Lonely Planets

by David Grinspoon

  lovers.

  So Earth was born a burning sauna with the thermostat jammed into

  the red zone—a tumultuous, noisy place with mountains continually fall-

  ing from space at supersonic speeds, spraying showers of molten metal

  and rock, which then rained back down through the suffocating sky.

  Finally, about 100 million years after the beginning of planet forma-

  tion, the primordial pounding from space began to subside, depriving

  the steam atmosphere of its sustaining energy source. The surface

  began to cool, and the steam to condense.

  It rained. For a thousand years it rained, filling Earth’s oceans for the

  first time.

  Peace on Earth at last, or so it would seem.

  However, when Earth formed out of smaller bodies, the finish was

  not tight, but scattershot, like an unrehearsed or wasted band trying to

  end a song. The gathering of Earth tapered off gradually and irregu-

  larly. Toward the end, after the first rains came, there were quiet spells

  during which the planet was spared the trauma of large impacts. The

  carbon-rich ocean settled down to begin the fragile dance of organic

  evolution. Repeatedly, these placid periods were rudely punctuated by

  late hits from space that shattered the calm and plunged the world back

  into chaos. The last few giant impacts took a huge toll. An impactor

  more than two hundred miles in diameter falls to Earth with enough

  energy to boil off the entire ocean and heat the planet’s surface to thou-

  sands of degrees. There were probably four or five of these impacts

  between 4.4 and 3.8 billion years ago, each returning Earth to steam,

  endangering any complex organics that had, during the interregnums,

  made tentative steps toward life.

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  Several times the world was lulled into an apparent cease-fire, and

  began the chemical climb toward life, only to be thrust back into hell.

  Oceans formed and were blasted back into steam, which then con-

  densed and fell again in thousand-year rains. No one knows how far

  chemical evolution progressed in the many calm periods between. Life

  may have gotten started more than once, only to be destroyed by mas-

  sive impacts. If life did start many times, it died out every time but one.

  This we know, because we are such close kin to all life on Earth. The

  biochemical evidence shows clearly that all Earth’s diverse organisms

  are related and stem from a single origin. We are family. Bugs, slugs,

  and us—we all share a single origin that occurred sometime around

  4 billion years ago, soon after the last of the mighty, ocean-boiling

  impacts.

  Do you ever wonder what might have happened if you had met your

  true love too soon, or too late? Though the thought is disconcerting,

  sometimes the most important things in life come down to luck and

  timing. The same is true of the life of our planet. Since life may have

  started several times, only to be snuffed out by massive impacts, we

  may owe some deeply ingrained features of our basic biochemical

  machinery to the random timing of the last giant collisions. Maybe

  some of life’s earlier doomed experiments were quite promising. If fate

  had allowed one of these other beginnings to inherit the Earth, then

  evolution on Earth could have proceeded along a very different path.

  Perhaps large animals and intelligence would have come along after

  only 1 billion years, instead of 4 billion. By now we could have been off

  roaming the galaxy, with Earth a fond but distant memory. Or, perhaps

  life would have evolved much more slowly, and we would never have

  made it beyond the bacterial stage in the entire 10-billion-year life of

  our Sun.

  H O L D I N G W A T E R

  There is still some mystery about the source of Earth’s life-giving

  oceans—the ultimate headwaters for our rivers and seas. Where in the

  solar system was the water before it landed here? Was it locked up in

  the rocky bodies, orbiting in Earth’s vicinity, that smashed together to

  form the bulk of the planet, escaping as steam when the world got

  large? Did it arrive frozen in comets, plunging headlong from the cold

  fringes of outer planetary space, each exploding upon crashdown like a

  Earth Birth

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  snowball shot into a blast furnace? Or did it waft gently down to Earth,

  stowed away on tiny grains of interplanetary dust?

  These questions about planetary water have been an obsession of

  mine since grad school: where it comes from, where it goes, what kind

  of planets end up with it, and what it does to climate, geology, and biol-

  ogy. Why do we care where it came from, since we know we got it?

  Because water is key to so much of what makes Earth Earth. Water

  erodes the mountains, keeps plate tectonics* chugging smoothly along,

  and governs global climate. Most important, water is essential for life as

  we know it. The same triangular molecule—a tricycle with one big oxy-

  gen and two little hydrogen wheels—also plays host for the 3-D carbon

  dance party that carries our genetic memory down through time, com-

  poses our bodies and, perhaps, forms our every thought. Understanding

  how the planets of our solar system have gained and lost water can help

  us nail down the conditions that other planets need to come alive.

