This is the second of a two-part dive into the story of oceans on Earth and elsewhere, following my review of Ocean Worlds. That book gave a deep history of how our oceans shaped Earth and life on it and briefly dipped its toes into the topic of oceans beyond Earth. Alien Oceans is the logical follow-up. How did we figure out that there are oceans elsewhere? And would such worlds be hospitable to life? Those are the two big questions at the heart of this book. If there is one person fit to answer them, it is Kevin Peter Hand, a scientist at NASA’s Jet Propulsion Laboratory and their deputy chief for solar system exploration.
Alien Oceans: The Search for Life in the Depths of Space, written by Kevin Peter Hand, published by Princeton University Press in March 2020 (hardback, 248 pages)
A major question in astrobiology is whether the evolution of life on Earth is a fluke, or whether life is bound to pop up wherever conditions are favourable. Hand very neatly frames this in the bigger history of science. Over the centuries, we figured out that the laws of physics, chemistry, and geology work beyond Earth. But “when it comes to biology, we have yet to make that leap. Does biology work beyond Earth?” (p. 15). What we have learned is that life as we know it needs water. And though there is no shortage of theories on the origins of life, oceans are very likely where it started, and thus a logical first place to start looking for answers.
If you have any interest in astrobiology, you will probably have heard of the concept of a habitable zone or Goldilocks zone where, based on the distance to a star, conditions for life are just right. Not so close as to be too hot, nor so far as to be too cold. Earth obviously falls in that zone. Next to many minor insights, Alien Oceans had three major eye-openers for me. This was the first one:
There are other Goldilocks zones.
Depending on the details of their orbit, moons can experience such strong tidal tugs from their parent planet that the constant squeezing and stretching of the rock creates enough heat through internal friction to sustain liquid water. The physics of water helps, as it has a seemingly mundane but rather unusual property. Ice floats. When water solidifies, its density decreases slightly. What this means for moons is that the liquid water exposed to the cold of deep space freezes and forms a protective icy shell. Most liquids do not have this useful property. When they freeze, they sink to the bottom exposing more liquid until all of it is frozen solid. To top it off, ice is also a good thermal insulator, helping such ocean worlds retain heat. Maybe I have been hiding under a rock, but this was revelatory for me. Suddenly, the amount of cosmic real estate suitable for life has increased quite dramatically. And we have some of it right here on our doorstep.
“Over the centuries, we figured out that the laws of physics, chemistry, and geology work beyond Earth. But “when it comes to biology, we have yet to make that leap. Does biology work beyond Earth?“”
The existence of oceans in our solar system and how we gathered the evidence is one of the two major threads running through this book. Hand examines this in detail for Jupiter’s moon Europa, which has been studied in the most detail. Three types of data are typically gathered: spectroscopic, gravimetric, and magnetometric. This is where Hand gets fairly technical, though, fortunately, he extensively uses comparisons with everyday concepts and technologies to help you understand the underlying (astro)physics. Without retreading his careful explanations, in Europa’s case, these different strands of data all converge on a moon with an icy shell and a substantial subsurface ocean some 80–170 km thick as the best explanation. Mixed in with this narrative are the details and many technical setbacks of the Galileo mission that are nail-bitingly tense in places.
Similar missions and measurements have been done for Saturn’s moons Enceladus and Titan, Jupiter’s moons Ganymede and Callisto, Neptune’s moon Triton, and Pluto. The evidence for oceans gathered so far gets less robust in this order, but there are some notable variations on the theme. Enceladus ejects spectacular plumes of water and carbon compounds that were photographed and sampled by the Cassini–Huygens mission. Ganymede, meanwhile, is so large that the bottom of its ocean might consist of an exotic form of dense ice, ice III, formed at very high pressures not seen on Earth, meaning its ocean is sandwiched between two layers of ice.
So you have found exo-oceans. Now what? Can we expect to find life here? That is the second major thread. Hand identifies five conditions for life to emerge: a solvent such as water, chemical building blocks, an energy source, catalytic surfaces, and time. Interestingly, there is a gap between two schools of thought. The top-down explanation deconstructs life backwards in time until we arrive at an RNA world, but how did that get started? The bottom-up explanation has shown that life’s basic building blocks such as amino acids exist in space, but how do we go from there to larger functional molecules?
“There are other Goldilocks zones. Moons can experience such strong tidal tugs that the heat released by internal friction can sustain liquid water.”
