The Planets

by Dava Sobel

  Not one of them set foot on the far side, though they all saw its strange terrain first-hand from lunar orbit, and remain the only humans ever to have done so. They could have exclaimed any expletive or sentiment in the private thrill of that revelation, since traveling behind the Moon cut off their radio contact with Houston and the rest of the world. Apollo command module pilots, who stayed in orbit while the landing parties worked the surface, experienced a profound solitude over the far side, out of touch with all civilization—including their teammates—for forty-eight minutes out of every two-hour loop around the Moon. The far side of the Moon is the one place in the whole Solar System deaf to Earth’s radio noise.

  Like the hidden half of any being, the lunar far side bears scant resemblance to the face the Moon shows the world. More craters abound there, overlapping in profusion, and one sees hardly any of the smooth dark expanses of pooled lava that characterize the near side. The thicker crust on the Moon’s back apparently checked the expulsion of lava from within.

  All geologic ferment on the Moon ceased about three billennia ago, after the late heavy bombardment cleared the Solar System of most menacing massive projectiles. Today, a ton-mass meteorite strikes the Moon no more than once in three years, on average. The occasional Moonquake can be confidently dismissed as a weak reaction to tidal stress, not the stirrings of a living planet with a liquid core.

  Only micro-meteorites continue to fall steadily on the dead Moon, thickening the dust by a millionth of a millimeter per year. This influx constitutes the major tectonic force now at work on the Moon. Selenologists call it “gardening,” because the new arrivals mix and turn over the sterile lunar “soil” as they insert themselves into it. The gentle process barely disturbs the present still life on the Moon—the arrays of scientific instruments, the litter of spent rocket stages, the three parked rover vehicles.

  Among the personal talismans intentionally left behind, a posed snapshot of an astronaut and his family calls attention to itself. Someone took care to wrap that photo in plastic for protection—as though anything could happen to it on the Moon’s arid, uneventful surface, where a bootprint enjoys a life expectancy of a million years, and every dust particle savors of immortality.

  *Valued as exotica, a single carat of Moon rock sold at auction in 1993 for $442,500. Similarly, a site map of the Moon’s Descartes highlands, only slightly used by Apollo 16 astronauts and bearing smudges of lunar dust, brought $94,000 at a sale in 2001.

  *A second, which once divided a mean solar day into 86,400 equal parts, is now defined as the time a confined cesium-133 atom takes to complete 9,192,631,770 natural vibrations. Since 1972, the International Earth Rotation Service has added twenty-four leap seconds, always inserted into the first moments of January or July.

  SCI-FI

  Call me “It,” or call me “Allan Hills 84001,” my given name—even “Thing from Mars” will suit. Although I am only a rock and cannot answer, allow me this conceit of conscious identity for the space of these few pages, that I may speak for Mars, whence I traveled via chance and the laws of physics.

  Of the thirty-four Martian meteorites definitively identified to date, I am by far the most ancient, and the only one to show, under microscopic examination, internal shapes and residues similar to those formed by primitive terrestrial bacteria. These findings have made me the most studied rock of all time.

  One might surmise I had been contaminated by Earth life during the thirteen thousand years I lay in the Antarctic ice fields before scientists collected me there in 1984. The scientists certainly assumed contamination, until they ruled out the possibility, concluding in near disbelief that it was more likely I had once sheltered small beings on my home planet—creatures perhaps already extinct when an asteroid impact flung me from the Martian surface sixteen million years ago.

  My story, consonant with the history of Mars in human regard, seems riveted on Martian life, despite my vagueness on this subject. I have little to say of fossil life, and even less to contribute to conjecture about life on Mars today. Therefore I make no bold claims, lest I be lumped in the company of such fiercely imagined aliens as the giant sand worms of Arrakis, or the wild thoats, green men, and great white apes of Barsoom.*

  My Martian origin, however, I state as indisputable. My constitution mirrors the chemical makeup of rocks and dust examined in situ on the planet’s surface and from close orbit by visiting spacecraft. Traces of gases, trapped in glassy bubbles within my matrix, exactly match the sampled atmosphere of Mars, element for element and in the same relative abundance of rare isotopes. My foreign nature could never have been proven before the current age of spacefaring, and yet I came to Earth without benefit of any artificial conveyance.

