Lonely Planets

by David Grinspoon

  egy for astrobiology is highly focused on two places that surely have

  liquid water within: Mars and Europa. But what if we tried to devise an

  *Or if we receive a signal (see chapter 18).

  †Remember Kepler’s deduction of lunar cities based on the “anomalous” circular shapes of craters. . . .

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  exploration strategy based on a more general definition of life as an

  evolving system of complexity thriving under stable and vigorous con-

  ditions of thermodynamic disequilibrium? What if we decided that the

  main criteria was to look for places where a lot is happening? The two

  most compelling targets under these alternative criteria would be Io,

  Jupiter’s hypervolcanic moon, and Venus, Earth’s “twin,” which (as

  described in chapter 11) seems to be, geologically, very much alive.

  Saturn’s moon Titan also gets an honorable mention. We don’t really

  know the level of geologic activity, but it does have a thick atmo-

  sphere, and a surface pooled with juicy hydrocarbons. If it wasn’t so

  darn cold there, permanently well below the freezing point of water,

  I’d rate it as the number one candidate for life. As it is, who knows?

  Reid Thompson (chapter 7) pointed out that intermittent lakes of

  organic-rich, liquid water appear on Titan for thousands of years

  whenever an occasional large impact melts a portion of the icy surface.

  It is conceivable that there could be life on Titan today, using liquid

  methane/ethane lakes as a fluid medium. I will certainly be paying

  attention on the morning of January 14, 2005, when the Huygens

  probe, now on its way to Titan, attached to the Cassini Saturn orbiter,

  descends through the hydrocarbon hazes, methane clouds, and thick

  nitrogen atmosphere to land or maybe even splash down, sniffing the

  air and sending back pictures all the while. There’s a nonnegligible

  chance that something living will fly, crawl, or float through the

  Titanian scene.

  M A R S I S D E A D : L O N G L I V E M A R S

  From a living worlds perspective, the new wave of interest in life on

  Mars is highly questionable. If Earth’s drastically out-of-equilibrium

  atmosphere is anything close to typical for living worlds, Mars does not

  qualify. Mars today has a highly equilibrated atmosphere of almost

  pure CO2.

  If we regard life on Earth as synonymous with Gaia, with the global

  biosphere that infects and affects all of Earth so deeply, so exuberantly,

  and if that is what we are looking for on other planets, then we already

  know the answer about Mars. It is not enough to identify places on

  another planet with conditions that overlap with those where organisms

  can live on Earth. Whether or not Mars has little pockets of water, organ-

  ics, and local energy flows somewhere, in the Gaian sense, it is dead.

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  Further, large areas of Mars are covered with craters that date back

  to the earliest days of the solar system. A living worlds perspective sug-

  gests that such a surface is incompatible with life. Ironically, the famous

  “Mars rock,” ALH84001, the meteorite with the “fossil” worms, may

  provide the best evidence that Mars is dead. Radiometric dating sug-

  gests that this rock crystallized 4.5 billion years ago, when the planets

  were newly formed. No rocks on Earth are anywhere near that old. A

  living world has no 4.5-billion-year-old rocks.

  With its rusty surface and stale (equilibrated) atmosphere, Mars

  seems very unlike a world where life is thriving. If life exists at all it is

  barely hanging on in isolated outposts, and it hasn’t taken over the

  thermodynamic state of the atmosphere, and the global geochemical

  cycles, as has life on Earth.

  In his writings, Lovelock distinguishes the birth of Gaia from the ori-

  gin of life. This idea, of an origin of life separate from the birth of a liv-

  ing world, has interesting implications for life elsewhere. It is possible

  that a planet could develop life, but never become a living world. If self-

  regulating Gaia is responsible for Earth life’s longevity, then we need to

  find other places where this kind of global organism has evolved, not

  merely places where the origin of life might once have occurred.

  Can a planet be a little bit alive? For a world to be alive, in the sense

  that Gaia is, life must be deeply ingrained in the physical functioning of

  the planet. This suggests that life, as a global property, is something

  that a planet either has or doesn’t have, a distinct state of being, just as

  an animal’s body is either dead or alive. An animal can be “barely

  alive,” but not for long. Maybe in the period after the origin of life and

  before the origin of Gaia (or something like it), a planet is “barely

  alive,” in a fragile state that either achieves Gaia-hood or quickly dies

  out. During this vulnerable stage, the continuance of life depends on

  luck, on the environment’s not changing too rapidly or extremely, until

  the expanding web of feedbacks grows to become a global, self-

  regulating system, like an organism. The image of a dying organism

  experiencing global systems failure may be more accurate in picturing

  life’s extinction from a planet than that of isolated colonies of intrepid

  survivors trying to hold out against all odds.

