Mariner 9 revealed Martian surface features that were clearly carved by
flowing liquid. Mars was again teasing us with an almost Lowellian
glimpse of a warmer, wetter past, and daring us to fantasize about
Martians who had somehow survived the great drying. The Viking pro-
gram of two orbiters and two landers, which arrived at Mars in the
bicentennial summer, 1976, was sold, mostly, as search for life.
V I K I N G S O N M A R S
With Viking, exobiologists finally had a chance to try out some of the
life-detection experiments they had been developing since the early
1960s. The most precious cargo—the essential core of Viking—was the
biology instrument: a thirty-pound cube about the size of a microwave
oven. It was one of the most complex machines ever built: a miniature,
self-contained, automated laboratory complete with sample chambers,
nutrient containers, canisters of radioactive gases, mini grow lights,
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valves, and Geiger counters. That little cube contained about forty
thousand parts.
A mechanical sample arm scooped Martian soil into the biology
instrument, which contained three separate experiments. One was Gil
Levin’s Labeled Release experiment, which tested to see if Martian
microbes would eat our offerings of radioactively labeled nutrients.
These “nutrients” were a broad mixture of organics including amino
acids, nucleic acids, and vitamins—the kind of glop you could buy
today at Vitamin Cottage. This Martian health food, jokingly called
chicken soup, seemed so good that any organic Martians would not be
able to resist a taste.
The second experiment was Gas Exchange, in which wet nutrients
were mixed with soil and any gases given off were analyzed so that the
tiniest breath or belch of a growing microbe would be detected. The
third, Pyrolytic Release, exposed soil to gases with labeled carbon, then
heated the sample to a high temperature, essentially cooking any life
within,* and sampling the gases given off to see if any of the labeled
carbon had been eaten by some form of life.
One aspect of these experiments seems strange in hindsight. Except
for Pyrolytic Release, they were designed to find life that would thrive
in Earth-like conditions, rather than Martian conditions. They were
“wet experiments” using liquid water to encourage growth. Because
liquid water cannot exist under Martian surface conditions, the sam-
ples had to be heated and pressurized to get the water to stick around.
Why go to Mars and look for something that can only survive in a
warmer, wetter climate? The designers of the experiments were influ-
enced by the idea that Mars goes through periodic climate changes. We
know that the Martian spin axis wobbles over long timescales, but we
really don’t know how dramatic the climate swings are. The theory was
that if Mars oscillates between its current severe climate and much
more moderate conditions, it might have creatures that can persist in
dormant form, for tens of thousands of years if necessary, holding out
for the return of warmer, wetter days. The Viking experiments simu-
lated the Martian conditions expected during such a thawing. As Sagan
put it in The Cosmic Connection, Mars could harbor “sleeping beau-
ties, awaiting a somewhat wet kiss from Viking.”
*Pyrolysis is a technical, scientific term for “burning the crap out of something.”
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In addition to the three experiments in the biology instrument, the
search for life was aided by the cameras and an instrument called the
GCMS, for Gas Chromatograph Mass Spectrometer, which searched for
traces of organic molecules in the Martian dirt. Cameras carried one
important advantage: the only assumption they required about the nature
of the life there was that it should not look exactly like rocks or dirt. The
disadvantage, of course, is that most life we know of is microbial, so it’s
possible to land on a planet that is thick with life and not see any of it.
Sagan successfully agitated for photographic sequences optimized to
search for moving creatures. Carl actually hauled snakes and turtles out
to the Great Sand Dunes National Monument in Colorado and pho-
tographed them crawling across the sand with the spare Viking camera,
to practice for the possibility of finding Martian “macrofauna”—large,
visible organisms. Shortly after this, while my parents and I were visit-
ing Carl’s Ithaca home, he pulled out his new pictures of desert animals
bizarrely smeared by the slow-motion camera and showed us how the
distortion revealed the speed and direction of motion. If Viking did see
turtles, snakes, or, as he liked to say, “silicon giraffes,” Carl was not
going to be caught unprepared.
The Viking cameras worked wonderfully. Seeing those pictures mate-
rialize in real time, strip by strip across a screen, was unforgettable. Just
thinking about it brings me right back to being sixteen and in love with
space. But the Vikings never did catch sight of anything crawling
through the dust.
