Lonely Planets
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made Pasteur a household name.) Pasteur’s results, published in 1860,
convinced many scientists that spontaneous generation does not occur.
So then, whence came life?
Perhaps it arrived here in meteorites. During the nineteenth century,
when scientists figured out that these strange, charred lumps of rock
and metal fall from beyond the Earth, meteorites became the subject of
intense study. Since the 1830s we have known that they contain organic
matter. Some scientists, assuming that organic materials can only be
produced by living organisms, took this to be both evidence of extrater-
restrial life and the solution to the riddle of Earth life’s origins. It came
out of the sky. Riding on meteorites.
No one argued this more eloquently than William Thomson—
a.k.a. Lord Kelvin—the Scottish physicist who invented the absolute-
temperature scale that scientists use to measure temperatures above
absolute zero in “degrees Kelvin.” Thomson compared the origin of life
on Earth to the rapid blooming of a newly formed and initially barren
volcanic island. The seeds must drift in from elsewhere. He argued (cor-
rectly) that every year many tons of meteorites fall to Earth from space.
Some of these, he suggested, must be the fragments of planets once rich
with life:
“Hence and because we all confidently believe that there are at pres-
ent, and have been from time immemorial, many worlds of life besides
our own, we must regard it as probable in the highest degree that there
are countless seed-bearing meteoric stones moving about through
space. If, at the present instant, no life existed upon this earth, one such
stone falling upon it might, by what we blindly call natural causes, lead
to its becoming covered with vegetation. . . . The hypothesis that life
originated on this earth through moss-grown fragments from the ruins
of another world may seem wild and visionary; all I maintain is that it
is not unscientific.”
The idea that Earth life was seeded from elsewhere was refined and
advanced in the first years of the twentieth century by Svante
Arrhenius, a polymathic Swedish chemist who was often far ahead of
his time. He barely graduated from university in 1884, earning scorn
A Wobbly Ladder to the Stars
47
from his professors for unorthodox ideas about the electrical conduc-
tivity of solutions. These same ideas earned him the Nobel Prize in
chemistry in 1903. That year, emboldened by success to venture farther
out on the limbs where both the fruits and the dangers of speculation
can be found, he published his theory of life’s origin from outer space.
His name remains the one most closely associated with the concept of
panspermia.
Arrhenius agreed with Kelvin that life was seeded from space. He did
not think, however, that meteorites were the most likely carriers. Life
would be unlikely to survive either the violent collisions that produce
meteorites or the heating and shock that occur when these rocks fall to
Earth. Instead, he proposed that seeds were carried throughout the uni-
verse in tiny particles of dust. Noting the way a comet’s dusty tail is
blown away from the Sun by the gentle pressure of sunlight, he pro-
posed that this “radiation pressure” was the force that distributed “liv-
ing seeds” to the planets.
Arrhenius calculated that the Sun’s radiation pressure could blow
seeds from Earth to Mars in twenty days, to Jupiter in eighty days, and
to the nearest star (Alpha Centauri) in nine thousand years. He argued
that these interplanetary transit times were short enough for the seeds
to remain viable: “In this way, life would be transferred from one point
of a planetary system, on which it had taken root, to other locations in
the same planetary system, which favor the development of life.”
And what of the much longer travel times between the stars? The
exceedingly frigid temperatures of interstellar space, he suggested,
would freeze-dry the traveling seeds, preserving them to survive even
these epic journeys.
Arrhenius developed his ideas on panspermia much further in his
book Worlds in the Making: The Evolution of the Universe (1908),
which used detailed theories to argue that life develops inevitably on
numerous worlds in our solar system and others. For his sweeping syn-
thesis of astrophysics, chemistry, and biology, Arrhenius could be con-
sidered the first astrobiologist.
The idea of panspermia, like one of Arrhenius’s intrepid interstellar
seeds, is hard to kill. But panspermia does not solve the problem of
life’s origin, it just removes it from Earth. Even if life came here from
elsewhere, it still originated somewhere. Pushing it off into outer space
merely relocates the mystery.
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L I F E I S C H E M I C A L
Another take on the origin of life arrived with the new discipline of bio-
chemistry. As the extraterrestrial-life debate was waged between the
optimism of physicists and the pessimism of biologists, it made sense
that new approaches should arise from the field that bridged the con-
ceptual gap between atoms and organisms.
Proteins (organic chemical components of all living cells) were first
isolated from cellular material in the first years of the twentieth cen-
tury. Around that time many simple biochemical reactions were dupli-
cated in laboratory flasks, adding to a growing sense that life is, funda-
mentally, chemistry. This more sophisticated version of spontaneous
generation renewed hopes of finding the key to life’s origins in special
brews of chemicals native to the primitive Earth.