This was the second major eye-opener for me: “Our environment is full of chemical disequilibrium […] there are reactions just waiting to happen. […] The metabolisms that drive life accelerate reactions in the environment, releasing energy faster than would have occurred without life” (p. 144). Hand takes a leaf out of Nick Lane’s book The Vital Question (which, shame on me, I still have not read) when he enthusiastically concludes that “the why of life is metabolism” (p. 146), offering the universe a pathway to increase entropy faster. These are remarkable ideas that give a whole new meaning to philosophical questions on the meaning of life.
The third and final eye-opener concerns the need for a catalytic surface, which is where Hand circles back to oceanographic exploration here on Earth, a recurrent theme in this book. When the submarine Alvin discovered hydrothermal vents in 1977 and found them teeming with life, these quickly became a popular alternative explanation to warm tidal pools as a place where life could have started. These so-called black smokers are powered by magma rising to the surface at mid-ocean ridges, jetting out superheated water of over 400 °C. Though volcanism and tectonics are, or sometimes were, common processes on many solar system bodies, there is another option. Alkaline vents, first discovered in 2000 at the Lost City hydrothermal field, are powered by exothermic (energy-releasing) reactions between water and mineral-rich rock, heating water to a more gentle 70–100 °C. All these need are the right rocks with cracks in them so water can percolate down.
“In addition to hydrothermal vents, alkaline vents are another place where life might have started. All these need are the right type of rock with cracks in them so water can percolate down, no plate tectonics needed.”
Hand raises many other interesting questions towards the end of the book, of which I will mention just three. One, life’s metabolic reactions require so-called oxidants, oxygen being “the most glorious of oxidant” (p. 162), but how would these get down into subsurface oceans? Two, how contingent or convergent is the evolution of life’s biochemistry? Carbon is a suitable building material for life as it is “hands down the best team player on the periodic table” (p. 212). But does physics restrict us to these options, or can we sketch a periodic table of life with other, weirder possibilities? And three, how should we seek for signs of life? What makes a good biosignature? This is discussed far more in-depth in Life in the Cosmos, but Hand considers three types of evidence.
Alien Oceans limits itself to oceans in our solar system, not touching on the topic of exoplanetary oceans. Given this is not Hand’s expertise, that is reasonable. He also glosses over the question of what aliens might look like, though he speculates on the likelihood of intelligent life in ice-covered subsurface oceans. Even without these topics, Alien Oceans is information-dense, requiring me to make a summary, and then a summary of that summary while preparing this review. Nevertheless, it is an intellectually very rewarding book and the many analogies make it accessible. I enjoyed it as a follow-up to Ocean Worlds but it is a fine standalone book. Terribly fascinating, Alien Oceans makes a convincing case for exploring the moons in our solar system in the search for extraterrestrial life.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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Life most likely originated in the oceans, and it is to oceans that astronomers are looking to find life elsewhere in the universe. With the publication last year of Kevin Peter Hand’s Alien Oceans, I decided this was the right time to finally review Ocean Worlds, a book that I have been very keen to read ever since buying it some years ago. This, then, is the first of a two-part dive into the story of oceans on Earth and elsewhere.
Ocean Worlds: The Story of Seas on Earth and other Planets, written by Jan Zalasiewicz and Mark Williams, published by Oxford Press in December 2017 (paperback, 302 pages)
Palaeobiologists Jan Zalasiewicz and Mark Williams have previously collaborated on The Goldilocks Planet. Here, they provide a deep history of our oceans. As soon as I tucked in, it became clear that they go deeper than Eelco Rohling did in the previously reviewed The Oceans: A Deep History, a book that focused heavily on palaeoclimatology. Even though most of the action in Ocean Worlds takes place on Earth, and the wider universe is only considered in the opening and closing two chapters, the book is characterised by an almost cosmic perspective on the subject. The writing of Zalasiewicz and Williams is such that I felt as if was surveying major developments in the history of our universe from an elevated, slightly detached, almost omniscient position. The result is thrilling and at times awe-inspiring. What follows are some of the big questions and outrageously fascinating topics they consider.
To have an ocean we first need water. Hydrogen was an immediate byproduct of the Big Bang. Oxygen, however, did not appear until after the universe had gone through its first cycle of stars being born and dying, as its creation required nuclear fusion. Likely, the formation of water had to wait for a few hundred million years, though some have argued it could have started much sooner. As is usual when dealing with processes that took place in such a distant past, opinions are divided and there are several reasonable scenarios.
“the book is characterised by an almost cosmic perspective on the subject […] The result is thrilling and at times awe-inspiring.”
With water present in the universe, how did Earth acquire its oceans? After all, “There is a wild card here, which surely had an impact” (p. 18). We have good evidence that our proto-Earth, called Tellus by some, was hit by a small planetoid, Theia, with the resulting debris forming our current Earth–Moon system. This event would likely have obliterated what early oceans we had, if any. Various authors have proposed that certain meteorites (carbonaceous chondrites) or comets might have subsequently been water’s cosmic delivery vehicle.