  After studying this problem for a couple of decades, I’ve concluded

  that getting enough water was not the problem. Water was everywhere

  in the early solar system. The tricky part for any young planet is hang-

  ing on to it. The more we learn about planet formation, the more it

  seems that the planets must have all lost large amounts of water during

  their earliest phases. Especially when most of the water was exposed in

  a hot, extended steam atmosphere, it would have been vulnerable to

  getting stripped away to space in several different ways.

  The Sun, in its wild youth, radiated furiously at energetic far-

  ultraviolet wavelengths, heating Earth’s upper atmosphere and causing

  a steady breeze of hydrogen to blow back into the interplanetary void.

  No hydrogen, no water. The late phases of the Earth’s assembly must

  also have taken their toll. Explosive impacts can blow bits of a planet’s

  atmosphere right out into space. When Earth was a steaming lava ball,

  its water supply was susceptible to these losses.

  Later, after the rains, the remaining water was in much less danger of

  being lost to space. Once it was condensed into surface oceans, Earth

  could keep its precious water closer to its chest. Good thing, or our life-

  giving oceans could have gone the way of those of Venus or Mars, a

  topic I’ll soon get back to.

  *Earth’s outer, solid “Lithosphere” is broken into about a dozen plates which drift around, driven by internal heat, making mountains, causing earthquakes, and recycling the planet’s surface. We call this activity plate tectonics.

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  S E T T I N G T H E S C E N E

  Wherever the water came from, Earth was awash in it from the start.

  As the sterilizing steam rained down for the last time, the curtain
rose

  on a new drama. What was it like here when our planet first became

  infused with something that could be called life?

  The young Earth was heavily cratered and largely covered in oceans.

  The circular rims of the largest craters formed rugged mountain arcs

  jutting sharply from the sea. Still reeling from the mighty collision that

  had made the Moon, Earth spun rapidly, completing a day and night in

  about five hours.* A dim Sun and a nearby, looming Moon bolted

  across the sky. The atmosphere that remained after the steam con-

  densed was largely composed of carbon dioxide.

  Baby Earth needed all the help she could get to keep warm, as several

  factors conspired to threaten a deep freeze. In its early days, the Sun was

  much dimmer than it is now. Throughout its life, our star has gradually

  been brightening as the hydrogen in its core burns slowly to helium,

  increasing the Sun’s density, and requiring a hotter nuclear flame to fight

  off gravity. If the modern Sun is a hundred-watt bulb, at the time of

  Earth’s formation it was only about a seventy. This wimpy Sun could not

  have kept Earth very warm without help. If the greenhouse effect had

  been as feeble then as it is now, the oceans would have completely frozen

  over. Earth would have been the solar system’s largest skating rink, and it

  is questionable whether life of the kind we know would have formed.

  The earliest known rocks, some 3.8 billion years old, are constructed

  from deposits of water-borne sediments, betraying the presence of a liq-

  uid-water cycle well before that date. Clearly, something was keeping the

  young Earth warm against the weak Sun and the frigid vacuum of space.

  Given that our neighboring planets Venus and Mars have atmo-

  spheres of nearly pure carbon dioxide, the consensus these days is that

  the early Earth also had a thick atmosphere consisting mostly of CO2.

  As you well know from presidential debates and cereal boxes, CO2 is a

  greenhouse gas that warms a planet. Back then, the dreaded greenhouse

  effect was a good thing† that efficiently trapped the weak sunlight,

  keeping Earth cozy and warm. Global warming to the rescue.

  *The moon itself, dragging on Earth through the tides, has gradually slowed us to our current twenty-four-hour day.

  †It still is, actually. Without it we wouldn’t be here. But anthropogenic global warming could quickly cause too much of a good thing.

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  A weak young Sun was not the only problem for a newborn Earth

  struggling against the elements to stay warm. Making matters worse

  were the thick clouds of obscuring dust constantly lofted high into the

  air by the continuing bombardment. Though the early, fearsome ocean-

  vaporizers, the two-hundred-mile-wide monsters, were finally gone

  from the inner solar system, a steady rain of lesser impacts continued to

  pepper the planets. Many of these were big enough to cause severe

  environmental changes.

  An object only a few miles across falling from space will raise enough

  dust to darken the skies globally for a couple of years. When this hap-

  pens, most incoming sunlight is absorbed by dust in the upper atmo-

  sphere and the surface grows dark and cold. Even now, such events still

  happen every 10 to 100 million years. The last big one was the “K/T

  impact” 65 million years ago, which did in the dinosaurs.* Long ago,

  when the solar system was still sweeping up the mess from planet for-

  mation, impactors came much more frequently. Between 4.3 and 4.1

  billion years ago, several objects this large were hitting Earth every cen-

  tury, causing enormous temperature oscillations at Earth’s surface. A

  thick cloud of light-absorbing dust intermittently shrouded our planet.

