Earth’s landmass biospheres to any landmass biosphere on any planet in contradistinction to any oceanic biosphere on any planet.)

Part of what makes these discoveries so interesting is that we are finding potentially “habitable zones” for life as we know it (in contradistinction to Weird Life, i.e., life as we do not know it) far beyond the habitable zone in which something like the terrestrial biosphere can occur (i.e., the circumstellar habitable zone, or CHZ). Thus these discoveries are re-shaping our conception of the habitability of a planetary system — not only our own planetary system, but the habitability of any planetary system whatever.

Recently there has been a lot of attention directed at Saturn’s moon Enceladus, which we now know to have a subsurface ocean, like several of the Jovian moons. (Cf. Does Enceladus support life? 7 key facts and Saturn’s Geyser Moon Shines in Close Flyby Views) NASA’s Cassini spacecraft has flown past Enceladus (on 28 October 2015), close enough to pass through the plumes of gas and ice that are erupting from Enceladus’ subsurface liquid water ocean, sampling the plumes with the spacecraft’s gas analyzer and dust detector instruments. As this information is analyzed over the coming months and years, it will provide new insight into the interior structure of Enceladus and its subsurface ocean.

We also know that Ganymede, a Jovian moon that is the largest moon in the solar system, has a subsurface ocean (cf. NASA detects subsurface ocean on solar system’s largest moon). It has been known for some time that Europa, another Jovian moon, has a subsurface ocean as well. I previously wrote about Europa’s subsurface ocean in Europa’s Cryosphere and Hydrosphere.

When we someday possess the technological wherewithal to explore within the many subsurface oceans within our solar system, in addition to the possibility of finding other life, I think that we will discover quite complex internal structures, and that these internal structures will be different in the different subsurface oceans, accordingly whether they are heated by the tidal forces of the gas giants they orbit, by an internal molten core, by radioactive decay, or by some combination of these sources of energy.

These internal structures, will, in turn, be implicated in any life that is found, since we know that on Earth the organic chemistry of microbial life has a close relationship with the inorganic chemistry of the lithosphere, hydrosphere, and cryosphere. With or without life, these subsurface oceans are likely to be geologically complex environments with all manner of internal structure unprecedented in our knowledge of terrestrial ocean environments.

Life in Subsurface Oceans

In another post I discussed an origins of life experiment that has produced organic molecules in conditions replicating the extreme cold of extraterrestrial space, noting earlier origins of life experiments that have shown the importance of cold as well as of heat in the formation of organic molecules. These subsurface oceans of Europa, Ganymede, Enceladus, and other moons of our solar system (and perhaps even on rogue planets wandering in interstellar space, perhaps ejected from their planetary system by a gravitational interaction with a passing star) will have strong chemical interactions between their heated core (by whatever means heated) and their extremely cold icy shells. It is in such an environment of complex geological and chemical interactions that one would expect to find the conditions that would independently give rise to the emergence of life.

As a thought experiment we can imagine an alternative history of Earth in which the iced over “snowball Earth” never thawed out, but retained its oceanic biosphere under a layer of ice many kilometers thick. The kind of oceanic biosphere that would have emerged in this context, cut off from any expansion onto land, would look very different from the biosphere that we have on Earth today. One can think of glaciation as the retreat of the biosphere before the expansion of the cryosphere, and the opposite climatological development of warming as the retreat of the cryosphere before the expansion of the biosphere. The two are locked in a relationship that develops over geological time.

Another scenario for a thought experiment, taken from another theory of terrestrial glaciation, might involve a nearly entirely glaciated Earth, but with a small “slushy” zone near the equator (a “slush-ball Earth”), where the sun shone on a small patch of open water, with floating icebergs. Here life might have had an opportunity to find its way out onto the ice-covered surface, or even into the skies of our planet. Again, this would have resulted in a very different counterfactual biosphere.

For either of these counterfactuals to have profoundly have impacted the development of life we needn’t even suppose that the Earth be permanently locked in an “icehouse” climate, as indeed would be unlikely given the geologically active lithosphere of Earth. But if a severe “icehouse” event endured for a sufficiently long period of time, or if it occurred repeatedly, or it it occurred later in the development of life (which nearly was the case during the recent glacial maxima of the Quaternary), any of these scenarios could have pushed life on Earth in a different direction than that which it did, in fact, take. One suspects that, given what we now know about the abundance of water in the universe, and the number of planets within the habitable zones of stars, that these scenarios have occurred millions and millions of times over, each time with a slightly different result.

