Lonely Planets

by David Grinspoon

  tempered engineer an ulcer, Galileo made it to Jupiter in late 1995 and, seemingly against all odds, it worked. Unlike the Voyagers, which were

  flyby missions, Galileo is an orbiter. When it got to Jupiter, it fired its powerful rocket perfectly, slowed down, and established permanent

  residency, becoming (as far as we know) the first artificial moon of

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  Jupiter. For eight years it circled Jupiter, caroming among the Jovian

  moons, scanning and photographing each of them on repeated close

  passes.

  Now Galileo, its mission completed, is crippled but triumphant, near

  senile from radiation damage and hobbling through its final orbits with

  that pathetically crumpled main antenna still in tow, its failing gyros

  barely able to keep the ship oriented. As of this writing, Galileo is

  scheduled to be intentionally crashed into Jupiter in September 2003.

  Why don’t we just leave it in orbit? Because it might someday smash

  into Europa, contaminating that world with flecks of plutonium and,

  conceivably, some stowaway bacteria. It seems unlikely, but who can

  say? Diving Galileo into Jupiter while we are still able to control it is the responsible thing to do.*

  A S U R P R I S E I N S I D E

  Galileo carried the first digital camera ever flown in space and, while

  bouncing among Jupiter’s moons, made photographs with a level of

  sharpness and detail new to space exploration. The strange beauty of

  these distant worlds raises some questions: Why should these places,

  where no terrestrial eye can ever before have wandered, be beautiful to

  us? What structures, deep within our brains or deep within the physical

  universe that created both Gaia and Europa, are resonating when we

  gaze upon an alien landscape for the first time and find exhilarating

  beauty? Would alien souls feel it, too?

  What we have found here is something much more profound than

  simply a random collection of odd worlds. These large, complex moons

  are really planets in their own right, and Galileo is humanity’s first

  exploration of a new planetary system. As you might expect, many

  comfortable preconceptions have been completely overturned. Much of

  what we thought we knew about comparative planetology turns out to

  be wrong. This includes our previous notions of where interesting geo-

  logical—and biological—activity may be found in the universe. The

  moons of Jupiter, as glimpsed by the Voyagers and explored by Galileo,

  turn out to be a much more active and diverse gang of worlds than our

  previous theories of planetary evolution would have led us to believe.

  *Wouldn’t it be ironic if in our effort to protect the Europans we ended up nailing some poor Jupiterian gas creature?

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  We thought that only large, rocky worlds like Earth could have

  active geology, and that we would find only dead ice worlds out this far

  from the Sun. But who needs a star when you’ve got a giant planet like

  Jupiter? Several of those moons are seething with internal heat, rest-

  lessly churning inside and out. It is Jupiter’s influence that creates this

  heat and activity. Just as our Moon tows the oceans around the Earth,

  the huge gravitational pull of Jupiter yanks the insides of these moons

  around, creating the internal heat that drives the furious volcanic activ-

  ity of Io and continually warms and cracks the ice on Europa.

  This surprising activity is facilitated by the tight, polyrhythmic

  orbital dance continually executed by the three innermost giant moons.

  Io orbits Jupiter twice for one Europa orbit, and similarly Europa laps

  Ganymede twice each orbit. They are locked together tighter than the

  tightest rhythm section in Jamaica. Every time they pass, they grab at

  each other with gravitational arms, but Jupiter pulls them back into

  line. This rhythmic back-and-forth keeps them flexing, pumping energy

  into their interiors. The heat of the dance keeps their insides hot and

  their faces young and fresh. The greatest heating goes to those caught

  most deeply in Jupiter’s gravitational spell. Thus we see an evolution of

  planetary style, each moon getting progressively colder, and its surface

  older, as we travel outward from innermost Io past Europa, Ganymede,

  and Callisto. The only moon that receives no internal heating from this

  intricate multipartner dance is outermost Callisto. Like an older chap-

  eron watching the young ones dance and bloom, Callisto comes closest

  to our original expectations of all the Jovian moons—an ancient, dead,

  heavily cratered ice world.

  The active nature of these moons was actually predicted right before

  the Voyager discoveries. In a triumph of theory, and an example of

  good timing, three planetary scientists* modeled the effect of tidal heat-

  ing on Jupiter’s moons and predicted that Io would be volcanically

  active. Their paper was published in Science one week before Voyager 1

  arrived at Jupiter. When Voyager 1 flew past Io, dang if they weren’t

  right there: giant volcanic plumes shooting off into space from the

  edges of the little red moon. The plumes astonished everyone (including

  the scientists who had made the prediction).

  Europa, too, gets a workout on the inside from those Jovian tides,

  but we don’t know exactly how much heating it gets. Some models pre-

  *Stan Peale, Pat Cassen, and Ray Reynolds.

