Science & Tech

Searching for Life in the Outer Solar System

Jupiter’s icy moon Europa, photographed by the Galileo spacecraft. (NASA/JPL-Caltech/SETI Institute)
In his engaging new book Alien Oceans, NASA scientist Kevin Peter Hand draws a blueprint for the grand quest.

Alien Oceans: The Search for Life in the Depths of Space, by Kevin Peter Hand.
Princeton, 204 pages, $27.95.

Perhaps we are the only ones. Perhaps the origin of life is hard, and life is rare. Or perhaps we live in a universe teeming with life — a biological universe of incredible diversity across planets, moons, stars, and galaxies. Perhaps our tree of life — the singular, center of biology as we know it — is revealed to be but a tiny twig, on a tiny branch, joined to a vast and grand tree of life connecting the beauty of all life in the known universe.

— Kevin Peter Hand

You Earthlings are all alike. Whether humans, turtles, wasps, trees, mushrooms, tardigrades, or bacteria, you all use the same DNA, RNA, ATP biochemical operating system. You offer some interesting diversity, that is true. But are you all there is to life?

Kevin Peter Hand, a scientist with the Jet Propulsion Lab, really wants to get the answer. In his engaging new book, Alien Oceans: The Search for Life in the Depths of Space, he lays out why, where, and how we can do so.

English, French, Spanish, German, and Polish all use the same system for encoding information: the Latin alphabet. Greek and Russian employ alphabets that differ significantly but still work according to the same basic principles. That is because all these scripts have a common origin. If all you knew were European languages, you might think that variation on such writing systems exhausted the possibilities. Chinese, though, on the other hand, utilizes an information technology with no resemblance to any Western phonetic alphabet. It accomplishes the same function but does so in a fundamentally different way.

There are, as Hand explains, fundamental reasons why we might expect that life everywhere uses the same carbon- and water-based chemistry we see here. But Earth life is far more restricted in its format than such considerations alone require. Specifically, it all uses the same DNA-RNA alphabet for encoding genetic information from one generation to the next — the Latin alphabet, if you will. That works well enough, but could life elsewhere be using Chinese? And what could that mean if it does?

To find out, Hand says, we need to find alien life, and the place to find it is the outer solar system.

Liquid water is the essential environment for life. That understood, scientists in the past defined the “habitable zone” around any star as that distance — near or far, depending on the brightness of the star — at which temperatures allowing for the existence of liquid water might prevail. In our solar system that meant no closer to the sun than Venus and no farther from it than Mars.

Such thinking, however, was upended in 1979 by NASA’s Voyager probes, which discovered vast liquid water oceans on Jupiter’s large moons Europa, Ganymede, and Callisto. Covered by ice but heated from below by tidal forces, the volume of their seas dwarfs that of Earth’s oceans, by a factor of more than ten. In 2005, an additional ocean, on Saturn’s small moon Enceladus, announced itself in spectacular form to NASA’s Cassini probe by shooting a fountain of liquid water hundreds of kilometers into space. Erupting through a crack in Enceladus’s south polar ice cover, the geyser turned into snow, which then fell back down to cover the moon’s Antarctic. From the theoretical foundation based on these observations, it is now believed that Saturn’s giant moon Titan almost certainly has an ice-covered water ocean hidden beneath its surface methane-ethane seas, and that there are probably many additional icy ocean worlds waiting to be found around Jupiter, Saturn, Uranus, Neptune, and Pluto. Furthermore, the Kepler space telescope has discovered myriads of ice worlds orbiting other stars, some of whose kind have no doubt been ejected by planetary interactions to cruise interstellar space.

Could such plentiful worlds harbor life? Fifty years ago, science would have answered no, because their ice covers ten kilometers thick block out all light, making photosynthesis impossible. But in 1977, oceanographers using the submarine Alvin discovered a new kind of ecosystem on Earth, one that draws its energy not from sunlight but from the thermal and chemical energy released by deep-ocean volcanic vents. In the course of his research, Hand has visited such locales himself, including the fantastical hydrothermal “Lost City,” one kilometer beneath the Atlantic Ocean, whose abundant and varied life forms the author describes in lyrical terms. So, in a word, Hand’s answer is yes, and he goes on to speculate on all sorts of interesting possibilities, ranging from microbes to intelligent octopi.

How can we ever get to see them? NASA has a funded mission on the books right now, Europa Clipper, scheduled to launch around 2023. It will go into orbit around Jupiter toward the end of the decade. Making numerous close passes by Europa, this probe will return a lot of useful data via remote sensing, conceivably even detecting organics within the streaks of materials that seem to have emerged from below, through cracks in the moon’s surface. NASA is also funding a team including Hand to do a preliminary design of Dragonfly, a mission that would send a radioisotope-powered helicopter to Titan. Titan has one seventh Earth’s gravity but four times its atmospheric density — flight conditions so favorable that if you were there (and dressed warmly — surface temperatures are about -180 degrees Centigrade) you could strap wings on your arms and fly like a bird. Presumably, using pontoon landing gear to deal with the moon’s liquid hydrocarbon seas, Dragonfly would be able to fly and repeatedly land all over Titan, taking photographs from the air and samples from the ground. If oceanic living material has emerged from cracks in the ice floor beneath Titan’s hydrocarbon seas, Dragonfly may be able to find it.

But if we really want to meet the icy ocean world’s aliens of the deep, we are going to have to melt our way through their ice covers. Using 300-watt radioisotope units of the kind that powered Voyager and Cassini, and that will power Europa Clipper and Dragonfly, it could take a decade or more for a probe to melt its way down through an ice cover ten kilometers thick. That’s too long, which is why no such mission is on the books. Although Hand does not discuss it, there is a technology that could do the job — space nuclear-fission reactors. Together with Los Alamos National Lab, NASA has finally begun work on one such reactor design, which they call kilopower. Currently rated to ten kilowatts, but scalable to 100, a kilopower orbiter could deeply probe Europa or Enceladus with high-powered active sensing, reporting back science with a data rate far exceeding anything currently possible. Landed on an icy moon, a kilopower system could provide the juice to dramatically cut a melt probe’s travel time through the ice. Space nuclear power would also be of tremendous utility in supporting NASA’s plans for human bases on the moon and Mars. For the sake of both human exploration and outer-solar-system science, then, let’s hope that NASA starts moving this program forward with greater urgency.

Is the universe alive? Or is it dead? Are we what life is? Or are we part of a much vaster tapestry of possibilities? To paraphrase Hamlet, there are more things in heaven and Earth than are dreamt in our philosophy. Hand calls on us to probe the depths of alien oceans to discover them. I agree. Biology, the study of self-organizing matter, may well prove to be the premier science of the 21st century. Expanding our concept of life’s true possibilities could have untold and unlimited benefits. But that is not why we should seek the truth about life. We should do it because we would be less than human not to.


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