A Thought Experiment in Subsurface Ocean Worlds

The astrobiological interest in subsurface ocean worlds is their potential for hosting life as we know it, i.e., life consisting of macromolecules closely related to terrestrial DNA, even if distinct in detail. In a previous post I explained what it would be like for life to be “life as we know it” but different in detail:

“…if life in the outer solar system is to be found, and it is significantly different from life of the inner solar system, how do we recognize it as life? How different is different? It is easy to imagine life that is different in detail from terrestrial life, but, for all intents and purposes, the same thing. What do I mean by this? Think of terrestrial DNA and its base paring of adenine with thymine, and cytosine with guanine: the related but distinct RNA molecule uses uracil instead of thymine for a slightly different biochemistry. Could something like DNA form with G-U-A-C instead of G-T-A-C? Well, if we can consider RNA as being ‘something like’ DNA, then the answer is yes, but beyond that I know too little of biochemistry to elaborate. As several theories of the origins of life on Earth posit the appearance of RNA before DNA, the question becomes whether the ‘RNA world’ of early life on Earth might have also been the origin of life elsewhere, and whether that RNA world matured into something other than the DNA world of terrestrial life.”

We can construct a great many interesting scenarios around life in such subsurface ocean worlds, whether strangely similarly to terrestrial life or spectacularly alien from anything we know — a continuum of life either from the same root, and having since diverged, or a “second genesis” (or multiple geneses, G1, G2, …, Gn,…) at any degree of distance from terrestrial life. There is another possibility, however, and I would like to consider this possibility as a thought experiment.

The Setting of the Thought Experiment

Suppose we investigate the subsurface oceans of Europa, Enceladus, Ganymede, and any others that we might find in our solar system. In this thought experiment, we posit that the subsurface oceans of Europa, Enceladus, Ganymede, Titan, and others are found to have no life in them whatsoever, whether related to terrestrial life, or unrelated to terrestrial life; that, while these environments have interesting geological and chemical features, all of these environments are entirely sterile. We posit that no second genesis occurred in our planetary system, and no dispersal vector (such as lithopanspermia) conveyed terrestrial life to these other worlds (or vice versa). These oceans may have organic chemicals them, but derived exclusively from non-biological natural processes.

Furthermore, suppose in addition that, while these environments are demonstrably sterile, they are compatible with life as we know it. In other words, if terrestrial life were “seeded” into these potential biospheres that are nevertheless sterile, this terrestrial life could take hold, grow, and reproduce on these alien worlds. More narrowly: suppose there is at least one terrestrial species that could flourish in the subsurface oceans of the moons of the gas giants in our planetary system. It would be likely that, if any terrestrial organism could survive in an extraterrestrial ocean, it would be some extremophile, though we should also acknowledge the possibility that a wide variety of terrestrial organisms could flourish on other worlds, and that indeed the interaction of these organisms would be a factor in their survival and reproduction, as is the case on Earth.

First Thought Experiment

Suppose that human beings introduce terrestrial life into sterile extraterrestrial environments. Specifically, suppose that we introduce those terrestrial microbes compatible with the subsurface ocean environments on Europa, Enceladus, Ganymede, and elsewhere. Further suppose that these terrestrial organisms flourish in their new environment, rapidly multiplying. (Call this thought experiment 1A, or TE1A.) In so doing, in so “seeding” other worlds with terrestrial life, we have expanded the range of terrestrial life beyond Earth, and become an agent, an extraterrestrial dispersal vector, for terrestrial life in the wider universe. We could call this “experimental astrobiology.”

This terrestrial life acclimating to a distinct world, however, will be subject to different selection pressures than life on Earth, and will likely speciate rapidly in this new environment. In so doing — in proliferating and speciating — terrestrial life on some other world will begin to create a biosphere for itself, changing both life and world in the process. Terrestrial life will not remain terrestrial in extraterrestrial environments, although it will be traceable to the same terrestrial root, and that essential DNA sequence that Sean Carroll called “immortal” and which has, “survived through an immense arc of time, and life will continue to depend upon this core set of genes as it evolves in the future” (cf. Last Universal Common Ancestor), will continue to furnish the core macromolecules of this terrestrial life on other worlds, even as this life adaptively radiates in its new environment and take on very different forms, altering the genetic “periphery” but not the “core” genes essential to, and perhaps definitive of, life.

