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In all candor, I should admit that my initial interest in Kevin Peter Hand’s Alien Oceans: The Search for Life in the Depths of Space (Princeton University Press) was largely escapist. The premise sounded intriguing: some of the moons orbiting other planets in our solar system offer conditions in which life could have evolved, with the ocean beneath the ice shell of Jupiter’s satellite Europa being an especially plausible candidate. I had seen occasional reference to the possibility over the years -- enough to inspire curiosity, if not any sense of urgency. But the book arrived at exactly the right moment, when a little distraction from events on this planet was at its most craveworthy. The life of a space squid may be fraught with peril, but I doubt it’s as terrifying as any given nursing home right now.

While diverting, Alien Oceans is anything but fanciful. (That space squid is my imagining, not the author’s.) An astrobiologist and planetary scientist at NASA’s Jet Propulsion Laboratory, Hand keeps the ratio of speculation to confirmed information always to a manageable low -- with an emphasis on how the discovery of life elsewhere in the universe would line up with how our own planet’s history is currently understood. Besides the expected astronomy lessons, the book requires side trips into microbiology, climate science, oceanography, a little astrophysics and more basic chemistry than the reader might be expecting. The volume includes several color plates of the moons of Jupiter, Saturn and Neptune, mostly from photographs taken by the Galileo and Voyager spacecraft in the 1980s and '90s. Most are stunning.

In the popular-science writing of a few decades ago, at least, the possibility of finding life elsewhere in the solar system seemed effectively off the table. Readers of H. G. Wells and Edgar Rice Burroughs had once found our closest planetary neighbors to be credibly habitable, but the suspension of disbelief became impossible with the discovery that it rains sulfuric acid on Venus and that Mars has a Saharan landscape. The remaining planets are at such extremes of temperature to preclude harboring life even a single-celled organism. So speculation over where life might have evolved other than our world turned to the possibility of finding Earth-like exoplanets (that is, planets orbiting other stars). And that search has gone well enough that Wikipedia now has a page listing potentially habitable exoplanets in “the Goldilocks zone” of their respective solar systems. They are at a suitable distance from their suns for liquid water to exist: conditions are neither too hot nor too cold, but just right.

Hand’s argument is that a number of moons orbiting the outer planets of our own solar system fall within the Goldilocks parameters. When trying to imagine this, the moon in our own sky is not a helpful reference point: it lacks the mass, and hence the gravitational pull, for an atmosphere to form. By contrast, Jupiter’s satellite Europa and Saturn’s Titan are big enough to be planets in their own right. In spite of being fairly small -- as well as covered with impact craters like those on Earth’s satellite -- Saturn’s Enceladus is, the author says, “alive with geological activity, sending jets of salt-rich water hundreds of kilometers out into space.” That suggests the presence of an ocean beneath its surface. Triton, which orbits Neptune, has an icy surface and a thin nitrogen atmosphere; plumes of black gas containing organic material erupt from its surface, which Hand indicates probably covers another ocean.

The fluids and ices vary in composition across the various moons -- Triton’s ocean, for example, may be rich with ammonia -- and liquid water is not always part of the mixture. But the potential for life may be a matter of the availability of a liquid solvent with suitable chemical properties, rather than of H20 in particular. (The distinction between polar versus nonpolar molecules plays a part in this, but it is not susceptible to miniaturized paraphrase here.)

“Our cells, and the cells of all life as we know it,” writes Hand, “are fancy bags of salty water … It could be that many forms of life, perhaps on Titan and other worlds, function with cells that are fancy bags of nonpolar liquids, like methane and ethane, encased in a membrane of polar molecules … The chemistry of weird, ammonia-based life could be chugging away on these worlds without any care for my earthly bias and failure of imagination.”

In the opening pages, the author narrates his voyage in a very small submersible to the bottom of the Atlantic Ocean -- far beneath the reach of sunlight but surprisingly rich in bioluminescent creatures, especially in the vicinity of hydrothermal vents on the seafloor. The vents send “clouds of fluids jetting out into the ocean at temperatures well beyond boiling,” although the pressure is too high for them to do so. “The vents erupt hydrogen, methane, hydrogen sulfide and a host of metals,” Hand writes, “many of which provide tasty treats for microbes,” which form the basis of the local food chain. Some larger organisms “have also developed symbiotic relationships with the microbes -- hosting the microbes within their bodies in exchange for the microbes detoxifying the water.” And all without the benefit of photosynthesis.

The existence of comparable vents on the floors of distant lunar oceans seems a reasonable guess, especially given that the enormous gravitational pull of Jupiter or Saturn creates tectonic strains on their moons. Data collected from a flyby of Saturn’s moon Enceladus by the Cassini spacecraft in October 2015 seem to have confirmed the activity of a superheated vent -- spewing out a combination of salts and gases “giving shape to a chemically rich ocean with a hydrothermally active seafloor,” according to Hand.

None of which makes it a certainty that life has evolved on any of these heavenly bodies -- and it will be quite some time before we know either way, if we ever do. But the author leaves us with news of a development that he is involved with: the Dragonfly mission, green-lit by NASA in July of last year. "If all goes well," he says, "a robotic spacecraft will parachute into Titan's atmosphere in the year 2035. It will fire up its propellers and land softly on Titan's surface. Once there, it will use a suite of instruments to look for biosignatures on Titan and investigate the geology, geochemistry, and geophysics of its surface and subsurface." Once done, it will fly to another site, "and another, and another, over the course of roughly three years." It sounds like the stuff of daydreams.

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