A chemical origin of life became widely accepted after 1936 when the
Russian biochemist Aleksandr Ivanovich Oparin published his land-
mark book Origins of Life. Oparin, no doubt influenced by the dialec-
tical materialist philosophy permeating the Moscow air, postulated an
inevitable historical process in which conditions on the young Earth
caused the molecules of life to rise up and organize out of nonliving
matter.
The prevailing view of Earth’s earliest environment at the time
included an atmosphere composed mostly of carbon dioxide (CO2),
similar to that which we already knew to exist on neighboring Venus.
Oparin argued for a very different kind of ancient atmosphere, rich in
methane (CH4) and ammonia (NH3), gases which had recently been
detected in the atmospheres of Jupiter and Saturn.
This proposed change in the early atmosphere, from CO2 to CH4 and
NH3, has a crucial effect on the social behavior of carbon atoms. In an
environment rich in hydrogen compounds such as methane and ammo-
nia, called a reducing environment, carbon atoms will tend to grab on
to each other, forming the giant carbon conga lines and group carbon
hugs we call complex organic molecules. Carbon behaves very differ-
ently in an oxidizing atmosphere richer in carbon dioxide or oxygen
(O2). The carbon is seduced by oxygen’s pull and ign
ores its own kind.
Organic molecules don’t stand a ghost of a chance.*
*Yes, it’s true. Our precious oxygen is lethally toxic to the basic molecules of life. We’ll return to this chemical irony in a later chapter.
A Wobbly Ladder to the Stars
49
Oparin described how, on an early Earth with a reducing environ-
ment, simple organic compounds formed and began reacting with one
another. This led to “chemical evolution” in which the more stable (or
“fit”) molecules hang around, accumulating and evolving further. The
result was a rich soup of chemicals that gradually increased in size and
complexity until the organic molecules essential to forming the first liv-
ing cells were abundant in the ponds and oceans of the juvenile Earth.
Origins of Life was a watershed in modern thought about life’s
beginnings, strongly influencing both astronomical and biological
beliefs about the primitive Earth for the rest of the century. Although
Oparin’s book was strictly about Earth, the theory described the inex-
orable chemical development of life from conditions believed to exist
generally on young planets. The cosmic consequences were inescapable.
In the 1930s, we knew precious little of the actual conditions on other
planets in our solar system, and even less about the primitive environ-
ments on these planets way back when life on Earth began. It seemed
probable that early conditions were similar on all planets, so Oparin’s
chemical evolution seemed like a universal life-generating theory.
In 1953, at the University of Chicago, Nobel laureate Harold Urey
and his grad student Stanley Miller realized they could test Oparin’s
thesis experimentally. Urey, one of the fathers of modern planetary sci-
ence,* created the subfield of cosmochemistry, in which we follow the
chemical forms of matter through the stages of cosmic evolution. As the
first to improve upon the nebular hypothesis using sophisticated chemi-
cal modeling, he cleverly deduced what the planets were made of when
they condensed out of the solar nebula. He concluded that the early
Earth was rich in methane, ammonia, hydrogen, and water—a picture
similar to Oparin’s.
Miller, then a beginning student, wanted to try simulating the natural
creation of organic chemicals on the early Earth. Urey was skeptical,
but he agreed to let Miller attempt a preliminary experiment to test the
concept. The setup was simple: brew up some primitive air, zap it with
simulated lightning, and see if anything happens. They mixed ammo-
nia, methane, and water in a flask and sparked it up. After a few days
they were both astounded to find their experimental flask full of an
ugly, sticky, brown goo. The gunk turned out to be made of amino
*And my “intellectual grandfather”: Urey was thesis adviser to my thesis adviser, John Lewis.
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acids—the building blocks of protein, the stuff of life! This finding far
exceeded the ambitions of their initial mock-up investigation, which is
now inscribed in textbooks as the Miller-Urey experiment.
The astonishing result suggested that unremarkable conditions and
processes on the primitive Earth would inexorably have produced the
molecules of life. Of course, that was Oparin’s original thesis twenty
years earlier, but a bird in the lab is worth two on the page: experimen-
tal proof is more convincing than the most sophisticated theoretical
conjecture. Miller and Urey had not actually created life in the lab, but
by producing life’s crucial building blocks from garden-variety chemi-
cals, they removed what had seemed a fundamental barrier to the spon-
taneous generation of life from nonlife on Earth or elsewhere. Like
Oparin, Miller and Urey did not at first discuss the extraterrestrial
applications of their work. However, ammonia, methane, and water
were known to be among the most abundant compounds in the uni-
verse. Embedded in the results of Miller-Urey was the clear implication
that the steps to life here were the result of common cosmic processes.