Even with terrestrial life introduced, these environments will never become terrestrial, and the longer that these environments are left to develop on their own, the more they will diverge from terrestrial life, and the more environments will diverge from terrestrial habitats. We will continue to recognize this as “terrestrial” life only because we know ourselves to have been the vector that introduced life into the novel context and have watched the entire development of this life as it has taken on novel forms not represented on Earth. We might call this process of introducing terrestrial life into compatible but sterile extraterrestrial niches bioforming or bioengineering, (or, since “bioengineering” is already in use, “extraterrestrial bioengineering” or EBE, or even ETBE, or even planetary-scale bioengineering). Does this planetary scale bioengineering have moral consequences? Does the meaning and value of life change when life takes on new forms and expands into new niches as a result of human agency?

Do we have the right to begin a process that will transform life and other worlds in ways that we cannot predict? Does the question of right even enter into the possibility of bioengineering sterile worlds? There has been much discussion in the science press recently about protecting other worlds from terrestrial microbes, which apparently is a recognition of the intrinsic value of life on other worlds. (NASA even has an “Office of Planetary Protection.”) Certainly life on other worlds, including other worlds in our planetary system, would be of incalculable value to science, but is this life possessed of moral value? Recall that, in this thought experiment, we are explicitly excluding the possibility of life on other worlds, and have stipulated that the worlds into which we have seeded terrestrial life are sterile ab initio. In the parameters of this thought experiment, the possibility of destroying or displacing the life of other worlds does not arise — until later.

Suppose that we had transferred terrestrial life into subsurface oceans in our planetary system, and that this life proliferated and created a biosphere in an ocean that was previously sterile. (TE1A) Suppose furthermore that this experiment was judged to be a failure, or to have resulted in an undesirable outcome. (TE1B) A decision is made to terminate the experiment and to re-sterilize the oceanic biosphere created by human intervention. Is there here a question of our moral right to end an experiment in life by terminating the life that we propagated? Is there any moral objection to sterilizing a biosphere of which we are the authors, or, at least, the origin and the triggering event? How would this be different from sterilizing a petri dish? Would we feel differently on this question depending upon the size the biosphere? Would we feel differently on this question depending on the length of life of the biosphere? Might we feel justified in sterilizing this biosphere in a period of, say, less than 100 years, but if the experiment were allowed to run for several hundred years, we might feel less justified in bringing an end to this biosphere?

Suppose the nascent biosphere endured for less than a month. (TE1C) Would extirpation sting our collective conscience less that if the biosphere had endured for a decade, or a century? Suppose the biosphere that took root on an alien world turned out to be highly robust, and it took years of intensive effort to end this experiment, so that the effort began to feel like an extermination. (TE1D) Would this change our perception of our re-sterilization of a human-induced biosphere? Suppose that, in order to terminate the experiment of an alien biosphere of terrestrial life we employed weapons of mass destruction — say, we built a series of neutron bombs equally spaced in orbit around the moon, so that the entire moon would be bathed in deadly radiation when the bombs were simultaneously detonated? Would this be a moral use of WMD?

Second Thought Experiment

Suppose, as before, that the subsurface ocean environments that we find on other worlds in our solar system are sterile, but instead of the above scenario suppose that they are not compatible with terrestrial life, even with terrestrial extremophiles. However, suppose that with a little creative genetic tampering, we can produce a variation on terrestrial life that can survive in subsurface ocean environments. (TE2A) Suppose that these genetically altered organisms are introduced into subsurface ocean worlds, where they flourish.

These life forms will be, ab initio, distinct from terrestrial life, although derived from terrestrial life. May we consider life that is derived from terrestrial life but different from terrestrial life to be an extension of the terrestrial biosphere? And when this artificial life undergoes adaptive radiation in its environment, speciating and further diverging from its distant terrestrial ancestor, at what point does it cease to be life as we know it, and recognizably traceable to life on Earth? There is an obvious sorites paradox here, with life, originally made distinct by human effort, gradually becoming more distinct until it no longer can be accounted an extension of terrestrial life.

Something like this may have already occurred within our planetary system. If life got its start in our planetary system very early, perhaps even before Earth had formed, or at least fully formed, it might have been distributed around our solar system and subsequently diverged so that, when other life in our planetary system is discovered again, extraterrestrial life seems to me life as we do not know it, but only as a result of billions of years of evolutionary divergence under radically distinct selection pressures. A scenario like this would involve subsurface oceans inhabited by microbes, and so is distinct from our thought experiment, but the conditions of our thought experiment can shed some light on this other possibility as well.

The same scenarios considered in the first thought experiments can also be reformulated for the changed conditions of this second thought experiment, so that (TE2B) the experiment in introduced artificially altered life into a subsurface ocean was judged to be a failure, and some action is taken to undo what has been done, and that (TE2C) this experiment may have lasted a very short time, or a much longer time, so that the expiration of the artificially altered life artificially induced into a subsurface ocean is difficult to undo and our effort to return the subsurface ocean to its pristine and sterile condition puts us in the role of planetary-scale exterminators (TE2D).

