water or vegetation. These early missions also confirmed that Mars has
The Planets at Last
53
only a wisp of an atmosphere and is far too cold for liquid water. The
polar caps were made not of water ice but of dry ice (frozen CO2).
Thus even while humans stepped onto another world and Clarke and
Kubrick carried our imaginations out to “Jupiter and beyond the infi-
nite,” hope for an inhabited Mars was at an all-time low. But, the three
early Mariners had photographed only 10 percent of the surface up
close. Perhaps there was still room for surprise. However, the general
vibe in the early seventies was that Mars was old and dead like the
Moon, an eternally lifeless place.
All hopes for life on Mars, and Martian exploration in general, rested
with Mariner 9, which was to be the first human-built spacecraft to orbit another planet. Mariner 9 promised, if it worked, to expose the nature
of the Red Planet definitively by photographing the entire surface.
Mariner 9 reached Mars, entered orbit as intended, turned on its
cameras, and saw . . . absolutely nothing! Mars was not ready to
divulge his secrets quite yet and had chosen to shroud himself in a
global cloud of obscuring dust. Mars has a habit of working itself into
a tizzy of violent winds and thick dust clouds that encircle the entire
planet every few years, but the global dust storm that greeted Mariner
9, the “great dust storm of 1971,” was one of the most intense we’ve
ever seen, causing some to wonder if Mars was hiding something.*
Slowly, after many weeks, the dust began to settle and Mars revealed
itself from the top down, with the highest mountains peeking first
through the settling pall. The first features to appear—four huge dark
spots near the equator—gradually emerged as gigantic volcanoes. The
largest of these, which turns out to be the largest volcano anywhere in
the solar system, was named Olympus Mons—Mount Olympus, the
home of the Greek gods. As the dust cleared further, a new Mars was
revealed: not the uniform dead world seen by the early Mariners, but a
complex, varied planet with vast, jagged canyons dwarfing any similar
features on Earth; wide volcanic plains covering much of the northern
hemisphere; polar caps ringed by intricate layered terrain; and what
appeared to be large networks of dried-up river valleys covering much
of the ancient southern highlands.
Parts of those antediluvian southlands are devoid of features other
than craters, at least if you don’t look too closely. By sheer dumb luck,
all the Mariners of the sixties had completely missed the most interest-
*The face!
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ing features on Mars. This experience taught us a lesson about the dan-
gers of drawing global conclusions from incomplete coverage.
I remember Carl Sagan showing up at our house with glossy prints of
brand-new Mariner 9 images and kvelling over them proudly as if they
were baby pictures. I caught his enthusiasm like an incurable disease.
My parents let me tack one of these pictures up in my room, and
though it has yellowed a little, I still have it. It shows the great volca-
noes just emerged from the dissipating global cloak of dust—an image
full of the promise of continued revelation.
The Mars of Mariner 9 is, in many ways, the Mars we know today. It
is not a dead world like the Moon, or a living world like the Earth, but
caught somewhere in between. Though many parts of its surface are
heavily pockmarked with craters, revealing billions of years of geologic
inactivity, ancient floods have also left their mark. The atmosphere,
thin as it is, supports vigorous weather and continued erosion by wind-
blown sand. Breezes blow and seasons change.
Mariner 9 also gave us strong hints of past climate change on Mars.
The ancient valleys appeared to have been carved by rainfall, but no rain
can fall in today’s thin, frozen air. When the rivers ran, the atmosphere
must have been thicker and warmer. Why did it change, and what hap-
pened to all the water? Mars, it seemed, started out more like the Earth,
but had somehow gone cold and dry (shades of Percival Lowell). Might
Mars and Earth have been similar long ago, when life was getting started
here? If so, perhaps life sprang up on both worlds. Given the impressive
ability of evolution to adapt to changing environments, might Mars still
support some kind of life? This new hope spawned by Mariner 9 gave us
the lift needed to launch the Viking program.
Viking was the most ambitious and expensive planetary exploration
program to date. It consisted of two orbiters and two landers—all suc-
cessful. All four spacecraft were crammed with scientific instruments,
but the centerpiece of the program, the raison d’être of the missions
and ultimate source of their lavish funding, was the search for micro-
bial life in the Martian soil. Each lander carried a package of three biol-
ogy experiments.
The Viking landers set down in the summer of 1976. Along with
other space-heads the world over, I was transfixed by the first pictures
materializing on TV monitors. My teenage friends and I were at least
briefly distracted from sex, drugs, and rock ’n’ roll as the panoramic
photos of dusty, rock-strewn, dune-filled landscapes gave us our first
The Planets at Last
55
Image unavailable for
electronic edition
good sense of what it might feel like to stand on the surface of another
planet, gazing at the horizon.