The instrument that, in the end, seemed to most surely rule out the
presence of life was not part of the biological package or the cameras. It
was the GCMS, which gave us the ability to detect even trace amounts
of organic molecules mixed into the dirt. At each of the two Viking lan-
der sites, separated by forty-two hundred miles, it failed to find any at
all, down to the parts per billion* level. That incredible, disconcerting absence of organics at the two Viking lander sites was damning for life
on the entire planet. The fitful winds of Mars are constantly blowing
dust around the globe, so Viking sampled much more than just those
two sites. A planet with life anywhere, at least life that resembles ours
at all, should, it seems, have some organic material spread around its
surface. Mars has, to a close approximation, none.
*Pronounced “bill yun.”
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The Viking results at first seemed puzzling and ambiguous. For each
of the three biological experiments, the criteria for a positive
response—meaning that there was life—was established in advance.
Such an approach is necessary to keep us honest. If we didn’t have
agreed-upon requirements for success, then it would be easier to twist
the results to reach a desired conclusion. Ironically, all three experi-
ments in the biology package produced positive results at first. Viking
detected life according to these previously determined criteria!
When fed our “chicken soup,” something in the Martian dirt imme-
diately began exhaling labeled carbon dioxide. The Geiger counters
went crazy, but then quickly settled down. This created some brief
excitement, and a media feeding frenzy. Yet, ultimately, when the red
dust settled, the results as a whole didn’t add up to the signature of life
so much as an unexpected kind of chemistry in the soil. At least we
think we can distinguish a biological growth pattern f
rom the pattern
of a chemical reaction that just burns up the available fuel and then
stops. Further, if you first heated the Martian soil to killing tempera-
tures (320°F) and then repeated the experiments, you still got positive
results. That doesn’t seem like the response of living organisms. So,
unless the life there is so different that we just don’t know how to
approach it, Viking found no life on Mars. A highly oxidizing surface
was considered the most probable explanation for all of the Viking
results, though this consensus conclusion is not by any means proven.
Gil Levin still shows up at astrobiology meetings, insisting to anyone
who will listen that his Labeled Release experiment did indeed detect
microbial life in the Martian soil.
Norman Horowitz, a veteran exobiologist and lead investigator for
the Viking Pyrolytic Release biology experiment, wrote in To Utopia
and Back, his poignant book about the Viking search for life, “The fail-
ure to find life on Mars was a disappointment, but it was also a revela-
tion. Since Mars offered by far the most promising habitat for extrater-
restrial life in the solar system, it is now virtually certain that the Earth
is the only life-bearing planet in our region of the galaxy. We have
awakened from a dream.”
The Viking biology experiments illustrated just how hard it actually
is to search for life on another world. In retrospect the experiments
might seem naive. But let’s face it, any exobiology investigation will
always be a bit of a stab in the dark. Most of them will fail, until some-
day, one succeeds spectacularly. It takes courage to so publicly, and
Exobiology: Life on the Fringe
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expensively, put your ideas to the test. Viking now seems like a rite of
passage we had to go through to bring our thoughts about life in the
universe to a new level of sophistication. The next time we send experi-
ments specifically meant to search for life somewhere, be it Mars,
Europa, or elsewhere, we’ll try to cast a wider net.
In the aftermath of Viking, there was another backlash, and throughout
the 1980s and early 1990s exobiology persisted as a fringe element within
planetary science, tolerated but not tightly embraced. It took a while for
it to again become permissible to talk about life on Mars. But, clearly, we
can’t give it up for long. Soon, the question became, how can there be life
that would not have shown up in Viking’s instruments? Of course there
are answers. In fact, after any negative test for life, we can always ask,
“What kind of life might there be that we would have missed?” and we
will always be able to come up with answers. What if Viking had only
scratched the sterile surface of a world brimming with bugs who stay a
few feet underground, hidden from the deadly radiation, the oxidizing
surface chemicals, and Viking’s sample arm? There might even be some
underground hot springs where life is beautiful all day long. The idea was
born that life can persist, hiding out in isolated oases. This idea still dom-
inates contemporary discussions of life on Mars.
U N C L E C A R L A N D T H E L I T T L E G R E E N M E N
One who was never afraid to keep pushing exobiology as a reason for
planetary exploration was Carl Sagan. By the late 1960s he was gaining
prominence as both a scientist and a popularizer, spreading his vision of
modern astronomy infused with cosmic philosophy (stripped of its
explicit spirituality). Carl promoted planetary exploration and the
search for life with the passion of a true believer. In 1971, Time maga-
zine called him “exobiology’s most energetic and articulate spokesman.”