However it got here, the first major effect it had was kick-starting plate tectonics. The early Earth was hot, but without the lubrication provided by water, the heat-venting mechanism of plate tectonics was not in place. How did molten rock make its way to the surface? Some scientists argue that it was through simple vertical conduits, so-called heat pipes, which would have made for a radically different surface topography: “the fundamental proportions of land area and ocean area […] would have been utterly different to today’s familiar patterns” (p. 34). Though, again, this idea is contested by others. The puzzle of when plate tectonics started, possibly 3 billion years ago, relies on truly ancient rocks, 3.5 to 3.8 billion years old, of which we have precious few remaining in places such as Australia and Greenland.
Beyond those earliest days, Ocean Worlds has much interesting material about later episodes. Life likely started in the oceans, this much I knew, but these were iron seas. Water without oxygen can hold large amounts of dissolved iron, and early organisms used this in their biochemistry to generate energy. This was the realm of the Archaea: the salt-tolerant, heat-loving, chemoautotrophic microbes for whom oxygen was poison and the Great Oxygenation Event murder. It was also a time when banded iron formations (BIFs) were built up, relevant to us today as they formed the ore deposits providing most of our iron and steel. Though, as clarified here, their formation was anything but straightforward. Other fascinating episodes are the Messinian Salinity Crisis, some 5.6 million years ago, when the Mediterranean repeatedly dried up, leaving behind kilometre-thick salt layers that reduced global ocean salinity.
“However it got here, the first major effect [water] had was kick-starting plate tectonics.”
Of course, a book about oceans has to consider current human impacts. With due diligence, the authors tackle the problems of overfishing, shifting baselines, trawling, litter, ocean warming, oxygen loss, and acidification, and conclude that: “there currently seems not the faintest chance of stopping carbon emissions over many decades, let alone overnight” (p. 191). Does this sound gloomy? I prefer the word “sobering”. Consider, they write, that the “more-than-tripling of human population” (p. 183) was enabled by the invention of the Haber–Bosch process and the plentiful artificial fertiliser it made available. To this, they add geologist Peter Haff’s argument of the technosphere that resonated with me. “The 7 billion humans on Earth today are kept alive only through the continuous action of an enormous, globally interlinked system of transport and communication, metabolized by the use of vast amounts of energy […] Without it, most of us would not be alive—and therefore we are forced to keep it going” (p. 197).
If that was not sobering enough, what really made me feel small was when they pulled back from our timescale and the current “brief ecological wrecking spree” (p. 195), to the long-term future. Our oceans are not forever. As the Sun grows hotter they will evaporate, though the “end of the oceans is not likely to be simple” (p. 207). Whether through a moist greenhouse phase where water is gently siphoned off into space by solar winds, or a runaway greenhouse hot enough to melt rock, a dry future awaits, and plate tectonics will once again grind to a halt. As this process “is unlikely to simply just stop, smoothly and without fuss” (p. 211), expect some extraordinary landscapes.
“Our oceans are not forever. […] Whether through a moist greenhouse phase where water is gently siphoned off into space by solar winds, or a runaway greenhouse hot enough to melt rock, a dry future awaits.”
Amidst these grand, cosmic scenes, the authors highlight the human stories behind this research. Such as the pioneering contributions to oceanography by the people on board the HMS Challenger expedition, the mapping of the seafloor by Marie Tharp, or the work of Wally Broecker who established a link between ocean currents and rapid climatic changes. And while Svante Arrhenius is better remembered for linking historical changes in carbon dioxide concentrations to past ice ages, both he and Fritz Haber tried to extract gold from sea water. Unsuccessfully, I might add.
In the last two chapters, the authors turn their gaze to the skies once more, discussing past and present oceans inside and outside of our solar system. With the many exoplanets discovered by the Kepler space telescope, “We are on the verge of not just a new chapter in oceanography—or exo-oceanography, if you like—but of setting up an entirely new library of oceans, for the diversity and complexity of cosmic oceans will be beyond anything that we can dream of” (p. 264).
I explore this topic more in-depth in my review of Alien Oceans. But, as a warming-up exercise and a proper deep history of oceans, Ocean Worlds is a fantastic book that strikes the right balance. Zalasiewicz and Williams present fascinating science with enviable ease, without smoothing over the fact that science is rarely a straightforward affair, proceeding by means of conflicting scenarios and competing hypotheses. The deep-time perspective and big questions asked make this one awe-inspiring book.
Other recommended books mentioned in this review:
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