More specifically, life is a process that consumes energy and pro-
duces gases that are “out of equilibrium” with the rest of the atmos-
phere. These gases don’t last long because they react quickly with
their surroundings. Methane does not belong in the atmosphere of
Mars, just as oxygen doesn’t belong in the atmosphere of Earth.
Oxygen here is a product of green plants. What (or who) is producing
the methane on Mars?
When I first heard about the methane observation, I was highly skep-
tical, but at the same time I felt my pulse quicken. Could this really be
the faint breath of underground colonies of Martians?
We’ve been wrong about Mars many times. Mars is so like Earth in
some respects, and so close at hand. As if it is our only friend in a large,
empty universe, we sometimes project too much onto poor old Mars.
We are hungry for signs of life, and this hunger is dangerous. In science
the desire to find a certain answer can lead us astray. I describe a few of
these wrong turns in chapter 3 of this book, such as the late 1950s “dis-
covery” of chlorophyll—the green stuff in green plants—on the Red
Planet. This sensational announcement (in Science magazine) was
greeted without skepticism at the time because of the prevailing view
that the seasonal color changes observed through telescopes were
Foreword: It Came Out of the Sky
xiv
caused by vegetation. (We now know that they are caused by wind-
blown dust.) The chlorophyll was later discredited, its “fingerprint”
shown to be caused by compounds of deuterium (heavy hydrogen) in
our own atmosphere. Half a century earlier, the scientific community
had been briefly enthralled,* and then greatly embarrassed, by Percival
Lowell’s claims of finding intelligently designed canals crisscrossing the
map of Mars. The canals either conveniently disappeared just as our
telescopes and cameras improved enough to truly see them, or (more
likely) they were never there at all.
Given this history of false starts and retreats, we’ve learned not to
lightly declare that the Martians have at last been found. Or have we?
In the spring of 2004 the teams reporting on the methane detections all
openly speculated on underground Martian life as a likely source. Such
speculation is in part facilitated by an attitudinal pendulum within sci-
ence that has again swung toward acceptance of the possibility of life
on our neighboring planet.
The methane claim was quickly bolstered by several independent
observations. In addition to the detection by Mars Express in orbit, it
has now been seen by two different groups of ground-based observers
using some of the best telescopes on Earth, in Hawaii and Chile. So the
methane, it seems, is there. But does it really mean life on Mars?
When you actually look at the numbers, the evidence is not immedi-
ately convincing because the quantity of methane is so tiny. Seen in the
infrared, methane has a distinct and strong signature. In an atmosphere
of carbon dioxide it stands out like a blood stain in a fresh snow bank.
So, having scrutinized Mars in the infrared for decades, why haven’t we
found it before? The signal is, in fact, very weak, implying that the
methane is extremely scarce. The data suggest something like ten parts
per billion (ppb) methane in the Martian air, so for every billion mole-
cules of carbon dioxide (CO2) there are, apparently, ten molecules of
methane (CH4). That’s hardly a methane mother lode. Yet, there must
be a source. And that is the part that sets our minds spinning. On Earth
the main sources of methane are biological ones. Methane doesn’t last
long in our air either, but bacteria living in rice paddies and in the guts
of cows (for example) supply a constant trace. Could underground bac-
teria on Mars be the culprits here?
Given “Sagan’s law” that “extraordinary claims require extraordi-
* For a couple brief decades, that is.
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nary evidence,” we are obliged to search for other, more mundane
explanations before we trumpet the news (once again) of finding life.
How do we know whether we should accept, or rule out, an alternative
explanation for something as potentially important as methane on
Mars? Many times a “back of the envelope” calculation is sufficient—
an exercise in which we plug in rough but reasonable estimates for the
important quantities and see if the answer we get is in, or at least near,
the ballpark.
For example, a friend e-mailed me, suggesting that maybe the tiny
residue of methane is simply leaking from the small collection of
derelict spacecraft we’ve left on the planet. Yet this can quickly be ruled
out with the nearest envelope: given that the entire Martian atmosphere
weighs about 2.5 x 1016 kilograms, or about 25,000 trillion kilos, this
means that ten parts per billion methane, as small as that sounds, still
adds up to about 90 million kilos of methane. So, if we had 90,000
spacecraft on Mars (as opposed to about a dozen), each weighing a
thousand kilos and each composed entirely of methane gas (not a rec-
ommended construction material), then this could work as an explana-
tion. Envelopes are good for reductio ad absurdum arguments, which
tell us where not to waste our time.
