Lonely Planets

by David Grinspoon

  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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  L o n e l y P l a n e t s

  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

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  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
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  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

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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

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  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-

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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.

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  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