  The collision that launched my journey dug a hole in Mars several miles wide. Astronomers think they have identified that particular crater on satellite images of Mars, near a small valley in the southern highlands. The violence of the impact lofted tons of crustal rock into the sparse atmosphere at high speed, and all the fastest-moving pieces—the ones accelerated past the local escape velocity of three miles per second—hastened out of the planet’s grip forever.

  As a Martian from a heavily cratered region, I was acquainted with meteorite strikes, and in fact already bore a fracture scar from having been crushed and reheated in a previous impact. But now I found myself become a meteorite, or more correctly a meteoroid, that is to say, a true space wanderer loosed from one world and not yet landed at another. Sixteen million years of seemingly aimless rambling at length brought me close enough to Earth to be captured by its gravity, which is three times stronger than the pull of Mars. By long odds I should have disappeared into an ocean, the fate of most meteorites that survive their fiery descent to Earth’s surface, but instead I fell near the South Pole, during the last Ice Age, on a bed of frozen water.

  Snows came and covered me, folded me into the slowly flowing glacier, and together we crept forward for thousands of years. Not until we reached the Allan Hills, and tried to climb them, did the sharp cliffs and arctic winds pluck me from the ice to leave me lying exposed again.

  The scientists arrived on seven snowmobiles, in dragnet formation, hunting dark rocks on the blue-white ice, confident that all such finds would prove extraterrestrial, whether from the Moon, the Asteroid Belt, or Mars. Even though I measure no larger than a squarish softball, or a four-pound potato, they spotted me easily by color contrast. “This green rock,” I appeared to them in that dazzling expanse of ice and light, only later fading to gray, “dull gray,” in the laboratory.

  I was airlifted to the United States, to the Johnson Space Center in Houston, Texas, where my age was established by two independent mother-daughter radio-isotope measurements, one analyzing the proportion of samarium that had decayed to neodymium inside me, while the other pursued the radioactive transformation of rubidium to strontium. Both assays yielded the same result, citing a lapse of 4.5 billion years since the time I had crystallized, though these tests said nothing of my provenance. At first examiners took me for an igneous rock from the asteroid Vesta, but later they fired a narrow beam of electrons at some of my grains, exciting my near-surface atoms to emit X-rays that revealed the wider truth of my alien composition, especially the form of iron I contain, which positively identified me as Martian.

  My extremely advanced old age sets me apart from other known Martian meteorites. At 4.5 billion years, I quadruple the age of the second-oldest in the group, which suggests that I am a piece of Mars’s original planetary crust. No comparable Earth rock has yet been discovered, for the most ancient of these does not exceed 4 billion years, and only a single rock retrieved from the Moon, the so-called “Genesis Rock,” rivals my extraordinary antiquity.

  A sturdy relic of the Solar System’s earliest days, I have maintained myself, virtually unaltered, through eons likely to have seen me pulverized by impact or melted in a volcano and subsequently reincarnated after cooling.

  Mars is a
great respecter of longevity. Most of Mars’s surface endures today much as it always has, while Earth and Venus go on reinventing theirs through constant upheaval. Yet Mars is no slavish preservationist like the Moon or Mercury, whose static vistas are shaped almost entirely by outside forces. On the contrary, my planet, a globe only half the size of Earth, raised the tallest mountains in the Solar System, carved vast labyrinthine valleys, inundated its lands with liquid water, and then froze to a desert of spectacular dunes in a palette of reds, yellows, and browns so vivid as to make Mars, seen from afar, glow like an orange star.

  The Martian landscape hosts a desert more dust than sand, and when its fine, smooth, iron-rich particles of rusted dust hang in the sky like a haze of smoke, they share their color with the air. The pinkish atmosphere, consisting predominantly of carbon dioxide, exerts a barely perceptible pressure at ground level, only one-hundredth that of Earth, but how its winds do spur the dust to action! Lone dust devils spiral up and snake across the open spaces. Masses of dust rise in whirling sallow storm fronts fit to rage for days, and sometimes grow into global tempests that envelop the whole of Mars for months, until the dust-laden air finally tires of its burden.