The Viking cameras and almost all the other instruments worked
flawlessly. The mission was amazingly successful and greatly enriched
our knowledge of the atmosphere and surface of Mars. But the biology
experiments were a bust. Though some early, puzzling readings pro-
vided brief, exciting moments of hope, the sum of all the results was
convincingly negative: there is no life on Mars. At least no life that we
knew how to search for. At least not in the surface soils at the two loca-
tions where the Vikings landed.
Hopes of finding life on Mars were demolished for two decades. But
the Viking biology experiments, while failing to nourish any Martian
microbes, gave us plenty of food for thought. How do we design an
experiment to look for life on another planet when we’ve only observed
it on this one? It’s not a simple proposition. The question forces us to
think deeply about what life really is—about the essential features that
would transcend the specific natural history of one world.
U N C O V E R I N G V E N U S : W O R L D G O N E W R O N G ?
During those years of the first reconnaissance missions, the rest of the
solar system didn’t prove any friendlier to life as we know it. Mariner
2, the first machine (from Earth) to successfully visit another world,
flew by Venus in December 1962 and radioed back news that was dis-
heartening, at least for carbon-based creatures on Earth looking for
close company or a nearby vacation paradise. The surface of Venus is
/>
hot as a kiln and dry as bones. There, organic molecules would fare
about as well as a snowball in hell. The warm, wet, richly inhabited
56
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world that had dwelled in our scientific fantasies for hundreds of years
was not to be found on Venus.
The drastic differences between Earth and Venus pose a huge chal-
lenge for our young science of comparative planetology. We are close
neighbors, almost the same size and apparently built of the same mate-
rials overall, yet we have followed very different evolutionary paths.
Most strikingly, Venus lacks just those features of Earth that seem most
crucial to the survival and comfort of creatures like us: lots of water
and a climate in the right range to keep it liquid. The temptation is
strong to regard Venus as a world gone wrong, since Earth is so right—
at least for us. We generally assume, although we can’t yet prove, that
Venus and Earth were very much alike at the start. But Venus, closer to
the warming of the young Sun, suffered a “runaway greenhouse effect”
early on. The young Venusian oceans boiled off into space in a global-
warming disaster of mythical proportions, leaving “Earth’s twin” a
dried-up, burnt-out shadow of her former self. Admittedly, this analysis
is rife with Earth-bound bias. A sentient Venusian sulfur slug might
have a different perspective, but solar system history is written by the
survivors.
The atmosphere of Venus is tricky to explore, with its acid clouds,
ferocious winds, and turbulence that would make a United flight into
Denver feel like a pony ride. But if you think that’s bad, try exploring
the surface. Obscured by clouds whether viewed from Earth or from
orbit, it is difficult to probe in person, or even in robot, since we don’t
yet know how to design machines that can survive there for long with-
out frying. The Soviet Union’s persistent and methodical planetary
exploration program was much more successful on Venus than on
Mars. Two craft, Venera 9 and 10, built like big, round diving bells packed with refrigerants, made it to the surface in 1975 and snapped
several pictures before surrendering to heat death. These photos, taken
a year before Viking, were the first ever returned to Earth from the sur-
face of another planet. They depicted gently rolling scenes strewn with
volcanic-looking rocks, a little loose dirt hinting at some form of ero-
sion, and a dull, cloudy sky off in the distance.
I found these barren, warped, rocky vistas to be slightly repulsive yet
also enticing. Their unsettling otherworldliness was enhanced by the
strange bits of alien Russian space technology rimming the foreground,
and by the unusual geometry of the pictures, in which the camera cap-
tured a curving swath that dipped close to the ground in the center of
The Planets at Last
57
the frame but out to the distant horizon on the edges. It was not a land-
scape that made you want to pack a lunch and bound across it, yet
those shady, distorted rock fields were somehow compelling. Like a
piece of a fading dream you want to remember, this vague, tantalizing
glimpse of an unexplored world made me want to see more. Little did I
know at that time (I was a sophomore in high school) that I’d spend
years of my adult life trying to unravel the story of Earth’s twisted sis-
ter, Venus.
We didn’t get our next direct glimpse of the surface of Venus until
1982, when I was about ready to graduate from college. This time
(Venera 13 and 14) the pictures were in color, and the rocks were cast in red by the murky sunlight filtering through the thick clouds and
crushing atmosphere. The strange, ruddy quality of the light served as a
further reminder that these volcanic vistas were not of this world.