A more promising possibility is volcanic venting. On Earth volcanoes
burp great quantities of methane into the atmosphere. However, Earth
is a volcanically active planet at present and Mars, overall, is not. My
favorite candidate for a nonbiological source is the steady rain of mete-
ors. Organic material falls from space all the time on Mars, as on all
other planets. We don’t know the precise rate at which this space gunk
is entering Mars’s atmosphere, but we can make some reasonable infer-
ences based on the observed rate on Earth and applying what we know
about orbits and the gravitational reach of Mars. When I put this all
together on the back of a nearby envelope, I find that the amount of
organic carbon landing on Mars each year is likely close to the needed
supply rate for the observed methane. Decaying organic matter is a
classic source of methane—think swamp gas. My oversimplified calcu-
lation tells me that the infall of meteors, and the subsequent release of
organic gas as they break down in the atmosphere, could provide the
right trickle of methane. No bugs required.
Is the methane falling from the sky? Maybe. Maybe not. With future
space missions we’ll eventually be able to measure the rate at which
meteors supply Mars with organics. Much sooner than that, we’ll have
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xvi
better observations of the methane. Where, exactly, in the atmosphere
does it reside? Is it concentrated near the surface (implying an under-
ground source)? Is it coming from specific locations? The latter is the
most important observation we can make in the near future. If we find
it venting from certain surface features, this dest
roys my “falling from
the sky” idea and gives us important clues to the conditions below-
ground that may be fostering the chemical activity—be it geological or
biological—supplying the doomed molecules.
As is often the case at scientific conferences, the most interesting dis-
cussions took place in the hallways between sessions. There was much
chatter about the hot new methane observation but little consensus on
the right interpretation. Should we take it as a sign of life? Nobody
was arguing that the idea was ridiculous. So here we had a possible
new sign of life on Mars (discovered, I was aware, in the few short
months since Lonely Planets was first published) being taken seriously
by this skeptical crowd. This stunning development heightened my
awareness of the fluid state of our ideas about life in the universe and
the breathtaking pace of discovery in the solar system.
Whether or not the methane turns out to be the breath of Mars bugs,
our attempts to explain it will certainly further our ideas about how to
detect life elsewhere. It often happens in science that our difficulty in
understanding a new observation exposes the weaknesses in our previ-
ously agreed-upon ideas, ushering in a period of confusion that ulti-
mately leads to new insights. The difficulty we are having in interpreting
the methane discovery exposes the inadequacy of the well-accepted “dis-
equilibrium equals life” protocol for identifying planets with life. How
much disequilibrium does it take to signal the presence of life? This
leads to some fascinating questions about the relationships between
planets and life. Can a planet be a little bit alive? Or is there some
threshold amount of biological activity required for a robust biosphere?
In my view, the methane is probably not a sign of life. (I’d sure love
to be wrong about this.) For reasons I describe in this book, I don’t
expect that the signs of life on a planet with an atmosphere will be fee-
ble or subtle. I believe that if life has survived on a planet for many bil-
lions of years (as it has on Earth and must have on Mars for there to be
life today), then it will have become deeply intertwined with the atmos-
phere of that world in a way that will make the atmospheres of living
worlds distinct—flagrantly distinct—from those of nonliving worlds.
As of this writing, both rovers are still crawling over new Martian vis-
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tas and calling home daily with their latest dispatches. They have both
remained remarkably healthy in the killing Martian cold, but will they
survive the coming winter? Spirit has developed a bum wheel, and
Opportunity is tempting fate with daring exploits in a scary-looking
crater. They may roam for another year, or die in the coming weeks, but
either way they have, by any measure, far surpassed our expectations. I
wish them well.
Up above, in Martian orbit, Mars Express, Mars Global Surveyor,
and Mars Odyssey are all still operating, and plans are being drawn up
for a more ambitious Mars rover mission, to launch in 2009 and search
more directly for signs of past organic life. It’s a busy time on Mars,
and—life or no life—we need to keep exploring. Mars seems to be
telling us that it once had conditions in which living creatures could
have thrived. So how could we not go and look for fossils? Can you
imagine—the chance to study the traces left by evolution on another
world and compare them with the shells and bones of Earth?