  Bright white ice caps at the planet’s poles advance and retreat over the ruddy ground in a rhythmic seasonal cycle of changing weather. Between the poles the land divides itself into unequal halves, with most of the ancient, heavily cratered highlands concentrated in the south, where I came from, and the younger, lower-lying plains in the northern hemisphere. Those far northern plains lie so low that the planet looks lopsided, with its south pole four miles farther from the equator than the north pole.

  Just north of the equator, mighty Olympus Mons attained its altitude of Alps atop Rockies upon Himalayas early in Martian history, when the leftover heat of planetary coalition escaped in eruptions of lava copious enough to build a dozen monstrous mountains and scores of smaller ones. Since then Mars’s peaks have received many crater hits on their flanks, but suffered hardly any erosion. The white water-vapor clouds around their summits drop no damaging rains down the mountainsides, and the visiting winds carry only fine, smooth particles of clay dust, almost too soft to wear the rock away.

  East of Olympus, ancient faults split thousands of miles of ground asunder to gouge the grand canyons of the Valles Marineris. Landslides widened these chasms and rushing waters deepened them, molding islands shaped like teardrops on their floors, but all the planet’s steep-sided valleys stand empty now that the Martian water supply has disappeared from view.

  The warmer, wetter climate of the distant past may have ended violently, when the impacts that excavated Mars’s widest, deepest basins blew away the water vapor and nitrogen that had once thickened the atmosphere. Then the liquid water fled the surface by every available means, evaporating and dissipating into space, flowing down into hidden aquifers, hibernating as subterranean permafrost.

  My own experience of Mars dates back to a time of liquid water. Between 1.8 and 3.6 billion years ago, as narrowly as cosmochemists can estimate, water from Martian hot springs bathed me, permeated the cracks I had sustained in earlier shocks, and lined the cracks with signature veins of carbonate minerals. These mineral deposits now account for one-tenth of my bulk makeup, and all the signs of life inside me reside within them.

  Startling and unprecedented as my cargo of genuine extraterrestrials might be, science embraces the possibility. Whatever forces sparked the emergence of life on Earth, three to four billion years ago, could have done the same on Mars at that same early epoch. Even supposing that only Earth alone among all the planets ever gave birth to life, it is still conceivable that at least one archaeobacterium left Earth, sealed in a spore-like state of suspended animation inside some meteoroid, and arrived at Mars by the same train of circumstances that delivered me here. Surely sufficient time has elapsed in the life of the Solar System for such a sequence of events to have played out, perhaps even repeated itself.

  To read the evidence lining my fractured interior is to stretch the limits of intuition and instrumentation alike. High-resolution scanning electron microscope images show bacteria-like colonies of minuscule sausage-shaped forms, including one with segments like those of a worm. After close-up photos appeared in news reports worldwide in 1996, however, further investigation suggested that the suspected micro-fossils were neither Martian nor terrestrial remains, but the artifacts of lab procedures that had been used to prepare my samples for study. The processing had caused textural changes that inexplicably copied the outline of familiar life forms—the way a windblown mesa on Mars may per chance assume the contours of a human face.

  Three other promising indicators of life, including my content of organic molecules called polycyclic aromatic hydrocarbons, failed to provide conclusive proof. Still remaining to be explained, or explained away, are the tiny grains of magnetite around my carbonate globules. No inanimate process, so far as anyone knows, yields this type of pure magnetite, which is produced on Earth by aquatic bacteria of the strain MV-1. The dark, singular crystals are all that now sustain my proffered hope of Martian life, yet they suffice.

  The long-accepted likelihood of Mars as an abode of alternate life draws strength from the planet’s solid ground and Earth-like pattern of days and nights. A Martian day—a sol—lasts just a little more than half an hour longer than a day on Earth, since the two bodies spin at nearly the same rate. They also tilt on their axes at almost the same angle, with Mars tipped 25 degrees and Earth 23.5, which accounts for their similar passage through seasons over the course of each year.