The Miller-Urey experiment, by establishing that the origin-of-life
question is subject to experimental inquiry, generated not only a flask
of dark, promising sludge, but a cottage industry. Investigators trying
to discover the essential early steps of life have endlessly varied the for-
mula of the gaseous brew, following evolving ideas about the primitive
atmosphere, and they’ve zapped these mixtures with all kinds of energy
that might have been present on the young Earth, including ultraviolet
radiation and simulated asteroid-impact explosions. To this day, the
resulting brown goos are eagerly analyzed like the precious elixir of life.
Just as all later theoretical discussions of the chemical origins of life on Earth or anywhere in the universe can be seen as refinements of
Oparin’s ideas, all experimental efforts in the field are variations on the
theme begun by Miller and Urey.
These rigorous scientific results returned to the study of extraterres-
trial life much of the legitimacy it had lost in the aftermath of the
Lowell affair. Just as the nebular hypothesis predicted that planets were
a natural byproduct of the formation of stars, Oparin’s theory and the
Miller-Urey experiment implied that life itself is a natural byproduct of
the formation of planets.
The Planets at Last
4
The desire to know something of our neighbors in
the immense depths of space does not spring from
Image unavailable for
idle curiosity nor from thirst for knowledge, but
electronic edition
from a deeper cause, and it is a feeling firmly
rooted in the heart of every human being capable
of thinking at all.
—N IKOLA T ESLA , 1901
Fly me to the moon
Image unavailable for
Let me play among those stars
electronic edition
Let me see what spring is like
On Jupiter and Mars©
—OSCAR HAMMERSTEIN
B E I N G T H E R E
The 1950s were our last age of interplanetary innocence. These were
the final moments of a 350-year stretch of blissful ignorance between
telescopes and spacecraft—between figuring out what the planets were
and learning what they were like. Like a first date that leaves you smit-
ten and full of hopeful fantasy, the planets were easiest to idealize when
we knew little about them.
We knew enough chemistry to believe that life came naturally to
planets with the right conditions, but we had only hints of the actual
environments on other planets. Scientists wondered in print if we
would find “astroplankton” on the Moon, or plants and animals on
the surfaces of Mars or Venus. As we were poised to enter the universe,
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our ideas about what we would find there were still greatly influenced
by wishful thinking and simple extrapolation.
The few clues we had were often interpreted to encourage hope for
nearby life, even while growin
g spectroscopic evidence implied that the
atmospheres of Venus and Mars were not at all Earth-like and suggested
that both were severely lacking in water. Many scientists were willing to
believe that this evidence was not conclusive and to suspend judgment
about life on other planets until we could go and see for ourselves.
Since the 1960s, we’ve finally been able to travel to the other planets,
sending robotic extensions of our eyes and noses to radio back their
findings. Early results threw buckets of cold water on our dreams of
comfortable, Earth-like environments with abundant life. Viewed up
close, the warm oceans of Venus dissolved into a choking sulfuric incin-
erator. The vegetable patches of Mars were mirages of windblown dust
on a frozen, sterile desert.
When I was a child, my imagination was fired up by pictures of new
worlds being explored for the first time. The images and stories of real
spacecraft exploration blended smoothly in my adolescent brain with the
worlds of science fiction. I had seen Neil and Buzz jump down a ladder
onto the bright, dusty Moon when I was in the fourth grade, so the voy-
age to Jupiter in 2001: A Space Odyssey did not seem unreal. I dreamed
of spaceships and extraterrestrials and thought about how I would be
forty in the year 2000. The Future. I imagined that one day I would travel
to other worlds, following the trail of alien life. It helped that my parents
were socially enmeshed in the Boston scientific community, and friends
like Isaac Asimov, Carl Sagan, and Fred Whipple regularly dropped by
our house with news of the latest discoveries or setbacks. I followed the
ups and downs of planetary exploration as closely as other world-
changing developments, such as Vietnam and the breakup of the Beatles.
At age eleven, in 1971, I was gripped by the drama of Mariner 9, a
turning point in our understanding of Mars. Between 1965 and 1969,
three other American spacecraft had reached the Red Planet, pho-
tographing small areas of the surface as they sped past on brief “flyby”
missions. These craft, Mariner 4, 6, and 7, had shocked and disappointed with their pictures of a Mars that looked very much like the
surface of the Moon. Hopes for life on Mars were dashed on the rocks
of an ancient, cratered landscape that looked dead as doornails. There
was no sign of recent geologic or atmospheric activity, let alone running