Meanwhile, the Cassini spacecraft has just entered Saturn orbit,
returning its breathtaking first close images of the mighty rings, with
their picture-perfect waves and ripples. These gorgeous patterns look
so much like mathematically simplified computer models that I am
now completely convinced that the Pythagoreans were right: God
is math. In October 2004, Cassini will make its first close encounter
with the enigmatic, organic-rich moon Titan. Later, it will release the
Huygens probe, which, in January 2005, will descend through the
clouds and crash (or splash) onto Titan, photographing and sampling
the air all the way down. Titan, as I describe herein, is a long shot for
some unknown kind of extreme cold–adapted life, but a safer bet as a
place that will teach us more about organic evolution in an environ-
ment that in many ways resembles the young Earth on the eve of life.
Back on the home planet, recent outbreaks of UFO sightings have
been reported in Iran and Mexico, but nobody outside the world of
dedicated “UFOlogists” seems convinced. Our radio searches for signs
of intelligent life continue to grow in power and reach, but as yet the
aliens have not called in.
Most discussion of life elsewhere focuses on the possibility of finding
simple, microbial life. Yet SETI (Search for Extraterrestrial Intelligence)
was also represented there under the astrobiology big top. Seth
Shostak, from the SETI Institute, reported on the completion of Project
Phoenix just two weeks before our conference, which listened to the
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xviii
radio emissions from 750 stars between 1995 and 2004, finding, alas,
only noise and no signals. He pointed out that since Earth has been
“leaking” radio and television signals for sixty years now, there are a
thousand stars that are close enough to have noticed us and responded
by now. None have. Perhaps this doesn’t tell us much. Our galaxy
alone has some hundred billion stars, so this sample represents less than
a millionth of a percent. At least we know that not every single star in
our galaxy is occupied by creatures who instantly answer any faint
radio signal with a powerful reply, though in our case this could be due
to the content of our programming.
Shostak was upbeat. SETI researchers are famously vague when it
comes to making predictions of success. The standard line is that we
could hear something any day, or it could take centuries or millennia,
but we should keep on searching, because we can. So it came as a sur-
prise to hear Shostak make a much more specific prediction—that SETI
will succeed within twenty years if it is to succeed at all. Given the
exponential increases in our listening power, he suggested, within two
decades we will have the capability to search for signals from a large
fraction of the stars in our galaxy. So, he said, we are certain to find
them soon if they are there at all. It is always refreshing to hear some-
thing new at a SETI meeting, but I found myself wondering if this pre-
diction doesn’t somehow give us too much credit.
Even farther out there (in a good way) were the speculations of
Steven Dick, NASA’s chief historian, who raised the intriguing, perhaps
disturbing, possibility that we may live in a “postbiological” universe,
where extraterrestrial intelligence need not imply extraterrestrial life.
This could come about if, on most planets, biology gives way to
machines that outlive, outthink, and outevolve their slimy organic
pre-
cursors. Dick suggested that it may be the machines who inherit the
universe, and that in most places this may already have happened. Does
intelligent life always cede its future to intelligent machines? Does it
always seek out other life? Or does it usually destroy itself in an orgy of
shortsighted technological cleverness? Is it always compelled to move
beyond its home planet to colonize other worlds?
Questions about the behavior of intelligent life elsewhere inevitably
lead back to questions about our own nature and future. Our evening
debate on the wisdom and feasibility of the future terraforming of
Mars led back to discussions of environmental ethics and the human
role on Earth. What responsibilities, to any indigenous life-forms and
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to the life of Earth, including our descendants, do we carry with us as
we explore Mars and contemplate going there to live? If we purpose-
fully alter Mars to increase its ability to support life, will it be a dese-
cration or a restoration, a salvation or a contagion? Do we display
obnoxious hubris even to ask the question?
Joining us on the terraforming panel, virtually at least, by satellite
hook-up from his home in Sri Lanka, was one of the science fiction
heroes of my youth, Arthur C. Clarke. Unfortunately, the satellite connec-
tion wasn’t working very well, and the comments of Sir Arthur (who, by
the way, invented the communications satellite) were almost completely
unintelligible. We all would have liked to hear what Arthur had to say
on the matter, but his main role for the evening became one of providing
amusement to the audience by interrupting the rest of us at awkward
moments with little electronic bleeps, blurps, and word fragments, like
some intermittent alien signals tantalizingly close to pure noise. Perhaps
somebody up there was having some fun, reminding us that for all our
talk about future high-tech wonders, our early twenty-first-century tech-
nology is still full of bugs. Why worry about terraforming Mars when
we can’t get a reliable phone connection between Sri Lanka and
California? They can put a man on the moon but . . . In fact, during the
entire evening, the only thing I was sure I clearly heard Clarke say about
terraforming was, “Well, I think we should ask the Martians first.” I
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