  The wider compass of the Martian orbit naturally extends each season, as Mars takes 687 Earth days to make its longer, slower way around the Sun. All the seasons are cold ones, with an average annual global temperature of 40 below zero, compared to Earth’s 59 degrees Fahrenheit. Prevailing cold, however, need not rule out the possibility of life, considering all the seemingly inhospitable niches on Earth—inside volcanic vents on the ocean floor, in oil reservoirs, in buried rock salt—that serve as homes for tube worms, blue-eyed pink vent fish, and other known extremophiles.

  The orbits of Earth and Mars bring the two planets within thirty-five million miles of each other every fifteen to seventeen years, enlarging Mars three-fold in telescopic views at these times, and thus imposing a natural tempo on the pace of early discovery. With Mars near in August of 1877, for example, its long-suspected moons revealed themselves at last as two small, dark companions, Phobos and Deimos, practically at the limit of detection, and traveling so fast that lunar months on Mars could be reckoned by these satellites in a matter of hours.

  During that same 1877 planetary encounter, networks of straight-sided Martian canali were sighted from Italy and plotted on new maps of Mars. The Italian “channels” translated into English “canals” in time for the next close approach, in 1892, when an American enthusiast insisted he saw several hundred canals, and soon attributed their existence to the desperate irrigation efforts of a dying race.*

  The fixed idea of a Martian alter-ego guided preparations for the next Mars approach of August 1924, when civilian and military broadcasters proposed a three-day radio silence, in order to listen for intelligent signals from Martians. The U.S. Army designated its chief signal officer to attempt decoding any intercepted transmissions, and although the value of his services went untested in that effort, British and Canadian wireless operators reported several unidentified radio beeps. Meanwhile, observers in the Swiss Alps directed a greeting to Mars in the form of a lens-amplified light ray reflected off the snow-covered slopes of the Jungfrau, and astronomers confirmed that the moving bright spots seen through improved telescopes were clouds in the Martian atmosphere.

  Rather than wait decades for the planets to align themselves in such research-favorable positions, planetologists and rocket scientists of the 1960s began taking advantage of ideal launch opportunities, which arise every twenty-six months, to send a series of flybys, orbiters, and la
nders to Mars. These spacecraft followed the efficient, purposeful route of the Hohmann transfer orbit trajectory, calculated to carry them from Earth launch to intersection with Martian orbit less than a year later, just in time to intercept the planet at that point.*

  Mishaps have prevented half the Mars-destined craft from either gaining their difficult objective or functioning profitably there, including three intended landers that accidentally crash-landed and destroyed themselves on contact. Among the numerous successes, however, five landers set up both stationary and roving field laboratories, automated to take in Martian air and soil samples for analysis.

  Vikings 1 and 2, the first pair of twin robot scientists from Earth to search for life on Mars, reached the golden plains of Chryse and Utopia in the summer of 1976, while I lay still entombed in the wintry Antarctic. They settled on landing sites named for classical fantasies and vague nineteenth-century impressions of my native world. Even now that on-site surveys have made sense of Mars’s actual topography, many romantic allusions persevere in the otherwise logical nomenclature scheme instituted by modern areographers. Thus the large dry river valleys discovered in the early 1970s, such as Ares Vallis and Ma’adim Vallis, recognize the war god Mars or the word “star” in various human languages—the sole exception being Valles Marineris, the greatest valley of all, which honors its discoverer, Mariner 9, the first artificial satellite ever to orbit a planet other than Earth. The smaller valleys take their names from Earthly rivers, either classical or actual. (Evros Valles, near my former home, shares its name with a river in Greece.)

  Large, ancient Martian craters, newly visualized, now bear the names of scientists and science-fiction writers, including Burroughs and Wells, and small craters the names of small Earthly villages with populations of fewer than 100,000. On the smallest level, individual surface rocks spotted in the intimate photos taken by landed spacecraft have assumed whimsical names from cartoons and storybooks, including Calvin and Hobbes, Pooh Bear and Piglet, Rocky and Bullwinkle, or nicknames based on their appearance: “Lunchbox,” “Lozenge,” and “Rye Bread.” Although my own name is specific and descriptive, occasionally I have been called “Big Al” and other such convenient nicknames in closed-door discussions among researchers.