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  dict that the icy surface is just a thin shell, a few kilometers thick, over

  a deep liquid ocean. Others suggest that the icy shell is tens of kilome-

  ters thick, but increasingly, nearly everyone accepts the likelihood of a

  deep ocean beneath the ice.

  Ever since Voyager we’ve wondered just how active Europa might be

  and what really goes on there, and science fiction writers have riffed on

  the theme of Europan life. Now, seen through Galileo’s digital eyes, it

  turns out to be arguably the weirdest world we’ve yet explored. On

  first viewing, its planetwide system of dark fissures and raised bands

  looks like a global tangle of roots. The face of Europa seems eerily

  unnatural, with animated landscapes suggesting arrested flow. At every

  scale, from hemispheric to close-up, the globe is covered with what

  could almost be veins, like a giant, frozen mutant leaf.

  Europa looks alive. Its bright, lineated surface is composed of fresh-

  water ice crisscrossed by a global system of fissures and cracks, formed

  as the surface pulsates with the tide. Galileo has now shown us the sur-

  face at a magnification a thousand times Voyager’s best. In the most

  detailed photos we see icebergs, which have apparently broken loose,

  floated, and jostled before freezing in place again. This adds to a

  mounting pile of evidence for an ocean of liquid water beneath the ice.

  In fact, we now think that Europa may have more water than all of

  Earth’s oceans combined. And, on Earth at least, where there is water,

  there is life. Does anything swim through Europa’s icy seas?

  As far as I know, the possibility of life in underground oceans on icy

 
moons was first mentioned in 1975 by Guy Consolmagno, another

  John Lewis graduate student, in his master’s dissertation at MIT. Guy

  (who is now a Jesuit priest as well as an active planetary scientist) was

  studying the thermal evolution of ice moons. His calculations showed

  that they should sometimes develop layers of liquid water within. The

  appendix of his thesis ended, “But I stop short of postulating life in

  these oceans, leaving that to others more experienced in such specula-

  tions.”* This passing comment in a student dissertation went mostly

  unnoticed, but the possibility of life on Europa became a hot topic after

  Voyager’s discoveries. Arthur C. Clarke made it one of the main themes

  of his novel 2010, published in 1982 when the new images from

  Voyager were fresh on our minds.

  *Reflecting on this now, Guy wrote to me, “So I guess that makes me the first person NOT to predict life in these oceans!”

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  electronic edition

  With Galileo’s further hints of a deep, dark ocean, Europa has

  emerged as the most promising place to look for water-based extra-

  terrestrial life in our solar system. Indeed, this bizarre little moon can

  serve as a test bed for our current assumptions about life.

  We are currently planning a mission that we hope to launch some-

  time in the next decade to orbit Europa. It will determine definitively

  whether an ocean is flowing within. We’ll also look more closely at the

  surface for life-revealing chemicals mixed into the ice. If we do confirm

  the presence of liquid water, the next step will be to go ice fishing. We’ll

  go back and land a self-disinfecting probe that can slowly melt through

  the icy crust. Then, when we break through to open water, we’ll go

  exploring in our solar system’s deepest ocean.

  G R A V I T Y ’ S R E I G N

  We thought the Jovian system would be a fossil, a freeze-dried remnant

  of the early solar system. Instead close-up spacecraft investigations

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  revealed a living, evolving planetary system. The unanticipated activity

  and youthfulness of these worlds implies that gravitational energy may

  be of equal or even greater importance than solar energy in fueling life

  in our universe.

  The likelihood of oceans on ice-moons like Europa suggests a whole

  new concept of habitable zones. If there can be water, and perhaps life,

  on Europa, then the same could be true on moons orbiting giant plan-

  ets anywhere in the universe. This would mean that distance from a

  star matters less in defining habitability. Such “gravitational habitable

  zones” could be common and large, implying a universe with a lot

  more real estate having favorable conditions for life as we know it.

  The idea of life on Europa has got us all thinking about life that

  could be independent of starlight. Meanwhile we’ve discovered

  extremophile organisms on Earth that live deep below the surface and

  have little interest in the Sun. This may brighten the corners for life and

  planetary habitability.

  We animals are completely enmeshed in the part of the Earth’s bio-

  sphere that lives off the Sun. We’re just a minor outgrowth of the green

  world and the ubiquitous, hidden microbial world—we’re those weird

  brainy things that crawled out of the compost heap after the oxygen

  waste piled up. Its hard for us to imagine life that is truly isolated from

  the solar influences that drive and so thoroughly shape our world.

  The idea of life underground is not new, of course. Subterranean

  creatures have filled our mythologies and occasionally our scientific

  fantasies as persistently as have creatures living beyond the sky. Hell, in

  the seventeenth century Fontenelle speculated about microbes living

  underground on the Moon or elsewhere, slowly eating rocks. Yet now

  we have new reason to wonder if perhaps his conjecture was prescient,

  and surface life is not the only game in town.