In the case when the experiment is judged to be gone awry, and we seek to return a subsurface ocean world to its pristine and sterile state, do we have any less moral claim to do this because it is qualitatively biologically different from us, from terrestrial life? Should there be any moral claim at all on biological consanguinity? Of course in this context we must interpret consanguinity in terms of kinship relations of increasingly broad scope: our relationship to other organisms on Earth (i.e., our slight difference from them) is usually taken as a license to ignore all moral constraints in our dealings with other species, so does our greater (biological and genetic) distance from artificially altered microbes on another world give us even greater license, or even greater responsibility? Can we recognize our kinship with these organisms, and if we can recognize our kinship with such different organisms, can this or ought this to influence our conception of our closer kinship with other terrestrial species?

There are no definitive answers to these questions. The moral experience of human beings to date does not prepare us to answer these questions, and the extant systems of ethics do not yet address questions such as this. We must approach these questions, if we approach them at all, as research questions in moral philosophy, and as an opportunity to interrogate, to cultivate, and to expand our moral intuitions beyond default responses due to the evolutionary psychology native to beings of planetary endemism; these are questions that transcend planetary endemism in a radical way.

Some Reflections on the Thought Experiments

It has long been a staple of science fiction that, when human beings eventually reach other worlds, we may have to modify ourselves in order for our descendants to live elsewhere in the universe. This idea is flawed in several ways — we don’t have to live on the surface of a planet when we can fashion ourselves perfectly comfortable artificial habitats — but it seems to return time and again in discussions of any human diaspora in the cosmos. The above thought experiments show how we may face much the same moral issues posed by this idea much sooner, and within our own planetary system. However, we may experience this moral problems at one remove, not directly affecting our own species, but directly affecting environments in which we cannot live, never can live, and directly affecting other terrestrial species.

Experiencing a moral dilemma at one remove is itself interesting, and could be the setting for several thought experiments in turn. Being at one remove from the dilemma gives us a certain distance, and, if we like, we can codify this distance by placing further controlled conditions on the thought experiments, such as a proviso that no human being will ever be either helped or harmed by our experiments in anthropogenic biospheres (or, if you like, anthropogenically induced biospheres, or AIB). Other conditions might also be held to obtain to a similar end.

In both of these scenarios, human beings become agents of panspermia, so that even if panspermia has not occurred in the past, it may yet occur in the future. And it can further be pointed out that any panspermia presided over in the future by human agents will be a form of directed panspermia, i.e., intelligently directly panspermia, especially in the case of the second thought experiment above, in which the resources of science and technology must be brought to bear in order to acclimate terrestrial life to a non-terrestrial environment.

This is a point that I previously sought to make in The Moral Imperative of Human Spaceflight, in which I wrote, “After extraterrestrialization, human dispersal vectors will add to the range of existing life into what is otherwise the sterile absence of any ecosystem while supporting the human presence that makes this dispersal possible.” (I addressed this point at greater length in Extraterrestrial Dispersal Vectors.) I also wrote in the same paper, “It is the role of conscious agents in the natural history of value to be exapted by nature. We are a dispersal vector not only for life, but also for consciousness, civilization, and value.” If this is a reasonable extrapolation of the human role as a dispersal vector for terrestrial life, thought experiments could be constructed, parallel to those formulated above, that address consciousness, meaning, civilization, and value.

The Thawing of Distant Oceans

With these thought experiments we are not yet at an end of our discussion of distant oceans — not only distant oceans on distant worlds orbiting distant stars (the universe is probably full of them), but distant oceans within our own solar system. At the present time, the distant oceans of our solar system are subsurface oceans, but this will not always be the case. In the fullness of time, when the sun expands into a red giant star at the end of its life, the habitable zone will expand outward (even if briefly on the scale of cosmological time), and these icy worlds will thaw out and their subsurface oceans may become actual oceans of open water.

The idea of the habitability of planets of a red giant has been around for a few years. Paul Gilster wrote about the habitable zones of red giants in Habitability Around Red Giants, reviewing the paper “Habitability of Super-Earth Planets around Other Suns: Models including Red Giant Branch Evolution” by W. von Bloh, M. Cuntz, K.-P. Schroeder, C. Bounama, and S. Franck. Interestingly, this paper makes the point that, “Habitability was found most likely for water worlds, i.e., planets with a relatively small continental area.” This strikes me as particularly interesting in terms of our own solar system, since many of the moons of Jupiter are believed to have substantial subsurface oceans beneath many kilometers of ice. Indeed, there is probably more water on Europa, Ganymede, and Enceladus (a moon of Saturn, rather than Jupiter), than on Earth.