  How can we be so sure about all this? After all, we’re talking about a

  time that is, as I’ve already admitted, older than any preserved surface

  on the planet, and older than the oldest Earth rock ever found. Aren’t

  we just guessing? Nope. Fortunately a well-preserved record extends

  back to this time. You can see it with your own eyes on any moonlit

  night.

  Our nearest neighbor has not had nearly as interesting or eventful a

  life as has Earth. While Earth’s surface has continually been remade by

  mountain building, weather, and life, destroying all traces of the earliest

  rocks and landscapes, the Moon is dead. Geologically, meteorologi-

  cally, and biologically, it’s dead. It just sits there, passively taking what-

  ever space tosses its way, never washing its face with rain or regenerat-

  ing its skin with plate tectonics. The moon’s entire surface is much,

  much older than any place on Earth, and it has been getting shot up

  with craters for billions of years. Consequently, its pockmarked face

  preserves a record of the intense bombardment that hammered both

  Earth and Moon when remnants of the preplanetary swarm still men-

  *The “Cretaceous/Tertiary impact event” which ended the Cretaceous geological age and ushered in the Tertiary.

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  aced the inner solar system. The Moon serves as a cosmic rain gauge,

  recording the environment of near-Earth space back to circa 4.1 billion

  years ago. (Before that, the cratering itself was so vigorous that it con-

  tinually obliterated all traces of earlier surfaces, so we have no direct

  record of the bombardment rate for the first half billion years of our

  planet’s existence.)

  The pockmarked face of the Moon tells us, without a doubt, that

  conditions on Earth’s surface were dominated by the effects of explo-

  sive impacts right up to the time when life here first got started. The

  entire globe oscillated between periods of freezing dark gloom and hot-

  ter spells when the skies cleared and the surface was bathed in intense,

  deadly solar ultraviolet irradiation (the protective ozone layer would

  not be invented for billions of years).

  These surface conditions were not healthy for children or other living

  things. For this reason, we think that life may have originated deep

  underground, or at the bottom of the oceans, places that provided nat-

  ural fallout shelter from the cosmic bombs still wreaking havoc at the

  surface. A currently popular location for life’s origins is at hydrother-

  mal submarine vents on the ocean floor. Plenty of chemical energy was

  supplied by the hot, mineral-rich waters pouring out of these vents, and

  the deep ocean was relatively immune to the extreme environmental

  hazards plaguing the surface at the time when life seems to have gotten

  its start. As the impact storm raged above, the first glimmerings of life

  on Earth may have been safe and warm below the storm in an octopus’s

  garden beneath the waves.*

  *If life can get started at the bottom of a planet’s ocean, caring little about hazards plaguing the surface environment, this could have interesting implications for life beyond the Earth. Keep this in mind when, in a few chapters, we return to the question of life on Europa, Jupiter’s oceanic moon.

  Life Itself

  7

  What do they call it . . . the primordial soup? The

  glop? That heartb
reaking second when it all got

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  together, the sugars and the acids and the ultravio-

  electronic edition

  lets, and the next thing you knew there were tanger-

  ines and string quartets.

  —EDWARD ALBEE, Seascape

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  All is but a woven web of guesses.

  electronic edition

  —XENOPHANES

  W H A T I T I S

  Speaking of life, I suppose I’ve danced around the question long

  enough. What is it? I was hoping you wouldn’t ask.

  Next question?

  Okay, how about this? Life is something that eats, grows, repro-

  duces, and evolves.

  Except for when it isn’t. There is no airtight definition. Paradoxical

  exceptions are easy to find. What about a mule, or a cat that has been

  “fixed”? They can’t reproduce, so does that mean they aren’t alive?

  Certainly their individual cells are performing the functions listed

  above. So, if you are sterile, your cells are alive but you are not?

  Perhaps you are alive if you are made of living subunits. If that’s the

  case, where do we draw the line? Is the Earth alive? How about the

  universe?

  Further, individuals do not evolve (not in the biological sense). Yet no

  being alive today could ever have existed without a long evolutionary

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  saga written in the lives and deaths of countless generations of ances-

  tors. It seems that aspects of the above definition do not apply to indi-

  vidual life-forms, but work well for the larger continuum of life. In fact,

  the existence of a living organism requires the existence of a larger bio-

  sphere. If there is no such thing as an individual organism independent

  of a biosphere, then perhaps we don’t need a definition of life that

  works for individuals.

  If we just say that life is something that eats, excretes, and makes

  more of itself, then you could say that a forest fire is alive. Perhaps it is,

  a little bit. But, can something be a little bit alive, or would that be like

  being a little bit pregnant? Intuitively, it seems like an all-or-nothing

  deal. Alive or dead, with no in-between.

  Try this: Life is a self-perpetuating, self-contained chemical phenom-

  enon that extracts or manufactures high-energy nutrients from its envi-