  In this view, it is hard to imagine life existing for 4 billion years on a

  planet without participating thoroughly in planetary evolution. A living

  worlds perspective suggests that life cannot hang on for billions of

  years isolated in underground hot springs. Life either thoroughly infests

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  a planet, or it is not there at all. The signs of life on a planet will not be

  subtle. As on Earth, life will shout out its existence in the air itself. For

  this reason, I’d be (pleasantly) surprised if we find life on Mars.

  Mars may once have evolved living organisms but never become a

  living world, before environmental changes doomed all life there. Mars

  did not have as long as Earth did to make this change before it froze

  over and lost most of its air and water to space. Maybe the difference

  between Earth and Mars tells us something about how long it takes to

  transform a planet into a living world.

  A new wrinkle in all of this has appeared since Lovelock’s work, and

  Viking’s observations, in the 1970s: the discovery of an extensive and

  deep underground biosphere on Earth. This discovery is partly responsi-

  ble for the renewed interest in life on Mars and elsewhere in the solar

  system. Even if conditions on the surface are inhospitable, life may be

  underground. I don’t rule this out, but over long timescales any gases

  emitted by living things inside a planet will diffuse out into the atmo-

  sphere. We should be able to detect underground life from the disequi-

  librium nature of those gases. Can a planet have internal life but no signs

  of it on the surface or in the atmosphere? I doubt it. I regard the absence

  of flagrant disequilibrium in the Martian atmosphere as a likely sign that

>   Mars is dead—not mostly dead or almost dead or just dead on the out-

  side but completely dead. Perished. Deceased. An ex-biosphere.

  These considerations reveal a contradiction within current astrobio-

  logical thinking. It is widely held that Mars may have life, regardless of

  the lack of disequilibrium seen in its atmosphere, because there are

  probably layers deep inside where liquid water exists. Yet the com-

  monly agreed upon signs of life on exoplanets, primarily the detection

  of atmospheric oxygen, requires that life drastically alter an atmo-

  sphere from a Mars-like state.

  I can’t think of anything I’d rather be wrong about, but I think Mars

  is a dead world if ever there was one. The Gaia hypothesis suggests that

  biologically generated gases will permeate the atmospheres of living

  worlds. Complexity theory suggests that for a planet to support life it

  must have an active, evolving, recycling surface where self-organization

  can flourish. Mars fails both tests. The planet’s ruddy complexion, eas-

  ily visible to the naked eye on a clear night, shows that it has literally

  rusted everywhere, with none of the active chemistry found on the sur-

  face and in the air of a living world like Earth.

  I’m a skeptic about life on Mars, but also an enthusiastic advocate of

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  Mars exploration. We’ll keep exploring Mars for other good reasons: it

  is beautiful, mysterious, nearby, and a relatively easy place for human

  beings to survive with a decent space suit.

  In fact, Mars, because it is almost surely dead, has certain advan-

  tages. On a planet like Earth, nothing lasts long, because a living planet

  eats its past. Our frozen neighbor planet has wonderfully preserved

  traces of ancient epochs that have long been erased from Earth. Mars,

  then, surely holds important clues to our own past. We might even find

  fossils there, remnants from a brief, early flowering of life before the

  billion-year winter set in.

  The other important advantage of a dead Mars is that we could be

  free to import life there without violating any strong ethical principles.

  We could become the Martians, but should we? Only when we are sure

  there are none already there.

  L I F E O N V E N U S A N D B E Y O N D

  We may need to look beyond our solar system to find another example

  of a living world, but in our ignorance, we cannot yet rule out some

  nearby places. Europa is an obvious place to look. As I’ve mentioned,

  my favorite underdog places for biology in the solar system are Venus

  and Io. Both have active chemistry and vigorous flows of energy and

  matter. And where something’s flowing, maybe something’s growing.

  What draws my attention to Venus is that geologically it is a vibrant

  world, pulsing with volcanic eruptions, bathed in a chemically fertile

  disequilibrium atmosphere. I first suggested that some unexplained

  Venusian phenomena might possibly be signs of life in my book Venus

  Revealed. This book was published in 1997, just before exobiology

  became astrobiology and such speculations became not only respectable

  and tolerated but sometimes even encouraged with research grants.* At

  the time I was consciously floating the idea of life on Venus to see if any-

  one would bite. It has taken a while, but some of these ideas have

  recently been picked up and cited in the peer-reviewed literature, and

  others have been used in science fiction novels.†

  Though still not a place where you and I would be comfortable with-

  *I hurriedly added a mention of the new Martian “fossils” as the book went to press.

  †Two SF novels of Venusian life that explicitly credit Venus Revealed are The Quiet Invasion by Sarah Zettel and Venus by Ben Bova.