To uncover the story of Venus we needed global maps. Normal
orbital cameras using visible light are useless for that purpose, so, like
bats in flight or whales navigating the dark ocean depths, we must use
echolocation to map the contours of the cloud-covered Venusian ter-
rain. In 1979, the American spacecraft Pioneer Venus entered orbit,
bouncing radio waves off the surface and recording the echoes to
assemble our first crude global maps. These maps were a tease. You
could see a lot of interesting structure, but you couldn’t really tell what
you were looking at. When I was a student at Brown University in the
early eighties, one of my first research jobs was to help analyze these
indistinct but enticing maps. We could make out numerous circular fea-
tures dotting the Venusian plains. Were these impact craters or volca-
noes? A lot rested on the answer to this question, as we did not know if
the surface of Venus was ancient and full of craters like the Moon and
the southern half of Mars, or young, restless, and volcanically active
like the Earth.
After a decade of this torturous game of blindman’s bluff (during
which I got a Ph.D. in Tucson and then a postdoctoral fellowship at
Ames, a NASA research center south of San Francisco), we got another
spacecraft into orbit. Magellan, launched in 1989, mapped Venus for
four years using cloud-penetrating radar. With these greatly improved
radar eyes we saw towering volcanoes, vast plains flooded with lava,
and a surface intermediate in age between ancient Mars and youthful
Earth. Magellan did for Venus what Mariner 9 had done for Mars, giving us a first clear global view. In many ways, our state of understand-
58
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ing of Venus today is where our knowledge of Mars was in the seven-
ties, after Mariner 9 and before Viking. Venus is a bit easier to get to but a lot harder to explore. It will take some new, advanced technology
for long-lived landers like those of Viking to survive the sweltering con-
ditions on Venus. I expect to see it happen.
G A S G I A N T S A N D I C E M O O N S
During the 1970s and 1980s our eyes were opened to the rest of the
solar system by the epic travels of the Voyagers. Launched in 1977, the
year after the Viking landings, the two Voyagers flew by Jupiter in
1979 and Saturn in 1980. One craft, the indomitable Voyager 2, made
it to Uranus in 1986 and Neptune in 1989. These missions took
decades of hard work and intense planning, but the excitement was dis-
tilled into brief, manic “encounters” lasting only several days each, as
one of the Voyagers would race past one of these giant planets, fever-
ishly snapping pictures of its cloudy surface and its entourage of
moons. Then, having safely radioed the bounty home, the spacecraft
would quickly recede into the lonely depths of interplanetary space,
heading for the next new world. During each encounter, multiple
worlds were transformed instantly from obscure telescopic subjects
into concrete, detailed places. The stunning pictures from these bursts
of revelation will be treasured by humankind forever.
For planetary scientists the Voyager encounters were peak, formative
experiences, and the trajectories of those two spacecraft through the
/> outer reaches of our solar system became entwined with the trajectories
of our lives. When Voyager 2 reached Jupiter, I was a nineteen-year-old
undergraduate assisting the team of scientists who retrieved and ana-
lyzed the photos beaming back from deep space. At the Uranus
encounter I was participating as a twenty-six-year-old graduate stu-
dent, and at Neptune I was a postdoc pushing thirty.
These encounters became bonding experiences for our community, part
scientific conference, part family reunion, part soap opera. Each time our
beloved robot craft plunged through yet another new system of worlds
there was a gathering of the tribes as scientists and reporters descended on
the Jet Propulsion Laboratory (JPL) in Pasadena, California, where the
pictures and other information came down. Friendships formed and
solidified. Romances began and ended. Some of those who were instru-
mental early in the mission were no longer with us at the later encounters.
The Planets at Last
59
Politicians and entertainers would show up to join in the fun, satisfy
their curiosity, or make political hay from the stunning success of the
Voyager project. One surreal morning at the Neptune encounter, after
staying up all night watching the first close pictures come down from
Triton, Neptune’s schizoid frozen moon, my colleagues and I staggered
out into the too bright California sunlight, and I could swear we stum-
bled upon Vice President Dan Quayle (who is not a rocket scientist) try-
ing to milk the occasion by delivering an astonishingly insincere speech
to a politely inattentive crowd.
Voyager revealed an outer solar system much more varied than we
had expected and expanded our ideas about where we might find life.
The most delightful surprises involved the myriad diverse moons orbit-
ing these giant planets. The life stories of these small worlds turned out
to be more complex and interesting than we had surmised. Surprisingly,
several of them showed signs of recent geological activity.
The Galileo spacecraft orbited Jupiter from 1996 to 2003, dropping
further hints suggesting (though not yet proving) the existence of a
liquid-water ocean beneath the surface of Jupiter’s moon Europa. This
icy moon became a major focus of our remaining hopes for alien life
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