Throughout exobiology’s fickle fortunes in the 1960s and 1970s,
Sagan stood firm in his belief that biological concerns were central to
both public and scientific interest in space exploration. His aggressive-
ness and skill as a communicator kept exobiology in the face of the
planetary community during the years when many hoped it would just
go away. He reminded generations of planetologists that life is really
why we care.
Carl helped planetary science and exobiology erase the Lowellian
stain, but he somehow managed to generate a backlash of his own
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within science. College professors would take cheap shots at him in
class. At Brown in 1981 my cosmology professor loved to point out in
his lectures that he thought Carl’s science was all wrong. This Sagan-
bashing among scientists began after The Cosmic Connection became a
best-seller and peaked after the Cosmos television series aired in 1980.
There were several reasons for this. Popular works, no matter how
innovative, creative, or substantial they might be, are suspect in the
academy. There is a regrettable but pervasive attitude that any work
that can be understood by a nonscientist is a waste of time for profes-
sionals. There was also the unavoidable jealousy engendered by
celebrity status in a world where teachers are criminally underrewarded
and thinkers think themselves underappreciated. There was resentment
toward someone who had become a multimillionaire popularizing dis-
coveries that were often the result of someone else’s hard work. But
undoubtedly much of this scorn was simply the price for embracing
exobiology.
At times, Sagan took a fair amount of slack from his colleagues, even
as his visibility and popularity among the wider public soared. Many
who have now enthusiastically embraced the new astrobiology move-
ment were among those who snickered at Carl and dismissed exobiol-
ogy as too flaky to be considered real science. Once, at a conference in
Silicon Valley in the eighties, we were sitting at a table during a coffee
break: me, Carl, his graduate student Chris Chyba, and scientist X—a
prominent and accomplished planetary theorist. Carl expressed enthu-
siasm for the exobiological implications of Dr. X’s new results. X said,
rolling his eyes, “Well, Carl, I like little green men as much as anyone,
but . . . ” and went on to give his more sober assessment of the true
importance of the work, which had nothing to do with exobiology.
Now X has become a recognized leader in astrobiology.
Sagan’s experience was emblematic of the scientific ambivalence
toward exobiology. Scientists enjoyed the public interest but feared the
potential for ridicule. Nobody wanted his research to be seen as a
search for “little green men.” At planetary science conferences the
occasional presentation about exobiology was sometimes met with gig-
gles in the back of the room. Most scientists who dabbled in exobiol-
ogy were careful to cover their asses (and their research assets) by also
doing some more “serious” science. In NASA’s official statements of
goals and objectives, looking for life was always mentioned as one goal,
but never as the goal.
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Students excited about the search for life quickly learned that this
was not the way to gain the respect of established scientists. When I
was in grad school, which wasn’t all that long ago, exobiology was
semitaboo. I felt pushed and pulled in different directions. Sagan urged
me to take my work in the direction of exobiology, but some of my pro-
fessors at Arizona took a dim view of this. When I told Carl that I was
doing some modeling of the effects of “impact-generated dust clouds”
(dark dirt splashed up into the atmosphere when big rocks land from
space) on the early Earth climate, he encouraged me to consider the
implications for the origin of life. When Carl was excited about some-
thing you were doing, it was impossible not to respond, and we ended
up collaborating on a study of large impacts and the conditions for
early life. I would write up my results and send them to Carl in draft
form. A month or two later I’d get my draft back, marked up in Carl’s
precise, minuscule handwriting, contributing everything from persnick-
ety punctuation points to paragraphs of precise prose. We concluded
that the early Earth, around the time of life’s apparent origin, was fre-
quently shrouded in a cloud of dark dust, which would have kept the
surface frozen much of the time.
In this kind of science you have to make some fairly crude assump-
tions, because there is so much we don’t know about the environment
of the early Earth. The payoff is a new and interesting conclusion about
the environment where life first emerged. Much of exobiology (and
now astrobiology) depends on assumptions that are hard to test, and
which either lead you astray or lead you to some new answers on the
really big questions.* Depending on your attitude, this kind of science
is either flaky or cutting edge.
One time I got up my courage to ask the Great and Powerful
Professor Don Hunten—a leader in the theory of planetary atmo-
spheres—for his opinion of some calculations that I had done for an
upcoming talk. It was to be one of my first invited talks, at an interna-
tional conference in Helsinki, and I wanted to make sure that I had
done things right. The topic was “The Effect of Asteroid and Comet
Impacts on the Early Terrestrial Environment,” and Sagan was my
coauthor.
*You know, like how do you pronounce Io and Uranus, and is the hokeypokey really what it’s all about?
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