  Life may be something that frequently, or even ubiquitously, happens

  inside planets. Perhaps biology can be a purely internal planetary phe-

  nomenon. If life begins underground, it makes the origin of life any-

  where else in the universe less dependent on surface conditions. If all

  you need is some internal energy and some liquid water, then most

  planets must have the right combo at some point in their history, since

  planets are born hot and wet. The classical concept of the habitable

  zone starts to seem like a bourgeois notion invented by self-centered,

  Sun-worshiping surface dwellers. Can a planetary biosphere that loses

  its surface water retreat inward and persist, subterranean and home-

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  sick, but still alive? In some cases an “inhospitable” surface may be

  only the thick, protective skin covering a thriving underground world.

  Astrobiologists agree that there are three essential ingredients for hab-

  itability: (1) organic molecules (the building blocks), (2) liquid water

  (the medium), and (3) a source of energy (the spark of life). Based on our

  studies of comparative planetology, how rare might such a combination

  be? Organic matter seems to be ubiquitous, falling from the sky every-

  where in the universe, so this requirement serves only to rule out places

  with environmental factors that directly destroy organics. No, on sec-

  ond thought, it doesn’t even do that. Otherwise, Earth, with its organic-

  burning oxygen atmosphere, would be lifeless. Opportunistic life might

  find a way to derive energy from whatever it is that destroys organics, as

  we do on Earth with high-octane poison oxygen.

  The belief that water is necessary and sufficient rests on the assumption that suitable energy sources will be common. How reasonable is this?

  Well, the presence of liquid water actually implies some kind of energy

  source, so the two requirements aren’t completely separate. Our two

  examples of oceanic planets are Earth and Europa. Earth is wet because it

  is in the right place to soak up plenty of solar energy. Europa is wet

  because of the release of tidal energy. So, both ways that we know of for

  a planet to stay watery also come with built-in, long-term energy supplies.

  But is tidal energy sufficient to drive a biosphere, or does life need a

  sun? Picture the recently discovered dense colonies of bottom-dwelling

  life clustered around the “black smoker” volcanic vents on Earth’s

  ocean floors. The seafloor on Europa may also have volcanic vents,

  driven by internal tidal heat, which could provide the chemical fuel for

  a native biosphere.*

  Another idea was proposed by Chris Chyba, a former student of Carl

  Sagan’s, now at the SETI Institute in Mountain View, California.

  Chyba’s idea makes use of the punishing radiation at Europa’s surface.

  The powerful magnetosphere of Jupiter whips charged particles into a

  frenzy; all of the inner moons exist within a raging storm of radiation.

  At Europa, an astronaut in a space suit would receive a lethal radiation

  do
se every twelve minutes.† Could a Europan biosphere somehow har-

  *Indeed, Europa on the inside may be like a less hyperactive version of fiery, volcanic Io, doused with an outer shell of ice and water.

  †Which is one reason why crashing a small amount of plutonium onto the surface may not really be as bad as it sounds.

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  vest this potent energy source? Radiation breaks chemical bonds, which

  is why it is often lethal. It must be inducing chemical changes in the

  Europan crust. Chyba proposed that the radiation rips apart water ice

  molecules, liberating hydrogen to drift off into space, and leaving

  behind oxygen and various oxidized compounds. Perhaps this oxygen

  is eventually mixed down into the ocean where indigenous creatures,

  their fragile bodies protected from the radiation by miles of ice,

  breathe, harvesting the chemical fruit of the irradiated surface.

  The intense radiation at the surface of Europa can be regarded as a

  threat to life or as a source of energy that could drive a biosphere.

  There is an important concept here that we can generalize to help us

  think about life in the universe beyond the biases of our terrestrial per-

  spective. Paradoxically, a deadly environmental factor may create

  opportunity, if you can control the slide toward destruction. We live by

  burning ourselves in oxygen, but slowly, slowly. It’s a fine line between

  a deadly chemical or radiation and a bountiful source of energy. Life

  may adapt by staying at a safe distance, reaping the flow without being

  destroyed by it. Just as our biosphere runs on nuclear energy, keeping

  its distance from the solar reactor, underwater life on Europa might use

  the intense radiation at the icy surface while avoiding its direct effects.

  A whole category of potential adaptations might make use of such

  “dangerous,” “lethal” energy sources. Life would need to take advan-

  tage of the inevitable flows induced by any source of energy, while

  avoiding the destruction that comes from getting too close to the fire.

  One thing we know about life is that it is inventive. What seems like a

  death ray to us may be a meal ticket for suitably adapted creatures.

  B I O S P H E R E T W O

  That first voyage by robotic submarine through Europa’s briny sea,

  sometime in the next couple of decades, with all the folks back home