Alien Oceans The Search for Life in the Depths of Space

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.

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 too hot, nor 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 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.

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?

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 when he enthusiastically concludes that "the why of life is metabolism" (p. 146), offering the universe a pathway to increase entropy faster.

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 hydrothermal vents teeming with life were discovered in 1977, these quickly became a popular alternative explanation to warm tidal pools as a place where life could have started. These 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 arecommon 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.

Hand raises many other interesting questions towards the end of the book that I will skip over for the sake of brevity. 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. Even without this topic, Alien Oceans is information-dense. Nevertheless, it is an intellectually very rewarding book and the many analogies make it accessible. Terribly fascinating, Alien Oceans makes a convincing case for exploring the moons in our solar system in the search for extraterrestrial life.