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  out a well-designed suit or terrarium, Venus may have some of the

  essential characteristics of a living world. In fact, some of the very qual-

  ities that seem, at first glance, to doom Venus’s prospects for life may

  conceivably help to support a more alien kind of life. That “chemically

  corrosive” atmosphere would certainly not be kind to organic mole-

  cules, but it reveals dynamic interactions between the surface and

  atmosphere.

  If we are looking for a specific kind of life that we are familiar with,

  we had best look elsewhere. In terms of carbon biochemistry on the

  surface, Venus is deader than burnt meat. But if we are looking for the

  kind of chemically charged environment where self-organization could

  thrive, and the kind of ongoing geologic and atmospheric activity that a

  Gaia-like planetwide network of organisms could come to participate

  in, then Venus deserves a closer look.

  If there is life on Venus, unless it uses a radically different kind of

  chemistry than we do, it probably lives in the clouds, thriving on the

  chemical energy created by absorption of UV light, and deriving nutri-

  ents from the active chemical cycles connecting the atmosphere and

  clouds to the surface and interior.

  One of the arguments against cloud life on Venus is that there is no

  life in the clouds of Earth, except for bugs that are just passing through,

  blowing in the wind. However, recently the Austrian biologist Birgit

  Sattler has found evidence of a population of microbes that are repro-

  ducing in clouds over the Alps. She is planning further experiments to

  verify this result. If confirmed, it has important implications for possi-

  ble life on any cloudy world, not just Venus.

  Astrobiologists have discussed the conditions in Venusian clouds

  with respect to the impressive acid tolerance found in some terrestrial

  extremophiles. Some Earth bugs would likely be able to live in the

  clouds of Venus. Comparing conditions there with the comfort range of

  Earth’s extremophiles is a worthwhile exercise, but it may not really

  constrain the habitability of the Venusian clouds. Anything living there

  is not going to be an Earth extremophile but a Venusian, and there will

  be a million ways in which it will be better adapted to its own world

  than anything that evolved on Earth.

  Venus’s clouds are complex, stable, global in extent, and populated

  with a menagerie of unidentified particles and strange, moving patterns

  of light-absorbing materials. Could these be photosynthetic organisms?

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  I’ve thought a lot about the Venusian clouds and published papers

  about their evolution, construction, materials, particle sizes, climate

  effects, and chemical sources. I think there could be life there. This

  doesn’t mean that I think there is life there. I would be stunned to learn that there is, but the supposition certainly doesn’t violate any scientific

  principle that I can think of. If there could be life in clouds in one place that we know, then given plenitude and biological opportunism, there

  is cloud life somewhere in the galaxy.

  Is it an accident that Earth has the outstanding activity level that it

  does and also happens to be the one living world we know? I don’t think

  so. The living worlds hypothesis (my theo
ry, which is mine) suggests that

  life is most likely to be found where vigorous activity is discovered. Not

  just any activity, but cyclic flows of energy inside and outside a planet,

  phase changes, and physical flows across surfaces. According to this

  view, we should explore those places where a lot is happening. Io and

  Venus are the winners. Europa, Titan, and Mars are all worth a look.

  Ganymede and Triton are runners-up. Pluto? We’ll see.

  Y O , I O

  While we’re considering heretical ideas about life in the solar system,

  let me speak in defense of my other favorite underdog biosphere: Io. If

  it’s active geology that makes a world viable, then have I got one for

  you. Sure Io has some drawbacks—its position deep within Jupiter’s

  intense radiation belts, an apparent lack of water, and an atmosphere

  so thin that it would seem like a vacuum to us.

  But, Io can barely contain itself. This innermost large moon roils and

  seethes with such intense volcanic activity that its insides are constantly

  overflowing, coughing up silicate and sulfuric lava. The surface is an

  ever shifting collage of green, white, and red plains. Io is the most geo-

  logically alive place in the solar system. On a world where the geology

  changes as fast as the weather does on Earth, who needs an atmo-

  sphere? There is plenty of cycling and flow in the incontinent continen-

  tal crust. Superhot flows of molten rock plow into vast fields of frozen

  sulfur compounds, violently vaporizing at the margins and sending sul-

  furous plumes blasting into space, only to snow back down on the vol-

  canic surface. Could there be evolving complexity, perhaps leading to

  biological evolution at some level within that churning mass? If there is

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  any truth to the living worlds hypothesis, which posits a relationship

  between geological vigor and biological potential, then Io is one to

  watch.

  When we think about life within the moons of Jupiter, our water fix-

  ation keeps us focused on Europa. Nobody talks about life on Io. Yet,

  energetically and thermodynamically, Io is a lot more promising. The

  problem with Europa might be a lack of energetic flows to drive biol-

  ogy. If only Io and Europa could join forces, just think what could be

  accomplished with Europa’s watery conditions and Io’s heat flow.