As the sun grows in size and eventually sterilizes Earth, its habitable zone will move outward, first making Mars a pleasant environment, perhaps for hundreds of million years, and then eventually making Mars too hot. Mars will pass through its optimal habitable conditions, and would be tropically hot if still habitable. Probably Mars could be kept habitable by some Dyson dots screening the sun and cutting insolation, or by other geoengineering technologies no longer effective on Earth.

Even when the sun’s habitable zone is pushed beyond the inner rocky planets, there will be the water-rich moons of Jupiter and Saturn. Here the kilometers of ice will ultimately melt. Much of this water will be lost to space, as these moons have little gravity to retain an atmosphere, but there is a lot of water on these moons, and it may be possible that the largest moons will form an atmosphere from the water vapor and outgassing from volatiles from beneath the surface of these moons. (Titan has managed to retain an atmosphere, and it is smaller than Ganymede, which latter is larger even than Earth’s moon.)

There is a well-known quote from Roger Revelle (well-known, at least, to climate scientists, and I myself previously cited this quote in Scaling up Origins of Life Research) concerning the non-reproducibility of planetary-scale phenomena, such as the quantity of carbon dioxide that human beings have been releasing into the atmosphere since the industrial revolution:

“Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future.”

While we cannot reproduce planets and vary their conditions at will in order to experiment with entire biospheres, nature does this for us. At least, we may know nothing of other biospheres at present, but nature has been prodigal in its production of stars and planets, as I implied in Cosmology is the Principle of Plenitude teaching by Example.

We are only now starting to learn about the structure of other planetary systems. (in On the Likely Existence of “Random” Planetary Systems I noted that the present stage of planetary science has been called the Golden Age of Exoplanet Discovery.) The next stage in this development, once we possess the technology, will be to learn about exoplanet biospheres, if there are any.

We can look outward in order to find examples of possibilities not exemplified in our own solar systems, but we can also look forward and backward in time within our own solar system in order to find different conditions than those that obtain at present. The future development of our solar system will, over time, exemplify many variables that will force changes to the bodies of our planetary system, including the size and temperature of our sun, which will in turn change the location of the sun’s habitable zone.

When, near the end of its life, the sun swells into a red giant star, its habitable zone will be displaced from its current distance, bathing the small rocky planets of the inner solar system in clement temperatures, instead favoring the outer gas giant planets with clement temperatures. This has been written about in Friendly giants have cozy habitable zones too by Matt Williams (19 May 2016), which was a popular exposition of Ramses M. Ramirez, et al. Habitable Zones Of Post-Main Sequence Stars, The Astrophysical Journal (2016). The abstract of the paper includes the following observation:

“ A planet can stay between 200 million years up to 9 Gyr in the post-MS HZ for our hottest and coldest grid stars, respectively, assuming solar metallicity. These numbers increase for increased stellar metallicity.”

In the above, “MS” means main sequence and “HZ” means habitable zone. Metallicity refers to the elements in the stars other than hydrogen and helium, so that the later the generation of star concerned, and thus the higher the metallicity, the longer such a star would have planets in its red giant habitable zone. The longer an outer planet (or moon) remains in the habitable zone of a red giant, the more likely that life could find (or make) itself a home under these conditions off the main sequence.

Will we someday sail the Jovian oceans long after Earth has become uninhabitable? Will we water-ski on Europa? Will we ride wet-bikes on Ganymede? Para sail on Enceladus? If human civilization is still viable, and still in the vicinity of our home planetary system billions of years hence when these moons thaw out, we could guide their development to habitability — perhaps with water sports in mind.


Addendum added Wednesday 08 November 2017: A new paper on the subsurface ocean of Enceladus, Powering prolonged hydrothermal activity inside Enceladus, by Gaël Choblet, Gabriel Tobie, Christophe Sotin, Marie Běhounková, Ondřej Čadek, Frank Postberg, and Ondřej Souček, notes that, “The release of heat in narrow regions favours intense interaction between water and rock, and the transport of hydrothermal products from the core to the plume sources.” In other words, chemical complexity possibly conducive to the evolution of life may obtain, and be present in the observable plumes. This was further elaborated in The internal ocean of Saturn’s moon Enceladus could be old enough to have evolved life, finds study, by Monica Grady.


My astrobiology thought experiments have included:

These astrobiological thought experiments prompted further related posts:

Not directly related, but relevant, are:

I have also written several related posts on astrobiology:


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